Molybdenum seems to be something that just won’t go away! Most people feel it is one of the reasons Iran appears willing to ship its indigenously enriched LEU first to France to be further purified and then to Russia to be enriched to the 19.75% U235 the TRR now uses. (See the latest ISIS technical report for the best numbers on refueling the TRR. See my post on the medical aspects of production of 99Mo for the other facets.) Another, speculative, reason might be Iran’s desire not to appear to be capable of enriching to 20%. Of course, there might be very good reasons why Iran is not capable of producing the UF6 of the required purity for the TRR fuel. First, we need to discuss the contamination standards for different levels of enrichment for reactor grade fuel. It is possible that the requirements for weapons grade uranium are even greater, but I’m still working on understanding that.
ASTM Standards for Reactor Grade Fuel
As has been pointed out before, molybdenum is capable of attacking and corroding many of the internal components of gas centrifuges; more capable than the corrosive uranium hexafluoride. It is also a neutron poison that can seriously affect the power of a nuclear reactor since it has a relatively high thermal neutron absorption coefficient. For both these reasons, the ASTM International Organization sets standards for, among other things, the nuclear energy industry. This includes the allowable molybdenum contamination for various forms and enrichment levels of uranium compounds that will are used in the civilian sector. These specifications are shown in the below:
| ASTM Standard | Enrichment Level | Maximum Molybdenum Contamination (ppm) | Form of Uranium |
|---|---|---|---|
| C753-04 | < 5% | 250 | UO2 |
| C787-06 | Natural | 1.4 | UF6 |
| C1462-06 | More than 5%, less then 20% | 100 | U metal |
Note that reactor fuel using UO2 and having an enrichment level less than 5% has considerably larger allowable amount of the neutron “poison” molybdenum than the fuel for reactors with 20% enriched uranium. That must imply that the 100 ppm (parts per million) is set by the neutron physics of the reactor rather than worries about the potential damage to centrifuges. (If it was expressed in terms of UF6 at 20% enrichment, it would be 149 ppm.) Its not clear, however, that the 250 ppm for UO2 with enrichment less than 5% is determined by reactor physics or fears of damage to centrifuges. That would be nice to know.
It does turns out, however, that the specification for uranium hexafluoride, using natural uranium, of 1.4 parts per million maximum molybdenum contamination would lead to 20% enriched uranium (metal) with 99.2 ppm assuming that all the molybdenum goes with the U235. This is just under the threshold for use in a the appropriate type of nuclear reactor. The corresponding Mo contamination for 5% enriched uranium is 16 ppm, starting out with natural uranium, well within the specifications.
Speculations on Iranian Purification
So why does it appears like France going to be purifying the Iranian LEU before being shipped to Russia for further enrichment? One possibility is that Iran is able to purify its natural UF6 to a degree which produces acceptable 5% (or less) enriched fuel but not to the point where it could be enriched to 20%. This tacitly assumes the Iranian UF6 is not within the ASTM specs but that the resulting uranium oxide is.
Working backwards, this could mean that Iran can only purify its UF6 to between 1.5 and 30 ppm molybdenum. If it was just at 1.5 ppm, presumably it could somehow be squeaked cleaner. I would therefore guess Iran’s Mo contamination is closer to 30 ppm, which produces just acceptable UO2 fuel for nuclear power plants using 3.5% U235 but could not be used at all in the TRR fuel. If Iran tried to use UF6 with contamination on the high side of this estimate, the molybdenum contamination would reach about 1500 ppm by the end of the enrichment process. Presumably that could damage the centrifuges as well as effectively poison the TRR. (By the way, for a very interesting list of Iranian publications on their nuclear research, see the bibliography complied by Mark Gorwitz available on the FAS site.)
Let us look into things the West could do to make it harder for this 20% enriched uranium to be used for a bomb. In doing so, we assume for the moment that Iran’s yellowcake purification process cannot produce a UF6 product purified better than 30 ppm molybdenum. If the West returned the U3O8 TRR fuel with the maximum amount of molybdenum contamination, 100 ppm, as allowed by the ASTM specifications, then Iran could simply run it through its uranium conversion process (it is, after all, just the major component of yellowcake) and get it down to 30 ppm. Enriching that to 90% U235 would increase the molybdenum contamination to 137 ppm in UF6. That is certainly acceptable in their centrifuges since they appear to reach that during their LEU production. (Failing to further purify the TRR fuel might lead to damage of centrifuges.)
Would that UF6 be acceptable to be used in a bomb? This is when the neutron poisoning effects need to be studied. Of course, Iran would need to convert the resulting weapons grade UF6 to uranium metal and they could, with a certain additional time, purify this to 30 ppm. So the big question is: is this acceptable from the neutronics point of view for a bomb?
Well, you see that molybdenum is a fast fission byproduct anyway, so there is no way to avoid having a lot of the stuff around. Notwithstanding this objection, I believe this is NOT a problem also because:
First, neutron poison is not such a bad thing in small amounts because as it burns up, it increases criticality. For HEU instead of plutonium, it’s not clear that you care anyway.
Second, the amount is fantastically small and therefore would require some kind of huge absorption cross section to cause much poisoning at all.
Third, the fast fission neutron spectrum is over 1 MeV wide and while narrow and strong resonances (although not strong enough) might be possible, this absorption would not be strong across the whole neutron energy range. This is totally different than a thermal poison.
Fourth, although not quite the data we want, there are no such high cross section neutron absorbing reactions for any of the moly isotopes at 14.7 MeV. The biggest neutron absorbing cross sections are for the lowest isotopes – not surprisingly – adding up to maybe 200 millibarn, less for the higher isotopes.
Fifth, if inert, a diluent of 137 ppm would only increase the critical mass by 270 ppm and therefore the pit diameter by about 90 ppm. This is less than one part in 10,000. It is measurable, but very unlikely to be inside their must-hit tolerancing for a (first) bomb.
So, I think NOT a problem. For the centrifuges or for a reactor, I definitely see the concern though.
Thanks John, thats very convincing. I suspected it would not be an issue for a bomb at this level but Im glad you thought about it.
Surely the best thing “the West could do to make it harder for this 20% enriched uranium to be used for a bomb” would be to ensure it gets loaded into a reactor and bathed in thermal neutrons.
In short order it’ll be contaminated with a menagerie of nasty hot fission products.
I just eyeballed the cross-section data from JAERI, and it looks like natural Mo will have a perfectly unremarkable fission-spectrum neutron capture cross-section of 50 millibarns or so. And nuclear weapons are driven entirely by fast, i.e. fission-spectrum neutrons. Test devices that attempted to use even weakly-moderated neutrons in the resonance range, delivered underwhelming results.
So I concur, there’s no great problem in using Mo-contaminated uranium in a bomb. In that application, it’s just an inert diluent.
And I’m suspicious of the claim that Mo contamination is any great barrier to HEU production. Anyone who can produce natural uranium feedstock (metal, oxide, or hexafluoride) of <100 ppm Mo, can produce arbitrarily high enrichment levels at the same purity – just run the output stream through whatever purely chemical process you used on the feedstock.
Perhaps not the same machinery, out of concern for contamination, and tedious if you have to do it at intermediate stages to protect the centrifuges. But the volume of the output, or moderately-enriched intermediate, stages is substantially lower than for the feed, so it’s not likely to be any sort of showstopper.
If the Iranians, or anyone else, are producing enriched uranium with >>100 ppm molybdenum, it’s either because they just plain can’t produce any sort of uranium at that level of purity (unlikely), or because they don’t care to repurify the output of the enrichment cascade. And that’s somewhat worrisome, because people who are honestly trying to fuel a reactor, do seem to care.
I agree with John Field about the impact on a weapon.
1. I don’t have any data to hand (JEF or ENDF) but a little googling confirms some dusty memories: At energies >> 1 MeV most isotopes have similar cross sections of a few barns (or less). Certainly at 5 MeV molybdenum has scattering cross sections around 2 – 5 barns (see the 1974 data from Argonne at http://www.osti.gov/bridge/servlets/purl/7350458-FpQAsB/7350458.pdf). So whatever Mo might be around, it’s mostly going to scatter neutrons, not absorb them.
2. The lower fission product yields for U-235 peak right around atomic masses 95 – 100. So there’s no point in seeking super low levels of Mo in a weapon because soon after initiation you’re going to get pots of the stuff anyway as fission products.
3. The thermal absorption cross section for molybdenum is around 10 barns; so yes, it does mop up a few neutrons in a reactor. But, as John says, this just makes it something of a burnable poison. There’s nothing about that that makes it a showstopper – a competent core designer would undoubtedly add a little extra excess reactivity if he knew how much Mo he had to deal with – so I’m sure the ASTM limits are simple doing what they say on the tin: they’re convenient STANDARDS for the designer so that so long as the fuel is in accordance with the ASTM limit he can essentially disregard the effects of Mo. (Just like if you buy a standard steel you can assume its composition / heat treatment – and hence its strength – and therefore you can plug ‘n play standard designs).
Overall, conforming to ASTM molybdenum levels would make reactor designs a bit cheaper and quicker, in that you wouldn’t have to do bespoke accounting for the Mo, but I think it’d be a long way down the list for a weapon designer.
With regard to centrifuges, I wonder if we’re looking down the wrong end of the telescope? If the Mo plates out on the centrifuges and unbalances them – leading to a crash – then that would make a commercial enrichment process less economic, and thus be seen as a major issue. But if you want to build a weapon, and can’t remove the stuff chemically, then you just create a sacrificial cascade where you WANT it to plate out, thus protecting the rest of your process. It’ll cost a few hundred more centrifuges, but so what? No-one’s counting the costs for a strategic weapons programme. Add a little bit of laser monitoring, and you just collect the Mo on your sacrificial rotors, changing them as they approach dangerous imbalance. Even if the Mo concentrates in the U-235 it’s not a problem: the fact that it’s more concentrated than it was in the natural uranium just means that you send it off for chemical separation again, where the new concentration gradients mean you can remove some more.
Admittedly all this messing around adds to costs, delays and lowers overall U-235 yield (after accounting for losses), but if you’re playing a long game to get the bomb that doesn’t matter (just remember Saddam and his Calutrons). The important point is that the molybdenum issue shouldn’t be seen as an absolute roadblock: instead it’s just a hindrance that time and resources can overcome.
Geoff,
There must have been a technical basis for senior US and UK officials in Geneva last week telling reporters that the fuel elements that would be returned to Iran by France could not be further enriched for weapons purposes. John Bolton claims (WSJ, Oct 5 2009)that this is flatly untrue. Your exploration of the moly issue is very helpful, but I wonder if there are not other ways the fuel can be fashioned that would make it impractical for further enriching.
IRAN, FRANCE, AND IRAN’S EUP:
For the record, this appeared in our pages on October 6:
—RUSSIA, FRANCE, THE US, AND THE IAEA HAVE A “FORMULA” for assisting Iran in
obtaining 19.75%-enriched uranium fuel for its TRR reactor in Tehran, Russian
Foreign Minister Sergei Lavrov said October 5, according to the Interfax news
agency. Lavrov said Iran’s low-enriched uranium would be used as the basis of
the TRR fuel. According to P-5 officials, Russia would enrich the uranium from
3.5% to 19.75% and France would fabricate the TRR fuel assemblies. (The P-5 is
comprised of the UN Security Council’s five permanent members — China,
France, Russia, the UK and the US.) Before the uranium is enriched in Russia,
some officials said, France might also purify the Iranian enriched uranium
feedstock. Beginning in 2005, Iran encountered difficulties in removing
impurities such as molybdenum from its uranium hexafluoride, or UF6, processed
at the Uranium Conversion Facility at Isfahan and intended for feedstock for its
enrichment plant at Natanz. Last year, IAEA sources said that Iran had solved
the impurities problem as required for power reactor fuel enriched to 3.5% U-
235. For TRR fuel, however, sources said, Iran would have to remove more of the
impurities from feedstock processed in Iran, since the concentration of the
impurities in enriched uranium product varies directly proportionally with the
enrichment level of the fuel. When a deal was reached last week with Iran for
supply of the fuel, commercial sources and some P-5 officials said that the
simplest solution for the P-5 to provide Iran the fuel would not be to enrich
the Iranian feedstock exported to Russia. Instead, it would be simpler for
Russia to blend down uranium from a large Russian inventory of material enriched
to 36% U-235, they said.
A followup item appeared in Nucleonics Week on October 8. We had requested on October 7 confirmation from Areva that it would not only fabricate the TRR fuel, as P-5+1 officials announced on October 1 in Geneva, but would also remove the impurities from Iran’s EUP before the uranium was shipped to Russia for re-enrichment to 19.75%. As of end of the day in Europe on October 8, Areva has not responded to us.
On October 7, P-5+1 officials and commercial sources told us that it was likely that Iran had operated UCF to remove metallic fluoride contamination sufficient for enrichment to 3.5% for PWR fuel, but had not removed the impurities to the extent necessary for meeting ASTM quality standards for fuel enriched to nearly 20%.
It is other contaminants that I would worry if building a bomb. The added neutrons from Al and the like by alpha bombardment make the device more prone to early initiation (altho at its worst a HEU core is less prone than a run-of-the-mill Pu core).
Also, I do not understand how making 20% uranium metal creates a barrier to further enrichment.
Yes, it needs to be re-fluorinated, but big whoop. The amount of material will fit in a briefcase.
Maybe the issue is that it will be irradiated. If so, in a breakout that is only a hindrance not a wall.
Adding a modest percentage of U-232 to the fuel, would make it rather intensely radioactive, perhaps to the extent of killing whichever technicians are tasked with stripping the stuff out of fuel elements and running it through an enrichment cascade. And, as a light uranium isotope, it would be devilishly hard to remove and would be concentrated by further enrichment.
It might be possible to do this while leaving the complete fuel assemblies safe enough to handle, but there are obvious diplomatic and political problems with such a scheme – especially if we don’t explicitly tell the Iranians that their fuel has been transformed into a poison that will kill anyone who mishandles it.
The only thing that would seriously impair use of the fuel in a bomb, would be a fast neutron absorber such as Boron-10, and that would be fairly trivial to remove chemically.
And if there’s some sort of heavy-metal impurity that would e.g. corrode centrifuges, that can be removed by running the stuff through a filter made of centrifuge material.
I don’t see any way of reliably and safely “denaturing” low-to-medium enrichment uranium. Anyone who can finish the enrichment process, will have little difficulty with any denaturant save U-232.
Quite possibly the “technical basis” is that the fuel would have to be removed from the fuel element structure for any further enrichment, and thus technically the “fuel elements” could not be further enriched.
Deceptive and misleading, but that’s hardly unheard of when politicians and diplomats talk to reporters. And there’s clearly more to this deal than has been made public so far.
A nice article of the effect of u232 is Hippel and Kang, U-232 and the Proliferation-
Resistance of U-233 in Spent
Fuel