E. D. COLLINS, G. D. DEL CUL and B. A. MOYER, Oak Ridge National Laboratory, USA

Abstract: The United States inventory of used nuclear fuels contains approximately 2 to 5 wt % fission products, depending on the extent of fuel burnup during irradiation, with the greater amounts produced in higher burnup fuels. For reprocessing of used nuclear fuels, fission products are more often divided into categories according to their chemical and radiological properties. Advanced reprocessing includes further separations processes to enable capture and disposal of the volatile fission product elements in improved solid waste forms, as well as additional separations processes being developed (1) to enable recovery and recycle of the remaining minor transuranium element actinides, neptunium, americium, and curium, and (2) to segregate the lanthanide fission products and the intermediate-lived heat-generating radionuclides, 137Cs/137mBa and 90Sr/90Y.

Key words: separation/extraction techniques, voloxidation, transition metal behavior, lanthanide recovery, cesium-strontium isolation.

7.2 Introduction

The United States inventory of used nuclear fuels contains approximately 2 to 5 wt % fission products, depending on the extent of fuel burnup during irradiation, with the greater amounts produced in the higher burnup fuels. The fission products are produced predominantly in two mass fractions, typically centered around a peak of mass 90-100 in the light fraction and 135-145 in the heavy fraction. For reprocessing of used nuclear fuels, the fission products are more often divided into categories according to their chemical and radiological properties, as illustrated in Table 8.1.1,2

The properties shown in Table 8.1 represent used fuels from light water reactors (predominantly thermal-neutron-driven fission of 235U). The fission product elements — tin, antimony, and tellurium — are included here but would become more important when produced in higher concen­tration from fast neutron-driven fissions. The radioisotopes of significance shown in Table 8.1 were confined to long-lived and intermediate-lived isotopes that are most significant in reprocessing used fuels that have been stored for five years or more after reactor discharge. The significant

Nd 3 4,2 143,144,145,146,148,150 Pm 3 Sm 3 2 149,152,154 Eu 3 2 151,153 Gd 3 2 155,156,157,158,160 Transition metals (35%) Zr 4 90,91,92,94,96 Nb 5 3 Mo 6 5,-2 95,97,98,100 Tc 7 5,4 Ru 4,3 8,6,2 101,102,104 Rh 3 5,4,1 103 Pd 2 4 105,106,108,110 Ag 1 2 109 Cd 2 111,112,114,116 In 3 Sn 4,2 117,118,119,120,122,124 Sb 3 5,-3 121,123 Те 4 6,-2 125,126,128,130

147 (2.62у)

154 (8.8y), 155 (4.71 у)

106 (with Ru-106)

125m (with Sb-125)

8.1 Used fuel component separations in current plants.

radionuclide-decay isotope pairs that are in secular equilibrium are shown in Table 8.1 because, in effect, they double the radioactivity emitted by the parent radioisotope.

In established reprocessing plants (Fig. 8.1), the volatile fission products, xenon and krypton, are vented to the environment, while the iodine is trapped from the off-gas and then released to the sea as liquid waste. Carbon-14 is scrubbed from the off-gas and released to the sea as low-level liquid waste or converted to insoluble barium carbonate and encapsulated in a solid waste form, such as cement. After dissolution of the fuel compo­nents, the cladding hulls and hardware are removed by screening and put into a solid waste form (compacted metal or grout) for subsequent geologic disposal. The dissolved fuel solution still contains finely divided undissolved solids (UDS), composed of variable portions of the transition metal ele­ments. The UDS are removed by centrifugation and disposed in vitrified high-level waste, along with the soluble fission products that are separated from the uranium and plutonium products by means of solvent extraction processes.

Advanced reprocessing will include further separations processes to enable capture and disposal of the volatile fission product elements in improved solid waste forms. Also, additional separations processes are being developed (1) to enable recovery and recycle of the remaining minor transuranium element actinides, neptunium, americium, and curium, and (2) to segregate the lanthanide fission products and the intermediate-lived heat-generating radionuclides, primarily 137Cs/137mBa and 90Sr/90Y. Details of the advanced fission-product recovery processes are described below. The actinide recovery processes are described in other chapters.