Pellet-Cladding Mechanical Interaction of PWR Fuel Rod Under Rapid Power Transient.

Abstract

A refabricated commercial PWR fuel rod (active length: 0.12m) base-irradiated 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 irradiation capsule at the Nuclear Safety Research Reactor (NSRR) in the Japan Atomic Energy Research Institute (JAER1), the rods were irradiated 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 (PCM1) evaluated 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) against 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 PWR fuel rods from view point of PCMI failure prevention.2. Fission gas release (FGR) of the tested PWR fuel rod was 0.17% for base-irradiation 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 observed magnitude of FGR was insufficient to induce stress corrosion cracking on the tested fuel rods.