Evaluation of core-wide fuel pin burst during LOCA in APR1400

Abstract

Core-wide fuel pin burst fractions in a typical APR1400 nuclear power plant during a loss-of-coolant accident (LOCA) are assessed. For the assessment, fuel pin based power distribution at an initial and an equilibrium core of APR1400 is analyzed. Fuel cladding burst probability curves represented as a local peak power are established up to 70 MWd/kgU fuel burnup. Main uncertainty parameters for pin burst related to the fuel and thermal–hydraulic (TH) performance are identified. Monte Carlo method is used for combining uncertainties. FRAPTRAN and MARS-KS integrated code is employed in this study. In the APR1400 core, power ratio of the fuel pin between a target fuel pin and surrounding ones is ranging 0.899–1.131 within ±2 standard deviation. This power ratio has induced about 1.0–2.1 kW/ft burst power change. In the fuel uncertainties, fuel thermal conductivity, fission gas release (FGR), cladding yield stress shows a relatively strong influence on the burst power change. Groeneveld critical heat flux (CHF) and Chen transition boiling correlation shows a strong influence among TH uncertainty parameters. Established cladding burst probability curves reveal that fresh fuel has a relatively low burst power. But, it increases rather steeply until burnup reaches 5–10 MWd/kgU. Then, the burst power is reduced slowly until the lowest power is attained at the fuel burnup of 70 MWd/kgU. Evaluation of fuel pin burst fraction shows that the most susceptible fuel burnup for the core-wide pin burst is fresh one. Beginning-of-cycle (BOC) at the initial core seems to be the most vulnerable during a reactor life time.