Patent Number: 041561473
Section: summary

This invention relates to neutron absorbing articles. More particularly, it relates to such articles which comprise neutron absorbing boron carbide particles and diluent particles bound together in a matrix of cured phenolic polymer in a form suitable for absorbing neutrons from nuclear material, such as spent nuclear fuel. It is well known that products of the radioactive decomposition of nuclear materials are harmful to human life and to the environment about such materials. Accordingly, where nuclear materials have been employed shielding has often been utilized so as to lower the level of radioactivity in surrounding areas. Nuclear fuels employed in nuclear reactors to produce electric power diminish in activity to such an extent as they are consumed that periodic replacement is required to maintain reactor operations at specification rates. To increase the capacities of storage pools, such as have been employed in the past for temporary storage of such removed fuel and other nuclear wastes, the spent fuel has been stored in the pools in racks with neutron absorbing material surrounding it. Such racks and the storage of nuclear materials, such as spent fuel from nuclear power plants, in them have been described in U.S. patent application Ser. No. 854,966, filed Nov. 25, 1977, by McMurtry, Naum, Owens and Hortman, the disclosure of which is hereby incorporated by reference. The McMurtry et al. application describes boron carbide-phenolic resin neutron absorbers which are preferably in long thin flat plate form and are of exceptionally high neutron absorbing capabilities because of their high contents of B.sup.10 from the boron carbide particles therein. Although such products have met with acceptance by operators of nuclear power generating installations, in which they have been successfully employed, sometimes the greater neutron absorbing capabilities thereof are not required and on other occasions neutron absorption specifications may be lower than those for the McMurtry et al. neutron absorbers. Because it is the B.sup.10 in the boron carbide particles of the boron carbide-phenolic polymer compositions which is the active neutron absorber the absorption properties of boron carbide particles-phenolic polymer product may be lowered by diminishing the quantity of boron carbide therein and increasing the phenolic polymer content accordingly. Although such method allows the production of neutron absorbers of various activities by variations in the boron carbide:phenolic polymer ratio in the neutron absorbing articles made, the physical properties of the product as well as the neutron absorbing power thereof vary and accordingly, to meet specifications, it may often be necessary to make allowances for such variations in the design of the fuel storage racks or other environments wherein the nuclear material to be shielded is present. Such design variations often are not feasible. Additionally, different processing techniques will often have to be employed when the proportions of boron carbide and phenolic resin, from which the final cured polymer matrix is made, are changed. Thus, at high proportions of phenolic resin in the desired final product it may be necessary to utilize different and more expensive manufacturing techniques because, especially when liquid resin is utilized, the "green" article or plate first made from the boron carbide-phenolic resin mixture may not retain its desired form during the curing process unless it is held under a pressing or compacting pressure, which is not practical for the preferred simple oven cures of such articles. Because of the disadvantages accompanying properties changes due to variations in the ratio of boron carbide particles to phenolic resin in neutron absorbing articles containing such materials along and because of difficulties encountered in processes for the manufacture of such changed articles the present invention is especially advantageous. In accordance with this invention a neutron absorbing article comprises boron carbide particles, diluent particles and a solid, irreversibly cured phenolic polymer cured to a continuous matrix binding the boron carbide particles and the diluent particles. In such products usually the total content of the boron carbide particles and the diluent particles is a major proportion of the article and the content of the cured phenolic polymer is a minor proportion. By means of the present invention neutron absorbing articles or plates can be made, utilizing mixtures of boron carbide particles and diluent particles with phenolic resin, the mixture of which can be pressed to green article form, and which articles can be subsequently cured efficiently and easily in an oven with a plurality of others. Because the diluent particles behave similarly to boron carbide particles, except for their lack of neutron absorbing capability, the manufacturing methods employed need not be changed and products of varying neutron absorbing powers may be manufactured, utilizing the same equipment and processes but changing the mixtures of boron carbide and diluent particles utilized. Also, the products made will have the desired physical and chemical characteristics for successful use as neutron absorbers in storage racks for installation in storage pools for spent nuclear fuel.