ANALYSIS AND IMPLEMENTATION IMPACT OF SOME SELECTED CHF MODELS AS USED IN THE RELAP5/MOD3 CODE

摘要

In the framework of nuclear plant design and safety,rnone of the most important parameters to take into account forrnestablishing the highest power available from a Nuclear PowerrnPlant is the value of Critical Heat Flux.rnExtensive research has been carried out to developrnempirical, semiempirical and theoretical methods to representrnthis phenomenon, and to provide best-estimate values of CHFrnunder different conditions, especially near to the operationalrnconditions of PWRs and BWRs. In the literature/1/,/2/ there arernat present more than 300 different correlations that coverrnnearly every instance of the occurrence of CHF, but anrnanalytical theory for grouping the experimental data is stillrnlacking. In the thermal-hydraulic system codes which arerndesigned for simulating the accidental transients in NuclearrnPower Plants, the CHF value is calculated by empirical orrnsemi-empirical correlations based on a number of experimentalrndata sets.rnIn this paper, some of the most widely usedrncorrelations in thermal-hydraulic system behaviour codes arernanalysed and compared with each other in order to show therndifferences in their modelling of the phenomenon. Moreover,rnthe different performance of these selected correlations asrnimplemented in the thermal-hydraulic transient computer codernRELAP5/MOD3 is examined and analysed. In order to analysernthe behaviour of the models over wide range of conditions,three different boil-off transient tests are considered. Inrnaddition, how the possible discrepancies in the models couldrninfluence the prediction of a transient in a plant is alsornanalysed.rnTo achieve the above-mentioned objectives,rncomparisons among the different models were performed inrnthe following steps:rn- Initially, the selected correlations were tested in two typicalrnfields of interest during a transient:rna) low mass flow and low pressure (near to pool boilingrnconditions), typical of such slow transients as a small-breakrnLOCA, mid-loop operation, etc.rnb) high mass flow and high pressure (film boiling conditions),rntypical of a large-break LOCA in which CHF conditionsrnare rapidly reached after the rupture of coolant pipe.rnIn this step, comparison of the selected correlationsrnwas made outside of the system code in order to see whetherrnthe main trends of the CHF phenomenon are followed.rn- In the second step, the selected correlations werernimplemented in the transient system code, and thernimplementation tested on a simple benchmark problem.