The fuel rods in one of Japan's damaged nuclear reactors have been temporarily fully exposed from their coolant, raising the risk of overheating and a meltdown.
A spokesman at the Fukushima plant said today that Unit 2's rods were briefly exposed.
Sea water was being channelled into the reactor to cover the rods again.
Unit 2 of the Fukushima Dai-ichi plant is the latest reactor to lose its ability to cool down.
The other two reactors at the plant are facing a meltdown and authorities are racing to cool them with sea water.
And the crisis has prompted Switzerland to suspended plans to build and replace nuclear plants.
Energy Minister Doris Leuthard said the ban would affect all "blanket authorisation for nuclear replacement until safety standards have been carefully reviewed and if necessary adapted."
Three sites for new nuclear power stations had been approved after the plans were submitted in 2008.
"Safety and well-being of the population have the highest priority," said ms Leuthard, who ordered the Swiss Federal Nuclear Safety Inspectorate to analyse the exact cause of the accidents in Japan and draw up new or tougher safety standards "particularly in terms of seismic safety and cooling."
She said no new plants could be allowed until those experts report back.
Earlier in Japan a second explosion in three days rocked the plant, sending a massive cloud of smoke into the air and injuring 11 workers.
The blast was felt 25 miles away, but the plant's operator said the radiation levels at the affected unit were still within legal limits.
The blast was in Unit 3, which workers have been trying to cool with sea water after a system failure in the wake of the earthquake and tsunami.
It triggered an order for hundreds of people to stay indoors, said Chief Cabinet Secretary Yukio Edano.
Operators knew the sea water flooding would cause a pressure buildup in the reactor containment vessel - and potentially lead to an explosion - but felt they had no choice if they wanted to avoid a complete meltdown. In the end, the hydrogen in the released steam mixed with oxygen in the atmosphere and set off the blast.
The inner containment shell surrounding the Unit 3 reactor was intact, Mr Edano said, allaying some fears of the risk to the environment and public. But the outer building around the reactor appeared to have been devastated, with only a skeletal frame remaining.
Tokyo Electric Power Company, which operates the plant, said radiation at Unit 3 was well under the level where a nuclear operator must file a report to the government.
A similar explosion occurred on Saturday at the plant's Unit 1, injuring four workers, causing mass evacuations and destroying much of the outer building.
Shortly after today's explosion, Tokyo Electric warned it had lost the ability to cool Unit 2. Hours later, the company admitted that the fuel rods had been fully exposed.
It was trying to channel sea water into the reactor to cover the rods, cool them down and prevent another explosion.
More than 180,000 people have evacuated the area in recent days, and up to 160 may have been exposed to radiation - pouring misery onto those already devastated by the twin disasters.
States of emergency have been declared at six Fukushima reactors, where the main cooling systems and backup generators have been knocked out. Three are at Dai-ichi and three at the nearby Fukushima Daini complex.
Most attention, though, has been focused on Dai-ichi units 1 and 3, where operators have been funnelling in sea water in a last-ditch measure to cool the reactors. A complete meltdown - the melting of the radioactive core - could release radioactive contaminants into the environment and pose major, widespread health risks.
Mr Edano said no Fukushima reactor was near that point, and he was confident of escaping the worst scenarios.
International scientists say there are serious dangers but little risk of a Chernobyl-style catastrophe. Chernobyl had no containment shell around the reactor.
"The likelihood there will be a huge fire like at Chernobyl or a major environmental release like at Chernobyl, I think that's basically impossible," said James Stubbins, a nuclear energy professor at the University of Illinois.
And, some analysts noted, the length of time since the nuclear crisis began indicated that the chemical reactions inside the reactor were not moving quickly toward a complete meltdown.