Comparative Study of Primary-to-Secondary Leakage in a SG in case of Multiple Tube Ruptures Using RELAP5/Mod3.3

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Abstract This study investigates a Steam Generator Tube Rupture (SGTR) scenario in a VVER-1000 / B320 Nuclear Power Plant (NPP), using the RELAP5 / MOD3.3 thermal-hydraulic code. The analysis is based on Unit #6 of the Кozloduy NPP. The scenario considers natural disasters induced failure leading to multiple SGTR, causing primary coolant leakage into the secondary side. The consequence of that will increase secondary-side pressure and may cause the opening of the Steam Dump Valve to the Atmosphere (BRU-A). A critical case examined when the BRU-A remains open, allowing irreversible release of radioactive coolant to the environment and loss of reactor cooling capability. Calculations are carried out up to a core exit temperature of 1200 °C, at which zirconium–steam reactions become self-sustaining. This article presents thermohydraulic analyses of primary-to-secondary leakage under a range of scenarios involving double-ended tube ruptures in a Steam Generator (SG) #1, located on the cold collector side. The initiating event considered is defined as a double-ended break involving a variable number of tubes in SG#1. Two cases were analysed: a double-ended rupture of 270 tubes (Case #1) and a double-ended rupture of 540 tubes (Case #2). The reference case is based on a previously investigated scenario [1], involving a double-ended rupture of 270 tubes in SG#1 on the cold collector side. In the model, each tube rupture is represented by an equivalent break with an inner diameter of 13 mm. The analysis shows the influence of additional rupturing tubes to the initial number of tubes which could open the BRU-A. Results highlight additionally the complex interaction between primary system depressurization, secondary system pressurization, radioactive contamination of the secondary loop, and loss of coolant inventory. The study underscores the critical importance of accurate event diagnosis and timely operator response, as inappropriate actions may compromise containment barriers and increase the risk of radiological release.

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