Office for Nuclear Regulation

MSSS Retrievals - Arrangements for Detection of Leakage and Escape of Radioactive Materials

Executive summary

Purpose of Intervention

The Magnox Swarf Storage Silo (MSSS) is a legacy facility on the Sellafield site that was built in the 1960’s to store Intermediate Level Waste under water and is currently assessed as one of the highest risk nuclear facilities in the Nuclear Decommissioning Authority estates. In order to safely accelerate hazard and risk reduction, Sellafield Ltd (SL) has commenced a programme of work that aims to retrieve the waste from the MSSS and transport it for storage in a more modern facility at the Sellafield site.

SL has constructed Silo Emptying Plant № 2 (SEP 2) in the MSSS that will be used to extract the waste. This plant is currently undergoing inactive commissioning and will be brought into service on compartment 10 for active commissioning by retrieving waste from this compartment in due course. Given the safety significance of the activities, complexity and novel nature, ONR has introduced a hold point prior SEP2 being moved onto compartment 10.

Movement of SEP 2 and subsequent waste retrieval operations have the potential to cause leakage and escape of contaminated silo liquors into the ground adjacent to the MSSS. SL has developed arrangements to manage the risk of loss of containment resulting from SEP movement and waste retrieval operations.

The purpose of the intervention was to confirm SL’s implementation of these arrangements. The findings of this intervention will be used to inform ONR’s regulatory decision whether to permission the release of this hold point.

Interventions Carried Out by ONR

Licence Condition (LC) 34 is concerned with leakage and escape of radioactive material and radioactive waste. Furthermore LC34 (2) requires that the licensee shall ensure, so far as is reasonably practicable, that no such leak or escape of radioactive material or radioactive waste can occur without being detected, and that any such leak or escape is then notified, recorded, investigated and reported.

This inspection was carried out to confirm SL’s implementation of its LC34 (2) compliance arrangements that ensure so far as is reasonably practicable that any suspected leakage from the MSSS is detected and then investigated and reported.

 This inspection focused on the arrangements within the MSSS to acquire and analyse data relating to leakage of radioactive materials. It also considered the arrangements to manage, confirm and report any leakage. This intervention did not inspect any equipment that MSSS has deployed to detect leakage as this has been considered in recent permissioning inspections.

The inspection was based on ONRs inspection guidance for LC34, NS-INSP-GD-034 Revision 5 and comprised discussion with licensee staff, and examination of documentation and records.

Explanation of Judgement if Safety System Not Judged to be Adequate

No safety system was inspected during this intervention.

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

SL’s MSSS Leak to Ground Risk Management Plan and supporting arrangements have been sampled and SL demonstrated to ONR’s satisfaction that these documents have completed due process and have been implemented. 

ONR reviewed SL’s arrangements to identify possible leakage from MSSS. A discrepancy between liquor level recording devices in the Original Building was identified that makes true liquor height determination uncertain. This could lead to confusion in demonstrating that an operating rule that requires a minimum liquor level is not breached. SL has accepted actions to ensure that the liquor level recording devices are recalibrated to ensure consistent liquor height measurement. These actions have been recorded in a level 3 regulatory issue.

SL has a sufficient borehole monitoring capability, but it was noted that there have been numerous instances where boreholes and blind tubes are rendered inaccessible by other work thus preventing samples being taken at the appropriate time. This has led to gaps in SL’s near-MSSS ground contamination data and may adversely affect early detection of leakage from MSSS. SL has been advised that it should prioritise access to the relevant boreholes and capture this in its arrangements. This action has also been recorded in the level 3 regulatory issue.

New blind tube monitoring equipment has been procured and is being commissioned. When commissioning has been completed, it is considered that SL will be able to obtain the relevant necessary data from its blind tubes to support detection and monitoring of in-ground gamma dose-rate. However, at this time, ONR considers this capability to still be in development and that it must be fully deployed prior to SEP2 being moved onto compartment 10.   

SL has constituted an expert body that has responsibility for scrutinising the data obtained from the various leak detection and monitoring capabilities, however SL was unable to provide sufficient evidence that the expert body was objectively considering the data relating to MSSS containment performance. Failure to objectively consider the data may lead to confirmation bias and subsequent failure to recognise potential for a leak to be occurring. SL has accepted ONR’s advice and regular level 4 meetings have now been arranged with the Ground Environment Review Meeting (GERM) so that ONR can consider the deliberations of the GERM.

Should the GERM confirm that a leak is occurring, it was not clear from MSSS’s arrangements how such an event would be formally reported to ONR as required by LC34(2) through its LC7 arrangements. SL has been required to demonstrate how the MSSS arrangements will ensure that a confirmed leak of radioactive materials from MSSS will be reported in accordance with SL's arrangements made under licence condition 7. This action has also been recorded in the level 3 regulatory issue.

Conclusion of Intervention

I have concluded that SL has adequate capability to acquire data relating to leakage of radioactive materials from MSSS and considers that it has credible plans to improve this capability. However SL has not demonstrated that it is adequately analysing and investigating or reporting potential instances of leakage or escape, as required by LC34(2). For this reason, the intervention is rated as AMBER. I have raised a level 3 regulatory issue to monitor SL’s completion of the agreed improvement actions.