Nuclear Technology / Volume 154 / Number 1 / April 2006 / Pages 107-116
Technical Paper / Materials for Nuclear Systems
The purpose of 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 physicochemical behavior of overheated fuel elements under different flooding/cooling conditions, and to create a database 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 1.024 m. The Zircaloy-4 rod cladding and the grid spacers were identical to those used in Western-type light water reactors (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. Failure of the absorber rod cladding was detected at the same temperature in both experiments, i.e., at ~1555 to 1585 K. After a B4C oxidation phase at ~1720 to 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 and to production of CO, CO2, and CH4 compared to a bundle without a control rod. The total amount of hydrogen released during the flooding, i.e., cooling, phase was, however, significantly larger in QUENCH-09 (~0.400 kg) than in QUENCH-07 (~0.120 kg). 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 min) that caused the enhanced zirconium oxidation in the cooling phase of QUENCH-09. This is the revised and updated version of the paper that was presented at the 2004 International Meeting on LWR Fuel Performance in Orlando, Florida, on September 19-22, 2004, under the title "Results of the QUENCH-09 Experiment Compared to QUENCH-07 (LWR-Type Test Bundles with B4C Absorber)."