A power excursion, which resulted in a reactor trip on overpower, occurred at the McMaster Nuclear Reactor (MNR) during a refuelling operation. MNR is a pool type research reactor currently operating at 2 MW. Following replacement of a shim safety rod control assembly, the fuel assemblies were being returned to the core. After loading the 5th fuel assembly, the shim safety rods were withdrawn to check that the reactor was not critical. The rods were then driven to the 85% withdrawn position instead of required 40% (safeguard position). On insertion of the 6th fuel assembly a blue glow was seen and the reactor tripped on overpower. The Log N trip had been bypassed to avoid spurious trips while moving irradiated fuel into position for loading into the core and the bypass had not been turned off. The power transient maximum was estimated to be about 300% full power. MNR is analyzing this incident to establish energy output, fuel temperature etc. and will be monitoring fuel behaviour. Procedures related to refuelling are being reviewed and revised. Rating is INES level 2 from Table II - Degradation of defense in depth, with an high/expected initiator (reactor trip due to reactivity excursion) and just adequate safety function availability (Level 2 rather than 3 was chosen on account of the full containment provision and the smaller hazard potential to the public of the pool type reasearch reactor compared to a power reactor).
Location: MCMASTER RESEARCH REACTOR Event date: Tue, 04-01-1994