Office for Nuclear Regulation

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Planned system based inspection of the shield door systems within THORP

Executive summary

Purpose of intervention

In accordance with the Office for Nuclear Regulation's (ONR's) Sellafield Strategy, each year ONR performs a series of planned system based inspections (SBIs) targeted at key safety significant systems on those facilities with the potential to give rise to offsite effects to the public. The purpose of this inspection was for ONR to determine the adequacy of implementation of Sellafield Ltd.'s (SL's) safety case claims in respect of the Thermal Oxide Reprocessing Plant's (THORP's) shield doors safety systems.

Interventions Carried Out by ONR

Between 26 - 27 October 2016, ONR carried out a planned 2 day inspection of the THORP shield doors safety systems utilising specialists from the following technical disciplines:

In order to determine the adequacy of implementation of the licensee's safety case claims in respect of this system, ONR examined evidence regarding the implementation of SL's arrangements for five pre-defined licence conditions (LCs), as listed below. These LCs have been selected in view of their importance to nuclear safety and are defined within ONR's formal process for system based inspection. Whilst LC 34 (leakage and escape of radioactive material and radioactive waste) is normally included for assessment as part of system based inspections, I judged that it was not applicable to this inspection due to the nature of the system under inspection and hence has not been considered further in this report.

The inspection involved reviewing the applicable safety case claims and sampling, through a combination of document reviews, discussions with operators and plant inspections, suitable evidence to determine compliance against these LCs on the plant.

ONR assessed compliance in THORP against the following LCs using the applicable ONR inspection guidance:

In addition I carried out a number of short information-sharing sessions with senior members of THORP's staff.

Explanation of Judgement if Safety System Not Judged to be Adequate

N/A - this system is judged to be adequate. However, there are specific improvements that SL should make as described in the next section.

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

Whilst some minor opportunities for improvements to SL's compliance against LCs 10, 24, 27 and 28 were identified in this inspection, it is my view that these licence conditions are all adequately implemented with regards to the shield door systems. Consequently, it is my opinion that a rating of GREEN (no formal action) is appropriate for these licence conditions.

However, with regard to LC23 (operating rules), based on the sampling undertaken, I have awarded a rating of AMBER (seek improvement). This is due to a number of issues including:

The introduction of revised interlock key lock-out arrangements without full substantiation in the supporting safety case;

A lack of adequate auditable close-out of identified safety shortfalls and of learning from experience from past shield door issues, both at THORP and elsewhere on the Sellafield site

THORP's use of so-called supplementary clearance certificates, as these do not at present provide suitable visibility to SL staff in regard to the plant configuration.

Despite these shortfalls, I do not recommend that the safety case supporting this system be subjected to an ONR safety case assessment earlier than would otherwise be planned. This is in part because the safety case is about to be re-written as part of SL's Event Driven Review process (in preparation for THORP's operations following the cessation of fuel reprocessing) and ONR intends to perform detailed assessments of the safety case at that time.

Conclusion of Intervention

I have identified three specific shortfalls in regard to the THORP shield doors safety systems relating to operating rule compliance. Overall however, I am content that the shield doors systems, once wider improvements enacted by SL are taken into account, adequately implements the requirements of the supporting safety case.

I have raised four Regulatory Issues as a result of this inspection which I will use to track SL's timely return to full compliance. Three of these relate to the Amber rating I have awarded against LC23, while the fourth relates to a minor shortfall related to training which I intend to progress under normal regulatory business.