Cracking in the gas circuit expansion baffles of a Magnox nuclear power plant was identified during an inspection.  This portion of the gas circuit is external to the environmental shielding.

The objective of the work was to establish the root cause of cracking and to perform a fitness for service assessment.  A review of operation and inspection data was followed by the application of Finite Element Analysis (FEA) and Virtual Crack Extension (VCE) procedures to establish rate and direction of crack growth.

It was concluded that thermal transients were the prime stress drivers, leading to fatigue.  Crack growth was demonstrated to be relatively slow and the cracks were shown to grow into a thick portion of the baffles where the thermal driving force diminishes.

Continued safe operation demonstrated assuming normal conditions prevailed and costly repairs in an irradiated environment were, therefore, avoided.