R/TSO/FBR/03: Design & Operational Safe

To provide the Russian nuclear safety organisation GAN with Western know-how and to assist in its assessment of safety justifications for fast reactors.
Reports which were to be developed:

1. Report on United Kingdom (UK) safety justification methodology (AEAT).
2. IPSN (Institut de Protection et de Suretuclre) comments on UK safety justification methodology.
3. Report on Russian Federation (RF) safety justification methodology (NRS).
4. Review of plant documentation (AEAT).
5. Report on RF safety improvement programme (NRS).
6. Prototype Fast Reactor (PFR) safety improvement programme (AEAT).
7. Review of Russian Federation safety improvement programme (AEAT).
8. Identification of topics for Task 4 (AEAT).
9. Review of RF safety improvement programme (IPSN).
10. Report on the method of treating specific BN600 safety issues (NRS).
11. Review of BN600 specific safety issues (AEAT).
12. Review of PD10 treatment of BN600 safety issues (IPSN).

Project activities in tasks:

• Task 1: Identification of possible improvements to the Russian safety justification methodology for BN600.
• Task 2: Assessment of Russian safety improvement programme.
• Task 3: Identification and assessment of any improvement in treatment of particular BN600 safety issues.
• Task 4: Transfer of information on Western treatment of the safety issues identified in Task 4.

Tasks 1 & 2:

The main aspects of the safety justification methodologies in the UK and RF have been reviewed and compared, together with the safety philosophy and regulatory practices adopted in the two countries.
In the RF the licence applicant designs, analyses, constructs and operates the plant in accordance with prescriptive regulatory criteria, rules and standards. The regulatory approach practised in the UK places on the applicant the responsibility to propose both the plant design and the acceptance criteria against which it will be analysed, constructed and operated.

The advantage of the UK approach is that it allows more flexibility but it may also create uncertainty as to what design solutions may be found acceptable, particularly for plant types for which there is little or no national experience.

GAN RF has developed an extensive regulatory library with three tiers of documents addressing basic safety criteria as well as detailed requirements for design and analysis. In contrast, the UK practice relies on the Safety Assessment Principles, which are non-prescriptive and apart from a few statutory limits do not place mandatory requirements for plant design or its safety analysis.
This difference in basic philosophy between the two systems is further reflected in the way the regulatory bodies in the two countries operate. Significant differences exist, for example, in the involvement of the regulatory body in the day-to-day activities at the plant.

Task 3:

Within this task, information was transfered on the PFR and BN600 safety improvement programme and a comparison of the Russian safety improvement programme was made. The main conclusions of the review of the Russian safety review programme are as follows:

• All the main types of potential accident which are assessed in Western fast breeder reactors (reactivity incidents, sodium fires, loss of decay heat removal capability, sodium/water reactions, fuel cladding failures, loss of primary circuit containment) have been assessed for BN600. The assessments themselves have not been reviewed;
• The BN600 list of reactor protection system trip parameters is typical of fast reactors. Only one parameter, present on PFR and Phenix reactors, is missing. That is there is no automatic trip on failed fuel detection. It is possible that the BN600 safety case includes a justification of this;
• The improvements made to BN600 have been mainly motivated by incidents which have occurred on the BN600 and BN350 plants but have also included modifications aimed at improving plant reliability, plant safety and the economy of the fast reactor fuel cycle. Although different in detail from improvements made to the UK and French fast reactors, the philosophy is similar. Evidence that the programme has been successful is provided by the high availability of the BN600 reactor. It is also evident that the operation of BN600 has led to significant advances in knowledge of fast reactor systems;
• The intended future improvements are in the areas which are likely to be of most benefit in improving the safety performance of BN600, in particular with regard to upgrading the reactor protection system;
• The purposes of experiments carried out at BN600 are similar to those experiments, which have been carried out on PFR (verification of reactor kinetics parameters, demonstration of natural circulation through the core, verification of Na/H2O detection systems efficiency etc.).

Task 4:

The specialist topic chosen for task 4 was reactivity transients and information transfered between AEAT, IPSN and GAN RF on the treatment of this topic in the UK, France and the RF. A meeting was held in Risley to present and discuss the specialist topic and project documents were produced on the treatment of reactivity transients in the UK, France and the RF.

Task 5 (created in addition):

Presentations on the project were made to GAN RF during the course of the project. The main presentations were at Risley in January and June 1998 and in Moscow in August 1998. The conclusions and recommendations from the project are presented in the next two sections.

Conclusion:

The main findings and conclusions of the project are summarised hereafter:

1. The decommissioning of Russian Federation fast reactors is becoming an issue of importance to RF and the fast reactor design and operating organisations. Following discussions with the GAN region representatives, during the Task 3 meeting at BN600, a summary report on decommissioning PFR was provided.
   Although the final decommissioning of BN600 is some time away GAN appreciated, in accordance with IAEA guidelines, that decommissioning plans should be made 5 years before commencing decommissioning. More detailed guidance, based on Western experience would be beneficial, particularly in the light of the ongoing decommissioning of PFR, and a recommendation how to develop this has been made.
2. During this project there was only a limited opportunity to explain Western risk assess methodologies, including As Low As Reasonably Achievable (ALARA), as applied to fast reactors. New RF normative documents are available and the Ural region GAN representative is currently preparing guidance documents.
3. Following presentation of a more deterministic approach by IPSN it was clear that there could be a benefit in applying the Lines of Defence (LoD) approach to existing plant such as BN600 and, complimentary approach to PRA currently being used for BN800.
4. The treatment of BN600 reactor kinetics transients was reviewed by IPSN and AEAT, however, no detailed analysis was carried out of any of the BN600 accident sequences by AEAT or IPSN further work in this area may be beneficial to GAN RF to provide confidence in the approach used for the BN600.