RU2184995C2 - Method for servicing fast homogeneous reactor - Google Patents

Abstract

FIELD: servicing nuclear reactors. SUBSTANCE: minor actinides are extracted from spent nuclear fuel. For initial charge of reactor core use is made of fuel mixture of plutonium and aluminum granules incorporating 1-2 atom percent of plutonium and the rest of aluminum. Criticality of core reducing with first-charge fuel burn-up is maintained by adding granules of minor actinides. Plutonium burnout yields fuel in the form of metal alloy of minor actinides and aluminum containing 3-3.5 atom percent of minor actinides and 96.5-97 atom percent of aluminum. Fuel composition obtained is maintained till end of burnout process of accumulated minor actinides. Then minor actinide burn-up in core is compensated for by plutonium. Combustion of minor actinides results in fuel in the form of metal alloy containing 1-2 atom percent of plutonium and the rest of aluminum. Composition of fuel mixture obtained is maintained till end of combustion of available plutonium. Proposed method enables destruction of highly radioactive and long-living products of nuclear fuel irradiation (minor actinides). EFFECT: enlarged range of structural materials for core container manufacture; reduced accumulated stock of weapon plutonium. 3 cl, 1 dwg

Description

 The invention relates to nuclear engineering, in particular to methods for operating fast homogeneous nuclear reactors.

 A known method of operating a fast homogeneous nuclear reactor, which consists in the fact that the active zone is loaded with metal fuel containing plutonium (Kessler G. Nuclear Power, M., Energoizdat, 1986, p.182).

 During the operation of this reactor, not only electricity and heat are obtained, but also secondary nuclear fuel is produced, which can be used not only in fast neutron reactors, but also for the production of nuclear weapons.

 At present, programs for the development of a "peaceful atom" in almost all industrialized countries are being frozen. Moreover, in some countries (for example, Sweden), they are not only not going to build new nuclear reactors, but they are also shutting down operating, not exhausted, reactors. In this regard, the long-standing issue on the agenda of the disposal and disposal of radioactive waste is even more acute.

 The end of the Cold War and the cessation of the arms race also does not reduce the stockpiles of radioactive materials and exacerbates the situation with their disposal and disposal.

 The problem of disposal and disposal, among all other reasons, is very significantly aggravated by the fact that during the burning of nuclear fuel it forms long-lived radioactive elements - minor actinides (MA), due to which the processing process is complicated many times and the choice of options and places is narrowed burial of radioactive waste.

 To implement this method, it is necessary to use heterogeneous reactors in which the active zone is loaded with glass-coated heat-generating elements. Because of this, the fuel cycle becomes more complicated and more expensive and the costs of implementing the method itself increase. In addition, a high degree of fuel burnup cannot be achieved in the glass-covered fuel elements due to fuel swelling and the release of gaseous fission products, which leads to the destruction of the shells of the fuel elements and the ingress of radioactive products into the circulation circuit.

 The closest set of essential features to the proposed solution is a method of operating a fast homogeneous nuclear reactor, which consists in the fact that the processed nuclear fuel in the form of granules is loaded into the active zone, then the fuel in the active zone is melted, a part of the fuel is removed from the zone, granulated, separated from a liquid metal coolant, and again served in the active zone (RF patent 2031455, CL G 21 C 1/00, 3/42, 1990).

 The use of homogeneous reactors in this method allows us to simplify and reduce the cost of the technology for the manufacture of fuel elements. The problems associated with the danger of destruction of the shells of the fuel elements are also removed.

 However, when implementing this method, excess fuel is also accumulated in the fast reactor containing plutonium and minor actinides, and volatile and gaseous fission products.

 In the light of the cessation of the arms race, an increase in the amount of plutonium is undesirable because, firstly, there is nowhere to use plutonium, secondly, the problem of radioactive waste disposal becomes more acute every year, and thirdly, old measures need to be improved and new measures developed, aimed at ensuring the safety of plutonium.

 The presence of fission products in the circulation loop leads, due to the difficulty in retaining gaseous substances, in general, and the great length of the circuit, in particular, to the danger of fission products getting outside the nuclear power plant.

 However, the most significant drawback of this method is that the method not only does not allow the burning of minor actinides in secondary fuel, but also leads to their accumulation.

 The task to which the invention is directed is to increase the safety of nuclear energy, including environmental safety, and its economy.

 Implementation of the proposed method will destroy highly radioactive and long-lived products of nuclear fuel irradiation - minor actinides, without increasing the melting temperature of the fuel in the core, reduce the mass of produced plutonium and radioactive waste to be disposed of.

 The specified technical result is achieved by the fact that in the method of operating a fast homogeneous nuclear reactor, which consists in the fact that spent nuclear fuel in the form of granules is loaded into the active zone, then the fuel in the active zone is melted, some of the fuel is removed from the zone, it is granulated, and separated from the liquid metal coolant, and again served in the active zone, minor actinides are extracted from the spent fuel and granulated, granules of plutonium are mixed uniformly with aluminum granules, a fuel mixture is obtained containing 1-2 at. %% plutonium, the rest is aluminum, which is used as fuel for the initial loading of the active zone, the initial loading fuel is melted in the active zone and the reactor is brought to a critical state, part of the burnt fuel is discharged, fission products are removed and compensated burnout of plutonium by adding minor actinide pellets to the fuel; after burning out of plutonium, fuel is obtained in the form of a metal alloy containing 3-3.5 at. %% minor actinides and 96.5-97 at. %% aluminum, and continue to operate a fast homogeneous reactor, maintaining the resulting fuel composition until the end of the process of burning accumulated minor actinides, in addition, by the fact that plutonium and aluminum are used to make a metal alloy containing 1-2 at. %% plutonium, the rest is aluminum , which are then granulated, as well as by the fact that the burnout of minor actinides in the core is compensated by the addition of plutonium granules to the fuel, after combustion of the minor actinides, the fuel is obtained in the form of a metal alloy containing 1-2 at. %% p Lutonium, the rest is aluminum, and they continue to operate a fast homogeneous reactor, maintaining the resulting fuel mixture until the end of the combustion process of the existing stock of plutonium.

 To clarify the essence of the invention can be considered, for example, such a thermal hydraulic circuit of a fast homogeneous nuclear reactor, which is presented in figure 1.

 A fast homogeneous nuclear reactor contains an active zone 1, which is located in the container 2. The walls of the container 2 can be made cooled “clean”, that is, non-radioactive, liquid metal coolant circulating in a separate circuit (not shown in the drawing).

 In the active zone 1 can be installed starting assembly 3 and / or regulatory bodies 4.

 Openings 6 are made in the bottom 5 of container 2 and a gas chamber 7 is located under it, which during reactor operation is filled with some neutral gas, for example, argon, ending in a pool 8 with a liquid metal coolant (for example, sodium).

 The gas chamber 7 is connected to a device 9 for supplying a neutral gas and, through a pipe 10, to a device 11 for cleaning gaseous and volatile fission products.

 The pool 8 through a dispersed heat exchanger 12 is in communication with a vortex hydrocyclone 13 connected to the active zone 1, and using a pipe 14, which is equipped with a normally open valve 15, with an intermediate heat exchanger 16.

 The intermediate heat exchanger 16 is followed by a low-pressure manifold 17, which is in communication with the chamber 18 for collecting the liquid metal coolant above the core 1 and, through a circulation pump 19, with a high-pressure manifold 20, to which injections 21 of the fuel circuit and other devices are connected.

 To enter the fuel into the circulation circuit on the pipeline 22, leading from the high pressure manifold 20 to the hopper 23 with the "fresh" fuel connected to the dispersed heat exchanger 12, a valve 24 is installed.

 The burned-out fuel is removed into the bunker 25 for storing the spent fuel through a pipeline (not shown in the drawing), "embedded" in the circulation circuit in the area between the dispersed heat exchanger 12 and the vortex hydrocyclone 13.

 A bypass pipe 26 with a normally closed valve 27 is also extended to the bunker 25.

 The fast homogeneous nuclear reactor depicted in the drawing is operated as follows.

 Minor actinides are separated from the components of the spent fuel (for example, in a smelting bath) and granules are made from them.

 At the factory or immediately before loading fuel into a homogeneous fast reactor, plutonium granules (Pu) are uniformly mixed with aluminum granules (Al) and a fuel mixture containing 1-2 at. %% plutonium is obtained, the rest is aluminum, so that in future it can be used for the initial reactor loading.

 To reduce the number of operations conducted directly at the industrial site of a nuclear power plant, it is possible at the beginning (for example, in the factory) to produce a Pu + Al alloy containing 1-2 at. %% Pu, the rest is Al, and then granulate it.

 Then, before starting up the reactor, the container 2 of the active zone 1 is filled with fuel pellets Pu + Al, each of which will contain 1-2 at. %% Pu, 98-99 at. %% Al.

The initial loading of the reactor with a fuel mixture of plutonium and aluminum is due to the desire to lower the melting temperature of the fuel, because if at the beginning of the campaign to place fuel consisting of actinides and aluminum, to achieve criticality it is necessary that the minor actinides in the fuel mixture be at least 12, 5 at.%, Which, according to calculations, increases the melting point of the fuel to 900 ^o C. Such a high melting point significantly narrows the choice of structural materials for the container of the active zone, makes p The factor is more expensive, and also reduces its reliability and safety.

 As for the quantitative indicators of plutonium in the fuel mixture, they are selected on the basis that, with a plutonium value of less than 1 at.%, The fuel mixture in the core does not reach criticality, while when the plutonium value is more than 2 at.%, The reactivity becomes excessively high , which leads to lower reactor safety, unjustified complication and cost of the reactor control and protection system.

 The fuel is introduced into the circulation loop hydraulically by opening valve 24. Fuel from the hopper 23 is sent by a liquid metal coolant (sodium) stream to a dispersed heat exchanger 12, and from it to a vortex hydrocyclone 13. In the hydrocyclone 13, the fuel granules are separated from sodium and then fed to core 1. After that, valve 24 closes.

 The cooling circuit (not shown) of the container 2 of the core 1 may be provided, for example, with an electric heater (not shown). The liquid metal coolant heated by an electric heater is supplied to the container 2 and heats and melts the fuel granules through the walls. The initial melting of the fuel can also be achieved using the launch assembly 3 and / or regulatory bodies 4.

 The fuel melted in the core 1 through holes 6 in the bottom 5 of the container 2 flows in thin jets into a gas chamber 7 filled with an inert gas, where, under the action of gravity, the fuel jets burst into droplets. Drops of liquid fuel fall into pool 8 with sodium, granulate without using any auxiliary equipment, and heat it. The high temperature of liquid fuel and the large surface of the jets and droplets of liquid fuel contribute to the effective exit of gas and volatile fission products in the gas chamber 7. Connecting to it a system for cleaning gas and volatile fission products 11 can significantly improve the safety level of the reactor.

 From the pool 8, the fuel granules are again transferred through a dispersed heat exchanger 12 to a vortex hydrocyclone 13 through a dispersed heat exchanger 12, where they are separated from the sodium heated by them, which goes into the intermediate heat exchanger 16 and transfers heat to the second circuit there.

 Granules of fuel, together with a small part of sodium (volume fractions of fuel and coolant are approximately equal), are lowered in chamber 18 to the upper part of core 1, limited by the level of molten fuel, and enter the molten fuel of core 1. Sodium as a lighter fraction (sodium 20 times lighter than fuel), pops up and is diverted from the chamber 18 to the low pressure collector 17, and the fuel granules under the influence of gravity move down the core 1 from top to bottom and melt.

 From the intermediate heat exchanger 16, the cooled sodium through the low pressure manifold 17 enters the circulation pump 19, from it into the high pressure manifold 20, and from there the sodium enters the injections 21 of the fuel circuit and other devices and, in particular, the hopper 23, from which as plutonium burns out, granules of minor actinides begin to enter the circulation loop to maintain the reactor in critical condition.

 Fuel with burnt plutonium, containing solid and partially gaseous and volatile fission products, is diverted to the hopper 25, for which a pipeline 26 is opened with a normally closed valve and pipeline 14 is closed with a normally open valve 15. After that, valves 15 and 27 are brought to their normal position.

 To ensure the equilibrium composition of the fuel in the core 1 during operation of a fast homogeneous reactor, the MA content in it is gradually increased to 3-3.5 at. %%, and the remaining volume of the core 1 is filled with aluminum.

 With this composition of the fuel mixture, the reactor enters a stationary mode of operation with excess reactivity close to zero. In this mode, when the MA content in the fuel is less than 3 at.%, The fuel mixture in the core volume does not reach criticality, while when the MA content in the fuel is more than 3.5 at. % high excess reactivity appears, which leads to a decrease in reactor safety.

 After the fuel reaches the specified equilibrium composition, i.e. MA - 3-3.5 at. %%, Al - 96.5-97 at. %%, operation of a homogeneous fast reactor can continue until all actinides accumulated in the stockpile are burned.

 At some point in time, however, the reserves of actinides will come to an end, and in the active zone 1 some of them will remain. Then, in order to burn out these residues, and at the same time to reduce the accumulated plutonium reserves, as well as if the reactor has not exhausted its resources in order to continue to receive electricity and heat, they start adding plutonium granules to the fuel from actinides and aluminum, compensating for the burning out of actinides and thereby supporting the reactor in critical condition. After burning minor actinides, the fuel is obtained in the form of an alloy containing 1-2 at. %% plutonium and 98-99 at. %% aluminum, and until the end of the campaign, the reactor continues to run on fuel of this composition.

 Thus, the application of the proposed method will destroy highly radioactive and long-lived products of nuclear fuel irradiation - minor actinides, reduce the mass of produced plutonium and radioactive waste to be disposed of, expand the selection of structural materials for the manufacture of the core container and reduce its cost, increase reliability, safety and efficiency the reactor.

Claims (3)

 1. The method of operation of a fast homogeneous nuclear reactor, which consists in the fact that spent nuclear fuel in the form of granules is loaded into the active zone, then the fuel in the active zone is melted, some of the fuel is removed from the zone, granulated, separated from the liquid metal coolant and again fed into the active zone, characterized in that minor actinides are extracted from the spent fuel and granulated, plutonium granules are mixed uniformly with aluminum granules, and a fuel mixture containing 1-2 at. %% plutonium, the rest is aluminum, which is used as fuel for the initial loading of the active zone, the initial loading fuel is melted in the active zone and the reactor is brought to a critical state, part of the burned fuel is discharged, fission products are removed, and plutonium is burned out by adding minor pellets actinides, after burning plutonium get fuel in the form of a metal alloy containing 3-3.5 at. %% minor actinides and 96.5-97 at. %% aluminum, and continue to operate a fast homogeneous reactor, maintaining the resulting fuel composition until the end of the process of burning accumulated minor actinides.  2. A method of operating a fast homogeneous reactor according to claim 1, characterized in that a metal alloy containing 1-2 at. %% plutonium, the rest is aluminum, which is then granulated.  3. A method for operating a fast homogeneous nuclear reactor according to claim 1 or 2, characterized in that the burnout of minor actinides in the core is compensated by adding plutonium granules to the fuel, after burning minor actinides, the fuel is obtained in the form of a metal alloy containing 1-2 at. %% plutonium, the rest is aluminum, and they continue to operate a fast homogeneous reactor, maintaining the resulting fuel composition until the end of the process of burning the existing stock of plutonium.