Nuclear physics calculation with as-built data of IFA-651 for the IMF-MOX fuel irradiation test in HBWR

As a part of activity for future fuel development project in KAFRI, the irradiation program of the test IMF-MOX was launched as a KAERI's joint international program with Paul Scherrer Institute (PSI). The test MOX fuel rods were loaded into HBWR(Halden Boiling Water Reactor) in May and are going to be irradiated for five years since late June this year. PSI fabricated the test MOX rods with attrition mill device developed by KAERI. The test fuel assembly rig contains total six test fuel rods: three MOX rods and three inert matrix fuel rods. For comparison purposes, one of three MOX rods was fabricated by BNFL and the other two MOX rods were manufactured jointly by KAERI and PSI. Three inert matrix fuel rods were fabricated with (Zr-Y-Er-Pu) oxide by PSI. One of the three IMF rods was fabricated with wet process and the other two rods were fabricated with dry process. As-built data for test fuel rods are a little different from the design data described in the report KAERI/TR-1210/99. The irradiation position and the surrounding of the test fuel rig in HBWR were determined. So, the neutronic characteristics of the test fuel rig was recalculated with as-built data and real surrounding condition. One of the major changes is the plutonium content of IMF-Co from 0.935 gm/cc to 0.830 gm/cc. So the relative linear power of IMF-Co is reduced from 1.04 to 1.00 which is about average of the test rods in IFA-651. Due to the change of environment, the relative linear power of IMF-ET is increased from 1.01 to 1.06, which that of MOX-Att-ET is decreased from 0.97 to 0.93 however, the linear power densities for all test rods will not exceed the limiting value of 250w/cm for the first irradiation cycle. The actual neutron spectrum of IFA-651 is very similar to the previous one. So the CD-functions for the test rods are not significantly changed form the old ones. The role of Dy203 at both is to reduce the thermal flux peak at the end of the pellets. Since the axial power peak occurs at the fuel stack mid-point for both cases, using Dy203 pellets does not reduce the axial power peaking factor. However, the Dy203 end pellets reduce the second power eak at the ends of fuel, it make the axial power shape flat