Copyright R. Zamenhof, 2013.
THE JOINT COMPREHENSIVE PLAN OF ACTION (JCPOA) NUCLEAR AGREEMENT WITH IRAN: WHAT iran and the U.S. have AGREED UPON AND THE SCIENCE BEHIND IT--as it was in FEBRUARY 2015
Gas centrifuge "farm" for separating uranium-238 from uranium-235. The vertical tubes are the centrifuges, having internal elements that spin very fast along the vertical axis. Uranium Hexafluoride gas (with natural uranium) is passed into the centrifuges. Uranium-235 and uranium-238 are then separated by virtue of the slightly different centripetal forces acting on the two slightly different nuclear masses. Uranium-238 becomes concentrated at a larger radius within the centrifuge than uranium-235. The two uranium isotopes are then separately extracted from the centrifuges.
Posted on April 3, 2013 by Dr Simple Science
Iran has always claimed that it is enriching uranium as a necessary step toward providing various civilian services, such as radioisotopes for nuclear medicine, a civilian nuclear power program, and a civilian nuclear research program. However, this has clashed with widespread international belief that Iran’s claims are simply a cover for a much more nefarious goal, that of joining the Nuclear Club of nations possessing nuclear weapons–-without being invited, and, more specifically, for "wiping Israel off the face of the earth".
The recently agreed upon Joint Comprehensive Plan of Action (JCPOA) between Iran and the U.S., together with five of the other principal nuclear nations, has caused a kerfuffle in the U.S. Congress and strong condemnation by Israel. Israel's Prime Minister, Bibi Netanyahu, is highly skeptical about the value of the JCPOA agreement and still insists that the U.N. “draw a red line” beyond which Iran’s nuclear development should not be tolerated by the international community and, if violated, might result in preemptive strikes by Israel against Iran's uranium enrichment and plutonium conversion facilities--as has already occurred on two previous occasions. Mr. Netanyahu has also warned that as early as this summer, despite the JCPOA agreement, Iran’s uranium enrichment and nuclear warhead fabrication facilities are expected to be moved to dispersed underground locations, making it far more difficult, if not impossible, to achieve successful verification, therefore rendering the JCPOA agreement largely ineffective.
Since uranium enrichment by Iran and its progress towards nuclear weapon acquisition has produced a substantial amount of public fear in past years, I will try to clarify some of the scientific facts behind these issues.
Summary of the JCPOA Agreement Between the U.S. and Allied Nations and Iran
* The primary uranium-235 enrichment site in Iran is Natanz (see map above). Under the JCPOA agreement, Natanz will be permitted to operate 5,060 uranium enrichment centrifuges. This is 25% of Iran's 20,000 currently operating centrifuges and will constitute older, much less efficient models that will be far slower in enriching uranium than the current ones.
* Iran's uranium stockpile of uranium will be reduced by 98% to 300kg (660lbs) for 15 years, and will not be allowed to exceed an enrichment level of 3.67% (see below for further discussion of enrichment).
* The Arak reactor (see map above), which according to its original design would have been a source of fissile plutonium-239 for manufacturing at least one nuclear weapon per year, will be transformed to produce far less plutonium than before and of a poorer quality. Fundamentally, this would limit plutonium-239 production and make it virtually impossible to fabricate any plutonium-239 based nuclear weapons.
* All spent fuel from the Arak reactor that could potentially be reprocessed to recover plutonium-239 will be sent out of the country under a rigorous IAEA inspection protocol. In fact, Iran will ship out all spent fuel from all of its power and research reactors, preventing the accumulation of any spent fuel from which plutonium-239 could be extracted, and will not engage in any activity associated with the reprocessing of spent fuel to obtain plutonium-239, even for research purposes.
U.S. and the 5 Allied Nations' Obligations
* On the part of the U.S. and the five other allied nations in the JCPOA agreement, the commitment given to Iran in response to Iran's acceptance of the above conditions is to end the severe sanctions that were imposed on Iran in 2010. Quoting a memorandum from the U.S. State Department, "These sanctions were designed: (1) to block the transfer of weapons, components, technology, and dual-use items to Iran’s prohibited nuclear and missile programs; (2) to target select sectors of the Iranian economy relevant to its proliferation activities; and (3) to induce Iran to engage constructively, through discussions with the United States, China, France, Germany, the United Kingdom, and Russia in the “E3+3 process,” to fulfill its nonproliferation obligations".
Specific Technology of Uranium and Plutonium Processing for Nuclear Reactor Fuel and Nuclear Weapons Manufacture
Uranium mined from the ground contains various uranium isotopes, including uranium-235, the uranium isotope needed for the manufacture of fission nuclear weapons and nuclear reactor fuel. The "raw" uranium is first combined with hydrofluoric acid, which reacts with the uranium to produce uranium hexafluoride gas. The uranium hexafluoride gas is fed into centrifuges that spin the gas at extremely high speed (see the second picture above). By centripetal force, the heaviest uranium isotope, uranium-238, is forced to the outer wall of the centrifuge where it is extracted, while uranium-235 remains along the central axis of the centrifuge where it is separately extracted. However, this approach to uranium-235 separation is notoriously slow, so to offset this problem a very large number of very large advanced design centrifuges operating simultaneously is required (see third picture above). Limiting the number and efficiency of Iran's centrifuges is the first step in the JCPOA agreement in preventing Iran from enriching uranium to the level where fission nuclear weapons could be manufactured. The next step is limiting the enrichment level of Iran's entire stockpile of uranium to prevent its further enrichment to weapons grade uranium. The final step is to modify the cores of those nuclear reactors which are capable of rapidly producing high quality plutonium-239, which can also potentially be manufactured into fission nuclear weapons, and sending out of the country all spent reactor fuel rods to prevent Iran from extracting plutonium-239 from the burned fuel.
Principles of Uranium Enrichment and Uranium-235 Fission Rates for Nuclear Reactors vs. Fission Weapons
"Raw" or “natural” uranium, as it comes out of the ground, consists of about 0.7% uranium-235 and about 99% uranium-238 (there are a few unimportant additional isotopes of uranium present that account for the remaining 0.3%). Although the uranium-235 and uranium-238 uranium isotopes are chemically identical, uranium-235 is "fissile", whereas uranium-238 is not. Uranium-235 is the only naturally occurring fissile isotope that is suited for the manufacture of nuclear weapons and nuclear reactor fuel.
A fissile isotope is one whose nucleus can be induced to break apart, or “fission”, following bombardment by nuclear particles called neutrons. In the case of uranium-235, the nucleus absorbs a neutron that makes it unstable and causes it to break apart. In doing so, it emits 2-3 outgoing neutrons, accompanied by a tremendous amount of energy, mainly as heat and light, but also as ionizing radiation (the kind of radiation that is potentially harmful to humans). Each of the 2-3 outgoing neutrons can now be considered as 2-3 incoming neutrons in the next uranium-235 nuclear interactions, that once again each produce 2-3 more uranium-253 fissions with the emission of 2-3 more outgoing neutrons and more energy. Therefore, the uranium-235 fissions-rate grows "exponentially" with time and, if not controlled, continues until all uranium-235 has been used up.
Uranium-235 for Nuclear Reactor Fuel
Since for each uranium-235 nucleus to fission requires one neutron to be absorbed—i.e., removed from the neutron population--each fission event increases the neutron population by 1-2 neutrons (2-3 neutrons produced in each fission, minus the one neutron that is absorbed to initiate each fission). If each fission event on average absorbed one neutron and emitted only one neutron that could initiate the next fission, the neutron population with time would remain roughly constant; this is the safe, “steady-state” condition under which nuclear reactors typically operate and produce energy. But to ensure that steady-state condition, one of the outgoing neutrons, on average, in each fission event must be "blocked" from causing further fissions. This blocking to some extent occurs naturally, due to the presence of uranium-238, which can absorb neutrons but does not undergo fission. But it is also achieved in a more controlled way using "control rods", which can be inserted into the uranium fuel to various depths. Control rods contain the isotope boron-10, which very strongly absorbs neutrons. Therefore, the fission rate in the uranium fuel is "tuned" by the precise positioning of the control rods in the core to maintain a safe and constant fission rate. If, on the other hand, the fission-rate were to increase exponentially in a nuclear reactor’s core, dangerous overheating could result, with the further possibility of more serious consequences such loss of core coolant followed by core meltdown. This is what happened in the Japanese Fukushima daiichi reactor accidents in 2011.
Uranium-235 for Nuclear Fission Weapons
In contrast, to manufacture uranium-235 fission weapons, we want the maximum amount of fission energy produced in the shortest possible time; which means we need to let the fission rate rapidly increase unchecked until all the uranium-235 has been used up. This means that following "triggering" of the fission process, the uranium-235 fission-rate would increase at a higher and higher rate, i.e., "exponentially". In a nuclear fission weapon, the lack of control rods is insufficient to ensure that the growth in fission-rate is as rapid as possible. But because, as already mentioned, natural uranium is mainly composed of neutron-absorbing--but non-fissile--uranium-238, potential problems are encountered in trying to utilize uranium fission for nuclear weapons. The large amount of uranium-238 significantly “eats up” the fission neutrons produced by the uranium-235, down-regulating the uranium-235 fission-rate just as control rods do in a nuclear reactor core. So the presence of uranium-238 is highly undesirable if the uranium is to be used for a nuclear weapon where the fission-rate must increase as rapidly as possible. Therefore, for the manufacture of nuclear weapons, the uranium-235 enrichment level is increased to at least 20%, but more commonly to 90% or higher, to minimize the deleterious (sometimes referred to as "poisoning") effect of uranium-238.
[For similar reasons, although raw or natural uranium (with a uranium-235 fraction of about 0.7%) could, in principle, be used as a cheap and very safe fuel for nuclear reactors, much better fuel-to-energy conversion efficiency is obtained with slightly higher enrichment levels of 0.9-2%.]
The Politics of Uranium Enrichment
From the perspective of the recent JCPOA agreement, if Iran were committed to maintaining its 660 lb stockpile of uranium at an enrichment level of 20% or lower--as it was permitted to do in a previous deal proposed by Russia and initially accepted by Iran about 10 years ago--there is a theoretical possibility that Iran still could manufacture nuclear fission weapons; but more realistically, it could use its outdated 5,060 permitted centrifuges to moderately elevate the enrichment of the stockpiled uranium to bring it into a range where nuclear fission weapons could more easily be manufactured. To prevent this from possibly occurring, as part of the JCPOA agreement Iran's stockpile of uranium is not permitted to exceed an enrichment factor of 3.67%; from which, as already mentioned, it would be impossible to manufacture nuclear fission weapons.
However, for the operation of nuclear reactors that can produce plutonium-239 from uranium-238 (plutonium-239 is an alternative fissile isotope suitable for nuclear fission weapon manufacture), a 3-4% uranium enrichment level is ideal, maximizing the efficiency and, therefore, the speed of uranium-to-plutonium conversion. That is why a component of the JCPOA agreement consists of disabling the ability of Iran's nuclear reactors from the production of plutonium-239.
Iran’s uranium enrichment issue, together with the “hair trigger” of Israel’s decision of possibly preempting an Iranian nuclear attack on Israel by a destruction of all of Iran’s nuclear capabilities, is probably the most dangerous military crisis the U.S. has faced since the Cuban missile affair. We can only hope that the recent diplomatic progress between the U.S. and Iran will lead to a nuclear stand-down between Iran and the Western powers and Israel to give the world another decade or two of nuclear peace.
Uranium-235 can be enriched to higher percentages that the 0.7% level in naturally occurring uranium. Up to about 20% enrichment, the uranium can only be used as fuel for nuclear fission reactors, but at enrichments levels greater than 20%, the possibility exists that the uranium can be used to manufacture nuclear fission weapons. Much lower uranium enrichment levels of 3-4%--permitted under the current JCPOA agreement--though precluding the direct manufacture of nuclear fission weapons, if used as nuclear reactor fuel could, in principle, lead to the rapid production of plutonium-239, a fissile isotope of plutonium from which nuclear fission weapons can be manufactured as they can from uranium-235. Under the JCPOA agreement "Export" of used nuclear fuel rods out of Iran should, in principle, prevent this from happening.
The concepts of 1) a steady-state fission-rate, maintained through the use of control rods in nuclear reactors, and 2) an exponentially increasing fission-rate, desirable in nuclear fission weapons, are both determined by the fate of the additional neutron produced with each uranium-235 fission, over and above the one outgoing one neutron that is necessary in a steady-state fission rate. With that "extra" neutron absorbed either by control rods or by the uranium-238 present in the uranium, a steady-state fission-rate is achieved, which is the operation mode of nuclear reactors. However, if that extra neutron is not removed and becomes available to cause further uranium-235 fissions, an exponentially increasing fission-rate results results which, if allowed to proceed unhindered, results in a nuclear fission explosion.
The 3.67% enrichment level of Iran's stockpile of uranium-235, permitted under the JCPOA agreement, although too low to enable nuclear fission weapon manufacturer (as well as being precluded from being further enriched by the restrictions on the number and type of the centrifuges Iran is permitted to retain) can, nevertheless, be manufactured into very efficient nuclear reactor fuel. Such efficiently fueled nuclear reactors can potentially rapidly produce plutonium-239, from which nuclear fission weapons can also be manufactured. However, one component of the JCPOA agreement consists of disabling the nuclear reactors that are capable of the rapid production of high-quality plutonium-239, and in addition requiring the export of all burnt up fuel to prevent the covert extracting of plutonium-239 from the uranium fuel residue.
Many countries, in particular Israel, consider the JCPOA agreement much too favorable to Iran, with too many loopholes permitting Iran's continued development of nuclear weapons; but, as president Obama has stated, "It was either this deal or no deal". The world can only hope and pray (if that is our disposition) that this agreement will be honored by both sides, which would greatly alleviate the international nuclear tensions that presently exist.
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