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Iran's Vice-President F.A. Davani at the IAEA June 2011, Photo credit: Dean Calma / IAEA
During the Iranian nuclear crisis, the concept of a 'zone of immunity' has been widely used. Iran's prospective entry into this zone is considered to be a red-line, the crossing of which would or should precipitate an Israeli military strike against Iranian nuclear facilities.
Take, for instance, Ronen Bergman's widely read January report in the New York Times:
[Israeli Defence Secretary Ehud Barak] warned that no more than one year remains to stop Iran from obtaining nuclear weaponry. This is because it is close to entering its 'immunity zone' - a term coined by Barak that refers to the point when Iran's accumulated know-how, raw materials, experience and equipment (as well as the distribution of materials among its underground facilities) - will be such that an attack could not derail the nuclear project.
Despite the increasing salience of the concept of the zone of immunity in policy discourse, it remains lacking in coherence and clarity.
The first problem is that 'derail' has no specific meaning. If it denotes 'terminate', and refers to an irreversible cessation of Iranian nuclear enrichment, then Iran is already in the zone of immunity. As US Chairman of the Joint Chiefs of Staff General Martin Dempsey explained in August, an Israeli attack would 'clearly delay but probably not destroy Iran's nuclear programme'. Michael Hayden, the Bush administration era CIA Director, had said in January 2012 that 'The Israelis aren't going to [attack Iran] ... they can't do it, it's beyond their capacity'.
If 'derail' denotes 'delay', then the central question is: what length of delay in Iran's nuclear activities would constitute a derailment? Without specifying the precise period, the concept of the zone of immunity blurs to the point where it has little analytical utility or explanatory power.
The second question is why the US or Israeli ability to 'derail the nuclear project', as Bergman puts it, should be declining over time. The answer to this usually makes reference to three activities, which are rarely distinguished with care:
1. The hardening of Iran's underground Fordow enrichment facility
2. The accumulation of low enriched uranium (LEU), and particularly that enriched to 20 per cent (20% U-235) i.e., nine-tenths of the way to weapons-grade
3. The increase in the number of installed centrifuges i.e., latent enrichment capacity
The first of these - the hardening of Fordow - directly increases Iran's ability to withstand attack. Fordow lies an estimated 80 metres underground, and even the most penetrative of US munitions, the Massive Ordnance Penetrator (MOP), is not certain to damage targets at that depth. Discussions of the zone of immunity frequently observe that the zone comes later for the US than for Israel, because of the former's greater military capabilities. This reinforces the idea that the primary determinant of a zone of immunity is the physical fortification of Fordow. Yet there is almost no evidence, beyond a handful of un-sourced sentences in news reports, that Iran is actively and successfully strengthening Fordow's defenses. If Fordow's (in)vulnerability to attack is broadly constant, why should Iran be any closer to the zone of immunity today than it was months ago?
This is where the second and third criteria, uranium stockpiles and enrichment capacity, are relevant. Concerns over the zone of immunity have typically been driven by reports of increases in these two factors. The IAEA's new report on Iran, dated 30 August indicates that Iran has progressed considerably in both these regards. Iran's stockpile of 20% U-235 has doubled since February (to 189.4kg), and Iran has doubled the number of installed - though not necessarily active - centrifuges at Fordow since May (to 2,140). Even though Iran is not using the centrifuges added since May, this still represents a formidable increase in its enrichment capacity.
An increase in nuclear materials and installed centrifuges does shorten Iran's breakout timeline i.e., the time it would take Iran to produce a bomb were it to take the decision to do so. But what is the relationship between the breakout timeline and a zone of immunity?
Assuming that Israel does retain some ability to cause meaningful damage to Fordow, there are two reasons why a shorter breakout timeline might be relevant to a zone of immunity.
First, if an attack only destroys a certain proportion of Fordow's contents, then more pre-attack materials and centrifuges would translate into more surviving, post-attack quantities with which to break out. In other words, an expanded programme could 'absorb' more of any strike, leaving a rump programme capable of breakout.
This may very well be a factor in Israel's assessment of the zone of immunity, but it would seem to require an unrealistic degree of foresight and precision in military strikes, particularly so given the prevailing uncertainty over whether Fordow could be successfully penetrated at all (rather than sealed off by the obstruction of exits). It therefore seems reasonable to postulate that any attack would probably have to be premised on an ability to neutralise almost all of Fordow's contents, not just a middling proportion. Else, Israel would be at high risk of engendering a bombed, embittered Iran with sufficient (or near-sufficient) short-term capability to produce weapons-grade uranium.
However, it is unclear whether there can be such a linear relationship between strike efficacy and programme size. It seems unlikely that an additional 1,076 centrifuges (the amount added since May) are likely to make a qualitative difference to Israel's ability to damage Fordow. The available evidence does not allow an accurate assessment of this.
A second, more pertinent argument is that a shorter breakout timeline might allow Iran to pre-empt (note: not withstand) an attack, either by diverting LEU to a clandestine site for further enrichment, or by enriching existing LEU at Fordow to weapons-grade, and then diverting the resultant fissile material.
In each of these cases, Iran could only be said to be 'immune' from attack if it could undertake and complete these steps prior to being caught and attacked. However, Iran is subject to frequent IAEA inspections, including 'short-notice announced inspections', typically on two-hours notice.
Iran is only using older, less sophisticated IR-1 centrifuges that slow its maximum breakout pace.
Moreover, Iran has taken steps that likely lengthen its breakout timeline. Iran has converted or is converting 97.9kg of 20% U-235, over half its entire stockpile, for fuel plates for the Tehran Research Reactor. Although this process is not irreversible, it does make it much harder for Iran to use this quantity for weapons purposes. Iran is left with just over 91kg of 20% U-235, which is not enough for even one bomb.
The overarching point is that a timeline measured in months, rather than weeks, is far in excess of the time that it would plausibly take for the detection of breakout. Even if Iran did succeed in getting together enough weapons-grade uranium, it would take another year or so to fabricate a useable nuclear device, and longer still to fabricate a warhead for a ballistic missile. If Iran were to attempt any of this in conjunction with the expulsion of IAEA inspectors or the diversion of nuclear materials 'in plain sight', it would almost certainly face a serious military campaign led by the United States, which could do far more damage than Israel.
It should also be noted that US confidence in its detection abilities is crucial. If Iran achieves a very high degree of nuclear latency by continuing to shorten its breakout time, even slight reductions of Western visibility into the Iranian programme - such as that which would result from the expulsion of IAEA inspectors from Iran - could trigger preventive military strikes.
Finally, there is one more important aspect to Iran's nuclear progress. In August, The Wall Street Journal, citing both the IAEA and a variety of other sources, reported that that Mohsen Fakhrizadeh, head of Iran's alleged clandestine weapons programme until 2003, had returned to work. That pre-2003 weapons programme was outlined in some detail in the IAEA's November report. It is not clear whether Fakhrizadeh's 'return' adds to the activities of what has become, in the years after 2003, a 'dormant' programme comprising mostly less sensitive, dual-use activities. Iran has repeatedly blocked the IAEA from interviewing Fakhrizadeh, which adds to suspicion about his past and present role.
Ehud Barak's aforementioned definition of the zone of immunity included 'accumulated know-how'. Fakhrizadeh's activities could be reasonably placed under this category. To the extent that these activities shorten the time it would take to eventually fabricate a bomb from fissile material, should Iran take such a path, this does reduce the overall breakout time. If the breakout time were eventually shortened below the time it takes to detect and respond to breakout, then such alleged, clandestine research would be taking Iran into the zone of immunity.
However, there is no evidence to suggest that the breakout time is being shortened to that degree. Moreover, the fact that Fakhrizadeh's return has been detected indicates the degree to which Iran's nuclear programme is transparent to interested parties, and therefore how difficult it would be for Iran to break out without detection. Conditional on some sort of amnesty for past violations of its NPT obligations, it would be in Iran's own interest to cooperate more fully with the IAEA. Its failure to do so has been the principal enabler of the greatly tightened sanctions regime since the end of 2011, and is likely to engender still more punitive measures.
The New York Times reported on the new IAEA findings by declaring that 'Iran is close to crossing what Israel has long said is its red line: the capability to produce nuclear weapons in a location invulnerable to Israeli attack'.
Unless the specific nature of that invulnerability is specified in much greater detail, the zone of immunity should be understood primarily as a rhetorical device - including as an Israeli effort to persuade allies to tighten the sanctions regime against Iran - rather than a clearly defined condition into which Iran will definitively enter.
 GOV/2012/37: Implementation of the NPT Safeguards Agreement and Relevant Provisions of Security Council Resolutions in the Islamic Republic of Iran (Vienna: International Atomic Energy Agency (IAEA), August 30, 2012), 13-14,< http://isis-online.org/uploads/isis-reports/documents/Iran_report_--_August_30_2012.pdf >.
 Ibid., p.8.
 Ibid., 13.
 David Albright and Christina Walrond, Iranian Production of 19.75 Percent Enriched Uranium: Beyond Its Realistic Needs, ISIS Report (Institute for Science and International Security (ISIS), June 15, 2012), 6-7, >.
 GOV/2011/65: Implementation of the NPT Safeguards Agreement and Relevant Provisions of Security Council Resolutions in the Islamic Republic of Iran (Vienna: International Atomic Energy Agency (IAEA), November 8, 2011).