Development of the method of substantiation and analysis of the passive heat removal system with iInjector-condenser (PAHRSIC) for VVER NPP.

One of the most important tasks of safety enhancement of the Russian Nuclear Power Plants (NPP) is the analysis and modernization of the VVER-440 safety systems.

In Russia a project of the Passive Heat Removal System with Injector-Condenser (PAHRSIC) was under development for the VVER-440 units. PAHRSIC allows to solve a problem of residual heat removal in accident situations and to improve a safety level of the operating units.

PAHRSIC has no analogues, and for its incorporation it’s necessary to make a full substantiation of its efficiency on the base of the modern methods and the best-estimate thermal-hydraulic codes.

The analysis of applicability of PAHRSIC is based on modeling of emergency situation with the help of a thermal-hydraulic code CATHARE. At the analysis of applicability of PAHRSIC the beyond design accidents are considered caused by initial events, not taken into account in the design project, or multiple failures of safety systems, arising at various kinds of disturbances, such as earthquake, fire, flooding etc.

At the analysis of applicability of PAHRSIC the expert estimations on reliability of the given system and its cost are given.

Global objective of the project is the safety enhancement of the VVER and PWR NPPs by means of creation of new generation passive safety systems.

Alongside with the global purposes, the project seeks to reach the following special objectives:

• Adaptation of experience of western countries (Framatome-ANP) and capabilities of the best-estimate thermal-hydraulic code CATHARE for the substantiation of the design decisions of PAHRSIC.
• Demonstration calculations of emergency operation with use of PAHRSIC for one of the operating VVER-440 units.

The special objectives of the project were fully accomplished: using the experience of the consultant in the code CATHARE usage. The theoretical substantiation of PAHRSIC operability was confirmed for the VVER-440 NPPs. The results will possibly be used in modernization of VVER-440 and VVER-1000 NPP, as well as in creation of the new generation NPP designs.

The actual results at the end of project were:

• Method of the analysis of applicability of PAHRSIC for the NPP units with VVER has been developed;
• It’s been confirmed by means of CATHARE code calculation that PAHRSIC operation helps to improve the safety of VVER-440 NPPs.

The technical tasks performed were:

1. Analysis and a choice of the accidents and failures, consequences of which can be limited by means of PAHRSIC.
2. The description of a principle of action and a model for PAHRSIC calculation. There are two variants that were considered.
3. Initial data deck choice and approval with the Consultant to make calculations of the thermal-hydraulic processes of VVER-440 with code CATHARE.
4. Development of the PAHRSIC model for calculation with the CATHARE code. Running the test calculation.
5. Linkage of the PAHRSIC model to the VVER-440 model for the CATHARE calculations. Running the test calculation.
6. Analysis of the accidents. Following calculations were performed:
   • 6.1. Calculation of accidents without PAHRSIC operation.
   • 6.2. Calculation of accidents with operation of 4 trains of PAHRSIC without its power regulator.
   • 6.3. Calculation of accidents with operation of 2 trains of PAHRSIC without its power regulator.
   • 6.4. Calculation of accidents with operation of 4 trains of PAHRSIC with its power regulator.
   • 6.5. Calculation of accidents with operation of 2 trains of PAHRSIC with its power regulator.

All the planned technical tasks have been realised during the project period.

The conclusions of the study were the following:

• Operation of four PAHRSIC trains installed on the four steam generators from six allows removing the reactor residual heat with conservation of the primary circuit tightness (the pressuriser valves stay closed).
• Use of passive PAHRSIC power regulator decreases the rate of the reactor cooling.
• PAHRSIC is twice redundant: operation of two PAHRSIC trains out of four also allows maintaining the primary tightness but the active measures are needed earlier.

The next step would be to install a pilot system to one of the first generation VVER-440 reactors, e.g. Kola NPP. The problem may be to convince the plant management of the need for another system after many upgrading measures have already been implemented in the reactors in question.

Results of the analysis and the developed recommendations were transmitted to the interested organizations of the Ministry of Atomic Energy of the Russian Federation for their further application at the modernization of the VVER-440 units (NPP of Kola, etc) and at the development of the new generation NPP, including PWR.