Irradiation behaviour of Th-Pu and U-Pu MOX fuels

Abstract

Thoria based mixed oxide (MOX) is the candidate fuel for the Advanced Heavy Water Reactor (AHWR) being developed in India for thorium utilization. An experimental fuel pin cluster comprising of twelve fuel pins of PHWR design, filled with fuel pellets of different chemical compositions (UO₂, ThO₂, (Th-6.75%Pu)O₂ and (U-3%Pu)O₂ ) was test irradiated in the Pressurized Water Loop (PWL) of CIRUS. The pellets were encapsulated collapsible Zircaloy-2 cladding. The achieved peak burnup of the fuel was 10,400 MWd/Te(HM). After irradiation, the fuel pins of the cluster were examined using various non-destructive and destructive techniques to compare the performances of the fuel pins. The techniques used for post irradiation examination (PIE) included visual examination, dimension measurement, ultrasonic and eddy current testing, gamma scanning, fuel pin puncturing and fission gas analysis and microstructural examination using optical and scanning electron microscopy. Metallography was carried out to observe the extent of restructuring in fuel and examination of the cladding was aimed to study the cladding oxidation, hydride distribution and fuel clad interaction. PIE did not reveal any abnormality or defect in the cladding of fuel pins. Maximum change in fuel pin diameter was observed in U-Pu MOX fuel pins and the minimum change in Th-Pu MOX pins. The difference in the compositions of the fuel pins and their position in the core resulted in the observed variation of ¹³⁷Cs activity. The fission gas release in the fuel pins was low. Metallography did not reveal any fuel restructuring. The oxide layer on the outer surface of the cladding was about 2 µm in thickness in all the fuel pins. The oxide layer thicknesses observed on the inner surfaces of the claddings in case of Th-Pu and U-Pu MOX fuel pins were 1.5 µm and 3.3 µm, respectively. Photomacrographs of the fuel sections did not indicate migration of fission products in the fuel implying low temperature operation. This paper describes the results of PIE of fuel pins containing (Th, Pu)O₂, (U, Pu)O₂ and UO₂ fuel pellets in the cluster and compares the irradiation behaviour of MOX fuels with that of UO₂.