Andy GrottoNon-Weapons Grade HEU Bust in Slovakia

Two weeks ago, a story broke involving the arrest in Slovakia of two Hungarians and one Ukrainian for allegedly trying to peddle around 1 pound of weapons-grade highly-enriched uranium.

Days later, the incident was reported as involving 481.4 grams of a substance containing uranium 235, with a Slovak lab technician saying “Preliminary examinations showed it was low-enriched uranium,” which contains U-235 but can’t be used in a bomb. Big difference.

Well, a State Department official confirmed to me yesterday that the material was definitely not HEU.

When the story first broke, I was skeptical of the initial claim by the Slovak police that the level of enrichment was 98.6%. The precision of the number rubbed me the wrong way, given that the bust had just occurred, but it also struck me as a bit high. I’m not a physicist, but my understanding is that to the extent HEU is used in non-weapons applications — such as research reactors, propulsion, and radioisotope production — it is almost always closer to 90% U-235. (I’m sure a reader will correct me if I’ve got this wrong…)

Anyway, this seems to be one of those cases where authorities rush to worst-case judgments to hype their accomplishment. The Slovak police deserve credit for the bust, but crying wolf is never a good thing.

Addendum: And the extraordinary precision of that 98.6% figure? A little birdie told me today that the NRDC’s Tom Cochran had it right two weeks ago when he told the New York Times that the figure is the confidence in the radiation detector measurement, not the [level of] enrichment. Way to go Tom!


  1. thermopile (History)

    FAS has an article about this, located here:

    and a picture of the Canberra NaI detector used to get the spectrum located here:

    If you do the math (see comments in the article) based on those pictures, it’s regular Uranium.

  2. Yale Simkin (History)

    What struck me was the constant yammering that this stuff, even if not bomb fuel, is the heart of a fearsome dirty bomb.

    Ok, would I want to breathe HEU, LEU, or DU? No freakin’ way, but it is not The End Of Civilization As We Know It.

    If it were Pu238 or C060, then get outta my way.

  3. Eric Hundman (History)

    I’m glad to hear that the 98.6% enrichment level was shown to be false, since I also found it to be oddly high and too precise.

    As to your statement about research reactors, etc: 90% U-235 is the optimal enrichment level for most weapons designs, but HEU reactors often use considerably lower levels of enrichment, ranging from 21%-90% U-235. Most Russian naval nuclear reactor designs, for instance, have used enrichment levels between 21-45%. I believe research reactors and radioisotope production also tend to use closer to 40% enrichment.

  4. Andy Grotto (History)

    Thanks Eric. I should have been more precise in my post: I know that HEU in non-weapons applications is often much lower than 90% U-235. What I’m less certain of is whether HEU as high as 98.6% U-235 has any non-weapons purpose. My understanding is that it does not, but I’m not 100% sure.

  5. Alex W. (History)

    If you Google down from “9X% enriched”, the highest I was able to find references to was 93% enriched, used for really experimental reactor arrangements (like nuclear rockets). Obviously that doesn’t mean a whole lot but I think it is interesting that “93% enriched” lights up the DOE Information Bridge with results but nothing higher gets anything. Note that I didn’t search for fractions of percents, just whole numbers.

  6. George Adams (History)

    Following the links suggested by thermopile, by my calcs it is clearly DU—maybe an aircraft balance weight, maybe a recovered US armor-piercing round.

  7. Yale Simkin (History)

    93.5% enriched uranium is “Oralloy”, which is the standard for weapon fuel.

    It is abbreviated “Oy” (without irony) and is a code name from WWII, meaning Oak Ridge Alloy.

    Sometimes Oralloy is used to refer to any HEU, but that just confuses things.

    Natural uranium was tuballoy.

    235 U was “25” and 239 Pu was “49”

  8. Yale Simkin (History)

    The highest enrichment I have so far discovered is in criticality tests run at Los Alamos that used 97.68% 235 U (and which was annoyingly labeled as “oralloy”)

  9. Eric Hundman (History)


    Good question. What I know agrees with what Alex and Yale have dug up: the difference between 90% and 98% should only be significant for rather esoteric nuclear research.

    Even in those cases, it is probably possible to use LEU instead, though possibly at some cost to efficiency. Several studies have concluded (with various caveats) that most if not all research reactors that currently use HEU could switch to LEU fuel. While I believe they all refer to fuel enriched to ~90% U-235, it seems reasonable to me to assume the findings would apply to slightly higher levels of enrichment.

    In such cases, the reasoning for using higher enrichment levels would be economic, not necessarily technical. Some reactors that use HEU would not otherwise need to change fuel over their lifetimes, for example, so from a purely economic perspective switching fuel out would be an unnecessary added cost. Lower enrichment might also make the desired isotopoes more difficult to extract in certain cases.

  10. Yale Simkin (History)

    For the moment, if we exclude military weapons and military propulsion from consideration, then the need for HEU almost evaporates.

    If, that is, “civilian” atomic power is allowed to obey the rules of real market forces and thus evaporates as an option.

    Without that bogus prop you don’t need HEU or Pu. No critical assemblies and arrays, no “research” reactors, no fast reactors, no reactor engineering schools, no enrichment, no reprocessing, no trade in sensitive materials, etc.

    The minimal remainining need for isotope reactors and similar technologies, whether they use bomb-usable materials or not, since they would no longer support either the local weapons or power industry, could be directly under the physical control and management of international agencies.

    If you do that, then any possession of nuclear usable materials or technologies becomes unambiguously weapon related.
    A country caught messing with that stuff would have to directly anwer for its actions.

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