A method of integrating thermal-hydraulic analysis and probabilistic assessment for safety evaluation and screening

Y. Guan et al., A method of integrating thermal-hydraulic analysis and probabilistic assessment for safety evaluation and screening, NUCL TECH, 133(3), 2001, pp. 290-309
Citations number
Categorie Soggetti
Nuclear Emgineering
Journal title
ISSN journal
0029-5450 → ACNP
Year of publication
290 - 309
SICI code
A method of integrating traditional thermal-hydraulic (TH) analysis with pr obabilistic assessment (PA) (called the TH-PA method) has been developed. T his method allows for an exhaustive search through a Set Of individually de veloped but subsequently linked logic models to screen and identify acciden t scenarios. The logic models consist of a probabilistic risk assessment (P RA) used for probabilistic screening purpose and an ensemble of integrated behavior logic diagrams (IBLDs). The PRA model represents the functional/lo gical relationships of the components and accident scenarios, the same way as is modeled in the conventional PRAs. The IBLDs hierarchically represent system interactions/dependencies due to TH phenomena and human actions. Thi s hierarchy also shows causal factors and consequences of plant states, and identifies induced system failures. The TH-PA method relies on two types o f scenario screening: probabilistic screening (PA screening) and TH screeni ng. The PA screening eliminates scenarios with low frequencies (e.g., <10(- 10)/reactor-yr). The traditional frequency-based screening method used in t he PRAs has been adopted for PA screening. The TH screening eliminates scen arios that do not expect to result in core uncovery. For the TH screening, a simple accident trajectory approach has been devised. A trajectory repres ents the collapsed liquid volume fraction in the reactor primary system as a function of primary pressure. The trajectories are based on simple mass a nd energy conservation equations (if the TH-PA method is applied to a syste m where mechanical energy transfer is important, momentum conservation shou ld also be considered). The roles of each plant system are then identified by indicating whether the system is a "source" ol a "sink" for mass and ene rgy at a given time during accident progression. Based on an input set brat represents the plant system failures and the stage of the transient, the a ccident trajectory is developed. The accident trajectory allows for the eva luation of safety significance of scenarios. The trajectory also determines whether the core becomes uncovered, should the input conditions (i.e., con ditions described by the input set) remain unchanged.