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The semiofficial Iranian website nuclearenergy.ir has produced a report on breakout times (link no longer works; try this), which have been consuming the attention of Iran-watchers in the West and possibly the P5+1 negotiators. The infographic above is a summary. The site has also posted a summary of the findings, with two infographics. They refer to John Kerry’s claim of two months for breakout, but not to calculations from the Institute for Science and International Security (ISIS). This is appropriate, because the ISIS calculations are not official. But it’s worth comparing the two, since influential people are quoting the ISIS numbers.

Both use the concept of “significant quantity,” the amount of highly-enriched uranium (HEU) or plutonium necessary to make a bomb. But this is not an easily definable number. Iran calls it “significant amount,” and both take it to be something like 25 kilograms of HEU. ISIS doesn’t consider plutonium. The centrifuge and separative work unit (SWU) performance numbers are similar. The ISIS calculations rely on the stockpiles of partially enriched uranium in September 2013. One difference between the two calculations is that ISIS uses these stockpiles as the basis for further enrichment, and the Iranian calculation seems to be from unenriched uranium. Enrichment to 3.5% takes more SWUs than enrichment of 3.5% uranium to 90+%.

ISIS calculates only the amounts of UF6 equivalent to one SQ. But a bomb needs metallic uranium, not UF6, requiring several chemical transformations, plus casting and machining. In those chemical transformations, there are always losses, and Iran has reported some high losses in those processes. Susan Voss pointed out those losses here at Nuclear Diner over a year ago. The Iranian estimates are similar to what Voss estimated.

The differences, however, are as much in Iran’s ability to do what is needed to get to those amounts as in the calculations of amounts of fissionable material and SWUs. To produce a significant quantity of enriched uranium and form it into a bomb requires more steps and equipment than are discussed in the ISIS report, which focuses on the enrichment process only. IAEA inspectors are likely to see that installation and modification of equipment, especially with enhanced inspections.

One of the problems that would have to be addressed for breakout is the potential for reaching a critical mass as the enrichment increases. A criticality accident during production would be very serious. It would not be a nuclear explosion, but rather a rapid release of a high neutron flux that would kill people and damage equipment. This report summarizes criticality accidents in the US and Russia. Criticality accidents can happen at all processing stages if uranium enrichment is high enough. Like the definition of significant quantity, “high enough” depends on surrounding circumstances.

Because ISIS considers only the enrichment process, they consider only the possibility of criticality there (p. 3):

This time period [two weeks]… assumes the operators would modify or change the cold traps in some of the withdrawal sections of the plants so that they could hold 60 and 90 percent enriched uranium safely without an undue risk of the HEU becoming critical, a serious accident which would threaten the entire breakout operation.

But other storage and processing would have to be modified as well to handle enriched uranium. For example, when I worked on canning plutonium, the size of the cans was determined by the separation needed to prevent criticality. The Iranians would need quantities of lead and other shielding, along with special racks and other storage devices. Additionally, as the Iranian report points out, the conversion equipment would have to be sized in relation to criticality considerations (p. 5).

The Iranian report points out that Iran does not have much of that conversion equipment needed to convert UF6 to the uranium metal needed for a bomb. The coming startup of a plant is being hailed to convert UF6 to oxide and thereby make it more difficult to convert back for enrichment. However, that conversion would also be the first step to making uranium metal.

There are many ways for inspectors to recognize an attempt to produce a bomb:

  • Diversion of uranium-containing materials from planned uses
  • Changing the configuration of the piping connecting the centrifuges
  • Building new equipment
  • Changes in configuration of processing equipment and storage to protect against accidental criticality

These kinds of things are already being covered by IAEA inspections; the Joint Plan of Action now in effect increases inspections, as a final agreement is likely to do. Because essential facilities do not yet exist, breakout time calculations are misleading. The times to build these facilities are in the range of months to years; getting them operating will require more months.

With such inspections, Iran would have to have a secret facility, as Jeffrey Lewis has pointed out. Iran has built covert facilities before, so this is a real concern. It can partly be mitigated by additional inspections.

The Iranian report is probably closer to the truth than the ISIS estimates, although it glosses over a couple of points. Curtailment of Iran’s nuclear program gives no absolute guarantees, because covert facilities are possible. It is better to build trust through a reasonable agreement on a significant peaceful program for Iran with stringent inspection.

Photo: A row of Iranian nuclear centrifuges shown on the state-run Press TV, with portraits of Ayatollah Ali Khamenei and the late Ayatollah Ruhollah Khomeini in the background. (Reuters)

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