Support to Rostechnadzor and its TSOs with the purpose to review documents justifying safety of Russian NPPs

The Federal Environmental, Industrial and Nuclear Supervision Service of Russia (Rostechnadzor) is a federal executive authority which implements functions on adoption of regulatory documents, control and supervision in the field of the environmental protection as regards confinement of negative man-induced impact including state regulation of safety in the use of atomic energy. Responsibilities and liability of this Authority are defined in the Decree of the Government of the Russian Federation. Since it was established, Gosatomnadzor of Russia, at present time Federal Environmental, Industrial and Nuclear Supervision Service of Russia (Rostechnadzor), is supported by different international organisations in the improvement of itsactivities. With this co-operation, the system of regulatory documents has been modernised and a number of basic regulatory documents, guides and recommendations has been developed. Review of a number of upgrading NPPs has been carried out jointly with Western European experts. In the frame of this work, Russian experts have received and mastered main Western European codes for accident analysis and other important calculations. All this has improved safety of the Russian NPPs and is used by the Rostechnadzor to improve its regulatory activities

To strengthen the Environmental, Industrial and Nuclear Regulatory Authority of Russia (Rostechnadzor), with the development of the already transferred Western European regulatory methodology and practices according to the Russian culture, mentality and legislative environment;
To improve managerial, scientific and technical capabilities of Russian experts belonging to the Regulatory Authority and its TSOs;
To develop regulatory and technical capabilities with reference to Plant IDSA (In-Depth Safety Assessment), in the definition of the SAR content and in SAR evaluation.
To improve the Russian licensing process and the interface and co-operation with licensee, industry and scientific-technical institutions.

The project consisted of three tasks:

Task 1: Improvement and pilot application of analytical simulators for review of safety justification of NPP

Task 2: Improvements in methods of analyses and classification of operational events with respect to their impact on safety. Update of safety performance indicators analyses to optimise review process and inspection activities

Task 3: Computational research of emergency processes.

Task 1 was focused on the development and application of analytical simulators for the last generation of operated power units with VVER-1000 reactor and power units with VVER-440/213 reactor. Two Russian experts, detached for 14 days to GRS Berlin, were assisted in modifying the existing analytical simulators for a unit with VVER-1000 reactor in such a way that it fits the special features of the Volgodonsk-1 NPP reactor facility, in performing two accidents calculations with the AS and in the preparation of the draft of the technical report and in determining future work. During a 14 days detachment to GRS Garching two other experts were assisted by discussions and joint work in the development of graphical user interfaces for VVER-440/213 reactor (Kola-3), in selecting adequate and useful pilot applications for testing the newly developed analytical simulator.

Within Task 2 the technical report providing detailed analysis of European practices in analyses and classification of operational events has been worked out and the recommendations for improving operational safety assessment methods and the operational experience feedback process in Russia were elaborated. The selected subset of recommendations has been applied within the assessment of the safety state of Leningrad 1 NPP.

Task 3 “Computational research of emergency processes” consisted of several subtasks dealing with analysis of different possible accidents:
“Analysis of accidents with the failure of emergency reactor protection system (ATWS) in VVER-1000 reactor”. Two ATWS scenarios were selected for analysis:
Main steam line break
Closure of the main turbine valve
Both scenarios were calculated with ATHLET (GRS) applying its point kinetics model and also with the KORSAR code, where the point kinetics and 3-D neutron kinetic models were applied. The results of KORSAR calculations with point and 3-D kinetics models were compared and analysed in depth. These results gave the basis to develop the recommendations concerning applicability of the point kinetics model for ATWS analyses.

Analysis of accidents at low power and shutdown states of VVER-1000 reactor

Thermal hydraulic analyses were performed with the ATHLET code for VVER-1000 reactor, Balakovo-3, behaviour under following accident conditions started in cold reactor state:
Break of normal cool-down line;
Long-lasting station blackout.

Analysis of accidents leading to VVER-1000 reactor pressure vessel thermal shock

(Inadvertent opening of pressurizer safety valve; Steam line break) Thermal hydraulic analyses were performed with the analytical simulator based on coupling of the ATHLET, CMS and CONDRU codes. Detailed investigation of the local impact of cold coolant plume on the reactor down-comer steel wall was performed with the GRSMIX code as a post-processor of ATHLET data.

Analysis of accident with RBMK-1000 MCP header rupture

Thermal hydraulic analyses of large break LOCA at RBMK-1000 (Leningrad-2) NPP were performed. Accidents initiated by rupture of pipes larger than DN300 are considered for the given plant as a beyond design basis accidents. However, according to preliminary assessments made by the utility, the ECCS after modernisation is able to prevent damage of fuel rods during these accidents.

Analysis of hydrogen distribution in VVER-1000 containment under LOCA conditions.

Containment thermal hydraulic analyses were performed with COCOSYS code applying to the Balakovo-3 NPP.
Above groups of accidents were selected for the project because they are considered as significant contributors to risk and there is a lack of experience in Russia in implementation, presentation and evaluation of their analysis because many of them are not included in the design basic accidents (DBA) in Russia. That is why for concerning analysing presently in Russia neither clear instructions nor requirements exist. The recommendations developed in the project may compose the basis of the corresponding Rostechnadzor’s regulatory document.

Comments

During the project substantial progress was made in the development, improvement and application of Analytical Simulators, development of recommendations for the update for the review process of operational safety justifications and development of recommendations for the review of deterministic accident analyses and performing pilot thermal hydraulic analyses for these accident scenarios.