RU2444796C1 - Method for removing channel uranium-graphite nuclear reactor from service - Google Patents

Abstract

FIELD: power industry.

SUBSTANCE: method for removing the channel uranium-graphite nuclear reactor from service, which is installed in concrete shaft formed with walls and bottom, which has upper and lower steel structures with holes, graphite brickwork, side steel structures, cavity with san backfilling between side steel structures and walls of concrete shaft, which involves removal of available reactor equipment; reinforcement of the reactor bottom with reinforced damp-proof concrete; formation of protective coating above the reactor and charge. Platform for arrangement of reactor plant is chosen so that the enveloping geological structure and engineering facilities form safety barriers which are enough for removal from service as per the in-situ burial; at that, containing formed with natural and artificial barriers is used for burial of radio waste in the matrix from fine composition on the basis of clay minerals.

EFFECT: invention allows removing from service the channel uranium-graphite nuclear reactor under conditions of absence of regional burial sites for radioactive waste containing long-lived radionuclides, with reduction of volumes of removal works, radiation exposures on the personnel and the period of final insulation of cumulative activity.

5 cl, 1 dwg

Description

The invention relates to the nuclear industry in terms of technology for decommissioning of channel uranium-graphite nuclear reactors and can be used to reduce the radiation impact on the environment, the population and personnel.

A uranium-graphite reactor, after its transfer from a nuclear-hazardous to a nuclear-safe state, is a radiation hazardous object and requires the determination of a strategy (method) for its decommissioning.

In world practice, the concept of decommissioning according to the elimination option, which implies the achievement of the final state of the reactor site, the Brown Lawn, is currently accepted as the main one. This option involves the dismantling of equipment and the release of buildings and structures not intended for further use, the processing and removal of all radioactive waste from the territory and bringing it to a condition suitable for the needs of nuclear energy, for example, for the construction of a radioactive waste storage facility or other economic activity, for example, creating an industrial park.

The implementation of this concept is possible both within the framework of the immediate withdrawal strategy, and with the option of delayed dismantling. The strategy for immediate decommissioning of power units in comparison with delayed dismantling has its drawbacks. In this case, the dismantling works are carried out on equipment having higher levels of radioactivity, which requires the adoption of more serious and costly measures to protect the personnel performing these works.

The main way to ensure minimum radiation exposure limits from a shutdown reactor is to delay dismantling after a period of safe storage. According to this method, the reactor structure is brought into a nuclear-safe state and left under observation for a certain period of time - from 30 to 100 years or more, and then dismantled. During long-term storage, the decay of radionuclides leads to a decrease in the activity and dose rate at the facility and facilitates the dismantling process (Elagin Yu.P. Regulation of decommissioning of nuclear power plants. - Nuclear Technology Abroad, 2007, No. 1, pp. 3-12).

A uranium-graphite reactor, for example, an ADE-type industrial uranium-graphite reactor, after its transfer from a nuclear-hazardous state to a nuclear-safe state, is a structure consisting of graphite masonry composed of columns of graphite blocks and supporting metal structures (MK) surrounding it, also performing radiation protection functions.

In the masonry, upper and lower metal structures there are coaxial holes that form the path of the technological channel for the placement of fuel rods. Moreover, in a nuclear-safe state, fuel rods from the reactor are removed. The entire reactor structure is located on the basis of a concrete shaft, the walls of which perform the function of radiation protection and the supporting structure. Between the side metal structures and the walls of the mine there is a sand backfill.

A known method of decommissioning industrial uranium-graphite reactors, comprising two stages. At the first stage: the state of the reactor equipment (RO) is evaluated within the reactor mines; the volume, labor and priority of dismantling are determined; In accordance with the design and construction documentation, organizational and technical measures are developed and implemented to maintain the RO in a safe condition. At this stage, dismantling works and work to create and strengthen protective barriers that reliably isolate the reactor from the environment are carried out within the reactor shaft. All concreting works are carried out using waterproofing concrete. Dismantling of activated RO can be carried out without delay from the start of decommissioning with appropriate justification and development of radiation protection measures, or after exposure necessary for the decline in activity of the RO to be dismantled. At the time of decommissioning, including the exposure time of the reactor, it is required: monitoring the state of the RO inside the reactor shaft; maintenance of systems ensuring the safety of the reactor; environmental monitoring. The duration of the 1st stage is determined by the duration of the work and is 3-5 years excluding the exposure time of contaminated equipment. Stage 2 (monitoring) provides for work related to monitoring: the temperature of the graphite masonry, the relative humidity of the masonry, the decrease in radioactivity, the mechanical properties of the metal, and the corrosion state of the supporting metal structures of the reactor space. Monitoring and maintenance of localized reactor equipment, supporting metal structures have been performed for at least 100 years. [The concept of decommissioning PUGR ADE-2 - JSC GI VNIPIET, Inv. No. 06-05769, 2007] is a prototype.

The specified method (prototype) has the disadvantages:

- significant amounts of dismantling work in radiation hazardous conditions;

- large volumes of radioactive waste generated during dismantling of reactor equipment;

- the terms of the decommissioning of the reactor are stretched to 100 years, which is unacceptable from the point of view of shifting the solution to future generations;

- the lack of scientifically sound disposal methods and an industry-accepted program on methods for handling radioactively contaminated graphite;

- the inexpediency of reburial of graphite masonry, the activity of which is determined by long-lived radionuclides, in specially designed storage facilities, usually calculated for 50 years of operation;

- lack of national, regional repositories for radioactive waste containing long-lived radionuclides. The volume of such waste will increase by an order of magnitude compared with the period of operation of a power unit.

The present invention is to develop a method that is free from the disadvantages of the prototype.

The present invention is aimed at achieving a technical result consisting in decommissioning of a channel uranium-graphite nuclear reactor in the absence of scientifically sound methods for the disposal of radioactively contaminated graphite, regional repositories for radioactive waste containing long-lived radionuclides, with a reduction in the volume of dismantling work, dose loads on personnel and deadline for the final isolation of accumulated activity.

To obtain the specified technical result in the proposed method of decommissioning a channel uranium-graphite nuclear reactor installed in the walls formed by the walls and the base of the concrete shaft, having upper and lower metal structures with holes, graphite masonry, side metal structures, a cavity with a sand filling between the side metal structures and the walls concrete mine, including dismantling of accessible reactor equipment, reinforcing the base of the reactor with reinforced waterproofing concrete, the formation of a protective overlap above the reactor and the shaft, preliminary at the construction stage, the site of the reactor installation is selected so that the surrounding geological structures and engineering structures form a multi-barrier safety system sufficient for decommissioning according to the on-site disposal option. At the final stage of the life cycle of a channel uranium-graphite nuclear reactor, the containment formed by natural and artificial barriers is used for the disposal of radioactive waste generated during decommissioning of a reactor installation and rehabilitation of the industrial site.

In order to reduce the permeability of sand filling with radionuclides, a gel-forming solution with an inorganic ion exchanger is injected into it.

The radioactive waste generated during decommissioning is predominantly finally isolated in the reactor space, limited on one side by graphite masonry and on the other hand by biological protection elements, in a matrix of finely dispersed clay minerals.

If necessary, existing security barriers are strengthened and additional ones are created, based on the implementation of the concept of deeply echeloned (multi-barrier) protection.

In addition, mining, which forms a natural containment that is able to absorb significant external and internal loads and influences, is mainly chosen as the site of construction of the reactor installation.

The implementation of the present invention makes it possible to significantly reduce the time and cost of decommissioning the reactor without shifting the solution to future generations, as well as significantly reduce the risks of uncontrolled spread of radionuclides outside the industrial site of a uranium-graphite nuclear reactor and the integral dose burden on personnel.

The figure shows a diagram of a reactor installation after the implementation of the present invention on the example of an industrial uranium-graphite reactor ADE-2.

The reactor is located in the mine workings. The mountain range forms a natural safety barrier 8, which, together with existing and additionally created protective barriers, will ensure the fulfillment of modern radiation safety requirements. In addition, the massif performs the function of the main structural element of the underground structure, which is able to absorb significant external and internal technological loads and impacts. A gelling solution containing an inorganic ion exchanger is injected into the sand bed 5. The reactor space 3 is used for the disposal of radioactive waste generated during decommissioning of reactor equipment. Radioactive waste is finally isolated in the reactor space in a matrix of a finely dispersed composition based on clay minerals by layer-by-layer filling of the reactor space, eliminating the formation of local voids. After filling the reactor space and reinforcing the existing safety barriers, access to the mine working in which the reactor is located is stopped by filling the free space with rubble stone 9. Thus, in the final state, the reactor is protected by a multi-barrier system that provides reliable isolation of radionuclides from the environment, combining existing barriers (graphite masonry 1, casing 2, concrete shaft 6, surrounding rock formation 8) and newly created protective barriers (backfill with clay 3, injection putting a gelling solution containing an inorganic ion exchanger into the sand backfill 5, concreting the subreactor space 7, overlapping, sealing the reactor shaft 10).

Claims (5)

1. Method for decommissioning a channel uranium-graphite nuclear reactor installed in a concrete shaft formed by the walls and the base, having upper and lower metal structures with holes, graphite masonry, side metal structures, a cavity with sand filling between side metal structures and concrete mine walls, including dismantling accessible reactor equipment, reinforcing the base of the reactor with reinforced waterproofing concrete, forming a protective overlap above the reactor and the shaft, excellent in that the site of the reactor installation is selected so that the surrounding geological structures and engineering structures form safety barriers sufficient for decommissioning according to the option of on-site disposal, while the contour formed by natural and artificial barriers is used for the disposal of radioactive waste in a matrix of finely divided compositions based on clay minerals. 2. The method according to claim 1, characterized in that the existing barriers are strengthened by injecting a gelling solution with an inorganic ion exchanger into the sand backfill. 3. The method according to claim 1, characterized in that the localization and final isolation of the main radioactively contaminated components of equipment, building structures, radioactive waste is carried out within the reactor shaft. 4. The method according to claim 1, characterized in that the reactor is located in a mine working, forming a natural containment, which is able to absorb significant external and internal technological loads and impacts. 5. The method according to claim 1, characterized in that when forming a deeply echeloned (multi-barrier) protection, a combination of existing barriers (casing, concrete shaft surrounding the mine workings, etc.) and newly created protective barriers (backfilling with clay, concreting sub-reactor space, overlap, pressurizing reactor shaft, etc.), and the total number of barriers is not less than six.