Office for Nuclear Regulation

ONR reviews criticality safety across UK licensees

28 November, 2023

Criticality safety management at UK nuclear licensed sites has met required standards, inspections by the Office for Nuclear Regulation (ONR) have concluded.

What is criticality?

Nuclear materials, including enriched uranium and plutonium, can undergo a fission chain reaction under certain conditions, which releases large amounts of energy as nuclei are split apart.

This state of ‘criticality’ can be useful if controlled – for example, in a reactor operating normally – but is particularly hazardous if uncontrolled, referred to as a ‘criticality accident’.

ONR’s Radiation Protection and Criticality (RP&C) specialism focuses on regulation to protect workers and the public from ionising radiation, including criticality accidents, on and around nuclear sites, and via the transport of radioactive materials.

A series of broad cross-site ONR inspections in 2021 and 2022 found dutyholders had complied with the required legal standards.

The inspections had three key aims:

  1. To identify how licensees could improve their management of criticality safety, including legal non-compliances if present, and work with them to address these issues;
  2. To highlight good practice within individual licensees and share this more widely across the sector; and
  3. To influence improvements both across external sites and in ONR’s internal guidance.

They provided valuable opportunities for inspectors to trial the use of ONR’s new Technical Inspection Guide (TIG) on criticality safety.

Based on their recommendations, ONR criticality specialist inspectors will continue to engage with the UK Working Party on Criticality (WPC) to support the development of strengthened safety guidance.

Adam Nichols, a nuclear safety inspector within the RP&C specialism, presented the findings at the UK Society for Radiological Protection (SRP) annual conference in April 2023, as well as presenting to almost 100 attendees at a WPC webinar in November.

Eoin Flannery, another RP&C nuclear safety inspector, delivered a presentation on the findings at the International Conference on Nuclear Criticality (ICNC) 2023 in Sendai, Japan, in October, which fellow inspector Greg O’Connor also attended and supported as a member of the organising committee.

Eoin Flannery (l) and Greg O’Connor (r) at ICNC 2023.

ICNC is the premier nuclear criticality safety conference in the world, giving experts the opportunity to share operational experience and innovative approaches internationally.

ONR was a member of the organising committee for ICNC 2023 and will also be a member for ICNC 2027, which will take place in Manchester, UK.

Gareth Thomas, ONR’s Professional Lead for RP&C, said: “The cross-site criticality inspections provided an opportunity to examine compliance and highlight good practices at the highest criticality-risk sites, for the first time in ten years.

“The learning gained is helping our inspectors to conduct more strategic, intelligence-informed and risk-based engagements with dutyholders. This will help support improvements in criticality safety both in the UK and overseas.

“I would like to thank Clive Ingram, who led the project, as well as Greg O’Connor, Eoin Flannery, Adam Nichols and all involved in carrying out the inspections and sharing our findings.”