MOX fuel

Source: Wikipedia, the free encyclopedia.

Mixed oxide fuel, commonly referred to as MOX fuel, is

low-enriched uranium fuel used in the light-water reactors that predominate nuclear power
generation.

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

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

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

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

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

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]

  • Plutonium oxide is substantially more toxic than uranium oxide, making fuel manufacture more difficult and expensive.
  • As plutonium isotopes absorb more neutrons than uranium fuels, reactor control systems may need modification.
  • MOX fuel tends to run hotter because of lower thermal conductivity, which may be an issue in some reactor designs.
  • Fission gas release in MOX fuel assemblies may limit the maximum burn-up time of MOX fuel.

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

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

See also: BN-800 reactor

Because the fission-to-capture ratio of high energy or

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

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

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

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

See also: Thorium fuel cycle

MOX fuel containing

boron worths, CVR and plutonium consumption."[18]

See also

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. ^ a b "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. 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. ^ a b c d "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. ^ a b Björk, Klara Insulander; Fhager, Valentin (June 2009). "Comparison of thorium-plutonium fuel and MOX fuel for PWRs". p. 487. Retrieved 11 October 2017.

External links

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