Correction: This op-ed says that no action has been taken on recommendations by a National Academy of Sciences committee in 2006 to reduce the risks of spent nuclear fuel pools by mixing hot fuel with cold, older fuel and adding sprayers to dispense cooling water if needed. Steps along these lines have been implemented at U.S. reactors since the report was issued.

In 2006, a National Academy of Sciences committee recommended two simple steps to prevent spent nuclear fuel from catching fire: putting old, cool fuel next to the new, hot fuel discharged from a reactor, and adding sprayers that could dispense water if the cooling water in the pool was lost. But no such action has been taken, either in the United States or in Japan — where the most deadly danger at Fukushima nuclear plant since the recent earthquake and subsequent tsunami has been the risk that uncovered spent fuel in the storage pools would catch fire, spreading radioactive material miles downwind. Nor has much of the older spent fuel been moved out of pools into safer dry casks made of steel and concrete — another possibility to reduce the risk.

The radioactive steam that rose over Fukushima should be a searing reminder of the costs of failing to identify such dangers and fix them. A serious blow has been dealt to public confidence in the nuclear industry and its overseers.

Every country operating nuclear facilities needs to undertake an urgent review — by an independent international team, not by the companies that own the plants or the agencies that have long regulated them — of whether there are risk-reduction steps as compelling as those the academy recommended that have not been taken. (Indeed, another simple academy recommendation that was not followed was that a group independent of both the U.S. nuclear industry and its regulators should review the security of spent fuel pools.)

The European Union has announced that its member states will work together to review the safety of all E.U. reactors in the coming months. The rest of the world must do likewise — as well as invite separate teams to review security.

The risk is not just accidents but attacks. Al-Qaeda has repeatedly considered sabotaging nuclear facilities. The 2006 study focused primarily on the danger that terrorists might succeed in draining the water from a spent-fuel storage pool, the same outcome raising risks in Japan. Moreover, al-Qaeda has long sought to get stolen nuclear material to make a crude nuclear bomb — which government studies in the United States and elsewhere have repeatedly concluded a sophisticated group might be able to do if it got enough weapons-usable nuclear material.

Nuclear facilities around the world are much less prepared for security incidents than for accidents. While U.S. reactors are required to have armed guard forces, many reactors abroad — and even some sites with potential nuclear bomb material — have none. One senior U.S. nuclear official I spoke to last fall described security for most of the reactors he had visited abroad as “frightening.” Everyone in the civilian nuclear industry is taught to focus on safety from day one, while on security, nuclear workers and managers might get a half-hour briefing once a year. All this needs to change.

At the nuclear security summit President Obama convened last spring, leaders from 47 countries agreed on the goal of securing all vulnerable nuclear materials worldwide within four years — but a great deal remains to be done to realize that ambition.

The International Atomic Energy Agency (IAEA) provides safety and security reviews for countries that request them — but it will need more money and additional experts to carry out these assessments on the scale required. It will take time for the IAEA to assemble teams, and major obstacles are likely to include complacency, secrecy, sovereignty concerns and bureaucratic impediments.

As it outlines questions, it should start here:

l Why haven’t operators of nuclear plants been required to rearrange the fuel in reactors and provide sprayers, as the academy recommended?

l Why was the Fukushima site required to have only eight hours of battery supply in case it lost power? Why are some U.S. reactors allowed to have only four?

l Why were the backup diesel generators down low, where they might be swamped by a tsunami, rather than up high?

l Why aren’t reactors and sites with potential nuclear bomb material in all countries required to be protected against the kinds of attacks and theft attempts that terrorists and criminals have shown they have the capability to carry out?

l Shouldn’t all such facilities be required to have on-site armed guards, capable of holding off an attack until off-site forces arrive?

The task is urgent. While the odds are against another accident occurring tomorrow — more than two decades elapsed between Chernobyl and Fukushima — no one knows when terrorists might choose to strike.

Ultimately, regular independent, international reviews should be the norm in nuclear operations worldwide. All countries must demonstrate that they are doing everything practicable to prevent the next Fukushima — or something far worse.

Matthew Bunn, an associate professor at the Harvard Kennedy School and a former adviser in the White House Office of Science and Technology Policy, is the author of “Securing the Bomb 2010: Securing All Nuclear Materials in Four Years.”