Nuclear fuel cycle
The nuclear fuel cycle is the series of actions and industrial processes that are connected with the utilization of uranium as a fuel for the production of electricity from its fission in nuclear power plants.
Uranium is a relatively common element – a slightly radioactive metal that occurs in the Earth's crust. It is about 500 times more abundant than gold and about as common as tin. It is present in most rocks and soils as well as in rivers and in sea water.
Nuclear fuel cycle can be divided into the following stages (click on links under the image to expand stage details):
1. Uranium mining & treatment
Uranium ore is extracted in underground or open pit mines. The ore may contain from 0.1% to 3% of uranium. The greatest amounts of uranium ore are extracted in Canada, Australia and Kazakhstan. By crushing and chemical treatment (leaching), the so-called „yellow cake” is obtained, which contains more than 80% of uranium.
2. Conversion and enrichment
Uranium compounds present in the yellow cake are converted in a gaseous form (uranium hexafluoride - UF6) suitable for enrichment of uranium 235. Uranium, which is present in natural resources, particularly consists of two isotopes: U-238 and U–235. There is only a very little concentration of fissionable U–235 in natural uranium 238 (0.7% in average); therefore it is necessary to enrich its concentration in the fuel up to 5%. The use of centrifuges is commercially the most common process of enrichment.
3. Fuel fabrication
UF6 is chemically treated into UO2 (uranium dioxide) powder. It is then pressed and sintered at high temperature (1,400°C) into a ceramic pellet form, which are hermetically encased in zircalloy tubes. As many as 126 these tubes form a fuel rod. Operation of one VVER-440 reactor requires 7 to 9 tons of uranium fuel. Fresh nuclear fuel does not pose any radiation risk, as it is a weak source of radiation, and is activated only in a nuclear reactor.
4. Fuel use in a reactor
The energy released by fission of uranium in the reactor is removed by coolant (water) and then converted into electricity in a turbine generator. Fuel in the reactor must always be flooded with water, otherwise it might be overheated and at temperatures above 1,500°C fuel cladding starts melting, at temperatures above 2,500°C even the fuel melts down. Some of the U-238 in the fuel is turned into plutonium in the reactor core. The main plutonium isotope is also fissile and this yields about one third of the energy in a typical nuclear reactor.
5. Storage of spent fuel
After 5 to 6 years of operation in the reactor, the spent fuel is moved to a spent fuel pool, which is located immediately adjacent to the reactor. Here, it is cooled down, and its radiation level decreases. Water provides excellent radiation shielding and also absorbs residual heat generated by the spent fuel.
After about 5 years of cooling down the spent fuel, it can be transported to the storage of spent fuel in Bohunice, where it is stored in water ponds. This storage is operated by the state owned company JAVYS, a.s. where all of the nuclear spent fuel, produced in Slovakia, is stored. It is expected that in the near future the storage capacity will be expanded by a dry storage (special containers cooled just by natural air circulation).
6. Fuel reprocessing
Spent fuel contains about 95% uranium, 1% plutonium and 4% highly radioactive fission products that incept in the reactor. Fuel can be recycled in reprocessing plants, where it is separated into three components: uranium, plutonium and high level waste. Uranium and plutonium can be repeatedly used during a fresh fuel production, containing a mixture of fissile isotopes of U and Pu (so called MOX fuel).
The reprocessing process, however, is finance and energy demanding, therefore there are only a few reprocessing plants in the world. Spent fuel from Slovak NPPs is currently not being reprocessed, but in the future such a process is not excluded.
7. Final solution for spent fuel
Worldwide accepted final solution for safe storage of nuclear fuel spent is its disposal in special containers in deep geological repository. Strict requirements are applied, when finding a suitable site to build a deep repository in rock or sediment environment. Even though at present, there are no final deep repository facilities available in the world, several countries (e.g. Finland, Sweden, and Switzerland) have already built demonstration facilities, where they study and declare safety and feasibility of such solutions. Introduction of the first deep repository is expected around 2020 in Finland.
From Slovakia’s perspective,there is no urgent need for final solution, as the total volume of spent fuel is relatively small and can easily and safely be kept in the storage for a long time. In terms of statutory legislation, Slovenske elektrarne set aside future costs associated with the management of spent nuclear fuel, radioactive waste and decommissioning of its nuclear power plants. National Nuclear Fund manages these resources and ensures the implementation of tasks arising from the national back-end strategy of peaceful use of nuclear energy in Slovak republic.
Under the current national program for the final part of peaceful nuclear energy utilization, even in Slovakia a geological survey is conducted to identify possible sites for the construction of the repository. It is assumed that the final repository could be available by around 2065.
Further, other options of using the spent fuel by new technologies are investigated. New types of so-called fast-breeder reactors are being developed that in the future will use spent fuel from reactors, currently in operation. This seems to be the optimal solution of the back-end of nuclear fuel cycle.