Study of transients in a natural circulation liquid metal cooled ADS

The objective of this paper is to study the transient behavior of an accelerator driven system (ADS) cooled by natural circulation of liquid metal in the primary circuit. The case studied is based on the conceptual design of 1500 MWth fast energy amplifier presented by the European Nuclear Research Centre (Rubbia, et al., 1995). The mathematical model integrates computational fluid dynamics (CFD) with neutronics. CFD is used to solve the natural circulation flow including the turbulence model. A sub-model of the core calculates the heat transferred between the fuel pins and the coolant in the hydraulic channel and also determines the reactivity feedback. Neutronics is modeled with point kinetics theory including six groups of neutron precursors and a decay heat model according to the ANS standard (1979). The analyzed transients are characteristic scenarios of nuclear power plants operation, e.g. start up, shut down and lost of heat sink. The calculations are performed for two different coolants (sodium and lead-bismuth) that can operate within the same reactor geometry and offer similar performance during steady operation (Ceballos, et al. 2004). The results show that inherent safety capabilities of natural circulation are questionable during some of the events. High cyclic loads in the cladding and structures, thermal shock and high thermal stress can be foreseen in this type of reactor; therefore, additional solutions are required in order to assess the safe operation. (author)