Application of the Monte Carlo method to develop a model to estimate the internal contamination of pipes in a nuclear reactor

The dose reduction program at a Nuclear Power Plant (NPP) includes the utilization of a gamma spectrometry device for on-site measurements. The equipment uses a heavily shielded HP Germanium detector coupled with high count-rate pulse-height counting electronics. The on-site spectra acquisition is an essential tool to monitor the radionuclide concentration deposited onto the inner side of the process pipes of the reactor. The determination of this activity is very important to plan the ALARA actions that must be taken in order to decrease the collective doses received by exposed professional workers. Nevertheless, the analysis using direct measurements presents the problem of detector calibration, because they have to be done in places with difficult access and high dose rates. It is convenient to apply a computer solution, such as the Monte Carlo method, for the detector efficiency calibration. A model has been developed using the MCNP code, based on the Monte Carlo method. Results have been compared with experimental measurements in order to validate the model. Validation for point and disk sources as well as for volumetric sources has been published in previous works. In this paper, the model developed for the calibration of the detector to perform contamination measurements in a pipeline is presented. Results of simulation should be compared with experimental measurements using a portion of pipeline contaminated at its inner surface with some radionuclides covering a wide energy range. The validation of the model will permit to extend it to other situations throughout the plant.