Analysis of accidents leading to local flow stagnation in parallel fuel channels of RBMK-type reactors

• Published in: Nuclear engineering and design 2011-12, Vol.241 (12), p.5127-5137
• Main Authors: Kaliatka, A., Uspuras, E., Vaisnoras, M.
• Format: Article
• Language: English
• Subjects: Applied sciences; Channels; Controled nuclear fusion plants; Cooling systems; Energy; Energy. Thermal use of fuels; Exact sciences and technology; Fission nuclear power plants; Fuels; Installations for energy generation and conversion: thermal and electrical energy; Mathematical models; Nuclear engineering; Nuclear fuels; Nuclear power generation; Nuclear reactor components; Nuclear reactors; Preparation and processing of nuclear fuels
• ISSN: 0029-5493; 1872-759X
• DOI: 10.1016/j.nucengdes.2011.08.060

► The coolant flow stagnation in the fuel channels of RBMK-1500 nuclear reactor was analysed. ► The analysis was performed using RELAP5, system thermal-hydraulic computer code. ► The recommendations for operator actions to prevent fuel overheating are provided. ► The main ideas may be used for RBMK-1000 reactors, operating in Russia. The RBMK-type nuclear power reactors, still operating in Russia, are graphite-moderated with vertical fuel channels, using low-enriched nuclear fuel. The main challenge, which leads to the overheating of the fuel assemblies, fuel channels and other core components in channel type nuclear reactors, is a misbalance between heat generation in core structures and heat sink, which can appear due to the loss of coolant accident. In this accidental case, the emergency core cooling system ensures the core cooling. In RBMK-type reactors this system consists of hydro-accumulators and a number of pumps, taking water from large water reservoirs. This equipment injects water into the fuel channels through the group distribution headers at high pressure. However, the direct supply of cold water from emergency core cooling system into fuel channels is possible only if check valves on group distribution headers are closed properly. If these check valves failed, the part of water would be lost through the break, the flow stagnation in channels could occur, which might lead to overheating of fuel assemblies in the fuel channels. This paper presents the results of deterministic safety analysis, performed using system thermal hydraulic code RELAP5. Using this code the reactor cooling system of RBMK-1500 was modelled and analyses of loss of coolant accidents with failure of few check valves in group distribution headers were performed. The results of the calculations are used for the development of symptom-based emergency operating procedures for RBMK-1500. The basic principles that describe the complex distribution of water flows in vertical forced circulation circuit with parallel fuel channels can be adjusted for the RBMK-1000 reactors, still operating in Russia.