تحقیق از آزمایش نفوذ هوا NACOK با استفاده از کدهای مختلف تجزیه و تحلیل سیستم

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.

Air ingress accident, which will result in graphite oxidation reaction of fuel elements and reflectors so as to weaken the structural strength, damage the graphite structure integrity of the core bottom, impact retention capacity of coated particle and produce the flammable gas mixtures, is considered as one of the severe hypothetical accidents for the high temperature gas-cooled reactor (HTR). The heterogeneous chemical reactions between graphite and oxygen include: Complete oxidation View the MathML sourceC+O2→CO2 ΔH=−394 kJ/mol Turn MathJax on Incomplete oxidation View the MathML source2C+O2→2CO ΔH=−222 kJ/mol Turn MathJax on Boudouard reaction View the MathML sourceC+CO2↔2CO ΔH=171 kJ/mol Turn MathJax on The main homogenous reaction referring to the air ingress accident is: View the MathML source2CO+O2→2CO2 ΔH=−564 kJ/mol Turn MathJax on With low steam content in the gas mixture, the effect of the following reactions can be almost ignored: Water gas reaction View the MathML sourceH2O+C⇔H2+CO ΔH=131 kJ/mol Turn MathJax on Water shift reaction View the MathML sourceH2O+CO⇔H2+CO2 ΔH=−41.6 kJ/mol Turn MathJax on The mass transfer and diffusion play important role in the reaction rate between the gas and the porous graphite. Accordingly, the chemical reaction could be divided into the following three types: I. CR (chemical regime) at low temperature The reaction rate is very slow, the gas transfer process in the graphite can be neglected and the reaction takes place in total graphite homogeneously. II. IPDR (in-pore diffusion controlled regime) at middle temperature The gas transport process cannot be neglected. The porous diffusion and chemical reaction both determine the gas convection rate. III. BLDR (boundary layer diffusion controlled regime) at high temperature The chemical reaction takes place at the graphite boundary and the boundary layer diffusion determine the corrosion rate. The temperature range for above three reaction types are approximately as follows: CR: <500 °C IPDR: 500–900 °C BLDR: >900 °C The graphite/gas reaction characteristics are described as shown in Fig. 1 (Moormann et al., 1982).The NACOK (Naturzug im Core mit Korrosion) facility was built at Jülich Research Center in Germany to study the oxidation behavior of graphite blocks with air flow driven by natural convection (chimney effect) in the event of a complete rupture of the coaxial duct or a tube break in the HTR primary system. This paper presents a preliminary analysis performed to investigate the NACOK air ingress experiment carried out on October 23, 2008 (Nissen et al., 2008), which simulates the chimney effect and studies the corrosion of graphite blocks with two different geometries and types. A detailed simulation model of this NACOK facility was developed at NRG with the SPECTRA code and at INET, Tsinghua University of China, with the TINTE code. The calculations were performed to validate those codes for the HTR air ingress scenarios.

The main findings from the present studies are: • Results are very sensitive to the oxidation model applied. • The best results are obtained when the oxidation correlation of Roes is applied. This is the only correlation which takes into account gas velocity. The effect of gas velocity on oxidation should be studies in the future. The simulation results show that SPECTRA and TINTE codes are capable to model the NACOK experiment with a good agreement, and the preliminary code-to-code validation is also successfully carried out. The analysis results will be further investigated for study of the fundamental phenomena and prediction of the consequences of such an air-ingress accident. Besides, more research work, including parameter sensitivity analysis, modeling refinement, etc., should be performed to improve the simulation accuracy in the future.