RESULTS OF THE QUENCH-09 EXPERIMENT COMPARED TO QUENCH-07 (LWR-TYPE TEST BUNDLES WITH B4C ABSORBER)

The QUENCH experimental program at the Karlsruhe Research Center is to investigate the hydrogen source term that results from quenching an uncovered core, to examine the physico- chemical behavior of overheated fuel elements under different flooding/cooling conditions, and to create a data base for model development and code improvement. The QUENCH-07 and -09 test bundles consisted of 21 rods, 20 of which were electrically heated over a length of 1024 mm. The Zircaloy-4 rod cladding and the grid spacers were identical to those used in Western-type LWRs whereas the fuel was represented by ZrO2 pellets. In both experiments the central rod was made of an absorber rod with B4C pellets and stainless steel cladding, and of a Zircaloy-4 guide tube. The absorber rod failed at the same temperature in both experiments, i.e. at ~1555-1585 K. After a B4C oxidation phase at ~1720-1780 K and a subsequent transient test phase to well above 2000 K, cooling of the test bundle was accomplished by injecting saturated steam at the bottom of the test section. The presence of the B4C absorber material in the central rod triggers the formation of eutectic melts, i.e. melts that are formed far below the melting point of metallic zircaloy (~2030 K), and the oxidation of boron/carbon/zirconium-containing melt can lead to increased amounts of hydrogen, compared to a bundle without control rod. The total amount of hydrogen released during the flooding, i.e. cooling, phase was, however, significantly larger in QUENCH-09 (~400 g) than in QUENCH-07 (~120 g). It is conjectured that it is mainly the period of steam starvation prior to the cooling phase of QUENCH-09 (steam flow reduction from 3.3 to 0.4 g/s for a duration of ~11 minutes) that caused the enhanced zirconium oxidation in the cooling phase of QUENCH-09.