Office for Nuclear Regulation

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THORP inspection at Sellafield

Executive summary

Purpose of Intervention

In accordance with the Office for Nuclear Regulation's (ONR) Sellafield Strategy, each year ONR performs a series of planned System Based Inspections (SBIs) targeted at key safety significant systems. The purpose of this inspection (THORP System Number 16) was for ONR to determine the adequacy of implementation of Sellafield Ltd.'s (SL's) safety case claims associated with the centrifuge fines management system and zircaloy fire hazards. The THORP inspection plan identified the principal "sub-systems" as being; centrifuge fines cooling, fines management and the zirconium explosion risk.

Interventions Carried Out by ONR

Between 10 and 11 October 2017, ONR carried out a planned System Based Inspection of the THORP centrifuge fines management system utilising specialists from the following technical disciplines:

In order to determine the adequacy of implementation of the licensee's safety case claims for the selected system, ONR examined relevant evidence relating to six pre-defined licence conditions (LCs). These LCs were previously selected by ONR as being germane to System Based Inspections because of their importance to nuclear safety and the nuclear safety case, as defined within ONR's formal process for System Based Inspection.

The System Based Inspection reviewed applicable safety case claims and identified and examined relevant evidence, to determine whether the licensee complies with the licence conditions, whilst adequately delivering the requisite safety functional requirements.

The inspection was undertaken on a sampled basis, via a combination of document reviews, plant inspection and discussion with plant operators and maintenance staff. The pertinent points of the System Based Inspection are recorded within this intervention record (IR).

We undertook the System Based Inspection to determine whether THORP adequately complies with the following LCs, whilst being cognisant of applicable ONR inspection guidance as follows:

Explanation of Judgement if Safety System Not Judged to be Adequate

This system is judged to be adequate.

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

From the evidence sampled during the inspection, we consider that SL has adequately implemented those claims in the safety case that relate to the centrifuge fines management system. Consequently, it is our opinion that for this System Based Inspection a rating of Green (no formal action) is appropriate for license conditions 10, 23, 24, 27, 28 and 34.

There were no significant shortfalls identified during this inspection. We did however raise a minor shortfall against LC27 and LC28, regarding an item of Safety Related Equipment (SRE), which was identified within the implemented safety case, but was not included within the engineering schedule or proof test schedule. We recognised that this is a non-basket safety measure that is safety related (not a Safety Mechanism). We have raised a level 4 regulatory issue to ensure that this matter is addressed in a proportionate manner to an appropriate timescale (RI 5801).

We also raised three minor observations that we suggested SL may wish to consider. Given the minor significance of these observations we do not consider it proportionate for any formal follow up action to be raised, and we will not be following these up formally with SL.

Conclusion of Intervention

From the evidence sampled during the inspection, we judge that SL has adequately implemented the relevant claims in the safety case and that the formal arrangements for each of the LCs inspected are being adequately implemented.

We conclude that the safety system is adequate and it fulfils the requirements of the safety case.