In Japan and the nearly 40 other countries in Asia, North America and Europe that operate nuclear power reactors, the amount of spent fuel has been mounting since the first commercial plants started in the 1950s. There are now about 270,000 tons of used fuel in storage, much of it at reactor sites where critics say safety and security needs to be improved.

Each year, at least 12,000 tons of used fuel is added to the pile. About 3,000 tons is reprocessed to remove unburned uranium for reuse as reactor fuel, and to extract plutonium for mixed oxide reactor fuel. But after reuse, this fuel must also be stored.

The cheapest and easiest method of temporary storage for used fuel is in reinforced concrete pools. Water keeps it cool and prevents leakage of radiation.

However, the huge earthquake and tsunami that crippled the Fukushima No. 1 nuclear power plant last month caused water levels in some cooling ponds to become dangerously low. This has prompted a worldwide review of spent fuel storage because about 90 percent is in ponds. Some spent fuel has been there for decades and many reactors' pools are either full or becoming overcrowded.

The remaining 10 percent of spent fuel is in dry cask storage. This is safer but more costly. Spent fuel that has already been cooled for at least a year in a pool is locked away inside big steel casks or concrete vaults, surrounded by inert gas.

Unlike water ponds, dry cask storage does not need to be mechanically cooled. Advocates say it should be expanded as an interim measure, until long-term storage systems can be agreed and built.

This, too, is becoming more urgent as the pile of used fuel mounts. After 40 or 50 years, the heat and average radioactivity have fallen to about one thousandth of the level at removal from the reactor. Handling and storage become easier.

However, some of the by-products of nuclear fission in reactors remain extremely dangerous. Plutonium, for example, has 15 different forms, or isotopes. Some are created in the reactor as uranium atoms split and generate fission energy.

They include plutonium-239, which takes 24,000 years to lose half its radioactive potency. In concentrated form, plutonium-239 is also used to make advanced nuclear weapons.

Most countries with nuclear power programs agree that disposal deep underground in geologically stable rock caverns is the best long-term method of storing used fuel.

Sweden and Finland are pace-setters. The former is committed to spend $ 3.8 billion constructing a national underground storage, starting in 2015. The first intake of used fuel is expected in 2020. Finland has a similar plan.

However, getting local agreement for underground storage can be difficult. The United States has the world's sole operating deep repository in the state of New Mexico. But it only accepts waste from nuclear weapons research and production.

In 1987, Congress authorized development of underground storage for high-level waste from U.S. civilian reactors beneath Yucca Mountain in Nevada. Although $ 12 billion in taxpayers' money was spent on the project, the Obama administration canceled it in 2009 in the face of local opposition.

Even if the project is reactivated, it will take at least 10 years before the facility starts accepting atomic waste. Meanwhile, the amount of spent fuel in the U.S. is already greater than the planned storage capacity at Yucca Mountain.

Japan, too, has struggled to confirm a site for underground storage because of its vulnerability to earthquakes.

There is also a deeper question to be settled by governments: should secure underground storage be permanent or just long-term, in case future nuclear technologies can turn the spent fuel into a valuable new source of energy. Used reactor fuel still contains much of its original uranium and over half its original energy content.

Beyond this is another question that concerns countries that decide to develop relatively small-scale nuclear power programs, but do not want to be saddled with the costs and responsibilities of long-term waste storage.

The Association of Southeast Asian Nations (ASEAN) should provide an answer if multiple member states opt to develop nuclear power, despite the risks exposed at Fukushima. Indeed, the crisis over nuclear safety in Japan could help forge a common policy on nuclear energy in Southeast Asia, including spent fuel storage and disposal.

In 2009, 14 small nuclear power nations in Europe formed a working group to develop a regional used fuel storage model for possible adoption by next year.

An alternative path for Southeast Asia could be fuel leasing, in which local nuclear power plant operators hire fabricated fuel from foreign suppliers and send it back to them after use.

Either way would show that ASEAN is committed to safe, secure and peaceful development of atomic power.