Office for Nuclear Regulation

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Extension to the Safety Case for the Reactivity Effects of Boiler Tube Failure

Executive summary

Purpose of intervention

During normal operation of an advanced gas cooled reactor oxidation results in the gradual loss of reactor core graphite. This has potential implications for the shutdown and hold-down of the reactor in the event of a boiler tube leak due to the effects on reactivity. The safety case addressing this issue for Dungeness B (DNB) currently limits operation to 6.2% graphite weight loss (GWL). EDF Energy Nuclear Generation Limited (NGL) has been developing a safety case to extend operation to 8% GWL which is intended to be sufficient to cover operation through to the statutory outages in 2017 and 2018.

This intervention enabled NGL to present, in a meeting, its extension to the safety case prior to assessment by ONR.

Interventions Carried Out by ONR

The intervention was intended to provide ONR with:

The intervention relates to Licence Condition 23 which concerns operating rules and includes a requirement for the licensee to produce an adequate safety case to demonstrate safety and identify any necessary conditions and limits. ONR will be assessing the adequacy of the safety case extension in due course.

Explanation of Judgement if Safety System Not Judged to be Adequate

No safety system inspection was undertaken, hence this is not applicable.

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

Production of the safety case is on target and it will be presented to the April 2014 Nuclear Safety Committee (NSC) as a Category 1 safety case. The safety case raises the extant safety case limit of 6.2% GWL to 8% GWL. This is intended to cover the period through to the 2017 statutory outage for Reactor 21 and the 2018 statutory outage for Reactor 22.

With respect to operation beyond 2017/18 NGL reported that a strategy paper is being developed that will be presented to the NSC in September 2014. Early engagement with ONR is planned.

The increase in GWL limit from 6.2% in the extant safety case arises from a combination of the use of new reactor physics methods and revised assumptions about the different fault scenarios. I will consider the validity of the revised methods and assumptions in my planned assessment of the extension to the safety case.

During the meeting one ONR Issue was raised on NGL to consider whether the variability of GWL in the core has any significant implications for the extension to the safety case.

Conclusion of Intervention

The meeting provided a useful overview of the extension to the safety case for the reactivity effects of boiler tube failure and met the objectives of the intervention.

I recommend that ONR assesses NGL's extension to the safety case for the reactivity effects of boiler tube failure faults which proposes to increase the GWL limit for Dungeness B to 8% in a timescale commensurate with the start-up of Reactor 21 following its ongoing statutory outage.