A Transient Subchannel Analysis Code

8 Development of a Transient Subchannel Analysis Code and Application to Flow Decreasing Events 

The radial heat transfer model is almost the same as that of SPRAT-DOWN (see Fig. 4.2.6). The coolant enthalpy is calculated by solving an energy conservation equation. The heat fluxes on the fuel rods and the water rod walls that have been calculated at the previous time step are utilized the same as in SPRAT-DOWN. This does not cause a problem by keeping the time step reasonably short. 

The semi-implicit scheme is chosen to solve the mass and momentum conservation equations where only the pressure is implicitly solved. The Runge-Kutta 

Setting of the fuel power, inlet temperature, mass and pressure  

The advection of coolant in a time step is limited below one-third of the mesh size, which means that the Courant number is below one-third. The diffusion in a time step is limited below one-third of the mesh size, which means that the diffusion number is below one-third. 

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The change of cross flow within a subassembly may occur during transients. The MCST may change from the result of the single-channel calculation. A transient subchannel analysis code was developed and the safety analysis of a Super LWR was carried out [70]. The temperature rises from the steady state value are about 20°C at the abnormal transients and about 130°C at accidents. The maximum values still stay below the MCST criteria for transients and accidents. The development and application of the transient subchannel analysis code are sununarized in Sect. 6.8. 

K. Yoshimura, Development of a Transient Subchannel analysis code and safety analysis for the Super LWR, Graduate thesis, the University of Tokyo (2009) (in Japanese)... 

In order to investigate the influence of cross flow in the fuel assemblies on the safety margin, a transient subchannel analysis code for the Super LWR is prepared. It is found that the safety margin decreases at the flow decreasing events by considering the cross flow but the Super LWR still has a considerable safety margin. 

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