Development of Analysis Method for Plutonium Amount and Burn up by Measurement of Xenon Isotopic Ratio in Dissolver Off-Gas at Reprocessing Facility

Okano, Masanori ; Kuno, Takehiko ; Charlton, W. S.*; Wells, C. A.*; Hemberger, P. H.*

According to achieve the International Atomic Energy Agency's safeguards criteria, development of simple and rapid determination method of Pu amount in the spent fuel has been required. It is known that stable xenon isotopes produced with fission reflect spent fuel characteristics (fuel type, burn up, Pu amount). Since dissolver off-gas which is released with dissolution of spent fuel in reprocessing facility, contains most of gaseous fission products, xenon isotopic ratio in dissolver off-gas sampled from the stack would be expected to provide a new measurement method for application to safeguards inspection. We have studied a new technique to evaluate Pu isotopic ratio and its amount in the fuel from Xe isotopic ratio, which is related to burn up. The technique is also applicable for evaluating burn up credit to achieve efficient critical safety control. Six batches of off gas from dissolution process, DOG: Dissolver off gas, were obtained with press pump from the stack in Tokai Reprocessing Plant at BWR fuel (approx. 30GWD/MTU) reprocessing campaign. Xe isotopic ratio was determined with Gas Chromatography- Quadrupole Mass Spectrometer, GC-QMS. Xe concentrations in the DOGs sampled at the stack were 1,000 to 10,000 ppm, which were enough concentrations for determining minor Xe isotopes. We evaluate burn up and Pu amount with NOVA, Noble Gas Environmental Monitoring Application code, developed by Los Alamos National Laboratory. Inferred burn up was realized 94% of confidences level, which is the conventional definition based on the standard deviation of the results. We also calculated Pu amount in the spent fuel by ORIGEN code based on declared burn up. Based on the two different evaluations for Pu amount, we found that there is 7% difference between NOVA and ORIGEN code. It is summarized that there is possibility of developed method being used newly burn up monitoring and safeguards technique.