Office for Nuclear Regulation

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Wylfa Level 4 - NPSC 4807 Annual Review of the Safety Case for the Integrity of the Reactor 1 Graphite Core 2013 Addendum 10

Executive summary

Purpose of intervention

A level 4 meeting was held on 13th May 2014 to allow Magnox Ltd to present addendum 10 of their graphite core integrity safety case for Wylfa Reactor 1 following their annual review.

Interventions Carried Out by ONR

The main points of the meeting can be summarised as discussions on the:

Explanation of Judgement if Safety System Not Judged to be Adequate

N/A

Key Findings, Inspector's Opinions and Reasons for Judgements Made

I judge that the PSR of Wylfa R1 graphite core is served primarily by the annual safety case review up to end of power generation. Beyond this the threat to the safety functions of the core are largely diminished.

The licensee presented the claim that reduced power operation reduces the probability of graphite brick cracking by reducing the thermally generated stresses during shut-down transients. I judge this claim appears sensible and intuitively makes sense.

Based on the slides presented the weight loss of the trepanned samples as a function of cumulative irradiation was consistent with the overall trend in weight loss for the whole core.

Conclusion of Intervention

Overall the information presented at the meeting combined with the intervention performed during the 2014 R1 outage in March lead me to conclude that the current addendum of the safety case adequately supports the safe operation of the Wylfa R1 graphite core.