Dawn of the Atomic Age

A New Century - A New Universe

The turn of the century marked a profound revolution in the development of science and our understanding of the fundamental principles of the natural world.

During the nineteenth century classical physics - the laws of motion, electromagnetic ffields, and thermodynamics - had reached an advanced state of development. Chemistry had also reached a considerable degree of sophistication but on a largely empirical basis, the fundamental basis of chemistry remained mysterious. Much had been learned about the Earth and solar system as well. Estimates of the age of the Earth had risen from about 6000 years in the late eighteenth century to tens or hundreds of millions of years; and the view that life, the Earth, and the rest of the solar system had arisen in a single great upheaval in recent times had been replaced by the idea of gradual change over eons.

To some it seemed that science, especially physics, was reaching such a state of maturity that few fundamental principles remained to be discovered. But there were problems. Essentially nothing was known about the fundamental structure of matter that gave rise to the Periodic Law and other chemical behaviors - the very existence of atoms was largely conjectural. Geology and astronomy seemed in serious conflict since the apparent age of the geologic record could not be reconciled with the only power source for the Sun then conceivable, gravitational contraction, which would exhaust itself in mere millions of years. An important part of classical thermodynamics was stubbornly resisting resolution - the properties of blackbody radiation. In fact by the end of 1900s it had become clear that within the existing framework of physics no solution of the blackbody problem was possible (the untenable prediction made by existing physics was termed the "ultraviolet catastrophe"). Something important was missing.

Advancing experimental technique in the seemingly well understood field of electricity and magnetism gave the first clues to the new universe. In 1895 Wilhelm Konrad Roentgen at the University of Wurzburg discovered X-rays. He had been conducting experiments involving high voltage currents in an evacuated tubes. The penetrating radiation he discovered was wholly new and unexpected.

The following year (1896), by serendipitous accident while investigating X-rays, Henri Becquerel (at the Museum of Natural History in Paris) discovered radioactivity in a piece of uranium salt. This discovery provided for the first time direct evidence of the fundamental structure of matter, and also revealed the existence a totally new source of energy independent of the Sun's rays, or of chemical fuels, and vastly more concentrated than either.

Discoveries followed rapidly. Marie Skladowska Curie and her husband Pierre Curie immediately began isolating sources of radiation from uranium ore. This led to the discovery of polonium in 1896, and radium in 1897. Different types of radioactive emissions were soon identified: in 1899 Becquerel found that at least some of the radiation emissions were electrically charged, and Ernest Rutherford further distinguished two types of charged emissions - alpha and beta rays, Paul Villard identified neutral gamma rays.

During this time another key discovery was in the making - the development of Quantum Theory. Two threads led to the foundation of the theory, one theoretical and one experimental. The theoretical development was by Max Planck at the University of Berlin. In pursuing the perplexing problem of blackbody radiation, he developed a theory announced in 1900 that successfully predicted the observed blackbody spectrum. This theory postulated that matter could only absorb or emit energy in arbitrary units or "quanta". In 1898 J.J. Thomson detected the emission of electrons when a metal surface is illuminated by ultraviolet light - the photoelectric effect. The properties of this phenomenon could not be explained, particularly a metal-dependent frequency threshold for the emissions. Albert Einstein united these threads with his theory of the photoelectric effect in 1905 which proposed the existence of the photon - quantized light (for which he received the Nobel Prize). Also in 1905 Einstein formulated his Special Theory of Relativity, one aspect of which (the equivalence of mass and energy) began to give some insight into the origin of the atomic energy that had been revealed by the discovery of radioactive decay.

These developments had also greatly extended the understanding of the Earth and Sun. In 1905 Rutherford and Boltwood used the ratio between radioactive isotopes and their decay products to data a rock to 500 million years old. This great age sharpened the conflict with classical theories of solar development, but radioactivity also offered a resolution. Perhaps some atomic transformation process, not then understood, was the source of the Sun's brilliance and longevity.

The New Universe Explored

With the hints given by these new discoveries, and the powerful new probes of matter offered by the newly discovered ionizing radiations, more discoveries followed swiftly.

Rutherford soon demonstrated that alpha particles were in fact helium atoms, minus their electrons.

In 1906 Rutherford began a series of experiments at McGill University where he was now professor, and continued at the University of Manchester. In these he experiments he studied how alpha rays were scattered by thin layers of mica and gold.

The age of the Earth jumped again in 1907 when Boltwood identified a piece of uraninite as being 1.64 billion years old.

In 1911 Rutherford published his conclusions drawn from the alpha scattering experiments - that nearly all of the mass of the atom is concentrated in a tiny positively charged region in the center called the nucleus.

J.J. Thomson discovers isotopes of neon in 1912, showing that the atoms of the same element could have different masses.

Although it was realized late in the nineteenth century that the identities of chemical elements were related to the number of electrons that each atom contained (the atomic number), it was difficult to determine this number accurately for most elements. In 1913 H. G. J. Moseley demonstrated that by studying X-ray emissions, the atomic number could be easily measured.

It was now possible to study the relationship between the atomic charge (the atomic number) and the atomic mass. Evidence began to accumulate that there were two principal contributors to the mass of the atom and the nucleus, one that was positively charged (later called the proton), and one that was neutral (the neutron).

Also in 1913, Niels Bohr made a key theoretical breakthrough. He devised the "Bohr atom" - a planetary model of the hydrogen atom with the electron orbiting the positively charged nucleus - that explained studying the spectrum of light emitted by hydrogen atom. This model was based on the quantum theory, and was consistent with the atomic structure observed by Rutherford.

Although physics and science continued to advance (Einstein completed the General Theory of Relativity during this period for example) there was a temporary doldrum in key discoveries about the structure of matter lasting for several years. This is partly explainable by the calamity of the First World War that disrupted all of Europe. Some of the destructive effects of the war on science were quite direct - the young genius Moseley perished in the trenches of Gallipolli.

Discovery of the Neutron

On June 3, 1920 Ernest Rutherford gave his second Bakerian Lecture in London, and in the course of this lecture he speculated on the possible existence and properties of the neutron. This is apparently the earliest public proposal of the idea of positive and neutral particles composing the atomic nucleus.

In 1921 the American chemist H.D. Harkins coined the term "neutron" in a proposal of nuclear structure. Rutherford published further work on the idea in this same year. Little progress was made on developing the idea, or proving its existence for the next several years.

In 1930 two German physicists, W. Bothe and H. Becker, observed unusually penetrating radiation being emitted from beryllium metal when it was bombarded by alpha particles. On 28 December 1931 Irene Joliot-Curie (Marie and Pierre's daughter) reported on these same emissions, but like Bothe and Becker, believed them to be energetic gamma rays. Joliot-Curie discovered that these emissions produced large numbers of protons when they passed through paraffin, or other hydrogen containing materials, something never observed (and apparently impossible to explain) with gamma rays.

Over a ten day period, from February 7 to 17, 1932 James Chadwick conducted a series of experiments that conclusively demonstrated that these unusual emissions were actually neutrons. Using this new potent new tool, rapid progress on the structure of matter began to be made.

Although radioactive decay releases an enormous amount of energy compared to chemical processes, this energy release is gradual and cannot be modified to any significant degree. The possibility of "atomic energy" as a source of human controlled power thus came into existence as a concept, but without any known means of bringing it about - even in theory. On September 12, 1933 this changed.

On that day the brilliant Hungarian physicist Leo Szilard conceived the idea of using a chain reaction of neutron collisions with atomic nuclei to release energy. He also considered the possibility of using this chain reaction to make bombs. These insights predate the discovery of an actual chain reaction process - fission - by more than six years.

Invention and Discovery: Atomic Bombs and Fission

Leo Szilard and the Invention of the Atomic Bomb

It would be logical to assume that the discovery of fission preceded the invention of the atomic bomb. It would be normal also to expect that no single individual could really claim to be "the inventor", since the possibility sprang naturally from a physical process, and required the efforts of many thousands to bring it into existence. Many descriptions of the origin of atomic bombs can be found that logically and normally say exactly these things.

But they are not correct.

The idea of "invention" does not usually require the physical realization of the invented thing. This fact is clearly recognized by patent law, which does not require a working model in order to award a patent. It is common for inventions to require additional discoveries and developments before the actual thing can be made. In these cases, an invention may fairly have more than one inventor - the originator of the principle idea, and the individual who actually made the first workable model.

In the case of the atomic bomb there is clearly one man who is the originator of the idea. He was also the instigator of the project that led ultimately to the successful construction of the atomic bomb, and was a principal investigator in the early R&D both before and after the founding of the atomic bomb project - making a number of the key discoveries himself. By any normal standard this man is the inventor of the atomic bomb.

This man is Leo Szilard.

On September 12, 1932, within seven months of the discovery of the neutron, and more than six years before the discovery of fission, Leo Szilard conceived of the possibility of a controlled release of atomic power through a multiplying neutron chain reaction, and also realized that if such a reaction could be found, then a bomb could be built using it.

On July 4, 1934 Leo Szilard filed a patent application for the atomic bomb In his application, Szilard described not only the basic concept of using neutron induced chain reactions to create explosions, but also the key concept of the critical mass. The patent was awarded to him - making Leo Szilard the legally recognized inventor of the atomic bomb.

Szilard did not patent this prescient and tremendously important idea for personal gain. His motive was to protect the idea to prevent its harmful use, for he immediately attempted to turn the idea over to the British government for free so that it could be classified and protected under British secrecy laws.

On October 8, 1935 the British War Office rejected Szilard's offer, but a few months later in February 1936 he succeeded in getting the British Admiralty to accept the gift. Szilard's actions in attempting to restrict the availability of the atomic bomb, are also the earliest case of nuclear arms control. Later, when the possibility of a German atomic bomb had been shown to be nonexistent, Szilard campaigned vigorously against the use of the bomb.

The Discovery of Fission

With the discovery of the neutron by James Chadwick in February 1932 a scientific gold rush ensued to discover what effects would be produced by bombarding different materials with this new particle. Over the next several years, teams of researchers in several countries (especially one headed by Enrico Fermi in Rome) bombarded every known element with neutrons and recorded scores, even hundreds, of new radioactive isotopes.

On May 10, 1934 Fermi's research group published a report on experiments with neutron bombardment of uranium. This was the first such investigation to be reported on. Several radioactive products are detected, but positive identifications were not made. Interpreting the results of neutron bombardment of uranium became known as the "Uranium Problem" since the large number of different radioactivities produced defied rational explanation. The dominant theory was that a number of transuranic elements never before seen were being produced, but the chemical behavior as well as the nuclear behavior of these substances were unexpected and confusing.

The first statement of the correct resolution of the Uranium Problem was published by German chemist Ida Noddack in September. Her letter in _Zeitshrift fur Angewandte Chemie_ argued that the anomalous radioactivities produced by neutron bombardment of uranium may be due to the atom splitting into smaller pieces. No notice of this suggestion was taken.

Fermi discovered the extremely important principle of neutron behavior called "moderation" on October 22, 1934. Moderation is the phenomenon of enhanced capture of low energy neutrons, as when they are slowed down by repeated collisions with light atoms.

o        December 1935, Chadwick won the Nobel Prize for discovery of the neutron.

o        In November-December 1938, the Otto Hahn and Lise Meitner correctly unravel the Uranium Problem. Hahn determines conclusively that one of the mysterious radioactivities is a previously known isotope of barium. Working with Meitner, they develop a theoretical interpretation of this demonstrated fact. On December 21, 1938 Hahn submits a paper to _Naturwissenschaften_ showing conclusive evidence of the production of radioactive barium from neutron irradiated uranium, i.e. evidence of fission.

o        In the first few weeks of January, word of the discovery traveled quickly in Europe.

o        January 13, 1939 - Otto Frisch observed fission directly by detecting fission fragments in an ionization chamber. With the assistance of William Arnold, he coins the term "fission".

o        By mid January Szilard heard about the discovery of fission from Eugene Wigner, and immediately realized that the fission fragments, due to their lower atomic weights, would have excess neutrons which must be shed. The multiplying neutron chain reaction that he had postulated had finally been discovered.

o        January 26, 1939 - Niels Bohr publicly announces the discovery of fission at an annual theoretical physics conference at George Washington University in Washington, DC. This announcement was the principal revelation of fission in the United States.

o        January 29, 1939 - Robert Oppenheimer hears about the discovery of fission, within a few minutes he realized that excess neutrons must be emitted, and that it might be possible to build a bomb.

o        February 5, 1939 - Niels Bohr gained a crucial insight into the principles of fission - that U-235 and U-238 must have different fission properties, that U-238 could be fissioned by fast neutrons but not slow ones, and that U-235 accounted for observed slow fission in uranium. At this point there were too many uncertainties about fission to see clearly whether or how self-sustaining chain reactions could arise. Key uncertainties were:

1.        The number of neutrons emitted per fission, and

2.        The cross sections for fission and absorption at different energies for the uranium isotopes.

For a chain reaction there would need to be both a sufficient excess of neutrons produced, and the ratio between fission to absorption averaged over the neutron energies present would need to be sufficiently large.

The different properties of U-235 and U-238 were essential to understand in determining the feasibility of an atomic bomb, or of any atomic power at all. The only uranium available for study was the isotope mixture of natural uranium, in which U-235 comprised only 0.71%.

o        March, 1939 - Fermi and Herbert Anderson determine that there are about two neutrons produced for every one consumed in fission.

o        June, 1939 - Fermi and Szilard submit a paper to _Physical Review_ describing sub-critical neutron multiplication in a lattice of uranium oxide in water, but it is clear that natural uranium and water cannot make a self-sustaining reaction. This paper is the first experimental evidence of neutron multiplication.

o        July 3, 1939 - Szilard writes to Fermi describing the idea of using a uranium lattice in carbon (graphite) to create a chain reaction. This is the first proposal of the graphite moderated reactor concept.

o        August 31, 1939 - Bohr and John A. Wheeler publish a theoretical analysis of fission. This theory implies U-235 is more fissile than U-238, and that the undiscovered element 94-239 is also very fissile. These implications are not immediately recognized.

o        September 1, 1939 - Germany invades Poland, beginning World War 2.

The Manhattan Project (and Before)

Prelude to the Manhattan Project

At the same time that the key discoveries in neutron physics and neutron reactions were occurring (see Invention and Discovery: Atomic Bombs and Fission, the political situation around the world was deteriorating. The two key developments were of course:

·         the armed expansionism of military dominated Japan (commencing with invasion of Manchuria in September 1931), and

·         the rise and expansion of Nazi Germany (commencing with Adolf Hitler's appointment as Chancellor in January 1933, and his acquisition of dictorial power in March).

The deterioration was more general than that though. Political and social repression, instability, and military violence were rising throughout Europe - Italy, Spain, Central Europe; and Stalinism was reaching a fevered pitch in the Great Purges (1936-38).

The rush of ominous events is too thick to enumerate in a brief overview but some principal ones are:

o        Nuremburg Laws begin severe persecution of Jews March 1936:

o        Occupation of the German Rhineland July 1937:

o        Japan invades China November 1937:

o        The Axis Alliance is created by a pact between Germany, Japan, and Italy March 1938:

o        the Anschluss (occupation of Austria by Germany) September 1938:

o        German occupation of the Sudetenland in Czechoslavakia

It is against this background that Szilard fretted about the possibility of an atomic bomb. The discovery of fission came just as Germany was girding itself to abandon expansion by intimidation and resort to armed conquest.

World War II erupted at a moment when the promise of atomic energy had progressed from being possibile to being probable. It was not clear whether this energy could be released explosively however.

Szilard, as always, was both a man of vision and a man of action. Well known among European physicists, Szilard drafted a letter in consultation with Albert Einstein that was addressed from Einstein to President F.D. Roosevelt and which warned hime of the possibility of nuclear weapons (the "Einstein Letter"). This letter was delivered to FDR on October 11, 1939, and ten days later the first meeting of the Advisory Committee on Uranium (the "Briggs Uranium Committee") was held in Washington, DC on Pres. Roosevelt's order.

Due largely to persistent official lack of interest, the progress on the subject was desultory and inconclusive in the United States. The next key developments occurred in the United Kingdom.

During February 1940, expatriot physicists Otto Frisch and Rudolf Peierls, living in the UK prepared a theoretical analysis of the possibility of fast fission in U-235. Their report contains the first well grounded (although rough) estimates of the size of a critical mass ("a pound or two") and probable efficiency, and proposed practical schemes for bomb design and the production of U-235. This "roadmap" for fission weapon development would be elaborated upon and modified to a spectacular degree in the coming years, but it remains basically sound.

So persuasive is the report by Frisch and Rudolf Peierls' that a study committee is formed at the highest levels of goverment (eventually code-named the MAUD Committee) on April 10. By December the MAUD Committee would issue a key report selecting gaseous diffusion as the most promising method of uranium enrichment.

Through 1940 and well into 1941, work accelerated in the U.S., and important discoveries accumulated although official interest and support languished. In February, 1941 Philip Abelson began actual development of a practical uranium enrichment system (liquid thermal diffusion) and on February 26 Glenn Seaborg and Arthur Wahl discover plutonium. During March the first American measurements of the U-235 fission cross section allow Peierls to calculate the first experimentally supported estimate of a critical mass for U-235 (18 lb as a bare sphere, 9-10 lb when surrounded by a reflector).

By July 1941 plutonium was demonstrated to be a superior fissile material, and the MAUD Committee completed its final report, describing atomic bombs and project propsals for building them in some technical detail.

On September 3, 1941, with PM Winston Churchill's endorsement, the British Chiefs of Staff agree to begin development of an atomic bomb. But it is not until December 18, after months of bureaucratic struggling and the U.S. entry into the war, that a U.S. project to investigate atomic weapons (as opposed to "study fission") finally gets underway.

This Manhattan Project predecessor, code named the S-1 project, was headed by Arthur H. Compton. The core group of scientists that would lead the development of the atomic bomb had coalesced well before this, and was already working as hard as resources allowed on the problem.

In January 1942, Enrico Fermi's on-going work with graphite and uranium was transferred to a new secret project, code named the Metallurgical Laboratory (Met Lab) at the University of Chicago. In April Fermi begins design of CP-1, the world's first (human built) nuclear reactor.

Throughout early and mid 1942, fundamental neutron physics research proceeded, as did work on developing industrial scale processes for producing fissile materials. But it became increasingly obvious that since this was to be an industrial scale project, a proven project manager was called for. Furthermore, since it was a weapons project, it need to be brought under an organization experienced in producing weapons.

On June 18, 1942 Brig. Gen. Steyr ordered Col. James Marshall to organize an Army Corps of Engineers District to take over and consolidate atomic bomb development. During August Marshall created a new District organization with the intentionally misleading name "Manhattan Engineer District" (MED), now commonly called "The Manhattan Project".

The Manhattan Project

Despite its official founding in August, the Manhattan Project really began on September 17, 1942 when Col. Leslie Richard Groves was notified at 10:30 a.m. by Gen. Brehon Somervell that his assignment overseas had been cancelled. Groves, an experienced manager who had just overseen the collosal construction of the Pentagon, seized immediate and decisive control. In just two days he resolved issues that had dragged on for months under Compton. On September 18 Groves ordered the purchase of 1250 tons of high quality Belgian Congo uranium ore stored on Staten Island, and the next day purchased 52000 acres of land to be the future site of Oak Ridge. Groves was promoted to Brigadier General on September 23. By September 26 Groves had secured access to the highest emergency procurement priority then in existence (AAA).

The era of weak, indecisive leadership was over.

Groves' pushy, even overbearing, demeanor won him few friends among the scientists on the Manhattan Project (in particular a special enmity developed between Groves and Szilard). Many detested him at the time, considering him a boor and a buffoon. It was only after the war that many scientists began to appreciate how crucial his organizational and managerial genius was to the MED.

During the fall, while Fermi built CP-1 in Chicago, Groves took the fissile material programs out of the hands of the scientists and placed them under the management of industrial corporations like DuPont and the Kellog Corporation. He ordered construction begun immediately on the fissile material production plants, even though designs and plans had not yet been drawn up, realizing that the same basic site preparation work would be required no matter what.

On October 15, 1942 Groves asks Dr. J. Robert Oppenheimer to head Project Y, the new planned central laboratory for weapon physics research and design. The site for which he selected on November 16 at Los Alamos, New Mexico.

Oppenheimer, a professor of physics at Berkeley, had demonstrated a special skill at leading groups of scientists during the S-1 program, which Groves quickly took notice of. Oppenheimer and Groves developed a good relationship, each recognizing how critical the other was to the project.

On December 1, 1942, after 17 days of round-the-clock work Fermi's group completed CP-1 (sooner than planned) when Fermi projected that a critical configuration had been reached. It contained 36.6 metric tons of uranium oxide, 5.6 metric tons of uranium metal, and 350 metric tons of graphite.

On December 2, 1942 - 3:49 p.m. CP-1 went critical and was allowed to reach a thermal output of 0.5 watts (ultimately it was operated up to a maximum power level of 200 watts).

In January, 1943 Groves acquired the Hanford Engineer Works, 780 square miles of land on the Columbia River in Washington for plutonium production reactors and separation plants. During March Los Alamos began operations as the staff arrived.

During the remainder of 1943, work continued on the construction of the plutonium production facilities (reactors and chemical processing) at Hanford, and the uranium enrichment plants (gaseous diffusion and electromagnetic separation) at Oak Ridge. A large experimental graphite reactor (the X-10) was also constructed at Oak Ridge to provide research quantities of plutonium, and went critical on November 4. Refinement of gun-assembly based weapon designs continued at Los Alamos. Preliminary implosion research also proceeded, initially at a low level of effort, but after promising early results at an accelerated rate late in the year.

The first attempt at large scale uranium enrichment, the electromagnetic Alpha tracks at Oak Ridge, went on-line in the fall, but failed completely. By the end of the year complete rebuilding was ordered.

Also in the fall, Project Alberta began. Its purpose was to prepare for the actual combat delivery of atomic weapons by conducting weapons delivery tests, modifying aircraft for carrying the atomic weapons, and organizing and training flight crews and field teams for weapons handling.

In 1944 work proceeded on all fronts:

·         weapon development

·         fissile material production

·         combat delivery preparations

In January a major problem surfaced with the diffusion barriers intended for the K-25 gaseous diffusion plant at Oak Ridge. The process then being developed for barrier production seemed unpromising, and Groves to switched planned production to a new process creating months of delays in equipping K-25 for operation. Abelson, then in the process of constructing a thermal diffusion uranium enrichment plant, learned about the problems with the Manhattan Project's gaseous diffusion plant, and leaked information about his technology to Oppenheimer.

On April 5 the first sample of X-10 reactor produced plutonium arrived from Oak Ridge. Emilio Segre immediately began monitoring its spontaneous fission rate. By April 15 his preliminary estimate of a spontaneous fission rate indicated that it was far too high for gun assembly. The report was kept quiet due to limited statistics, and observations continued.

By mid-May 1944, six months after the start of accelerated implosion research, little progress towards successful implosion had been made. The experimental and theoretical work on the problem had been reorganized a number of times, and resources devoted to it kept expanding. New IBM calculating equipment was now being put to use. At this point two British scientists joined Los Alamos who had important impacts on the implosion program. Geoffrey Taylor (arrived May 24) pointed out implosion instability problems (especially the Rayleigh-Taylor instability), which ultimately led to a very conservative design to minimize possible instability problems. James Tuck brought the critical idea of explosive lenses for detonation wave shaping.

On June 3, after visiting the thermal diffusion uranium enrichment pilot plan at the Naval Research Laboratory, a team of Manhattan Project experts recommended that a plant be built to feed enriched material to the electromagnetic enrichment plant at Oak Ridge. On June 18 Groves contracted to have S-50, a liquid thermal diffusion uranium enrichment plant, built at Oak Ridge in no more than three months.

On July 4, 1944 Oppenheimer revealed Segre's spontaneous fission measurements to the Los Alamos staff. The neutron emission for reactor-produced plutonium was too high for gun assembly to work. The measured rate was 50 fissions/kg-sec, the fission rate in Hanford plutonium is expected to be over 100 times higher still.

This discovery was a turning point for Los Alamos, the Manhattan Project, and eventually for the practice of large scale science after the war. The planned plutonium gun had to be abandoned, and Oppenheimer was forced to make implosion research a top priority, using all available resources to attack it. A complete reorganization of Los Alamos Laboratory was required. With just 12 months to go before expected weapon delivery a new fundamental technology, explosive wave shaping, had to be invented, made reliable, and a enormous array of engineering problems had to be solved. During this crisis many foundations for post-war science were laid. Scientist- administrators (as opposed to academic or research scientists) came to the forefront for running large scale research efforts. Automated numerical techniques (as opposed to manual analytical ones) were applied to solve important scientific problems, not just engineering applications. The dispersal of key individuals after the end of the war later carried these insights, as well as the earlier organizational principles developed at Los Alamos throughout American academia and industry.

July 1, 1944 - The Manhattan Project was granted the highest project-wide procurement priority (AA-1).

July 20, 1944 - The Los Alamos Administrative Board decided on a reorganization plan to direct the laboratory's full resources on implosion. Instead of being organized around scientific and engineering areas of expertise, all work was organized around whether it applied to implosion, or the uranium gun weapon, with the former receiving most of the resources. The reorganization was completed in less than two weeks.

During August Groves made his first estimate of bomb availability since the beginning of the Manhattan Project (the estimate was mid-spring 1945). Also this month the Air Force began modifying 17 B-29s for combat delivery of atomic weapons.

September 1944 marked a difficult period:

·         K-25 was half built, but no usable diffusion barriers had been produced. The Y-12 electromagnetic enrichment plant was operating at only 0.05% efficiency. S-50 enrichment plant began partial operation at Oak Ridge, but leaks prevented substantial output. The total production of highly enriched uranium to date was only a few grams. The only workable bomb design at hand, the gun-type weapon, required U-235 which has no proven practical production methods available.

·         Plutonium production had not yet begun, but the production techniques appeared to have a high probability of success. However plausible approaches to building a plutonium bomb did not yet exist.

·         Project Alberta on the other hand has moved into a new phase as Air Force Lt. Col. Paul Tibbets began organizing the 509th Composite Group, which would deliver the atomic bombs in combat, at Wendover Field, Utah. [Interestingly, the 509th stayed together after the war and exists to this day (1999) as a US Air Force Strategic Command bomber force.]

Then, a new crisis struck the plutonium production effort. On September 26 the first full scale plutonium reactor, the B pile, at Hanford was completed and loaded with uranium. This reactor contained 200 tons of uranium metal, 1200 tons of graphite, and was cooled by 5 cubic meters of water/sec. It was designed to operate at 250 megawatts, producing some 6 kg of plutonium a month. On this day Fermi supervised reactor's first start-up. After several hours of operation at 100 megawatts, the B pile inexplicably shut down, then started up again by itself the next day. Within a few days this was determined to be due to poisoning by the highly efficient neutron absorber Xenon-135, a radioactive fission product. The B reactor, and others under construction, had to be modified to add extra reactivity to overcome this effect before production could begin.

October 27, 1944 - Oppenheimer approved plans for a bomb test in the Jornada del Muerto valley at the Alamagordo Bombing Range. Groves approved the plan 5 days later, provided that the test be conducted in Jumbo.

By the end of the year things start looking up;

·         Y-12 output had reached 40 grams of highly enriched uranium a day in November, then to 90 grams/day in December.

·         In Mid-December the first successful explosive lens tests established the feasibility of making an implosion bomb.

·         December 17, 1944 - The D pile went critical with sufficient reactivity to overcome fission product poisoning effects. Large scale plutonium production begins.

The Y-12 Plant The D-Reactor at Hanford

By the start of 1945 the Manhattan Project had 'turned the corner'. The uranium bombs seemed assured of success in a matter of months. The prospects for the plutonium bomb were looking up although meeting an August 1 deadline imposed by Groves was far from certain. However, allied military successes against Germany and Japan made it a horse race to see whether it would matter to the war effort.

January, 1945:

·         Y-12 output reached an average of 204 grams of 80% U-235 a day; projected production of sufficient material for a bomb (~40 kg) was July 1.

·         Usable barrier tubes began arriving at the K-25 plant. The first stage of the K-25 plant was charged with uranium hexafluoride and began operation.

·         The Dragon experiment (January 18), conducted by Frisch, created the world's first assembly critical through prompt neutrons alone (i.e. it reached prompt critical). The largest energy production for a drop was 20 megawatts for 3 milliseconds (the temperature rose 6 degrees C in that time).

The K-25 Plant

 February, 1945:

·         The F reactor went on-line at Hanford, raising theoretical production capacity to 21 kg/month.

·         Uranium gun design was completed and frozen. Only planning for deployment and combat use once the U-235 was deliveredwas now required.

·         Plutonium began arriving from Hanford.

·         Tinian Island was selected as the base of operations for atomic attack.

·         A meeting between Oppenheimer, Groves, and Los Alamos division leaders (February 28) fixed the design approach for the plutonium bomb. The next day the powerful Cowpuncher Committee was organized to "ride herd" on implosion bomb development.

March, 1945:

·         S-50 thermal diffusion plant finally began enriching uranium in quantity.

·         Oppenheimer officially froze explosive lens design (March 5).

·         By mid month the first evidence of solid compression from implosion was observed (5%).

April, 1945:

·         April 3 - Preparations began at Tinian Island to support the 509th Composite Group, and to assemble the atomic bombs.

·         April 11 - Oppenheimer reported optimal performance with implosion compression in sub-scale tests.

·         April 12 - President Roosevelt died of a brain hemorrhage.

·         April 13 - Pres. Truman learned for the first time of the existence of atomic bomb development from Secretary of War Henry Stimson.

·         April 25 - Truman received first in-depth briefing on the Manhattan Project from Stimson and Groves.

·         April 27 - The first meeting of the Target Committee was held to select targets for atomic bombing. Seventeen targets are selected for study: Tokyo Bay (for a non-lethal demonstration), Yokohama, Nagoya, Osaka, Kobe, Hiroshima, Kokura, Fukuoka, Nagasaki, and Sasebo (some of these were soon dropped because they had already been burned down).

May 1945:

·         May 7 - The 100-ton test was conducted. 108 tons of Composition B, laced with 1000 curies of reactor fission products, were exploded 800 yards from Trinity ground zero to test instrumentation for Trinity. This was the largest instrumented explosion conducted up to this date.

·         May 8 - V-E Day. Germany formally capitulated to the allies.

·         May 9 - The draft of general procedures for atomic bombing were completed.

·         May 10 - Target Committee reconvened. The target list was shortened to Kyoto, Hiroshima, Yokohama, and Kokura Arsenal.

·         Mid-May - Little Boy was ready for combat use, except for the U-235 core. It was estimated that sufficient material would be available by 1 August.

·         May 25 - Operation OLYMPIC, the invasion of Kyushu (the southern Japanese island), was set for November 1.

·         May 28 - Target Committee met with Lt. Col. Tibbets in attendance. Tibbets estimated that by Jan. 1, 1946 all major cities of Japan will have been destroyed by fire bombing. The target list was now Kyoto, Hiroshima, and Niigata.

·         May 30 - Sec. of War Stimson ruled out Kyoto, the ancient capital of Japan, as a target for atomic attack.

June, 1945:

·         June 10 - 509th Composite Group crews began arriving on Tinian with their modified B-29s.

·         June 24 - Frisch confirmed that the implosion core design is satisfactory after criticality tests.

·         Late June - LeMay estimated that the Twentieth Air Force would finish destroying the 60 most important cities in Japan by Oct. 1.

July, 1945: Final preparations began at the New Mexico test site, the Jornada del Muerto at the Alamagordo Bombing Range, for the first atomic bomb test, code named Trinity. The date was set for July 16.

·         July 3 - Casting of the U-235 projectile for Little Boy was completed.

·         July 7 - Explosives lens casting for Trinity was completed.

·         July 10 - The best available lens castings were selected for Trinity.

·         July 11 - Assembly of Gadget, the first atomic bomb began.

·         July 12-13 - The plutonium core and the Gadget components left Los Alamos for the test site separately. Assembly of Gadget began at 1300 hours on July 13. Assembly of Gadget's explosive lens, uranium reflector, and plutonium core was completed at Ground Zero at 1745 hours.

·         July 14

o        Gadget was hoisted to the top of the 100 foot test tower, and the detonators were installed and connected. Final test preparations began.

o        Little Boy bomb units, accompanied by the U-235 projectile, were shipped out of San Francisco on the USS Indianapolis for Tinian.

o        The only full scale test of the implosion lens system (before Gadget) was conducted. Initial analysis indicated failure, but Bethe later corrected mistaken calculations and found that the measurements were consistent with optimum performance (he also discovered that the test instrumentation was incapable of distinguishing success from failure).

·         July 16 - At 5:29:45 a.m. Gadget was detonated in the first atomic explosion in history. The explosive yield was 20-22 Kt (initially estimated at 18.9 Kt), vaporizing the steel tower.

 

Part I -- History: Nuclear Power

"...you had uranium in the rocks, in principle, an inexhaustible source of energy -- enough to keep you going for hundreds of millions of years. I got very, very excited about that, because here was an embodiment of a way to save mankind. I guess I acquired a little bit of the same spirit as the Ayatollah has at the moment."

--Alvin Weinberg, former head of the Oak Ridge National Laboratory and nuclear reactor designer, 198115

I am sure we are agreed that the ultimate survival of America is dependent on intellectual vigor and on spiritual deeprooting -- not on specific devices which are always for the moment. The atom has no ethics of its own any more than it has politics. The future of the scientists' America, and yours and mine, lies fundamentally with education -- that which is taught to the young in our schools -- that which is taught throughout life in the media of general communication by the contemporary writers. Fundamental are respect and zeal for scholarship, a lively regard for moral values, and a love of truth. And of these the last is, of course, the greatest.

--Lewis Strauss, AEC Chairman, 195416

Chapter 1: Romance with the Atom

All forms of transportation will be freed at once from the limits now put upon them by the weight of present fuels....

Instead of filling the gasoline tank of your automobile two or three times a week, you will travel for a year on a pellet of atomic energy the size of a vitamin pill ... The day is gone when nations will fight for oil....

The world will go permanently off the gold standard once the era of Atomic Energy is in full swing ... With the aid of atomic energy the scientists will be able to build a factory to manufacture gold.

No baseball game will be called off on account of rain in the Era of Atomic Energy. No airplane will by-pass an airport because of fog. No city will experience a winter traffic jam because of snow. Summer resorts will be able to guarantee the weather and artificial suns will make it as easy to grow corn and potatoes indoors as on a farm.

--David Dietz, Science writer, 194517

The control of fire was central to the development of cities -- that is, of civilization. Its use on a large scale directly and indirectly, through diverse carriers of energy such as steam, is the foundation of modern industry and commerce. In the eighteenth and nineteenth centuries, steam power enabled the centralization of manufacture by the use of mechanized transport to draw huge quantities of raw materials, such as cotton and jute, as well as food from around the globe into the world's new manufacturing centers in Europe. The first fuel to be used on a large scale for industry and transport was coal; it continues to occupy a large place in the world's energy supplies today. In the late nineteenth century it was joined by petroleum; in the twentieth natural gas was added to the fossil fuel mix.

The potential for the application of energy to transform life for vast numbers of people was demonstrated in the second part of the nineteenth and first half of the twentieth centuries in a number of radical and graphic ways. Electric lights illuminated the night. Rapid travel over large distances became commonplace, first via railroad and steamships and then also via trolleys, buses, and cars. This mechanized mobility was symbolized by Phileas Fogg, Jules Verne's nineteenth century fictional voyager, who went around the world in eighty days and returned home punctually. Farm mechanization reduced the need for farm labor; cities grew; occupations and specializations multiplied.

In the cities, automobiles and street cars rapidly replaced horse drawn carriages. And the domestic scene was transformed, for those who could afford it, by central heating and numerous appliances that reduced the burdens of physical labor. The possibility that life for ordinary people around the world could one day be very comfortable, even luxurious, was no longer theoretical -- it was being practically realized everyday by large numbers of people of European origin and also by a small minority in the colonized countries. The prospect that such a life would be available to all seemed to depend on nothing more than human ingenuity in the application of science and technology and on the availability of sufficient natural resources, chief among which were fuels.

But these historic changes also carried the seeds of misery and destruction. Consolidation of farms threw people off the land. Machines threw people out of work. In many of the countries that were colonies of Europe, the destruction of cottage industries actually reduced the proportion of people working in non-agricultural occupations. At the same time, there was little work to be had on the land. From time to time, as in the 1930s in the United States, there were vast and sudden displacements of people from farms to urban areas, accelerating trends started by industrialization. Indigenous cultures, whose knowledge of the natural environment is, in many ways, still unparalleled by science were destroyed around the world. Unemployment became a permanent feature of the world economy.

Despoilation of the environment was occurring on a scale as grand as the huge industries that were springing up. Air pollution was, in many places, literally breathtaking. For instance, in London the air often got so bad that episodes of smog came to be called "peasoupers" after their resemblance to pea soup: visibility was typically reduced to a few yards. Thousands of people died of respiratory diseases as a result of the London "peasouper" of 1952. The public outcry accompanying the deaths and suffering led to the initiation of unprecedented pollution control regulations in Britain. The general recognition of potential damage to the entire atmosphere due to a build up of carbon dioxide from fossil fuel burning was still about three decades away, however. (18)

As the exploitation of resources and the trade in them became global, so did the wars for their control. To a considerable extent, these global wars had their roots in the dependence of western economies on cheap imported primary commodities and in the competition between them for these resources. After World War I, oil rapidly became the most crucial strategic primary commodity. Much of the prelude to World War II, including the Japanese bombing of Pearl Harbor, many of battles during that war, and much of the wartime strategy of the antagonists revolved around the control of oil resources that had become the lifeblood of the war machine. (19)

By the middle of the twentieth century, with the colonies in Asia and Africa on the verge of political independence, people throughout the world were seeking to achieve the level of material standards of living that had already become a reality for a substantial minority of people in western Europe and the United States and would soon be realized by a majority. But would there be enough resources for all, given the already high and rising levels of consumption in Europe and the United States and the dependence of western economies on imported primary commodities, especially oil?

Einstein's discovery early in the twentieth century that matter and energy were equivalent, expressed by the famous equation E = mc2, came in the middle of this immense and unprecedented technological, political, economic, and military ferment. H.G. Wells, in The War of the Worlds, wrote about bombs that might destroy cities and entire civilizations. But there were also visions of unlimited amounts of energy for everyday life. Einstein's equation showed that a small amount of matter was theoretically equivalent to a huge amount of energy: just one gram of matter, if completely converted to energy, was equivalent to roughly 3,000 metric tons of coal. (20)

If only some way could be found to change matter into energy, the days of deprivation would be over! The Pharaohs needed slaves to do their bidding. Modern life would not need to be cruel to be affluent. Small bits of dead matter could take the place of slaves and everybody could be happy ever after -- at least so far as material matters were concerned. Life would be free of drudgery. Convenience and creativity would flourish in the ample leisure time that everyone would enjoy.

In the late 1930s, the fission of uranium -- that is, the splitting apart of its nucleus into smaller nuclei of light elements -- was discovered and the possibility of converting matter into energy on a large scale started to move from the realm of science fiction and improbable theory to reality

The practical harnessing of fission energy required the splitting of a large number of uranium atoms -- in a controlled sustained way for nuclear power production, or all at once for a bomb. The Hungarian scientist Leo Szilard had realized well before fission was discovered in the laboratory in 1938 that a nuclear chain reaction would be the basis for nuclear energy production, whether commercial or military. In such a reaction, each fission would generate another without any external inputs, so that once initiated, fission reactions would continue until some other factor intervened to stop them.

Uranium appeared capable of sustaining a chain reaction because each fission released more than one neutron, a neutral particle that could penetrate the outer parts of an atom to reach its tiny nucleus. After the experimental demonstration of fission in Germany in late 1938 and its confirmation in the United States in 1939, the main question that remained was: could a nuclear chain reaction be realized in practice? If so, a large sustained release of energy could be achieved. The requirement for achieving a nuclear explosion was even more stringent since each fission would have to generate more than one fission in a very short time. In this way, the number of fissions would multiply very rapidly, resulting in a huge explosive release of energy.

The first chain reaction took place in an "atomic pile," as nuclear reactors were initially called, at the University of Chicago in December 1942. However, a minimum amount of nuclear material, called a critical mass, was necessary to sustain a chain reaction. The most basic physics questions had been answered. The immense engineering job of making nuclear energy a practical reality for explosive or commercial applications remained.

It was thought early on that the widespread use of nuclear energy would be complicated by an important resource limitation. While heavy elements could be fissioned to yield energy, only one element that occurred in nature in substantial quantities could sustain a chain reaction. That element was uranium. There was a further difficulty. It was discovered that only one naturally-occurring isotope of uranium, called uranium-235, could sustain a chain reaction (see box below). However, about 99.3 percent of natural uranium consists of uranium-238, which cannot sustain a chain reaction. Uranium-235 is only about 0.7 percent of natural uranium. Still, just one gram of uranium-235, when completely fissioned, yielded as much energy as 3 metric tons of coal, which is more than annual average household energy requirement for home heating in the United States.

Isotopes of elements

Elements occur in variants called isotopes. All isotopes of an element have essentially identical chemical properties, which are determined by the number of protons in the nuclei of the element's atoms. Protons have positive electrical charges. The number of protons in a nucleus is normally equal, at ordinary temperatures, to the number of electrons that surround the nucleus in that atom. But the nuclei of elements can also contain varying numbers of neutrons, which are electrically neutral particles slightly heavier than protons, and far heavier than electrons. Changing the numbers of neutrons in the nucleus changes the properties of the nucleus and the overall weight of the atoms of an element. Variants of an element whose nuclei have the same number of protons but different numbers of neutrons are called isotopes of that element.

Some heavy nuclei are rendered highly unstable and split apart after absorbing a slow neutron, having essentially no kinetic energy. Such isotopes are said to be fissile. Other heavy nuclei require incoming neutrons (or other particles) to have a large amount of energy before they will split apart. These isotopes are fissionable, but not fissile. In general, fissile isotopes are required to sustain chain reactions, and hence to build nuclear reactors or nuclear weapons. Uranium-235 is essentially the only naturally-occurring fissile material. (21) Uranium-238 is fissionable but not fissile.

 

But uranium-238 was soon found to possess another remarkable property that made it seem at least as important a substance as uranium-235. When uranium-238 absorbs a neutron, it is transmuted, in two steps, to a fissile element that is present in nature only in the minutest quantities: plutonium-239 (see Appendix A). This meant that a nuclear reactor could be used to do two things at once. First, it could generate energy by fissioning uranium-235 in a chain reaction. Second, it could at the same time convert non-fissile uranium-238, which was 140 times more plentiful than uranium-235, into fissile plutonium-239. There is so much uranium-238 in nature that it could, if converted to plutonium-239, far outstrip fossil fuels as an energy source. Limitless energy supply seemed within the reach of mankind, a prospect that gave rise to fervent, almost religious declamations by scientists about the deliverance of mankind.

The first nuclear engineering achievements were made by the U.S. military's crash program to develop the atom bomb during World War II, known as the Manhattan Project. Uranium-235 provided the explosive energy in the bomb that destroyed Hiroshima; plutonium-239 powered the Nagasaki bomb. The Manhattan Project also showed that it was possible to build large nuclear reactors, to produce plutonium in them, and subsequently to chemically separate the plutonium from fission products and the remaining uranium.

As the United States entered the post-war era, millions of Americans believed that their lives or the lives of soldiers personally near and dear to them had been saved because the atom bombings of Japan had ended the war early. U.S. leaders saw in nuclear weapons the potential to move the world in a political direction of their choosing. The immense technological feats that the U.S. had accomplished during World War II were exemplified most dramatically for all (including Stalin) by the Manhattan Project. Now they would be applied to making the United States by far the most militarily powerful country in human history and also to the material salvation of mankind. Nuclear energy was in the center of that military and economic prospect. America's romance with the atom had begun. (22)

But before commercial nuclear energy could save mankind, some problems, seemingly mundane, remained. They would come to dominate the fate of nuclear power. The devil, it turned out, was in the details:

These problems seem clear enough in hindsight. But how many were apparent in the early days? Was the romance with the atom a case so intense that it blinded engineering judgment? Was it propaganda waged for economic or military purposes? Or was it a mixture of both?

 

Basics of Nuclear Physics and Fission

A. Structure of the Atom

The atoms of which every element of matter is composed have a nucleus at the center and electrons whirling about this nucleus that can be visualized as planets circling around a sun, though it is impossible to locate them precisely within the atom. The nuclei of atoms are composed of protons, which have a positive electrical charge, and neutrons, which are electrically neutral. Electrons are electrically negative and have a charge equal in magnitude to that of a proton.

The number of electrons in an atom is normally equal to the number of protons in the nucleus. As a result, atoms of elements are normally electrically neutral. The mass of an atom lies almost entirely in its nucleus since protons and neutrons are far heavier than electrons.

Free neutrons are unstable particles which decay naturally into a proton and electron, with a half-life of about 12 minutes.

neutron ===> proton + electron + a neutrino

However, it is remarkable that neutrons, when they exist together with protons in the nucleus of atoms, are stable. Protons are about 1,836 times heavier than electrons, and neutrons are about 1,838 times heavier than electrons. The energy balance in the decay of a neutron is achieved by the anti-neutrino, a neutral particle that carries off surplus energy as the neutron decays. The nominal mass of an atom of an element is measured by the sum of the protons and neutrons in it. This integer is called the mass number. The nominal mass of an atom is not affected by the number of electrons, which are very light. Hence the nominal mass, based on the mass number, approximates the actual atomic mass. The number of protons in the nucleus, which determines the chemical properties of an element, is called the atomic number. Elements are arranged in ascending order of atomic number in an arrangement called the periodic table. The term derives from the tendency to periodicity of chemical properties deriving from arrangements of electrons in atoms.

B. Radioactive Decay

The nuclei of some elements are not stable. These nuclei are radioactive, in that they emit energy and particles, collectively called "radiation." All elements have at least some isotopes that are radioactive. All isotopes of heavy elements with mass numbers greater than 206 and atomic numbers greater than 83 are radioactive.

There are several ways in which unstable nuclei undergo radioactive decay:

Often, there is still excess residual energy in the nucleus after the emission of a particle or after electron capture. Some of this residual energy after radioactive decay can be emitted in the form of high-frequency electromagnetic radiation, called gamma rays. Gamma rays are essentially like X-rays and are the most penetrating form of radiation. 1 It should be noted that the emission of gamma rays does not change the mass number or atomic number of the nucleus -- that is, unlike radioactive decay by emission of particles, spontaneous fission, or electron capture, it does not cause the transmutation of the nucleus into another element.

Each quantum, or unit, of a gamma ray (or other electromagnetic energy) is called a photon. Gamma rays are like light, except that they are much higher frequency electromagnetic rays. Photon energy is directly proportional to the frequency of the electromagnetic radiation. Photons of gamma rays can damage living cells by splitting molecules apart or ionizing elements in them.

Many heavy nuclei emit an energetic alpha particle when they decay. For instance uranium-238 decays into thorium-234 with a half-life of almost 4.5 billion years by emitting an alpha particle:

92-uranium-238 ====> 90-thorium-234 + alpha particle (nucleus of 2-helium-4)

The mass number of uranium-238 declines by four and its atomic number by two when it emits an alpha particle. The number before the element name is the atomic number and that after the element name is the mass number. The totals of the atomic numbers and the mass numbers, respectively, on both sides of the nuclear reaction must be the same. (This is like balancing a chemical equation, in which the number of atoms of each element on both sides of the reaction must be equal)

In beta decay, the atomic number increases by one if an electron is emitted or decreases by one if a positron is emitted. For instance thorium-234, which is the decay product of uranium-238, in turn beta-decays into protactinium-234 by emitting an electron:

90-thorium-234 ====> 91-protactinium-234 + beta particle (electron)

The nuclei that result from radioactive decay may themselves be radioactive. Therefore, some radioactive elements have decay chains that may contain many radioactive elements, one derived from the other. (See Uranium Factsheet for a diagram of the decay chain of uranium-238.)

The radioactive decay of nuclei is described probabilistically. Within any given time period, a particular unstable nucleus has a fixed probability of decay. As a result, each radioactive element is characterized by a "half-life," which is the time it takes for half the initial atoms to decay (or transmute into another element or nuclear state). At the end of one half-life, half the original element is left, while the other half is transformed into another element. After two half-lives, one fourth of the original element is left; after three half-lives one eighth is left, and so on. This results in the build-up of decay products. If the decay products themselves decay into other elements, a whole host of radioactive materials come into being. The decay products of radioactive elements are also called daughter products or progeny.

C. Binding Energy

Nuclei are tightly bound together by the strong nuclear force and each nucleus has a characteristic binding energy. This is the amount of energy it would take to completely break up a nucleus and separate all the neutrons and protons in it. Typically, binding energy increases by several megaelectron-volts (MeV) for every proton or neutron added to a nucleus. (Since protons and neutrons are constituent particles of nuclei, they are known collectively as nucleons.) The release of nuclear energy derives from the differences in binding energy between the initial nucleus (or nuclei) and relative to the end-products of the nuclear reaction, such as fission or fusion.

The electrons that whirl around the nucleus are held together in their orbits by electrical forces. It takes on the order of a few electron-volts to dislodge an electron from the outer shell of an atom. The "binding energy" of a nucleon is on the order of a million times greater. Electrons are the particles the enable chemical reactions; nucleons take part in nuclear reactions. The huge differences in binding energy are one measure of the differences in the quantities of energy derived from nuclear compared to chemical reactions.

It must be stressed that the binding energy is the amount of energy that would have to be added to the nucleus to break it up. It can be thought of (approximately) as the amount of energy liberated when a nucleon is drawn into the nucleus due to the short range nuclear attractive force. Since energy and mass are equivalent, nuclei with higher binding energy per nucleon have a lower atomic weight per nucleon.

The key to release of nuclear energy from fission of heavy elements and fusion of light elements is that elements in the middle of the periodic table of elements, with intermediate mass numbers have a higher binding energy per nucleon (that is a lower atomic weight per nucleon). Therefore when a heavy nucleus is fissioned, the resultant products of the nuclear reaction have a slightly smaller combined nuclear mass. This mass difference is converted to energy during nuclear fission.

D. Nuclear Fission

Nuclear energy is produced by the conversion of a small amount of the mass of the nucleus of an atom into energy. In principle, all mass and energy are equivalent in a proportion defined by Albert Einstein's famous equation

E = mc2

where E stands for energy, m for mass and c for the speed of light. Since the speed of light is a very large number--300 million meters per second--a small amount of mass is equivalent to a very large amount of energy. For instance, one kilogram (about 2.2 pounds) of matter is equivalent to

E = 1 kg x (3 x 108 meters/sec)2 = 1 x 3 x 108 x 3 x 108 joules
= 9 x 1016 joules

This is a huge of amount of energy, equivalent to the energy content of over three million metric tons of coal.

Heavy atoms such as uranium or plutonium can be split by bombarding them with neutrons.2 The resultant fragments, called fission products, are of intermediate atomic weight, and have a combined mass that is slightly smaller than the original nucleus. The difference appears as energy. As explained in the previous section, this mass difference arises from the binding energy characteristics of heavy elements compared to elements of intermediate atomic weight. Since the binding energy of the fission products per nucleon is higher, their total nucleonic mass is lower. The net result is that fission converts some of the mass of the heavy nucleus into energy.

The energy and mass aspects of the fission process can be explained mathematically as follows. Let the total binding energy of the heavy nucleus and the two fission products be Bh, Bf1, and Bf2, respectively. Then:

Amount of energy released per fission Er = (Bf1 + Bf2) - Bh

Amount of mass converted to energy = Er/c2 = {(Bf1 + Bf2) - Bh}/ c2

This energy appears in various forms: the kinetic energy of the neutrons, the vibrational energy of the fission fragments, and gamma radiation. All of these forms of energy are converted to heat by absorption in with the surrounding media in the reactor, mainly the coolant and the moderator (for thermal reactors). The most basic fission reaction in nuclear reactors involves the splitting of the nucleus of uranium-235 when it is struck by a neutron. The uranium-235 first absorbs the neutron to yield uranium-236, and most of these U-236 nuclei split into two fission fragments. Fission reactions typically also release two to four neutrons (depending on the speed on the neutrons inducing the fission and probabilistic factors). One of these neutrons must trigger another fission for a sustained chain reaction. The fission reactions in a nuclear reactor can be written generically as follows:

U-235 + n ==> U-236

U-236 ===> fission fragments + 2 to 4 neutrons + 200 MeV energy (approx.)

The uranium-236 nucleus does not split evenly into equal fission fragments. Rather, the tendency, especially with fission induced by thermal neutrons, is for one fragment to be considerably lighter than the other. Figure 9 (not available in on-line version of report) shows the distribution of fission products due to fission with the slow neutrons and fast neutrons. It can be seen that the fission product atomic numbers are concentrated in the ranges from about 80 to 105 and from about 130 to 150 in thermal reactors. An example of a fission reaction is:

92-U-235 + n ==> 92-U-236

92-U-236 ===> 38-strontium-90 + 54-xenon-144 + 2 neutrons + energy

While many heavy nuclei can be fissioned with fast neutrons, only a few can be fissioned with "slow" neutrons. It turns out that, with some exceptions, like plutonium-240, only nuclei that can be fissioned with slow neutrons can be used for sustaining chain reactions. Isotopes with nuclei that can be fissioned with zero energy neutrons (in practice neutrons with low energy, or "slow neutrons") are called fissile materials. Generally these are the odd-numbered isotopes, such as uranium-233, uranium-235, plutonium-239, and plutonium-241. Other heavy nuclei, like uranium-238, can be fissioned with fast neutrons, and so are fissionable, but not fissile.

There are only three fissile isotopes of practical importance: uranium-233, uranium-235, and plutonium-239. Of these, only uranium-235 occurs naturally in significant quantities. The other two occur in trace quantities only.

E. Fertile Materials

To obtain plutonium-239 and uranium-233 in amounts useful for nuclear energy production, they must be manufactured from materials that occur in relative abundance. Plutonium-239 is produced from reactions following the absorption of a neutron by uranium-238; uranium-233 is produced by neutron absorption in thorium-232. Uranium-238 and thorium-232 are called fertile materials, and the production of fissile materials from them is called breeding. The reactions for plutonium-239 are

92-U-238 + n ===> 92-U-239

92-U-239 ====> 93-Np-239 + beta particle (electron)

93-Np-239 ====> 94-Pu-239 + beta particle (electron)

For uranium-233 the reactions are:

90-Th-232 + n ===> 90-Th-233

90-Th-233 ===> 91-Pa-233 + beta particle (electron)

91-Pa-233 ====> 92-U-233 + beta particle.

The symbol Pa stands for the element protactinium.

(See Chart of Chemical Names on IEER's On-Line classroom page for this and other chemical names and their symbols.)

1. The terms alpha, beta, and gamma radiation, and X-rays were coined because scientists did not know the nature of these kinds of radiation when they were first detected.

2. Nuclear fission can also be induced by bombardment of the nucleus by electrically charged particles, such as alpha particles. However, the nucleus is positively charged and alpha particles are also positively charged. Since positive charges repel each other, these types of fission reactions are more difficult to accomplish than reactions with neutrons. Fission can also be induced by bombarding the nucleus with energetic gamma rays (photons). This process is called photofission.

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