Benchmark calculations of JACS code system for the criticality experiment of pellet-solution coexisting low-enriched uranium fuel

Criticality experiments of the systems simulating a fuel dissolver were carried out at PNL in USA. The systems were described as triangular arrays of 4.3 wt% 235U-enriched uranium dioxide rods partially immersed in uranyl nitrate solution of the same 235U enrichment containing soluble gadolinium. In our summary report of benchmark calculations with the use of JACS code system, the neutron multiplication factors were calculated to be below 0.95 for 8 cases out of all the 12 cases of the experiment systems. In this report, recalculations based on the experiment report as well as analyses according to the international benchmark exercises set by OECD/NEA are carried out, which show the following 3 points. (1) The present calculations yielded 1.3%Δk higher values on the average for the neutron multiplication factor than the previously reported values, which were considered to have been obtained by an erroneous treatment for Dancoff correction method; (2) There was an inconsistency in the reported composition of uranyl nitrate solution, which is well remedied by treating as without free acid; (3) The average of the neutron multiplication factor calculated with the JACS code system became 0.980 for the above treated 12 cases of the experiments. (author)