Authors & Affiliations
A.I. Leypunsky Institute for Physics and Power Engineering, Obninsk, Russia
In the reactor with substantial heatup, the fuel element cladding temperature is primarily specified by the coolant temperature, and the studies in the direction of bringing analytical and experimental coolant temperatures closer together appear topical from the standpoint of ensuring reliable and safe operation of NPP. Heatup can be calculated with the use of a particular model and compared with the result of the direct measurement of temperatures. Since the coolant heatup occurs in the array of gaps between the fuel elements, then the most immediate and inevitable impact on the result will be caused by the core structural elements’ production tolerances. With the low spread of basic operating parameters of the test facility in the experiment and low temperature measurement spread compared to the calculation result spread due to core structural elements’ production tolerances, the typical statistical characteristics of the spread of the results (due to deviations of key parameters within the range of tolerances) tentatively indicate the potential limit of calculation accuracy. The analysis of calculation results spread for a number of models may reveal a more plausible model of the phenomenon description.
Calculation was made of coolant heatup in the channels in the operation modes of the KM-1 reactor-test facility on the assumption of maintaining the design flow area of channels in high-power modes using three models of core flow distribution (design distribution, average flow rate in each channel, flow distribution assessed with the use of least-square method “individual flow rate”). Statistical characteristics of calculated heatup were determined via the comparison of calculated and controlled heatups.
The spread of calculation results with account for “individual flow rate” corresponding to the mode under consideration turned out to be minimum, but at the same time the statistical characteristics of disagreement with the experiment reached the maximum spread. In supplementing the “ind ividual flow rate” with the consideration of heat-induces distortion of heat-exchange channels using the proposed statistical model, the least calculation result spread was obtained, as well as the max imum agreement of statistical characteristics with the spread due to random deviations of input parameters within the range of production tolerances. The model of heatup formation in the core channels with account for heat-induces distortion of heat-exchange channels on these grounds appears most plausible.
The performed analysis has demonstrated that the formation of calculated heatup spread, in a comparable degree is caused by both the neglect of non-uniformity of coolant distribution over the core (as part of reactor header system, with design flow areas maintained), and the neglect of non uniformity of coolant distribution in the channels due to heat-induced distortion of core components.
core, fuel element, production tolerances, statistical characteristics, flow area, heatup, coolant, flow simulation, experiment
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