Office for Nuclear Regulation

Sellafield – Inspection ID: 52728

Executive summary

Date(s) of inspection:

  • May 2023

Aim of inspection

The Office for Nuclear Regulation (ONR) regulates Sellafield Site in accordance with the ONR Sellafield strategy. One of the three key outcomes of the strategy is accelerated hazard and risk reduction (HRR) across the Sellafield site, targeting those projects that deliver the biggest HRR. One of those projects is the retrieval of stored radioactive waste from the magnox swarf storage silo (MSSS), an intolerable risk legacy facility ONR has placed in significantly enhanced regulatory attention.

Sellafield Ltd. MSSS retrievals programme to remove bulk waste over several decades includes replacing the current nitrogen gas supply system with a new nitrogen generation and storage plant (NGSP), initially supplying silo compartment C10. ONR has placed a regulatory hold point (HP), known as HP2a, on bringing the NGSP into service on C10 to support retrieval and pressurised inerting. Release of HP2a will allow the company to commence bringing the NSGP into service, which is planned to be completed in several months after HP release.

The purpose of the intervention was to judge the adequacy of Sellafield Ltd.’s readiness to commence operation of NGSP to support waste retrieval and pressurised inerting on C10. ONR judged that for HP2a it was proportionate to utilise inspection to provide the main permissioning recommendation.

The inspection was undertaken against Licence Condition (LC) 22, modification or experiment on existing plant, utilising eight other unrated LCs to judge compliance.

Subject(s) of inspection

  • LC22 – Modification or experiment on existing plant – Rating: Green

Key findings, inspector’s opinions and reasons for judgement made

The inspection focused on the hazards introduced by bringing the NGSP into service on C10 and the measures by which Sellafield Ltd. ensures that the associated risks are reduced so far as is reasonably practicable. The areas of focus were chronic and acute hydrogen management, and pressurised inerting (PI). PI is one of the lines of protection for the risks arising from acute hydrogen release resulting uncontrolled waste thermal excursions.

Human factors specialist, chemical engineering specialist inspectors and I obtained evidence from discussions with members of Sellafield Ltd.’s staff involved in HP2a and from sampling key documents. We utilised the following LCs to form our judgement on compliance with LC22.

  • LC10: training.
  • LC12: duly authorised and other suitably qualified and experienced persons.
  • LC21: Commissioning
  • LC23: operating rules.
  • LC24: operating instructions.
  • LC26: control and supervision of operations.
  • LC27: safety mechanisms, devices and circuits.
  • LC28: examination, inspection, maintenance and testing.

The inspection was informed by the early engagement inspection of MSSS NGSP the specialist inspectors and I undertook in February 2023. We did not identify any issue arising from the visit that needed us to consider further regulatory action.


On the basis of the evidence sampled, I judge that Sellafield Ltd. has provided evidence for adequately implementing LC22 compliance arrangements for bringing the MSSS NGSP into operation to support C10 retrievals and PI. Noting the ONR guidance on inspection ratings, it is my opinion that an inspection rating of green (no formal action) is appropriate.
We judged that the strategy to permission release of HP2a based on the evidence gained from inspection is still valid.