Nuclear Fuel Cycle

Nuclear Power currently accounts for about 10% [1] of the world’s electricity supply.  As a low carbon “always on” source of electricity, there is a growing interest in building nuclear power plants, and in new developments such as Small Modular Reactors and Advanced Nuclear Reactors [2].  Current generation reactors predominantly use uranium based nuclear fuel; however, some are also able to use Mixed Oxide (MOX) fuel which is manufactured by recycling used nuclear fuel which is removed from reactors when its efficiency drops after about five years.

The full nuclear fuel cycle is depicted in figure 1 and safe and secure transport is required between many of the steps identified. Such transport has been ongoing for many decades with an enviable safety record.  Nuclear fuel cycle transports are commonly designated as either front end or back end. The front end covers all the operations from the mining of uranium to the manufacture of new fuel assemblies for loading into the reactors, i.e. the transport of uranium ore concentrates to uranium hexafluoride conversion facilities, from conversion facilities to enrichment plants, from enrichment plants to fuel fabricators and from fuel fabricators to the various nuclear power plants. The back end covers all the operations concerned with the spent fuel which leaves the reactors, i.e. the shipment of spent fuel elements from nuclear power plants to reprocessing facilities for recycling, and the subsequent transport of the products of reprocessing. Alternatively, if the once-through option is chosen, the spent fuel is transported to interim storage facilities pending its final disposal.

Uranium is a naturally occurring element that exists in varying concentrations in the earth’s crust.  Where the concentration is sufficiently high (typically 0.1% or higher) to make extraction economic, Uranium ore is extracted by mining and is processed into Uranium Ore Concentrate (UOC) which is then transported to the next stage of the cycle. The transport of UOC is typically undertaken in 210l steel drums (figure 2) which are in turn packed into 20’ ISO shipping containers (figure 3).

Uranium consists primarily of two isotopes, U-235 and U-238.  When extracted from ore, there is only about 0.7% U-235.  For use in most current reactors this needs to be increased to around 5% and for some future reactor designs, higher enrichments of up to 20% U-235 will be required.  Enrichment needs the Uranium to be converted into Uranium Hexafluoride (UF6) also known as HEX which can be turned into a gas at a relatively low temperature for the enrichment process.  UF6 is transported in specially designed cylinders (figure 4) which are designed to meet the transport regulations [3] and to be heated to enable the UF6 to be extracted from the cylinder.  During transport, the UF6 is a solid.

Typically UF6 is enriched in gas centrifuges.  These work by spinning the gas at a high speed which partially separates the lower mass U-235 from the higher mass U238.  By passing the gas through a series of these centrifuges, the UF6 can be enriched to the desired enrichment for the fuel to be manufactured.  The enriched UF6 is then transported for deconversion and fuel fabrication.  Enriched UF6 is classed as fissile material and the cylinders used to transport it are therefore smaller (figure 5).

The enriched UF6 is deconverted to Uranium Oxide which can then be used for fuel manufacture.  This enriched UO2 is transported in packages which are designed to ship the powder with higher enrichment than UOC.

The UO2 powder is typically pressed into pellets which are in turn loaded into stainless steel or zirconium alloy rods. These rods are then built into a structure called a fuel assembly which is carefully manufactured to meet the requirements of the reactor into which it will be loaded.  Further details on fuel manufacture are available on the WNA website. The fuel assemblies are then loaded into a transport package for shipment to the nuclear power plant.

After approximately five years in a nuclear reactor, the fuel becomes less efficient and is removed and replaced with fresh fuel.  Used fuel is highly radioactive and still generates significant heat.  It is therefore typically stored in a cooling pond or ventilated dry store at the reactor for an initial cooling period.  After initial cooling, the fuel can be transported or transferred to a storage cask for longer on-site storage if it is removed from the pond or dry store.  The transport (or storage and transport) casks have thick walls to shield people and the environment from the radiation and are designed to withstand severe accidents.  These packages meet the “Type B” requirements of the transport regulations [3].  Spent fuel is transported either for disposal (normally in a deep geological disposal facility) or for reprocessing.

Used fuel still consists mainly of uranium (approximately 96%) although the enrichment will have dropped to less than 1%.  Fission products (waste) account for around 3% and plutonium 1%.  It is possible to separate the uranium, plutonium and waste.  The concentrated high level waste can then be turned into a form suitable for disposal and the plutonium and uranium recycled as MOX fuel.  Transport of Plutonium and MOX fuel is carried out in robust packages which are also designed to meet the type B requirements of the transport regulations [3].  There are also additional security requirements for the transport of these materials to meet the requirements of the Convention on Physical Protection of Nuclear material CPPNM [4].

There is a wide range or radioactive waste material from very low level waste such as protective overclothes worn by nuclear plant workers through to the high level waste from reprocessing.  These are treated in different ways such as incineration, disposal in shallow landfill, recycling of metals to separate reusable metal from the radioactive waste through to disposal in a deep geological facility for long lived high level waste. In each case, the disposal or recycling route is determined by the type of material and its radioactivity.  Similarly the packaging used to transport the waste is commensurate with the radiological hazard it presents.  Very low level waste may be transported in strong plastic sacks (similar to bags used for bulk deliveries of building materials), whereas high level waste is transported in robust type B packages with thick shielding to protect the public and the environment.

The design and performance standards for packages used for the transport of radioactive material, including nuclear fuel cycle material, are defined in the International Atomic Energy Agency (IAEA) Regulations for the Safe Transport of Radioactive Material [3].  The regulations have been in place since the 1960s and are regularly reviewed and updated.  This has resulted in a history of over 60 years of safe transport or radioactive materials.  The regulations take a graded approach to safety with the level of protection increasing with the radioactive hazard (Figure 9). For low hazard materials, such as a single dose of a radiopharmaceutical for injection into a patient, the excepted package may be simply a robust cardboard box with absorbent material to soak up any spillage.  For high level waste, a Type B package would be required with appropriate shielding to protect the public and the environment.  In addition, the design of the package must ensure that it meets the severe accident conditions defined in the regulations covering impact, fire and immersion in water.

As described in the paragraph above, the safety of transport of radioactive materials is generally ensured by the package design, regardless of the mode or transport (road, rail, air or water).  There is one exception to this, however.  For spent fuel, high level waste and plutonium transported by sea, there are additional requirements for the vessel used which are defined by the International Maritime Organisation (IMO) in the Code for the Safe Carriage of Irradiated Nuclear Fuel, Plutonium and High-Level Radioactive Wastes in Flasks on Board Ships (INF Code). The provisions of the INF Code mainly cover ship design, construction and equipment.

There are three levels, INF1, INF2 and INF3, depending on the quantity of material carried. INF3 is the highest IMO safety rating for ships carrying irradiated nuclear fuel, plutonium and high level radioactive wastes cargoes (Figure 10). The main safety features of these ships are shown in Figure 11 and include:

• double hull construction around the cargo areas with impact resistant structures between hulls;
• at least two sets of safety-related equipment such as navigation, communication, cargo monitoring, electrical and cooling systems so the ship can use the second system if a fault develops;
• satellite navigation and satellite tracking;
• extra fire detection and firefighting equipment including a hold flooding system.

Further details of INF3 class vessels can be found on the PNTL and SKB websites.

Nuclear Power currently accounts for about 10% [1] of the world’s electricity supply.  As a low carbon “always on” source of electricity, there is a growing interest in building nuclear power plants, and in new developments such as Small Modular Reactors and Advanced Nuclear Reactors [2].  This demand will increase as nuclear power usage increases to meet the demands of decarbonisation.  Current predictions are that nuclear power capacity will grow by around 60-70% between 2020 and 2050 [1].  None of this will be possible without ongoing safe and secure transport of the radioactive materials used in the nuclear fuel cycle.  The transport of nuclear fuel cycle materials is carefully regulated in accordance with the IAEA transport regulations [2] and has an excellent safety performance stretching back over 60 years. The nuclear transport industry is fully committed to maintaining its record of safety, security, and environmental protection in the transport of these materials.

1. https://world-nuclear.org/information-library/current-and-future-generation/world-energy-needs-and-nuclear-power
2. https://world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-power-reactors/small-nuclear-power-reactors
3. https://www.iaea.org/publications/12288/regulations-for-the-safe-transport-of-radioactive-material
4. https://www.iaea.org/publications/documents/conventions/convention-physical-protection-nuclear-material-and-its-amendment