RELAP5 Analysis of Two-Phase Decompression and Rarefaction Wave Propagation Under a Temperature Gradient

The capability of RELAP5 to model single- and two-phase acoustic wave propagation is demonstrated with the use of fine temporal and spatial discretizations. Two cases were considered: a single-phase air shock tube problem that was simulated, resulting in a shock wave and a rarefaction wave that lie within 1% error of the analytic solution, and pressure oscillations observed by Takeda and Toda in a two-phase decompression experiment in a pipe under a temperature gradient.
Whereas the agreement for the single-phase case is excellent, some discrepancies were observed in the two-phase case:

1. Thermal nonequilibrium and the associated delay in the bubble growth were identified as the cause for the dispersion of the rarefaction wave as it becomes trapped inside a two-phase fluid region. The short timescale of the experiment justifies the use of a bubble diameter that is one order of magnitude smaller than the standard RELAP5 predicted bubble diameter, which is calibrated for longer transients.

2. The initial depressurization undershoots seen in the Takeda and Toda experiment were overpredicted by the RELAP5 model. Improved agreement with the experiment was obtained by altering the discharge coefficient in the choked flow model to account for uncertainties in the discharge geometry and/or the choked flow model at low pressure.

By adjusting these parameters RELAP5 produced markedly better comparisons with the experimental data. These results illustrate two generic shortcomings of nuclear reactor system codes, i.e., the absence of a dynamic model for the interfacial area concentration and uncertainty in two-phase choked flow modeling. However, it is remarkable that RELAP5 could predict the complex dynamics of the two-phase acoustic phenomena in the Takeda and Toda experiment in spite of these shortcomings.