Improvements in residual heat removal reliability in the GCFR demonstration plant

Reliability of decay heat removal is an important safety consideration in the gas-cooled fast breeder reactor (GCFR). The design evolution of the residual heat removal (RHR) systems over the past few years has been markedly aided by system reliability analyses to the point where there is confidence that loss of coolable core geometry can be classified as a beyond-design-basis accident. This evolution proceeded in three steps. First, the reliability-limiting features in the total combination of RHR systems were investigated and the need for improvements in the reliability of the main loop cooling system for RHR as well as in the physical separation of RHR support systems between the main loops and the core auxiliary cooling systems (CACS) was established. Secondly, a wide range of RHR options for the main loop cooling system were investigated resulting in the adoption of a new reference concept for the main loop RHR system. The third and last step then consisted of an evaluation of the reliability aspects of natural circulation decay heat removal in an upflow GCFR design. The major conclusion from this study is that decay heat removal can be reliable in the GCFR. Furthermore, the current limitations of natural circulation RHR reliability have been identified, and means to optimally exploit natural circulation have been defined