Risk-informed Safety Margin Characterization for A Large Break Loss-of-coolant Accident of Nuclear Power Plants and Associated Peak Cladding Temperature Margin Evolution

Liang, T
(2019) Risk-informed Safety Margin Characterization for A Large Break Loss-of-coolant Accident of Nuclear Power Plants and Associated Peak Cladding Temperature Margin Evolution. Master of Philosophy thesis, University of Liverpool.

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The evaluation of methodology for large break loss of coolant accident (LBLOCA) licensing analysis involves two kinds of methodologies, namely deterministic methodology and risk-informed methodology. In the deterministic methodologies required for design-basis LBLOCA analysis, only epistemic or calculation uncertainty is addressed by either the conservative appendix K approach or the BEPU approach. Calculation uncertainty generally consists of physical model uncertainty and plant status uncertainty. In the risk-informed methodology, not only the epistemic uncertainty but also the aleatory are addressed by conducting a peak cladding temperature (PCT) load spectrum of LBLOCA. According to the existing 10 CFR50.46, LBLOCA is one of the most essential design-basis accidents (DBA) and a deterministic methodology shall be applied to perform LBLOCA analysis based on a so-called surrogate sequence. Without considering how low this sequence occurrence probability is, this surrogate sequence satisfies all the required licensing assumptions. However, in the to-be-issued 10 CFR 50.46a, the LBLOCA will be categorized as accidents beyond design basis and the peak cladding temperature margin shall be evaluated in a risk-informed manner. According to the risk-informed safety margin characterization methodology (RISMC), a process has been suggested to evaluate the risk-informed PCT margin. Following the RISMC methodology, a load spectrum of peak cladding temperature for LBLOCA has been generated for the Taiwan’s Maanshan Nuclear Power plant and 14 probabilistic significant sequences have been identified. It was observed in the load spectrum that the conditional PCT generally ascends with the descending sequence occurrence probability. With the load spectrum covering both aleatory and epistemic uncertainties, the risk-informed PCT margin can be evaluated by either expecting value estimation method or sequence probability coverage method. It was found that by comparing with the traditional deterministic methodology, the PCT margin evaluated by the RISMC methodology can be enlarged by 38.3-42.6 K. Besides, to have a cumulated occurrence probability over 99% in the load spectrum, the occurrence probability of the sequence referred is about 5.07*10-3, whereas for the traditional surrogate or licensing sequence generally applied in the deterministic methodology, the occurrence probability is only about 5.46*10-5. Finally, observed from the evolution of LBLOCA methodologies, the safety margin of a LBLOCA can be released from traditional Appendix K methodology by (i) relaxing plant bounding state assumption (DRHM), (ii) performing realistic LOCA analysis with statistical consideration of both model uncertainties and plant status uncertainties (BEPU), and (iii) relaxing licensing sequence assumption and evaluating the peak cladding temperature margin in a proper risk-informed manner (RISMC).

Item Type: Thesis (Master of Philosophy)
Divisions: Faculty of Science and Engineering > School of Engineering
Depositing User: Symplectic Admin
Date Deposited: 12 Jul 2019 14:49
Last Modified: 15 Sep 2021 07:10
DOI: 10.17638/03045424
URI: https://livrepository.liverpool.ac.uk/id/eprint/3045424