Office for Nuclear Regulation

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THORP, Sellafield Site, Cumbria, To conduct a System Based Inspection with Sellafield Limited

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) systems for the prevention and warning of criticality events.

Interventions Carried Out by ONR

ONR carried out a two day inspection between 9 and 10 February 2016 of the THORP Criticality Incident Detection (CID) System and plant safety mechanisms whose function is to prevent a potential criticality event.  Specifically, we inspected the CID (which detects potential criticality events and activates alarms) and alpha monitoring (AM) and absolute neutron monitoring (ANM) equipment (which continuously monitor for and prevent unsafe concentrations of fissile material occurring in non-geometrically safe areas of the plant). The inspection utilised specialist inspectors 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, we examined evidence regarding the implementation of SL’s arrangements in respect of the six licence conditions listed below. These have been selected in view of their importance to nuclear safety and are defined within ONR’s formal process for system based inspection. 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 with the following Licence Conditions (LCs) using the applicable ONR inspection guidance:

No detailed inspection was however carried out against LC34 (leakage and escape of radioactive material and radioactive waste) since the system being inspected does not directly control or contain nuclear matter.

Explanation of Judgement if Safety System Not Judged to be Adequate

N/A - The system has been judged to be adequate.

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

It is my opinion, based on the sampling undertaken, that SL has demonstrated that the CID system and Alpha/Neutron detection and preventative measures have been adequately implemented in accordance with safety case requirements.

It is my opinion that, whilst some potential minor improvements were identified to SL, with regard to LCs 10, 24, 27 and 28, the licensee was able to provide appropriate evidence of compliance with these LCs for this system. Consequently, I awarded an IIS rating of 3 (Adequate) for these licence conditions.

With regard to LC23 (operating rules) it is my opinion that an IIS rating of 4 (Below Standard) is warranted. The is because the safety case for the CID system appears to fall short of ONR guidance in that it:

As such I have raised a formal regulatory issue for SL to produce an adequacy statement justifying and reviewing the current status of the CID system.

Conclusion of Intervention

Based on my sampling inspection, I judge that SL has demonstrated that THORP’s CID system and AN and ANM equipment have been adequately implemented in accordance with the requirements set out in SL’s safety case. The need for an adequate safety case supporting the CID system will be regulated through the above-mentioned regulatory issue.  Other, more minor improvements will be regulated as part of normal regulatory business.