RU2688137C1 - Method of handling of spent reactor graphite nuclear uranium-graphite reactor - Google Patents

Abstract

FIELD: processing and recycling wastes; nuclear physics.

SUBSTANCE: invention relates to the field of solid radioactive wastes treatment, in particular to the spent graphite (SG) at decommissioning of uranium-graphite reactors. Method of handling spent reactor graphite during decommissioning of uranium-graphite reactors involves preliminary temporary holding of graphite masonry, dismantling of masonry, packing of spent graphite into containers and burial in near-surface or deep burial place. Prior to dismantling of graphite masonry, water-based acrylic primer is impregnated with graphite blocks.

EFFECT: invention makes it possible to create additional protective safety barriers during dismantling of SG, transportation of graphite blocks and packing of SG into containers and to reduce collective radiation doses of personnel when handling SG during decommissioning of uranium-graphite reactors.

1 cl, 1 dwg, 1 tbl

Description

The invention relates to the field of solid radioactive waste (RW), in particular, the management of spent graphite (OG) during the decommissioning of uranium-graphite reactors (UTR).

One of the most pressing problems associated with the decommissioning of nuclear power plants with uranium-graphite reactors is the problem of handling radioactive graphite. The total amount of reactor graphite at decommissioned NPPs in Russia will be ~ 40,000 tons.

Graphite is used as a moderator in uranium-graphite reactors. As an example, the active zone of the RBMK-1000 uranium-graphite reactor is constructed of densely standing graphite columns of 250 * 250 * 8000 dimensions with a vertical opening 114 mm in diameter across the entire height of each column. Each column consists of 250 * 250 * 600 graphite blocks. The total mass of the total exhaust gas at the 1st unit of the Leningrad NPP is distributed as follows:

In the process of decommissioning a nuclear power unit of the RBMK type, all of the graphite from the graphite stack goes into the category of RW 1 ÷ 4 class.

Given the combustibility of graphite, its storage requires special security measures. In addition, carbon is one of the most common elements of living systems. Therefore, when released into the environment, it can become part of living systems and subject it to internal irradiation, leading to various negative consequences.

Radionuclides in graphite during operation appear as a result of:

1 - neutron activation of impurities of graphite and the matrix carbon itself, as well as activation of the gas mixture cooling graphite;

2- falling into the masonry of fission products of nuclear fuel and corrosion products, as well as the fuel itself in violation of the tightness of technological channels (TC). Cells in which TK depressurization occurred are known from the reactor operation logs.

Graphite, the radioactivity of which is determined by radionuclides of activation origin, are, as a rule, classified as RW 2-4. The class 2 solid RW is determined by the long-lived radionuclide C-14 carbon with a maximum specific activity of no higher than 10 ⁶ Bq / g. The third and fourth classes of solid RW are determined mainly by Co-60 cobalt.

When injected into the graphite stack of uranium-235 and transuranic radionuclides in case of accidental NM leakage from process channels, the formation of 1st and 2nd class RW is possible. The sites with these radioactive contaminations are located in separate places of the masonry. These sites are few, they are limited in volume. With an increase in the operating time, the number of these areas after the fuel breakthrough may increase, and contamination of the nearest neighboring graphite columns is possible. Federal Law of the Russian Federation No. 190-FZ of 07/11/2011 requires underground disposal of solid, long-lived radioactive waste of class 2 to a depth of more than 100 m.

Until recently, there was no uniform, environmentally safe and reliable way to handle such graphite in the world experience, with its final disposal as solid RW, especially for RBMK high-power uranium-graphite reactors.

All exhaust gas treatment options begin with the temporary exposure of the exhaust gas bed after fuel assembly removal (10-20 years old, applied to all reactors) to reduce radiation loads on personnel due to decay of the main short-lived γ-radionuclides, especially ⁶⁰ Co, after which they immediately follow stages of the treatment of exhaust gases.

There is a method of dealing with exhaust gas, which consists in the underground burial of the entire reactor at the place of its operation with the adoption of safety measures for leaching long-lived nuclides for more than 300 years. In this case, the graphite stack is not removed from the reactor installation. (AO Pavlyuk. Technical solutions and experience of JSC “UDC UGR” on handling of irradiated graphite during decommissioning. Moscow. 21.22.11.2017. Http://www.atomeco.org/mediafiles/u/files/2017/ materials / 06_ATOMEKO_Pavlyuk_A.O..pdf). This technical solution was carried out in the Siberian Chemical Combine. The implementation of this method became possible due to the fact that the upper level of the graphite stack in these reactors is several meters below the zero mark, which made it possible to create a “green platform”.

For UGR high power reactors (RBMK-1000, RBMK-1500), the floor level of which is above the zero level of the central hall by ~ 20 meters, this method of exhaust gas handling is not suitable.

A similar method of processing on small UGR is known - the Kurchatov Institute, where the exhaust gas from the masonry was partially dismantled, and the remaining part of the masonry, together with the reactor vessel, was removed with a crane and moved to the place of further processing (AA Abramov, VV Vagin, and others . The elimination of large nuclear and radiation-hazardous facilities in dense residential buildings in Moscow (Kurchatov Institute Research Center. 2015).

The disadvantage of this method is the mandatory appearance of radioactive dust, which occurs when dismantling the graphite blocks of masonry and moving them to the place of further processing. In addition, the method of removing the entire reactor vessel with the remaining graphite used for small reactors is not applicable to RBMK -1000 and RBMK -1500, due to the large size and weight of the reactors.

Closest to the claimed method to the technical nature and essential features is the method of handling spent graphite, including preliminary temporary exposure of graphite stack, dismantling of the stack, packaging of spent graphite in containers and disposal at a near-surface or deep disposal point (M.A. Tuktarov, L A. Andreeva, AA Romenkov. Conditioning of reactor graphite of decommissioned uranium-graphite reactors for disposal purposes, NIKIET JSC, Moscow, 2016. http: //www.atomic-ener gy.ru/articles/2016/06/08/66585 The scheme for dealing with exhaust gas is shown in Fig. 1. The choice of an exhaust gas disposal strategy at the near-surface or deep disposal site of radioactive waste (RWDF) is determined by a number of technical and economic factors, including location of the nuclear installation (NI), availability of transport infrastructure from NI to RWDF, radionuclide composition of waste, etc.

The disadvantage of the closest analogue is the formation of radioactive dust during the dismantling of graphite stack, transportation of graphite blocks and packaging of exhaust gases in containers. This is due to a change in the mechanical properties of graphite during operation. With long-term irradiation of the RBMK graphite stack with fast neutrons and with thermal loads on graphite columns, the graphite of the reactor loses its density, becomes cracked, loose and brittle, especially in the areas of the central opening of the column, located closer to the technological channel. In graphite columns, bends appear in the direction from the center of the reactor, and longitudinal cracks appear in the columns at the end of their life. Therefore, during any mechanical operation, radioactive dust is formed during dismantling and transportation. Dust contains high concentrations of β-radionuclides, in particular tritium, ³⁶ Cl and radioactive ¹⁴ C, which practically do not decay, during the exposure time of the reactor.

The problem solved by the invention is to create additional safety barriers for dismantling spent graphite, transporting graphite blocks and packaging the exhaust gas in containers

The essence of the proposed technical solution lies in the fact that the method of handling spent reactor graphite during the decommissioning of uranium-graphite reactors, including preliminary temporary exposure of graphite brickwork, dismantling of brickwork, packaging of spent graphite in containers and disposal at a near-surface or deep disposal point, is proposed before dismantling the graphite stack, impregnate the graphite blocks with an acrylic water-based primer.

In addition, it was proposed to apply 2-3 layers of a primer with drying after each impregnation on the surface of the channels in graphite blocks, and immediately, when excavating graphite columns, impregnate their outer surfaces by applying a single primer layer.

In the proposed technical solution, the following distinctive feature is used: prior to disassembling the graphite stack, the graphite blocks are impregnated with an acrylic water-based primer.

In order to substantiate the compliance of the claimed distinctive feature of the invention with the criteria of novelty, inventive step, we give the following. Compared with the closest analogue, the impregnation of graphite blocks with an acrylic water-based primer allows you to create additional safety barriers when dismantling spent graphite of a power unit for transporting graphite blocks and packing exhaust gases into containers. This is due to the fact that the sprayed acrylic water-based impregnation quickly and deeply penetrates the pores and cracks, reliably fixes the loose surface layer of graphite adjacent to the process channels, preventing the formation of radioactive dust at various stages of the handling of spent reactor graphite. The time interval between the application of the layers of impregnation is selected according to the technological passport on the impregnation. The expected consumption of acrylic primer deep penetration ~ 250-350 g / m ² depending on the absorbency of graphite and the number of coating layers.

When the graphite column is removed from the masonry, the graphite blocks will rub against the outer surfaces of the neighboring graphite columns, generating radioactive dust into the airspace of the central hall. To prevent this process, it was proposed to additionally apply a quick-drying acrylic coating on the outer surfaces of graphite blocks directly during the excavation of the column in the central hall of the power unit.

After impregnation of channels and external surfaces, graphite blocks can be moved, laid in containers and prepared for burial.

The use of a water-based deep penetration primer allows for environmental and fire safety during work.

Impregnation of graphite blocks with an acrylic water-based primer was tested on real graphite blocks of the GR-280 type (not used in operation) with dimensions of 250 * 250 * 600 with an internal through-hole with a diameter of 114 mm.

The block was cut across into two equal halves, with dimensions of 250 * 250 * 300. The impregnation was carried out with a paint sprayer from the outer and inner surfaces of both halves of the block. An acrylic primer (brand Knauf-Multi Grund) was used. Exposure between applying a layer of primer 1 day.

Estimation of the propensity of the surfaces of graphite blocks to dust formation was determined as follows:

One half of the graphite block face 250 * 300 fit on a flat surface, on plastic wrap. The second half moved with friction through the first 50 times, with pressure on the first half of the block only with a weight of the second half ~ 27 kg. The number of movements of one half of the graphite blocks relative to the other was chosen on the basis that each graphite column with a height of 8 m, when removed from the masonry, will rub on adjacent columns by two sides.

The collected graphite dust and chips, formed as a result of the abrasion of the halves of the blocks, were weighed on an RE-260 electronic scale.

The tests have shown that the impregnation of graphite blocks with an acrylic water-based primer can reduce dust formation by a one-time treatment by 18 times. When applying two and three layers of primer reduction is 25-30 times. The application of the following layers is not economically feasible.

Thus, the use of the proposed technical solution allows you to create additional protective safety barriers when dismantling spent graphite, transporting graphite blocks and packing exhaust gases into containers and reducing collective radiation doses to personnel when handling spent reactor graphite during decommissioning of uranium graphite reactors

Claims (2)

1. Method of handling spent reactor graphite during decommissioning of uranium-graphite reactors, including preliminary temporary holding of graphite stack, dismantling of stack, packing of spent graphite into containers and dumping at a near-surface or deep disposal point, characterized in that before dismantling graphite stack it is carried out impregnation of graphite blocks with a water-based acrylic primer. 2. A method according to claim 1, characterized in that 2-3 layers of primer are applied to the surface of channels in graphite blocks with drying after each impregnation, and directly upon excavating graphite columns, their outer surfaces are impregnated by applying one layer of primer.

Images