Cracking and relocation behavior of nuclear fuel pellets during rise to power

Abstract

Out-of-pile experiments and analyses of pellet thermal deformation and in-pile analyses of fuel rod gaps were carried out to elucidate pellet relocation behavior during fuel rod operation. Behavior of cracked pellets, subjected to cladding restriction, was evaluated by analogy to compaction of granular materials. From this work a gap model was proposed to express gap closure during rise to power. The concept of a critical void ratio was introduced for determination of the hard PCI (pellet-cladding interaction) onset condition. The following results were obtained within the range of the design parameters and operating conditions of typical BWR fuel rods. 1. (1) When the rod power, P, reached P = 60 W/cm, pellets simultaneously cracked and increased their diameters (pellet jump). The amount of pellet jump, ΔD j, could be given by ΔD j ( μm) = 3.6 D, where D is pellet diameter (mm). 2. (2) The gap G( B)( μm) could be given from the following relocation, assuming that K is the gap change (μm) caused by phenomena which do not directly participate in pellet cracking (e.q. swelling, creep); G(B)=C ′ 1(2−C 3) e −C 2A−C ′ 1 (1−C 3) e −(C 2A+C 4B) where A = ( P−60), B is the burnup (GWD/T), C 1 = G 0 − ( ΔD j + K), G 0 is the initial gap (μm), C 2 = 0.0039, C 3 = 1.41 and C 4 = 0.95. 3. (3) The hard PCI occurred when the void ratio of the sectional area inside the cladding decreased to about 0.25%.