Reliability-based design optimization of passive poison injection system (PPIS) for advanced heavy water reactor

Abstract

In nuclear power plants (NPPs), the use of passive safety systems such as accumulators, condensers, evaporative heat exchangers, and gravity driven cooling systems provide enhanced safety and reliability. In addition, they eliminate the huge costs associated with the installation, maintenance and operation of active safety systems that require multiple pumps with independent and redundant electric power supplies. As a result, passive safety systems are preferred in advanced reactor concepts. In current NPPs, safety systems which are not participating in day–day operation, are kept isolated, require a signal to open the valve. It is proposed to replace these valves by passive components and devices such as self-acting (passive) valves, rupture disks, etc. Some of these innovative passive valves, which do not require external power, have been recently designed, developed and tested at rated conditions. These valves are proposed to be used in several passive safety systems of indigenous advanced heavy water reactor (AHWR). One crucial and important passive safety system which has been designed for AHWR is passive poison injection system (PPIS). Since it is passive, it not only provides higher reliability, but also ensures safe shutdown of the reactor in case of insider threats or malevolent acts leading to disabling and failure of wired shutdown systems of the reactor. Since the passive systems developed are meant for nuclear reactor applications, they must be reliable enough for its desired operation under all reactor operating conditions. As these systems are indigenously designed with no prior plant operating experience, assessment of reliability of passive systems is an important task for ensuring extensive use in nuclear power plants. In view of this, it is necessary to define a roadmap for estimation of reliability of passive systems to be used in AHWR. As a case study, the reliability assessment has been carried out for passive poison injection system of AHWR. Two failure modes of PPIS namely, (1) probability of failure on demand and (2) probability of spurious actuation of system are considered. Analysis has been carried out for evaluating the demand failure probability and spurious injection probability for five proposed schemes for design comparison. An experimental facility along with prototype of passive valve named as poison injection passive valve (PIPV) has been successfully developed for demonstration of PPIS of AHWR. Based on the F-distribution, testing scheme for valve was suggested: minimum of 9088 cycles, if no failures are observed or 14,360 cycles, if one failure is observed to ensure the target unavailability of 3.3E−4 with 95% confidence. This paper briefly describes about the passive shutdown system, i.e., PPIS and its reliability assessment along with the probable configurations of active and passive valves under development for AHWR.