Beryllium to the Rescuehttps://www.armscontrolwonk.com/2499/trr-refueling-and-proliferation-barriers
Based solely on reactor uranium loading, the best match for Iran’s TRR fuel rod density (of 2.96 grams of uranium percubic centimeter, gU/cm^3, according to the Iranian publications) isalloying uranium and beryllium in the form of UBe13 (i.e. thirteen atomsof beryllium for each atom of uranium). Perhaps not surprisingly, theresulting density of the alloy is very similar to the uranium oxidepowder Iran current uses (4.37 g/cm^3 as opposed to the current 4.76g/cm^3: not even the mechanical support structure of the rods will needto be changed. Replacing the oxide with a beryllium compoundimmediately increases the time it would take Iran, if it chose, toconvert it to UF6 for further enrichment.
It is very doubtful that Iran already has much experience withindustrial scale beryllium processing. And while it is conceivable thatIran could acquire lab-scale expertise at separating uranium fromberyllium using natural uranium in its tests, it would be a majortechnological challenge to take experience and turn it into a processfor efficiently separating 150 or so kilograms of uranium. After all, theRussian nuclear industry doubted its ability to reprocessuranium-beryllium alloyed spent fuel rods as late as 2006.
If UBe13 alloy fuel rods represent a very significant time andtechnology barrier to further enrichment, they do not represent anabsolute barrier. That is where the other components to proliferationresistance come in. Clearly, an increased time barrier is mosteffective when it is accompanied by either periodic or continuousmonitoring of the fuel supply; something that was already in place forexisting fresh fuel stocks. For most of the time Iran had fresh, 19.75%enriched U3O8 fuel rods sitting around waiting to be used (since 1992)it did not have a significant enrichment capacity. Now that it has, itis only natural that the periodicity of the monitoring be increased.