The findings are set out, for the three topics, under the headings used during the discussions: areas of good practice, shortfalls, areas for improvements and, developments and emergent issues.

On good practice. There were general observations:

• The industry noted that ONR was able to take a balanced view on safety case arguments through its assessments;
• ONR guidance was thought to provide the right words and aspirations for defect tolerance assessments;
• The industry said that there was generally good engagement with ONR and participation in committees and industry bodies.
• There were areas where the industry’s good practices could be reflected in ONR guidance:
• Where current metallurgical practices allowed for more rigorous control of alloying elements and tramp elements than the design codes stipulated and these improvements had been identified as ONR expectations within the generic design assessment (GDA) process;
• Expectations for representative materials data, particularly on fracture toughness;
• Responses to regulatory observations on avoidance of fracture demonstrations within the GDA process.

On shortfalls.There were areas where ONR guidance was said to be vague and open to interpretation and the industry asked for further guidance:

• On what was meant by ‘adequate margin’ in defect tolerance assessments (DTAs)—it was noted that the currently accepted defect size margin of two was set a number of years ago and it was not clear why this was still appropriate or how it aligned with the selection of conservative inputs such as infrequent transients and lower bound material properties.
• On ONR expectations for the integrity of safety significant structures, systems and components (SSCs) for which a safety case did not discount the possibility of gross failure—these SSCs could carry a lesser burden of requirements than those for which the highest reliability claims were made.
• The industry attendees said that it was difficult for a duty-holder to take credit for improvements beyond the basic safety objective (BSO).

On areas for improvement.Qualified inspections that were not stipulated in design codes could provide assurances that SSCs were free of detectable defects such that some inspections stipulated by the codes were superfluous. In these cases, licensee’s safety cases could present arguments for relaxations of the inspection regimes stipulated by the design codes.

• The use of probabilistic methods and the concept of target reliability;
• Ground rules for a defect tolerance assessment;
• The adequacy and use of plastic deformation based design methods;
• Advice and guidance for the assessment of thin walled components;
• Guidance on the applicability of design codes for advanced nuclear designs;
• Guidance on the structural integrity of thin walled components;
• Whether there was sufficient active research and development of structural integrity understanding and failure methodology development and, if not, what were the key issues to address?

Developments and emerging issues.It was noted that ONR guidance had been developed for the assessment of submissions from a single mature licensee and that, with new nuclear build, the number and variety of licensees, prospective licensee and requesting parties had increased. Therefore the implicit assumptions that had been developed for highest reliability plant warranted further explanation.

• On good practice: (ONR notes the general observations for which no action needs to be taken.) In the areas where the industry suggested that ONR could give further advice, this can be provided in TAG 16 by citing Generic Design Assessment (GDA) Findings and/or Regulatory Observations.
• On shortfalls: (i) To provide guidance on the adequacy of margins in DTAs, ONR can refer to the judgments that emerged from the Sizewell B inquiry and cite Generic Design Assessment (GDA) Findings and/or Regulatory Observations in TAG 16. (ii) ONR accepts that the integrity of safety significant structures, systems and components (SSCs) for which a safety case does not discount the possibility of gross failure carries a lesser burden of requirements than those for which the highest reliability claims were made and ONR will consider the clarity of its advice in this area and revise it if appropriate. (iii) ONR notes that it is difficult to take credit for improvements beyond the basic safety objective (BSO). However, ONR also notes that this reflects the ALARP principle and, accordingly, ONR does not intend to change its advice in response.
• On areas for improvement: ONR notes (A) the suggestion to relax inspections where qualified inspections that exceed code requirements provide the necessary assurances and (B) the identified areas for the industry to take forwards. ONR does not see an immediate requirement to provide further guidance in response to these proposals.
• On developments and emerging issues: (i) ONR held the workshop in response to the increased number and diversity of its duty-holders and will consider revising its advice (as above). (ii) ONR notes that no new issue emerged for which there was a pressing need for new guidance or advice.

ONR welcomes the industry’s identification of areas to take forwards, largely related to shortfalls and areas for improvement. ONR will continue to engage with the industry (as licensees, requesting parties, technical support companies and code making bodies) as they progress.

ONR thanks the FESI for facilitating the event which ONR has found worthwhile. ONR and FESI thank the delegates for their constructive and well informed contributions, particularly those who made long journeys to attend.