Character and manufacturing problems of transport cask for spent nuclear fuels: radiation shielding design

In the shielding problem of the transport casks for spent nuclear fuel, first radiation sources are evaluated. In gamma-ray sources, attention must be paid to energy spectra in particular, and in neutron sources, radioactive curium is important as its burn-up increases exponentially. Next, effective multiplying factor is calculated, and the increase of neutrons in the system is taken into consideration. In the shielding design, neutrons are more problematic than gamma-ray, and the material effectively attenuating neutrons and its arrangement must be considered. The photoelectric effect which is the process of absorbing gamma-ray and the Compton scattering which is the process of reducing energy are both the interaction with electrons, and the material with larger mass is superior in shielding capability. Lead is commonly used for shielding primary gamma-ray. Iron as the strength member of transport casks has considerably large capture cross section for thermal neutrons, but the shielding of capture gamma-ray is required. In order to solve the problem of capture gamma-ray, lead is used as an inelastic scattering material instead of iron, and it is also conceivable to add the material absorbing low energy neutrons, such as boron-10. In case of the casks for transporting neutron sources, attention is paid to the shielding material in particular. It is desirable to carry out mock-up experiment with the casks of actual scale containing spent fuel, and to collect and analyze the data. (Kako, I.)