Study of the development of high-performance shielding material

It is necessary to make the shield structure of marine reactors lighter and smaller for a nuclear ship to be put into practical use. Meanwhile, efforts have been made to reduce the exposure of employees in nuclear facilities on basis of the as-low-as-reasonably-achievable spirit. Development of high-performance shielding materials is desirable to solve these subjects. Studies have been carried out to obtain new materials that have good shielding properties for both neutrons and gamma rays. The most effective shielding material for nuclear reactors is probably obtained by mixing hydrogenous materials, heavy metals, and thermal neutron absorbers. Inelastic scattering by heavy metal and elastic scattering by hydrogen are quite effective to slow down fast and intermediate neutrons, and the absorbers reduce secondary gamma rays as well as thermal neutrons. Ultraviolet curing monomers such as long-chain aliphatic acrylate, lauryl acrylate, and stearyl acrylate were selected as hydrogenous materials. They are all monofunctional monomers and tend to make fairly elastic solid materials. Polyfunctional monomers were not used up to now because they tend to make rather hard but breakable solid materials. Lead and boron compounds were dissolved in the monomers. They were chosen from lead nitrate, lead borate, boric acid, lead powder, etc. The composition of raw materials was determined empirically so that the density of the mixture was 2.2 g/cm3. This density was determined to make the attenuation of fission gamma rays to the same degree as that of fission neutrons. Although calculations by the ANISN discrete ordinates code indicated that the shielding effect of boron did not change very much in the range between 0.2 and 5 wt%, a rather large amount of boron compound was added to prevent the precipitation of lead powder or lead compound. By coating the liquid mixture on a vinyl sheet and irradiating ultraviolet rays, a sheet of solid material having thickness of 1 to 2 mm was obtained. Repeating this procedure, the authors finally obtained material samples having a thickness of ≅1 cm. calculations by a computer code of fuzzy linear programming methods indicated that the optimum atomic densities were 5.78 x 1022, 3.71 x 1021, and 2.78 x 1021 for hydrogen, lead, and boron, respectively. However, the atomic densities of the best samples up to now are 5.02 x 1022, 3.40 x 1021, and 6.79 x 1021, respectively. Further experiments are now being put into practice to increase the amount of hydrogen