Code-to-code Comparison between ATHLET-CD and MELCOR for SBLOCA Severe Accident Scenario in Generic German PWR

The evolution of a hypothetic SBLOCA severe accident scenario for a generic German PWR of type Konvoi is investigated by means of the two severe accident computer codes ATHLET-CD and MELCOR. The simulation results derived from the both codes are compared and possible reasons for deviations between the results are analyzed.
For the assessment of scenarios with core degradation, correct prediction of physical phenomena and timings, such as beginning of core degradation, time of relocation of molten core material to lower head or time of vessel failure, is of particular interest. Validation of the obtained simulation results against experimental data is very limited simply due to the fact of unavailability of full integral experiments, especially for the late in-vessel phase of severe accident scenarios. Therefore two equivalent plant models with similar geometry, initial and boundary conditions were build, one for ATHLET-CD 3.0A and one for the MELCOR 1.8.6 code. A 50 cm² cold leg SBLOCA scenario was selected for the code-to-code comparison. Due to the assumed multiple failures of safety systems, the scenario develops into a severe accident scenario with beginning of core melting approximately 3 h after beginning of the transient.
The general comparison of the main parameters between both codes shows that qualitatively good agreement is reached. Similar time spans until the start of core heat-up are predicted by both codes. Initiation of zircaloy oxidation and the total amount of produced hydrogen agree. During the late in-vessel phase more significant deviations are identified, e.g. in the process of relocation to lower head.
In a second calculation, the effectiveness of mobile pump injection to the primary circuit as additional accident management measure is investigated. The results obtained by both codes show concordantly that a commercial high power fire pump system can be applied to stop the progression of the accident, with only slight degradation of the absorber rods.