This earthquake will be denoted further as margin earthquake. In the paper, a procedure has been developed for evaluation of the margin to liquefaction, assuming that an earthquake happens, with maximum horizontal acceleration (PGA) 1.67 (or 1.4) times larger than that of the DBE. Even the question has not been answered, whether an earthquake characterised by 1.4 or 1.67 times the design basis peak ground acceleration, vibratory effects of which the plant can withstand thanks to seismic margin, will not induce liquefaction at soil sites and loss safety functions.
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How to define the margin to liquefaction and how large should be the margin to be sufficient are open questions and actual research tasks. Ĭontrary to the margin with respect to the earthquake vibratory motion, the margin with respect to liquefaction and the issue of potential liquefaction caused by beyond-design basis earthquakes are not sufficiently investigated up to now.
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Similar conclusion was made on the basis of seismic PSA for Paks NPP. However, if it is the case, liquefaction could be an essential contributor to the core damage. Basic finding of these analyses was that the liquefaction is generally not a safety issue. Liquefaction hazard has been considered with some extent for 41 operating NPPs at soil sites in the U.S. In case of operating at soil sites NPPs, reevaluation of the liquefaction hazard can be subject of periodic safety review or a focused review, as it has been done at several NPPs after Fukushima accident. For screening out the hazard, the factor of safety to the liquefaction ( FS L) due to design basis earthquake (DBE) is applied, which should be calculated by conservative methods (see ). In case of new plant, if the potential for soil liquefaction is recognized, the site shall be qualified for unacceptable, unless proven engineering solutions are available for the soil improvement. Liquefaction is one of those secondary effects of beyond-design basis earthquakes that should be investigated and evaluated for nuclear power plants (NPPs) at soil sites (see ). The catastrophe at the Fukushima Daiichi Nuclear Power Plant caused by the Great Tōhoku earthquake followed by a huge tsunami on 11th of March 2011 revealed the fundamental experience that the plants designed in compliance with nuclear standards can withstand effects of the vibratory ground motion due to disastrous earthquake exceeding several times the design basis one but may fail due to effects of phenomena accompanying or generated by the earthquakes. These acceptance criteria are based on the conservatism of the established nuclear design standards and justified by extensive studies. In the European practice, a margin exceeding by 40% the design basis PGA has to be justified. for new plants, a HCLPF capacity of at least 1.67 times the design basis peak ground acceleration (PGA) is required to be demonstrated. In case of earthquake, the High-Confidence-of-Low-Probability-of-Failure (HCLPF) could be the measure of the seismic margin. According to the regulations, best-estimate approach can be adopted for the evaluation of this margin.
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This abrupt change to severe plant condition is termed as ‘cliff-edge-effect’.Īn essential question is how large the margin should be to be accepted as adequate for complying with above requirements. The design has to guaranty that a severely abnormal plant behaviour could not be caused by plant response to a small deviation of magnitude of external events. It should be demonstrated that those values of the parameters of natural hazards will highly probable not be reached, for which the sudden loss of capability of the mentioned above items would occur, which would result in abrupt and severe change of the plant condition. According to generally accepted design requirements of nuclear power plants (NPPs), the design shall provide for an adequate margin to protect items ultimately necessary to prevent an early radioactive release or a large radioactive release in the event of levels of natural hazards exceeding those considered for design basis.