Fig. 10-1 Profiles of I-129 amount in adsorbent column It was confirmed that the adsorbent was also effective for iodine removal in the actual spent fuel dissolution process.

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Fig. 10-2 Np and Tc separation performances at respective steps in PARC process Np and Tc are extracted by TBP, in addition to U and Pu, at co-decontamination step. They are further efficiently separated from U and Pu, by using n-butylaldehyde, as a reductant, and nitric acid stripping solution with high concentration.

JAERI has been developing an advanced reprocessing flow sheet called "PARC process" at NUCEF, which would realize both the process simplification and complete confinement of long-lived nuclides simultaneously. To confirm the performance of its separation efficiency, approximately 1.5 kg each of spent fuels of 8,000 MWD/tU burn-up (200 day's irradiation in a power reactor) and 31,300 MWD/tU burn-up (900 day's irradiation) were so far subjected to experimental runs, separately. Iodine (I-129) was released into the dissolver off-gas and then collected in the absorbent columns loaded with silver impregnated silica-gel (AgS). Figure 10-1 shows the concentration profile of iodine captured on the adsorbent, indicating the iodine being concentrated near the entrance of the columns. Radioactive carbon (C-14 in the form of CO2 in the off-gas), which is formed from nitrogen (N-14) (an impurity in fuels) through its irradiation in the reactor, was trapped in alkaline solution for measurement. The results indicated that the concentration of nitrogen was several ppm in the fuels. The experiments also showed that the valencies of plutonium (Pu) and neptunium (Np) in the spent fuel solution were hexavalent and that hexavalent plutonium was effectively extracted into TBP organic solvent along with uranium (U). At the newly installed Np and Tc separation steps, more than 95% of Np and Tc in the organic solvent were selectively back-extracted into nitric acid solution and thus separated from U and Pu (Fig. 10-2). Butyl amine compounds (oxalate and hydrogen carbonate) which have been selected as a salt-free detergent (not producing the secondary radioactive waste), were tested to confirm their performance to remove radionuclides remaining in the organic solvent deteriorated by radiation damage and other causes. More than 99.9% of alpha- and beta-emitters were successfully removed from the deteriorated organic solvent. On the basis of the results so far obtained, further experiments are planned on the treatment of the high burn-up spent fuels (up to 45,000 MWD/tU and 1,400 day's irradiation) and the "thermal MOX" fuels in order to confirm the performance of the PARC process and thus to aid the enhancement of economy and reliability of a reprocessing plant, in its turn, of a nuclear fuel cycle as a whole. For the long-lived radionuclides thus confined in the process, a concept has been proposed to convert them into stable or short-lived nuclides by transmutation, whereby their impact to the environment would be reduced.

Reference G. Uchiyama et al., Development of an Advanced PUREX Process (PARC process), Proc. ENC'98, 623 (1998).

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