Patent Number: 044951400
Section: summary

BACKGROUND OF THE INVENTION This invention relates to the nuclear reactor art and has particular relationship to light-weight power plants for mobile or vehicular propulsion applications where a nuclear reactor is a primary source of energy. Such power plants are shown in Thompson-Pierce U.S. Pat. No. 4,057,465 and Thompson-Spurrier-Jones U.S. Pat. No. 4,088,535. The nuclear reactor included in such power plants is typically gas cooled. Usually the cooling gas is helium. An important consideration in dealing with nuclear propulsion plants is reliable shutdown of the reactor by control of its reactivity under normal and abnormal conditions. Adequate reliable reactivity control is available for normal shutdown. But a different problem is presented on the occurrence of an emergency which results in failure that affects the reactor so that normal reactivity control is insufficient. This invention concerns itself with the need for reliable shutdown under such emergency conditions. It is an object of this invention to compensate for the insufficiency of the control and thereby provide for reliable shutdown of the reactor thereby precluding the happening of a catastrophe during the emergency. Typically an emergency can arise when water penetrates into the core of nuclear reactors of certain types. For example a ship propelled by a light-weight power plant including an epithermal-neutron, gas cooled, nuclear reactor is sunk. The penetration of water into the core of the reactor is possible because the ship may sink to a depth, for example exceeding 600 feet, at which the containment of the reactor may rupture or for other reasons. The water may materially increase the nuclear reactivity of the reactor and thus may lead to a nuclear excursion and a serious catastrophe. it is an object of this invention to effectively and permanently deactivate the nuclear reactor supplying the primary energy to the propulsion plant of a ship when the ship is sunk and to accomplish this purpose before water can flood the core of the reactor. SUMMARY OF THE INVENTION In accordance with this invention the nuclear reactor of the propulsion power plant of a vehicle which has suffered an accident is deactivated permanently by impregnating the core with a refractory poison, specifically boron, boron carbide (or a boron-carbon polymer), boron nitride (or a boron-nitrogen polymer) or a metal boride. The isotope of boron which is an effective poison is B.sub.10. There is 18.83% B.sub.10 in natural boron. While the core may be impregnated with sufficient natural boron to effectuate permanent deactivation, it is desirable in the interest of reliable deactivation in the required short-time interval that the boron used be enriched in B.sub.10. Typically boron is enriched by gas diffusion of boron trifluoride etherate. Typically the material is converted into diborane (B.sub.2 H.sub.6) with an enrichment of up to 85%. Diborane is a gas at room temperature; it has a melting point of -165.5.degree. C. and a boiling point of -92.5.degree. C. In the practice of this invention, diborane is reacted with unsaturated hydrocarbons, such as acetylene or alkyl hydrocarbons, to form heat sensitive carboranes, alkyl boranes, or alkyl diboranes. Appropriate products of these reactions (triethylboron, for example) are stored for injection into the coolant of the nuclear reactor on the occurence of an emergency. Or the individual reactants are injected into the coolant stream where they react and are carried along with their reaction products by the coolant through the core. Alternatively diborane is reacted with ammonia producing boron-nitrogen compounds which are also carried by the coolant through the core. At the lower or core inlet temperatures these compounds react to form less volatile boron-containing compounds that condense on the walls of the coolant channels penetrating the pores of graphite moderated reactors. These polymeric compounds are unusally stable to water and aqueous acids. At the elevated temperature of the core the boron-carbon and the boron-nitrogen compounds dissociate producing boron, boron carbide and boron nitride which adhere to the core. The latter materials are highly refractory. Boron has a melting point of 2300.degree. C. and a boiling point of 2550.degree. C. and is insoluble in water. Boron carbide, B.sub.4 C, has a melting point of 2450.degree. C. and does not boil at 3500.degree. C. and is insoluble in water. Boron nitride sublimates at 3000.degree. C., is insoluble in cold water but dissociates slightly in hot water. Diborane is highly reactive. When heated it generates higher and less volatile hydrides. The reactions are shown in the diagram below: ##STR1## Reference is made to K. Wade, Electron Deficient Compounds, Nelson 1971, pp. 71, 86. Diborane begins to decompose at temperatures as low as 300.degree. C. to form, with increasing temperatures, higher and more stable hydrides. When heated at 600.degree.-800.degree. C., the hydrides decompose to boron and hydrogen. For example, the pyrolysis of diborane at 800.degree. C. is used as a production method for high purity boron. Although the reactor core exit temperature is 800.degree. C. or higher, a high purity deposit is not required to deactivate a reactor. Diborane and other boron hydrides are more effective when reacted with unsaturated hydrocarbons or ammonia, particularly as the products of these reactions dissociate into the refractory compounds at lower temperatures than 800.degree. C. In the presence of a reagent, for example, an acetylene or an alkyl hydrocarbon, the boron hydrides form other metal organic compounds. ##STR2## where R is H in the case of acetylene or an alkyl radical. Diborane and alkylboranes react to form a variety of alkyldiboranes. EQU B.sub.2 H.sub.6 +R.sub.3 B.fwdarw.RB.sub.2 H.sub.5 +R.sub.2 B.sub.2 H.sub.4 +R.sub.3 B.sub.2 H.sub.3 R.sub.4 B.sub.2 H.sub.2 where R is an alkyl radical. Higher boron hydrides react with unsaturated hydrocarbons to form heat-sensitive carboranes. A reaction of a hydride with acetylene is as follows: ##STR3## Carboranes decompose in the following manner: ##STR4## When diborane and ammonia are heated together, boron-nitrogen oligomers or polymers of ill-defined composition form. ##STR5## The pyrolysis of a boron-nitrogen compound, hydrazine-borane at 200.degree. C. results in the formation of a polymeric compound that is unusually stable to water, but decomposes slowly above 200.degree. C. to an unidentified compound with the empirical formula (HBN).sub.n as follows: ##EQU1## Ref: H. Steinberg and R. J. Brotherton, Organoboron Compounds, Vol. 2, Wiley & Sons, 1966. The following diagram shows the reactions between diborane and ammonia: ##STR6## Reference is made to Wade above. Boron nitride is the ultimate product of the decomposition of the reaction products shown in the above diagram. As the coolant carries the above compounds into and through the core the higher hydrides, carboranes and boron-nitrogen compounds deposit in the perforations at the lower or cold-leg core inlet temperatures. These compounds progressively break down to boron, borocarbons and boron-nitrogen compounds and to boron carbide or boron nitride at the higher core temperatures. Hydrogen liberated in these reactions may react with the graphite core to form volatile hydrocarbons that also react and combine with or entrap the boron compounds. Boron alkyls begin to decompose thermally above 200.degree. C. Among the compounds which break down to provide boron carbide, triethyl boron requires special note. The following reactions occur: ##STR7## Typical of the compound B.sub.x C.sub.y H.sub.z is B.sub.5 CH. All of the compounds in this series form rapidly and irreversibly and are stable, non-volatile solids insoluble in water. Boron nitride is isoelectronic with graphite; i.e., except for mass and nuclear charge, the molecules of boron nitride and graphite resemble one another. They have the same number of valence electrons and should have similar orbitals. Thus, the structural properties of boron nitride and graphite are analogous and boron nitride produced by the thermal breakdown of aminoboranes or other boron-nitrogen compounds should be readily accommodated by the graphite core and firmly adhere to the porous walls of the reactor coolant passages. Some aminoboranes are commercially available; boron imides and borazole can also be used. The following reactions are typical: ##STR8## With respect to the Borazole, reference is made to Gmelin's Handbuch der Anorganischen Chemi, 8th ed., Verlag Chemie, Weinheim/Bergstrasse No. 13 (1954). Refractory boron containing compound also may be deposited through the thermal decomposition of a metal borohydride. The metal borohydrides are volatile at low temperatures and decompose in the range of 200.degree.-400.degree. C. For example, thorium borohydride has been decomposed at 300.degree. C. leaving an amorphous, metallic deposit of ThB.sub.3.83. Other borohydrides also leave metallic borides and probably uncombined boron upon thermal decomposition. Reference is made to Powell, Oxley and Blocher, Vapor Deposition, John Wiley & Sons, 1966, p. 345. Typical reactions are: EQU Th(BH.sub.4).sub.4 .fwdarw.ThB.sub.4 +2 H.sub.2 EQU Hf(BH.sub.4).sub.4 .fwdarw.HfB.sub.2 +2 B+8 H.sub.2