MOX fuel

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Mixed oxide fuel, commonly referred to as MOX fuel, is nuclear fuel that contains more than one oxide of fissile material, usually consisting of plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an alternative to the low-enriched uranium fuel used in the light-water reactors that predominate nuclear power generation.

Contents

For example, a mixture of 7% plutonium and 93% natural uranium reacts similarly, although not identically, to low-enriched uranium fuel (3 to 5% uranium-235). MOX usually consists of two phases, UO2 and PuO2, and/or a single phase solid solution (U,Pu)O2. The content of PuO2 may vary from 1.5 wt.% to 25–30 wt.% depending on the type of nuclear reactor.

One attraction of MOX fuel is that it is a way of utilizing surplus weapons-grade plutonium, an alternative to storage of surplus plutonium, which would need to be secured against the risk of theft for use in nuclear weapons. [1] [2] On the other hand, some studies warned that normalising the global commercial use of MOX fuel and the associated expansion of nuclear reprocessing will increase, rather than reduce, the risk of nuclear proliferation, by encouraging increased separation of plutonium from spent fuel in the civil nuclear fuel cycle. [3] [4] [5]

Overview

In every uranium-based nuclear reactor core there is both fission of uranium isotopes such as uranium-235, and the formation of new, heavier isotopes due to neutron capture, primarily by uranium-238. Most of the fuel mass in a reactor is uranium-238. By neutron capture and two successive beta decays, uranium-238 becomes plutonium-239, which, by successive neutron capture, becomes plutonium-240, plutonium-241, plutonium-242, and (after further beta decays) other transuranic or actinide nuclides. Plutonium-239 and plutonium-241 are fissile, like uranium-235. Small quantities of uranium-236, neptunium-237 and plutonium-238 are formed similarly from uranium-235.

Normally, with low-enriched uranium fuel being changed every five years or so, most of the plutonium-239 is "burned" in the reactor. It behaves like uranium-235, with a slightly higher cross section for fission, and its fission releases a similar amount of energy. Typically, about one percent of the spent fuel discharged from a reactor is plutonium, and some two-thirds of the plutonium is plutonium-239. Worldwide, almost 100 tonnes of plutonium in spent fuel arises each year.

Reprocessing the plutonium into usable fuel increases the energy derived from the original uranium by some 12%, and if the uranium-235 is also recycled by re-enrichment, this becomes about 20%. [6] Currently plutonium is only reprocessed and used once as MOX fuel; spent MOX fuel, with a high proportion of minor actinides and plutonium isotopes, is stored as waste.

Existing nuclear reactors must be re-licensed before MOX fuel can be introduced because using it changes the operating characteristics of a reactor, and the plant must be designed or adapted slightly to take it; for example, more control rods are needed. Often only a third to half of the fuel load is switched to MOX, but for more than 50% MOX loading, significant changes are necessary and a reactor needs to be designed accordingly. The System 80 reactor design, notably deployed at the U.S. Palo Verde Nuclear Generating Station near Phoenix, Arizona, was designed for 100% MOX core compatibility, but so far has always operated on fresh low enriched uranium. In theory, the three Palo Verde reactors could use the MOX arising from seven conventionally fueled reactors each year and would no longer require fresh uranium fuel.

Fast neutron BN-600 and BN-800 reactors are designed for 100% MOX loading. In 2022, the BN-800 was fully loaded with MOX fuel for the first time. [7]

According to Atomic Energy of Canada Limited (AECL), CANDU reactors could use 100% MOX cores without physical modification. [8] [9] AECL reported to the United States National Academy of Sciences committee on plutonium disposition that it has extensive experience in testing the use of MOX fuel containing from 0.5 to 3% plutonium.[ citation needed ]

Spent MOX fuel

The content of un-burnt plutonium in spent MOX fuel from thermal reactors is significant – greater than 50% of the initial plutonium loading. However, during the burning of MOX the ratio of fissile (odd numbered) isotopes to non-fissile (even) drops from around 65% to 20%, depending on burn up. This makes any attempt to recover the fissile isotopes difficult and any bulk Pu recovered would require such a high fraction of Pu in any second generation MOX that it would be impractical.[ why? ] This means that such a spent fuel would be difficult to reprocess for further reuse (burning) of plutonium. Regular reprocessing of biphasic spent MOX is difficult because of the low solubility of PuO2 in nitric acid. [10]

As of 2015, the only demonstration of twice-recycled, high-burnup fuel occurred in the Phénix fast reactor. [11]

Current applications

A used MOX, which has 63 GW days (thermal) of burnup and has been examined with a scanning electron microscope using electron microprobe attachment. The lighter the pixel in the right hand side the higher the plutonium content of the material at that spot SEMofusedMOX.jpg
A used MOX, which has 63 GW days (thermal) of burnup and has been examined with a scanning electron microscope using electron microprobe attachment. The lighter the pixel in the right hand side the higher the plutonium content of the material at that spot

Reprocessing of commercial nuclear fuel to make MOX is performed in France and to a lesser extent in Russia, India and Japan. In the UK THORP operated from 1994 to 2018. China plans to develop fast breeder reactors and reprocessing. Reprocessing of spent commercial-reactor nuclear fuel is not permitted in the United States due to nonproliferation considerations. Germany had plans for a reprocessing plant at Wackersdorf but as this failed to materialize, it instead relied on French nuclear reprocessing capabilities until legally outlawing the transport of German spent fuel for reprocessing in 2005. [12]

The United States was building a MOX fuel plant at the Savannah River Site in South Carolina. Although the Tennessee Valley Authority (TVA) and Duke Energy expressed interest in using MOX reactor fuel from the conversion of weapons-grade plutonium, [13] TVA (currently the most likely customer) said in April 2011 that it would delay a decision until it could see how MOX fuel performed in the nuclear accident at Fukushima Daiichi. [14] In May 2018, the Department of Energy reported that the plant would require another $48 billion to complete, on top of the $7.6 billion already spent. Construction was cancelled. [15]

Thermal reactors

Most modern thermal reactors using high burn up uranium oxide fuel produce a quite significant proportion of their output towards the end of core life from fission of plutonium produced by neutron capture in uranium 238 earlier in the life of the core, so adding some plutonium oxide to the fuel at manufacture is not in principle a very radical step. About 30 thermal reactors in Europe (Belgium, the Netherlands, Switzerland, Germany and France) are using MOX [16] and an additional 20 have been licensed to do so. Most reactors use it as about one third of their core, but some will accept up to 50% MOX assemblies. In France, EDF aims to have all its 900 MWe series of reactors running with at least one-third MOX. Japan aimed to have one third of its reactors using MOX by 2010, and has approved construction of a new reactor with a complete fuel loading of MOX. As 2011, of the total nuclear fuel used, MOX provides about 2%. [6]

Licensing and safety issues of using MOX fuel include: [16]

About 30% of the plutonium originally loaded into MOX fuel is consumed by use in a thermal reactor. In theory, if one third of the core fuel load is MOX and two-thirds uranium fuel, there is zero net change in the mass of plutonium in the spent fuel and the cycle could be repeated; however, there remains multiple difficulties in reprocessing spent MOX fuel. As of 2010, plutonium is only recycled once in thermal reactors, and spent MOX fuel is separated from the rest of the spent fuel to be stored as waste. [16]

All plutonium isotopes are either fissile or fertile, although plutonium-242 needs to absorb 3 neutrons before becoming fissile curium-245; in thermal reactors isotopic degradation limits the plutonium recycle potential. About 1% of spent nuclear fuel from current LWRs is plutonium, with approximate isotopic composition 52% 239
94Pu
 , 24% 240
94Pu
 , 15% 241
94Pu
 , 6% 242
94Pu
 and 2% 238
94Pu
 when the fuel is first removed from the reactor. [16]

Fast reactors

Because the fission-to-capture ratio of high energy or fast neutrons changes to favour fission for almost all of the actinides, including 238
92U
, fast reactors could use all of them for fuel. All actinides can undergo neutron induced fission with unmoderated or fast neutrons. A fast reactor is therefore more efficient than a thermal reactor for using plutonium and higher actinides as fuel.

These fast reactors are better suited for the transmutation of other actinides than thermal reactors. Because thermal reactors use slow or moderated neutrons, the actinides that are not fissionable with thermal neutrons tend to absorb the neutrons instead of fissioning. This leads to buildup of heavier actinides and lowers the number of thermal neutrons available to continue the chain reaction. A subcritical reactor with an external neutron source could either be run in the fast neutron spectrum (without the need for highly enriched fuels as otherwise common in fast reactors) or use thermal neutrons to breed fissile materials, compensating the loss of neutrons by increasing the flux from the neutron source.

Fabrication

Plutonium separation

The first step is separating the plutonium from the remaining uranium (about 96% of the spent fuel) and the fission products with other wastes (together about 3%) using the PUREX process.

Dry mixing

MOX fuel can be made by grinding together uranium oxide (UO2) and plutonium oxide (PuO2) before the mixed oxide is pressed into pellets, but this process has the disadvantage of forming much radioactive dust.

Coprecipitation

A mixture of uranyl nitrate and plutonium nitrate in nitric acid is converted by treatment with a base such as ammonia to form a mixture of ammonium diuranate and plutonium hydroxide. After heating in a mixture of 5% hydrogen and 95% argon will form a mixture of uranium dioxide and plutonium dioxide. Using a base, the resulting powder can be run through a press and converted into pellets. The pellets can then be sintered into mixed uranium and plutonium oxide.

Americium content

Plutonium from reprocessed fuel is usually fabricated into MOX within less than five years of its production to avoid problems resulting from impurities produced by the decay of short-lived isotopes of plutonium. In particular, plutonium-241 decays to americium-241 with a 14-year half-life. Because americium-241 is a gamma ray emitter,[ citation needed ] its presence is a potential occupational health hazard. It is possible, however, to remove the americium from the plutonium by a chemical separation process. Even under the worst conditions, the americium/plutonium mixture is less radioactive than a spent-fuel dissolution liquor, so it should be relatively straightforward to recover the plutonium by PUREX or another aqueous reprocessing method.[ citation needed ]

Curium content

It is possible that both americium and curium could be added to a U/Pu MOX fuel before it is loaded into a fast reactor or a subcritical reactor run in "Actinide burner mode". This is one means of transmutation. Work with curium is much harder than americium because curium is a neutron emitter, the MOX production line would need to be shielded with both lead and water to protect the workers.

Also, the neutron irradiation of curium generates the higher actinides, such as californium, which increase the neutron dose associated with the used nuclear fuel; this has the potential to pollute the fuel cycle with strong neutron emitters. As a result, it is likely that curium will be excluded from most MOX fuels. A subcritical reactor such as the Accelerator Driven System could "burn" such fuels if the problems associated with their handling and transportation are solved. However, to avoid power excursions due to unintended criticality, the neutronics must be known precisely at any given point in time, including the effect of build-up or consumption of neutron emitting nuclides as well as neutron poisons.

Thorium MOX

MOX fuel containing thorium and plutonium oxides is also being tested. [17] According to a Norwegian study, "the coolant void reactivity of the thorium-plutonium fuel is negative for plutonium contents up to 21%, whereas the transition lies at 16% for MOX fuel." [18] The authors concluded, "Thorium-plutonium fuel seems to offer some advantages over MOX fuel with regards to control rod and boron worths, CVR and plutonium consumption." [18]

See also

Related Research Articles

<span class="mw-page-title-main">Nuclear fuel cycle</span> Process of manufacturing and consuming nuclear fuel

The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the front end, which are the preparation of the fuel, steps in the service period in which the fuel is used during reactor operation, and steps in the back end, which are necessary to safely manage, contain, and either reprocess or dispose of spent nuclear fuel. If spent fuel is not reprocessed, the fuel cycle is referred to as an open fuel cycle ; if the spent fuel is reprocessed, it is referred to as a closed fuel cycle.

<span class="mw-page-title-main">Nuclear reprocessing</span> Chemical operations that separate fissile material from spent fuel to be recycled as new fuel

Nuclear reprocessing is the chemical separation of fission products and actinides from spent nuclear fuel. Originally, reprocessing was used solely to extract plutonium for producing nuclear weapons. With commercialization of nuclear power, the reprocessed plutonium was recycled back into MOX nuclear fuel for thermal reactors. The reprocessed uranium, also known as the spent fuel material, can in principle also be re-used as fuel, but that is only economical when uranium supply is low and prices are high. Nuclear reprocessing may extend beyond fuel and include the reprocessing of other nuclear reactor material, such as Zircaloy cladding.

<span class="mw-page-title-main">Breeder reactor</span> Nuclear reactor generating more fissile material than it consumes

A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. These reactors can be fueled with more-commonly available isotopes of uranium and thorium, such as uranium-238 and thorium-232, as opposed to the rare uranium-235 which is used in conventional reactors. These materials are called fertile materials since they can be bred into fuel by these breeder reactors.

<span class="mw-page-title-main">Fast-neutron reactor</span> Nuclear reactor where fast neutrons maintain a fission chain reaction

A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons, as opposed to slow thermal neutrons used in thermal-neutron reactors. Such a fast reactor needs no neutron moderator, but requires fuel that is relatively rich in fissile material when compared to that required for a thermal-neutron reactor. Around 20 land based fast reactors have been built, accumulating over 400 reactor years of operation globally. The largest of this was the Superphénix Sodium cooled fast reactor in France that was designed to deliver 1,242 MWe. Fast reactors have been intensely studied since the 1950s, as they provide certain advantages over the existing fleet of water cooled and water moderated reactors. These are:

A subcritical reactor is a nuclear fission reactor concept that produces fission without achieving criticality. Instead of sustaining a chain reaction, a subcritical reactor uses additional neutrons from an outside source. There are two general classes of such devices. One uses neutrons provided by a nuclear fusion machine, a concept known as a fusion–fission hybrid. The other uses neutrons created through spallation of heavy nuclei by charged particles such as protons accelerated by a particle accelerator, a concept known as an accelerator-driven system (ADS) or accelerator-driven sub-critical reactor.

<span class="mw-page-title-main">Integral fast reactor</span> Nuclear reactor design

The integral fast reactor is a design for a nuclear reactor using fast neutrons and no neutron moderator. IFR would breed more fuel and is distinguished by a nuclear fuel cycle that uses reprocessing via electrorefining at the reactor site.

<span class="mw-page-title-main">Nuclear fuel</span> Material fuelling nuclear reactors

Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission.

<span class="mw-page-title-main">Fertile material</span>

Fertile material is a material that, although not fissile itself, can be converted into a fissile material by neutron absorption.

<span class="mw-page-title-main">Plutonium-239</span> Isotope of plutonium

Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 is also used for that purpose. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in thermal spectrum nuclear reactors, along with uranium-235 and uranium-233. Plutonium-239 has a half-life of 24,110 years.

Plutonium (94Pu) is an artificial element, except for trace quantities resulting from neutron capture by uranium, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was synthesized long before being found in nature, the first isotope synthesized being 238Pu in 1940. Twenty plutonium radioisotopes have been characterized. The most stable are plutonium-244 with a half-life of 80.8 million years, plutonium-242 with a half-life of 373,300 years, and plutonium-239 with a half-life of 24,110 years. All of the remaining radioactive isotopes have half-lives that are less than 7,000 years. This element also has eight meta states; all have half-lives of less than one second.

<span class="mw-page-title-main">Minor actinide</span> Category of elements in spent nuclear fuel

A minor actinide is an actinide, other than uranium or plutonium, found in spent nuclear fuel. The minor actinides include neptunium, americium, curium, berkelium, californium, einsteinium, and fermium. The most important isotopes of these elements in spent nuclear fuel are neptunium-237, americium-241, americium-243, curium-242 through -248, and californium-249 through -252.

<span class="mw-page-title-main">Thorium fuel cycle</span> Nuclear fuel cycle

The thorium fuel cycle is a nuclear fuel cycle that uses an isotope of thorium, 232
Th
, as the fertile material. In the reactor, 232
Th
 is transmuted into the fissile artificial uranium isotope 233
U
 which is the nuclear fuel. Unlike natural uranium, natural thorium contains only trace amounts of fissile material, which are insufficient to initiate a nuclear chain reaction. Additional fissile material or another neutron source is necessary to initiate the fuel cycle. In a thorium-fuelled reactor, 232
Th
 absorbs neutrons to produce 233
U
. This parallels the process in uranium breeder reactors whereby fertile 238
U
 absorbs neutrons to form fissile 239
Pu
. Depending on the design of the reactor and fuel cycle, the generated 233
U
 either fissions in situ or is chemically separated from the used nuclear fuel and formed into new nuclear fuel.

<span class="mw-page-title-main">Spent nuclear fuel</span> Nuclear fuel thats been irradiated in a nuclear reactor

Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor. It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and, depending on its point along the nuclear fuel cycle, it will have different isotopic constituents than when it started.

<span class="mw-page-title-main">Weapons-grade nuclear material</span> Nuclear material pure enough to be used for nuclear weapons

Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon and has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nuclear weapons are the most common examples.

Plutonium-241 is an isotope of plutonium formed when plutonium-240 captures a neutron. Like some other plutonium isotopes, 241Pu is fissile, with a neutron absorption cross section about one-third greater than that of 239Pu, and a similar probability of fissioning on neutron absorption, around 73%. In the non-fission case, neutron capture produces plutonium-242. In general, isotopes with an odd number of neutrons are both more likely to absorb a neutron, and more likely to undergo fission on neutron absorption, than isotopes with an even number of neutrons.

Uranium-236 (236U) is an isotope of uranium that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived radioactive waste. It is found in spent nuclear fuel and in the reprocessed uranium made from spent nuclear fuel.

<span class="mw-page-title-main">Liquid fluoride thorium reactor</span> Type of nuclear reactor that uses molten material as fuel

The liquid fluoride thorium reactor is a type of molten salt reactor. LFTRs use the thorium fuel cycle with a fluoride-based molten (liquid) salt for fuel. In a typical design, the liquid is pumped between a critical core and an external heat exchanger where the heat is transferred to a nonradioactive secondary salt. The secondary salt then transfers its heat to a steam turbine or closed-cycle gas turbine.

Reprocessed uranium (RepU) is the uranium recovered from nuclear reprocessing, as done commercially in France, the UK and Japan and by nuclear weapons states' military plutonium production programs. This uranium makes up the bulk of the material separated during reprocessing.

<span class="mw-page-title-main">Nuclear transmutation</span> Conversion of an atom from one element to another

Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed.

Remix Fuel was developed in Russia to make use of Mixed Recycled Uranium and Plutonium from spent nuclear fuel to manufacture fresh fuel suitable for widespread use in Russian reactor designs.

References

  1. ↑ "Military Warheads as a Source of Nuclear Fuel - Megatons to MegaWatts - World Nuclear Association". www.world-nuclear.org. Archived from the original on 2013-02-24. Retrieved 2008-09-06.
  2. ↑ "U.S. MOX program wanted relaxed security at the weapon-grade plutonium facility". 11 April 2011.
  3. ↑ "Is U.S. Reprocessing Worth The Risk? - Arms Control Association". www.armscontrol.org.
  4. ↑ "Factsheets on West Valley · NIRS". 1 March 2015. Archived from the original on 20 March 2011. Retrieved 6 September 2008.
  5. ↑ Podvig, Pavel (10 March 2011). "U.S. plutonium disposition program: Uncertainties of the MOX route". International Panel on Fissile Materials. Retrieved 13 February 2012.
  6. 1 2 "Information from the World Nuclear Association about MOX". Archived from the original on 2013-03-01. Retrieved 2011-05-22.
  7. ↑ Реактор БН-800 полностью перешел на МОКС-топливо
  8. ↑ "Candu works with UK Nuclear Decommissioning Authority to study deployment of EC6 reactors". Mississauga: Candu press-release. June 27, 2012. Retrieved 5 December 2013.
  9. ↑ "Swords into Ploughshares: Canada Could Play Key Role in Transforming Nuclear Arms Material into Electricity," Archived 2013-10-03 at the Wayback Machine in The Ottawa Citizen (22 August 1994): "CANDU ... reactor design inherently allows for the handling of full-MOX cores"
  10. ↑ Burakov, B. E.; Ojovan, M. I.; Lee, W. E. (2010). Crystalline Materials for Actinide Immobilisation. London: Imperial College Press. p. 58.
  11. ↑ Natarajan, R. (2015). "Reprocessing of spent fast reactor nuclear fuels, Natarajan". Reprocessing and Recycling of Spent Nuclear Fuel: 213–243. doi:10.1016/B978-1-78242-212-9.00009-5.
  12. ↑ Rücknahme radioaktiver Abfälle aus der Wiederaufarbeitung (In German)
  13. ↑ TVA might use MOX fuels from SRS, June 10, 2009
  14. ↑ New Doubts About Turning Plutonium Into a Fuel, April 10, 2011
  15. ↑ Gardner, Timothy (12 October 2018). "Trump administration kills contract for plutonium-to-fuel plant". Reuters.
  16. 1 2 3 4 "NDA Plutonium Options" (PDF). Nuclear Decommissioning Authority. August 2008. Archived from the original (PDF) on 2011-05-25. Retrieved 2008-09-07.{{cite journal}}: Cite journal requires |journal= (help)
  17. ↑ "Thorium test begins". World Nuclear News. 21 June 2013. Retrieved 21 July 2013.
  18. 1 2 Björk, Klara Insulander; Fhager, Valentin (June 2009). "Comparison of thorium-plutonium fuel and MOX fuel for PWRs". p. 487. Retrieved 11 October 2017.
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