R2.03/02B Thermal hydraulic code validation - Experimental tests (AP ref.7.1.2.3)

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.

This project built on experience from Bistro project (TACIS BIS/00/017/011-N) “Development of experimental programme on PSB RBMK integral test facility for validation of thermal hydraulic system codes aimed at safety analysis of NPPs with RBMK-type reactors” that was impended in 2000.

Russian nuclear power plants were designed in the sixties and seventies and in accordance with the industrial and regulatory standards existing at that time. Based on world-wide evolution on improving the safety standards, as well as results from investigation of causes and consequences of NPP accidents, new design requirements emerged. There were two basic concerns, such as how to correctly verify results of deterministic analysis in the safety analysis report for the plant accidents and anticipated transients, and how to properly reflect these new design requirements in the safety upgrading programme for existing nuclear power plants.

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.

A PSB RBMK integral test facility had been constructed in the Electrogorsk Research and Engineering Center (EREC, Moscow Region) to provide for experimental verification and validation of T/H codes used for RBMK safety analysis. 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.

This project aimed at enhancing nuclear safety of RBMK reactors through validation and precertification of basic thermal hydraulic codes used in the Russian Federation in safety analysis of RBMK type reactors. The specific objective of this project was to improve the validation status of thermal‐hydraulic codes for RBMK reactor safety analysis.

A PSB RBMK integral test facility had been constructed in Electrogorsk Research and Engineering Center (EREC, Moscow Region) as part of the effort to address these issues. The PSB RBMK test facility is a large-scale integral testbed which models one loop of the multiple forced circulation circuit (MFCC) of the RBMK-1000 reactor.

Project activities involved performing a number of scaled experiments to simulate anticipated accidents and transients in RBMK at the large-scale integral test facility PSB RBMK. Obtained experimental data were used for validation of basic thermal hydraulic codes currently in use in the Russian Federation for RBMK safety analysis. Results of performed activities provided a basis for certification of the codes in the Russian Federation Regulatory Authorities – Rostechnadzor (former GOSATOMNADZOR).

Besides that a methodology for sensitivity and uncertainty analysis was developed and implemented to provide a quantitative appreciation of the confidence level that can be attributed to the code computational results.

The project was implemented through the present contract (96672) with the Electrogorsk Research and Engineering Center (EREC), the organisation which owns and operates the PSB RBMK test facility, as well as contracts (see 125745 and 199955) with an EU organisation able to provide experts in the field to follow-up the work performed by EREC and to provide expertise and advice to EREC, the Beneficiary and the Commission in order to help ensure the achievement of the expected results of the project.

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 analyzed and quantified and the most effective measures to minimize these uncertainties were identified. A methodology for the performance of such a sensitivity and uncertainty analysis was developed, documented and implemented.
All outstanding requirements were identified for
o experimental data needed to provide complete code validation, and subsequent certification by GAN for use in RBMK safety analysis,
o 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 European Commission contracted EU experts through a Framework Contract (see contracts 125745 and 199955) supplementary to Tacis contract 96672 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. FW Contractor task was 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, review technical reports from the above contract and 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 European Union.

Lessons learned from the Tacis R2.03/02 project showed that the operators of the 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 existing configuration of the facility. The performed experiments were effective in supplying required data for the code assessment. Additional pretest analyses of the selected transients that were carried out to evaluate the experiments in the actual test facility configuration were strictly necessary.