Chernobyl
Andrew
L Daley
Department of Mechanical Engineering
University of New Brunswick
Fredericton, NB
Abstract
This report deals with the
Chernobyl Power complex explosion. It
will deal with the sequence of events which lead to the explosion focusing on
operator decisions and their effect on the explosion process. The findings of this report include the cause
of the accident. The operator made
decisions which led to a dangerous situation but the underlying cause is a
design flaw in the RBMK-1000 reactor.
This flaw will be discussed and explained. The effect the accident had on the outside
world in the context of radioactive release and other environmental and social
damages will be presented. An analysis
of the ethical issues surrounding the tragedy will be examined.
1.0
Accident
Timeline
The explosion at the Chernobyl
Nuclear Generating Station in the Ukraine
occurred on April 26, 1986. After the fact, a timeline of events that
contributed to the accident was pieced together by investigators. A summary of this timeline follows. The story begins because the reactor was
scheduled to be shutdown on a routine matter.
During this shut down a test was planned to help determine the turbine
power characteristics in the event of loss of steam supply from the
reactor. In other words, if the reactor
had to be shut down and steam lost to the turbine, inertia will keep the
turbine spinning and will continue to supply power via the generator. The test was to determine how long this power
would last and continue to power the auxiliary systems of the plant (pumps,
etc.) before alternative sources of power could be switched on.
On April 25 at 1:06 AM the scheduled shutdown began. The power level of the reactor was slowly
decreasing from its 3200 MWt nominal value. At 3:47 AM,
The reactor was stabilized at 1600 MWt. At 2:00 PM
two events happened. The emergency core
cooling system was shut down so it would not interfere with the test. The power was also scheduled to drop, however
the grid operator refused to allow this as he had determined that the supply
was needed. The shutdown did not
continue until 11:10 PM once it was
safe to continue lowering the reactor power.
Fifty minutes later, at 12:00 AM
there was a shift change.
The new shift continued to lower
the power. By 12:28 AM the level had reached 500 MWt. At this point the operator switched
maintenance control over the reactor to an automatic system. Consequently the power dropped rapidly to
only 30 MWt! At 12:32, in response to the drop in power, the
operator retracted several control rods in order to increase the nuclear chain
reaction. Although there was a minimum
requirement that 30 control rods be in the reactor at all times, it is
estimated that the operator disregarded this requirement and less than 30 rods
were in.
By 1:00
AM, the reactor power had risen to 200 MWt. At this point everything appeared to be
normal to the operators, so preparation for the test was resumed. Between 1:03 AM
and 1:07 AM additional cooling pumps
were switched on. As a result, the water
level in the steam drum was reduced. At 1:15 AM an automatic trip system was
deactivated in the steam drum. At 1:18 AM the feed water flow to the drum (make
up water) was increased to raise the water level. At 1:19 AM
even more control rods were withdrawn from the core in order to raise power and
increase steam pressure. In order to
stabilize the water level in the steam drum, the operator subsequently reduced
the feed water flow. This resulted in a
lower rate of heat removal from the core.
As a result, steam started to form in the reactor itself at
approximately 1:22 AM. Although this was abnormal, the operator was
under the impression that the reactor was stable. He gave the go ahead to continue with the
test procedure.
At 1:23
AM the feed valves to the turbine were closed to begin the coast
test. As a result of the valve closing
the expected response of the system was for a pressure increase in the core
which would reduce steam quantity, thus lower the reactivity of the core. To compensate for this, the operator removed
control rods from the core.
Unfortunately, the expected decrease in reactivity did not occur. Removal of the control rods, instead of
keeping reactivity constant, led to a marked increase in reactivity. At 1:23:40 AM,
steam began to form uncontrollably in the core.
The operator realized this and 5 seconds later pressed the emergency
button to rapidly insert control rods into the core. Unfortunately the emergency process was of
finite time, usually taking 10-12 seconds for the rods to fully enter the core
and kill the chain reaction. At 1:23:44 AM the reactor power rose to 100 times
its designed value. Over the next few
seconds fuel pellets started to shatter and the fuel rods ruptured. Steam built up more rapidly and eventually
exploded, tearing the roof off the reactor.
A second explosion occurred but is unknown in origin. Fuel vapour is the suspected cause of the second
explosion.
2.0 Causes of the Accident
Why did the reactor explode? One cause that is obvious is operator
error. At no time should the control
rods have been removed to such an extent, and by playing around with coolant
flow and feed water flow, the problem was exaggerated. Another problem was a faulty test
sequence. The delay ordered by the grid
operator necessitated a change in schedule and the test was conducted by a
shift that was not supposed to be conducting the test. Had the original operator been at the
controls, perhaps the disaster would not have occurred. All of these are reasons for the disaster,
but they do not get to the heart of the problem.
A design flaw in the RBMK-1000 type reactor
was unforgiving of all the operator errors and is, in fact, the cause of the
disaster. The RBMK is the name given to
the Russian design for a type of nuclear reactor known as an LGR or light water
graphite reactor. This uses light water
as a coolant and graphite as a moderator.
The design flaw in the RBMK is a
characteristic known as “positive void coefficient”. The RBMK does not have a separate steam
generator; the steam is produced in the reactor itself (Boiling Water Reactor
or BWR) which is what makes it particularly susceptible to this
phenomenon. Positive void coefficient is
a measure of how the reactor reacts to voids, or steam, which may be produced
in the flow of coolant that is in the reactor.
If voids lead to an increase in reactor power then the reactor is said
to have a positive void coefficient. At
low power settings, less than about 20% nominal, on a RBMK a positive void
coefficient state is reached. There are other factors that affect the void
coefficient of a reactor; the relevant information here is that the positive
void coefficient is a dominant factor at low power loads. In other words, the positive void coefficient
was present during the test. If the
power in the reactor is increased or the cooling water flow is decreased then
steam production rises and therefore, because of positive void, the power rises
even further. Eventually this leads to
an unstable core and a very rapid rise in power.
Chernobyl
exploded because a sequence of events led to positive void occurring, causing
an unstable core which in turn caused the explosion. Essentially, due to operator decisions there
were excessive steam pockets produced in the reactor. Since more of the water
in the reactor is now steam the operating characteristics of the reactor
changed. Steam is less dense than
water and does not absorb neutrons as readily as water; therefore more neutrons
had the potential to cause fission.
Since the control rods were not inserted in sufficient quantity this
effect was exaggerated and the reactor soon became unstable. What occurred was runaway steam generation in
the core. As more steam was produced the
power level in the reactor increased and the rate of steam production increased
partly since steam is not as efficient a coolant as water. This process continued in a loop until the
core became unstable and the power suddenly spiked to 100 times nominal and the
temperature was so great that fuel shattered and the first steam explosion
occurred which blew the roof of the reactor and released radioactivity into the
atmosphere. This also allowed air into
the reactor which led to the graphite moderator catching fire. This released further radioactive substances
into the atmosphere for 9 days until the fire was brought under control.
3.0 Consequences
There were 83 fire fighters
involved with trying to contain the graphite fires, exposing themselves to the
high radiation doses. Thirty-one lost
their lives due to their efforts. The
fire was eventually extinguished by blanketing the fire with the use of
helicopters with neutron-absorbing compounds and fire-control material.
The three main radionuclides
released into the atmosphere were Cesium-134, Cesium-137, and Iodine-131. Although Iodine has a half-life of 8 days, it
can be transferred to humans through milk and leafy vegetables. Iodine collects in the thyroid gland and
causes complications with people that have small thyroid glands, such as
children. Both Cesium radionuclides have
longer half-lives, but will cause ingestion complications.
Radiation monitoring equipment at
a nuclear power plant near Stockholm, Sweden
detected high levels of radiation on April
28, 1986. This set off
speculation that a partial or total meltdown occurred at the Chernobyl
facility, since they were not providing information at that time. Neighboring countries received a great amount
of Chernobyl’s radiation,
especially neighboring Belarus. North-west winds and a rainy month of May led
to Belarus
receiving around 75 percent of all fallout due to the disaster. Agriculture continued in Belarus without
farmers or the public aware of the heavy contamination that had taken
place.
Of the 600 workers present on the
site during the morning of April 26,
134 workers received high doses of radiation. By the end of two months after the disaster,
28 workers were dead, followed by two others months later. Thyroid cancer numbers for children aged 15
and under rose by about 40 cases per million.
64 % of all Ukrainian thyroid cancer patients in this age group resided
in the most contaminated regions.
Following the incident, 116,000
people were evacuated from the area, with an additional 210,000 relocated
between 1990 and 1995. All agriculture,
forestry, and industry came to a halt in the area due to the contamination,
thus destroying the economy and future. The
country’s economy was damaged by an estimated 12 billion dollars.
Such psychological health
disorders as anxiety, depression, and helplessness have developed in the public
living in the area before being told to evacuate. This is mainly related to the lack of public
information after the accident, the stress of evacuation, and the concerns of
people and their children’s health. The
government would only allow limited amounts of information out each day, so
that the public would not think that the disaster was as big as it really
was. Citizens did not completely learn
what really took place until several years after.
3.0 Changes
in Reactor Design
Since the accident, the design of
the RBMK-1000 reactor has been investigated. The recommendations of the
investigation were to redesign certain aspects of the reactor to reduce the
effects of, or the chance of, a positive void coefficient. One immediate change
was to increase the number of manual graphite control rods from 30 to 45.
Increasing the number of these control rods increased the acceptable range of
operation. Also, 80 additional absorbers were added to the core to inhibit
operation at low power. These additional absorbers force the reactor into a
sub-critical state causing it to shut down. The fuel was increased in
enrichment from 2% to 2.4% to counteract the increased neutron abortion added,
allowing the reactor to produce the same amount of power with the increased
safeguards.
After the problems with the
positive void coefficient were addressed, thus reducing the risk of a necessary
shut down, steps were taken to make sure in the event of an emergency shut down
it would be quick and effective. This was done by reducing the time for the
scram rods (rods to shut down the reactor – similar to control rods) to enter
the core. This time was cut down from 18 to 12 seconds. The control/scram rods
were also redesigned to reduce the occurrence of the positive void coefficient
as the rods are lowered into the reactor. In addition to this, an additional
fast scram system was installed to start the shut down sooner. In addition to
these changes, the RBMK had improvements to its emergency systems and had
replacements of its fuel channel components and its process computer. Lastly,
the emergency safety systems were safeguarded from unauthorized alterations.
Following the accident, units one
through three continued operating due to Ukraine’s
power requirements. The Chernobyl
reactors were the only RBMK reactors operating in the Ukraine.
The last RBMK at Chernobyl reactor
was shut down in 2001. Construction on two additional RBMK reactors was halted
indefinitely following the accident. Currently all RBMK reactors are in the
process of or are already being decommissioned. Following the accident at Chernobyl,
the Eastern part of the world has partnered with the Western part of the world
to promote a culture of safety and to share valuable learning experience. Loans
from the United Nations are helping to build safer reactors for the Ukraine,
which is heavily dependent on nuclear energy.
The aforementioned changes were
also implemented in RBMK reactors in Russia
and Lithuania,
the only other places to use this specific type of reactor. Currently, there
are 13 RBMK reactors operational in these countries, each built to varying
levels of building codes and standards, each with varying amount of operational
life remaining.
4.0 Ethics and Chernobyl
Analyzing the Chernobyl
disaster from an ethical perspective is difficult. How much of the disaster is to be blamed on
poor ethical practice and how much is due to lack of knowledge? Nevertheless Chernobyl
serves as an example on how not to run a nuclear reactor.
The first thing wrong with Chernobyl,
which was really a common infirmity of the entire Russian program, was the
total lack of a safety culture. The
Russian program had a history of accidents, mostly minor, but safety was still
not a priority for the operation or design of facilities. After all, the RBMK did not even have a
containment building in the case of an explosion. This design deficiency clearly affected and
compounded the impact that this disaster had on the surrounding area. All other reactor designs in the world employ
a containment building so for the RBMK to leave one out, presumably as a cost
cutting measure, is clearly an ethical problem.
Positive Void Coefficient itself
does not kill a reactor. The CANDU
series also has a positive void (although this is a pressurized reactor
dissimilar to an RBMK design). The
problem is that the engineers and managers who designed and implemented the
coast test were either unaware of, or disregarded, the fact that PVC was
dangerous at loads experienced during the test.
Either way, lack of knowledge or lack of concern, is an ethically
abhorrent situation. If the engineers
were unaware of this reactor characteristic then they should not have been in
charge of designing tests, this lack of training and education is unheard of in
western nuclear facilities. If they were
not concerned with this reactor characteristic then the situation is even more
ethically at fault. They then
deliberately invoked a dangerous situation, putting workers and the public at
risk.
Also an educational concern is
the actions of the operator that lead to the disaster. Clearly the actions show that operators did
not understand the principle behind the operation of an RBMK but they also had
no regard for regulations and procedures.
A safety culture that stressed training and following the rules could
have helped prevent this disaster from happening.
Also suspect is the behaviour of officials after the
event. Perhaps due to cold war concerns
the disaster was not divulged immediately and international aid was not asked
for. The order to evacuate was not given
immediately, irradiating millions.
Firefighters were sent in with no protection to battle the reactor fire.
All of these decisions had no basis in
ethics and had action been taken rather than trying to hide the disaster
perhaps lives could have been saved
5.0 Conclusion
The Chernobyl
disaster was of unprecedented proportions and it effectively killed the nuclear
power industry. The disaster could have
been averted had one of a series of events had transpired differently however
it also showed that the Russian safety standards and safety culture was
lacking. Had the Soviet Union
had a comprehensive safety and ethical policy, similar to OSHA and an
engineering code of ethics, then perhaps this tragedy would never have
happened.
References
- The Chernobyl Accident, International
Chernobyl Research and Information Network (ICRIN), http://www.chernobyl.info/en/Resources/Links/Accident,
2004
- Chernobyl, Georgia
State University, http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/cherno.html,
2000
- Chernobyl
– Post Accident Changes to the RBMK, World Nuclear Association, http://www.world-nuclear.org/info/chernobyl/backfit.htm,
2001
- Chernobyl – Positive Void Coefficient, World
Nuclear Association, http://www.world-nuclear.org/info/chernobyl/voidcoef.htm,
2001
- Chernobyl – Introduction, World Nuclear
Association, http://www.world-nuclear.org/info/chernobyl/inf07.htm,
2001
- Chernobyl and Nuclear Power in the USSR,
Marples, D., Canadian Institute of Ukrainian Studies, University of
Alberta, Edmonton, 1986
- Government Proposes to Redefine Chernobyl
Exclusion Zone – Associate Press, http://www.atominfo.org.ua/news/resettlement2_aug_19_2003.htm,
2003
- Accident Analysis, Lithuanian
International Nuclear
Safety Center,
http://www.lei.lt/insc/sourcebook/sob11/sob114.html,
2004
- The Radioecological Consequences of the Chernobyl
Accident – The UNSCEAR Report, CNS Bulletin, Vol. 22, No. 1
- The Radioecological Consequences of the
Chernobyl Accident – Radioecological Aspects of Nuclear Power,
Kryshev, I. I., Nuclear Society International, Moscow, 1992