Improved burnup monitor in the Rokkasho reprocessing plant - 7-19

Burnup credit is taken in criticality safety control at the Spent Fuel Storage Facility (SFSF) and at the Fuel Dissolution Facility in the Rokkasho Reprocessing Plant. Spent fuel burnup value is measured using a burnup monitor at the SFSF. Storage rack types are selected with U²³⁵ residual enrichment derived from initial enrichment and measured average burnup value. At the Fuel Dissolution Facility, the necessity of neutron absorbers for nitrate solutions are judged based on initial enrichment and top 50 cm average burnup values. The burnup monitor was calibrated against operator declared burnup values through measurements of 100 spent fuel assemblies. The monitor has measured about 10 thousands spent fuel assemblies during these 10 years and operation is steady. The burnup monitor consists of two Ge detectors for gamma spectrometry, four fission counters for passive neutron counting and twenty ion chambers for relative burnup profile measurement on each opposite side of an assembly. The burnup monitor has under water driving units for fuel elevation, centering fuel position and setting fission counters on the surface of the fuel assembly. Presently, an improved type of burnup monitor is being developed for the purpose of reducing waste and exposure during maintenance work. The improved equipment has a few ion chambers at the top region only. They measure the axial profile of gross gamma-ray intensity while inserting an assembly into the equipment. The fuel elevating driver will be reduced. Fission counter driving units serve as fuel centering guides. The fuel scanning system determines effective fuel regions using depressions that appear at the position of spacers or grids in a measured profile. Thus, average burnup measurement is performed independently from the information relating to the position of the fuel handling machine. Moreover, some additional self diagnostic functions will be applied for gross gamma-ray scanning measurement. This includes for example, the checking of signal noise and fuel swinging during measurements. (authors)