Development and characteristics of the rim region in high burnup UO2 fuel pellets

Fission Gas Release Behavior from High Burnup UO2Fuels under Rapid Heating Conditions

“…Figure 14 summarizes the width of the developed rim structure as assessed from porous region described in literatures. 1,3,9,15,[40][41][42] Naturally, at the higher pellet average burnups above 70 GWd/tU, which approximately corresponds to the threshold burnup for the developed rim structure formation, rim width increases significantly. It reaches about 100-250 mm at 50-60 GWd/tU, 500 mm at 80 GWd/tU and 1,000 mm at 100 GWd/tU.…”

“…Rim structure, which becomes remarkable in the peripheral region of the pellet at pellet average burnups higher than [35][36][37][38][39][40] GWd/tU, is characterized by a marked decrease in UO 2 grain size at sub-micron size and an increase in porosity resulting from newly formed coarsened bubbles. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] From detailed analyses of local burnup and Xe distributions across UO 2 fuel pellets irradiated in LWRs and test reactors, 1,2,5,7,[11][12][13][14]17) and from recent systematic HBRP data from isothermally irradiated UO 2 fuel discs, 19,20) the threshold local burnup has been confirmed as approximately 70 GWd/tU for developed rim structure formation and the threshold temperature as less than 1,000-1,100 C. 2,20) Furthermore, from many studies of the rim structure formation mechanism, two processes have been proposed, those of polygonization 10,19,20) and recrystallization, 4,6...…”

Mechanistic model for the fragmentation of the high-burnup structure during LOCA

“…Above a certain burnup the matrix xenon content stabilises at about 0.25%, which is the solubility limit of xenon in UO 2 below 550 C [16]. However, secondary ion mass spectrometry (SIMS) [17], X-ray fluorescence (XRF) [18] and Knudsen cell measurements [19] showed that the xenon missing from the matrix is present in the HBS pores. Fission gas release from the HBS is thus assumed to be minimal if any, xenon and krypton depleted from the fuel matrix are considered to be stored in HBS porosity.…”

Lattice Parameter Changes for Spent UO2 Fuels With and Without Gd

“…The microstructural changes in the outer region, called the rim, of a fuel pellet has been a great concern with the development of high burnup fuel [1][2][3][4] . The structure of UO 2 can change during the operation of the reactor as well as during cooling after discharge.…”