Assessment of plutonium accumulation in mixer settlers and its effect on criticality safety for co-decontamination process in fuel reprocessing

Build up of Pu in a co-decontamination process of fuel reprocessing due to shift of process parameters from normal conditions and its effect on nuclear criticality of the system were investigated with computer codes. Being based on the flow sheet of a Purex reprocessing plant, the calculation shows that Pu accumulates mainly in the aqueous phase of extraction stages under the conditions in which concentration of U increases in the organic phase, i.e. a decrease of the extractant flow rate or the TBP concentration, an increase of feed flow rate or U concentration, or a decrease of scrub acidity. Relating the maximum Pu concentration and the time required for maximum build up to the change of the flow sheet conditions, an effective neutron multiplication factor k_eff of the system involving Pu build up was calculated for the first mixer-settler bank of a model plant. The analysis revealed that the extractor would become critical in approximately 45 h at earliest provided that shift of the parameters stayed within a specified range during operation. Required performance of the process control instruments, safety margin in concentration limit, and other operation safety considerations necessary for the prevention of Pu build up in the extraction bank are discussed. (author)