In-reactor performance of prototype SBR MOX fuel

As part of the international Callisto experiment, BNFL have undertaken the base irradiation, ramp testing and post-irradiation examination of two fuel rods, manufactured by BNFL's Short Binderless Route (SBR) for MOX fuel. Although of only short (lm) length, the rods were in other respects of standard PWR geometry. Base irradiation was performed in the Callisto loop of the BR2 reactor and achieved burn-ups of 17 GWd/tM, 28 GWd/tM peak pellet, at moderate to high power levels. No problems were encountered during base irradiation, confirmed by intermediate examination. In a separate BR2 capsule, a ramp test was then performed on one of the two rods. Again, the rod showed no problems and was discharged intact. The ramp test conditions employed were somewhat more onerous than the level corresponding to the best-estimate failure level for standard UO2 fuel. The survival of the rod confirms the general observation that the PCI failure resistance of MOX fuel is superior to that of standard UO2. The irradiation has been followed by a detailed programme of non-destructive and destructive hot-cell examinations on both of the rods. Profilometry, ECT and gamma-scanning confirmed the overall satisfactory performance of the rods and the absence of incipient damage. Puncture testing and gas analysis showed the fission gas release levels in both the ramped and unramped rods to be in line with what would be expected for a UO2 rod. The satisfactory microstructure of the fuel was confirmed via optical ceramography and autoradiography. This paper will describe the fuel manufacture, base irradiation and ramp test conditions, and will provide a summary of the results of the PIE programme. (author)