# RMPS Résumé de rapport

Project ID:
FIKS-CT-2000-00073

Financé au titre de:
FP5-EAECTP C

Pays:
France

## Reliability methods for passive safety functions

The objective of the RMPS project was to propose a specific methodology to evaluate the reliability of a thermal-hydraulic (T-H) passive system and to integrate this reliability in accidental sequence of Probabilistic Safety Analysis.

The project has developed a specific methodology for the evaluation of the reliability of passive system and its integration into the probabilistic analyses of accidental sequences. The methodology obtained deals with the following problems:

- Identification and quantification of the sources of uncertainties and determination of the important variables.

- Propagation of the uncertainties through T-H models and assessment of T-H passive system unreliability.

- Introduction of passive system unreliability in the accident sequence analysis.

Each step of the methodology has been described and commented and a diagram of the methodology has been presented. This methodology has been tested on three examples of passive systems: the Isolation Condenser System (ICS) of Boiling Water Reactor, the Residual Passive heat Removal system on the Primary circuit (RP2) of Pressurized Water Reactor and the Hydro-Accumulator (HA) of VVER 1000 reactors. Thermal-hydraulic calculations have been carried out with different codes (RELAP, ATHLET and CATHARE), and various methods of sensitivity analysis and reliability evaluation have been tested.

The Analytical Hierarchy Process has been chosen for the identification of the relevant parameters, which really affect the accomplishment of the system mission. The results obtained on the examples have shown the interest of sensitivity analysis for the determination, among the uncertain parameters, of the main contributors to the risk of failure of the passive system. They have shown also that it is possible to evaluate the reliability of the systems for specific situations, once the probability density functions of the input parameters are defined, in using Monte-Carlo or FORM method. The use of response surface methods, where the physical model is approximated by a simpler mathematical model is often necessary in order to reduce the number of calculations with the physical model.

The possibilities to integrate passive system reliability in a PSA sequence have been tested on an example. In a first approach, applied to a simplified PSA carried out on a fictitious reactor equipped with two types of safety passive systems, we have chosen an Event Tree (ET) representation of the accidental scenario. This methodology allows the probabilistic evaluation of the influence of the passive system on an accidental scenario and could be used to test the interest to replace an active system by a passive system on specific situations.

The developed methodology participates to the safety assessment of reactors equipped with passive systems. The development and the validation of a methodology of reliability analysis relative to passive safety systems are preconditions to the implementation of such systems on a nuclear reactor. This methodology is required to gain the necessary confidence of:

- The designers who define the architecture of reactors and safety systems. Indeed, the designers will accept new safety systems only if these systems remain at reasonable costs and with same efficiencies in comparison with the existing safety systems,

- Regulatory authorities that will have to accept the implementation of such systems on a nuclear reactor. feedback and organisational learning.

The project has developed a specific methodology for the evaluation of the reliability of passive system and its integration into the probabilistic analyses of accidental sequences. The methodology obtained deals with the following problems:

- Identification and quantification of the sources of uncertainties and determination of the important variables.

- Propagation of the uncertainties through T-H models and assessment of T-H passive system unreliability.

- Introduction of passive system unreliability in the accident sequence analysis.

Each step of the methodology has been described and commented and a diagram of the methodology has been presented. This methodology has been tested on three examples of passive systems: the Isolation Condenser System (ICS) of Boiling Water Reactor, the Residual Passive heat Removal system on the Primary circuit (RP2) of Pressurized Water Reactor and the Hydro-Accumulator (HA) of VVER 1000 reactors. Thermal-hydraulic calculations have been carried out with different codes (RELAP, ATHLET and CATHARE), and various methods of sensitivity analysis and reliability evaluation have been tested.

The Analytical Hierarchy Process has been chosen for the identification of the relevant parameters, which really affect the accomplishment of the system mission. The results obtained on the examples have shown the interest of sensitivity analysis for the determination, among the uncertain parameters, of the main contributors to the risk of failure of the passive system. They have shown also that it is possible to evaluate the reliability of the systems for specific situations, once the probability density functions of the input parameters are defined, in using Monte-Carlo or FORM method. The use of response surface methods, where the physical model is approximated by a simpler mathematical model is often necessary in order to reduce the number of calculations with the physical model.

The possibilities to integrate passive system reliability in a PSA sequence have been tested on an example. In a first approach, applied to a simplified PSA carried out on a fictitious reactor equipped with two types of safety passive systems, we have chosen an Event Tree (ET) representation of the accidental scenario. This methodology allows the probabilistic evaluation of the influence of the passive system on an accidental scenario and could be used to test the interest to replace an active system by a passive system on specific situations.

The developed methodology participates to the safety assessment of reactors equipped with passive systems. The development and the validation of a methodology of reliability analysis relative to passive safety systems are preconditions to the implementation of such systems on a nuclear reactor. This methodology is required to gain the necessary confidence of:

- The designers who define the architecture of reactors and safety systems. Indeed, the designers will accept new safety systems only if these systems remain at reasonable costs and with same efficiencies in comparison with the existing safety systems,

- Regulatory authorities that will have to accept the implementation of such systems on a nuclear reactor. feedback and organisational learning.