Assessment of Adequate Margin to Liquefaction for Nuclear Power Plants

Abstract

Design of nuclear power plant shall provide an adequate margin to protect items ultimately necessary to prevent an early large radioactive release in the case of earthquakes exceeding those considered in the design. An essential question is how large the margin should be to be accepted as adequate. In the practice, depending on the country regulation, a plant margin of at least 1.4 or 1.67 times the design basis peak ground acceleration is required to be demonstrated. The catastrophe at the Fukushima Daiichi Nuclear Power Plant revealed the fundamental experience that the plants designed in compliance with nuclear standards can survive the effects of the vibratory ground motion due to disastrous earthquake but may fail due to effects of phenomena accompanying or generated by the earthquakes. Liquefaction is one of those secondary effects of beyond-design basis earthquakes that should be investigated for NPPs at soil sites. However, the question has not been investigated up to now, whether a “margin earthquake”, vibratory effects of which the plant can withstand thanks to design margin, will not induce liquefaction at soil sites and will not result in loss of safety functions. In the paper, a procedure is proposed for calculation of the probability and margin to liquefaction. Use of the procedure is demonstrated on a case study with realistic site-plant parameters. Criteria for probability for screening and acceptable probabilistic margin to liquefaction are proposed. The possible building settlement due to margin earthquake is also assessed.

Applicability of Pseudoprobabilistic Method of Liquefaction Hazard Assessment for Nuclear Power Plants at Diffuse Seismicity Sites

“…ere are four VVER-440/213 units at the site, and construction of two new VVER units is foreseen. e site in this study is the same as that considered by Katona [7]. e seismotectonic features and seismic hazard assessments for the Pannonian Basin are presented in plenty of studies; see, e.g., Tóth et al [23,24] and Giardini et al [25].…”

“…It is important to know whether this margin earthquake with peak ground acceleration PGA ME and PGA ME ≤ 1.4(or 1.67) * PGA DBE would cause liquefaction. For the margin earthquake, the criterion P(liq | ME) ≤ 10 − 1 was proposed by Katona [7]. Here, it is conservatively assumed that the liquefaction could result in cliff-edge effect.…”

“…A simplified method for assessment of the margin to liquefaction has been proposed by Katona [7], which is based on the calculation of probability of liquefaction by the method proposed by Cetin et al [21] using a single pair of mean values of free-field maximum horizontal acceleration, a max , and moment magnitude, M w , taken from deaggregation results of the probabilistic seismic hazard assessment (PSHA). e application of the method has been demonstrated for a low-to-moderate seismicity nuclear site.…”

“…Furthermore, in the case of the operating NPPs at soil sites, a cost-effective method for assessment of the liquefaction hazard would be practical, when the liquefaction reveals as a possible safety issue recognised in the frame of periodic or ad hoc safety reviews. e aim of this paper is to analyse the conservatism of the method proposed by Katona [7] and justify its applicability for the aforementioned purposes, while the findings of Franke et al [22] will be accounted for. To achieve this goal, the results of Franke et al [22] obtained for the return periods of maximum 2475 years will be confronted with the results of analyses performed for the return periods T ≥ 10000 years typical for design and safety analyses for nuclear power plants.…”

“…To achieve this goal, the results of Franke et al [22] obtained for the return periods of maximum 2475 years will be confronted with the results of analyses performed for the return periods T ≥ 10000 years typical for design and safety analyses for nuclear power plants. e PSHA is the starting point of the studies performed by Katona [7] and Franke et al [22]. In both cases, the site is of low-to-moderate seismicity.…”