An indigenous uranium mining industry might provide early indication of a clandestine uranium or plutonium-based weapon program and is a sure indicator of at least the possibility. For the plutonium path, natural uranium could fuel a graphite- or heavy-water moderated plutonium-production reactor. A sizable research program involving breeder-reactors or the production of heavy water or ultra-pure carbon and graphite products might also be cause for concern, especially if such programs were not easily justifiable on other accounts.
Small research or power reactors with high neutron flux and significant amounts of uranium-238 in their cores can also be used to produce plutonium. However, a 40 to 50 MW(t) undeclared reactor (enough to produce plutonium for at least one bomb per year) should be easily discernible to overhead infrared sensors, at least if it is built above ground and located away from heavy industrial areas (such a location might be chosen for security and safety reasons anyway).
Inspections of safeguarded reactors, especially if carried out at more random intervals, might detect unnecessary placement of uranium-238 in or around the core, augmenting the rate of plutonium production. Similarly, inspections of CANDU-style reactors (a heavy-water-moderated reactor that can be refueled online) or of frequently shut-down LWRs should call attention to very low-bum-up fuel cycles, from which the plutonium produced is predominantly plutonium-239, the isotope best suited for weapons.
In general, the plutonium-production route, which involves reprocessing of spent reactor-fuel to extract plutonium, would be easier to detect than would be a small-scale clandestine uranium-enrichment facility. Plutonium and uranium from spent fuel (as well as enriched uranium from research reactor cores), is reclaimed by chopping up and dissolving the fuel elements in acid, subjecting the solution to solvent-extraction and ion-exchange processes, and chemically converting the plutonium and uranium in the resulting liquids to metallic or oxide forms. Methods for doing this, including the most common one, known as PUREX, involve various well-understood chemical processes that use characteristic groups of materials.
Detection of these materials, either by environmental sampling or by impactions, could indicate reprocessing activity. Some chemicals might also be observed through export monitoring; for example, high-purity calcium and magnesium, which are used in the metal-conversion step, are included in the Nuclear Supplier Group's new list of sensitive dual-use items to be subjected to export controls. In addition to the characteristic chemicals used in the PUREX process, effluents from reprocessing plants will contain telltale radioactive fission products, including radioactive isotopes of the noble gases xenon and krypton -- especially krypton-85 -- and possibly argon. Measurements made at the U.S. reprocessing facility at the Savannah River Plant in South Carolina have suggested that krypton-85 may be detectable, even from small facilities, at ranges of 10 kilometers or more. A single sampling station located in the direction of the prevailing winds close to a reprocessing facility can achieve detection probability of 90% according to a study done by the Krypton Verification Project at the Technical University (Technische UniversitSt) in Darmstadt, Germany. Tracing levels of krypton-85 in the atmosphere may be the best way of remotely detecting illicit plutonium separation at a nuclear fuel reprocessing plant. Technical means are available for the retention of krypton-85 and its immobilization, storage and disposal, having regard to the radiological hazards.
Analysis of plutonium samples or effluents from reprocessing could provide further evidence of weapon intent by revealing the fuel's irradiation level. For most types of reactor, a very low fuel-irradiation level would be a strong indicator of weapon activity. In addition, isotopic correlation techniques -- which compare the isotopic ratios of different samples of plutonium -- can provide sensitive indicators of plutonium production history or material diverted from one facility to another.
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