A Validation Model for the Transient Analysis of Tightly Coupled Reactors

Both the static and transient analysis of tightly coupled reactors differ from those of the loosely coupled systems, requiring use of at least two-dimensional multigroup, transport methods. Recently a few-group transient nodal method has been developed for tightly coupled reactors. Although the accuracy of the static part of this model can be tested easily, in the absence of a time dependent transport code, direct validation is not possible. In this study, an indirect test, making use of a static, multigroup, discrete ordinate code, to measure the accuracy of the few-group nodal (as well as any other approximate method) has been developed. The validation is based on the fact that, at any stage during a transient calculation, the transient equations can be converted to a static eigenvalue problem by replacing the time derivatives of group-flux and delayed precursor concentration with instantaneous "frequencies" multiplied into the quantities themselves. These time constants are edited at any time step from the transient analyzed. A test of whether the nodal transient calculation would agree with a transient transport calculation is then to see if the dynamic frequencies inserted into the time-dependent equation (thereby converting it to a static equation) will yield an eigenvalue of unity. Moreover