Pre-test calculational support for the QUENCH-13 experiment

The QUENCH experimental programme at FZ Karlsruhe investigates phenomena associated with reflood of a degrading core under postulated severe accident conditions, in the early phase where the geometry is still mainly rod-like. The latest large-scale bundle test, QUENCH-13, is the first in this programme to include a silver/indium/cadmium (SIC) control rod of prototypic PWR design. The effects of the presence of the control rod on early-phase degradation and on reflood behaviour are examined under integral conditions, while the opportunity is taken to measure, in realistic geometry, release of SIC aerosols following control rod rupture. These materials can affect the chemistry of fission products in the reactor circuit, and hence the radioactive source term to the environment in the event of containment failure. In particular, the sharp release of cadmium on control rod failure, which can involve some tens of percent of the inventory, is ill-defined experimentally. Pre-test calculations were performed to determine the test boundary conditions, such as the electrical power history to the bundle, the coolant flow, and the reflood timing and rate. The aim was to stabilise the bundle at maximum temperature of 1250 K, then ramped at about 0.25 K/s to give the best chance to measure the control rod aerosol release under controlled conditions, then to reflood, without provoking an oxidation excursion, at maximum bundle temperature of 1800-1850 K. A further aim was to check thermal conditions in the offgas pipe, where the aerosol instrumentation was situated. The calculational support was organised through the Source Term area of the EU 6 th Framework Network of Excellence SARNET, linking the experimental team at FZK with modellers at PSI, GRS and EdF. Following agreement of the target test conditions, the modelling teams used SCDAPbased codes, ATHLET-CD and MAAP4 respectively to help the definition of the test boundary conditions, and in the latter two cases to estimate the control rod aerosol release. The facility models used were benchmarked against data from previous QUENCH tests, while also the ATHLET-CD release modelling was checked against Phebus FP data. The experimental protocol took account of the recommendations from the pre-test studies. Benefit was gained in the cooperation through the use of independent codes by different organisations, in lending confidence to the test predictions, and in obtaining different perspectives on the test conduct. The experiment was successfully performed according to the agreed specification on 7 November 2007, and the results are to be analysed on a collaborative basis. Post-test calculations are planned following release of the definitive results. to t si on se ct