International Workshop on Operational and Regulatory Aspects of Criticality Safety
Assessing the Challenges of Nuclear Criticality Safety from an Operational and Regulatory Perspective

The OECD Nuclear Energy Agency (NEA) Committee on the Safety of Nuclear Installations (CSNI) Working Group on Fuel Cycle Safety (WGFCS) held an international workshop on Operational and Regulatory Aspects of Criticality Safety (ORACS). The workshop was hosted jointly by the United States Nuclear Regulatory Commission and the United States Department of Energy.


Working under the mandate of the CSNI, the objective of the WGFCS is to advance the understanding for regulators, technical support organisations (TSO) and operators of relevant aspects of nuclear fuel cycle safety in NEA member countries.

The objective of this workshop was to discuss and review current national activities, plans and strategies for maintaining and improving operational and regulatory approaches to criticality safety. Safety authorities and their TSO, fuel cycle facilities (FCF) operating organisations and international organisations were invited to share information on their approaches, practices and current developments.


Criticality safety is an integral component of ensuring the continued safe operation of current reactor and fuel cycle facilities, as well as addressing new and interesting challenges that are expected in the design of anticipated future nuclear facilities. Fuel cycle facilities (FCFs) represent a broad range of operations including uranium enrichment, fuel fabrication, radioisotope production, reprocessing, decommissioning, waste management (storage, handling, maintenance, and disposal of fissile materials including spent and damaged fuel) and transportation. All these operations exhibit the need to maintain a complete and thorough nuclear criticality safety (NCS) Program. The workshop discussions were intended to bring significant regulatory and operational aspects of these programs into focus.

Traditionally, NCS Programs have focused heavily on the preparation of deterministic criticality safety assessments based on computational modeling of normal and anticipated abnormal conditions to demonstrate subcriticality. These calculations form the basis for safety limits and controls embodying a defence-in-depth approach to safety known as the double contingency principle. In this deterministic approach, an operator assumes the failure of each control barrier occurs, without regard for the likelihood of occurrence, and demonstrates the system remains subcritical under the worst-credible condition that results. This is a very conservative approach that has resulted in a long record of safety but with the imposition of very large safety margins on regulated activities.

In some countries, a risk-informed and performance-based approach has been recently introduced in operations and regulations to ensure safety of FCFs. The integration of probabilistic risk analysis methods with the traditional deterministic approaches employed in criticality safety practice has resulted in numerous complexities and challenges, especially in facilities handling and processing fissile materials due to the complexity of the physical and chemical forms and the diversity of their processes and hazards. For the probabilistic approach, the added complexity makes it even more important to focus on the regulatory aspects of criticality safety. In addition, the integration of risk assessment requires operational aspects of criticality safety to be considered in addition to the analytical elements. This workshop presents a unique opportunity to discuss criticality safety from an operational and regulatory perspective, to complement the more traditional deterministic and computational context. The workshop was centered on the following focus areas:

  • national regulatory approaches;
  • operational NCS analysis;
  • operational considerations.

Scope and contents

The workshop addressed these issues, in order to provide an opportunity to discuss regulatory, industry and operational perspectives from participating countries.

National regulatory approaches:

  • regulatory criterion and guidance (sub-critical margins, prevention or mitigation of consequences, human and organizational factors, design principles, national reporting of events);
  • assessing NCS for proposed operations (verification of engineered and administrative controls);
  • implementation and evaluation of operations (audits, inspections, change management and configuration control);
  • post-operational clean-out criterion/methods.

Operational NCS analysis:

  • completeness: avoiding unanalysed and other improper conditions;
  • risk-informed versus traditional deterministic defence-in-depth framework;
  • consideration of human and organizational factors;
  • conservatism and uncertainties of NCS Analysis;
  • application case – A comparison of safety practices based on a practical application;
  • operational implications on the use of burn-up credit for spent fuel for storage, transportation and disposal.

Operational NCS implementation:

  • main practices, controls, and challenges for preventing inadvertent criticality events;
  • lessons learned from operating experience (including research and development needs) and potential regulatory gaps;
  • use of neutron poisoned fixtures for NCS;
  • operator training for NCS.

Participation was open to experts from FCF operating organisations, regulatory authorities and their technical support organisations, FCF designers and vendors, and industry associations and observers from OECD/NEA member countries.