Separate-effects experiments in the framework of the QUENCH program at KIT

M. Steinbrück, M. Große, J. Stuckert

Karlsruhe Institute of Technology, Institute for Applied Materials, Germany

Abstract

The most important accident management measure to terminate a severe accident transient in a Light Water Reactor (LWR) is the injection of water to cool the uncovered degraded core. Analysis of the TMI‑2 accident and results of various integral in-pile and out-of-pile experiments (CORA, LOFT, PHEBUS, PBF) have shown that before the water succeeds in cooling the fuel pins there could be an enhanced oxidation of the Zircaloy cladding and other core components that in turn causes a sharp increase in temperature, hydrogen production and fission product release.

The QUENCH programme at Karlsruhe Institute of Technology (KIT) has been started 1996 to investigate hydrogen generation, material behaviour and bundle degradation during reflood. The series of integral bundle experiments (16 tests were performed so far) was supported by separate-effects tests (SET) and code analyses. The main objective of the programme is to deliver experimental and analytical data for the development of quench and related models and for the validation of SFD code systems.

This paper presents the highlights of the separate-effects experiments conducted during the last decade at KIT, the former Research Centre Karlsruhe (FZK). The following topics will be touched:

§ Single-rod quench experiments

§ High-temperature oxidation of various cladding alloys, including advanced ones, in various atmospheres (steam, oxygen, nitrogen, air, mixtures)

§ Hydrogen absorption by zirconium alloys

§ Effect of steam starvation

§ Boron carbide absorber behaviour during severe accidents

§ Single-rod tests on AgInCd control rods

§ ZrO₂ failure criteria and interaction of metal melt with zirconia (and urania).

Oxidation of various materials is of special interest because it causes degradation of mechanical properties of structure materials and it can be additionally connected with the production of hydrogen and heat. So, the oxidation of zirconium alloy cladding is the main hydrogen source term during a severe accident. Furthermore, chemical interactions of the various core materials lead to liquefaction of core components at temperatures far below melting points of the single materials. For example, interactions between boron carbide absorber and stainless steel results in rapid melt formation at about 1250°C. Local melts may initiate early core degradation with release of fission products and further enhanced exothermal oxidation.