Key technological design study of a large LMFBR (I); Improvement of reactivity feedback modeling in SCC-L and analysis of plant thermal hydraulic behavior during ATWS accident

not registered; Ohshima, Hiroyuki

Reactivity Feedback Modeling in Super System Code (SSC) has been improved to analyze the whole plant thermal hydraulic response to an anticipated transient without scram (ATWS) in a liquid metal fast breeder reactor (LMFBR). First of all, two-dimensional (2D) fluid flow and heat transfer modeling of reactor upper plenum (UP) has been modified. The heat transfer between the coolant and the control rod driveline (CRD) can be evaluated based on the 2D, not one-point, temperature distribution calculated by the UP model. The CRD is included as a part of in-plenum structure, and the thermal expansion of it is evaluated assuming the elongation is proportional to the temperature rise of the CRD. The reactivity feedback effect is evaluated using the elongation and the control rod worth. SSC-L is now capable of treating the following reactivity feedback effects caused by; (1)fuel doppler, (2)sodium density, (3)fuel axial expansion, (4)thermal expansion of the in-core structure, (5)thermal expansion of the core support structure, and (6)thermal expansion of the CRD. Whole plant thermal hydraulics during the ATWS accident can be analyzed taking the reactivity feedback effect into consideration more realistically than ever. An ATWS accident, i.e. unprotected loss-of-heat-sink (ULOHS), has been analyzed using SSC-L as an example. It is impossible to mitigate the ATWS consequence without core damage if no design change is made. However, it is found that more than 7 minutes of grace time is available for the remedial action at least if the above mentioned reactivity model is used. The accident progression is not so rapid in general in the ULOHS accident. The relatively slow response implies reactor shutdown can be achieved by a manual scram or a shutdown system actuated at slower speed can be utilized for mitigating the ATWS consequence.In the present analysis of CRD thermal expansion, it is assumed that the deformation of the CRD is one-dimensional, linear and elastic. It ...