Greater-Than-Class C Low-Level Waste (GTCC LLRW) and GTCC-like wastes contain radionuclides that undergo radioactive decay. Radioactive decay is the process by which an unstable (radioactive) atom transforms itself into a more stable configuration, oftentimes as another atom. The atom produced by radioactive decay can either be another radioactive atom or a stable (nonradioactive) atom. The radioactive decay modes of most concern for the radionuclides in GTCC LLRW are alpha particle and beta particle emission, and the resultant gamma rays and X-rays that accompany these decay processes.
The radioactive half-life is the length of time for a given amount of a radionuclide to decrease to one half its initial amount by radioactive decay. Half-lives are constant for each radionuclide and can range from less than a second to billions of years. The half-lives of the radionuclides of most concern in GTCC LLRW range from several years to more than 10,000 years.
Ionizing radiation is energy that is carried by subatomic particles and photons capable of causing atoms to lose electrons and become ions. For example, this radiation is given off by the radioactive decay of unstable atoms, X-ray machines, particle accelerators, and nuclear reactions such as fission. Non-ionizing radiation, such as that emitted by most lasers, is different because it does not create ions when it interacts with matter but dissipates energy generally in the form of heat. The radiation associated with GTCC LLRW is ionizing radiation. The main types of ionizing radiation that may impact human health and the environment from the management of GTCC LLRW are:
- alpha particles,
- beta particles,
- gamma rays, and
An alpha particle consists of two protons and two neutrons and is identical to the nucleus of a helium atom. Because of its relatively large mass and charge, an alpha particle produces ions in a very localized area. An alpha particle loses some of its energy each time it produces an ion (its positive charge pulls electrons away from atoms in its path), finally acquiring two electrons from an atom(s) at the end of its path to become a helium atom. An alpha particle has a short range (several centimeters) in air and cannot penetrate a sheet of paper or the outer layer of skin. Alpha particles are a hazard only if they are taken into the body. A number of the radionuclides in sealed sources and other wastes decay by emitting an alpha particle (e.g., Plutonium-238, Plutonium-239, Americium-241, Curium-244).
A beta particle can be either negative (negatron) or positive (positron). Negatrons are identical to electrons and originate in the nucleus of an atom that undergoes radioactive decay by changing a neutron into a proton. The only difference between a negatron and electron is its ancestry. A beta particle originates in the nucleus whereas an electron is external to the nucleus. Unless otherwise specified, the term "beta particle" generally refers to a negatron, which is the context in which it is used for GTCC LLRW. Beta particles are much smaller and more penetrating than alpha particles, but their range in tissue is still limited. Although lower energy beta particles are generally a hazard only if taken into the body, high energy beta particles represent an external radiation hazard and can produce significant skin doses. Beta particles can pass through a sheet a paper or thin clothing, but are stopped by a thin layer of aluminum foil, plastic, or glass. Many of the radionuclides in GTCC LLRW decay by emitting a beta particle (e.g. Iron-55, Nickel-63, Cesium-137).
A neutron is one of the two primary building blocks of the nucleus, the other being the proton. A neutron has no charge, and high energy neutrons are very penetrating and present an external hazard. Neutrons can be produced by the fissioning or splitting of atoms. Some transuranic atoms such as certain isotopes of plutonium and curium fission spontaneously, i.e., the fission process occurs without the need for additional neutrons to initiate the process. These transuranic radionuclides are present in some GTCC LLRW. Shielding for high energy neutrons generally consists of materials having high concentrations of hydrogen including water, concrete, sheets of paraffin, and plastic.
A gamma ray is electromagnetic radiation (similar to visible light, but at a much higher energy in the electromagnetic spectrum) given off by the nucleus of an atom as a means of releasing excess energy, and is oftentimes released when an atom undergoes decay by emitting an alpha or a beta particle. Gamma rays are bundles (quanta) of energy that have no charge or mass and can travel long distances through air (up to several hundred meters), body tissue, and other materials. A gamma ray is extremely penetrating and represents an external hazard. A gamma ray can pass through a human body without hitting anything, or it may hit an atom and give that atom all or part of its energy. Because a gamma ray is pure energy, it no longer exists once it loses all of its energy. Some GTCC LLRW has very high concentrations of radionuclides that emit high energy gamma rays, such as cesium-137. These wastes must be remotely handled or adequately shielded to protect workers. Thick layers of concrete, lead, steel and other comparable shielding materials are necessary to stop the penetration of gamma rays.
An X-ray is the same as a gamma ray, but originates external to the atom by the movement of electrons between energy shells (from a higher to lower energy shell). The excess energy associated with this electron movement is released as an X-ray. X-rays have less energy than gamma rays, are less penetrating, and require less shielding. In all other aspects, X-rays behave in the same manner as gamma rays.
Radiation Dose Units
Radiation doses are typically reported in units of rem (an acronym for Roentgen equivalent man) or millirem (mrem), which is one one-thousandths of a rem. The rem is the product of the absorbed dose in rads (the amount of energy imparted to tissue by the radiation), and factors for the relative biological effectiveness of the radiation. For example, consistent with modern dosimetry, alpha particles are considered to be 20 times more hazardous than beta particles for the same energy deposition and hence have a quality factor of 20, whereas the quality factor for beta particles is one. The radiological impacts associated with management of GTCC LLRW will be evaluated in terms of the effective dose developed by the International Commission on Radiological Protection. The effective dose is the product of the dose to individual tissues and tissue-specific weighting factors (fractional values less than one) that indicate the relative risk of cancer induction or hereditary defects from irradiation of that tissue, summed over all relevant tissues. Use of effective dose allows for a direct comparison of the relative radiation hazards from various types of radiation that impact different organs of the body. This is referred to simply as dose in the EIS.
All organisms are being exposed to ionizing radiation from natural sources all the time. Exposure to background radiation and naturally occurring radioactive materials results in an annual dose of about 310 mrem/yr. Of this total, about 33 mrem/yr is due to external ionizing radiation from cosmic rays, about 21 mrem/yr is due to external radiation from terrestrial gamma rays, and radionuclides within the body (principally Potassium-40) contributing about 29 mrem/yr. About two-thirds of the background radiation dose is due to the inhalation of radon-220 and radon-222 gases and their short-lived decay products. Inhalation of radon gas contributes about 230 mrem/yr. Ingestion of food and water containing naturally occurring radionuclides account for only a few mrem/yr.
Medical and Industrial Sources
Man-made sources of radiation result in an average annual dose of about 310 mrem/yr, with most of this dose being attributable to medical uses of radiation and radioactive material. Medical and dental X-rays are used to detect problems such as broken bones and dental cavities, and sealed radiation sources are used to deliver very high, localized radiation doses to treat certain types of cancer. The largest contributor to the dose from medical uses is computed tomography (CT), which accounts for about 150 mrem/yr to an average individual. Lesser doses are associated with consumer products that produce radiation such as smoke detectors and television sets, and from travel in high altitude airplanes. Radioactive sources are used for a number of industrial activities such as to sterilize food products, inspect welds, and test materials, and these uses can result in doses to workers associated with these activities.
Primary Health Effects
Radiation exposures associated with management of GTCC LLRW and GTCC-like waste are expected to be limited to chronic effects and need not consider acute effects. The main health concern associated with chronic exposure to radiation is an increased likelihood of developing cancer, and this health effect is analyzed in detail in the Draft EIS. Additional health effects include genetic mutations, teratogenic effects such as mental retardation, heart disease, and circulatory problems. Large doses are required to cause acute effects, and no such exposures are expected for managing these wastes. Acute doses above 25 rads delivered over a short time period can induce a number of deleterious effects, including nausea and vomiting, malaise and fatigue, increased body temperature, blood changes, epilation (hair loss), temporary sterility, and others; bone marrow changes have not been identified until the acute doses reach 200 rads. There are no credible scenarios in which such acute doses are expected to occur.
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