Analysis of the dose rate produced by control rods discharged from a BWR into the irradiated fuel pool

Full text: BWR control rods have an estimated life of 15 years. Afterwards, they must be withdrawn from the reactor and stored into the irradiated fuel pool. The storage is disposed to hang up 12 rods in a device specially designed for this goal. Different devices are installed at various positions throughout the pool. These devices hanging up the irradiated control rods are placed close to the surface and walls of the pool. Therefore, a non-negligible dose can be received in the area surrounding the pool edges. The MCNP code based on the Monte Carlo method has been applied to model this situation and to calculate dose rates at points of interest. In order to simplify the model just one control rod has been modelled and then duplicated at the 12 positions where they are hanged up. As well the model for the control rod is simplified considering for the whole absorbent part of the rod a mix of CB4, stainless steel (tube gains and lining of the control rod blades) and H2O, while the rod handle is modelled with its real composition of stainless steel. This simplification permits us to reduce the number of surfaces in the model so that the maximum allowed by the MCNP code is not exceeded. For each device hanging up irradiated control rods, dose rates are calculated using MCNP at different points into and out of the pool. They are then summed for each point to obtain the total dose rate at this point. The source considered for each control rod into the pool is the activity calculated by MCNP taking into account the neutron activation of the rod into the reactor. These results are presented in another paper at this conference. Results are compared with dose rate measurements performed into the pool and close to its edges