Riassunto analitico
Safety analyses for RBMK reactors has to consider accident scenarios causing the lost of design boundaries against the fission products release. Severe transients involve the fuel pin damage and the potential pressure tube rupture, as it was experienced in Leningrad NPP unit 1 in 1975, Chernobyl NPP in 1982 unit 1 and Leningrad NPP unit 3 in 1992. The evaluation of the consequences of the pressure tube rupture due to accidental overheating is one of the key elements to address an RBMK safety analysis.
The fuel pin damage arises in case of high temperature condition, occurring when inside the fuel channel the cooling process is reduced. In case of coolant mass flow reduction, different events are expected to take place: thermal hydraulic crisis, fuel and cladding overheating, coolant overheating and evaporation, pressure tube overheating, pressure tube failure, fuel rod damage, Hydrogen and fission products release.
The present work is finalised to assess numerical models for the investigation of the damaging mechanisms of the RBMK Pressure Tube and fuel pin. The conducted activity, based on the modelling of a single fuel channel and its surrounding graphite column, is aimed at evaluating the transient conditions enabling the different damaging phenomena addressed before, varying the percentage of coolant flow reduction and the channel power.
The analysis involves specific competences of Thermal-Hydraulics and Structural Mechanics. This need was covered employing different codes for the investigation of each field. In particular:
• the RBMK fuel channel, graphite column, and He-N2 gas gap are modelled with the RELAP5 code;
• the fuel pin behaviour is predicted by FRAPCON 3 and FRAPTRAN1-2 codes.
Numerical models, analysis capabilities of the linked codes, and obtained results are discussed; a validation activity has been conducted too, and together with a comparison with available data from Leningrad NPP 1992 accident.
The extensive analysis performed to comprehend the physical phenomenology, as far as possible, allows to draw a failure map identifying the range of FC accidental conditions in which the cladding and Pressure Tube safety is expected, as well as the range of the accidental parameters enabling the different damaging mechanisms. Further results are:
• the Relap5/Mod.3.3 code capability to predict thermal-hydraulic phenomena occurring in the considered transient has been verified;
• the FRAP code capability to predict thermal-mechanical phenomena occurring in the considered transient has been verified.
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