2. THE INDUSTRIAL COMPLEX FOR URANIUM ISOTOPIC ENRICHMENT

2.1. General

Initially, the industrial complex for uranium isotopic enrichment was created in the USSR only to solve the problem of making nuclear weapon. Later, as the civilian nuclear power engineering arose, the capacity of the complex was partly reconstructed for production of low enriched uranium to supply nuclear power plants with reactor fuel. In 1988 the production of highly enriched uranium in the USSR ceased. At present the complex for uranium isotopic enrichment is used only for nuclear fuel production. The low enriched uranium is delivered now both to Russian nuclear fuel market and abroad.

The industrial complex for uranium isotopic enrichment includes:

• uranium hexafluoride production plants;
• uranium isotopic enrichment plants.

At the plants there are special departments for utilization, processing and storage of uranium radioactive wastes, generated in the process of the complex operation.

All the plants work in one whole balanced cycle, i.e. utilization of the plants for both primary uranium hexafluoride and low enriched uranium production is regulated by demands of the internal market of Russia and obligations on international contracts.

On January 1, 1997 two plants for primary uranium hexafluoride production and four plants for uranium isotopic enrichment were operated in Russia:

• The Urals Electrochemical Combine (UECC), Novoural’sk, Sverdlovsk Region - enrichment plant.

  Operation started in 1949.

• The Electrochemical Plant (ECP), Zelenogorsk, Krasnoyarsk Land - enrichment plant.

  Operation started in 1962.

• The Siberian Chemical Combine (SCC), Seversk, Tomsk Region:
• Enrichment plant, operated since 1953;
• UF₆ production plant, operated since 1954.
• The Angarsk Electrolysis Chemical Combine (AECC), Angarsk, Irkutsk Region:
• enrichment plant, operated since 1957;
• UF₆ production plant, operated since 1960.

All the listed enterprises are the State’s property and are subordinated to the Ministry on Atomic Energy of RF. They have right to do independent commercial business. There are no UF₆ production and uranium isotope enrichment enterprises in other countries of the former USSR.

Along with fulfilling their main task - primary and low enriched uranium hexafluoride production - the enterprises are also engaged in some related types of activity: storage and transportation (in packed form) of nuclear materials (uranium), process equipment decontamination, uranium radioactive waste management, including operation of uranium waste storage and disposal facilities at the enterprises’ territory, etc.

The production capacities of the industrial complex for uranium isotopic enrichment are distributed as follows:

All the enrichment plants started with gas-diffusion technology. In 1962 they began to introduce intensively gas centrifuges. In 1992 industrial operation of the gas-diffusion technology was completely ended, and now only gas centrifuges work at all the enrichment plants. The gas centrifuges do not need repair (correspondingly, opening of the internal process space is excluded) for all their service life, which is not less than 15 years.

The uranium isotopic enrichment process technology is continuous and highly automated. The process is carried out in large gas-tight systems under deep vacuum conditions.

By the degree of hazard to the environment the enrichment plants are at the same level as uranium and thorium mining enterprises and structures.

The uranium hexafluoride production plants include the main subdivisions:

• anhydrous hydrogen fluoride production;
• elementary fluorine production;
• uranium hexafluoride production.

To produce uranium hexafluoride high temperature fluorination of uranium raw materials by fluorine gas is used. The elementary fluorine gas is produced by electrolysis of HF in the potassium dihydrotrifluoride melt. At normal conditions the process system pressure does not exceed the atmospheric one.

Before ejection into the atmosphere the process gases undergo special cleaning, leading to formation of solutions, containing uranium and fluorine compounds. The solutions are transmitted to the liquid wastes processing section to recover uranium.

To produce uranium hexafluoride both natural uranium concentrate, oxides or tetrafluoride and slightly irradiated (the specific activity less than 22 000 Bq/g or 0,6x10³ Ci/kg) uranium oxide may be used as raw materials.

All the kinds of natural uranium raw materials are supplied by uranium ore mining and processing enterprises, both Russian and foreign. The oxide of slightly irradiated uranium is produced and delivered by the Siberian Chemical Combine.

Cessation of highly enriched uranium production resulted in emerging of "excessive" enrichment capacities. And this, together with low cost of separation work due to transition of the enrichment plants to centrifugal technology, allowed the plants to go to use of depleted uranium (former years "tailings" of the enrichment plants with uranium-235 content equal to 0,26-0,30 %) as feed material.

In these circumstances the utilization of the primary uranium hexafluoride production plants has fallen to 10-15 % of their capacity, whereas the enrichment plants’ utilization is kept at the nominal level.

The potential risk of uranium hexafluoride production and uranium isotopic enrichment works is connected with the following factors:

• nuclear risk, conditioned by enriched uranium compounds handling, probability of emerging of self-sustained chain reaction (SCR) of uranium nuclei fission;
• radiation hazard of uranium, its decay products while handling uranium compounds;
• chemical hazard of uranium hexafluoride and fluorine compounds, involved in the production cycle;
• general industrial risk, which may be aggravated under certain conditions by the above noted factors.

According to estimates of specialists, taking into account the nature and peculiarities of UF₆ production and uranium isotopic enrichment process technology, probable (hypothetical) accidents, conditioned by emerging of SCR, release of uranium hexafluoride or other uranium compounds, cannot have catastrophic consequences and may be characterized as local, taking place mainly in the limits of production areas.

2.2. The characterization of accumulated radioactive materials (RM) and radioactive wastes (RW) in the industrial complex as a whole and at separate enterprises

UF₆ production plants and uranium isotopic enrichment plants are the most environmentally clean links of the nuclear fuel cycle.

The main type of accumulated radioactive materials are "tailings" of uranium isotopic enrichment plants in the form of UF₆. They are kept in hermetic steel containers at special storehouses under continuous radiological control. As it was noted above, the radioactive materials are used now repeatedly as a feed material for the production of enriched uranium.

In the process of UF₆ production and uranium enrichment plants’ operation both liquid and solid radioactive wastes are generated. The liquid RW are managed so, that they are not accumulated and are not released into open water basins. The sewage waters, containing radionuclides with concentrations lower than PC _B , are not attributed to the category of liquid RW.

The data on amounts of the accumulated radioactive wastes and their activity since the moment of starting up the production works till 01.01.97 are presented in Tables 2.1 and 2.2.

The solid radioactive wastes are cakes, cinders, scrap, rubber technical articles, polyethylene, cotton articles, building garbage, sludges formed as a result of remelting decontaminated metallic equipment or liquid RW decontamination. In order to decrease accumulation of solid RW and return metals to the national economy they created sections for utilization of equipment at the enterprises of the complex. At the sections decontamination and cropping of the equipment is carried out, including partial remelting of decontaminated components.

Table 2.1

The amounts and activity of accumulated solid radioactive wastes, by 01.01.97

^*) - only medium specific activity RW

Table 2.2

The amounts and activity of accumulated radioactive sludges formed as a result of liquid RW decontamination,by 01.01.97

Sludges, formed as a result of the remelting and containing residual activity, are sent to solid RW storage and disposal facilities. The greatest part of the equipment, decontaminated to the level of sanitary standards’ specifications, is sent to civil enterprises as a scrap to be reprocessed.

The scheme of liquid RW management shown below gives an idea of the origin of radioactive sludges.

As far as the volumes of generated liquid RW are insignificant, at the enterprises there are no special waste-water disposal systems with continuous cycle of collection and transportation for processing of the liquid RW. All ladder waters, containing uranium, are collected in special reservoirs and then are periodically transferred into transportation vessels and sent for processing (uranium recovery) in special subdivisions located at the industrial sites of the enterprises.

After the uranium recovery to a residual concentration not higher than 2 mg/l the solutions are treated with lime milk and transferred in the form of lime slurry with uranium concentration less than 0,5 mg/l to slime settling basins, located in specially alotted areas at the enterprises, and built from special designs.

The actual concentration of radionuclides (uranium) in the liquid phase (clarified water) of the slime tanks is much lower (by 20-25 times) than the PC_B, established for drinking water.

As far as the slime settling basins are filled with solid material, they are put in dead storage by means of surface covering with soil. With that the actual radiation dose rate at a distance of 1,0 m from the covering soil surface is some tenths of mrem/h as compared to the permissible value of 2.8 mrem/h (0,028 mSv/h).

In the tables of the chapter some data related to UF₆ production and uranium isotopic enrichment plants at the SCC are absent. It is accounted for by the fact, that the SCC includes, besides the UF₆ production and uranium enrichment plants, a number of other plants, and the scheme of liquid and solid RW management and sewage waters removal to open water reservoirs is arranged so, that it is impossible to separate the radiation effects of the individual plants on the environment (the summary data for the SCC are presented in Chapter 7 of this Overview).

The RW activity depends on the content of uranium isotopes and products of their radioactive decay. The nuclide composition is determined by the type of raw material used: it may be either natural or slightly irradiated uranium.

According to radiation control data, for the complex as a whole the following nuclides are registered: uranium-234, 235, 236, 238, insignificant amounts of plutonium-238, 239, etc., as well as strontium-90, ruthenium-106, cesium-137, cerium-144.

The radionuclides releases into the environment (atmospheric air) are presented in Table 2.3. The values for all the enterprises are insignificant, either absolutely or as compared to calculated standard values of maximum permissible releases (MPR).

Table 2.3

The radionuclides ^*) releases into the atmospheric air in 1996

The similar situation is with radionuclides releases into the open water reservoirs (Table 2.4).

Table 2.4

The radionuclides releases into the open water reservoirs in 1996

^*) - The situation existing at the enterprise scheme of sewage waters removal does not allow to separate the data for individual plants.

^**) - No disposal of sewage waters, containing radionuclides, is carried out.

In 1996 all the enterprises, excluding the ECP, removed 45.2 million m³ sewage waters, mainly the equipment cooling waters, industrial and run-off pour sewerage. The concentrations of radionuclides in the sewage waters are, as a rule, by two orders of magnitude lower than the drinking water standards, certified by the NRB-76/87 Regulations.

The siting and erection of the solid RW disposal points is carried out in accordance with established order and is coordinated with regulatory bodies. Like the construction, the removal of the solid RW disposal points from service and putting them in prolonged storage are carried out under projects, worked out by specialized institutions and meet sanitary standards, presented in SPORO-85. The solid RW isolation and disposal are fulfilled in accordance with licenses, which are given by the regulatory bodies.

At the enterprises of the uranium enrichment industrial complex three types of storage facilities are used for the solid RW storage and disposal.

1. Points, designated for accumulation and disposal of low-level (I-st group of activity) wastes, are open rectangular pits or trenches. Their bottom and slopes are covered with 0,5-1 m thick waterproof clay layer, preventing radionuclides penetration into the environment.

Filling of the pits and trenches is carried out in loose, or in kraft-paper and flexible PVC bags to a level, about 0,5 m below the upper edge. Then the cavity is filled with soil, and the soil layer is compacted to the ground level. To protect the areas of the solid RW disposal points against flooding by flood waters, drainage canals are built, and access roads are covered with asphalt. In order to watch for probable contamination of ground waters by radionuclides, observation holes for sampling the waters are drilled around the disposal points areas and along perimeters of the pits and trenches.

2. Storage and disposal of low-level (2-nd group of activity) sludges, formed as a result of liquid RW decontamination, are carried out in special structures (slime collectors), i.e. banked-up pits, filled with loamy soils.

Bituminous concrete on sand and gravel bed or asphalt, covered by a bitimen layer, and then a protective soil layer are placed on the bottom and slopes of the pits. Lime suspension (pulp) is pumped to the pits by slime pipe-lines. Control for probable migration of radionuclides is conducted by ground water sampling from observation holes arranged along the slime collectors’ perimeter.

3. Isolation and disposal of solid medium-level RW (2-nd group of activity) are carried out in storage structures made in the form of reinforced concrete waterproof reservoirs (bunkers), lined with steel sheets, providing corrosion resistance. The bunkers’ design makes impossible ingress of atmospheric precipitation into the reservoirs and prevents radionuclides migration into the environment.

Around the trench burials, there is a network of observation holes to control the underground waters’ composition.

Specially equipped motor cars are used for solid RW transportation.

Radiation situation in the areas of RW storage sites can be evaluated in terms of uranium nuclide content in environmental objects. The monitoring data for 1996 are as follows:

All the sites of RW storage are located in the limits of industrial areas of the enterprises. They differ greatly in their volumes, filling level, time of commissioning, service period. The mean filling level is 98 % for trench type burials, 50 % - for slime collectors, 58 % - for bunker type burials.

Data of radiation monitoring, including control for Sr-90 and Cs-137 radionuclides content in soil as well as gamma-radiation exposure rates, show, that no territories were contaminated as a result of operation of the uranium enrichment industrial complex enterprises.

Background levels of gamma-radiation exposure rates were recorded for many years at the territories of sanitary-protective zones and observation zones of the enterprises.

In 1996 the gamma-radiation background exposure rates were as follows:

Data of measurements of Sr-90 and Cs-137 content in soil, made in 1996 at SCC, may serve as an evidence of insignificant effect of the uranium enrichment complex enterprises on the environment. According to the data of radiation monitoring in the area of location of UF₆ production and uranium enrichment plants, the radionuclides content in soil does not exceed 0,04 Ci/km² (1,5 kBq/m²) for Sr-90 and 0,3 Ci/km² (11 kBq/m²) for Cs-137 in the sanitary-protective zone of the enterprise and, respectively, 0,001 - 0,03 Ci/km² (0,04 - 1,1 kBq/m²) and 0,01 - 0,13 Ci/km² (0,4 - 4.8 kBq/m²) - in the observation zone.

REFERENCES

2.1. A.K.Kruglov, How the nuclear industry was created in the USSR, Moscow, TsNIIAtominform, 1994, 380 pp. (In Russian).

2.2. Radiation Safety Standards (NRB-76187), Moscow, Energoatomizdat, 1988.

2.3. Basic Sanitary Rules (OSP-72/87), Moscow, Energoatomizdat, 1988.

2.4. Sanitary Rules for Radioactive Waste Management (SPORO), Moscow, 1986.

2.5. Analytical information of enterprises on environmental effects of UF₆ production and uranium isotopic enrichment plants.