What is spent fuel assembly

Many fuel rods are arranged to make a fuel assembly. These fuel assemblies are about 14 feet long and weigh about kg or about lbs, depending on the type of reactor. Before use, nuclear fuel pellets are not radioactive, meaning you can stand next to unused fuel and be perfectly safe. These radionuclides are called fission products and have different half-lives — the time it takes for the radioactivity of a specified isotope to fall to half its original value.

Some common short-lived fission products have relatively low half-lives of around 30 years and include certain isotopes of cesium, strontium, neptunium, americium, and curium. This means that after spent nuclear fuel is taken out of a reactor and 30 years have passed, only half of these short-lived fission products are leftover.

Other elements produced have long half-lives and require isolation from the biosphere for hundreds of thousands of years. Fission products vary in radioactivity, but the ones that pose a larger risk to human health and the environment are long-lived fission products.

It is time to consider alternative options to address the nuclear waste challenge by working alongside communities for mutually beneficial solutions. Deep Isolation, Inc. What is spent nuclear fuel? Spent Fuel Pool. Fuel Rod Assembly Overview. Additional Resources. Deep Isolation Story It is time to consider alternative options to address the nuclear waste challenge by working alongside communities for mutually beneficial solutions.

They employ a vertical fuel channel design, and use CO 2 gas — a very weak moderator — as the primary coolant. AGR fuel assemblies consist of a circular array of 36 stainless steel clad fuel pins each containing 20 enriched UO 2 fuel pellets, and the assembly weighs about 43 kilograms.

Enrichment levels vary up to about 3. Stainless steel allows for higher operating temperatures but sacrifices some neutron economy. The assembly is covered with a graphite sheath which serves as a moderator. Eight assemblies are stacked end on end in a fuel channel, inserted down through the top of the reactor.

During refueling this whole stack is replaced. Fuel life is about five years, and refueling can be carried out on-load through a refuelling machine. It employs vertical pressure tubes just under of these, each about 7 metres long running through a large graphite moderator. The fuel is cooled by light water water, which is allowed to boil in the primary circuit, much as in a BWR.

RBMK fuel rods are about 3. Two bundles are joined together and capped at either end by a top and bottom nozzle, to form a fuel assembly with an overall length of about 10 metres, weighing kilograms. This has the effect of improving overall safety and increasing fuel burn-up. This new fuel can stay in the reactor for periods of up to six years before needing to be removed. Two BN units were planned in China. Fast neutron reactors FNR are unmoderated and use fast neutrons to cause fission.

The plutonium is bred from U during operation. If the FNR is configured to have a conversion ratio above 1 ie more fissile nuclei are created than fissioned as originally designed, it is called a Fast Breeder Reactor FBR. FNRs use liquid metal coolants such as sodium and operate at higher temperatures. See also Fast Neutron Reactor information paper.

Apart from the main FNR fuel, there are numerous heavy nuclides - notably U, but also Am, Np and Cm that are fissionable in the fast neutron spectrum — compared with the small number of fissile nuclides in a slow thermal neutron field just U, Pu and U A FNR fuel can therefore include a mixture of transuranic elements.

Also it can be in one of several chemical forms, including; standard oxide ceramic, mixed oxide ceramic MOX , single or mixed nitride ceramics, carbides and metallic fuels. Carbide fuels such as used in India have a higher thermal conductivity than oxide fuels and can attain breeding ratios larger than those of oxide fuels but less than metal fuels.

In each case the fuel composition for the seed and blanket regions are different — the central seed region uses fuel with a high fissile content and thus high power and neutron emission level , and the blanket region has a low fissile content but a high level of neutron absorbing material which can be fertile for a breeding design, eg U or a waste absorber to be transmuted.

BN fuel assemblies are 3. The central bundle is a hexagonal tube and for seed fuel houses rods, each 2. Blanket fuel bundles have 37 rods containing depleted uranium. BN fuel rods use low-swelling stainless steel cladding. They are capable of high fuel burn-up. Nuclear fuel operates in a harsh environment in which high temperature, chemical corrosion, radiation damage and physical stresses may attack the integrity of a fuel assembly. The life of a fuel assembly in the reactor core is therefore regulated to a burn-up level at which the risk of its failure is still low.

The radioactive materials with most tendency to leak through a cladding breach into the reactor coolant are fission-product gases and volatile elements, notably; krypton, xenon, iodine and cesium. Fuel leaks do not present a significant risk to plant safety, though they have a big impact on reactor operations and potentially on plant economics. For this reason, primary coolant water is monitored continuously for these species so that any leak is quickly detected.

The permissible level of released radioactivity is strictly regulated against specifications which take into account the continuing safe operation of the fuel. Depending on its severity a leak will require different levels of operator intervention:. A leaking fuel rod can sometimes be repaired but it is more usual that a replacement assembly is needed this having a matching level of remaining enrichment.

Replacement fuel is one cost component associated with failed fuel. There may also be higher operation and maintenance costs associated with mitigating increased radiation levels in the plant. Fuel management is a balance between the economic imperative to burn fuel for longer and the need to keep well within failure-risk limits.

Improving fuel reliability extends these limits, and therefore is a critical factor in providing margin to improve fuel burn-up. The reliability drive continues. These programs led to the accident-tolerant fuel program see below. Fuel failures in US power reactors are rare. The annual US failure rate is about one in one million i.

Fuel engineering continues to improve, e. Utilities themselves impose more rigorous practices to exclude foreign material entering primary coolant water. Global Nuclear Fuel GNF in had two million fuel rods in operation and claimed to have no leakers among them.

In the early s hydriding and pellet-clad interaction caused a lot of leaks. The s saw an order of magnitude improvement. Utilities must carefully balance the benefits of greater cycle length against higher front-end fuel costs uranium, enrichment.

Refuelling outage costs may also be higher, depending on length, frequency and the core re-load fraction. An equally important trend in nuclear fuel engineering is to be able to increase the power rating for fuels, ie, how much energy can be extracted per length of fuel rod.

Currently this is limited by the material properties of the zirconium cladding. The current annual demand for LWR fuel fabrication services is expressed as a requirement for about tonnes of enriched uranium being made into assemblies, and this is expected to increase to about t by Requirements for fuel fabrication services will grow roughly in line with the growth in nuclear generating capacity.

A parallel industry-wide focus on increasing fuel performance and reliability has also decreased the demand for fuel to replace defective assemblies. Plans to build many new reactors affect the demand on fabrication capacity in two ways.

The demand for reloads increases in line with the new installed reactor capacity, typically 16 to 20 tonnes per year per GW. Additionally the first cores create a temporary peak demand, since the amount required is about three to four times that of a reload batch in currently operated LWRs and some of the enrichment is less.

An average first core enrichment is about 2. Fuel fabrication services are not procured in the same way as, for example, uranium enrichment is bought. These are determined by the physical characteristics of the reactor, by the reactor operating and fuel cycle management strategy of the utility as well as national, or even regional, licensing requirements.

Most of the main fuel fabricators are also reactor vendors or owned by them , and they usually supply the initial cores and early reloads for reactors built to their own designs. Currently, fuel fabrication capacity for all types of LWR fuel throughout the world considerably exceeds the demand. It is evident that fuel fabrication will not become a bottleneck in the foreseeable supply chain for any nuclear renaissance.

The overcapacity is increased by countries such as China, India and South Korea aiming to achieve self-sufficiency. In May a European Commission staff report suggested that as a condition of investment, any non-EU reactor design built in the EU should have more than one source of fuel.

Conceptual design was completed in May , based on fuel provided by Westinghouse to Loviisa in LWR fuel fabrication capacity worldwide is shown in Table 1.

The back-conversion capacities are particularly unevenly distributed. For some fabricators it represents a bottleneck. Some fabricators do not have conversion facilities at all and have to buy such services in the market, while others with excess capacity are even sellers of UO 2 powder.

The MOX fuel market has weakened somewhat recently with the cessation of its use in Belgium, Germany and Switzerland moratorium , and the continued loading of MOX fuel in Japan has diminished in the aftermath of the Fukushima accident. It consists of depleted uranium about 0. This plutonium is reactor-grade, comprising about one third non-fissile isotopes. The pressing and sintering process is much the same as for UO 2 fuel pellets, though some plastic shielding is needed to protect workers from spontaneous neutron emissions from the Pu component.

This eliminates the need to manufacture pellets to high geometric tolerances, which involves grinding and scrap which are more complex to deal with for Pu-bearing fuels. Russian sources say vibropacked fuel is more readily recycled. It is distinct from MOX fuel in having low and incidental levels of plutonium — none is added. Fuel for these is in the form of TRISO tristructural-isotropic particles less than a millimetre in diameter.

Each has a kernel ca. This is surrounded by layers of carbon and silicon carbide, giving a containment for fission products which is stable up to very high temperatures. There are two ways in which these particles can be arranged in a HTR: in blocks — hexagonal 'prisms' of graphite; or in billiard ball-sized pebbles of graphite encased in silicon carbide, each with about 15, fuel particles and 9g uranium.

Either way, the moderator is graphite. Most of the experience with thorium fuels has been in HTRs. Previous production has been on a small scale in Germany. The fuel blocks are interspersed with yttrium hydride moderator elements. This is expected to produce TRISO fuel of "significantly higher quality and at costs that are substantially lower than other potential manufacturers.

X-energy is applying for a loan guarantee from the government for commercialization of a TRISO-based fuel supply chain and is expected to submit a licence application for a commercial plant by mid, though this may now be GNF's prerogative. HALEU may be metallic or oxide. HALEU can be produced with existing centrifuge technology, but a number of arrangements would need to be made for this, as well as for deconversion and fuel fabrication.

New transport containers would also be required as those for today's enriched UF 6 could not be used due to criticality considerations. Fuel development activities in the nuclear industry have largely focused on improving the reliability of standard zirconium-clad uranium oxide fuels. Accident tolerant fuel ATF is a term used to describe new technologies that enhance the safety and performance of nuclear fuel. ATF may incorporate the use of new materials and designs for cladding and fuel pellets.

Its objective is to develop new cladding and fuel materials that can better tolerate the loss of active cooling in the core, while maintaining or improving fuel performance and economics during normal operations. A priority of the EATF programme is to minimise the generation of hydrogen. Framatome in phase 2 of its PROtect enhanced ATF programme from has been developing a nuclear fuel concept, using a chromium-coated zirconium alloy cladding M5 combined with chromia-doped fuel pellets.

The fuel is expected to retain fission gases better and improve pellet-cladding interaction, and the cladding will better resist high-temperature oxidation. The GAIA spacer grid holding the fuel rods also has high mechanical fretting resistance. Entergy is also due to use them in Arkansas 1. These will also have chromium-coated zircalloy cladding and chromia-doped fuel pellets. Framatome is also continuing work on a silicon carbide cladding, and plans to use that cladding on chromia-doped pellets in lead test assemblies in about Both are for conventional UO 2 fuel and are designed to provide oxidation resistance and superior material behaviour over a range of conditions in BWRs.

The IronClad Fe-Cr-Al cladding has better mechanical strength at high temperatures, retains fission gases better than zirconium alloy and has less potential for hydrogen generation in an accident. The ARMOR-coated zirconium cladding provides enhanced protection of fuel rods against debris fretting. The initial EnCore fuel comprises high-density uranium silicide fuel pellets inside zirconium cladding with a thin coating of chromium making it more robust chemically.

In the second phase of EnCore, the higher temperature tolerance of silicon carbide cladding has potential for revised regulatory requirements, and Westinghouse sees this as a "game changer". After trials of lead test assemblies, Westinghouse intends to make full reload quantities available from Each assembly consists of 24 fuel rods with different combinations of cladding materials and fuel composition.

Each of the three TVS-2M assemblies contains twelve ATF rods with cladding composed of either zirconium alloy with a heat-resistant chromium coating, or chrome-nickel alloy. Also in research reactors Rosatom will continue to irradiate fuel rods with various combinations of cladding and fuel pellet composition which may include uranium-molybdenum alloy or uranium disilicide.

The Bochvar Institute is developing fabrication technology for uranium disilicide pellets. As well as high uranium density, it points to high thermal conductivity and low heat capacity of silicide fuel.

Independently of the US DOE and the international ATF program, Lightbridge is developing an advanced metal fuel concept that may have accident tolerant characteristics. But the higher melting point of uranium oxide has made it the preferred fuel in all reactors for half a century. However, metal has much better thermal conductivity than ceramic oxide, and recent research has turned back to metal fuel forms. The increased enrichment compensates for reductions in the initial fissile loading and in the derivative plutonium.

BWXT in the USA has now completed its assessment of feasibility and prepared a fabrication plan for manufacturing fuel samples. Each Lightbridge fuel rod consists of a central displacer of zirconium surrounded by a four-lobed fuel core with the cladding metallurgically bonded to it. For the hexagonal fuel assemblies for VVERs, the fuel rod is three-lobed. The shape of the rod provides increased surface area for heat transfer and the area between the lobes accommodated swelling during irradiation.

The rod has greater structural integrity than current tubes with ceramic pellets inside. The fuel operates at a higher power density than oxide fuels and the target burn-up is 21 atomic percent, about three times that of oxide fuels. The agreement was expected to see fabrication and characterization of prototype fuel rods using depleted uranium in early , with irradiation fuel samples using enriched uranium made later the same year.

Subject to final approval from the Norwegian Radiation Protection Authority, Lightbridge will test the fuel under prototypical PWR conditions in a pressurized water loop of Norway's 25 MWt Halden research reactor a boiling heavy water reactor. The initial phase of irradiation testing was to begin in using short samples to evaluate conductivity, and continue for about three years using 70 cm fuel rods to evaluate cladding and swelling. Tests aim to reach the burnup necessary for insertion of lead test assemblies LTAs in a commercial power reactor.

In March Lightbridge entered into an exclusive joint development agreement with Areva NP to set up a joint venture that would develop, fabricate and commercialize fuel assemblies based on the metallic fuel technology.

In November it announced an agreement on key terms for the US-based joint venture, creating "a viable and well-defined commercialization path" covering fuel assemblies for most types of light water reactor, including pressurized water reactors excluding VVERs , boiling water reactors, small modular reactors and research reactors. Commercial sales of the fuel were expected by However, in Lightbridge sought to terminate the joint venture and early in it was seeking new partners.

In March the two companies agreed to dissolve the joint venture, with the IP rights for the fuel reverting to the source companies. Lightbridge is working with four US nuclear utilities, and late in a letter of intent was signed with one of them for a lead test fuel assembly demonstration in a US commercial reactor, possibly by The fuel in this is designed to be rearranged but not replenished for 40 years.

This is the Radkowsky Thorium Reactor concept. No recent progress with it is known. Whereas normal fuel uses enriched uranium oxide throughout the fuel assembly, the new design has a demountable centre portion and blanket arrangement, with uranium-zirconium metal fuel rods in the centre and uranium-thorium oxide pellets in conventional fuel rods around it.