TOKYO — Japan’s struggle to contain the crisis at a stricken nuclear power plant worsened early Tuesday morning, as emergency operations to pump seawater into one crippled reactor temporarily failed, increasing the risk of a wider release of radioactive material, officials said.
With the cooling systems malfunctioning simultaneously at three separate reactors at the Fukushima Daiichi Nuclear Power Station after the powerful earthquake and tsunami, a more acute crisis developed late Monday at reactor No. 2 of the plant. There, a series of problems thwarted efforts to keep the core of the reactor covered with water — a step considered crucial to preventing the reactor’s containment vessel from exploding and preventing the fuel inside it from melting down.
The plant’s operator, Tokyo Electric Power, said late Monday that efforts to inject seawater into the reactor had failed. That caused water levels inside the reactor’s containment vessel to fall and exposed its fuel rods. The company said its workers later succeeded in infusing seawater in the pre-dawn hours of Tuesday morning, but fuel rods were exposed for at least several hours.
Workers had been having difficulty injecting seawater into the reactor because its vents — necessary to release pressure in the containment vessel by allowing radioactive steam to escape — had stopped working properly, they said.
But Tuesday morning Tokyo Electric announced that workers had succeeded in opening a malfunctioning valve controlling the vents, reducing pressure in the container vessel. It then resumed flooding the reactor with water.
The company said water levels were not immediately rising to the desired level, possibly because of a leak in the containment vessel. Still, a Tokyo Electric official said the situation was improving.
“We do not feel that a critical event is imminent,” he told a press conference.
The release of pressure appears to avert the immediate risk that the containment vessel would explode, creating a potentially catastrophic release of radioactive material into the atmosphere. But if the vessel is cracked and is not holding water properly, the risks of a large scale release of radioactive material would remain high.
In reactor No. 2, which is now the most damaged of the three at the Daiichi plant, at least parts of the fuel rods have been exposed for several hours, which also suggests that some of the fuel has begun to melt. Government and company officials said fuel melting has almost certainly occurred in that reactor, which can increase releases of radioactive material through the water and steam that escapes from the container vessel.
In a worst case, the fuel pellets could also burn through the bottom of the containment vessel and radioactive material could pour out that way — often referred to as a full meltdown.
“There is a possibility that the fuel rods are heating up and starting to melt,” said a Tokyo Electric spokesman told a late-night conference on Monday, televised on public broadcaster NHK. “It is our understanding that we have possible damage to the fuel rods,” he said.
By Monday night, officials said that radiation readings around the plant reached 3,130 micro Sievert, the highest yet detected at the Daiichi facility since the quake and six times the legal limit. Radiation levels of that magnitude are considered elevated, but they are much lower than would be the case if one of the container vessels had been compromised.
Industry executvies in touch with their counterparts in Japan Monday night grew increasingly alarmed about the risks posed by the No. 2 reactor.
“They’re basically in a full-scale panic” among Japanese power industry managers, said a senior nuclear industry executive. The executive is not involved in managing the response to the reactors’ difficulties but has many contacts in Japan. “They’re in total disarray, they don’t know what to do.”
The venting problems made it impossible for a time to administer the emergency remedy the plant operator had been using to control heat at the three crippled Daiichi reactors, all of which experienced failures in their electronic cooling systems. That remedy involves pumping in seawater to cool the fuel rods, then opening vents to release the resulting steam pressure that builds in the container vessel. When the vessel is depressurized, workers can inject more seawater, a process known as “feed and bleed.”
The extreme challenge of managing reactor No. 2 came as officials were still struggling to keep the cores of two other reactors, No. 1 and No. 3, covered with seawater. There was no immediate indication that either of those two reactors had experienced a crisis as serious as that at No. 2.
The United States Nuclear Regulatory Commission said Monday that the Japanese government had formally asked for assistance as it responds to the crisis in Fukushima. As part of a wider response, the United States has already dispatched two experts in boiling-water reactors, the type used at Daiichi. They are in Tokyo offering technical assistance to the Japanese, the commission said in a statement. The commission is considering further assistance, including providing technical advice, it said.
The situation at Daiichi was also complicated on Monday by another problem when the outer structure housing reactor No. 3 exploded earlier on Monday. A similar explosion destroyed the structure surrounding reactor No. 1 on Saturday. Live footage on public broadcaster NHK showed the skeletal remains of the reactor building and thick smoke rising from the building. Eleven people had been injured in the blast, one seriously, officials said.
Chief Cabinet Secretary Yukio Edano said earlier Monday evening that the release of large amounts of radiation as a result of the explosion at No. 3 was unlikely because the blast did not compromise the steel containment vessel inside the No. 3 reactor. But traces of radiation could be released into the atmosphere, and about 500 people who remained within a 12-mile radius were ordered temporarily to take cover indoors, he said.
“I have received reports that the containment vessel is sound,” Mr. Edano said. “I understand that there is little possibility that radioactive materials are being released in large amounts.”
Mr. Edano and other senior officials did not address the escalating crisis at reactor No. 2 later Monday or early Tuesday.
But the situation a reactor No. 3 was being closely watched for another reason. That reactor uses a special mix of nuclear fuel known as MOX fuel. MOX is considered contentious because it is made with reprocessed plutonium and uranium oxides. Any radioactive plume from that fuel would be more dangerous than ordinary nuclear fuel, experts say, because inhaling plutonium even in very small quantities is considered lethal.
In screenings, higher-than-normal levels of radiation have been detected from at least 22 people evacuated from near the plant, the nuclear safety watchdog said, but it is not clear if the doses they received were dangerous.
Technicians had been scrambling most of Sunday to fix a mechanical failure that left the reactor far more vulnerable to explosions.
The two reactors where the explosions occurred are both presumed to have already suffered partial meltdowns — a dangerous situation that, if unchecked, could lead to a full meltdown.
Hiroko Tabuchi reported from Tokyo, Keith Bradsher from Hong Kong and Matt Wald from Washington.