Patent Number: 
Section: description

The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventor of carrying out his invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the general principles of the present invention have been defined herein specifically to provide an improved nuclear shielding material that resists damage caused by radiation induced hydrogen production. The present invention is an improved nuclear shielding material that is initially flexible so as effectively to fill voids in radiation containment structures. The material is based on an amorphous organic matrix and is resistant to heat and radiation. Under very high temperatures the material is designed to undergo pyrolysis and transform into a strong ceramic material that retains the favorable radiation and hydrogen resistance of the original material. As such the composition consists of uniform mixture of up to seven different component groups. Abbreviated descriptions are given here with more detail below: 1) An organic polymeric elastomer matrix (ideally a two part self-polymerizing system)(about 10%-30% by weight of the final composition); 2) A gamma radiation shielding component (for example, tungsten carbide powder, 99% pure, 50-200 xcexcm average grain size preferred)(about 25%-75% by weight of the final composition); 3) A neutron absorbing/gamma blocking component (for example, boron carbide powder, 50-200 xcexcm average grain size preferred)(about 5%-10% by weight of the final composition); 4) A heat conducting component (diamond powder, 50-200 xcexcm average grain size preferred)(about 0%-5% by weight of the final composition); 5) A high temperature resistant component (silicon dioxide powder, 50-200 xcexcm average grain size preferred)(about 0%-5% by weight of the final composition); 6) A neutron absorbing/electrical conductivity-enhancing component (barium sulfate powder) (about 0%-5% by weight of the final composition); and 7) A hydrogen gas absorbing component (sponge palladium or other metals or intermetallic compounds that readily absorb hydrogen)(about 2%-8% by weight Sof the final composition). The first component (component group one) is a flexible organic matrix in which all of the other components are evenly suspended. The matrix material is preferably a flexible silicon rubber material (such as RTF 762 manufactured by the Silicon Division of General Electric Corporation). This organic elastomer is a two-part catalyst system so that all of the other component groups can be uniformly mixed together and then uniformly mixed into Part A of the RTF (xe2x80x98RTFxe2x80x99 stands for xe2x80x9croom temperature foamxe2x80x9d). Finally, Part B of the RTF is blended into the mixture, which is then injected into its final location where it foams, polymerizes and hardens. Alternatively, components 2-7 can be uniformly blended into a mixture. Then part A and part B of the RTF can be uniformly blended and that mixture rapidly blended with the 2-7 component mixture with the resulting mixture being injected into place before foam formation and heating has substantially occurred. The matrix provides the required flexibility, shock resistance and tensile strength to the material. Depending on formulation the matrix can exist in a porous or non-porous state. Non-porous matrices can be formed with RTV (xe2x80x9croom temperature vulcanizationxe2x80x9d) silicone rubber products. The advantage of the foam materials is somewhat lower weight and the ability to expand and fill voids upon injection into a structure. The goal is to eliminate all voids that are larger than about 5 mm because under intense radiation such voids can accumulate hydrogen gas and pose a danger of explosion. Alternatively, use of a non-foam matrix (e.g., RTV) can show increased strength and shielding ability, which may be advantageous under certain circumstance. An important consideration in the choice of RTF for the matrix material is the existence of aromatic radicals in the polymer. Various studies have shown that aromatic materials show a much higher radiation resistance than do, for example, polysiloxanes with mostly aliphatic radicals. A study on the radiation resistance of isoprene rubber demonstrated that the addition of polycyclic aromatic compounds greatly increased the rubber""s resistance to radiation. Benzantracene, diphenyl and phenantrene were shown to be the most effective. With such additives rubber irradiated in a vacuum was able to withstand a dose of 400 Mrad without appreciable structural deterioration. It is believed that aromatic rings afford a route for intramolecular transfer and dissipation of excitation energy. This significantly reduces the amount of hydrogen released on irradiation. That is, the aromatic carbonxe2x80x94carbon bonds involved in these polymers are resistant to radiation loads and environmental attacks. Polymers containing aromatic radicals, and especially benzantracene, diphenyl and phenantrene groups are especially preferred in the present invention. Other organic matrix elastomers and polymers are also usable in the present invention including siloxanes, silanols, vinyl elastomers (such as polyvinyl chlorides), and fluorocarbon polymers and elastomers. Again, polymers containing aromatic radicals are preferred. While the matrix provides basic strength and flexibility, the other six components provide various types of radiation resistance and/or enhancement to the basic mechanical-physical properties of the matrix. Component 2 provides significant shielding against gamma radiation. Gamma radiation shielding is important both because it limits the amount of dangerous gamma radiation exiting the shielded container (where is could be a biological hazard) and because the shielding limits the exposure of matrix material to strong radiation. Such exposure results in the gradual deterioration of the matrix and in the radiolytic production of hydrogen, which may result in a fire or explosion hazards. In situations with particularly high radiation fluxes as in containers for spent nuclear fuel, Component 2 can advantageously be supplemented with one or more additional shielding compounds. Such shielding compounds are generally powders of chemically pure heavy metals such as lead, tin, antimony, indium, and bismuth. These choices are a matter of balancing the opposing factors of cost, weight and requirement for shielding. While pure metal powders are useful, it is also advantageous to use salts of the shielding metals. Iodide salts can be especially advantageous because iodine itself is a good shielding material. Tungsten carbide is preferred as a primary shielding material (although metallic tungsten powder can also be used) because it is physically compatible with the matrix (i.e., the matrix polymers bind to the carbide) and because it can form a ceramic component under pyrolytic conditions. To this end oxides of heavy metals such as cerium and zirconium with high melting points (and even lighter ceramic compounds such as magnesium and aluminum oxide) are advantageously included to form a strong ceramic material. As is well understood in the art of refractory ceramics, it is important to avoid the inclusion of ceramic oxides that could form eutectic mixtures with low melting points. The addition of ceramic forming agents is optional and is based on the likelihood of the particular application resulting in sustained temperatures above about 900xc2x0 C. Component 3 has the primary task of absorbing neutrons. Because the organic matrix of the present invention is essentially transparent to neutrons, use of this invention without neutron absorbers could result in an increase in neutron flux as compared to other traditional shielding materials such as concrete. In some instances this could even result in a the danger of a chain reaction. The primary neutron absorber used is boron (but also see component 6). Boron is advantageously present as boron carbide because of the physical compatibility with the matrix. However, other forms of boron may also be used. For example, boron nitride may provide advantageous thermal conductivity and strength. In addition, more xe2x80x9cexoticxe2x80x9d neutron absorbers such as cadmium and gadolinium can be included to supplement the boron. Component 4, diamond powder, is partially responsible for high temperature resistance of the final product. The various shielding metals of the other components show relatively high thermal conductivity and help conduct heat out of the shielding material, thereby maintaining its favorable flexibility and related properties. However, diamond powder shows extremely high thermal conductivity and well as strength and thermal resistance (in a non-oxidizing atmosphere). Therefore, diamond powder can advantageously be included to help maintain temperature of the matrix below temperatures that would result in pyrolysis. Because the various shielding metals also contribute to thermal conductivity, it is possible to omit the diamond powder especially where the gamma shielding material is present in a metallic state. Component 5, silicon dioxide, is responsible for thermal resistance and strength at high temperatures. Should pyrolysis occur the silicon dioxide could form part of the newly generated ceramic. If other ceramic-forming metal oxides are included, this component can be omitted. Component 6, barium sulfate, is also an effective gamma radiation shield and a neutron absorber. In addition, it provides sufficient electrical conductivity to discharge free electrons released by interaction between the inventive composition and a strong radiation flux. These electrons can be involved in radiolytic breakdown and hydrogen production. Discharging or short-circuiting these currents can help avoid radiolytic breakdown and hydrogen formation. Since a primary purpose of component 3 is also neutron absorption, it is possible to omit component 6 particularly when metallic components are included as these components also enhance electrical conuctivity. Finally, component 7 is included to deal with hydrogen that forms despite the shielding materials and other additives used to minimize its formation. The xe2x80x9cgas suppressantsxe2x80x9d that make up component 7 are metallic and intermetallic compounds that readily absorb and bind hydrogen at relatively low temperatures and low partial hydrogen pressures. These materials include sponge palladium produced, for example, through the thermal decomposition of organo-palladium compounds and various readily xe2x80x9chydrogenatedxe2x80x9d metals such as lithium, calcium, scandium and titanium. Further, several of these are of sufficiently high atomic weight to also function as gamma shields. Of especial interest are intermetallic compounds such as the various lithium nickel (xe2x80x9clithiatedxe2x80x9d) compounds, lanthanum nickel compounds, samarium cobalt compounds, yttrium nickel compounds and yttrium cobalt compounds, all of which show significant ability to absorb hydrogen. In some situations, high radiation flux dictates that the hydrogen absorber-gas suppressant will become relatively rapidly saturated with hydrogen. When this occurs, hydrogen will diffuse through the inventive composition because the matrix material is quite permeable to hydrogen. The first thing that will occur is that pores in the material (pores are prevalent in the foam version) will fill with hydrogen. This could result in an explosion hazard as atmospheric oxygen and hydrogen can mix in the pores. However, this danger is considerably minimized by the small pore size of the foam. Generally the pores are smaller than the average effective trace length of radicals active in the hydrogen oxidation reaction (which amounts to several centimeters at atmospheric pressure). Therefore, the probability of developing a self-sustaining oxidation circuit is negligible due to quenching on the walls of the pores. The most likely scenario is that hydrogen will gradually infiltrate the pores and displace other gases therein. Eventually, there will be a steady escape of hydrogen from the surface of the material. Therefore, depending on the rate of hydrogen evolution, it may be necessary to provide some sort of ventilation system to safely gather and dispose of the escaping hydrogen. Finally, should thermal conductivity enhancers and other precautions fails to keep the composition at a temperature below 1,000xc2x0 C. or so the composition can undergo a pyrolytic transition (generally at 1,100-1,200xc2x0 C.) into an extremely strong ceramic. In the ceramic state the flexibility characteristics of the composition are largely lost; however, the overall shielding properties of the material are not significantly altered. If radiation and related conditions make the ceramic transition at all likely, provision should be made to exhaust the various gases released by pyrolysis. Ventilation systems provided to deal with hydrogen efflux could also serve to remove pyrolytic gases. While the possible ranges of components is fairly broad, following is a currently preferred xe2x80x9crecipexe2x80x9d for an effective nuclear shielding composition according to the present invention. The major component by weight is Component 2 (tungsten carbide powder of 99.99% purity) which makes up 55% by weight of the final composition. Component 3 is a mixture of boron carbide and boron nitride wherein the carbide makes up 4% and the nitride 1% by weight of the final composition. Component 4 is industrial diamond powder which makes up 0.5% by weight of the composition. Component 5 is quartz powder, which makes up 4.5% by weight of the final composition. Component 6 is barium sulfate which makes up 3% by weight of the final composition and component 7 is a gas absorber-suppressant which makes up 7% by weight of the final composition (this consists of an equal weight mixture of lanthanum/nickel and samarium/cobalt compounds to yield 4% by weight and further of hydrogenatable titanium to yield 3% by weight). These materials are thoroughly blended in an industrial mixer until the mixture is completely uniform. Then this mixture is thoroughly blended into RTF material Part A (an amount equivalent to 20% by weight of the final mixture). Finally, 5% by weight of the final composition of RTF Part B is blended in and the material is injected into a mold (or a cavity in a waste container) and allowed to polymerize. The inventive material is flexible and quite resistant to high temperatures and high radiation fluxes. If held at a high temperature it will transform into a strong ceramic especially if formulated with ceramic metal oxides as is understood by one of skill in the art. The composition is useful as a shielding component in any high radiation application. Especially suitable are nuclear power plants, nuclear fuel processing and reprocessing facilities and facilities for storage of spent nuclear fuels. For example, a good application of the present invention is as a shielding material in containers designed for transport and/or storage of spent nuclear fuels. One such container can be produced by making an container sized to hold a spent fuel rod assembly. The container is best fabricated from a strong and thermally/chemically resistant metal such as stainless steel. The container is fabricated with a double wall construction wherein a space exists between the inner wall and the outer wall. This space is filled by the composition of the present inventionxe2x80x94preferably in a foam formulation. That is, after the components are completely mixed with the silicone rubber Part A, the silicone rubber Part B is rapidly mixed in and the resulting mixture is injected into the space of the container. The mixture foams to completely fill the space and polymerizes to provide a resistant shielding material. A double-walled lid for the container is constructed along the same lines. The shielding material greatly attenuates the escaping radiation making transport and storage much safer. The following claims are thus to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention. Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiment can be configured without departing from the scope of the invention. The illustrated embodiment has been set forth only for the purposes of example and that should not be taken as limiting the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.