MELCOR 1.8.1 calculations of ISP31: The CORA-13 experiment

Abstract

The MELCOR code was used to simulate one of GRS's (a reactor research group in Germany) core degradation experiments conducted in the CORA out-of-pile test facility. This test, designated CORA-13, was selected as one of the International Standard Problems, Number ISP31, by the Organization for Economic Cooperation and Development. In this blind calculation, only initial and boundary conditions were provided. The experiment consisted of a small core bundle of twenty-five PWR fuel elements that was electrically heated to temperatures greater than 2800 K. The experiment comprised three phases: a 3000 second gas preheat phase, an 1870 second transient phase, and a 180 second water quench phase. MELCOR predictions are compared both to the experimental data and to eight other ISP31 submittals. Temperatures of various components, energy balance, zircaloy oxidation, and core blockage are examined. Up to the point where oxidation was significant, MELCOR temperatures agreed very well with the experiment --usually to within 50 K. MELCOR predicted oxidation to occur about 100 seconds earlier and at a faster rate than experimental data. The large oxidation spike that occurred during quench was not predicted. However, the experiment produced 210 grams of hydrogen, while MELCOR predicted 184 grams, which was one of the closest integral predictions of the nine submittals. Core blockage was of the right magnitude; however, material collected on the lower grid spacer in the experiment at an axial location of 450 mm, while in MELCOR the material collected at the 50 to 150 mm location. In general, compared to the other submittals, the MELCOR calculation was superior.

MELCOR 1.8.2 assessment: The DF-4 BWR Damaged Fuel experiment

“…These results indicate that MELCOR may have over-predicted oxidation during the melting stage and under-predicted oxidation after debris had been formed. Tile same results for hydrogen production were also noted in the MELCOR model of the CORA-l3 experiment [13]. The total hydrogen production calculated by MELCOR was 36.4 grams, which was well within the range of 33--40 grams given by the DF-4 PIE [6].…”

“…The results showed t09 sensitivity to this parameter, and it is recommended that this parameter be investigated as a part of any MELCOR modeling effort. (Both the LOFT LP-FP-2 [11] and the CORA-13 [13] assessments adjusted this parameter in the base case models. )…”