Oxygen potential of high burnup UO2

The oxidation state during irradiation and after discharge is a key property affecting nuclear fuel behaviour during normal operations and under accident conditions. During irradiation the oxygen potential DeltaG(O2) and the oxygen to metal ratio O/M of oxide fuel are not constant, but change with burnup due to the generation of fission products with different chemical properties replacing fissioned uranium and plutonium atoms and under the effect of the irradiation temperature. Moreover, non-uniform temperature and fission density conditions determine heterogeneous DeltaG(O2) and O/M in the fuel. At the fuel-Zircaloy cladding interface, the presence of zirconium metal can buffer excess oxygen. Few direct measurements of the DeltaG(O2) of irradiated fuel are reported. The oxygen potential of high burnup commercial LWR UO2 has been the object of several investigation campaigns performed at JRC-ITU. One problem affecting measurements on irradiated LWR fuel is the uncertainty in the burnup and irradiation temperature attribution for samples retrieved from different radial position of a fuel pellet. This paper describes the results of measurements performed on irradiated UO2 samples with well-defined burnup and irradiation temperature. The samples were from the High Burnup Rim Project (HBRP), an international project involving utilities and vendors from Japan, Europe and USA dedicated to the investigation of formation and properties of the high burnup structure (HBS) in LWR fuel. The HBRP UO2 disks were prepared in JRC-ITU and irradiated in the Halden reactor. Most disks were irradiated in special rigs without the pressure constraints associated with LWR cladded fuel rods. Extensive post-irradiation examination was performed at JRC-ITU in the frame of the project. The aim of the complementary oxygen potential measurements was to characterize the evolution of the oxidation state of the fuel as a function of burnup and irradiation temperature and, in particular, to assess possible effects associated with the formation of the HBS.

RONDINELLA Vincenzo; 

2015-07-17

American Nuclear Society (ANS)

JRC94525

0003-018X,   

http://epubs.ans.org/?p=trans:112,    https://publications.jrc.ec.europa.eu/repository/handle/JRC94525,