Plutonium in the DOE complex consists largely of plutonium metal pits (the spherical core of a nuclear warhead). But there are also over 33 tons of plutonium in other forms, including processing residues and pieces of metal scrap. Residues and scraps are stored in different chemical and physical forms and a variety of containers, many of which are unsuitable for long-term storage. Some plutonium remains in "process hold-up," (e.g., plutonium in ventilation systems and process vessels).

A number of potential problems, or "vulnerabilities" (see box), could arise from extended storage of excess plutonium in unsuitable containers or forms. The DOE is currently examining how to include excess plutonium² in its long-term plans for managing plutonium (known as "final disposition"). Since final disposition will likely take decades to complete, excess plutonium will require some Band-Aid measures in the meantime, such as processing or interim storage.

The term "vulnerability" has acquired a specific definition in the context of this DOE inquiry. Vulnerabilities in nuclear facilities are "conditions of weakness that may lead to unnecessary or increased radiation exposure of the workers, release of radioactive materials to the environment, or radiation exposure of the public."3

The Assessment

As the first step in deciding what to do in the short-term with plutonium residues and pits, the Office of Environment Safety & Health (ES&H) of the DOE is currently performing a vulnerability assessment. This assess-ment will provide the DOE with an understanding of ES&H issues surrounding the plutonium inventory. The assessment results will serve as the foundation for determining immediate, interim and final disposition actions.

Immediate corrective actions are needed to remedy pressing safety concerns. For instance, the solutions in degraded containers could be transferred to new containers to prevent leakage. Interim actions may be required to address vulnerabilities during the next 1 to 20 years. For example, the DOE has suggested converting plutonium nitrate solutions and plutonium oxide into metal "buttons" (disks of plutonium that are roughly the size of hockey pucks) at Savannah River Site (SRS) and Hanford. Some interim actions, however, are controversial. For example, the DOE has suggested that, from a technical perspective, pits are the ideal form for storage. But this option would contradict the non-proliferation objective of the project, since plutonium metal in pit form is essentially ready to fashion into a weapon. Finally, though the assessment project will not recommend actions for final disposition, the results of the study will help shape disposition policy in the future.

Some vulnerabilities are already known to exist. One of these relates to containers known as "cans" in which plutonium is deterioratin. A survey of 30 representative cans at the Los Alamos National Laboratory, in New Mexico, indicated that while none of the packages examined had yet failed, a third of the 30 containers showed various degrees of deterioration that could lead to failure if not properly stabilized and repackaged. There are thousands of storage containers.⁴ The ES&H study will also try to uncover vulnerabilities not yet identified.

The vulnerability assessment study will address several categories of plutonium, including process hold-up; plutonium metal; oxide; unirradiated reactor fuel and targets; weapons components and pits in DOE custody; scrap, residues, and compounds; and product solutions and laboratory samples. However, the study excludes plutonium in spent fuel. DOE has identified a total of 42 sites as possessing plutonium. Initial assessment of all these sites will rely on a set of written questions. In addition, 14 of the 42 sites will be visited.

The assessment will try to find out how much plutonium in its various forms is present at each of the 42 sites, and to analyze the physical barriers and administrative controls that would restrict the release of plutonium from the site ("barrier analysis"). It will identify adverse conditions both inside and outside of the facility that could aggravate environment, safety and health or security problems. Examples of such adverse conditions inside facilities include fire, equipment failure, and inadequate preventive maintenance. External adverse conditions could include aircraft crash, earthquake, and power failure.

The Hazards of Plutonium Storage

The principal hazard to human health from plutonium arises when it is inside the body. Plutonium emits almost all its energy as alpha radiation, which does not penetrate the outer, dead layer of the skin. But once inside the body, the alpha radiation from plutonium is able to damage cells. Even very small quantities of plutonium inside the body increase cancer risk substantially. When storing plutonium then, it is crucial to ensure that no plutonium can escape its storage container. The dangers to worker and public health from plutonium storage depend on the risk that plutonium may not be contained. That risk in turn depends on the physical and chemical properties of the various forms of plutonium (see Centerfold) and on their interaction with storage conditions and containers. See the box below, "Plutonium Storage: Why is it Hazardous?"

Evaluating the Results

When the survey stage of the assessment is complete, the problems at each site (materials, barriers, and adverse conditions) will be weighed against the compensatory measures (safety precautions such as alarm systems). The net result, or the "consequence analysis" will determine if and where there are vulnerabilities relating to contamination, exposure, and direct injury to workers and the public. It will also assess vulnerabilities in terms of environmental insult to the air, water, and ground. For instance, a vulnerability might consist of the potential contamination and exposure of workers (e.g., rupture of container in storage).⁵

Several actions can be taken to reduce risks from known vulnerabilities. For instance, preparation and certification of materials for storage are generally likely to reduce risks. The risks associated with storage of metal and oxide can be reduced by excluding all organic materials from the storage package. Finally, using leak-tested, hermetically-sealed, storage containers could also decrease risks.

This DOE initiative of determining environment, safety and health vulnerabilities and corrective actions for plutonium in the complex is vital. But, as with many past DOE efforts on ES&H matters, it has one crucial flaw. The scope of the effort excludes all consideration of what impact interim actions will have on the final disposition options for plutonium. Since interim processing could complicate final disposition and increase its costs, this is a serious omission. We urge DOE to address how their interim actions will affect each of the disposition options discussed in the accompanying editorial.

Plutonium Storage: Why is it Hazardous? Much of the plutonium in the DOE Complex is stored as plutonium oxide, which normally consists of small particles. If not contained, such particles can be easily dispersed and inhaled. Plutonium oxide has the ability to adsorb (to stick: see Dr. Egghead) water and organic molecules on its surface. If the container (and thus the plutonium inside it) is heated, or if chemical reactions within the container raise the temperature, any absorbed water on the plutonium may be released, building up the pressure in the container.. Pressurization can also occur when the adsorbed materials are slowly released over time. In addition, the adsorbed molecules are subject to radiation from the plutonium, which can chemically break them up (the process of radiolysis; see Dr. Egghead). Radiolysis can also cause problems in packaging materials; any plastic in the packaging, for example, may disintegrate. Unfortunately, the DOE wrapped and sealed many containers in plastic bags in an effort to keep out moisture. A breach of the primary containment would therefore put plutonium in contact with the plastic. Radiolysis of some types of plastic bags releases hydrogen and gaseous hydrochloric acid, both of which react with the container material and the plutonium metal. These reactions increase the risk of fires; some of them also release heat within the container. Such reactions in turn increase the risk that the plutonium will not be contained. Other material properties of plutonium may result in hazards in the DOE weapons complex First, plutonium in some cases is pyrophoric (spontaneously igniting in air: see Dr. Egghead). Clean plutonium metal does not burn at room temperature, but the higher temperatures associated with machining plutonium metal have caused numerous fires in the finely-divided plutonium metal machine scraps. Second, the decay of short-lived plutonium-241 in plutonium metal yields americium-241, which emits penetrating gamma radiation. Thus, worker exposure from gamma radiation during direct handling of some material may be a concern. Hazards associated with forms of plutonium other than metal or oxides are also likely to create environment, safety and health vulnerabilities. For instance, spills or accidental criticality are important concerns for plutonium nitrate solutions, such as those stored at the Savannah River Site, near Aiken, South Carolina.