EACS 2016 paper - Manageable Reactor Pressure Vessel Materials Control Surveillance Programme
journal contributionposted on 28.03.2017, 15:17 by E.A. Krasikov
EACS 2016 Paper No. 104
As a main barrier against radioactivity outlet reactor pressure vessel (RPV) is a key component in terms of safety and extended light water reactor (LWR) life. The surveillance programme (SP) calls upon to predict ahead RPV materials characteristics conservatively to guarantee RPV structural integrity without any compromise. General vice of existing SPs is an impossibility of SP changing and development during reactor operation (30, 60 and even more years). Up to day, approach based on initial hard nomenclature of surveillance specimens installed in capsules. Therefore, practically it is impossible to change anything in SP during RPV service life. Anachronistic principle of ahead of time, for some decades of years in advance fabrication and installation into reactor vessel the sets of surveillance specimens (SS) contradicts to request of RPV innovative monitoring technologies development during long-term operation.
Besides there is a deficiency of SP portliness relative to conditions of the RPV irradiation during operation. Most important is the discrepancy of the actual thermal condition of RPV wall from SSs irradiation temperature. This fact carries in the element of non-conservatism into the system of control. Ideally, surveillance metal has to be irradiated in contact with coolant. Metal placement in perforated capsules that is immediately in running water provides the minimum irradiation temperature and therefore guarantees the most conservative data on RPV metal mechanical properties getting. Clearly, that at this case there is no need in temperature monitors. Moreover, today there is no hard confidence in SS capsules integrity during RPV operation. In the event of capsule depressurization SSs damage occurs. At the same time in reality it is impossible to exclude environmentally assisted cracking of the primary circuit stainless steel components during 60 and more years of operation. Surveillance metal contacting with water in perforated capsules emulate RPV metal-water corrosion reaction appearance as a result of possible cladding cracking and hydrogen (as a corrosion product) - metal interaction. Therefore for materials susceptible to hydrogen embrittlement, the degree of SP conservatism grows.
We suggest to improve LWR SPs by means of passage from existing «hard» SPs to «flexible» manageable SPs (MSP) that would give the possibility of SP adaptation to requirements of time and to strengthen technical and scientific potential of investigators and researchers in the future. So, we believe that is no sense to leave present-day level of knowledge and technology in congeal state to next generation of researchers. Thus for new LWRs with the service life of 60 and more years we propose pass on from the SSs of routine nomenclature to MSP i.e. sets of archive materials coupons placed in non-hermetic containers and cooled directly by running water. It gives a perspective in case of need put into practice an innovative MSP taking into account the state-of-the-art safety standards, technical progress, present day level of science and technology. In support of the above-mentioned MSP conception 5 year duration prototype version of the MSP is under execution at operating commercial LWR.