Nitrous acid Purex process

Plutonium trifluoride. Plutonium trifluoride can be converted directly to plutonium metal, or it is an intermediate in the formation of PUF4 or PUF4 -PUO2 mixtures for thermochemical reduction, as described in Sec. 4.8. The stabilized Pu(III) solution, produced by cation exchange in one of the Purex process options for fuel reprocessing, is a natural feed for the formation of plutonium trifluoride, as is shown in the flow sheet of Fig. 9.9 [03]. A typical eluent solution from cation exchange consists of 30 to 70 g plutonium/liter, 4 to 5 Af nitric acid, 0.2 Af sulfamic acid, and 03 Af hydroxylamine nitrate. The sulfamic acid reacts rapidly with nitrous acid to reduce the rate of oxidation of Pu(III) to about 4 to 6 percent per day. Addition of ascorbic acid to the plutonium solution just before fluoride precipitation reduces Pu(IV) rapidly and completely to Pu(III). 

Reduction with ferrous ion was the reaction used in the first Purex flow sheets, at Hanford and Savannah River. The specific reductant used was ferrous sulfamate Fe(S03NHj)j, a compound selected because it stabilized ferrous ion against oxidation in a nitric acid-nitrous acid system. The process was satisfactory in all respects except its addition of extraneous, nonvolatile components to the wastes. 

In the PUREX process, the oxidizing property of nitric acid and the formation of nitrous acid are not favorable to maintain plutonium as a trivalent species. The sufficient amount of hydrazine is added to the system to ensure the stability of Pu(III) with the destruction of nitrous acid according to Equation 14.7. Despite the fact that hydrazine is particularly advantageous because the reaction products, N2/ N2O, and H2O, do not contribute to the volume of stored wastes (Schlea et al., 1963), the interaction of hydrazine and nitrous acid can initiate, in a TBP-nitric acid system, and under specific operating conditions, the formation of hydrazoic acid (HN3) which is a hazardous and potentially explosive compoimd (Equation 14.7) (Dukes and Wallace, 1962). Further oxidation leads to the formation of nitrous oxide and nitrogen gases as depicted in Equation 14.8... 

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