The 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.

Mining

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.

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.

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.

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 are 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.

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.

Nuclear Power Plant

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 nucleides 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.

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).

MOX Fuel Fabrication

Separated plutonium is converted into an oxide powder, packed in leak tight cans and transported to plutonium fuel fabrication facilities for the production of MOX fuels for Light Water Reactors and Fast Reactors. Because of the fissile isotopes ²³⁹Pu and ²⁴¹Pu, plutonium is used as substitute for ²³⁵U. But ²⁴¹Pu decays into the non-fissile and highly radioactive ²⁴¹Am. For this reason the utilization of plutonium for MOX fuel should ideally take place shortly after its separation from the spent fuel.

In general MOX fuel pellets are produced from UO₂ and PuO₂ powders in a similar way to the uranium fuel. In enriched uranium, the fissile material is inherently present in the fuel. In MOX fuel, the fissile material, plutonium, has to be added to the carrier material, uranium. This blending of two fissile / fertile materials is the most specific difference between uranium and MOX fuel manufacturing. The rod and assembly design of MOX fuel is universally based on, and also follows the evolutions of, uranium fuel design, with only minor modifications at most.

Electricity

Electricity production with conventional production forms, such as coal, oil and nuclear power, include four main stages. All of them generate electricity by: (1) heating water to (2) generate steam that (3) makes the turbine rotate to (4) enable the generator to produce electricity.

In a nuclear power plant, to produce the steam, the water in the reactor (known as cooling water) is heated up by fission chain reaction. Rarely, a gas like helium is used for cooling and transferring heat. When the nucleus of an atom of, for example, U-235 absorbs a neutron, it may split (or fission) into two pieces, giving off energy as heat and a few more neutrons to continue this nuclear chain reaction. This chain reaction is controlled to produce exactly the desired amount of energy.

The produced steam is extremely hot, several hundred Celsius degrees. Also the pressure is high. The steam is directed to turbines, making the turbine blades move and, thus, rotating the turbines at high speed. The kinetic energy of the rotating turbines is converted into electrical energy in generators. The electricity we use every day is distributed via grid.

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). 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.

HLW and 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.