There are basically two ways to make a nuclear bomb; both methods have been adopted in the United States and they both work.  The first consists of enriching natural uranium in the isotope 235, which is fissionable.  Its natural concentration at present is only 0.7%; the rest is non-fissionable U-238.  But when enriched to more than 60%, U-235 becomes suitable for construction of a weapon.  The Hiroshima bomb was a U-235 bomb.

The second method uses fissionable plutonium, which does not exist naturally but has to be manufactured.  For weapons purposes it is usually made in specialized reactors using uranium as the raw material.  The Nagasaki bomb was a plutonium bomb.  Although plutonium is toxic if particles are inhaled, its natural radioactivity (half-life about 20,000 years) is not dangerous and it can be handled safely as a metal.

In the case of Iran, much has been written about its enrichment of uranium in underground facilities that are well protected.  Iran employs centrifuges, although there are other methods that can be used and work well.  But Iran has also been constructing a nuclear reactor near Arak, capable of producing weapons-grade Pu-239, sufficient for about one bomb per year.

Arak is located about halfway between Isfahan and Tehran, somewhat to the west and close to the Iraqi border.  The reactor installation is above ground, fairly extensive, and has been photographed many times.

The Geneva Interim Agreement of 24 Nov 2013 between the P-5 (+1) nations and Iran includes both methods.  According to the White House press release, Iran has agreed to stop its construction of the reactor -- although Iran does not share this interpretation.  Negotiations are continuing to settle this dispute.  It should be noted that the Geneva Agreement permits Iran to continue uranium enrichment to low levels, essentially negating six separate UN Security Council resolutions that demand Iran stop all enrichment activities.  A WSJ op-ed (of 2 Dec) by former Secretaries of State George Shultz and Henry Kissinger comments on this and other shortcomings of Geneva.

Israel is not a party to the Geneva Agreement, which includes the United States, Russia, China, UK, France, and Germany; she may decide to bomb the Arak reactor and thereby eliminate one certain route for Iran to gain a nuclear weapon.  Recall that India's 1974 nuclear test explosion used Pu-239 made in the CIRUS "research" reactor, which had been constructed in the 1950s with US-Canadian assistance.

Israel has a lot of experience in destroying such reactors.  In 1981 they bombed Osirak, a similar reactor under construction with French help in Iraq.  In September 2007, in Operation "Mivtza Bustan" (Orchard), Israel destroyed a plutonium reactor at al-Kibar, Syria, being built with North Korean assistance, and financed by Iran.  There is no evidence of US participation in either operation -- though it did align with Western strategic objectives on nuclear non-proliferation.

[It is instructive to read about the meticulous care that went into planning and into the operation itself at  http://en.wikipedia.org/wiki/Operation_Orchard.   Ironically, the term 'bustan,' identical in both Arabic and Hebrew, is not of Semitic origin but a loan-word from Classical Persian.]

A successful Israeli strike would essentially "shuffle the Middle-East deck" and affect many ongoing operations, negotiations, and relationships.  Some of the consequences are hard to predict -- some others are not:

To bomb or not bomb: that is the question

After evaluating pros and cons, Israel may conclude that eliminating the Iraq reactor is a "no-brainer," taking advantage of unusual current opportunities that may last for several weeks but not much longer.  In any case, to avoid radioactive contamination, the reactor must be eliminated before it is fueled and turned on.

The operation itself is rather simple, compared to eliminating Iran/s underground uranium installations.  The reactor facility is above ground, and its location is well known.  One major obstacle is distance, which may require an aircraft-refueling operation somewhere along the route.  This problem can be solved by finding (at least temporary) allies who share the Israeli objective of reducing the Iranian nuclear threat.

The other problem may be the sophisticated air-defense system being installed in Iran by the Russians.  No one is quite sure if the S-300 system is already in place; in any case, Iran probably lacks experience in its operation, while Israel knows it fairly well, having worked on it with Cyprus, who bought such a system from Russia.  In my experience, any such defensive system can be overcome in various ways, requiring only a certain amount of ingenuity.

Of course, destroying the reactor may not be sufficient to affect Iran's drive for producing enriched uranium.  Howeverowever, Iran might be impressed by the fact that Israel doesn't allow its "red lines" to be crossed and that the oft-repeated statement "all options are on the table" is not an empty threat.  One can imagine that the White House, which has also used such phraseology, may thereby gain credibility -- even though the US would not participate in such an attack. 

In fact, it is unlikely that Israel will inform the US in advance of such an operation, fearing that leaks could jeopardize its success.  Nonetheless, it will be generally assumed that there is US connivance -- when in fact there may be none.

There is a small possibility, however, not to be discounted entirely, that Iran may decide to give up its drive for nuclear weapons.  They may be impressed by the fact that their vaunted anti-aircraft system can be overcome.  And they may fear the destruction of their elaborate and expensive enrichment operations.  One recalls that Moammar Qaddafi decided to give up his nuclear ambitions when faced with a similar tough decision. 

Of course, a much stronger and more difficult follow-up may be needed to cripple Iran's well-protected uranium facilities.  On the other hand, there may be a regime change in Iran -- or elsewhere.  One can always hope -- so any sort of delay improves the chances of such an important happening.  More likely, however, Israel must consider the other consequences of a successful operation -- be they military, political, or related to oil supply.  

Military 

It is unlikely that Iran will choose a direct military response, although they do have missiles that can reach Israel and even Europe.  Fortunately, these do not as yet have nuclear warheads.  [Recall that Iran had attacked the Osirak reactor in 1980, at the start of the Iran-Iraq war, a few months before Israel destroyed it.]

To avoid Israeli retaliation, a safer choice for Iran would be mobilize Hezb'allah or Syria to carry out an attack on Israel.  Syria is not a serious possibility right now, but Hezb'allah could prove to be a difficult problem for Israel.

Hezb'allah has been accumulating a huge stockpile of rockets, mostly short-range Katyushas, but also some longer-range Scud missiles.  Their arsenal is judged to number in the range of 50,000 -- constantly augmented from Syria by smuggling operations that Israel has not been able to stop entirely.  UNIFIL, the UN force responsible for preventing Hezb'allah rearmament, has been, predictably, a total failure.

Israel would need to consider possible counter-measures, since no anti-missile system I know of can defend against a mass attack involving tens of thousands.  Of course, one might ask: Why hasn't Hezb'allah attacked already -- and it sure ain't because of UNIFIL.

Deterrence would seem the best response, but artillery and aircraft are both ineffective and expensive against dispersed missile launch sites.  One possibility is to use very cheap Qassam-like, unguided rockets in an immediate counter-mass-response -- but that may lead to an unacceptable level of civilian casualties.  Another is for Israel to demonstrate a capability to eliminate Shi'ite Hezb'allah fighters, who are now engaged in Syria against Sunni rebels.

All things considered, Israel may conclude that now is the perfect time to take a chance on a Hezb'allah attack -- since its threat can only get worse over time.  Currently, Hezb'allah fighters are tied up in Syria, some 4000 strong, and vulnerable to Israeli air attacks; Hezb'allah cannot afford to lose that much manpower.

Political 

The overall window of opportunity is favorable for Israel -- at the moment.  Not only are Syria and Hezb'allah rather busy, but Jordan and Saudi Arabia are generally friendly, or at least uninvolved; Egypt is keeping Hamas under control, for the time being; and Turkey is facing internal political problems.  Shi'ite Iraq is defending against an Al Qaeda offensive.  So if there were ever a time to take risks, the constellation is favorable -- at least within the next few weeks.  Israel may acquire strange bed-fellows, but, hey, this is the Middle East.

An Israeli strike may also have a significant impact on the ongoing Israel-Palestinian negotiations.  The reaction of the Palestinian Authority on the West Bank is not fully predictable, nor are possible reactions of Israel's Arab citizens; but this may be as good a time as any to find out.  Within Israel itself, expect to see enhanced morale and other important psychological changes.

The US position is likely to be ambivalent -- with the public and Congress in enthusiastic support of Israel and its operations, while the White House may have some hesitation, related to the ongoing Arab-Israel peace negotiations.  More important perhaps, a successful Israeli strike would solve some tough problems for the White House -- without any military risk to the US.

The reaction of the UN and EU will be predictably negative, but there may be great sympathy for such an Israeli operation from Russia and East-European nations.  Canada and Australia will cheer from the sidelines.  There will be general relief not only among conservative Arab monarchies, but even in Sunni Pakistan.  No one wants nuclear proliferation by religious fanatics, such as the current Iranian regime of Twelver Shi'ites.

Oil Situation

Expect to see a short-lived price spike, but little else.  There will be no repeat of the so-called Arab oil embargo of 1973.  Iran needs the money and would not self-sanction its main source of foreign currency.

What about blocking the Straits of Hormuz or even attacks on Gulf oil producers?  Much too risky for Iran.  Any decrease in the world's oil supply would of course raise the world price, benefit non-blockaded producers, and hurt oil importers like China, India, and Japan.  What a great way for Iran to lose powerful foreign friends!

Note further that the United States ("Big Satan") and Israel ("Little Satan") are about to become fossil-fuel exporters and would gain "windfall profits."  The other cheerleaders for Iran might be extreme environmentalists: If the global price of oil were to double, it would raise the price at US pumps by $2 a gallon for a few months.  Heavily-taxed Europeans would see a smaller increase, percentage-wise.

Conclusion

Israeli Prime Minister Benjamin Netanyahu is the one who has to make the fateful decision that will determine the future of his country.  He will likely share that awesome responsibility with members of his Cabinet, many of whom are 'hawks.'  The coming weeks should be interesting.

The writer is an electrical engineer and physicist who has also been involved in pioneering missile and satellite work.  In past years he has visited Israel, Iran, and several Arab nations.  He has held several academic and federal positions, most recently as chief scientist of the US Dept of Transportation, concerned with aircraft security, air traffic control, and GPS applications.