Patent Number: 042254671
Section: summary

This invention relates to neutron absorbing articles and to assemblies incorporating a plurality of such articles in a container for nuclear material. More particularly, the invention relates to such articles which comprise boron carbide particles in a matrix of cured phenolic polymer and which are in a form suitable for use in assemblies for absorbing neutrons from spent nuclear fuel. Periodically nuclear fuels employed in nuclear reactors to produce power diminish in activity to such an extent that they have to be replaced so that the reactor in which they were employed may operate at specification rate. In the past the spent fuel was temporarily stored in pools of water in which the neutrons and other radiation emitted from the fuel were sufficiently absorbed so as to prevent harm to human life and the environment about the storage facility. Normally, the facility was only temporary and therefore did not have to have a capacity for a very large amount of spent fuel (one or two annual refuelings plus room for a full reactor fuel charge in case of an emergency or complete reactor recharging was usually sufficient capacity) because the spent fuel would be removed from storage periodically for reprocessing or disposal. However, with the moratorium on such reprocessing and the limitations imposed on such disposal operations, many utilities have found it necessary to increase the fuel storage capacities of their existing spent fuel pools. Increasing the capacity of a spent fuel storage pool may be effected in the obvious way, by increasing the size of the pool, but a more efficient and practicable method is to increase the neutron and radiation absorbing ability of the pool itself. Various materials are known to be effective neutron absorbers and of these boron has previously been recognized as exceptionally effective. The B.sup.10 content of the normally occurring isotopic mixture of B.sup.10 and B.sup.11 is the highly effective neutron absorbing component of boron carbide and has a neutron absorbing capability many times that of B.sup.11. Although boron is metalloidal in character it is not generally suitable for manufacturing sufficiently strong thin articles, such as long thin plates and therefore if it is to be employed as a neutron absorber it is usually in the form of its compounds or alloys. A particularly useful compound, having a large percentage of boron in it, is boron carbide, which is in the form of hard black crystals having a Mohs scale hardness of 9.3 to 9.5, a melting point of about 2,350.degree. C. and a specific gravity of about 2.6. Boron carbide has previously been fabricated into neutron absorbing articles by various high temperature methods. It also has been formulated with other materials to form neutron absorbers of improved physical properties and such absorbers can often be produced at lower temperatures. Boron carbide in aluminum (boral) has been employed as a useful neutron absorber and it has been suggested that particulate boron carbide be dispersed in polymeric matrices. Thus, U.S. Pat. No. 2,796,411 mentions the use of acrylate resins suitably impregnated or admixed with various boron compounds, including boron carbide, and made into sheets or various other structures. U.S. Pat. No. 2,796,529 describes a shield for radiation which has boron or its compounds included in a laminated structure with synthetic resinous materials such as polytetrafluoroethylene and polyethylene. Also mentioned therein as possible useful polymeric materials are urea formaldehyde, melamine formaldehyde and melamine-urea-formaldehyde synthetic resins. U.S. Pat. No.2,942,116 mentions neutron shields wherein resins attenuate fast moving neutrons, and boron compounds, such as borax, boric acid and boron carbide, are employed to absorb slow moving neutrons. U.S. Pat. No. 3,133,887 describes boron carbide particles in epoxy resin making a product which is useful as a neutron absorber. Additionally, various other patents refer generally to the employment of resinous materials as bonding agents for neutron absorbers but none appears to be more relevant to the subject matter of the present invention than those already mentioned. Because of the danger of radiation and the possibility that the spent fuel items, usually in vertical rod form, if stored too close together without effective absorption of slow moving neutrons, could exceed a "critical mass" and/or carry out a highly exothermic nuclear reaction, standards for neutron absorbing articles are extremely high. In addition to being effective absorbers, articles must be shown to be stable under usual storage conditions and resistant to physical shocks, temperature variations, radiation and contact with the aqueous medium in the pool in the event that leakage occurs. Also, resistance to galvanic corrosion, such as in the presence of stainless steel, which is often employed as a container for the absorbing article to keep it separate from the pool medium, is an important quality often required of the neutron absorber. Furthermore, it is not enough for the absorber only to be effective and stable but the absorbing power thereof should be accurately controllable so that desirably effective absorption to a pre-calculated extent is obtained. Finally, it is desirable for the absorbing article to be capable of being made by relatively simple and inexpensive techniques so that the cost of the absorber makes it competitive with similar articles made from boral and other such absorbing compositions. In accordance with the present invention a neutron absorbing article comprises boron carbide particles in which the boron carbide content is at least 90% by weight, preferably at least 94%, and which are substantially all of a size to pass through a No. 20 U.S. Sieve Series screen and a solid, irreversibly cured phenolic polymer, e.g., a phenol aldehyde condensation polymer cured to a continuous matrix about the boron carbide particles, with the proportion of boron carbide in the article being such that it contains at least 6%, preferably at least 7% by weight of B.sup.10 from the boron carbide content thereof. In preferred embodiments of the invention the neutron absorbing article is in long, comparatively thin plate form, the boron carbide contains little or no B.sub.2 O.sub.3 and iron and the phenolic resin of the polymer contains essentially no halogen, mercury, lead and sulfur or compounds thereof. Likewise, the boron carbide and phenolic resin do not come into contact with any halogens, mercury, lead or sulfur or compounds thereof in the fabrication processes. Also within the invention is an assembly of a plurality of the described neutron absorbing articles, preferably in long and comparatively thin plate form, in a container for nuclear fuel, positioned so as to absorb neutrons emitted by the nuclear fuel. Additionally a part of this invention is the use of the articles to absorb neutrons from nuclear material, which is preferably spent nuclear fuel.