Patent Number: 046817328
Section: claims

1. In a method of reducing the reactivity of and shutting down a gas-cooled graphite-moderated nuclear reactor having a reactor core containing fuel elements of nuclear fuel embedded in graphite and having graphitic surfaces along which a cooling gas can flow, the improvement which comprises the steps of: (a) forming quenching-element particles by enclosing and sealing in a sheath stable at temperatures below a predetermined shutdown temperature for said core, a neutron-absorbing substance which is in a gas phase above said predetermined shutdown temperature, the sealing of said particles by said sheath ceasing upon heating of said particles to a temperature above said predetermined shutdown temperature;  (b) incorporating a plurality of said particles in a graphite body permeable to said neutron-absorbing substance in said gas phase to form a quenching element, said graphite body having an outer surface permeable to said neutron-absorbing substance in said gas phase; and  (c) introducing at least one of said graphite bodies into said core of said nuclear reactor so that said body is present during normal operation but, upon an elevation of the temperature of said core above said predetermined shutdown temperature, said substance is liberated from said particles, penetrates through said body in the form of a gas and deposits on free graphite surfaces of said core, said particles each having a size sufficient to render them self-shielding againt neutron flux in said core.  quenching-element particles each having sealed sheaths stable at temperatures below a predetermined shutdown temperature for said core, and a neutron-absorbing substance sealed in said sheaths and which is in a gas phase above said predetermined shutdown temperature, the sealing of said sheaths ceasing upon heating of said particles to a temperature above said predetermined shutdown temperature; and  a graphite body permeable to said neutron-absorbing substance in said gas phase and in which a plurality of said particles are incorporated so that said particles are present during normal operation but, upon an elevation of the temperature of said core above said predetermined shutdown temperature, said substance is liberated from said particles, penetrates through said body in the form of a gas and deposits on free graphite surfaces of said core, said particles each having a size sufficient to render them self-shielding against neutron flux in said core, said graphite body having an outer surface permeable to said neutron-absorbing substance in said gas phase. 2. The method defined in claim 1 wherein said body is formed with outer configuration and dimensions corresponding to that of one of said fuel elements of said core. 3. The method defined in claim 1 wherein said particles are introduced into said core in said quenching elements in an amount such that said core contains a mass of said substance sufficient to survive burn-out of said nuclear fuel with a neutron absorption effectiveness diminishing with such burn-out of the fuel. 4. The method defined in claim 1 wherein said sheath is composed of a material selected from the group which consists of pyrolytic carbon, rare earth metals and rare-earth-metal alloys, and said substance is a halogen compound of gadolinium, samarium or europium. 5. In a gas-cooled graphite-moderated nuclear reactor having nuclear fuel elements in a core containing at least one quenching element, the improvement in which said quenching element comprises: 6. The improvement defined in claim 5 wherein said body has outer configuration and dimensions corresponding to that of one of said fuel elements of said core. 7. The improvement defined in claim 5 wherein said particles are present in said core in said quenching elements in an amount such that said core contains a mass of said substance sufficient to survive burn-out of said nuclear fuel with a neutron absorption effectiveness diminishing with such burn-out of the fuel. 8. The improvement defined in claim 5 wherein said sheath is composed of a material selected from the group which consists of pyrolytic carbon, rare earth metals and rare-earth-metal alloys, and said substance is a halogen compound of gadolinium, samarium or europium. 9. The improvement defined in claim 5 wherein said body contains a fissionable nuclear reactor fuel.