POST IRRADIATION EXAMINATIONS OF FMDP ATR MOX FUEL

The post irradiation examination (PIE) of the Advanced Test Reactor (ATR) MOX capsules containing MOX fuel fabricated from weaponsderived plutonium irradiated to 30 GWd/MT has shown excellent fuel performance. Two test capsule types were examined; one contained fuel that underwent a special thermal gallium removal process (TIGR) and the other contained fuel that did not undergo any special processing. Initial gamma scanning revealed normal capsule and fuel pin behavior. Fission gas measurements of the capsules and fuel pins demonstrated the integrity of the fuel pins and showed the fission gas release to be between 1.5 and 2.3%, within the expected range. Metallographic mounts were prepared from the fuel pins and fuel behavior was found to be in accordance with expectations, with no indications of significant fuel restructuring or abnormal swelling. As was expected from earlier PIE work, large plutonium agglomerates were present. Some were as large as 600 microns, several times that expected in contemporary commercial MOX fuel. No irradiation difficulties were noted with these agglomerates. The grain structure of the pellet was observed to change over the radius of the pellet, with larger grains in the pellet center. This effect was more pronounced in the TIGR treated material that also appeared to have a greater fission gas release. A SEM/Microprobe examination of the fuel and clad showed no adverse interactions and confirmed that the pellet grain structure of the TIGR treated material is behaving somewhat differently. Elemental mapping confirmed that the agglomerates were rich in plutonium and that fission products were localized in the agglomerates. Radiochemical analysis of the fuel confirmed the expected burnup values and showed no indications of significant migration of gallium to the clad.