OSLO — Japan's nuclear accident exposes the dilemma of whether to build power plants on tsunami-prone coasts or inland sites where water supplies are unreliable, a problem likely to be aggravated by climate change, experts say.

Many of the world's 442 nuclear power reactors are by the sea, rather than by lakes or rivers, to ensure vast water supplies for cooling fuel rods in emergencies like that at the Fukushima plant on Japan's east coast.

"It's quite a conundrum," said Ian Jackson, a nuclear energy fellow at Chatham House in Britain. "If you are in a geologically stable area, a coastal location is still the best option."

Japan was scrambling to avert a meltdown at the Fukushima plant after Friday's devastating quake and tsunami, which killed at least 10,000 people.

Inland, water supplies can be more vulnerable to heatwaves, floods, temperature swings and dam failures. Water is a prime consideration in siting decisions that include staying clear of geological fault lines, flight paths and cities.

A 2003 heatwave in Europe, for instance, forced Electricite de France to close or lower output at about half its 19 nuclear plants because of temperature limits on the water it returns to rivers such as the Rhone.

Excessively high temperatures can kill fish and other river life, as well as reduce output from the power plants.