Uranium oxide systems vaporization

Substantial quantities of nomadioactive material may be vaporized from the reactor core during this stage of an accident. In particular, control rod materials and burnable poisons may be vaporised. Constituents of structural materials such as steel and clad alloying agents as well as uranium oxides may be vaporised along with radionuclides. These nonradioactive materials add to the mass of condensable effluents from the core region and can affect the behavior of radionuclides both in the reactor coolant system and in the containment (See Chapter V). 

The uranium oxygen system is complex and has been the topic of numerous studies. Part of the problem with the studies has been the exact stoichiometry of the phases being examined (see earlier discussion about the numerous oxide phases of U). Most efforts have dealt with the dioxide system and several reviews have been made (Ackermann and Chandrasekharaiah 1974 and references therein, Weigel 1986). The solid monoxide of uranium is at best metastable. Vaporization of the dioxide yields the following major species U(g), UO(g), and UOjCg). Discrepancies with regard to the free energy of formation of U03(g)-one of the species that has been seen over UO2., t(s) - have been discussed. 

Of the wide range of possible combinations of uranium oxide, phosphoric acid, and water, two systems appear to be of special interest. The first, which consists of UO3 dissolved in an aqueous solution containing between 30 and CO w/o of phosphoric acid and pressurized with an oxygen overpressure, was used for the first Los Alamos Power Reactor Experiment (LAPRE-1). The properties of this solution are given in Chapter 3 and summarized in Table 7-8. The vapor pressure of this solution at the design operating temperature of LAPRE-1 (430 C) is 3600 psi. 

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. 

The binary systems actually and potentially important as nuclear fuel include oxides, carbides, nitrides, phosphides, and sulfides of uranium, plutonium, and thorium. An increasing amount of detailed information is becoming available on the phase equilibria of these compounds, but the relations existing between the composition (especially nonstoichiometric) and the vapor pressure (or activity) of each component are known only for a limited number of systems. 

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