PELLET-CLADDING MECHANICAL INTERACTION OF PWR FUEL-ROD UNDER RAPID POWER TRANSIENT

A refabricated commercial PWR fuel rod (active length: 0.12 m) base-ir radiated up to burn-up of 39 MWd/kg U and an unirradiated PWR fuel rod were prepared. After loading each test fuel rod into an atmospheric i rradiation capsule at the Nuclear Safety Research Reactor (NSRR) in th e Japan Atomic Energy Research Institute (JAERI), the rods were irradi ated with rapid increase of linear heat rating from 4 to 32 kW/m with ramp rate of 3 kW/ms. The results obtained are summarized as follows: (1) A magnitude of pellet-cladding mechanical interaction (PCMI) evalu ated by rod axial strain was 0.07% for base-irradiated PWR fuel and 0. 08% for unirradiated PWR one. The prevention of fuel rods from failure was partially due to relative low terminal power level (32 kW/m) agai nst defected fuel rod in TRANS RAMP II (60 kW/m) and partially due to the strong PCMI relaxation occurred in UO2 fuel in the course of rapid power increase. The PCMI decrease by relaxation which occurred during rapid power increase is quite new phenomenon, and may be useful for P WR fuel rods from view point of PCMI failure prevention. (2) Fission g as release (FGR) of the tested PWR fuel rod was 0.17% for base-irradia tion and 0.63% for rapid power transient. The principal mechanism for the former may be the recoil/knock out and that for latter may be the micro haircracking in UO2 fuel. Within this experimental scope, the ob served magnitude of FGR was insufficient to induce stress corrosion cr acking on the tested fuel rods.