Preparation of the spent fuel specimens: Spent UO 2 fuel specimens were prepared from a fuel rod discharged from the NPP(Y-2). An axial slice of the fuel pellet with a height of 3 mm and a thickness of 0.5 mm was cut along the diameter of the fuel (Fig. 1). The average burnup of the specimen was 41 GWd/tU, as measured by the Nd-148 method (ASTM, E321-96) 13,14. A burnable poison fuel, 5.98 wt % Gd 2 O 3-UO 2 , was also prepared from a fuel rod discharged from the NPP(K-3). The average burnup of the Gd-bearing fuel specimen was 41 GWd/tU. The slices of the spent fuels were embedded in