A layman's guide to the situation at the Fukushima No. 1 nuclear power plant
BY RYOMA KOMIYAMA STAFF WRITER
Q: What does this talk of a "meltdown" at the No. 1 reactor of the Fukushima No. 1 nuclear power plant mean?
A: A situation in which nuclear fuel rods have melted, fallen and accumulated on the bottom of a pressure vessel is called a "meltdown." The May 16 report from Tokyo Electric Power Co. indicated a high likelihood of meltdowns not only at the No. 1 reactor but at the plant's No. 2 and No. 3 reactors.
Q: We may be dealing with multiple meltdowns?
A: Nuclear fuel rods produce intense heat for some time after power generation has halted. The No. 2 and No. 3 reactors have not been supplied with sufficient cooling water since the tsunami triggered by the Great East Japan Earthquake crippled the plant. That is the same as the situation at the No. 1 reactor. It is very likely that most of the fuel rods at these reactors have melted.
Q: The incident happened two months ago. Why did TEPCO not know it was facing this situation much earlier?
A: When TEPCO repaired a water-level gauge at the reactor on May 10, it found little water was left in the pressure vessels. Nevertheless, the temperature inside the vessels was only about 100 degrees. If the fuel rods had been fully exposed, the temperature would have been much higher. That indicates that all fuel rods fell and have been submerged in water on the bottom (of the vessels).
Q: What happens if fuel rods melt?
A: Melted fuel rods cannot be easily cooled by water because they are completely deformed. They become like a chunk of concrete. Cooling them requires the continued injection of water. In this case, intense heat from melted rods on the bottom made holes in the metal pressure vessel, causing the contaminated water to leak.
Q: What are these holes like?
A: There are several of them. Their total size is about the same as a circle with a diameter of a few centimeters, according to TEPCO. As well as contaminated water, the melted fuel rods themselves could have passed through holes of that size.
Q: What if parts of the melted fuel have leaked?
A: In the Chernobyl nuclear accident in 1986, nuclear substances were scattered over a wide area, carried in smoke produced by the large fire at the plant. The Fukushima incident is not as grave as the Chernobyl accident, but it is much worse than the Three Mile Island nuclear incident in 1979. In the case of Three Mile Island, meltdown also occurred, but the nuclear fuel rods were contained in the pressure vessel. That prevented substantial amounts of nuclear material from leaking.
Meltdowns also likely occurred at No. 2, No. 3 reactors of Fukushima plant
The Fukushima No. 1 nuclear power plant seen from about 38 kilometers west of the facility and at an altitude of 7,300 meters (Eiji Hori)
Data shows meltdowns occurred at the No. 2 and No. 3 reactors of the Fukushima No. 1 nuclear power plant, creating huge problems for the plant operator that had presented a more optimistic scenario.
And like the No. 1 reactor, the melted fuel appears to have created holes in the pressure vessel of the No. 3 reactor, according to the data of Tokyo Electric Power Co. released May 16.
Goshi Hosono, special adviser to Prime Minister Naoto Kan, acknowledged the likelihood of meltdowns at the No. 2 and No. 3 reactors.
"We have to assume that meltdowns have taken place," Hosono said at a news conference May 16.
Haruki Madarame, chairman of the Nuclear Safety Commission, said in a separate news conference the same day that the meltdowns should not come as a surprise.
"When highly contaminated water was found at the No. 2 reactor building in late March, we recognized that a meltdown had taken place. So I informed the government," he said. "As for No. 1 and No. 3 reactors, we recognized that, given the processes that led to the accidents there, the same thing had occurred."
Immediately after the crisis erupted at the nuclear power plant in March, experts pointed out that meltdowns likely occurred at all three reactors.
But TEPCO's measures to contain the crisis have been based on the assumption of lighter damage to the reactor cores.
TEPCO had said it believed that only a portion of the nuclear fuel rods had melted. Now, it appears that all parts of the fuel rods have melted.
TEPCO recently said a meltdown likely occurred at the No. 1 reactor. But a TEPCO official on May 16 declined to comment on the possibility of meltdowns at the No. 2 and No. 3 reactors.
"We have yet to be able to grasp the entire situation at the plant," the official said.
A meltdown is a situation in which nuclear fuel melts and accumulates at the bottom of the reactor pressure vessel, which is located inside a containment vessel.
At the No. 3 reactor, the melted fuel may have burned through the pressure vessel to the containment vessel, the data showed.
TEPCO on April 17 released a schedule to reach a cold shutdown at the Fukushima plant within six to nine months.
However, given the latest data, the embattled company will have to drastically modify its plans.
If meltdowns have indeed occurred, more time will be needed to construct a system that cools the reactors. In addition, the company will be tasked with the huge chore of disposing of massive amounts of highly contaminated water.
TEPCO's latest data describes the situation immediately after the Great East Japan Earthquake and tsunami devastated the coast of the Tohoku region on March 11.
Release of the data, which had been kept at the central control room at the nuclear power plant, was delayed because it took time to restore power and remove radioactive materials attached to the papers.
According to the data, the pressure in the pressure vessel of the No. 2 reactor dropped at 6:43 p.m. on March 15. A similar drop in pressure also took place at the No. 3 reactor at 11:50 p.m. on March 16.
Those declines were apparently the result of holes made in the pressure vessels.
Previously, it was believed that water was leaking through holes at the bottom of the pressure vessels where measuring instruments and part of the control rod mechanisms were located.
Now, it appears that melted nuclear fuel formed new holes in the pressure vessels.
Radioactive materials, such as technetium, produced when nuclear fuel rods are damaged, have been detected in water in the No. 3 reactor building. That discovery has raised speculation that the melted nuclear fuel has breached the pressure vessel and landed in the containment vessel.
During the meltdown at Three Mile Island in the United States in 1979, the fuel remained in the pressure vessel. But work to remove the melted fuel from the pressure vessel, which started in 1985, took five years to complete. An additional three years were needed to confirm that radioactive contamination had been removed from the reactor.
Given the more serious situation at the Fukushima plant, some nuclear experts say more than 10 years will be needed to remove the melted fuel, eliminate the contamination and dismantle the reactors.
Fumiya Tanabe, a former senior researcher at what was then the government-affiliated Japan Atomic Energy Research Institute, said black smoke from the No. 3 reactor building in the days after the earthquake and available data on pressure showed early on that a meltdown had taken place.
"Before we saw TEPCO's data (released on May 16), we had been already aware of the possibility (of a meltdown)," he said.
Tanabe criticized TEPCO's recovery efforts and measures that were taken based on a situation that was much less serious than reality.
He said TEPCO's optimistic scenario led three workers to be exposed to highly radioactive water on March 24 and prevented measures to keep contaminated water from leaking into the sea through a trench at the No. 2 reactor building.
"In resolving serious accidents like those (at the Fukushima plant), it is a cardinal rule to work out recovery measures based on the worst possible situation," he said.
In Japan Reactor Failings, Danger Signs for the U.S.
By HIROKO TABUCHI, KEITH BRADSHER and MATTHEW L. WALD
Published: May 17, 2011
This article is by Hiroko Tabuchi, Keith Bradsher and Matthew L. Wald.
These articles explore warning signs that were unheeded or covered up before an earthquake and tsunami triggered a nuclear crisis in Japan.
TOKYO — Emergency vents that American officials have said would prevent devastating hydrogen explosions at nuclear plants in the United States were put to the test in Japan — and failed to work, according to experts and officials with the company that operates the crippled Fukushima Daiichi plant.
The failure of the vents calls into question the safety of similar nuclear power plants in the United States and Japan. After the venting failed at the Fukushima plant, the hydrogen gas fueled explosions that spewed radioactive materials into the atmosphere, reaching levels about 10 percent of estimated emissions at Chernobyl, according to Japan’s nuclear regulatory agency.
Venting was critical to relieving pressure that was building up inside several reactors after the March 11 tsunami knocked out the plant’s crucial cooling systems. Without flowing water to cool the reactors’ cores, they had begun to dangerously overheat.
American officials had said early on that reactors in the United States would be safe from such disasters because they were equipped with new, stronger venting systems. But Tokyo Electric Power Company, which runs the plant, now says that Fukushima Daiichi had installed the same vents years ago.
Government officials have also suggested that one of the primary causes of the explosions was a several-hour delay in a decision to use the vents, as Tokyo Electric managers agonized over whether to resort to emergency measures that would allow a substantial amount of radioactive materials to escape into the air.
But the release this week of company documents and interviews with experts provides the most comprehensive evidence yet that mechanical failures and design flaws in the venting system also contributed to delays. The documents paint a picture of increasing desperation at the plant in the early hours of the disaster, as workers who had finally gotten the go-ahead to vent realized that the system would not respond to their commands.
While venting would have allowed some radioactive materials to escape, analysts say that those releases would have been far smaller than those that followed the explosions at three of the plant’s reactors, which blew open containment buildings meant to serve as a first line of defense against catastrophe. The blasts may also have been responsible for breaches in containment vessels that have complicated efforts to cool the fuel rods and contain radioactive leaks from the site.
One reason the venting system at the plant, which was built by General Electric, did not work is that it relied on the same sources of electricity as the rest of the plant: backup generators that were in basements at the plant and vulnerable to tsunamis. But the earthquake may also have damaged the valves that are part of the venting system, preventing them from working even when operators tried to manually open them, Tokyo Electric officials said.
In either case, regulators in the United States and Japan will now need to determine if such systems at similar plants designed by G.E. need to undergo expensive and time-consuming retrofitting or redesign to allow them to function even in severe accidents.
“Japan is going to teach us lessons,” said David Lochbaum at the Union of Concerned Scientists. “If we’re in a situation where we can’t vent where we need to, we need to fix that.”
Officials from General Electric did not comment on Tuesday.
The seriousness of the crisis at the Fukushima plant became evident within hours of the quake and the tsunami that rushed over the plant’s sea wall.
Just 12 hours after the quake, the pressure inside Reactor No. 1 had reached roughly twice the maximum pressure the unit had been designed to withstand, raising fears that the vessels that house fuel rods would rupture, setting a possible meltdown in motion. With the pressure high, pumping in additional cooling water also was not possible.
The government became rattled enough that it ordered Tokyo Electric to begin venting. But even then, Tokyo Electric’s executives continued to deliberate, according to a person close to government efforts to bring the reactors under control. The exchanges became so heated, the person said, that the company’s nuclear chief, Vice President Sakae Muto, and the stricken plant’s director, Masao Yoshida, engaged in a “shouting match” — a rarity in reserved Japan.
Mr. Yoshida wanted to vent as soon as possible, but Mr. Muto was skeptical whether venting would work, the person said, requesting anonymity because he is still an adviser to the government and is not permitted to comment publicly. “There was hesitation, arguments and sheer confusion over what to do,” he said.
The executives did not give the order to begin venting until Saturday — more than 17 hours after the tsunami struck and 6 hours after the government order to vent.
As workers scrambled to comply with their new directive, they faced a cascading series of complications.
The venting system is designed to be operated from the control room, but operators’ attempts to turn it on failed, most likely because the power to open critical valves was out. The valves are designed so they can also be opened manually, but by that time, workers found radiation levels near the venting system at Reactor No. 1 were already too high to approach, according to Tokyo Electric’s records.
At Reactor No. 2, workers tried to manually open the safety valves, but pressure did not fall inside the reactor, making it unclear whether venting was successful, the records show. At Reactor No. 3, workers tried seven times to manually open the valve, but it kept closing, the records say.
The results of the failed venting were disastrous.
Reactor No. 1 exploded first, on Saturday, the day after the earthquake. Reactor No. 3 came next, on Monday. And No. 2 exploded early Tuesday morning.
With each explosion, radioactive materials surged into the air, forcing the evacuation of tens of thousands of earthquake survivors living near the plant, contaminating crops and sending a faint plume of radioactive isotopes as far as the United States within days. Aerial photos of the reactor buildings showed No. 1 and 3 had been blown apart and another was seriously damaged.
As the troubles mounted, Tokyo Electric and government officials conducted a series of news conferences that began to suggest the scope of the damage. The blasts, they said, probably caused breaches in containment vessels that are among the final layers of protection against meltdowns and even larger releases of radioactive materials.
Tokyo Electric in recent days has acknowledged that damage at the plant was worse than previously thought, with fuel rods most likely melting completely at Reactors 1, 2 and 3 in the early hours of the crisis, raising the danger of more catastrophic releases of radioactive materials. The company also said new evidence seemed to confirm that at Reactor No. 1, the pressure vessel, the last layer of protection, was broken and leaking radioactive water.
The improved venting system at the Fukushima plant was first mandated for use in the United States in the late 1980s as part of a “safety enhancement program” for boiling-water reactors that used the Mark I containment system, which had been designed by General Electric in the 1960s. Between 1998 and 2001, Tokyo Electric followed suit at Fukushima Daiichi, where five of six reactors use the Mark I design.
The company said that was the case this week, after a review of Japanese regulatory filings made in 2002 showed that the vents had been installed.
The fortified venting system addressed concerns that the existing systems were not strong enough to channel pent-up pressure inside the reactors in an emergency. Pressure would be expected to rise along with temperature, damaging the zirconium cladding on the fuel rods at the reactor core and allowing them to react chemically with water to produce zirconium oxide and hydrogen gas.
The new vents were designed to send steam and gas directly from the reactor’s primary containment, which houses the reactor vessel, racing past the usual filters and gas treatment systems that would normally slow releases of gas and eliminate most radioactive materials.
But the emergency vents were fitted with numerous safeguards, some of which require electricity to work, rendering them useless when all power is lost at a nuclear plant, experts say.
The most important of those safeguards are the valves, operated from a switch under lock and key in the control room, that must be opened for the vents to work. When a key is inserted into the keyboard in the nuclear reactor’s control room and turned, the valves are supposed to open, letting gases rush out of the reactor building.
Tokyo Electric has said the valves did not work at Fukushima Daiichi after the power failed.
That would suggest that operators of similar plants in the United States and Japan could protect reactors by moving generators to higher floors if the equipment is currently in places that could be affected by tsunamis or flooding from rivers.
But a redesign of the venting system itself might also be necessary.
The design is the result of conflicting schools of thought among United States nuclear officials, said Michael Friedlander, a former senior operator at several American nuclear power plants.
Mr. Friedlander said, referring to the Nuclear Regulatory Commission: “You have the N.R.C. containment isolation guys who want containment closed, always, under every conceivable accident scenario, and then you’ve got the reactor safety guys who need containment to be vented under severe accident scenarios. It is a very controversial system.”