Neutron absorbers composites materials

Boron is used as a reinforcing material for composites. It is used in the nuclear industry as a neutron absorber. Boron is used to harden metals and used as an oxygen scavenger for copper and other metals. Amorphous boron is used in pyrotechnic flares to produce a green color. Used as a catalyst in olefin polymerization and alcohol dehydration. The principal consumption pattern in the United States for boron is for the production of glass products with minor usage in the production of soaps and detergents. 

Metal borides and boron carbides of different compositions were used as neutron absorber materials—see patents (Hortman and Naum 1981 Lipp et al. 1981 McMurtry et al. 1981, 1982 Naum et al 1979 Owens 1980 Storm 1980 Wieczorek 1988)—and in the control rods of power reactors as well—see patents (Andrews 1987 Dixon et al. 1990). 

Hafnium-like boron is known to be a neutron absorber or neutron moderator element, and, therefore, composites of boron carbide, B4C, and hafnium diboride, HfB2, can be considered as nuclear materials. These boron compounds after sintering and °B/"B isotopic ratio adapting are found to be heterogeneous polyphone cermets useful for nuclear applications (Beauvy et al. 1999). Boron acid obtained from the °B enriched boron trifluoride also was used in nuclear reactors (Shalamberidze et al. 2005). Amorphous boron powders enriched both in °B and "B, boron carbide, and zirconium diboride (ZrB2) powders and pallets labeled with °B isotope And applications in nuclear engineering too. The °B enriched Fe-B and Ni-B alloys are useful for the production of casks for spent nuclear fuel transfer and storage. 

Properties Hard bik. cryst. insol. in water m.w. 55.26 dens. 2.510 m.p. 2350 C b.p. > 3500 C hardness (Mohs) 9.3 resist, to chem. attack dec. by molten alkalis at red heat does not burn in oxygen flame Toxicology Harmful dust avoid inhalation of dust or particles TSCA listed Uses Raw material for paint mfg. alloying agent mfg. of chem.-resist. ceramics grinding wheel abrasive neutron absorber reinforcing agent in composites for military aircraft control rods in nuclear reactors pol-... 

Anoher object of the invention is to provide a simple and sensitive means for controlling the neutronic reaction while at the same time producing a useful material by subjecting neutron absorbent material to intense neutron bombardment. This object is accomplished by dividing the reactor into a plurality of sections or cells, the central group of which comprises reactive composition between spaced portions of fertile material, and the outer group of cells comprises fertile material only. Thus a reactive zone is provided which is completely surrounded by an absorption zone of the particular fertile... 

Due to its neutron-absorbing efficiency, boron carbide is attractive as a neutron absorber material, and is used both in powdered and solid forms to control the rate of fission in nuclear reactors (Figure 4.19b)[530j. B4C mixed with other materials, such as aluminum metal or polyethylene plastic, is applied to protect it against oxidation in the reactor environment. AI-B4C metal-matrix composite plates (e.g., Boral, Bortec) have wide applications as isolators in spent fuel element racks, in the inner sections of reactor shields as shutdown control rods and neutron curtains, as shutters for thermal columns, and as shipping containers. 

The composition of boron carbide is approximately 80 atomic percent boron. The material is often considered as a source of boron, without the high reactivity of the latter. Like boron, B4C has a high neutron capture cross-section for thermal neutrons and a low secondary gamma radiation. As such, it provides an excellent neutron absorber and is used extensively to control the neutron flux in nuclear fission reactors, such as the boiling water, pressurized water, and fast breeding reactors. It is also used for the compact storage of spent fuel rods.l l... 

The major industrial applications of hexagonal boron nitride rely on its high thermal conductivity, excellent dielectric properties, self-lubrication, chemical inertness, nontoxicity, and ease of machining. These are, for instance, mold wash for releasing molds, high-temperature lubricants, insulating filler material in composite materials, as an additive in silicone oils and synthetic resins, as filler for tubular heaters, and in neutron absorbers. On the other hand, the industrial applications of cubic boron nitride rely on its high hardness and are mainly as abrasives. 

In PWR [2], most of the CEA use fingers (or pins or rods) fastened to a central cast spider assembly inserted from the top of the core in the fuel assemblies (Fig. 15.1). The number of fingers per CEA (about 20) and the number of CEA per core (about one for four fuel assemblies) depends on the core dimensions, fuel composition [due to neutron spectrum hardening, more control rods are required for mixed oxide fuel (MOX) fuels], and power. The neutron absorber materials are most often the Ag-In-Cd (AIC)... 

Up to now, we have discussed only monophasic materials. However, composites have been used as neutron absorbers for a long time and different concepts have been tested in order to improve some limitations of the classical materials. 

In accordance with the present invention, a novel process and apparatus for establishment of a self-sustaining neutron chain reaction of neutrons with a neutron fissionable isotope such as U, and 94 2 is provided. The invention is particularly advantageous since it may be applied to establishment of such a reaction in compositions such as natural uranium where the concentration of fissionable material is low. Thus, we have found that a self-sustaining reaction may be established by use of a suspension of natural uranium in a liquid moderator containing about 0.0025 to 0.04 atom of uranium per molecule of a moderator such as deuterium oxide or about 0.0013 to 0.02 atom of uranium per atom of deuterium. Where the liquid moderator is less efficient, and absorbs more neutrons than deuterium oxide, this range of uranium concentration is somewhat narrower. 

A primary object of the present invention is to provide a breeder system wherein a nuclear fission chain reaction is utilized to produce fissionable material at a rate 10 greater than the rate of consumption of fissionable material within the chain reacting composition. This is accomplished by neutron bombardment of fertile material adapted to undergo nuclear reaction productive of fissionable material as hereinafter described. Fertile iso-15 topes as herein defined are isotopes such as and U238 which are converted to thermally fissionable isotopes, and Pu 39, respectively, by nuclear reaction under neutron bombardment. These fertile isotopes are fissionable by fast neutrons and substantially nen-fission-20 able by slow neutrons (below about 1000 e.v.) and absorb neutrons fast or slow to undergo the above-mentioned nuclear reactions. 

According to the present invention the novel breeder system comprises a neutronic reactor wherein and 25 heavy water (D2O) neutron moderator are combined in a chain reacting composition surrounded by a neutron reflector of heavy water containing a fertile isotope or isotopes in solunon or in suspension. The fertile material absorbs neutrons emanating from the chain reacting 30 composition and is thus converted to thermally fissionable material. 

Referring now to FIG. 8, the system shown therein comprises inner and outer steel tanks 102 and 104, the 50 inner tank containing a plurality of composite rods 106 and the outer tank containing a plurality of composite rods 108, all of said rods being supported, as hereinafter described in detail, from a biological shield 110 composed of any suitable material adapted to absorb bio-55 logically harmful emanations, such as neutrons and alpha, beta, and gamma rays. 

In the Chicago Pile experiment, Fermi controlled the fission reaction by withdrawing rods made of cadmium. The rods absorbed neutrons as the rods were withdrawn, the fission reactions commenced. Nuclear power plants operate under fundamentally the same principle. The fissionable material employed in nuclear power plants is usually the uranium isotope U-235—the same fissionable material used in Little Boy, the bomb dropped on Hiroshima. In todays plants, cadmium and boron are the materials most frequently used in the composition of control rods. 

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