Data released by TEPCO on May 16 shows that apparently the isolation condenser, a cooling system that is supposed to protect the reactor after a shutdown, was manually shut down in unit 1 within 10 minutes of the earthquake.

When the quake hit at 14:46 on 2011-03-11, the power station lost its grid connection. Diesel generators sprang into life to provide backup power for the Residual Heat Removal System (RHR). Around 15:00 someone manually shut down the isolation condensor. About half an hour later tsunami hit the station. Within minutes the diesel generators failed and the RHR stopped. At that point the battery-operated pumps for the isolation condenser were the only system still able to remove heat from the reactor core, but the valve for this system had been closed by operators. From that point onwards the reactor core was entirely without cooling. Records show that at 18:10 the valve was open again. At 18:25 it was closed again and 21:30 it was open again. The isolation condenser finally failed at 01:48 on 2011-03-12, perhaps because its batteries ran out.

According to the AREVA presentation from 2011-04-07, the isolation condenser in unit 1 stopped at 16:36 on 2011-03-11, but perhaps that was based on incomplete or bad data from TEPCO.

See also:

• Worker error may have led to meltdown (Japan Times, 2011-05-17)

After pumping increased amounts of water into the reactor core of Fukushima 1 unit 1 with the aim of bringing it to a “cold shutdown”, TEPCO had to conclude that the water level in both the reactor core and the containment is much lower than expected and their plan appears not to be succeeding.

Previously it was believed that the lower half of the fuel rods in the reactor pressure vessel (RPV) was covered in water. Now it appears that part of the RPV is dry and the fuel rods (or what’s left of them) are fully exposed. However, due to the damage to the rods much of the uranium pellets inside may already have spilled into the bottom portion of the RPV and may thus receive some cooling after all. TEPCO had only recently revised its estimate of core damage for unit 1 down from 70% to 55%.

The plan to provide ouside cooling to the RPV by flooding the dry well of the containment surrounding it with enough water to raise the water level to above the top of the fuel rods has not worked yet. Even after pumping more than 10,000 m3 of water, the water level in the 7,400 m3 containment is still below half, not even reaching the bottom of the RPV, let alone its top.

If the water level doesn’t rise further because of leaks then it seems quite unlikely that it will be possible to repair those leaks using manual labour, especially at dose levels of 10 millisievert and more per hour. Even when it looked like the plan might work for unit 1, it was questionable if the same procedure could then be applied to unit 2 or 3, which were already assumed to have a leaky containment.

TEPCO has not yet discussed any alternative plan if flooding the containment won’t work for some or all of the units.

See also:

• Tepco: Leak Suggests Severe Damage (WSJ, 2011-05-12)

TEPCO has revised its estimates for the core damage in units 1, 2 and 3 of the Fukushima 1 nuclear power plant. It now estimates 55% (previously 70%) damage for unit 1, 35% (previously 30%) for unit 2 and 30% (previously 25%) for unit 3. The estimates are based on radioactivity levels measured inside the reactor containment.

TEPCO has experimented with pumping more water into the reactor core of unit 1 to fill the normally dry part of the containment vessel for cooling purposes:

TEPCO says it will raise the amount of water injected from 6 to 10 tons per hour for 6 hours, and then to 14 tons per hour. The temperature and pressure in the containment vessel will be monitored for 18 hours. The utility says it will decrease the flow back to 6 tons per hour by Thursday morning and then send robots into the reactor building to check for leaks. TEPCO also says it will make sure that the containment vessel, with the added weight of the water inside, can withstand strong aftershocks. The firm says robots on Tuesday detected radiation levels of up to 1,120 millisieverts per hour inside the No.1 reactor building. It says some contaminated water may be leaking from the reactor into external pipes. (NHK, 2011-04-27)

Radiation monitoring devices have been issued to 55 schools and kindergartens in Fukushima prefecture. Teachers will keep a log of radiation exposure of the children. At two schools in the city of Koriyama, some 50 km west of the wrecked reactor, workers are removing contaminated topsoil. The soil is first sprayed with water to prevent dust from rising through the work. The soil will be piled up and covered with sheets before later being moved to landfills.

One female worker in her 50s employed at Fukushima 1 received an accumulated dose of 17.55 mSv over the first quarter of the year, over three times the legal limit. She worked at the plant for 11 days after March 11. Most of the exposure was internal, through inhalation. By law female nuclear workers in Japan must not be exposed to more than 5 ms over any three month period.

See also:

• Core damage ratio of Unit 1 to 3 of Fukushima Daiichi Nuclear Power Station

Tepco is preparing for a “wet tomb” for unit 1 and 3 of Fukushima 1. It stepped up its water injection for unit 1 from 6 m3/h to 14 m3/h today to test for leaks. It is using robots to visually inspect the inside of the reactors for signs of water leaking.

By allowing water from the reactor pressure vessel (RPV) to overflow into the containment that surrounds it, almost all of the RPV will be immersed in water, which will carry away heat from the fuel rods inside the RPV through conduction. 7,800 tons of water will be injected. If the water starts leaking from the containment, the plan will have to be abandoned. Concern has been raised how the extra weight would affect the seismic stability of the building in the event of another major quake.

Radioactive water has been found in the basement of the turbine building of unit 4, which was shut down at the time of the tsunami and does not have any fuel loaded in its core. The water is assumed to have leaked across from adjacent unit 3.

Tepco has released a radiation map for the Fukushima 1 plant, showing known radiation “hot spots” around the site.

Power companies operating nuclear power stations in Japan have deployed vehicle-mounted mobile backup generators at their plants, but in many cases their capacity is too small to replace the fixed backup generators such as the ones that failed in Fukushima 1. Surprizingly, they don’t seem to be in much of a hurry to address that safety problem: Japan Atomic Power Co. is quoted by Kyodo as trying to purchase three generator trucks by March 2012 while Hokkaido Electric Power Co. wants to buy a second generator vehicle for its Tomari power station “within two years”.

See also:

• Tepco To Flood Damaged Reactors With Water
• Most Japanese reactors yet to have enough backups for stable cooling
• Fukushima 1 radiation map (Tepco, 2011-04-23)

Twentyfive years ago an accident at the Chernobyl unit 4 nuclear power station destroyed the reactor and released massive amounts of radioactivity into the environment. Contamination of vast areas of land, mostly in Belarus and Ukraine but also reaching as far as central and Western Europe, persists to this day. Most of the cesium-137 released that day has yet to decay.

It can be argued that the lessons of Chernobyl had been fading much more quickly than the radiation. With fear of Global Warming growing, the nuclear industry was pushing for new reactors to be constructed and old reactors, many of which had been constructed in the 1970s, to have their operating spans extended by another decade or two. One such reactor was Fukushima 1 unit 1, the oldest of the Fukushima reactors, which started up on March 26, 1971. Its original operating span of 40 years would have expired at the end of March 2011, but it had its license extended by another ten years in February. Unlike its siblings units 2 and 3, unit 1 was an older type called BWR3 (the others were of a type called BWR4). One difference between them is that the BWR4 has a Reactor Core Isolation Cooling (RCIC) system. During a loss of all grid and backup power a steam turbine running off decay heat in the core could still pump water from the Suppression Chamber into the reactor core. The BWR3 makes do with an isolation condensor, which relies on electric pumps to cool and condense steam from the reactor using cold outside water. Once the pumps stop, the isolation condenser stops cooling steam. With the RCIC at least liquid water was being injected as long as there was battery power to control valves and the water in the suppression chamber wasn’t boiling yet.

According to the AREVA report, cooling in the isolation condenser in unit 1 stopped at 16:36 on March 11, less than an hour after the backup diesel generators had failed. By contrast, the RCIC pump in unit 3 continued until 02:44 on March 13, about 35 hours after loss of backup power. In unit 2 the RCIC survived until 13:25 on March 14, some 46 hours after the accident.

Immediately after the emergency stop (“scram”) of the reactors, the fuel rods were still producing 6% of the original thermal output in decay heat, which is why cooling was desperately needed. After 24 hours the decay heat had dropped to 1% of original output. From there it takes another 4 days for the output to drop by half again (0.5%). Without adequate cooling, the water level inside the reactor drops and the steam pressure in the reactor pressure vessel (RPV) goes up. At some point a valve opens and steam is released into the suppression chamber (wet well) at the bottom of the reactor. Once the water level drops too far the exposed fuel rods overheat until zirconium cladding gets damaged and nuclear waste starts leaking from inside the fuel rods. At 1200C the zirconium rods start burning in a steam atmosphere, releasing hydrogen as a byproduct. It was estimated that in unit 1 between 300 and 600kg of hydrogen were produced that way (300-1000 kg in units 2 and 3, which hold more fuel). The hydrogen leaks through the water in the wet well into the dry well. This is the first containment of the reactor, which is designed for a pressure of a little over 4 bar. If the containment pressure gets too high, there is no alternative but to release steam and hydrogen from the containment into the environment, along with radioactivity from the damaged fuel rods.

Unit 1 was completely without cooling water for 27 hours (until 20:20 on March 12). Units 2 and 3 were without cooling water for 7 hours (until 20:33 on March 14 and 09:38 on March 13, respectively). Because of the far longer period without cooling, 70% of the fuel assemblies in unit 1 is estimated to be damaged, versus 30% in unit 2 and 25% in unit 3. Perhaps if unit 1 had also had an RCIC system like units 2 and 3, it could have avoided that high degree of fuel damage. All three units were designed before the Three Mile Island (TMI) accident that highlighted the risks of losing core cooling and consequent hydrogen buildup.

When the containment vessel is vented, the gases are normally scrubbed (filtered) before being fed into a pipe that goes up a tall steel tower located between two of the units (1 and 2, 3 and 4, 5 and 6). That is not what happened. In units 1 and 3 radioactive steam and hydrogen ended up leaking into the upper part of the reactor building, the service floor above the containment, where the spent fuel pool and a crane for refueling and unloading the reactor are located. After TMI, hardened pipes were installed in US boiling water reactors for venting gas, but apparently Tepco didn’t think it was worth spending money that could instead be paid to its shareholders as dividends. Perhaps the leak occurred because insufficiently robust venting pipes had already been damaged by the initial earthquake, or perhaps the pressure was too high because Tepco waited too long with venting, hoping to still get pumps going again in time and save its capital investment. Whatever the reason, had the venting occurred through the tower, the reactor buildings might have survived and the spent fuel pools wouldn’t be lying full of debris from the collapsed structure now.

As an engineer, I know how difficult it is to build and operate complex systems. Often you might hear engineers confess they’re glad what they’re designing wasn’t used to run a nuclear power station, or fly an aircraft or run medical equipment. It’s one thing when failure of technology causes loss of data or loss of money, but it’s quite another if people die as a result. That is a grave responsibility.

Most of us engineers are aware of our imperfections. It takes careful design and implementation to build mission-critical systems. For example, a friend of mine was working on the avionics of the Airbus A-320, the first fly-by-wire commercial aircraft. Airbus had multiple programmers implement the software in different programming languages and using different hardware platforms in order to ensure that it was as unlikely as possible that if one system failed the other would be in trouble too. Environments such as airlines and nuclear power stations require a corporate culture where safety is always the top priority. Not cost, not convenience, not customer expectations — only safety.

That is not how Tepco has been operating. They have been caught red handed, again and again. One of the most embarrassing cases even involved Fukushima 1 unit 1:

Faked pressure test
Yet in the most serious case of all, Tepco officials are alleged to have faked a pressure test designed to test the integrity of the containment building. The test involves pumping nitrogen gas into the building to increase the pressure to about three times atmospheric pressure, then taking pressure readings to measure the leak rate.

Regulations state that the leak rate must be less than 0.45% per day. However, at Fukushima I-1 in 1992, the company conducted its own tests before the government inspectors turned up, and discovered that the building might not pass the test. One source quoted in the Daily Yomiuri said that leak rates fluctuated from 0.3% to 2.5% per day.

Documents found at Hitachi by Tepco’s own investigative team describe a method to fake the test by secretly pumping in extra air from the main steam isolation valve. At the time, Hitachi had a contract to check Tepco equipment. It is alleged that Tepco officials followed this procedure when the government inspectors were checking the leak rate.

Japan: nuclear scandal widens and deepens (WISE, 2002-10-04)

It has been argued that nuclear power could never be safe in a country so criss crossed with earthquake fault lines as Japan is. But the forces of nature are not the only challenge facing nuclear technology. Chernobyl exploded because of operator errors. Fukushima failed because of mistakes made by the humans who designed and operated it. The Fukushima I nuclear disaster was a man-made disaster, with a natural disaster acting only as the trigger.

In 2001 and again in 2009, researchers had published evidence of past huge tsunamis in the region, which were ignored by the nuclear industry. Fukushima 1 was kept running, without any upgrade to its breakwaters. When the quake struck on March 11, 2011, unit 1 was set to operate until 2021, without the reactor core isolation cooling system that the other units have had since the 1970s and without the upgraded venting systems that US reactors have had since the 1980s.

Instead of thinking of ways to upgrade these ancients plants to at least make them a little bit safer, Tepco was being creative only in dodging vital inspections, for example to hide their leaking containment that was supposed to protect the tens of thousands of people living near the reactors. Extending the lifetime of the plants by another 10 or 20 years with the least amount of money spent was going to be profitable to Tepco shareholders. As a result, hundreds of square kilometers have now become a nuclear wasteland, 90,000 people have lost their homes and probably over a trillion yen (US$12 billion) will have to be spent on cleaning up the mess at the reactor site over a number of years.

See also:

• AREVA Fukushima report by Dr. Matthias Braun (2011-04-07)
• Information on Status of Nuclear Power Plants in
  Fukushima (JAIF, 2011-04-26)
• AP report on historical tsunami
• Japan: nuclear scandal widens and deepens (WISE, 2002-10-04)
• Culture of Complicity Tied to Stricken Nuclear Plant (New York Times, 2011-04-27)

Tokyo Electric Power Company (Tepco) president Masataka Shimizu was turned down twice when he requested a meeting with Fukushima prefecture governor Yuhei Sato who was very upset about the insufficient safety of Tepco’s nuclear power plants in the prefecture, which were not designed to cope with a tsunami the size triggered by the March 11 quake. Tepco was unable to reestablish cooling for too long, leading to core damage, forced venting of radioactive gas, hydrogen explosions that destroyed reactor buildings and spread radiactivity, a cracked containment, leaks of radioactive water, damage to reactor cores and to spent fuel pools.

Large parts of Fukushima prefectures are radioactively contaminated now. Agricultural and fishery products from the largely rural prefecture will have difficulties finding buyers, even if they were to meet Japanese safety regulations.

Some 80,000 people in a 20 km radius around Fukushima 1 (an area of about 600 square kilometers or 60,000 ha) have lost their homes and will now be prohibited on penalty of a fine of up to 100,000 yen (about US$1,200) or 30 days in jail from returning. The evacuation zone will now be extended to Iitate, Katsurao, Namie and parts of Minamisoma and Kawamata, which lie between 20 and 50 km northwest of Fukushima 1. Another 10,000 residents will be affected by that extension. In Namie, at the edge of the 20 km exclusion zone, backround radiation of 49.8 μSv/h was measured. The natural rate in Japan before the accident was between 0.02 and 0.06 μSv/h. Other notable readings were 3.5 μSv/h in Toyota town some 50 km to the west, 15.5 μSv/h in Iitate, some 30 km to the Northwest and 28.6 μSv/h in Namie 30 km to the Northwest. The current post-accident rate in Tokyo, 230 km to the south, is about 0.08 μSv/h, twice its pre-accident rate. Residents of the newly condemned areas have one month to move out.

Inside the exclusion zone even higher values have been measured, for example around 50 μSv/h in several locations 4-5 km from the plant and 110 μSv/h just over 3 km West of the plant. Under these circumstances, it seems almost unbelievable that Tepco was still talking about restarting units 5 and 6 of Fukushima 1, which lie only a few hundred meters from the wrecked units and had also been surrounded by seawater during the tsunami, but were shut down for maintenance at the time. Who is going to work in those plants and under what working conditions? As of April 21, the radiation level at the South side of the administration building located half way between units 1 and 5 was an astounding 490 μSv/h.

Prime minister Kan has made it clear that he does not want to see units 5 and 6 restarted, but made no mention of Fukushima 2, which is located about 11 km South of Fukushima 1 and 9 km inside its exclusion zone (and inside its own 8 km exclusion zone, which is completely enclosed by the larger 20 km exclusion zone imposed around Fukushima 1). Tepco is pushing for being allowed to restart Fukushima 2 as soon as possible, but Fukushima governor Yuhei Sato opposes the idea.

Before the disaster Japanese power companies had been planning to build enough new nuclear power stations to push the nuclear share of electricity generation from 30% to 50%. It remains to be seen, where they would like them built and what the local population and politicians will have to say about that. I am sure the residents of existing reactor sites are watching very closely as 90,000 of their fellow citizens are being evicted from their homes, their livelihoods destroyed, their communities devastated and ripped apart, their regional agricultural products shunned on supermarket shelves.

How much money do you have to pay a quiet town or village somewhere on the Japanese coast to agree to accept another nuclear power station? I still believe there are some things you can not buy with money. I grew up in Eastern Bavaria, some 30 km from the village of Wackerdorf, which the Bavarian state government had selected as the site of a huge nuclear waste reprocessing plant in the 1980s. Local unemployment was high, so the government thought the promise of jobs and local tax revenue would win over the locals, but nevertheless resistance turned out to be fierce. For years the struggle tore the country apart. Families from grandmother to grandchild would walk to the construction site fence every weekend to protest. Then came April 26, 1986: After the Chernobyl disaster the plan became a lost cause. It was officially withdrawn 2 years later.

I don’t expect Japan will rush to shut down its existing nuclear power plants. However, it will become next to impossible to build new reactors (14 are planned by 2030). Especially the oldest reactors and the most seismically vulnerable ones will receive increasing scrutiny because of a nervous local (and national) population. This is not the end of nuclear power in Japan, but it may be the beginning of the end.