TFTR Basement Radiation Fluence Calculations and Comparisons with Measurements

Abstract

The TFTR diagnostic basement, because of the existence of hundreds of penetrations through the roof and of various highly irregular structures in the area, is a typical environment where accurate modeling is particularly difficult. The detailed three-dimensional Monte Carlo calculations of the basement radiation fluences and comparisons with measurements that we have undertaken represent a severe test on the capabilities of some of the widely used radiation transport codes. The albedo version of the MORSE Monte Carlo code was employed to calculate the total neutron fluence above cadmium cutoff, which could be compared with the Bonner sphere measurements. Calculations of the detailed neutron and gamma-ray spectra, which could be compared with the NE-213 measurements were carried out with the MCNP Monte Carlo code. Results obtained with the albedo MORSE model are plotted as a function of the detector location along with some of the results of measurements using cadmium-covered Bonner spheres. The calculated results are seen to be 20 to 40% higher. This is consistent with the assumption of an empty basement. The agreement in this case is excellent in view of the rather crude approximations used in the calculations. Results of neutron fluences calculated with the quarter geometry MCNPmore » models are also well within the overall experimental uncertainties of the NE-213 measurements. The comparisons indicate that even for the extremely complicated geometry of the TFTR basement, realistic evaluations of the radiation fluences can be carried out with commonly used neutronic codes to accuracies that are comparable with measurements. These are achieved through proper choice of computational tools and appropriate approximations used in modeling the complicated geometry.« less