You are here

5. The MOX myths: only lies

Nuclear Monitor Issue: 
Special: The MOX Myth - The dangers and risks of the use of mixed oxide fuel

(April 11, 1997) The main arguments used by the nuclear industry1 to justify the use of plutonium as MOX fuel in LWRs are:

  1. It supports non-proliferation, by reducing the quantity of separated plutonium and by making the diversion of plutonium more difficult.
  2. It saves uranium by re-using plutonium and depleted uranium.
  3. It avoids the cost of storage of large stocks of plutonium and saves up to 10% on front-end fuel cycle costs.

These arguments will be dealt with below. It becomes clear that none of these arguments holds much truth. We will also look at the costs of MOX fuel compared to low-enriched uranium fuel.


5.1 Reduction of plutonium stocks?

Worldwide, discussion is growing about what to do with the surplus stockpiles of reactor-grade and weapon-grade plutonium. Each year about 70,000 kg of plutonium is generated in nuclear fuel in commercial nuclear reactors.2 About 30,000 kg so-called "civil" plutonium is annually separated from this spent fuel in reprocessing plants, adding to the already large amounts of separated plutonium. The military plutonium stocks are estimated at 300,000 kg, the quantity of separated civil plutonium is about 190,000 kg. From this plutonium, 49,000 kg has been re-used into MOX fuel for LWRs and FBRs.3 Russia and the US each have about 50 tons of weapon-grade plutonium surplus and are thinking of re-using this in MOX fuel in LWRs or CANDUs.

There are several options what to do with all the plutonium. One of the options is to burn the weapon-plutonium as MOX in LWRs and FBRs. The other option is to store the plutonium for the long-term, above ground or underground. To make the plutonium less accessible for potential diverters intending to use it for nuclear weapons, it can be mixed with nuclear waste and/or vitrified before storage.

One of the main arguments used in favor of MOX is that the plutonium stockpiles will be eliminated. However, this is not even true theoretically: to be able to eliminate the plutonium, the MOX fuel must be reprocessed and re-used many times, and so, slowly, the quantity of separated plutonium should be reduced. Apart from being quite expensive, each time the MOX fuel is used and reprocessed, the quality of the plutonium is degraded further and it will be more difficult to use it as fuel. That's why in practice, spent MOX fuel is not reprocessed and Pu is re-used only once. Technically it is possible to reprocess MOX fuel once (although it is not being done yet), but it raises technical problems and will be excessive expensive.4 The term "recycling" is misleading. Degradation of plutonium means that the share of the fissile plutonium isotopes, Pu-239 and Pu-241, in the total plutonium decreases. Reactor-grade plutonium contains about 65% fissile plutonium, degraded plutonium less than 65%. The standard MOX fuel contains about 5% fissile plutonium and in total 8% plutonium. Because about 92% of the MOX fuel contains depleted uranium, new plutonium is formed at the same time as the old plutonium is burned. As a consequence, after three years in the reactor the amount of plutonium in the MOX fuel has decreased by a mere 18%.5 Another limitation of the capacity of MOX to burn plutonium is the fact that the share of MOX in a reactor core is 20-30%. The other 70-80% of the core contains of enriched uranium fuel, in which so much new plutonium is formed, that the burning of 18% of plutonium in the MOX is more than compensated. The result is a net increase of plutonium. (see Table 5.1) The use of MOX only slows down the production of plutonium, but still helps the stockpiles of plutonium to grow.

Table 5.1 6 Pu balance in PWR with 30% MOX fuel in core

In contrast with the claim of the nuclear industry that the use of MOX fuel results in burning of plutonium, the opposite is true: even in nuclear reactors using MOX fuel, more plutonium is produced than burned. An example: A 1000-MW PWR, with 60 tons of nuclear fuel. Annually, one-third of the fuel, 20 tons, will be replaced. The new reloads contain 14 tons of LEU fuel with 4.2% enriched uranium and six tons of MOX fuel with 7% plutonium. After three years of MOX loading, the core contains 30% (18 tons) MOX fuel and 42 tons of LEU fuel and after four years an equilibrium situation is reached: each year as much MOX fuel is discharged as loaded. The spent LEU fuel will contain 1.4% Pu and the spent MOX fuel will contain 82% of the original Pu.



Year Reload Full core Spent fuel discharged Pu in (kg) Pu out (kg)
0 20 ton LEU 60 ton LEU 20 ton LEU with 200 kg Pu 0 200
1 14 ton LEU 54 ton LEU 20 ton LEU with 200 kg Pu 420 200
6 ton MOX
(420 kg Pu)
6 ton MOX
2 14 ton LEU 48 ton LEU 20 ton LEU with 200 kg Pu 420 200
6 ton MOX
(420 kg Pu)
12 ton MOX
3 14 ton LEU 42 ton LEU 20 ton LEU with 200 kg Pu 420 200
6 ton MOX
(420 kg Pu)
18 ton MOX
4 14 ton LEU 42 ton LEU 14 ton LEU with 196 kg Pu 420 540
6 ton MOX
(420 kg Pu)
18 ton MOX 6 ton MOX with 334 kg Pu

The net plutonium balance after three years of MOX loading is an increase of 120 kg Pu annually (in core: 420 kg of Pu; out of core: 540 kg of Pu), against an increase of 200 kg Pu/year without MOX fuel. PWRs with 30% MOX fuel annually produce 80 kg of Pu less than reactors without MOX, but still produce more Pu than they burn.
It appears that using 20% or 30% MOX fuel in LWRs and the percentage of Pu in the MOX don't make any difference in burning or producing Pu: after use always 82% of the Pu in MOX will still be present.

The claim that the use of MOX prevents proliferation is apparently false. First, the production of separated plutonium is not reduced. On the contrary, the use of MOX is the main justification to continue reprocessing spent fuel and the production of separated plutonium. Without the use of MOX an important argument for the nuclear industry to reprocess disappears. The conclusion can be that reprocessing and production of separated plutonium is stimulated by the use of MOX. If the nuclear industry is really aware of the dangers of plutonium and are willing to do something about it, they would stop producing it!
Secondly, proliferation dangers increase through the use of MOX: many thousands of kg of plutonium are transported by air, by ship and road and are stored and fabricated at many places. The danger of diversion thus increases. MOX plutonium can be separated easier from uranium in fresh MOX fuel than in spent fuel, and relatively easily be diverted for construction of crude nuclear bombs.7


5.2 It saves uranium?

Some 30 years ago, it was believed that uranium would soon become scarce and expensive. Nowadays there is an oversupply of uranium and prices are low. It is expected that the supply of uranium will be abundant in the coming years and prices will decrease more.8
The need for the nuclear industry to use uranium as efficient as possible does not exist any more. It is claimed that the use of MOX can save about 15% of uranium by recycling of plutonium and depleted uranium. At the moment, the amount of enriched uranium fuel used annually by all nuclear reactors is about 7,000 ton. The amount of MOX fuel used in 1996 was about 125 tons or 1.8% of all fuel loaded in LWRs.
In 2000, when an estimated 350 tons of MOX will be used, the share of MOX fuel of all fuel loaded in LWRs will be about 5%. This is not a very impressive saving of uranium.
If the industry was really willing to make better use of uranium (of course they would stop using it, but besides that), they would pay more attention to

  • Lowering of the Tails Assay during enrichment of uranium; and
  • Higher burn-up of fuel.

5.2.1 Lower tails assay

Natural uranium contains 0.7% fissionable uranium-235. LWRs use uranium enriched up to 4.5% U-235. The past 10 years the percentage of U-235 left in depleted uranium after enrichment, the so-called Tails Assay, has increased from 0.25% to 0.35% U-235. This means that 25% more natural uranium is needed to produce the same amount of enriched uranium.9 The choice for a higher Tails Assay is driven by economic motives: low uranium prices and high enrichment prices make it cheaper to use more uranium and less enrichment work.
If the nuclear industry was really concerned about a shortage of uranium, instead of making money, it could save 25% uranium by lowering the Tails Assay from the present 0.35% to 0.25% U-235.
This 25% saving is five times the amount of uranium saved by the expected use of MOX in the year 2000. If in the future uranium prices increase and uranium shortages are feared, a further reduction of the Tails Assay to 0.2% U-235 would save another 10% natural uranium.

5.2.2 Higher burn-up

Burn-up is defined as the amount of energy the discharged fuel has produced. It is expressed in MegaWattDays (MWD), instead of the more usual unit MWhour (MWh). One MWD is 24 MWh. To reach a higher burn-up, the enrichment level is increased from about 3.1% to 4% or more U-235.
About 10 years ago, the majority of LWR nuclear power plants reached a burn-up of about 25-30 MegaWattDay per kilo fuel (MWD/kg).10 Nowadays many reach a burn-up of 40-50 MWD/kg fuel. It is expected that most of them will reach 50-60 MWD/kg in the next 10 years.11 A burn-up of 60 MWd/kg fuel is seen as the limit for the present LWR fuel. In the US the Department Of Energy even hopes to develop advanced fuel, reaching a burn-up of 100 MWD/kg, with enrichment levels of at least 5%.12
The fact that almost all nuclear utilities make use of this possibility is caused by its economic benefits and by the fact that nuclear electricity is more expensive than other electricity producers. For example in the Netherlands the owner of the Borssele PWR applied for a higher burn-up, from 33 MWD/kg to 52 MWD/kg fuel and a higher enrichment, from 3.3% to 4% U-235. This was justified by the "significant savings on nuclear fuel costs".13 The increased burn-up results in 50% uranium savings, but the higher enrichment results in 30% more uranium use. The net result is a 20% saving in uranium use.

Problems with higher burn-up
The use of higher burn-up fuel has led to problems in many reactors. The main problems are:

  • More corrosion of the fuel rods because of higher radiation;
  • Deformation (bowing and swelling) of the fuel rods. This results in the sticking of control rods when they are lowered into the core to regulate the chain reaction;
  • Higher releases of radioactive gases such as tritium;
  • Longer cooling and storage time of spent fuel;
  • Difficulties with reprocessing; and
  • Difficulties with re-use of plutonium, separated from spent high burn-up fuel, in MOX fuel.14


5.3 MOX saves storage costs?

The nuclear industry maintains the claim that

  • the quantities of nuclear waste are less when plutonium is re-used;
  • Depleted uranium is also re-used in MOX and saves even more natural uranium;
  • the more energy is extracted from each kg of uranium, the less uranium ore has to be mined, the less the environmental effects will be; and
  • plutonium is responsible for the need of long-term storage of spent fuel. If the plutonium is separated from the spent fuel, the remaining waste doesn't need to be stored so long.

More instead of less problems
As we have seen in Chapter 5.1 MOX spent fuel will not be reprocessed. The result of the use of MOX fuel instead of uranium fuel is that when time passes by, reprocessed spent uranium fuel is gradually replaced by spent MOX fuel in the few countries where MOX is used. This will have consequences for the storage facilities.
Spent MOX fuel is much more radioactive than spent uranium oxide fuel. It contains 4%-6.5% plutonium, depending on the initial Pu percentage. This is on average five times more plutonium than spent uranium fuel. Spent MOX fuel contains also much higher amounts of transuranium isotopes: four times as much Neptunium-237 (half-life: 2.2 million years); nine times as much Americium-241 (half-life 430 years), which decays into Neptunium-237; 15-28 times more Curium-242 (highly radioactive) and 22 times more Curium-244 (half-life 18 years), changing into plutonium-240 (half-life 6,450 years).15

After 10 years, the heat generation from spent MOX fuel is twice as high as that of spent uranium fuel. After 100 years, it is even three times higher.16
Given the very long half-life of Pu-242 (380,000 years), and Neptunium-237 (2.14 million years) the storage of spent MOX is much more complicated than of normal spent fuel. Instead of a partial solution of the high level waste problem, MOX creates even bigger waste problems:

  • it needs more and longer cooling;
  • it has to be stored much longer;
  • it is more dangerous; and
  • the costs are therefore higher.


5.4 MOX fuel costs

Besides the disadvantages cited above, there is an additional cost disadvantage, for both civil as military plutonium. In most cost calculations of MOX fuel, plutonium is viewed as a free energy source. This, however, is economically not correct. The costs of reprocessing have to be accounted to the produced plutonium. The first aim of reprocessing has always been the separation of plutonium. In the case of civil reprocessing, the plutonium was primary meant for use as MOX in FBRs. When the FBR option failed, the plutonium policy shifted to the re-use of plutonium in LWRs. The sole aim of military reprocessing was production of plutonium for nuclear weapons. If surplus military plutonium is used as MOX in PWRs, it is no more than logic to incorporate at least a part of the historic production costs in the cost of MOX fuel.

We will consider the costs excluding and including reprocessing costs.
In the first case (excluding reprocessing costs), most calculations show higher costs for MOX fuel than for enriched uranium oxide fuel. MOX fuel, according to the German Institute for Energy Economics (EWI), costs US$2,614/kg. That's four to five times more expensive than standard uranium oxide fuel, which costs about US$523/kg.17 "World prices" for MOX fuel from civil plutonium are $2,587-$3,571/kg, according to EWI.18 This is five to eleven times the cost of uranium oxide. Another study mentions the cost of MOX fuel as $1,500/kg, compared with US$275/kg for enriched uranium oxide fuel.19
Reasons for this: the smaller scale of the MOX fuel fabrication plants; the extra measures necessary because of the much more radioactive plutonium, such as heavier shielding to protect the workers in the plant and preventing criticallity. Utilities in the US want to consider using military MOX only, if the more costs of MOX fuel are paid by the government.20 German utilities too want the excess costs of using MOX, if they ever use MOX made from Russian surplus weapon plutonium, to be compensated by US and European governments.21

Even without including the production cost of plutonium, the conversion of 50,000 kg of weapon plutonium into MOX fuel will cost US$1 billion-$5 billion, that is, US$20,000-$100,000/kg MOX fuel22. When reprocessing costs are included, the resulting MOX fuel prices are clearly much higher. This is no wonder as reprocessing is very expensive. Present prices of reprocessing spent LWR fuel range from US$1,569/kg23 to about US$1,000/kg of spent fuel.24 At the moment the standard MOX fuel contains 8% plutonium. To get 1 kg MOX fuel with 8% plutonium, 8 kg of spent LWR fuel (containing 1% plutonium) have to be reprocessed. The production cost of plutonium for MOX fuel are therefore about US$8,000/kg. When the extra costs of fabricating the MOX fuel, at least US$1,500/kg, are added, the price of MOX fuel is about US$9,500/kg.
Conclusions: "Civil" MOX fuel costs from twice to 11 times as much as uranium oxide fuel. "Military" MOX fuel costs 8.7 to 30 times as much as standard uranium oxide fuel. If reprocessing costs are included, MOX is more than 30 times as expensive.

The conclusion must be that reprocessing of spent fuel and re-use of plutonium as MOX doesn't have the advantages the nuclear lobby tells us. MOX fuel knows a number of additional problems and risks, which will be presented in the next chapter.



  1. For example BNFL in: Nuclear Europe Worldscan, no 7-8, 1994: 'BNFL's front-end fuel cycle services', p.92
  2. The Bulletin of Atomic Scientists, May/June 1994: 'Dangerous Surplus', p.39
  3. Energy and Security, No.2 1997; 'World Civilian plutonium inventories', p.14
  4. Pansters, D., 'Opwerking en hergebruik van plutonium', Technical University Eindhoven, August 1996, p.22
  5. Pansters, D., 'Opwerking en hergebruik van plutonium', Technical University Eindhoven, August 1996, p.21
  6. Pansters, D., 'Opwerking en hergebruik van plutonium', Technical University Eindhoven, August 1996, Appendix C.2, p.39
  7. Nuclear Free Local Authorities Bulletin, March 1997: 'Hazards of Mixed Oxide fuel', p.8
  8. Nuclear Fuel, 24 February 1997: 'U prices continue downward drift', p.17
  9. Küppers, C. and M. Sailer, 1993: 'MOX Wirtschaft oder die zivile Plutoniumnutzung', IPPNW, p.15
  10. Albright, D., F. Berkhout, W. Walker, 'Plutonium and Highly Enriched Uranium 1992, World Inventories, Capabilities and Policies', Oxford University Press/SIPRI, 1993, p.75
  11. Nuclear Energy Agency, '1995 Annual Report', Paris, 1996, p.26
  12. Nuclear Fuel, 24 February 1997: 'DOE program aimed at stretching fuel burnups to 100,000 MWD/MT', p.3
  13. Ministry of Economic Affairs, 8 August 1996: 'Ontwerp-beschikking Kernenergiecentrale Borssele', Den Haag, p.2
  14. WISE Newscommunique 468, 14 March 1997: 'Higher Burnup: Bigger Problems', p.6
  15. WISE Newscommunique 399, 15 October 1993: 'MOX and the Belgian Nuclear Industry', p.7
  16. Küpper, C. and M. Sailer, 'MOX-Wirtschaft oder die zivile Plutoniumnutzung', IPPNW, 1994, p.59
  17. Nuclear Fuel, 16 January 1995: 'German utility consultant advises scuttling post-baseload contracts', p.3
  18. Nuclear Fuel, 5 June 1995, 'Russian MOX plan would subsidize excess cost of MOX fuel cycle', p.5,6
  19. Nuclear Europe Worldscan, 5/6 1994: 'The plutonium challenge', p.50
  20. Nuclear Fuel, 16 December 1996: 'MOX: SOP to a dying industry or efficient disposal method?', p.6
  21. Nuclear Fuel, 24 October 1994: 'Bonn pushing for weapons Pu role in wake of smuggling escapades', p.9
  22. The Bulletin of Atomic Scientists, May/June 1994: 'Dangerous Surplus', p.39
  23. Nuclear Fuel, 16 January 1995: 'German utility consultant advises scuttling post-baseload contracts', p.3
  24. Nuclear Fuel, 1 January 1996: 'RT-1 operation faces cost crisis, uncertain future demand schedule', p.10