chapter  9
18 Pages

Design Basis

Sodium coolant remains in liquid state up to about 1160 K at ambient pressure, while the normal operating temperature in sodium-cooled fast reactor (SFR) does not generally exceed ~820 K. Hence, there is no need of pressurization to maintain the sodium in liquid state. This is why the design pressure of SFR system components is low. However, there exists a large temperature difference across the core (~150°C) between inlet and outlet sodium. This large temperature also reflects in the hot and cold legs of the sodium piping. In the case of pool-type reactors, this temperature difference coexists in the hot and cold sodium pools housed in the reactor vessel. During thermal transients such as power failure and pump trip event, the hot pool components and hot leg piping could face colder shock and the cold pool components and cold leg piping could face hot shock. In view of high heat transfer characteristics of sodium, the rapid changes occurring in the coolant are transmitted completely to the structure with insignificant film drop. Apart from these, austenitic stainless steels, the structural materials generally chosen, have low thermal conductivity, low thermal diffusivity, and large thermal expansion coefficients. These are factors responsible for causing certain structural mechanics failure modes of specialized nature.