1,147,290. Reactors. UNITED STATES ATOMIC ENERGY COMMISSION. 8 March, 1968 [8 May, 1967], No. 11333/68. Heading G6C. A pulsed nuclear reactor for producing bursts of fast neutrons comprises a molten salt fuel floating on a liquid metal coolant which is used to raise the fuel into an active core region surrounded by a neutron reflector. The molten salt becomes supercritical within the active core region, emits a short intense burst of fast neutrons, and then returns to a subcritical condition. As shown in Fig. 1, the reactor containment vessel comprises an upper section 1 and a lower section 2 communicating through a centrally positioned tubulation 3. The active core region 4 is surrounded by a graphite reflector 5 and a container-reflector 6 which is the upper end portion of the tubulation 3. The molten salt fuel 7 preferably comprises a LiF-UF 4 mixture, and molten Pb is preferred as the liquid metal coolant 8. Test materials to be exposed to neutron irradiation are disposed within a test cavity 16 located centrally within the core region 4. The fuel 7 which floats on the coolant 8 is driven upward through the tubulation 3 by means of a pulse of pressurized gas injected through a conduit 20 into the annular region 19 between the tubulation 3 and the lower section 2 of the containment vessel. When the fuel level is approximately three inches below the top of the reflector 5, the reactor becomes critical on prompt neutrons. As the remainder of the core region 4 fills, the reactor becomes supercritical to provide the desired burst of fast neutrons, the fuel temperature increasing rapidly causing the reactor to become subcritical due to the negative temperature coefficient of reactivity of the fuel. As the molten lead 8 continues to rise, the molten salt fuel 7 overflows into a heat exchange region 9 where it is cooled by direct contact with relatively cool molten lead pumped through piping 22 to discharge manifolds 10, 11. The heated lead flows through an air-cooled heat exchanger 23 and a freeze valve 24. When the entire mass of molten salt fuel 7 has been cooled to the starting temperature of 500‹ C., the freeze valve 24 is closed while pump 21 continues pumping lead into the region 9, thus displacing the fuel upward so as to flow back over the reflector 5 and down through the core region 4 to accumulate in the tubulation 3 where it floats on the coolant 8. The reactor is then ready for generating a new burst of neutrons. Deflecting vanes 12 disposed immediately above and annular plates 13 disposed immediately below the core region 4 contain material having a high neutron absorption cross-section, the former rapidly terminating the neutron chain reaction occurring in the molten salt fuel as it leaves the core region and the latter preventing the premature development of a chain reaction in the fuel before it enters the core region. Tiers of inverted cups (15) (Fig. 3, not shown) are supported by braces 14 within the core region 4, the cups retaining a portion of the expanding molten salt fuel and preventing its leaving the core region so as to partially compensate for the very large negative temperature of reactivity of the fuel. The cups are not completely filled and the size of the resulting voids is reduced during the neutron burst by the expanding fuel causing a positive reactivity insertion to occur due to the negative void coefficient of reactivity of the reactor. By proper selection of the number and size of the inverted cups and the pressure within the reactor prior to a burst, the negative temperature coefficient which is operative on the burst may be regulated to achieve the desired burst size.
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