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Nuclear Fuel cycle Overview

(Updated April 2021)

The atom fuel cycle is the series of industrial processes which involve the production of electrical power from uranium in nuclear power reactors.

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Uranium is a relatively common element that is uncovered throughout the world. The is mined in a number of countries and also must it is in processed prior to it deserve to be used as fuel for a nuclear reactor.Fuel removed from a reactor, ~ it has reached the end of its useful life, can be reprocessed for this reason that most is recycle for brand-new fuel.

Introduction

The various activities associated v the manufacturing of electrical energy from atom reactions room referred to jointly as the atom fuel cycle. The nuclear fuel cycle starts through the mining of uranium and ends through the disposal of nuclear waste. V the reprocessing of used fuel as an choice for nuclear energy, the stages kind a true cycle.

To prepare uranium for usage in a nuclear reactor, it experience the procedures of mining and milling, conversion, enrichment and fuel fabrication. This steps comprise the 'front end' of the atom fuel cycle.

After uranium has actually spent around three year in a reactor to produce electricity, the supplied fuel might undergo a further collection of steps consisting of temporary storage, reprocessing, and recycling prior to the waste developed is disposed. Collectively these procedures are known as the 'back end' that the fuel cycle.

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The nuclear fuel cycle

Uranium

Uranium is a contempt radioactive steel that wake up throughout the Earth's crust. That is about 500 times more abundant than gold and about as common as tin. The is current in many rocks and soils and also in numerous rivers and in sea water. That is, because that example, discovered in concentration of about four components per million (ppm) in granite, which provides up 60% of the Earth's crust. In fertilizers, uranium concentration have the right to be together high as 400 ppm (0.04%), and also some charcoal deposits contain uranium in ~ concentrations better than 100 ppm (0.01%). Many of the radioactivity associated with uranium in nature is in fact due to other minerals acquired from it by radioactive degeneration processes, and also which room left behind in mining and also milling.

There room a variety of areas approximately the world where the concentration the uranium in the floor is sufficiently high that extraction of the for usage as nuclear fuel is economically feasible. Such concentrations of mineral –including uranium –that can be extracted economically are described as ore.

For an ext information, see page on Geology the Uranium Deposits.

Uranium mining

The decision as to which mining method to use for a particular deposit is administrate by the nature of the orebody, and also safety and economic considerations.

Both surface ar (generally open pit) and underground mining methods are offered to recover uranium ore. In general, open up pit mining is used where deposits are close come the surface, and underground mining is generally used because that deposits in ~ depths higher than 120 m.

Since the wall surfaces of an open up pit mine should be sloped to avoid collapse, the forced holes are bigger in size than the ore deposit itself. Together a result, the quantity of product that must be eliminated in order to accessibility the ore might be really large. Secret mines have reasonably small surface ar disturbance and also the amount of product that have to be gotten rid of to access the ore is considerably less 보다 in the instance of an open up pit mine. Special precautions, consisting mainly of increased ventilation, are required in underground mines come protect versus airborne radiation exposure.

An boosting proportion of the world's uranium now comes from in situ leach (ISL) mining, whereby oxygenated groundwater is circulated with a very porous orebody come dissolve the uranium oxide and also bring it come the surface. ISL may use one of two people weak acid or alkaline options to keep the uranium in solution. The leached uranium oxide is then recovered from the solution as in a conventional mill.

For much more information, see page on Uranium Mining Overview.

Uranium milling

Milling, i beg your pardon is generally carried out close come a uranium mine, extracts the uranium from the ore (or ISL leachate). Most mining facilities encompass a mill, although whereby mines are close together, one central mill may procedure the ore from number of mines.

In a mill, the ore is crushed and also ground to a fine slurry i beg your pardon is leached in sulfuric mountain (or sometimes a solid alkaline solution) to allow the separation that uranium native the rubbish rock. It is climate recovered indigenous solution and precipitated together uranium oxide (U3O8) concentrate. After drying, and also usually heating, it is pack in 200-litre drums as a concentrate, periodically referred to together 'yellowcake' (though that is commonly khaki in colour). TheU3O8 concentrate frequently contains more than 80% uranium. The original ore, by comparison, might contain as small as 0.1% uranium.

U3O8is the uranium product which is sold. Around 200 tonnes is compelled to store a huge (1000 MWe) nuclear power reactor generating electrical energy for one year.

The remainder that the ore, containing many of the radioactivity and nearly all the rock material, becomes tailings, which space emplaced in engineered infrastructure near the mine (often in a mined out pit). Tailings must be isolated indigenous the environment due to the fact that they save on computer long-lived radioactive products in low concentrations and also can contain toxic materials such as hefty metals. However, the full quantity the radioactive aspects is much less than in the initial ore, and also their cumulative radioactivity will certainly be lot shorter-lived.

Conversion and also enrichment

The uranium oxide product of a uranium mill is not straight usable together fuel because that a nuclear reactor – added processing is required. Only 0.7% of natural uranium is 'fissile', or capable of experience fission, the procedure by which power is created in a atom reactor. The form (or isotope) the uranium i m sorry is fissile is uranium-235 (U-235). The remainder is uranium-238 (U-238)a.

For most kinds the reactor, the concentration the the fissile U-235 isotope requirements to be increased– typically to between 3.5% and 5% U-235b. Isotope separation, the physical procedure to concentrate (or ‘enrich’) one isotope family member to others, calls for the uranium to it is in in a gas form. The uranium oxide is first refined to uranium dioxide, which can be used as the fuel because that those types of reactors that do not need enriched uranium. The balance – many of the uranium oxide – is then converted into uranium hexafluoride, i beg your pardon is a gas at relatively low temperatures. The uranium hexafluoride is then drained into 14-tonne cylinders wherein it solidifies. These solid metal containers space shipped come the enrichment plant.

The enrichment procedure separates gas uranium hexafluoride into two streams: one being enriched to the required level and known as low-enriched uranium; the various other stream is increasingly depleted in U-235 and also is dubbed 'tails', or simply depleted uranium.

The main enrichment process in advertising plants offers centrifuges, with countless rapidly-spinning upright tubes. As they spin, the physical properties of molecules, particularly the 1% massive difference in between the two uranium isotopes, cause them come separate. A laser enrichment procedure is in the last stage the development.

The product of this stage of the nuclear fuel bicycle is enriched uranium hexafluoride, i beg your pardon is reconverted to create enriched uranium oxide. As much as this allude in the bicycle the fuel material can be considered fungible (though enrichment level vary), but fuel fabrication involves very specific design.

For an ext information, watch pages top top Conversion and Deconversion, and Uranium Enrichment.

Fuel fabrication

Reactor fuel is usually in the form of ceramic pellets. This are created from pressed uranium oxide (UO2), i m sorry is sintered (baked) at a high temperature (over 1400°C)c. The pellets room then encased in metal tubes to type fuel rods, which space arranged right into a fuel assembly all set for introduction into a reactor. The size of the fuel pellets and other components of the fuel assembly space precisely managed to ensure consistency in the characteristics and also behaviour the the fuel.

In a fuel fabrication plant good care is taken with the size and shape of handling vessels to prevent criticality (a restricted chain reaction release radiation). Through low-enriched fuel criticality is very unlikely, however in plants dealing with special fuels for research reactors this is a an essential safety consideration.

Some 27 tonnes of new enriched fuel is required every year by a 1000 MWe reactor.

For more information, see page on Fuel Fabrication.

Power generation and also burn-up

Several hundred fuel assemblies comprise the core of a reactor.dFor a reactor v an output of 1000 MWe, the core would contain about 75 tonnes the low-enriched uranium. In the reactor core the U-235 isotope fissions or splits, producing a lot of warm in a consistent process dubbed a chain reaction. The procedure depends on the existence of a moderator such as water or graphite, and is totally controlled.

Some the the U-238 in the reactor core is turned right into plutonium and also about half of this is additionally fissioned, providing about one-third that the reactor's power output (or much more than half in CANDU reactors).

As in fossil-fuel burning electrical power generating plants, the warm is provided to produce steam to journey a turbine and also an electric generator. With this process, a 1000 MWe unit offers over 8 billion kilowatt hours (8 TWh) of electrical power in one year.

To maintain effective reactor performance, about one-third of the invested fuel is removed annually or 18 months, come be replaced with new fuel.eThe length of fuel bicycle is correlated with the use of burnable absorbers in the fuel, allowing greater burn-up.

Typically, part 44 million kilowatt-hours of power are developed from one tonne of organic uranium. The production of this amount of electrical power indigenous fossil fuels would require the burn of end 20,000 tonnes of charcoal or 8.5 million cubic metres of gas.

An concern in operation reactors, and also hence specifying the fuel for them, is fuel burn-up. Fuel burn-up is measure in gigawatt-days (thermal) per tonne and also its potential is proportional to the level the enrichment. To date a limiting aspect has to be the physics robustness the fuel assemblies, and also hence burn-up levels have actually been restricted to around 40 GWd/t, request only around 4% enrichment. Through the development of equipment and also fuel assemblies, 55 GWd/t is now possible (with 5% enrichment), and 70 GWd/t is in sight (though this would call for 6% enrichment). The advantage of boosted burn-up is that procedure cycles can be much longer – approximately 24 months – and the variety of fuel assemblies discharged as supplied fuel can be decreased by one third. Connected fuel cycle cost is meant to be diminished by around 20%.

In CANDU reactors using natural uranium, burn-up is lot less, around 7.5 GWd/t, however in state of performance this is indistinguishable to practically 50 GWd/t for enriched fuel.

Burn-up in GWd/t is the standard measure for oxide fuels, and also 60 GWd/t U is identical to about 6.5 atom percent burn-up(i.e. about 6.5% the the original uranium atoms are melted directly, or indirectly via transformation to fissile plutonium). (With metal fuels, the atom percent metric is used, and a new light water reactor steel fuel is targeting 21 atom percent burn-up when it is deployed in 2020s.)

As with coal-fired power stations, around two thirds that the heat created is released, one of two people to a big volume of water (from the sea or large river, heater it a couple of degrees) or come a reasonably smaller volume of water in cooling towers, making use of evaporative cooling (latent heat of vaporization).

Used fuel

With time, the concentration that fission fragments and also heavy aspects in the fuel will increase to the point where the is no longer helpful to proceed using it. So after 18-36 months the used fuel is gotten rid of from the reactor. The amount of power that is produced from a fuel assembly varies v the type of reactor and also the plan of the reactor operator.Used fuel will generally have about 1.0% U-235 and 0.6% fissile plutonium (almost 1% Pu total), with approximately 95% U-238.f The balance, about 3%, is fission products and minor actinides.

When eliminated from a reactor, the fuel will be emitting both radiation, principally native the fission fragments, and also heat. The is unloaded right into a storage pond immediately nearby to the reactor to allow the radiation level to decrease. In the ponds, the water shields the radiation and also absorbs the heat,which is removed by turn the water through external heat exchangers. Offered fuel is organized in together pools for number of months and also sometimes countless years. It might then be transferred to naturally-ventilated dried storage, normally on site.

Depending on the policies of details countries, some offered fuel might be moved to central storage facilities. Whilst there is a clear motivation for interim storage, offered fuel must eventually either be reprocessed in order come recycle many of it, or prepared for long-term disposal.The longer it is stored, the much easier it is come handle, due to degeneration of radioactivity.

There room two alternatives for used fuel:

Reprocessing come recover and also recycle the usable portion of it.Long-term storage and final disposal there is no reprocessing.

Reprocessing

Used fuel tho contains around 96% that its original uranium, of which the fissionable U-235 content has been reduced to much less than 1%. Around 3% the the offered fuel comprises garbage products and also the continuing to be 1% is plutonium (Pu) created while the fuel was in the reactor.Reprocessing off uranium and plutonium from waste commodities (and from the fuel assembly cladding) by cutting up the fuel rods and dissolving lock in mountain to separate the assorted materials. It allows recycling the the uranium and plutonium right into fresh fuel, and also produces a significantly reduced amount of rubbish (compared v treating all provided fuel together waste). The remaining 3% the high-level radiation waste (some 750 kg every year native a 1000 MWe reactor) deserve to be save in liquid form and ultimately solidified.

For an ext information, see page on processing of supplied Nuclear Fuel.

Uranium and plutonium recycling

The uranium recovered native reprocessing, which generally contains a slightly higher concentration that U-235 than occurs in nature, deserve to be reused together fuel after conversion and enrichment.

The plutonium have the right to be directly made into mixed oxide (MOX) fuel, in i m sorry uranium and plutonium oxides room combined. In reactors that use MOX fuel, plutonium substitutes for the U-235 in regular uranium oxide fuel.

According to Areva, about eight fuel assemblies reprocessed deserve to yield one MOX fuel assembly, two-thirds of one enriched uranium fuel assembly, and around three tonnes of depleted uranium (enrichment tails) plus around 150 kg that waste. It avoids the need to purchase around 12 tonnes of organic uranium native a mine.

Another method of recycling plutonium and also uranium from reprocessing is Russia’s REMIX, not yet commercialized. Below a non-separated mix of both has actually some low-enriched uranium (17% U-235) added, to produce fuel with around 1% Pu-239 and 4% U-235.

Apart indigenous the incidental revolution of U-238 into plutonium in a common reactor, U-238 is no usable in today’s fuel cycle. However, in a quick neutron reactor it is fissionable, and also (more importantly) providing rise to plutonium, and also is as such potentially valuable. Increasingly, today’s offered fuel is being viewed as a future resource rather than a waste.

For an ext information, watch pages on combined Oxide Fuel MOX and Plutonium.

Waste

Waste native the nuclear fuel bicycle is categorized together high-, medium- or low-level based upon the quantity of radiation that it emits. This waste originates from a number of sources and also includes:

Low-level waste produced at every stages the the fuel cycle.Intermediate-level waste created during reactor operation and by reprocessing.High-level waste, i m sorry is waste containing the highly-radioactive fission assets separated in reprocessing, and also in plenty of countries, the used fuel itself. Separated high-level waste likewise contain long-lived transuranic elements.

After reprocessing, the liquid high-level waste can be calcined (heated strongly) to develop a dried powder, i beg your pardon is incorporated into borosilicate (Pyrex) glass come immobilize it. The glass is climate poured right into stainless stole canisters, each holding 400 kg of glass. A year's waste from a 1000 MWe reactor is had in 5 tonnes of together glass, or around 12 canisters 1.3 metres high and also 0.4 metres in diameter. These deserve to readily be transported and also stored, with ideal shielding.

The uranium enrichment process leads to the production of lot 'depleted' uranium, in which the concentration of U-235 is significantly less 보다 the 0.7% uncovered in nature. Little quantities the this material, i m sorry is mostly U-238, are provided in applications where its very high density characteristics are required, consisting of radiation shielding the production of MOX fuel. If U-238 is no fissile that is a low particular activity radioactive material and also some precautions must, therefore, be taken in its warehouse or disposal.

For much more information, see page on radiation Waste Management.

Used fuel and also separated waste: last disposal

At the current time, there room no disposal facilities (as protest to storage facilities) in procedure in which used fuel no destined because that reprocessing, and also the waste from reprocessing, can be placed.In either case the product is save in a solid, steady wasteform.

There is right now no pressing logistical require to establish such facilities, as the complete volume of such waste is fairly small. Further, the much longer it is save the simpler it is to handle, due to the progressive decrease the radioactivity.

There is additionally a reluctance come dispose of offered fuel due to the fact that it to represent a far-reaching energy source which could be reprocessed at a later date to permit recycling of the uranium and also plutonium.

A variety of countries space carrying out studies to recognize the optimum method to the disposal of offered fuel and waste from reprocessing. The general agreement favours its placement right into deep geology repositories, around 500 metres down, at first recoverable before being permanently sealed.

For an ext information, see web page on Storage and Disposal of radioactive Waste.

Other sources of nuclear fuel

In the 1990s uranium mines got a contender –in plenty of ways an extremely welcome –as army uranium come onto the civil industry under a US-Russian agreement. In the duration to 2013 fifty percent of the uranium supplied for power in the USA came from Russian military stockpiles, and worldwide about one-sixth the the sector was gave thus. In respectable 2013, the final distribution of low enriched uranium indigenous Russian TVEL's JSC Electrochemical Plant significant the perfect of Russia's commitments under the agreement typically referred to together the 'Megatons come Megawatts' programme.

Weapons-grade uranium in stockpiles collected during 1950s and 1960s has actually been enriched to an ext than 90% U-235 and must be diluted around 1:25 or 1:30 through depleted uranium (about 0.3% U-235) in order come be offered for civil power purposes. Weapons-grade plutonium may also be used to make mixed oxide (MOX) fuel for usage in ordinary reactors or in distinct reactors designed to 'burn' it because that electricity. Another US-Russian agreement covers disposition of army plutonium indigenous both countries into MOX fuel.

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For more information, see web page on armed forces Warheads as a resource of atom Fuel.

Material balance in the atom fuel cycle

The following numbers may be regarded as typical for the yearly operation that a 1000 MWe nuclear strength reactor typical of many operating today, using 4.5% enriched fuel and also with 45 GWd/t burn-up:g

MiningMillingConversionEnrichment
Anything indigenous 20,000 to 400,000 tonnes that uranium ore
249 tonnes that uranium oxide concentration (which includes 211 tonnes of uranium)
312 tonnes that uranium hexafluoride, UF6(with 211 tU)
35.9 tonnes the enriched UF6 (containing 24.3 t enriched U