Analysis of depressurization ability for IRIS containment during SBLOCAs using RELAP5 and CONTEMPT

Abstract

As an integral and modular pressurized water reactor (PWR), the international reactor innovative and secure (IRIS) eliminates the threat of large break LOCAs (LBLOCAs) by integrating all major reactor coolant system components into the reactor vessel and provides compact steel containment with a suppression pool. Unlike large reactor containment, IRIS containment has modest volume and compact internal structure. The analysis of traditional dry containment cannot be applied to the IRIS reactor. Despite the disappearance of LBLOCAs (exceeding 4 in), small steel containment still faces the threat of high–frequency small break LOCAs (SBLOCAs) (e.g., the largest possible 4 in chemical and volume control system (CVCS) line break and the lowest elevation 2 in direct vessel injection (DVI) line break). To identify the impact of these characteristics on the integrity of IRIS reactor containment, a safety assessment framework that combines deterministic and probabilistic methods is proposed for the pressure response of containment under SBLOCAs of IRIS reactor. First in the modeling phase, primary and secondary loop models have been built considering the design of IRIS, and accident consequences under typical SBLOCAs were evaluated using RELAP5/MODE4. A simplified model of containment was presented herein based on the primary structure of IRIS containment. Second in the input parameters identification and uncertainty quantification stage, the responses of containment were simulated using CONTEMPT–LT. Thus, the main factors affecting the depressurization capacity of the containment were identified, and the reasonable probability density distributions were determined. Third, in the uncertainty propagation stage, Monte Carlo method was used to map the uncertainty of input parameters to the output variable. Fourth in the sensitivity analysis and uncertainty analysis phase, the contribution of each input parameter was measured using sensitivity analysis, and their interactions were investigated. The range of deviations of the containment pressure response was measured using uncertainty analysis, and finally, the calculated results were combined with the containment fragility curve to evaluate containment failure probability.