Thermomechanical Model and Bursting Tests to Evaluate the Risk of Swelling and Bursting of Modified 9Cr-1Mo Steel Steam Generator Tubes during a Sodium-Water Reaction Accident

The MECTUB code was developed to evaluate the risk of swelling and bursting of Steam Generator (SG) tubes. This code deals with the physic of intermediate steam-water leaks into sodium which induce a Sodium-Water Reaction (SWR). It is based on a one-dimensional calculation to describe the thermomechanical behavior of tubes under a high internal pressure and a fast external overheating. The mechanical model of MECTUB is strongly correlated with the kind of the material of the SG tubes. It has been developed and validated by using experiments performed on the alloy 800. A change to tubes made of Modified 9Cr-1Mo steel requires more knowledge of Modified 9Cr-1Mo steel behavior which influences the bursting time at high temperatures (up to 1200°C). Studies have been initiated to adapt the mechanical model and to qualify it for this material. The first part of this paper focuses on the mechanical law modelling (elasticity, plasticity, and creep) for Modified 9Cr-1Mo steel and on overheating thermal data. In a second part, the results of bursting tests performed on Modified 9Cr-1Mo tubes in the SQUAT facility of CEA are used to validate the mechanical model of MECTUB for the Modified 9Cr-1Mo material. 1. Introduction and Context In the Steam Generator, the heat exchange tubes are the only physical barrier between the secondary sodium and the water. If there is a leakage in the heat exchange tubes, then the water and the sodium are into contact; thus an exothermic chemical reaction occurs between the two reactants. This reaction produces sodium hydroxide and hydrogen. Overheating of SG tubes can be due to the exothermic reaction. The temperature of the products of the reaction is high and it can reach the vaporization temperature of the sodium hydroxide (1390°C under 1 bar). The boiling temperature of the sodium is 880°C with a pressure of 1 bar. The heat exchange between the hot reaction products and the neighboring tubes results in a rise of the temperature of these tubes. Then the mechanical characteristics of the heated tubes decrease significantly; this leads the creep phenomenon to become not negligible. Due to the internal pressure of the water steam, the heat exchange tubes become significantly strained (swelling by a few tens %). Then the swelled tube may burst in a few tens of seconds, involving large breaks. This kind of accident can occur if the initial leak has enough time to evolve towards secondary and then a large leak: puncturing of another tube by wastage and propagating to other tubes. This scenario is only possible if the detection is late