Patent Number: 046817328
Section: summary

CROSS REFERENCE TO RELATED APPLICATION This application is related to the commonly assigned copending application Ser. No. 657,332 filed Oct. 3, 1984, based upon our German application P No. 33 35 888.5 filed Oct. 3, 1983. FIELD OF THE INVENTION Our present invention relates to a method of reducing the reactivity of a nuclear reactor core, even to the point of shutdown of the reactor and to a device for this purpose. More particularly, the invention relates to the reduction of the activity and shutdown of a gas-cooled graphite-moderated nuclear reactor of the type in which a cooling gas passes through a core containing nuclear fuel materials embedded in graphite and in which the fuel elements have a graphite surface. BACKGROUND OF THE INVENTION From U.S. Pat. No. 4,239,697 and the corresponding German Open Application DE-OS No. 27 53 928, it is known to reduce the reactivity and shutdown of a nuclear reactor by coating free surfaces of graphitic bodies containing fissionable fuel materials with a neutron-absorbing substance. Nuclear reactor installations generally comprise a number of control systems for regulating the reactivity of the reactor core and shutting down the chain reaction thereof. It is important that such shutdown systems be of such nature that they allow reactivation of the core in the event an emergency situation has been alleviated. In the aforementioned U.S. patent and the corresponding German application, there is described a gas-cooled nuclear reactor having graphitic fuel elements and in which the nuclear reaction is quenched by depositing a gadolinium-containing substance on the surfaces of the fuel elements by introducing that substance into the circulating cooling gases of the primary coolant contacting the fuel elements. The more gadolinium which is deposited upon the graphite surfaces of the fuel elements, the greater will be the absorption of thermal neutrons and the greater the reduction in reactivity. Since gadolinium in its natural isotropic mixture has the greatest neutron absorption cross-section for thermal neutrons of all naturally occurring elements, it suffices to deposit a comparatively small amount of gadolinium upon the fuel element. If it is desired to restart the reactor, the gadolinium may be desorbed by increasing the temperature of the reactor core and flushing it with gadolinium-free gas, by scrubbing or by a nuclear decomposition. Replacement of the gadolinium-cooled fuel elements by fresh fuel elements can also restore the reactivity. According to this earlier patent, the gadolinium compounds which are used can be applied as sols or solutions or in a gaseous state. Preferably an aqueous gadolinium acetate is used on the substance if applied as a gadolinium compound in a gaseous form, e.g. as gadolinium aluminum isopropoxide --Gd(Alc.sub.3 H.sub.7 O).sub.4).sub.3 -- or gadolinium tricyclopentadienyl --Gd(C.sub.5 H.sub.5).sub.3, the latter sublimating from the solid phase since it has no liquid phase. In their earlier system the substances were introduced through a bypass to the cooling gas circulation and, naturally, this poses no problem when the cooling gas circulation is intact. However, since a frequent cause of breakdown and the need for shutdown of the reactor is failure of the primary cooling system, the reliable introduction of the gadolinium compounds could not be ensured. Consequently, it is necessary to provide additional piping for the reactor to enable the introduction of the reactivity-reducing compounds. OBJECTS OF THE INVENTION It is the principal object of the present invention to provide an improved method of reducing the reactivity of a nuclear reactor of the type described whereby the need for additional piping is eliminated and which enables the reduction of reactivity to be effected reliably. Another object of this invention is to provide an improved method of shutting down a nuclear reactor of the gas-cooled graphite moderated type. Still another object of the invention is to provide an improved device or article for use in the improved method of the invention. It is also an object of the invention to provide a method for the purposes described which can be utilized even when there has been damage to the primary coolant circulation of a gas-cooled graphite moderated nuclear reactor. SUMMARY OF THE INVENTION These objects and others which will become apparent hereinafter are attained, in accordance with the present invention, in a method for reducing the activity and shutting down a gas-cooled graphite moderated nuclear reactor which comprises graphite fuel elements in which the nuclear fuel material is embedded in the graphite and the fuel elements are cooled by direct contact with a primary cooling gas and have a graphitic surface. According to the invention, at least one element is introduced into the reactor core which comprises a graphite body permeable to vapor of a neutron-absorbing substance and the neutron-absorbing substance is incorporated into this body in the form of particles from which the neutron-absorbing substance is released at in a gas form to temperatures above a predetermined temperature so that the gaseous neutron-absorbing material can enter the gas spaces of the reactor core and coat upon the graphitic surfaces of the fuel elements. To reduce the activity or shutdown the nuclear reactor, therefore, we introduce into the latter elements which are comprised of graphite and contain particles with the neutron absorbing substances. This neutron absorbing substances may hereinafter be referred to also as a shutdown substance or as an activity-reducing substance. The particles are so constituted that the neutron absorbing substance is released at a predetermined temperature which corresponds to the desired shutdown temperature in the reactor core to release the substance into the latter. This can be accomplished by providing the particles with a coating which becomes gas-permeable at the predetermined temperature or melts or otherwise is decomposed. The graphite body containing these particles is permeable to the gas phase of the neutron-absorbing substance and the latter substance. The coating and the graphite of the shutdown element are so related to one another that the shutdown substance which is capable of absorbing thermal neutrons can reach the fuel elements around the shutdown elements when the shutdown temperature is reached. The shutdown elements can be introduced into the reactor core together with the usual fuel elements. In that case, no further means for activating the shutdown by this mechanism is required since shutdown will ensue only upon attainment of the threshold temperature. However, we can also introduce the shutdown element into the core utilizing the mechanism for controlling the introduction and removal of fuel elements, as need arises. In the first case the reactor has an inherent shutdown system which is activated when the critical temperature is such so as to liberate the shutdown substance automatically even in the case of complete failure of the primary coolant circulation. The number of particles and their distribution in the reactor core or the number of particles per shutdown element and the distribution of the latter in the reactor core can be comparatively small because of the high neutron absorption cross section of the substance. Hence neutron losses during normal operation can be held relatively low. This is because the shutdown substance is highly concentrated at sparse localities. Indeed, we have found that a ratio of shutdown elements to fuel elements can be 1:1000 with effective shutdown response. The static inherent shutdown system described eliminates the need for any active measures in effecting shutdown and the invention also has the advantage that the simple replacement of a comparatively few fuel elements in existing nuclear reactors under construction and reactors in the planning stages be shutdown elements according to the invention and can suffice to provide the requisite security against failure without modification of the reactor or its design. According to a feature of the invention, the particles have such a size that they are practically self-shielding and thus are subjected to a minimum of decomposition in the neutron flux of the reactor under normal operation. The shutdown elements can have outer dimensions corresponding to those of a fuel element so that direct substitution is possible in the manner described. Furthermore, the particles should contain the neutron absorbing substances in such quantities that, even with decomposition in the neutron flux of the reactor, a sufficient quantity of the substance will remain at the end of the usefulness of the reactor charged to be effective until a few changes occurs. The coating is most advantageously that of a metal from the rare earth group or alloys containing rare earth elements, although coatings of pyrolytic carbons can also be used and the neutron absorbing substance is preferably gadolinium, but also can be samarium or europium, in the form most advantageously of halogen compounds. The arrangement whereby the quenching elements have particles with neutron-absorbing substances in amounts sufficient to survive the thermal neutron flux for at least the duration of the residence times of the fuel elements with which they are associated, enable the invention to be used with particle effectiveness for a nuclear reactor in which the fuel elements may be graphite balls containing the fissionable fuels and traverse the reactor in a single pass, e.g. when the reactor operates on the so-called OTTO-process. When the pure rare earth elements are used as coating materials, they can cover the melting point range between 800.degree. C. for samarium to 1650.degree. C. for lutetium, thereby establishing corresponding thresholds for release of the neutron absorption substance. Consequently, we can make the particles respond to relatively low temperatures as well as relatively high temperatures of potential failure of a particular reactor. Alloys of these metals can also be used and when the coating is constituted of gadolinium, samarium, europium or dysprosium, the coating also acts as a protective coating against nuclear decomposition of the absorbing substances since the coating also acts as a neutron getter or absorber. The use of pyrolytic carbons has been found to be advantageous because it becomes progressively more porous at temperatures above 1000.degree. C. and thus allows proportioning the reactivity reduction release of the absorbing substance at temperature increase. The higher the temperature in the rector core, therefore, the more of the absorbtion substance will be released and cooled onto the surfaces of the fuel element. As the temperature in the reactor core drops, the pyrolytic carbon coating becomes less permeable until eventually it hermetically seals. The preferred halides of the gadolinium, samarium or europium or other rare earth absorbing substances are fluoride, bromide and iodide. These halides are stable at high temperatures and pyrolytic decomposition does not occur prematurely or undesirably when the material is introduced into the particles or distributed in the hot reactor core. Another neutron-absorbing substance can be used as long as its vapor pressure and stability allows it to be utilized in the manner which has and will be described although the ones named have been by far found to be the best.