Permissible temperature in Fast Breeder Reactor vault concrete

This paper presents a detailed discussion on alterations in concrete properties and microstructure following exposure to moderately elevated temperatures after hardening. The normal aggregate concrete used in the vaults region of a 500 MeV commercial Prototype Fast Breeder Reactor (FBR) is designed for a continuous exposure temperature of around 63°C during normal operation but during power shutdowns the temperature could rise up to 83 °C. Because of the scarcity of information on the effects of high temperature on concrete and risks involved in the operation of reactors of this kind, the current practice is to provide an elaborate and often costly cooling system to control the temperature of the concrete within the limits permitted in the standards (65°C). In order to have a less stringent design of the cooling systems and for ensuring structural safety, the effects of sustained high temperatures below 100°C on concrete properties need to be understood. The amount of moisture in concrete and its migration are the two significant factors that affect the mechanical properties at high temperatures. Microstructural studies by scanning electron microscopy clearly indicate a drying out of the inner C-S-H, suggesting that the moisture from the gel pores migrates to the capillary pores. The proposed increase in the concrete vault temperature is based on the engineering properties such as compressive strength, modulus of elasticity and bond strength that can be assigned to the normal concrete while following the Indian regulatory guidelines of a Nuclear Power Plant (NPP) design.