Kuchler and associates [Кб, K7] of Farbwerke Hoechst have investigated the modifications necessary in the acid Thorex process to enable it to handle (1) the high concentration of fission products present in fuel with the burnups of up to 100,000 MWd/MT expected in fuel from the HTGR, AVR, and THTR, and (2) uranium concentrations of up to 20 percent in thorium, which may be used in these reactors when fissile uranium is diluted with 238 U to deter its use as a nuclear explosive. They found two difficulties with the acid Thorex process flow sheets previously used at Oak Ridge [B14] and Hanford [Jl]:

1. A second organic phase formed when the thorium concentration in first-stage solvent extraction feed was as high as 1.5 M.

2. Hydrolysis products of fission products precipitated when the feed was made acid-deficient.

To avoid these difficulties they reduced the thorium content of solvent extraction feed to 1.15 Af and developed a two-stage acid Thorex process. In this process thorium and uranium were coextracted from an acid feed to separate them from most of the fission products and then stripped back into the aqueous phase. By this means fission products were removed to such an extent that the Thorex process with acid-deficient feed could be used in the second stage without causing them to precipitate.

First stage. The flow sheet recommended by Kuchler et al. [K7] for the first stage of this two-stage process is shown in Fig. 10.22. Adjusted feed is 1.15 M in thorium and is assumed to contain from 4 to 20 percent as much uranium. The nitric acid content of feed is made from 0.7 to 1.1 M, depending on its uranium content. One volume of feed is extracted with 9.5 volumes of 30 v/o TBP in unit 1A, with eight extracting stages and eight scrubbing stages. One volume of 0.1 M HN03 is used for aqueous scrub, and 0.22 volume of 13 M HN03 is added to

the third extracting stage to complete extraction of thorium, as in the Hanford flow sheet Fig. 10.21. Uranium and thorium are returned to the aqueous phase by eight volumes of 0.01 Af HN03 in 16 stripping stages 1C. Aqueous product from 1C is concentrated and made 0.15 M acid-deficient in the evaporator and becomes partially decontaminated feed for the second stage.

Second stage. The second stage is shown in Fig. 10.23 with material quantities for the lower, 4 percent, uranium feed. In unit 2A, one volume of feed is extracted in eight stages with eight volumes of 30 v/o TBP and scrubbed in eight stages with one volume of 1 M HN03. The scrub contains 0.01 M H3PO4 to improve decontamination from protactinium and zirconium — niobium, as in the Hanford flow sheet Fig. 10.21. An additional scrub of 13 M HNO3 is added to the third extracting stage to complete recovery of thorium.

In unit 2B, thorium is returned to the aqueous phase by stripping in eight stages with 4.8

L_____ ^

Figure 10.23 Second stage of two-stage acid Thorex process for high-burnup fuel. (From К itchier et al. [K7].) volumes of 0.01 M HN03. Uranium is extracted from thorium product in eight stages by an additional 1.4 volumes of solvent.

In unit 2C uranium is returned to the aqueous phase by stripping in 16 stages with an additional 4.0 volumes of 0.01 M HN03.

Uranium product is further decontaminated by a third cycle of extraction with 5 v/o TBP in и-dodecane and stripping with 0.01 M HN03.

Process results. Decontamination factors observed by Kiichler et al. [K7] in processing 54,000 MWd/MT fuel with thorium/uranium ratio of 5.9, cooled 346 days, are listed in Table 10.19. Uranium losses were 0.012 percent to thorium product, 0.004 percent to solvent from 2C, and 0.0018 percent to solvent from third uranium cycle. Thorium loss was 0.025 percent to uranium product.

In these experiments, no mention was made of the disposition of the plutonium that will be present in fuel containing uranium irradiated to high burnup. This plutonium could either be

routed to high-level waste by adding ferrous sulfamate to the scrub solution for the second stage (as in Fig. 10.21) or could be made to accompany uranium into the third cycle. There, prior to extraction of uranium, plutonium could be reduced and made inextractable by addition of hydroxylamine.