Any industrial process produces some quantity of waste along with the product it is manufacturing, this is a fact of doing business, but the waste produced as a byproduct of nuclear power and weapons production has a number of characteristics which differentiate it from other forms of waste, chief among these is it’s radioactivity. Protecting both workers in the industry and the public from exposure to the harmful levels of radioactivity in nuclear waste is a primary concern. In all matters pertaining to disposal or storage of the waste, precautions must be taken to limit the potential risks and avoid contaminating the environment. Depending upon the level of activity of the waste special handling may be required in transporting and managing it. All of these considerations add to the difficulty and therefore the cost of dealing with the waste. The risk of injury, genetic damage, cancer or death from exposure to varying levels of radioactivity must be a serious consideration in waste handling.

The extended length of time that such precautions must be observed due to the relatively long half lives of a number of the isotopes present in nuclear waste is an additional safety concern and cost factor. The isotope with the longest half life is one of the most abundant as well, Uranium²³⁸ at 4,510,000,000 years. While the other isotopes involved have substantially shorter half lives they are still quite long. For example: Radium²²⁶ 1,620 years, Plutonium²³⁹ 24,400 years, Uranium²³⁴ 247,000,000 years, Thorium²³⁰ 80,000 years, Neptunium²³⁷ 2,100,000 years, Curium²⁴⁵ 9,300 years, Cesium¹³⁵ 2,000,000 years, Uranium²³⁵ 710,000,000, Americium²⁴³ 7,950 years, and Iodine¹²⁹ 17,000,000 years. One must remember also that half life does not mean the material will be safe in that many years, it simply means that half as much of the original isotope will exist. A rule of thumb has been established that after ten half lives the material will have decayed enough to be safe. Using this method even some of the shorter lived isotopes will remain hazardous for some time. For example: Strontium⁹⁰, half life 28 years would be safe in 280 years; Plutonium²³⁸, half life 86 years, would be safe in 860 years. This means that access to the waste disposal or storage must be controlled and that control maintained for an extended period. This period in some few cases would involve safe guarding the material from intrusions by man of from accidental exposure due to natural forces including earthquakes, volcanism, fire, tornado, etc. “The potential risk from the waste declines over time with the radioactivity, and with it the need for isolation lessens”. The majority of the isotopes will decay within several thousand years. Therefore the barriers to exposure must not depend upon the actions or behavior of man; they must be engineered into the system. This in itself is somewhat difficult since materials chosen for construction must maintain their structural integrity for fairly extended periods.

Inadvertent criticality is a threat from a percentage of waste. By critical we mean the start of a nuclear chain reaction in fissionable material. When this happens outside the reactor, the built-in controls are not available to prevent the reaction from running away. An uncontrolled critical mass emits large quantities of radiation including neutrons and gamma rays in levels that would be lethal to anyone nearby. In addition, it is possible for the correct concentration of fissionable material to become supercritical and explode. The explosion would not be an atomic explosion per se; it would be a non-nuclear explosion of the sort produced by TNT, but it would tend to spread radioactive material widely exposing more personnel if it occurred in an inhabited area and perhaps killing through blast effects. A critical mass is the minimum quantity of a fissionable material required to sustain a nuclear chain reaction; hence a sub critical mass is one just short of going critical. The difference between the two may only be the shape of the container. Geometry is an important factor in determining criticality. What is a sub critical mass in one container may be a critical mass in one that is shaped differently. If two containers are placed too close together, they may create a critical mass. The addition of a moderator, a substance such as water which tends to encourage the initiation and perpetuation of a chain reaction, may bring a sub critical mass to criticality. This effect is due to the slowing of neutrons by the moderator, which makes them more effective at splitting the nuclei of a fissionable element. Once the critical mass has been reached, fissioning begins nearly instantaneously. Anyone in the vicinity would be endangered but would not know it since there are no discernible signs that fissioning is occurring until it is well underway, at which point it would be too late—the damage would have already occurred. Once begun, fissioning increases exponentially producing heat, gamma radiation, neutron emissions, x-rays, alpha and beta emissions. As the mass nears supercriticalarity, it produces a noticeable light or glow that tends to be bluish white. If the reaction continues, it explodes and blows itself apart, halting the reaction but spreading the radioactive contamination. Inadvertent criticality is only possible if the waste contains fissionable elements such as Plutonium ²³⁹, Uranium ²³⁵, or Uranium ²³³. There have been a number of accidents due to inadvertent criticality in the US as well as other countries. The Tokamura Accident that killed a worker at a plant in Japan was this sort of accident.

Some types of nuclear waste produce significant amounts of heat as a product of radioactive decay. This heat must be considered in planning for storage or disposal and taken into account when choosing materials for containers or provisions must be made for cooling. Spontaneous boiling of liquid wastes can contribute to structural weakening of the container. Solid waste containers must be cooled with water or other means to prevent melting of the container or structural degradation. The quantity of heat energy produced is substantial and production continues for an extended period. The thermal output of a commercial reactor core is sufficient when fresh from the reactor to melt granite. Metal capsules containing Strontium⁹⁰ extracted from military reactor waste self heat to 806F. for a number of years. High level liquid waste in one gallon containers self heats to 392° F. The contents of waste tanks will, if left uncooled, boil themselves dry producing a caked sludge like substance at the bottom of the tank. This self-heating characteristic is particularly strong in fission products, relatively short lived isotopes that nevertheless produce a large amount of heat which must be provided for in the design of storage or disposal sites.

A somewhat related characteristic of some forms of waste is the pyrophoric tendency of Uranium and Plutonium metal. This means that these metals spontaneously burst into flame when they came in contact with air. Some non-radioactive metals and materials also have this characteristic –phosphorous, sodium, and magnesium among others. The ability of a waste material to combust spontaneously is directly related to its purity since materials in lower concentration have less or no pyrophoric ability. The percentage of the waste stream with high enough content is relatively small, and most are byproducts of nuclear weapons manufacture. Finely divided material is more likely to catch fire then larger pieces, for example dust and shavings from machining processes must be disposed of in metal buckets layered with sand to prevent them from burning.

Radioactive waste with water as a constituent tends to produce hydrogen gas as a result of radiation dissociation of water to hydrogen and oxygen. In liquid waste stored in underground or closed containers it is possible for the concentrations of hydrogen to reach explosive levels unless it is vented. Hydrogen is flammable and explosive if exposed to an ignition source such as a spark of static electricity. If an explosion occurs in a closed container, it could breach the container and disperse radioactive contamination to the environment.

A percentage of waste is contaminated by chemicals in its processing that themselves pose a threat of some sort. They may contain acids, volatile organic solvents, heavy metals, alkalis or inorganic compounds. These chemical constituents may be toxic, flammable, corrosive, reactive, carcinogenic, mutagenic or caustic. For example, tests at INEEL have detected metals including barium, cadmium, arsenic, zinc, beryllium, antimony, sodium, chromium, lead and mercury; inorganic salts including chlorides, sulfates and nitrates; and volatile organic solvents including: trichloroethene, tetrachloroethene, 1,2-dichloroethene, carbon tetrachloride, dichlorodifloromethane, tolulene, chloroform, 1,1,1-trichloride, benzene, xylene, polychlorinated biphenyls, benzoapyrene, diethylphthalate, acetone, methylene chloride, petroleum products, paint and hydrocarbons. At the Hanford site a large number of metals, organic solvents, acids and other chemicals have been found—zirconium, beryllium, lead, fluoride, nickel, silver, chromium, copper, bismuth, cadmium, sodium, sodium aluminate, sodium hydroxide, nitrates. Trichlorethylene, mercury, nitric acid, phosphoric acid, sulfuric acid, hydrofluoric acid, chromic acid, zinc, sodium silicate, methanol, carbon tetrachloride, acetone, acenophthene, PCBs, mercury, perchloroethylene, 1,1,1-trichloroethane and asbestos. Some of these materials are toxic or hazardous, some are relatively innocuous, they illustrate the wide range of chemical constituents that may be present in the waste stream.

The final characteristic of some types of nuclear materials and waste is their potential for diversion by outlaw nations or terrorists for the purpose of constructing a nuclear bomb or weapon intended to spread radioactive contamination with a non nuclear explosive. In order to construct a nuclear weapon, all that would be required is a supply of fissionable material and rather rudimentary technology. For this reason regulations have been established governing the storage, ownership, and disposal of ‘special nuclear materials’. Admittedly this category of waste is small compared to the total quantity of waste, but provisions for preventing its theft have been determined to be necessary since the 1960s. They are administered by the International Atomic Energy Agency. Inspectors physically inventory subject materials and utilize instrumentation and surveillance to monitor facilities where such materials are stored. Records are kept for fissionable material including reactor fuel, used fuel rods, metallic uranium and plutonium, and materials that could potentially yield such materials through processing. ”Accountancy, i.e., reporting by states of the whereabouts of the fissionable materials under their control….” Is a part of the IAEA program of voluntary safeguards and verification.

Nuclear waste often requires special handling due to its radioactivity or other characteristics. All waste falls into one of two categories based upon the level of risk it poses, either contact or remotely handled waste. Waste with low levels of radioactivity may be handled directly, workers may come in contact with the containers wearing a dust mask to prevent inhalation or ingestion of dust and disposable paper gowns, caps and foot covers are worn to prevent tracking material to uncontaminated areas. Disposable latex gloves are used to prevent contamination that could lead to ingestion. Much waste falls into this category.

Waste with higher levels of radioactivity cannot be directly handled and requires additional protective measures to prevent exposure of workers to radiation risks. Material that emits alpha radiation only may be handled using a glove box. This apparatus consists of a Plexiglas box with heavy rubber gloves permanently attached through holes in the front of the box. The box is sealed and has a positive pressure ventilation system with a series of filters connected to it, preventing escape of the material inside the box. To use a glove box, the worker inserts their hands and arms into the gloves enabling them to manipulate materials inside the box. All air exhausted from the box is carefully filtered and purified to prevent any airborne dust in the box from escaping. Glove boxes are often used for handling plutonium.