12.9. Interaction with Other SASSYS-1 Models

As mentioned in the introduction to this chapter, the voiding model interacts directly with several other major modules in SASSYS-1. This section will provide a more detailed discussion of these interactions.

The voiding model first makes use of information from another module by accessing initialization data from the prevoiding calculation. Quantities such as liquid sodium temperatures, pressures, and mass flow rates are computed in the prevoiding module and passed to the voiding model coding, once the prevoiding calculation has predicted that initiation of voiding is imminent. The voiding model then begins interacting with the transient heat-transfer module at the end of each heat-transfer time step. The heat-transfer calculation provides the cladding and structure axial temperature profiles required by the voiding model (\(T_{e}\) and \(T_{s}\), respectively). The voiding model estimates these temperatures at the end of the new heat-transfer time step by extrapolating the cladding and structure temperatures from the ends of the previous two heat-transfer time steps. It then computes the advanced time cladding and structure temperatures for each coolant time step by interpolating between the temperatures at the beginning and the end of the heat-transfer time step. During the course of the heat-transfer time step, the voiding model computes the time-integrated heat fluxes from the cladding (\(E_{e}\)) and the structure (\(E_{s}\)) and supplies them to the transient heat-transfer calculation at the end of the heat-transfer time step.

Reactivity feedback due to coolant voiding is provided by the voiding model through calculation of a spatially smeared liquid sodium density for each axial mesh segment. This density is passed to the point kinetics module, where it is used to compute the sodium void fraction in each segment. The reactivity change due to voiding is then computed from

(12.8‑1)

\[\rho_{c} = \sum_{I}^{}{\sum_{j}^{}{\left( \rho_{c} \right)_{jI}\alpha_{jI}}}\]

where

\(\left( \rho_{c} \right)_{jI} =\) reactivity worth of sodium in axial segment \(j\) of channel \(I\) (input through variable VOIDRA)

and

\(\alpha_{jI}\) = average sodium void fraction in segment \(j\) of channel \(I\).

Although the point kinetics module does not feed any information back directly to the voiding model, it does supply the transient heat-transfer module with information about the change in power, which in turn affects the computation of the cladding temperatures, and so the point kinetics model does indirectly influence the voiding model.