Nuclear fuel cycle



The nuclear fuel cycle, also called nuclear fuel chain, is the progression of spent nuclear fuel. If spent fuel is not reprocessed, the fuel cycle is referred to as a open fuel cycle (or a once-through fuel cycle). Likewise, if the spent fuel is reprocessed, it is referred to as a closed fuel cycle.

Fuel cycles

Once-through fuel cycle

  Not a cycle per se, fuel is used once and then sent to storage without further processing save additional packaging to provide for better isolation from the biosphere. This method is favored by six countries: the United States, Canada, Sweden, Finland, Spain and South Africa.[1] Some countries, notably Sweden and Canada, have designed repositories to permit future recovery of the material should the need arise, while others plan for permanent sequestration in a geological repository like Yucca Mountain in the United States.

Plutonium cycle

  Several countries are using the reprocessing services offered by isotopes.

Minor actinides recycling

It has been proposed that in addition to the use of plutonium, that the minor actinides could be used in a critical power reactor. Already tests are being conducted in which americium is being used as a fuel. [2]

A number of reactor designs, like the Integral Fast Reactor, have been designed for this rather different fuel cycle. In principle, it should be possible to derive energy from the fission of any actinide nucleus. With a careful reactor design, all the actinides in the fuel can be consumed, leaving only lighter elements with short half-lives. Whereas this has been done in prototype plants, no such reactor has ever been operated on a large scale, and the first plants with full actinide recovery are expected to be ready for commercial deployment in 2015 at the earliest.

However, such schemes would most likely require advanced remote reprocessing methods due to the neutron emitting compounds formed. For instance if neutron emission from a used fuel element which had included curium will be much higher, potentially posing a risk to workers at the back end of the cycle unless all reprocessing is done remotely. This could be seen as a disadvantage, but on the other hand it also makes the nuclear material difficult to steal or divert, making it more resistant to nuclear proliferation

It so happens that the neutron capture) changes in favour of fission as the neutron energy increases. Thus with a sufficiently high neutron energy, it should be possible to destroy even curium without the generation of the transcurium metals. This could be very desirable as it would make it significantly easier to reprocess and handle the actinide fuel.

One promising alternative from this perspective is an accelerator driven subcritical reactor. Here a beam of either photons will be generated. These high-energy neutrons and photons will then be able to cause the fission of the heavy actinides.

Such reactors compare very well to other neutron sources in terms of neutron energy:

As an alternative, the curium-244, with a half life of 18 years, could be left to decay into plutonium-240 before being used in fuel in a fast reactor.

 

Fuel or targets for this actinide transmutation

To date the nature of the fuel (targets) for actinide transformation has not been chosen.

If actinides are transmuted in a Subcritical reactor it is likely that the fuel will have to be able to tolerate more thermal cycles than conventional fuel. An accelerator driven sub critical reactor is unlikely to be able to maintain a constant operation period for equally long times as a critical reactor, and each time the accelerator stops then the fuel will cool down.

On the other hand, if actinides are destroyed using a fast reactor, such as an Integral Fast Reactor, then the fuel will most likely not be exposed to many more thermal cycles than in a normal power station.

Depending on the matrix the process can generate more transuranics from the matrix. This could either be viewed as good (generate more fuel) or can be viewed as bad (generation of more radiotoxic transuranic elements). A series of different matrices exist which can control this production of heavy actinides.

Fissile nuclei, like Uranium-235, Plutonium-239 and Uranium-233 respond well to delayed neutrons and are thus important to keep a critical reactor stable, and this limits the amount of minor actinides that can be destroyed in a critical reactor. As a consequence it is important that the chosen matrix allows the reactor to keep the ratio of fissile to non-fissile nuclei high, as this enables it to destroy the long lived actinides safely. In contrast, the power output of a sub-critical reactor is limited by the intensity of the driving particle accelerator, and thus it need not contain any uranium or plutonium at all. In such a system it may be preferable to have an inert matrix that doesn't produce additional long-lived isotopes.


Actinides in an inert matrix

The actinides will be mixed with a metal which will not form more actinides, for instance an zirconia could be used.

Actinides in a thorium matrix

Thorium will on neutron bombardment form uranium-233. U-233 is fissile, and has a larger fission cross section than both U-235 and U-238, and thus it is likely to produce very little additional actinides through neutron capture.

Actinides in a uranium matrix

If the actinides is incorporated into a uranium-metal or uranium-oxide matrix, then the neutron capture of U-238 is likely to generate new plutonium-239. An advantage of mixing the actinides with uranium and plutonium is that the large fission cross sections of U-235 and Pu-239 for the less energetic delayed-neutrons could make the reaction stable enough to be carried out in a critical fast reactor, which is likely to be both cheaper and simpler than an accelerator driven system.

Mixed matrix

It is also possible to create a matrix made from a mix of the above mentioned materials. This is most commonly done in fast reactors where one may wish to keep the breeding ratio of new fuel high enough to keep powering the reactor, but still low enough that the generated actinides can be safely destroyed without transporting them to another site. One way to do this is to use fuel where actinides and uranium is mixed with inert zirconium, producing fuel elements with the desired properties.

Thorium cycle

In the thorium fuel cycle uranium-238, thorium-232 is a fertile material.

After starting the reactor with existing U-233 or some other fissile material such as U-235 or Liquid fluoride reactor designs, the Pa-233 is extracted and protected from neutrons (which could transform it to Pa-234 and then to U-234), until it has decayed to U-233. This is done in order to improve the breeding ratio.

Uranium-233 is an excellent reactor fuel. Uranium-233 is superior to uranium-235 and plutonium-239 because it produces more neutrons per neutron absorbed (it has a high "beta" coefficient). Its absorption of neutrons (cross-section) also varies less with neutron energy than plutonium-239 or U-235. This stability suggests potential for high burnup, higher operating temperatures, and therefore more efficient conversion of heat to electricity.[citation needed]

When U-233 absorbs a neutron, it either fissions or becomes the next heavier isotope, U-234. The chance of not fissioning on absorption of a thermal neutron is about 1/7 (or even less than 10% according to another source), which is less than the corresponding capture/fission ratios for U-235 (about 1/6) or for Pu-239 or protactinium-231 (half-life 33,000 years) via the (n,2n) reaction on Th-232. [6] [7] [8] [9] Because the thorium/uranium-233 cycle produces a smaller amount of long-lived uranium dioxide cycle. [10]

Current industrial activity

Currently the only isotopes used as nuclear fuel are uranium in some countries, notably India.[11]

Heavy water reactors and graphite-moderated reactors can use enriched uranium, in which the ratio of U-235 to U-238 is increased. In civilian reactors the enrichment is increased to as much as 5% U-235 and 95% U-238, but in naval reactors there is as much as 93% U-235.

The term helium to release energy.

Front end

Main article: Uranium mining

Exploration

A deposit of uranium, such as free neutron, and the isotope is therefore said to be a "fissile" isotope. The nucleus of a U-238 atom on the other hand, rather than undergoing fission when struck by a free neutron, will nearly always absorb the neutron and yield an atom of the isotope U-239. This isotope then undergoes natural radioactive decay to yield Pu-239, which, like U-235, is a fissile isotope. The atoms of U-238 are said to be fertile, because, through neutron irradiation in the core, some eventually yield atoms of fissile Pu-239.

Mining

Uranium ore can be extracted through conventional mining in open pit and underground methods similar to those used for mining other metals. In situ leach mining methods also are used to mine uranium in the United States. In this technology, uranium is leached from the in-place ore through an array of regularly spaced wells and is then recovered from the leach solution at a surface plant. Uranium ores in the United States typically range from about 0.05 to 0.3% uranium oxide (U3O8). Some uranium deposits developed in other countries are of higher grade and are also larger than deposits mined in the United States. Uranium is also present in very low-grade amounts (50 to 200 parts per million) in some domestic fertilizers and other phosphate chemicals, at some phosphate processing plants the uranium, although present in very low concentrations, can be economically recovered from the process stream.

Milling

Mined uranium ores normally are processed by grinding the ore materials to a uniform particle size and then treating the ore to extract the uranium by chemical leaching. The milling process commonly yields dry powder-form material consisting of natural uranium, "yellowcake," which is sold on the uranium market as U3O8.

Uranium conversion

Milled uranium oxide, U3O8, must be converted to uranium hexafluoride, UF6, which is the form required by most commercial uranium enrichment facilities currently in use. A solid at room temperature, uranium hexafluoride can be changed to a gaseous form at moderately higher temperature of 134 °F (57 °C). The uranium hexafluoride conversion product contains only natural, not enriched, uranium.

Triuranium octaoxide (U3O8) is also converted directly to uranium dioxide (UO2) for use in reactors not requiring enriched fuel, such as CANDU. The volumes of material converted directly to UO2 are typically quite small compared to the amounts converted to UF6.

Enrichment

Main article: enriched uranium

 

The concentration of the fissionable isotope, U-235 (0.71% in natural uranium) is less than that required to sustain a nuclear chain reaction in light water reactor cores. Natural UF6 thus must be enriched in the fissionable isotope for it to be used as nuclear fuel. The different levels of enrichment required for a particular nuclear fuel application are specified by the customer: light-water reactor fuel normally is enriched to 3.5% U-235, but uranium enriched to lower concentrations also is required. Enrichment is accomplished using some one or more methods of isotope separation. gas centrifuge are the commonly used uranium enrichment technologies, but new enrichment technologies are currently being developed.

The bulk (96%) of the byproduct from enrichment is uranium hexafluoride (UF6).

Fabrication

Main article: Nuclear fuel

For use as nuclear fuel, enriched uranium hexafluoride is converted into alloy. The tubes are sealed to contain the fuel pellets: these tubes are called fuel rods. The finished fuel rods are grouped in special fuel assemblies that are then used to build up the nuclear fuel core of a power reactor.

The metal used for the tubes depends on the design of the reactor. zirconium. For the most common types of reactors, boiling water reactors (BWR) and pressurized water reactors (PWR), the tubes are assembled into bundles[13] with the tubes spaced precise distances apart. These bundles are then given a unique identification number, which enables them to be tracked from manufacture through use and into disposal.

Service period

Transport of radioactive materials

Transport is an integral part of the nuclear fuel cycle. There are nuclear power reactors in operation in several countries but uranium mining is viable in only a few areas. Also, in the course of over forty years of operation by the nuclear industry, a number of specialized facilities have been developed in various locations around the world to provide fuel cycle services and there is a need to transport nuclear materials to and from these facilities. Most transports of uranium hexafluoride (UF6) which is considered a gas. Most of the material used in nuclear fuel is transported several times during the cycle. Transports are frequently international, and are often over large distances. Nuclear materials are generally transported by specialized transport companies.

Since nuclear materials are spent nuclear fuel shipping casks are used which are designed to maintain integrity under normal transportation conditions and during hypothetical accident conditions.

In-core fuel management

A nuclear reactor core is composed of a few hundred "assemblies", arranged in a regular array of cells, each cell being formed by a fuel or control rod surrounded, in most designs, by a moderator and coolant, which is water in most reactors.

Because of the fission process that consumes the fuels, the old fuel rods must be changed periodically to fresh ones (this period is called a cycle). However, only a part of the assemblies (typically one-third) are removed since the fuel depletion is not spatially uniform. Furthermore, it is not a good policy, for efficiency reasons, to put the new assemblies exactly at the location of the removed ones. Even bundles of the same age may have different burn-up levels, which depends on their previous positions in the core. Thus the available bundles must be arranged in such a way that the yield is maximized, while safety limitations and operational constraints are satisfied. Consequently reactor operators are faced with the so-called optimal fuel reloading problem, which consists in optimizing the rearrangement of all the assemblies, the old and fresh ones, while still maximizing the reactivity of the reactor core so as to maximise fuel burn-up and minimise fuel-cycle costs.

This is a discrete optimization problem, and computationally infeasible by current combinatorial methods, due to the huge number of permutations and the complexity of each computation. Many numerical methods have been proposed for solving it and many commercial software packages have been written to support fuel management. This is an on-going issue in reactor operations as no definitive solution to this problem has been found and operators use a combination of computational and empirical techniques to manage this problem.

The study of used fuel

Main article: Post irradiation examination

Used nuclear fuel is studied in zirconate (BaxSr1-xZrO3).

 

Uranium dioxide is very insoluble in water, but after oxidation it can be converted to uranium trioxide or another uranium(VI) compound which is much more soluble. It is important to understand that uranium dioxide (UO2) can be oxidised to an oxygen rich hyperstoichiometric oxide (UO2+x) which can be further oxidised to U4O9, U3O7, U3O8 and UO3.2H2O.

Because used fuel contains alpha emitters (plutonium and the minor actinides), the effect of adding an alpha emitter (238Pu) to uranium dioxide on the leaching rate of the oxide has been investigated. For the crushed oxide, adding 238Pu tended to increase the rate of leaching, but the difference in the leaching rate between 0.1 and 10% 238Pu was very small.[15]

The concentration of uranium trioxide phases.[16]

By ‘cyclic voltammetry and AC impedance experiments, and these offer an insight into the likely leaching behaviour of uranium dioxide.[17]

Fuel cladding interactions

The study of the nuclear fuel cycle includes the study of the behaviour of nuclear materials both under normal conditions and under accident conditions. For example, there has been much work on how uranium dioxide as a function of distance from the centre of a 20 mm diameter pellet with a rim temperature of 200 oC. It is important to note that the uranium dioxide (because of its poor thermal conductivity) will overheat at the centre of the pellet, while the more thermally conductive other forms of uranium remain below their melting points.

 

Normal and abnormal conditions

The nuclear chemistry associated with the nuclear fuel cycle can be divided into two main areas, one area is concerned with operation under the intended conditions while the other area is concerned with maloperation conditions where some alteration from the normal operating conditions has occurred or (more rarely) an accident is occurring.

The releases of radioactivity from normal operations are the small planned releases from uranium ore processing, enrichment, power reactors, reporcessing plants and waste stores. These can be in a different chemical/physical form to the releases which could occur under accident conditions. In addition the isotope signature of a hypothetical accident may be very different to that of a planned normal operational discharge of radioactivity to the environment.

It is important to note that just because a radioisotope is released it does not mean it will enter a human and then cause harm. For instance the migration of radioactivity can altered by the binding of the radioisotope to the surfaces of soil particles. For example cesium binds tightly to clay minerals such as montmorillonite hence it remains in the upper layers of soil where it can be accessed by plants with shallow roots (such as grass). Hence grass and mushrooms can carry a considerable amount of 137Cs which can be transferred to humans through the food chain. But 137Cs is not able to migrate quickly through most soils and thus is unlikely to contaminate well water. It is important to note that colloids of soil minterals can migrate through soil so simple binding of a metal to the surfaces of soil particles does not fix the metal totally.

According to Jiří Hála's text book the distribution coefficient Kd is the ratio of the soil's radioactivity (Bq g-1) to that of the soil water (Bq ml-1). If the radioactivity is tightly bonded to by the minerals in the soil then less radioactivity can be absorbed by crops and grass growing on the soil.

One of the best countermeasures in dairy farming against 137Cs is to mix up the soil by deeply ploughing the soil. This has the effect of putting the 137Cs out of reach of the shallow roots of the grass, hence the level of radioactivity in the grass will be lowered. Also after a nuclear war or serious accident the removal of top few cm of soil and its burial in a shallow trench will reduce the long term gamma dose to humans due to 137Cs as the gamma photons will be attenuated by their passage through the soil.

Even after the radioactive element arrives at the roots of the plant, the metal may be rejected by the biochemistry of the plant. The details of the uptake of 90Sr and 137Cs into calcium ions on the uptake of the radioisotopes.

In livestock farming an important countermeasure against 137Cs is to feed to animals a little cesium in chernobyl fallout exists at [11], this is the Ukrainian Research Institute for Agricultural Radiology.

Release of radioactivity from fuel during normal use and accidents

The IAEA assume that under normal operation the coolant of a water cooled reactor will contain some radioactivity[20] but during a reactor accident the coolant radioactivity level may rise. The IAEA state that under a series of different conditions different amounts of the core inventory can be released from the fuel, the four conditions the IAEA consider are normal operation, a spike in coolant activity due to a sudden shutdown/loss of preasure (core remains covered with water), a cladding failure resulting in the release of the activity in the fuel/cladding gap (this could be due to the fuel being uncovered by the loss of water for 15-30 minutes where the cladding reached a temperature of 650-1250 oC) or a melting of the core (the fuel will have to be uncovered for at least 30 minutes, and the cladding would reach a temperature in excess of 1650 oC).[21]

Based upon the assumption that a PWR contains 300 tons of water, and that the activity of the fuel of a 1 GWe reactor is as the IAEA predict,[22] then the coolant activity after an accident such as the three mile island accident where a core is uncovered and then recovered with water then the resulting activity of the coolant can be predicted.

Releases from reprocessing under normal conditions

It is normal to allow used fuel to stand after the irradation to allow the shortlived and radiotoxic tritium are released from the fuel when it is dissolved, it has been proposed that by voloxidation (heating the fuel in a furnace under oxidizing conditions) the majority of the tritium can be recovered from the fuel.[15]

A paper was written on the radioactivity found in oysters found in the Irish Sea,[23] these were found by gamma spectrscopy to contain 141Ce, 144Ce, 103Ru, 106Ru, 137Cs, 95Zr and 95Nb. In addition a zinc activation product (65Zn) was found, this is thought to be due to the corrosion of magnox fuel cladding in cooling ponds. It is likely that the modern releases of all these isotopes from Windscale is smaller.

On-load reactors

Some reactor designs, such as RBMKs or CANDU reactors, can be refueled without being shut down. This is achieved through the use of many small pressure tubes to contain the fuel and coolant, as opposed to one large pressure vessel as in pressurized water reactor (PWR) or boiling water reactor (BWR) designs. Each tube can be individually isolated and refueled by an operator-controlled fueling machine, typically at a rate of up to 8 channels per day out of roughly 400 in CANDU reactors. On-load refueling allows for the problem of optimal fuel reloading problem to be dealt with continuously, leading to more efficient use of fuel. This increase in efficiency is partially offset by the added complexity of having hundreds of pressure tubes and the fueling machines to service them.

Back end

Interim storage

After its operating cycle, the reactor is shut down for refueling. The fuel discharged at that time (spent fuel) is stored either at the reactor site, commonly in a dry cask storage.

Transportation

Main article: Spent nuclear fuel shipping cask

Reprocessing

Main article: Nuclear reprocessing
See also: Used nuclear fuel

 

Spent fuel discharged from reactors contains appreciable quantities of fissile (U-235 and Pu-239), fertile (U-238), and other reaction poisons, which is why the fuel had to be removed. These fissile and fertile materials can be chemically separated and recovered from the spent fuel. The recovered uranium and plutonium can, if economic and institutional conditions permit, be recycled for use as nuclear fuel. This is currently not done for civilian spent nuclear fuel in the US.

Mixed oxide, or reprocessed uranium and plutonium and depleted uranium which behaves similarly, although not identically, to the enriched uranium feed for which most nuclear reactors were designed. MOX fuel is an alternative to low-enriched uranium (LEU) fuel used in the light water reactors which predominate nuclear power generation.

Currently, plants in Europe are reprocessing spent fuel from utilities in Europe and Japan. Reprocessing of spent commercial-reactor nuclear fuel is currently not permitted in the United States due to the perceived danger of nuclear proliferation. However the recently announced Global Nuclear Energy Partnership would see the U.S. form an international partnership to see spent nuclear fuel reprocessed in a way that renders the plutonium in it usable for nuclear fuel but not for nuclear weapons.

Partitioning and transmutation

As an alternative to the disposal of the transmutation.

Waste disposal

Main article: Radioactive waste

A current concern in the nuclear power field is the safe disposal and isolation of either spent fuel from reactors or, if the reprocessing option is used, wastes from reprocessing plants. These materials must be isolated from the biosphere until the radioactivity contained in them has diminished to a safe level. In the U.S., under the Nuclear Waste Policy Act of 1982 as amended, the Department of Energy has responsibility for the development of the waste disposal system for spent nuclear fuel and high-level radioactive waste. Current plans call for the ultimate disposal of the wastes in solid form in a licensed deep, stable geologic structure called a deep geological repository. The Department of Energy chose Yucca Mountain as the location for the repository. However, its opening has been repeatedly delayed.

See also

References

  1. ^ [1]
  2. ^ [2]
  3. ^ [3]
  4. ^ [4][5]
  5. ^ [6]
  6. ^ STATUS OF NUCLEAR DATA FOR THE THORIUM FUEL CYCLE. “Relative number of nuclei n = N(231Pa )/N(233U): Fast reactor 0.8×10-2 Thermal reactor 1.9×10-3... The problem is exacerbated by the fact that the secondary heavy nuclei produced in this cycle possess, as a rule, extremely unpleasant nuclear physics characteristics from the experimentalist’s point of view”
  7. ^ Method of increasing the deterrent to proliferation of nuclear fuels - Patent 4344912. “protactinium-231 which is normally found in the spent fuel rods of a thorium base nuclear reactor”
  8. ^ AN OVERVIEW OF R&D IN FUEL CYCLE ACTIVITIES OF AHWR. “the higher energy (n, 2n) reactions encountered by Th-232 during the irradiation in Th-U233 fuel also lead to the formation of long lived Pa-231... Pa-231 is of special concern in HLLW of AHWR because the pentavalent Pa-231 is capable to migrate much more in water/soil compared to other ions... the amount of Pa-231 produced in (Th-U233)O2 MOX fuel is ~ 3 gms/te at 20,000 MWd/t of burn-up... removal of protactinium has to be established using suitable solvents that is yet to be tested”
  9. ^ Nuclear Energy With (Almost) No Radioactive Waste?. “according to computer simulations done at ISN, this Protactinium dominates the residual toxicity of losses at 10 000 years”
  10. ^ [7]
  11. ^ [8]
  12. ^ [9]
  13. ^ [10]
  14. ^ A good report on the microstructure of used fuel is Lucuta PG et al (1991) J Nuclear Materials 178:48-60
  15. ^ V.V. Rondinella VV et al (2000) Radiochimica Acta 88:527-531
  16. ^ For a review of the corrosion of uranium dioxide in a waste store which explains much of the chemistry, see Shoesmith DW (2000) J Nuclear Materials 282:1-31
  17. ^ Miserque F et al (2001) J Nuclear Materials 298:280-90
  18. ^ Further reading on fuel cladding interactions: Tanaka K et al (2006) J Nuclear Materials 357:58-68
  19. ^ P. Soudek, Š. Valenová, Z. Vavříková and T. Vaněk, Journal of Environmental Radioactivity, 2006, 88, 236-250
  20. ^ page 169 Generic Assessment Procedures for Determining Protective Actions During a Reactor Accident, IAEA-TECDOC-955, 1997
  21. ^ page 173 Generic Assessment Procedures for Determining Protective Actions During a Reactor Accident, IAEA-TECDOC-955, 1997
  22. ^ page 171 Generic Assessment Procedures for Determining Protective Actions During a Reactor Accident, IAEA-TECDOC-955, 1997
  23. ^ A. Preston, J.W.R. Dutton and B.R. Harvey, Nature, 1968, 218, 689-690.

(Reference V. Artisyuk, M. Saito and A. Shmelev, Progress in Nuclear Energy, 2000, 37, 345-350)be-x-old:Ядзернае паліва

 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Nuclear_fuel_cycle". A list of authors is available in Wikipedia.