MOX Fuel Considerations

swl · Aug 1, 2011

What are the differences between the use of MOX fuel and "standard" fuel?
I've seen mention of the MOX fuel in news reports as being significant, but I've failed to understand their point of mentioning the use of plutonium in the fuel.

Are there safety issues?

Are there proliferation issues?

Are there other issues?

Thank you!

Astronuc · Aug 1, 2011

There is no substantial difference between MOX and UO2, although MOX tends to have higher TU isotopics. Neutronically they are similar in terms of power generation, i.e., power density. In mixing MOX in UO2 cores, the core/fuel designers attempt to match the power between the different types of fuel.

UO2 produces Pu239/Pu240 with irradation, i.e., spent fuel becomes MOX. There really isn't a proliferation issue.

Depending on the Pu-isotopic vector, there is some increased activity that must be considered during fabrication and inspection of the fuel.

Spent fuel is highly radioactive, whether it starts UO2 or MOX.

rmattila · Aug 2, 2011

Following is a translation of an answer given to the same question by the Finnish Nuclear Safety Authority (STUK) on their web page:

Also in "traditional" fuel containing enriched uranium when fresh, plutonium is produced during reactor operation in such quantities that about 1/3 of the total energy is produced by splitting plutonium in such fuel as well. When discharged, about one percent of plutonium remains in the fuel (by weight), and this can be "recycled" into manufacturing of fresh fuel in such a way that the plutonium collected from about ten assemblies will give enough "starting material" for one new assembly. Such fuel, called "MOX fuel", is being used in a number of NPP:s around the world, but so far uranium has been so cheap that plutonium recycling has not been an economical breakthrough. At Fukushima Dai-ichi unit 3, the first batch of 34 MOX assemblies was loaded among the about 500 uranium fuel assemblies last fall.

MOX fuel has certain safety challenges, which prevent hasty transition to its use:

1. Plutonium recycling requires transporting of highly radioactive spent fuel from the NPP to a reprocessing utility and back as MOX fuel. Transport of radioactive materials requires special arrangements in order to guarantee safety,
2. Reprocessing is a demanding process, and reprocessing utilities (in Europe: La Hague in France and Sellafield in the UK) contain lots of radioactive substances in different forms, and preventing them from escaping into the environment is a specific safety challenge.
3. Fresh MOX fuel radiates in such quantities that it's more difficult to handle than uranium fuel, which can be manipulated with thin gloves (which, as it comes, are needed to protect the fuel, not the hand).
4. Concerning final disposal of spent fuel, MOX fuel produces somewhat more decay heat, which requires a prolonged intermediate storage period or a reduced density in the final repository. (On the other hand, in the hour/day scale, which is important for accident situations, MOX fuel produces less decay heat than uranium fuel.)
5. Plutonium alters the physical properties of the reactor (so called reactivity feedbacks) to a degree, and the accident analyses of the NPPs must therefore be repeated with the new parameters in order to verify that the criteria set for ensuring fuel integrity are fulfilled in all required cases.
6. The efficiency of control rods is reduced by introduction of MOX fuel, so their efficiency must be ascertained prior to transition to MOX use. (As a side note, comments about the EPR reactor being "designed for MOX fuel" are to a large degree based on the increased number of control rods - not to the reactor having anything MOX specific in itself.)
7. Due to the different reactivity behaviour and heat conductivity of MOX fuel, the gas gap between the fuel pellet and the cladding will apparently contain a somewhat larger amount of fission products at high burnups than uranium fuel. Therefore, in some possible accidents the failed fuel rods may have a larger initial release of fission gases at the early stages of the accident. The difference disappears if the accident situation lasts longer, because the total amounts of fission products are close to each other in uranium and plutonium.
8. In the current nuclear reactor types (light water reactors) plutonium can not be recycled indefinitely, because eventually too large amounts of the isotope Pu-240 will collect in the plutonium. Therefore large-scale plutonium recycling is connected to the development of the next generation reactors operating on fast neutrons, the research on which has during the last couple of decades been almost nonexistent. During the last years, increasing worries about the sufficiency of the world uranium resources have led to a gradual reopening of fast reactor research projects and programmes.

Since the use of MOX fuel has several such extra challenges in comparison to regular uranium fuel, using it is naturally a more challenging operation than using uranium fuel, if the same safety level is to be maintained. However, when we're talking about an accident such as in Fukushima, there's no difference whether the fuel that is used in the reactors was initially based on enriched uranium or recycled plutonium, since the threat does not come from the isotopes of the fresh fuel but principally from the fission products (iodine, cesium etc.), and there's no significant difference in their numbers to either direction between uranium and plutonium fuel.

jim hardy · Aug 2, 2011

""I've seen mention of the MOX fuel in news reports as being significant, ...""

here's a link to the folks who i believe made that MOX fuel for Tepco.
http://www.belgonucleaire.be/uk/project.htm

To me as a not-nuclear-engineer (but have drank beer with some) a little plutonium in reactor fuel is like ethanol in gasoline. Pu makes the reactor slightly more "peppy" because of its shorter delayed neutron times but in the modest proportion used it seems benign enough.

If the MOX fuel was significant to the events i will be surprised.
IMHO - Astro and rmatt gave good answers.

""I've failed to understand their point of mentioning the use of plutonium in the fuel. ""
Mark Twain nailed that one:
"If a spectacle is going to be particularly imposing I prefer to see it through somebody else's eyes, because that man will always exaggerate. Then I can exaggerate his exaggeration, and my account of the thing will be the most impressive. "

old jim

Astronuc · Aug 3, 2011

I should have qualified my answer with the additional information that the enrichments in the TEPCO cores tend to relatively low by US/European standards, and the discharge exposures also tend to be relatively low, so the difference between MOX and UO2 is not significant from the standpoint of reactor performance.

The decay heat for MOX fuel is slightly higher than for UO2 at the same exposure. However, the 32 MOX assemblies in FKI, Unit 3 were in their first cycle, so the burnup was quite low - on the order of 5 to 6 GWd/tHM by my estimate. So I don't see this as significant to the event in Fukushima.

Enthalpy · Aug 5, 2011

"Could it be that" Pu fissions more readily with fast neutrons than ²³⁵U does, hence a ruined or molten core would be more reactive with Pu, even if water is lost? And that the stabilizing effect of the void coefficient is less good with MOx?

As for proliferation, new fuel isn't the same if it's MOx! Because it's not as hugely radioactive as used fuel, and Pu from MOx can be separated by chemical means, easier than isotopic enrichment. So stealing unused MOx is a way to grasp Pu. Not in military grade because of ²⁴⁰Pu proportion, but usable for a bomb.

zapperzero · Aug 16, 2011

Enthalpy said: ↑

So stealing unused MOx is a way to grasp Pu. Not in military grade because of ²⁴⁰Pu proportion, but usable for a bomb.

Bah humbug. You want plutonium, you buy a CANDU reactor.

jim hardy · Aug 17, 2011

Enthalpy wrote:
"""Could it be that" Pu fissions more readily with fast neutrons than 235U does, hence a ruined or molten core would be more reactive with Pu, even if water is lost? And that the stabilizing effect of the void coefficient is less good with MOx?""

Beyond my skills to answer that.
I assume the reactor physics guys would provide for degraded core conditions in their fuel assembly design.
But a question well stated is half answered.
Does this help ?

....What happens inside a MOX element of say 7.8%Pu surrounded by normal elements in absence of water and control rods?
Depleted U makes a decent reflector and i'd think wouldn't hardly moderate at all, so a 5% enriched reflector sounds like it'd be a fast reactor hot-rodder's dream.

basis for 7.8% : http://www.belgonucleaire.be/uk/mox.htm

A MOX element is nothing else than a fissile element that weights approximately 450 kg and is composed of a mixture of 415 kg of uranium oxide and 35 kg of plutonium oxide.

35/450 =7.777%

One would need to know something about the geometry - fuel pin diameter and spacing for starters and it'd still be quite a calc.
Then there's that sea salt.

Is there a BWR reactor engineer in the house?

old jim

rmattila · Aug 17, 2011

I have no practical experience in design of MOX cores, but I have the perception that MOX cores would have more negative void and doppler coefficients than UO₂ ones due to the large resonance absorption of Pu isotopes.

There's one report comparing MOX/UO₂ properties from point of view of PWR/BWR reactor safety.