Technical assistance for implementing Nuclear safety project R2.03/02: Thermo-hydraulic experiments at the PSB-RBMK integral Test facility

This contract was launched in parallel with TACIS project R2.03/02B (contract 96672). The European Commission launched, via a Framework Contract, the contracts 125745 and 199955 in order to provide the Beneficiary with an independent expertise for a series of experiments and code validation exercises that were specified in the project Terms of Reference of the TACIS contract 96672 with the Russian contractor EREC Electrogorsk.

The contract specific objectives were to consult the Beneficiary and EC on the way the work defined in the tasks of the above EREC contract was carried out by EREC Electrogorsk, to review technical reports from the above contract and to contribute to its reporting (i.e. to the deliverables of the EREC contract) and to provide advice in general to all parties according to the Nuclear standards and practices as applied in the EU.

The project was implemented by Resources and Logistics acting as the EC Framework Contractor.

TACIS Design Safety projects were service or consultancy type projects that were able to address research, design and engineering issues. They were usually implemented in such a way that an EU research, design or engineering organisation worked in close collaboration with Beneficiary country counterpart organisations on a specific technical problem.

The design safety projects therefore achieved a dual purpose of providing the necessary technical resources to solve the issue of concern whilst at the same time transferring know-how and developing capabilities within the Beneficiary country organisations wherever a lack had been identified. This dual purpose also helped to provide the sustainability of the project benefits that was a major objective of the TACIS programme.

In 1997, the Russian Nuclear Safety Authority – GOSATOMNADZOR (GAN) issued new guidelines for In-Depth Safety Assessment of VVER and RBMK reactors. Accordingly, the need to bring the level of the nuclear power plants safety to conformity with current safety requirements was originated.

For RBMK safety analysis, the western best estimate codes had been introduced, and Russian codes had been refined and updated. In addition, significant efforts had been dedicated to the validation of these codes. According to OECD-NEA, there remained some outstanding issues, particularly the need to extend the experimental database specific to the RBMK, and to improve the validation status of the codes used for RBMK analysis.

In response, the TACIS project R2.03/02B was launched with the aim to perform a number of experiments to simulate anticipated accidents and transients in RBMK at the large-scale integral test facility PSB RBMK. Obtained experimental data were to be used for validation of basic thermal hydraulic codes currently in use in the Russian Federation for RBMK safety analysis. Results of performed activities were intended to give the basis for certification of the codes in the Russian Federation Regulatory Authorities – Rostechnadzor (former GOSATOMNADZOR).

A PSB RBMK integral test facility had been constructed in the Electrogorsk Research and Engineering Center (EREC, Moscow Region). This test facility is a large-scale integral testbed which models one loop of the multiple forced circulation circuit (MFCC) of the RBMK-1000 reactor. The PSB RBMK test facility is the only large-scale installation that models the RBMK reactor loop. It includes six full-scale models of RBMK fuel channels and scaled models of other components. “PSB” is the abbreviation of the Russian name of “Full Scale Safety Facility”. The volumetric and power scale of the facility for one reactor loop is 1:140, whilst the volumetric and power scale for one fuel channel is 1:1. The facility is capable of providing high quality, RBMK specific, experimental data suitable for the validation of thermal hydraulic computer codes intended for application to RBMK safety analysis.

Analytical and experimental activities that were conducted in the frame of this project brought the following results:

Eleven groups of tests to be performed in the PSB-RBMK test facility were selected, adequately justified and carried out.
A system of T/H codes was selected for pre-and post-test analysis of the PSB-RBMK experimental results and verification of their predictive capabilities for the analysis of RBMK accident scenarios.
The uncertainties associated with the application of the code were analysed and quantified and the most effective measures to minimise these uncertainties was identified. A methodology for the performance of such a sensitivity and uncertainty analysis was developed, documented and implemented.
All outstanding requirements were identified for:

• experimental data needed to provide complete code validation, and subsequent certification by GAN for use in RBMK safety analysis,
• any requirements for further development of the code system selected for use in this project in order to improve its predictive capabilities.

A comprehensive review of methodologies used for the drafting of the TH sections of the RBMK Safety Analysis Reports was performed.
The EC Framework Contractor conducted an independent review of each deliverable for R2.03/02B. He performed the formal check of the reviewed documentations updated by EREC in accordance with the previous comments provided. He provided supervision, assistance and advice for the uncertainty analyses performed by EREC and for the development of the code validation matrix.

Lessons learned showed that the operators of the PSB-RBMK facility gained confidence with the hardware and the control systems of the installations. The reliability of the installation, after initial technical inconvenience, was continuously improved. In order to reproduce the expected phenomena it was necessary to calculate specific boundary and initial conditions taking into account the not nominal configuration of the facility.

The performed experiments were effective in supplying the data required for the code assessment. Additional pretest analyses of the selected transients to evaluate the experiments in the actual facility configuration were strictly necessary.

The success of the project can however be evidenced, ultimately, by the certification of the selected codes by the Russian nuclear regulatory authority (GAN) and their subsequent use in the preparation of improved safety analysis reports for the RBMK, which must also be accepted by GAN. This certification and application of the codes was, however, outside the scope of this project.