Main results of the research project entitled “VVER-1200/V491 safety analysis in the simultaneous occurrence scenario of loss of coolant accidents (LOCAs) and a partial failure of emergency core cooling system (ECCS) using RELAP5 code”

The project “VVER-1200/V491 safety analysis in the simultaneous occurrence scenario of loss of coolant accidents (LOCAs) and a partial failure of emergency core cooling system (ECCS) using RELAP5 code”, led by Hoang Minh Giang in collaboration with colleagues at Nuclear Safety Center, Institute for Nuclear Science and Technology (INST) was conducted in 2016. The purposes of the project include:

       (i)   To set up VVER-1200/V491 simulation models (input deck file) in terms of primary and secondary cooling systems, as well as associated safety systems;

       (ii)  To analyze several accident scenarios in design extension conditions such as the simultaneous occurrence of LOCAs and the loss of partly active safety systems to the ECCS;

      (iii)  To compare analysis results of the project with those of the corresponding scenarios presented in the feasibility study (FS) and safety analysis report (SAR) for the Ninh Thuan 1 nuclear power plant project.

 In order to fulfill the above-mentioned objectives, the research contents were designed with six items and implemented by the project team in twelve month duration as follows:

       (a) Data collection, analysis and processing for thermal hydraulics simulation of the VVER-1200/V491 reactor;

       (b) Simulation model establishment of the main elements of the VVER-1200/V491 reactor coolant systems, including primary and secondary systems;

       (c) Simulation model establishment of several safety systems regarding the simultaneous occurrence of LOCAs and a station blackout (LOCAs+SBO);

       (d) Construction of response threshold functions of safety systems in the LOCAs+SBO scenario;

       (e) Calculation and analysis of the VVER-1200/V491 steady state simulation;

       (f) Calculation and result analysis of several LOCAs+SBO scenarios.

At the end of the year, all research contents were successfully completed with acceptable results.

The project participants established VVER-1200/V491 simulation models composed of five sub-models: (1) hydro dynamics systems; (2) heat structures in the core, pressurizer and steam generator; (3) reactor kinetics input; (4) reactor control system and (5) passive heat removal system through steam generator. The simulation results showed a good agreement between the calculation values and the SAR design parameters for the VVER-1200/V491 reactor.

The VVER-1200/V491 simulation models in the case of steady state were taken into consideration and calculated in comparison with several main parameters of the reactor in SAR. The comparison result indicated that the relative deviation of main thermal hydraulics parameters was less than 10%.

Besides simulation models related calculation and analysis, the team analyzed LOCAs in design basis conditions to give valid results in comparison with the values presented in SAR. Accordingly, the analysis of the small break LOCA with an equivalent diameter of 100 mm and the double-ended guillotine break of the cold leg was performed.

The following simultaneous occurrence scenarios of LOCAs and the failure of the ECCS were then calculated in detail: (1) the small break LOCA along with the failure of high pressure injection system and (2) the large break LOCA along with the failure of low pressure injection system. The team also carried out safety parameter calculations for the VVER-1200/V491 in the double-ended break of the coolant line of the primary cooling system along with the failure of low pressure ECCS (LP ECCS).

The RELAP5 code was used for LOCAs calculation and analysis with corresponding initial and boundary conditions of the scenario as mentioned in SAR. The calculated results demonstrated the similarity to the SAR results and satisfied the safety criteria.

The main results of the project were presented in two research papers published in journals of science and technology in Vietnam, including:

1. Bui Thi Hoa, Hoang Tan Hung, Hoang Minh Giang, Safety Analyses of VVER-1200/V491 reactor for long term station blackout along with small LOCAs, Nuclear Science and Technology, Vol.6. pp.8-17, ( ISSN: 1810-5408 ) (2016).

2. Hoang Minh Giang, Hoang Tan Hung,A study on improvement of void fraction prediction in saturated region for PSBT sub channel benchmark using Ansys CFX 14.5, Journal of Science and Technology, Hanoi University of Industry, Vol.40, pp.28-33,  (ISSN: 1859-3585) (2017).

Despite the stoppage of the Ninh Thuan nuclear power project, it is necessary to continue the implementation of studies on nuclear safety such as safety analysis of thermal hydraulics in beyond design-basis conditions in order to affirm the response capability of reactors in severe accidents. This greatly helps Vietnam keep active in developing emergency preparedness and response plans for possible accidents from Chinese nuclear power plants located close to Vietnam-China border. 

INST