Patent Number: 046817328
Section: description

SPECIFIC DESCRIPTION In FIG. 1 we have shown a porous graphite body 14 forming a quenching element 10 which can be introduced into a gas-cooled nuclear reactor and in which particles 11 are embedded. Each particle 11, as is also apparent from this Figure, can comprise a rare earth metal shell 12 melting at the appropriate activation temperature and containing the neutron absorptive compound, e.g. a gadolinium halide as represented at 13. Such elements have been introduced into a nuclear reactor together with the fuel charge, the elements having the same dimension and shape as the fuel elements. When the desired threshold temperature is reached, the shells 12 melt and at the high temperatures the gadolinium halide is released to pass through the porous body 14 and coat gadolinium onto the graphite surfaces of the fuel elements. The particles shown in FIG. 2 comprise a pyrolytic carbon shell 15 enclosing the neutron absorptive halide 16. This shell is characterized by the fact that at about 1000.degree. C., with increasing temperature, the pores increasingly open to allow progressively increasing release of the neutron absorptive material. FIG. 3 represents a diagram of a core 17 of a gas-cooled nuclear reactor in which the fuel elements have been represented as rods 18 and are disposed in the core together with a rod 19 containing the particles of FIGS. 1 and 2 as described. While the rod 19 can be inserted into the core together with the fuel elements, it may also be introduced by a safety control 23 of the type used to insert a moderator rod. The more common application of the invention, however, is not a ball element nuclear reactor as described in the aforementioned U.S. patent and will be discussed below. The primary gas circulation is represented by the pump 20 and a secondary cooler 21. It will be apparent that the safety system of the invention is effective even upon failure of the primary coolant system. SPECIFIC EXAMPLES In the following specific examples 1-3, we will describe graphite elements of a type which pass through the reactor core of a piled ball reactor together with the fuel elements. The elements contain, as a neutron-absorbing substance, a halogen compound of a rare earth metal, for example a gadolinium, samarium or europium halogen compound, especially fluoride, bromide or iodide, or mixtures thereof. This shutdown substance is coated with or enclosed in (ensheathed in) a metal which has a melting point corresponding to the threshold for release of the reactivity-reducing substance. The latter then in a gaseous form can penetrate the graphite body in which the element or particle is embedded because of the porosity of the graphite and in the ambient space around the body can come into direct contact with the fuel elements to be absorptively deposited thereon. EXAMPLE 1 The shutdown element as a hollow ball formed of graphite whose size corresponds to the size of the fuel element balls of a stacked ball of a reactor. The absorption substance was gadolinium-III-bromide which was introduced in the form of particles with a particle size of less than 5 mm in the graphite element. The graphite shutdown element was added to a graphite ball filling simulating the fuel element packing of a stacked ball reactor. The filling was heated together with the shutdown element to 850.degree. C. As soon as this temperature was achieved, the absorptive substance was detected at spacings up to 750 mm from the shutdown elements, i.e. it had been dispersed to a volume with a diameter 1.5 mm in the pile so that the gadolinium was detected on the surfaces of the balls of this packing. EXAMPLE 2 A graphite element is provided with particles of the absorption substance in a sheath of metallic gadolinium. The gadolinium is practically impermeable at temperatures below the melting point. As to the absorption substance, however, the gadolinium sheath melts or fuses at a temperature between 1300.degree. C. and 1350.degree. C. and at this temperature liberates the absoprtion substance which was the gadolinium-III-bromide used in Example 1. The gadolinium-III-bromide upon melting of the metallic gadolinium sheath has a partial pressure of 0.28 bar and is effectively distributed in the fuel element bed of the reactor. It was found that the metallic gadolinium serving as a coating for the neutron absorption substance also serves as a protective shield from neuclitic burn of the enclosed absorption substance. With similar effects, we can use for high temperature shutdown of a nuclear reactor, europium which has a melting point of about 830.degree. C., samarium which has a melting point of 1070.degree. C. and dysprosium which has a melting point of about 1400.degree. C. For a shutdown temperature of about 1500.degree. C., erbium may be used which has a corresponding melting point. EXAMPLE 3 Naturally the halogen compounds of the rare earths which were employed must have an effective vapor pressure at the melting point at which the protective sheath decomposes. One criterion for the use of selection of these halogen compounds is their melting point, because above the melting point the vapor pressure generally increases very rapidly. This melting point should be close to the usual operating temperature of the nuclear reactor for which the graphite element is employed. For stacked ball reactors the most important shutdown range is a temperature between 900.degree. C. and 1300.degree. C. and the melting point of the following shutdown substances are thus given below so that the worker in the art may make use of the appropriate rare earth halide for the desired shutdown temperature: ______________________________________ SmBr.sub.3 melting point 913.degree. C. EuBr.sub.3 975.degree. C. GdBr.sub.3 1038.degree. C. GdJ.sub.3 1198.degree. C. GdF.sub.3 1228.degree. C. EuF.sub.3 1276.degree. C. SmF.sub.3 1305.degree. C. ______________________________________ EXAMPLE 4 A graphite element containing a particle of a shutdown substance of the type used in Examples 1-3 has a pyrolytic carbon sheath. The pyrolytic carbon coating, depending upon its quality, is not suddenly gas permeable at a temperature above 1000.degree. C. but generally tends to become permeable with increasing temperature. We are thus able to use this element to progressively reduce the reactivity by progressive release of the shutdown substance with increasing temperature of the reactor core. When the temperature of the reactor core drops below the threshold, as the pyrolytic carbon coating has progressively reduced porosity, eventually the release of the absorption substance is cut off. EXAMPLE 5 A graphite element contain particles of a diameter of 10 mm, the particles corresponding to those of Examples 1-3. On the pyrolytically coated particle of Example 4 the quenching element is supplied into the reactor core in a ratio to the fuel elements of 1:1000. The reactivity of the reactor core is reliably reduced below 0.1%. EXAMPLE 6 When such a particle (Example 5) comprises metallic gadolinium and contain gadolinium-III-bromide, after 1000 days in the reactor core substantially 100% of the shutdown substance in the particle remain active. Consequently, practically neuclidic burn-out occurs. EXAMPLE 7 A fuel element is provided with a particle of a diameter of about 1 mm of the shutdown substance in addition to its nuclear fuel particles. The incorporation of the shutdown particles in fuel elements is advantageous because they eliminate the need for additional shutdown elements. The absorptive substance is protected by self-shielding against neuclidic burn-out. With gadolinium compounds, the self-shielding effect with a sheath of a thickness of 0.01 mm is noticeable and is practically complete in a thickness of the compound of 0.5 mm which is satisfactory where the particle diameter is 1 mm. The burn-out of the absorptive material can be proportioned to the burn-out of the fuel element in which it is incorporated so that as the fuel element is depleted, the availability of the absorptive material is reduced. Where each fuel element contain a particle of the absorptive substance, we are able to eliminate errors of loading the reactor with the fuel absorption elements. Such fuel elements carry its safety particle from its production process to its reworking or final storage. Apart from the rear earth halogen compounds of Examples 1-3, we make use of other gadolinium compounds, samarium compounds as long as their vapor pressure and stability suffice for the purposes described.