Nitrate uranium dioxide-plutonium

Several components are required in the practical appHcation of nuclear reactors (1 5). The first and most vital component of a nuclear reactor is the fuel, which is usually uranium slightly enriched in uranium-235 [15117-96-1] to approximately 3%, in contrast to natural uranium which has 0.72% Less commonly, reactors are fueled with plutonium produced by neutron absorption in uranium-238 [24678-82-8]. Even more rare are reactors fueled with uranium-233 [13968-55-3] produced by neutron absorption in thorium-232 (see Nuclear reactors, nuclear fuel reserves). The chemical form of the reactor fuel typically is uranium dioxide, UO2, but uranium metal and other compounds have been used, including sulfates, siUcides, nitrates, carbides, and molten salts. 

In contrast to the direct formation of a uranate species, there is evidence that both uranium and plutonium dioxide form a soluble species in molten alkali metal nitrates in the presence of nitric acid vapor (8, H)) or ammonium nitrate (70. How-... 

We have determined that plutonium dioxide does not react with molten nitrates under the same conditions that uranium dioxide does. We have also determined that plutonium dioxide is not soluble in molten nitrates with the addition of 100% nitric acid vapor under conditions which did produce soluble uranium. This observation must be further verified under the various conditions which can produce the soluble uranium species. 

These mixed oxides can also be manufactured by mixing the uranium and nitrate solutions produeed during the reprocessing of spent nuclear fuels and converting these metal nitrate mixtures into a mixed oxide (coprecipitation). In this process the plutonium is first reoxidized, then gaseous ammonia and carbon dioxide are introduced into the aqueous nitrate mixture, whereupon ammonium uranyl-plutonyl carbonate is precipitated. This can be calcined to... 

Plutonium Dioxide in Molten Equimolar Sodium-Potassium Nitrate. The behavior of plutonium dioxide in molten alkali metal nitrates is an area of major concern. Claims that alkali metal plutonates are formed (1, 2, 3, 5, 6) are not substantiated by definitive analytical results. In some cases (5, 6), sodium peroxide was added as an oxidant to either an alkali metal nitrate melt (6) or to an alkali metal hydroxide melt (5). If the temperature is great enough, for example above 700°C, thermal decomposition of the nitrate melt produces peroxide species. Other studies (4, , 12, 17) do not claim formation of a plutonate species, but only state that an insoluble plutonium-containing compound exists. However, in all the references cited, the results were given for mixed uranium-plutonium dioxide definitive analytical results were not given. 

Mixed Uranium-Plutonium Dioxide in Equimolar Sodium-Potassium Nitrate. The behavior of two compositions of mixed uranium-plutonium dioxide has been investigated thus far at PNL. The first composition, designated material A, consists of 5.44% Pu02/94.56% UO2. The second composition, designated material B, consists of 27.56% Pu02/72.44% UO2. Both materials were acquired as pellets that had been sintered at 1700°C. The behavior of both mixed-oxide materials was studied under the same conditions used in the previous experiments. 

Although these studies are not complete, it appears that molten alkali metal nitrates will react with mixed uranium-plutonium dioxide material of varying composition. Higher melt temperatures and longer reaction times are required, however, as the plutonium enrichment is increased. The insolubility of plutonium dioxide must certainly be investigated further since its solubility in the molten phase upon addition of nitric acid vapor has been claimed in various patents (8, 10). It should... 

Our conceptual flowsheet for further decontamination of plutonium dioxide indicates oxidation of Pu02 with a nitrate melt containing peroxide. A plutonate species should form with this treatment. If formed, we expect the plutonate to be soluble in the melt upon addition of nitric-acid vapor. If this supposition is correct, then the plutonium could probably be recovered similar to uranium. Whether a plutonate or Pu02 would be obtained from the thermal decomposition of a soluble plutonate species is unknown for this system. 

Preliminary investigation has shown that most fission products are not soluable in alkali metal nitrate melts and that they are not dissolved by addition of 100% nitric acid vapor. If these characteristics are verified by further experiments, a fission product separation is easily envisioned. One could react the fuel with the molten nitrate, dissolve the uranate with the addition of 100% nitric acid, and separate the uranium from the remaining solids, which should consist of both plutonium dioxide and fission products. 

---