Improving the RETRAN-02 core modelling for the simulation of the rod ejection accident

The RETRAN-02 code was modified to accept a time dependent multidimensional power distribution as well as a time-dependent spatial Doppler and moderator feedback effects. This permits the use of the kinetic equations to calculate the reactor power. The time dependent power distribution function is obtained from an approximation to the 3-D time dependent diffusion equation. The code was then used to simulate the rod ejection accident (REA). A three RETRAN core model was developed. The first represents a single assembly from which the control rod is ejected; the second core region represents eight subassemblies surrounding the 'ejected' assembly; the third region represents the rest of the core. The improved modelling of the core performed in the present study, reveals that in addition to major differences in the hot channel state variables, there is also a difference in the system average transient parameters, such as power, pressure, flow and temperatures. The major impact is however the possibility to observe the transient behavior of the subassembly from which the control rod was ejected, as well as the subassemblies surrounding the 'ejected' assembly. The REA was analyzed under four different operating conditions, namely at the beginning and end of life at full and zero power. A sensitivity for the most important input parameters was also performed. From the results it was concluded that the HZP-EOL case produce the most serious consequences, however, even this case meets the imposed design limits. (author) 2 tabs., 18 figs., 16 refs