NFCSS, Details of the Modeling
Nuclear Fuel Cycle
Nuclear Fuel Cycle can be defined as the set of processes to make use of nuclear materials and to return it to normal state. It starts with the mining of unused nuclear materials from the nature and ends with the safe disposal of used nuclear material in the nature.
To produce energy from Uranium in a nuclear reactor, it must be passed through in a series of different processes. The complete set of processes to make nuclear fuel from uranium ore is known as front end of the nuclear fuel cycle. The processes in the front end of the nuclear cycle are mining and milling, conversion, enrichment and fuel fabrication.
After producing energy in the reactor, nuclear fuel becomes spent fuel. Spent fuel has also to be processed in a storage facility or in a reprocessing facility if it is being recycled. Temporary storage, reprocessing, long-term storage, or final storage of spent fuel are together called back end of the nuclear fuel cycle.
A basic schematic illustration of nuclear fuel cycle with recycling in thermal reactor is shown in below figure.
Mining & Milling: Uranium is an element that is widely distributed within the earth’s crust as ores. Its principal use is as the primary fuel for nuclear power plants. The uranium ore needs to be mined and then processed (milled) before being usable. Uranium ore is mined by open-pit or underground mining methods and the uranium is extracted from the crushed ore in processing plants or mills using chemical methods. Sometimes it is possible to pass chemical solutions to the ore beds and dissolve the uranium from the ore directly. This process is known as in-situ leaching. This is the first step in a nuclear fuel cycle. The feed for mining & milling process is uranium ore and the product is U3O8 compound, which is mostly called yellowcake due to its color.
Please click to see the list of commercially operating Uranium Milling Facilities from IAEA NFCIS database.
Conversion: The term conversion refers to the process of purifying the uranium concentrate and converting it to the chemical form required for the next stage of the nuclear fuel cycle. There are three such forms in common usage: metal, oxide (UO2) and uranium hexafluoride (UF6). UF6 is the predominant product at this stage of the nuclear fuel cycle since it is easily converted to a gas for the enrichment stage, as employed in world’s most common reactor types (LWRs). For the PHWR fuel cycle, which generally uses natural uranium oxide as the fuel, conversion to the UF6 is unnecessary. Uranium is purified and converted to UO2 or UO3. The Magnox fuel cycle uses natural uranium in metal form. So the feed for this stage is U3O8 concentrate, and the products are UF6, oxide (UO2 or UO3) or metal, in applicability order.
Please click to see the list of commercially operating Conversion Facilities from IAEA NFCIS database.
Enrichment: Uranium naturally consists of about 0.7% of 235U isotope which is the main energy source in thermal reactors. For LWR technology which is the most common reactor type, it is impossible to build a nuclear reactor with the natural occurrence of 235U, so the 235U content should be increased with a special process. This process is called enrichment. There two commercially available technologies: gaseous diffusion and centrifuge. Both techniques are based on the slightly different masses of the uranium isotopes nuclei. So the enrichment is defined as the process of increasing the amount of 235U contained in a unit quantity of uranium. The feed for this stage is Natural UF6 and the product is enriched UF6. The other output of the process is the uranium which has lower fissile content than the natural uranium. It is known as enrichment tail or depleted uranium.
Please click to see the list of commercially operating Uranium Enrichment Facilities from IAEA NFCIS database.
Fuel Fabrication: Enriched uranium in UF6 form is converted to UO2 powder to make fuel for LWR technology. This powder then is formed into pellets, sintered to achieve the desired density and ground to the required dimensions. Fuel pellets are loaded into tubes of zircaloy or stainless steel, which are sealed at both ends. These fuel rods are spaced in fixed parallel arrays to form the reactor fuel assemblies. The whole process is referred as fuel fabrication. The similar procedure is adopted for natural uranium oxide fuel for some reactor types. The feed of this process is enriched or natural uranium oxide powder and the product is fuel assembly.
Reactor: The reactor itself is irradiator for nuclear fuel. It burns the fuel, produce energy and spent fuel. There are currently 7 types of reactors in the world (classification is based on NFCSS assumptions): PWR, BWR, PHWR, RBMK, GCR, AGR, WWER. The feed for reactor is fresh fuel containing uranium and plutonium, in case of Mixed Oxide (MOX) fuel, for existing nuclear fuel cycle options. The product is the spent fuel consisting of new nuclides such as fission products (Cs, I, …), minor actinides (Np, Am, Cm) and Plutonium as well as the uranium. The biggest part of the spent fuel is still uranium.
Please click to goto the IAEA PRIS database web site.
Reprocessing: The spent nuclear fuel still consists of significant amount of fissile material that can be used to produce energy. The considerable amount of 235U is still contained in the spent fuel and there are new fissile nuclides that were produced during normal operation of nuclear reactor such as 239Pu. Some nuclear fuel cycle options consider taking out the fissile material from the spent fuel, refabricating it as fuel and burning in reactor. MOX fuel is the most common fuel that uses reprocessed material. Reprocessing process is based on chemical and physical processes to separate the required material from spent nuclear fuel. The feed of this process is spent fuel and the products are reusable material and High Level Wastes (HLW).
Please click to see the list of commercially operating Spent Fuel Reprocessing Facilities from IAEA NFCIS database.
Spent Fuel Storage: Spent fuel, which is not reprocessed, could be stored for temporarily for future use or could be stored for indefinitely. Spent fuel could be stored in pools (wet type) or in silos (dry type).
HLW Storage: The waste from fuel fabrication and reprocessing facilities are classified as HLW and requires careful treating. HLW is stored in special storage facilities after proper treatment.
Spent Fuel Conditioning: After a storage period in interim storage facilities (AR or AFR type), spent fuel will be prepared for reprocessing or conditioned for further storage or disposal. This process is performed by spent fuel conditioning facilities.
Please click to see the list of commercially operating Spent Fuel Conditioning Facilities from IAEA NFCIS database. (Please note that since this type has recently been added to NFCIS, there might not be any facility in the list.)
Spent Fuel Disposal: After being properly conditioned, spent fuel can be disposed in deep geological formations for an indefinite period of time until a non-hazardous level of radioactivity from the actinides and fission products is reached by decay.
Please click to see the list of commercially operating Spent Fuel Disposal Facilities from IAEA NFCIS database. (Please note that since this type has recently been added to NFCIS, there might not be any facility in the list.)
Related Industrial Activities: There are other related industrial activities to support nuclear fuel cycle activities such as heavy water production for PHWR fuel cycle or zircaloy tubing production.
Please click to see the list of commercially operating other facilities (Related Industrial Activities) from IAEA NFCIS database.
More information about the nuclear fuel cycle can be found at World Nuclear Association web site.