Core reflooding: Synthesis of the QUENCH program and its impact on code modelling

European Review Meeting on Severe Accident Research 2005 (ERMSAR-05), Aix-en-Provence, 14-16 Novembre 2005

M. Steinbrück, W. Hering, J. Stuckert, J. Birchley, E. Brunet-Thibault, T. Drath,
N. Seiler, K. Trambauer, M.S. Veshchunov.

The paper gives an overview on the status of the QUENCH program at FZK, including complementary bundle experiments and separate-effects tests, and in the second part, a discussion of the status of the main severe accident code systems used in Europe. The main objective of the program is to deliver experimental and analytical data to support development and validation quench and related models as used in code systems.
So far, ten QUENCH bundle tests have been performed. The main parameters of these experiments were: degree of pre-oxidation, initial quench temperature, reflood medium (water or steam) and flooding rate, influence of B4C absorber, steam starvation and air ingress. In four tests reflood of the bundle caused a temporary temperature excursion connected with the release of a significant amount of hydrogen, typically two orders of magnitude greater than in those “successful” quench tests in which cool-down was immediately achieved. Strong and global formation, relocation, and oxidation of melt were observed in all tests with escalation. The temperature boundary between rapid cooldown and temperature escalation was typically 2100-2200 K in the "normal" quench tests, i.e. tests without absorber and/or steam starvation. These factors were found to lead to escalation at lower temperatures.
All phenomena occurring in the bundle tests have been additionally investigated in parametric and more systematic separate-effects tests.
Seven SFD code systems are discussed with respect to the phenomena relevant to reflood scenarios: reflood progression, oxidation of zirconium alloy cladding materials, boron carbide absorber rods and melts, and the oxidation of Zircaloy cladding in air containing atmospheres. The codes are generally able to describe the reflood scenario satisfactorily as long as the bundle has an intact geometry. Recently, progress has been made regarding the modelling of B4C absorber oxidation, whereas the treatment of the oxidation of relocating/relocated melts as well as the oxidation in atmospheres containing air is under development or foreseen in the future for the most codes.