MODULE 5
After studying this module, the reader will be able to
Production of plutonium at the Hanford Site released over 100 radioactive substances into the environment for more than 40 years. Some substances contributed more than others to the radiation dose a person received. This module discusses how exposure to radiation occurs, how the body handles internal radiation exposure, and which tissues and organs received the main dose from radioactive materials released from Hanford.
In addition to Hanford radiation, radiation exposure comes from a variety of sources. These include medical uses of radiation; radioactive substances found in the environment, such as radon or cosmic rays; and nuclear fallout. This module, however, focuses on internal exposure from radionuclides released from Hanford.
Radiation exposure can be either external or internal. External exposure occurs when the radiation source is outside the body. Examples of this kind of exposure include standing in a cloud of radioactive gas, swimming in radioactively contaminated water, or being subjected to X-rays.
Internal exposure occurs when a radionuclide is ingested, inhaled, or enters the body through breaks in the skin. For most people exposed to Hanford's radioactive releases, the main route of exposure was internal for most of the radionuclides listed in Tables 1 and 2 in this section.
The Hanford Environmental Dose Reconstruction Project (HEDR) estimates that six radionuclides released into the air account for nearly all the radiation dose a person may have received through the air pathway. (For a description of the Dose Reconstruction Project, please see Module 3.) Five radionuclides are estimated to account for most of the dose a person may have received from the river pathway. These radionuclides are listed in Tables 1 and 2. Representative dose estimates for the eleven radionuclides are available from HEDR.
HEDR estimates that iodine-131 accounts for most of the radiation dose people received from the air releases. Most of this dose came from eating locally-grown, leafy green vegetables and fruit, as well as drinking milk containing iodine-131. Drinking Columbia River water and eating radioactively-contaminated fish were the two most important factors contributing to radiation dose from Hanford's river releases.
Once a radionuclide is inside the body, some of it may enter the bloodstream. The chemical properties of the radionuclide determine how the body handles the radioactivity. The body does not recognize the difference between a radioactive and non-radioactive substance. For example, strontium-90 is chemically similar to calcium and the body utilizes strontium in the bone in much the same way it does calcium.
When a radionuclide concentrates primarily in one organ, as when strontium concentrates in the bone, that organ receives a larger dose from the radioactive substance than do other organs or tissues. Other radionuclides, such as neptunium-239, which are not chemically similar to substances needed for the body's functioning, may also concentrate in different organs or tissues.
Some radioactive substances do not concentrate in one organ, but are distributed throughout the body. Tritium, for example, is a form of hydrogen. Hydrogen is part of the water molecules present throughout the body, so tritium delivers a dose to all tissues.
The dose to different parts of the body is determined by a number of factors, including the amount of radioactivity present and its distribution, solubility in the bloodstream, and the type and energy of the emitted radiation. Once the radioactive substance is taken into the body, it will continue to give off radiation until either the radioactivity has decayed or the body has eliminated the substance through normal metabolism. Both of these processes occur simultaneously.
The rate of decay of a substance depends on its half-life; the amount of time it takes for a radioactive substance to lose one-half of its radioactivity. Half-lives for different substances vary from millionths of a second to billions of years. An atom is no longer radioactive when it decays and becomes stable.
A radionuclide may be absorbed by organs and tissues other than the one in which it concentrates. The radioactive substance will give a radiation dose to the other organs or tissues, but the dose is typically much smaller. Iodine-131, for example, is concentrated by the thyroid gland, but also gives a dose to other organs and tissues, such as reproductive organs and breast tissue. However, the dose from iodine-131 received by the reproductive organs and breast tissue is much less than the dose to the thyroid. For example, the dose to breast tissue is 30,000 times less than the dose to the thyroid. A radiation dose to the ovary is nearly one million times less than a dose to the thyroid.
According to 1994 dose estimates from HEDR, releases from Hanford resulted in whole-body doses of 30 rem EDE or less. A whole-body dose is one in which approximately the same dose is received by each organ, as may happen with exposure to tritium. But some people-particularly those living near Hanford before 1960-may have received high doses to the thyroid gland or other organs. Doses to the thyroid gland between 1944 and 1951, for example, may have been as high as 870 rad for some children.
Both whole-body doses and organ doses increase a person's risk of cancer or other health problems. A radiation dose from the radioactive substances released from Hanford may have caused or could cause health problems. Because individuals were exposed to varying amounts of radioactive substances from the Hanford Site over many years, health effects may have resulted. However, very little is known from human health studies about low-dose radiation and health problems other than cancer. Current research methods may not be sensitive enough to detect a link between low-dose radiation and other health problems, if they exist. Hanford-related studies now underway may increase our knowledge about radiation doses and the relationship between iodine-131 doses and thyroid disease. Additional studies may help to identify other health problems reported by some people who were exposed to Hanford's releases.
Tables 1 and 2 refer to radioactive substances released into the air and into the Columbia River. The Tables list the current estimated amount of each substance released from Hanford (1944-1972), the main routes of exposure for each radioactive substance, the organs which received the main dose from the substance, and the physical half-life of each substance. Module 2 introduces the discussion of radiation health effects. Module 7 presents guidelines for evaluation of thyroid disease in people exposed to I-131. Modules 10 and 11 present discussions of possible health effects of exposure to selected radionuclides.
| Substance | Amount Released from Hanford | Main Routes of Exposure | Organs Receiving Main Dose | Half-life |
| Iodine-131 | 739,000 curies | ingestion | thyroid | 8 days |
| Ruthenium-103 | 1,160 curies | external inhalation | whole body lungs | 39.4 days |
| Ruthenium-106 | 388 curies | inhalation/ ingestion | lungs GI tract | 368 days |
| Strontium-90 | 64.3 curies | ingestion | bone surfaces red bone marrow | 28.8 years |
| Plutonium-239 | 1.78 curies | inhalation | lungs bone surfaces | 24,100 years |
| Cerium-144 | 3,770 curies | inhalation
ingestion | lungs
GI tract | 284 days |
| Substance | Amount Released from Hanford | Main Route of Exposure | Organs Receiving Main Dose | Half-life |
| Phosphorus-32 | 229,000 curies | ingestion | red bone marrow | 14.3 days |
| Zinc-65 | 491,000 curies | ingestion | whole body | 245 days |
| Arsenic-76 | 2,520,000 curies | ingestion | GI tract
stomach for infants | 26.3 hours |
| Sodium-24 | 12,600,000 curies | ingestion | stomach | 15 hours |
| Neptunium-239 | 6,310,000 curies | ingestion | GI tract | 2.4 days |
