Investigation of NACOK air ingress experiment using different system analysis codes

Abstract: Air ingress into to the core after the primary circuit depressurization due to large breaks of the pressure boundary is considered as one of the severe hypothetical accidents for the high temperature gas-cooled reactor (HTR). If the air source and the natural convection cannot be impeded, the continuous graphite oxidation reaction along with the formation of burnable gas mixtures resulting in the corrosion of the fuel elements and the reflectors might damage the reactor structure integrity and endanger the reactor safety. In order to study the effects of air flow driven by natural convection as well as to investigate the corrosion of graphite, the NACOK (Naturzug im Core mit Korrosion) facility was built at Jülich Research Center in Germany. A complete 2A-rupture of the coaxial duct in the HTR primary system, as well as the chimney effect caused by breaks in both upper and lower parts of the pressure boundary was simulated in the test facility. Several series of experiments and the related code validations (TINTE, DIREKT, THERMIX/REACT, etc.) have been performed on this facility since the 1990s. In this paper, the latest NACOK air ingress experiment, carried out on October 23, 2008 to simulate the chimney effect, was preliminarily analyzed at NRG with the SPECTRA code, as well as at INET, Tsinghua University of China with the TINTE code. The calculating results of air flow rate of natural convection, time-dependent graphite corrosion, and temperature distribution are compared with the NACOK test results. The preliminary code-to-experiment and code-to-code validation successfully proves the code capability to simulate and predict the air-ingress accident. In addition, more research work, including parameter sensitivity analysis, modeling refinement, code amelioration, etc., should be performed to improve the simulation accuracy in the future. [Copyright &y& Elsevier]