Expert Group on Integrated Performance Assessment


The management of radioactive wastes and, in particular, the safety assessment of radioactive waste disposal systems are areas of high priority for the NEA. The Working Group on Integrated Performance Assessments of Deep Repositories (IPAG) was set up in 1994 to provide a forum for informed discussion on the performance assessment (PA) of proposed deep repositories for radioactive waste. The group has recently ended in the third phase of its work. A common object of all IPAG studies is the collection of detailed information from national programmes through a questionnaire; the compilation and rationalisation of the information into a synthesis; and the identification of the lessons to be learnt.

Phase I: Feedback from IPAG exercises

In Phase I, the IPAG examined recently completed PA studies from ten organisations as a practical body of evidence to indicate the current status of PA and to shed light on what can and should be done in future studies. The major findings of this phase of the group's work were that traceability and transparency in the conduct of PAs were essential. 'Traceability' refers to the unambiguous and complete record of the decisions and assumptions made, and of the models and data used in arriving at a given set of results. 'Transparency' refers to the PA being clearly reported, so that the audience can gain a good understanding of what has been done, what the results are, and why the results are as they are. This is a more subtle, and audience-dependent, requirement. The difficulty of finding an unambiguous vocabulary for discussion of these issues was also addressed, along with recommendations for the essential elements of a safety assessment report. Completed in 1996, the resulting Lessons Learnt from Ten Performance Assessment Studies is available online (pdf, 395 kb).

Phase II: Regulatory experience of IPA reviews

In the second phase, IPAG-2, the goal was to examine the experience of peer reviews of integrated PAs, and especially reviews performed in support of regulatory assessment. These were examined from both the implementer and regulator points of view. Seventeen national organisations participated in this phase, where each had either carried out or reviewed a recent integrated PA study. The work, carried out mainly between May 1997 and October 1998, was focused by means of a questionnaire on the PA and review process, and examination and discussion of the answers. The final report makes recommendations and observations on the conduct of the review, aspects of the safety case, traceability and transparency, and regulatory guidance.

Phase III: Approaches and arguments for establishing confidence in safety and the overal results of IPAs

A third phase of the programme has evaluated different approaches and arguments for establishing technical and stakeholder confidence in the long-term safety of proposed deep repositories for radioactive waste. Establishing technical confidence requires the corroboration of both the disposal concept's intrinsic safety and its long-term performance. These two elements can be further articulated in the following checklist for establishing technical confidence in the disposal proposal:

  • the multi-barrier disposal system must be intrinsically robust;
  • disposal system data must be reliable and formally established;.
  • a logical, clear and systematic assessment approach must be implemented and independently reviewed;
  • the Integrated Performance Assessment (IPA) models employed must predict measurable parameters and produce intuitive results;
  • the safety case and the IPA analyses must demonstrate that the assumptions made are representative or conservative, and set out a clear strategy for handling uncertainty;
  • there must be clear feedback mechanisms on the site's design and its geological characteristics.

Findings show that communicating confidence arguments to stakeholders must be done in a clear and unambiguous way. The arguments need to be both tailored to the audience and consistent with both the assessment and the safety case. Different audiences have different information needs, and confidence has to be demonstrated, not just stated.

The IPAG-3 final report surveys international experience in achieving these goals. Twenty organisations from NEA member countries have participated in this initiative, providing a variety of viewpoints, institutional interests and national experiences. The report is entitled "Establishing and Communicating Confidence in the Safety of Deep Geologic Disposal".

Related publications

Establishing and Communicating Confidence in the Safety of Deep Geologic Disposal
Approaches and Arguments
Confidence among both technical experts and the public in the safety of deep geologic repositories for radioactive waste is a key element in the successful development of the repositories. This report presents the approaches and arguments that are currently used in OECD contries to establish and communicate confidence in their safety

Regulatory Reviews of Assessments of Deep Geologic Repositories
The goal of the IPAG-2 study was to examine the experience of regulatory reviews of IPAs, from both the implementer and regulator points of view. Ten implementer and seven regulatory organisations participated. This report presents the lessons learnt from their review experiences, and provides recommendations to aid future regulatory decision making

Lessons Learnt from Ten Performance Assessment Studies
Phase-1 of the IPAG study focused on the production, refinement and answering of a questionnaire on the submitted Performance Assessments, and examination and discussion of the answers.

Last updated: 12 January 2016