Yuanzineng kexue jishu (Oct 2023)
Study on Nitrogen Migration and Release Models under Pressure Relief Condition during LOCA
Abstract
Nitrogen pressurization is widely used in nuclear reactor (such as passive nitrogen-type accumulators and nitrogen regulator) because of passive mechanism. However, nitrogen pressurization brings new challenges. Loss of coolant accident (LOCA) can lead to rapid reduction of pressure in primary circuit of pressurized water reactor (PWR), putting dissolved nitrogen in the water into a supersaturated state, which contributes to nitrogen release. In order to prevent the accumulation of released nitrogen, which may act as a threat to reactor safety, the migration and release characteristics of nitrogen were studied. In this paper, a nitrogen wall release model was established, the physical significance of the release source term was explained and the specific process of nitrogen release was revealed. In the case of LOCA in primary circuit of a PWR, the pressure in the primary circuit decreases and the susaturated dissolved nitrogen degas from the liquid and form nitrogen bubbles on nucleate sites. The susaturated dissolved nitrogen continuously degas at the gas-liquid interface of the bubble, leading to the sustained growth of nitrogen bubble. After reaching departure diameter, the buoyancy force on the nitrogen bubble is greater than surface tension and it escapes from the wall finally. In this paper, the mathematical modeling of nitrogen dissolution equilibrium, dissolved nitrogen convection and diffusion, the susaturated dissolved nitrogen wall release and nitrogen bubbles upward floating was carried out. Combined with the one-dimensional lumped parameter method, the mass conservation of the control volume was analyzed and the one-dimensional nitrogen mass conservation equation was deduced. To solve the one-dimensional nitrogen mass conservation equation, a one-dimensional lumped parameter nitrogen migration solution program was written based on MATLAB. The program was verified by the experimental data of small break loss of coolant accident (SBLOCA). The trends of dissolved nitrogen concentration and void fraction under pressure relief condition during SBLOCA were simulated. The results suggest that susaturated nitrogen degas on the wall in the form of bubbles. The bubble grows at a rate proportional to t1/2 and also proportional to (C-Ci)12. The influence of the initial nucleation radius on the growth period of the bubble is negligible and the bubble rises uniformly in the main stream after departure. Under pressure relief condition during LOCA, the nitrogen wall release effect is significant and the diffusion process of dissolved nitrogen is weak. Controlled by nitrogen wall release, the average concentration of dissolved nitrogen decreased by 19.5% and the void fraction became 4.33 times of the original, which indicates that wall release has significant effect on circuit, potentially affecting reactor safety. The simulation results are in good agreement with experimental data of SBLOCA, proving the reliability of wall release model and program.
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