Fukushima had new vents, but system failed

The New York Times reports that the reactors at the Fukushima 1 nuclear power plant were equipped with an improved system for venting steam in an emergency, but it failed to work. Originally it had been reported that TEPCO did not retrofit the units that had been online since 1970s with the new designs introduced in the US in the 1980s. However, it appears to have done so between 1998 and 2001.

The problem was, the improved system relied on the same sources of electricity to operate valves as the cooling system, so when the cooling system stopped working as diesel generators failed and dangerous levels of steam pressures built up, the venting system designed to protect the containment wasn’t working properly either. Instead of one system protecting the public if the other system failed, both had a single point of failure, their dependence on diesel generators in the flooded turbine hall basements.

The executives did not give the order to begin venting until Saturday — more than 17 hours after the tsunami struck and six 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 a critical valve 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 from the accident’s early days.

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.

The venting system could also have been damaged by the earthquake.

According to the NYT, the new venting system bypasses filters that hold back much of the radioactivity.

When TEPCO was talking about venting the reactors, before the spectacular hydrogen explosions, they reassured the public that the release of gas would be “filtered”. Either they were misleading the public, or they were talking about the old venting system, which was suspected of not being able to cope with the pressure of an emergency release, which is the very reason the new system had been introduced.

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Fukushima cooling system switched off 10 minutes after quake

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.

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Fukushima reactor damaged before tsunami

According to a Kyodo news report, extremely high radiation levels were measured in unit 1 of Fukushima 1 nuclear power station the night after the quake, hours before radioactive steam was first vented from the containment vessel at 04:00 the next morning (2011-03-12).

Workers entered the No. 1 reactor building during the night to assess the damage only to hear their dosimeter alarms go off a few seconds later, sources at Tokyo Electric Power Co. said. Since they thought the building was filled with highly radioactive steam, the workers decided to evacuate.

Based on the dosimeter readings, the radiation level was about 300 millisieverts per hour, the source said, suggesting that a large amount of radioactive material had already been released from the core.

The source of the steam was believed to be the No. 1 reactor’s overheated pressure vessel.

But for that scenario to hold, the pressure in the reactor would have to have reached enormous levels — damaging the piping and other connected facilities. It should have taken much more time to fill the entire building with steam.

A source at Tepco admitted it was possible that key facilities were compromised before the tsunami.

(Japan Times, 2011-05-16)

According to the AREVA presentation by Dr Matthias Braun, the reactor isolation condensor in unit 1 stopped working at 16:36 on 2011-03-11, 55 minutes after the diesel generators had been knocked out by the tsunami. After that, pressure from boiling water built up in the reactor pressure vessel, from where it was vented into the containment via water held in the pressure suppression chamber.

It was only when pressure in the containment had built up far beyond its design limit that steam was first vented from it at 04:00 on Saturday, March 12. Radiation levels of hundreds of millisieverts per hour in the building hours before that suggest that steam from the pressure vessel or containment escaped hours before it was supposed to have come out.

Direct seismic damage to pipes, valves or other components from the quake at 14:46 is a plausible explanation for a premature radiation release while the containment was supposedly still serving its function of holding back radioactivity from the environment. The local intensity of the quake was either close to or in excess of the design specification the reactors had been built to. For example, maximum seismic acceleration at unit 3 was measured at 507 gal in East-West direction, versus the 441 gal it was designed for.

So how does it matter if parts of Fukushima prefecture became uninhabitable because of broken steam pipes or because of flooded diesel generators? Isn’t the outcome the same? It’s actually a very important difference: Nuclear power companies in Japan are now taking measures to evaluate tsunami risks in plants around the country. Units 4 and 5 of Hamaoka NPP in Shizuoka were recently shut down because they’re located right on top of a quake fault line, but the plan is to restart them around 2014, after tsunami defenses have been beefed up around them. The problem is, even if — thanks to raised sea walls — the diesel generators were not knocked out by sea water, but the violent shaking were to destroy some other vital part of reactor such as cooling pipes, the outcome could still be the same.

Unit 4 blown up by leak from adjacent unit 3

The Wall Street Journal reports that TEPCO now think the hydrogen explosion that destroyed the service floor above the spent fuel pool in unit 4 was caused by hydrogen leaking from the adjacent unit 3, which also exploded:

Tepco also released its analysis of a hydrogen explosion that occurred at unit No. 4, despite the fact that the unit was in maintenance and that nuclear fuel stored in the storage pool was largely intact.

According to Tepco, hyrogen produced in the overheating of the reactor core at unit 3 flowed through a gas-treatment line and entered unit No. 4 because of a breakdown of valves. Hydrogen leaked from ducts in the second, third and fourth floors of the reactor building at unit No. 4 and ignited a massive explosion.

Adjacent units 3 and 4 are connected, for example by sharing a venting tower for the release of radioactive gases, which is located between them.

Nuclear expert Arnie Gundersen believes it is plausible that the particularly violent explosion that destroyed the top of unit 3 may have involved a criticality event (i.e. a chain reaction) triggered by the well-publicised hydrogen explosion.

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TEPCO’s new cooling plan for Fukushima 1

After finding no water at all in the section of the reactor core of Fukushima 1 unit 1 that normally holds the fuel rods but instead finding 3000 tons of leaked water in the basement of the unit, TEPCO has abandoned its original plan and come up with a different plan to cool the reactor core.

Instead of trying to raise the water level inside the light bulb-shaped containment that surrounds the pressure vessel, which has not succeeded, it will spray water on to the reactor pressure vessel from above. The water will be pumped from the basement of the reactor, into which it appears to have been leaking and where it currently stands 4 m deep. It will be decontaminated to make it less radioactive before being recycled for cooling. A water decontamination unit built for TEPCO by French nuclear company AREVA is expected to arrive on Tuesday.

Reactors to be covered under tents

Meanwhile TEPCO has announced plans to cover units 1, 3 and 4 under tent-like polyester fabric sheets supported by a steel frame. The reactor building of unit 2 did not suffer a hydrogen explosion and therefore does not need covering. The tents may make it possible to filter radioactive steam and other emissions still rising from the reactor buildings whose cores are still beyond the boiling point and whose spent fuel pools are being cooled through water evaporation. Before the structure can be erected the vicinity of the blocks needs to be cleared of debris to make space for cranes and other construction equipment.

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Uphill struggle to cool Fukushima 1 unit 1

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.

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TEPCO sends workers into unit 1 of Fukushima I

TEPCO has taken the first step towards installing new heat exchangers in wrecked reactors at its Fukushima 1 nuclear power plant that have suffered core damage after a cooling system failure. The objective is to achieve “cold shutdown”, which means that cooling water in the reactor remains below 100 degrees C and the pressure is at atmospheric level. Currently the water inside units 1, 2 and 3 is still boiling.

During normal operation, sea water cools steam coming out of the power turbine that generates electricity. It condenses it back to water, which goes back into the reactor core to be turned into setam again. After a shutdown, electric pumps and a separate heat exchanger, known as the residual heat removal system (RHR), take over the core cooling function. TEPCO had been aiming to restore the RHR since about March 20, when it reestablished a grid connection into the turbine hall, but found that the pumps are no longer usable.

The new system will not cool the core directly, which after the fuel rods partially melted is highly contaminated, but instead will circulate water to the containment vessel that surrounds the core, cooling the central part of the reactor from the outside rather than from inside. To that purpose, water will be pumped from a pipe connected to the containment to a heat exchanger, which passes the heat to an external water cycle that will be cooled using air. The original cooling system depended on sea water for condensing steam in the inner cycle, but the sea water intake system was flooded and destroyed by the tsunami, along with most of the diesel generators that provided power to all emergency pumps.

Before any work can be performed inside the reactor building, which no human had entered since the hydrogen explosions around March 12-15, the level of radiation there has to be brought down considerably. To that purpose a pair of hose pipes has been laid through the airlock and hooked up to a filter. Until at least May 16 air will be pumped through a filter set up outside the building. US-made robots sent into units 1 and 3 measured radiation levels of up to 49 mSv/h in unit 1 and up to 57 mSv/h in unit 3 on April 18. The dose inside the airlock of unit 1 was 270 mSv/h, in unit 3 it was 170 mSv/h. The typical background radiation dose for a civilian is 2-3 mSv per year. TEPCO hopes to be able to reduce radiation inside the building by a factor of 20.

Work will start with unit 1, because unlike unit 2 and 3 its containment vessel is assumed to be intact. Unit 2 and 3 are at atmospheric pressure. If they can’t maintain steam pressure they may also not hold water once they’re flooded. Also, because of a damaged torus (pressure suppression chamber), a lot more radioactivity has been leaking from unit 2 than from unit 1 and 3. That makes unit 1 by far the best candidate for trying out ideas on how to regain control and limiting further damage to the environment. However, there is no guarantee that, even if everything were to go perfectly on unit 1, any of the lessons learnt could be applied to the other units, since they may be too badly damaged to be repaired this way. The worst “patient” probably will be unit 2, which seemed to be leaking the most water. It isn’t clear how TEPCO is planning to deal with the leaking containments or the damaged torus. One approach might be to pump leaking water from the basement of the building back into the cooling system. TEPCO estimates 6 to 9 months for its “cold shutdown” plan.

Fallout prediction system went blind

Meanwhile it was reported that two systems meant to collect data from nuclear reactors and make predictions for nuclear fallout in Japan have failed to perform as intended. The Emergency Response Support System (ERSS) was supposed feed data from reactors into the System for Prediction of Envionmental Emergency Dose Information (SPEEDI). Both systems combined cost JPY 28 billion (about US$350 million), but ERSS failed along with everything else in Fukushima 1 when power was lost. It has yet to gather any data on the damaged units.

Radiation found on ocean floor

Caesium levels about 1000 times higher than normal have been found on the ocean floor 15-20 km from the Fukushima power plant, NHK reports:

The plant’s operator, Tokyo Electric Power Company, conducted its first contamination analysis of the seabed near the plant using samples from 2 points 20 to 30 meters deep on Friday.

Samples collected about 15 kilometers north of the plant contained 1,400 becquerels of cesium-137 per kilogram and 1,300 becquerels of cesium-134.

Samples taken around 20 kilometers south of the plant contained 1,200 becquerels each of cesium-137 and cesium-134 per kilogram.

The samples from the 2 points were also found to be contaminated with iodine-131.

TEPCO says it’s difficult to evaluate the readings as there are no official limits for these substances, but it will continue monitoring the radiation levels and their impact on seafood.

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Fukushima watch 2011-05-01

The city of Koriyama in Fukushima prefecture, where top soil was removed from schools to reduce radiation exposure to children recently, is not only 50 km west of the Fukushima 1 nuclear power plant, it also lies in the heart of the central valley where most of the agricultural products of the otherwise mountainous region are grown.

Fukushima has already lost 90% of its fishing industry due to the tsunami. How much income will its farmers lose? Consumers can not be sure they will be safe if they eat crops produced in the region unless systematic testing takes place, but if high levels re found, farmers will demand compensation. How much will TEPCO have to pay and how much the government? In the 1950s nuclear power was sold to the public on the promise of being able to deliver electricity “too cheap to meter”. Now it could turn out as too expensive to measure.

Government advisor resigns in protest

Mr. Toshiso Kosako, a radiation export at the University of Tokyo has resigned as an advisor of the Japanese government, citing his opposition to raising permissible radiation exposure of school children. On April 19 the Ministry of Education and Scienced had announced a limit of 3.8 μSv/h (microsievert per hour), which is comparable to international maximum exposure rates of people working in nuclear power plants. Actually, 3.8 μSv/h times 24 hours times 365 days equals 33 mSv (millisievert), whereas in other countries the maximum permitted extra exposure on the job is 20 mSv, which comes on top of a natural radiation exposure of 2-3 mSv.

Radiation exposure of workers

Two workers at the Fukushima 1 nuclear power plant, employed by a sub-contractor of the power company, have been exposed to close to their maximum legal radiation dose:

The two workers have been exposed to 240.8 millisieverts and 226.6 millisieverts of radiation, respectively, when internal exposure is taken into account, among 21 workers exposed to over 100 millisieverts of external radiation since the crisis erupted following the magnitude 9 quake and tsunami, it said.

Under Japanese law, the Ministry of Health, Labor and Welfare has limited by an ordinance radiation exposure of each nuclear plant worker at 100 millisieverts a year in an emergency situation, but raised the limit to 250 millisieverts to cope with the Fukushima crisis on March 15.

The two were hospitalized for possible radiation burns to their feet after standing in water that contained radioactive materials 10,000 times the normal level while laying a cable underground at the troubled plant on March 24, the utility known as TEPCO said.

The limit for non-emergency work is 100 millisieverts over five years with no more than 50 per single year. These two workers received over 200 in a single month. The Ministry of Health and Welfare is planning to scrap the 50 mSv per year limit, leaving only the 100 mSv per five year limit in place for non-emergency workers. This is meant to make it possible to send workers from other reactors to Fukushima for cleanup work and still make it legal for them to absorb further radiation at regular work once they return.

Emergency power

TEPCO tested it’s mobile power generators at Higashidori nuclear power plant in Aomori prefecture on April 20 but revealed that the truck-mounted diesels had fuel for only 2 1/2 hours. If they have to provide power for longer they will need frequent refueling. In Fukushima 1 when the backup diesels failed it took about 9 days until grid power was restored inside the turbine halls to run electric pumps.

Radioactive contamination in unit 4 spent fuel pool

TEPCO has released updated numbers for radioactive contamination of the pool water in the spent fuel element pool of unit 4 of Fukushima 1. Here are the figures for 2011-04-29 (number for sample taken on 2011-04-13 in brackets):

  • Caesium 137 (half life: 30 years): 54,000 (was: 93,000 Bq/l)
  • Caesium 134 (half life: 2 years): 49,000 (was: 88,000 Bq/l)
  • Iodine 131 (half life: 8 days): 27,000 (was: 220,000 Bq/l)

Because 16 days have elapsed since the previous analysis, levels for iodine-131 should have halved twice (to about 55,000 Bq/l) while levels for both caesium isotopes should be virtually unchanged. Instead both caesium values dropped by more than 40% while Iodine is about 50% below the expected value. Where did the rest of the caesium and iodine go? There are several possible explanations.

Perhaps one of the analysises was simply wrong. That would not be out of the question, as TEPCO has had trouble with its figures before. More likely though, perhaps the water level was much lower on the first sample, meaning the current sample is diluted by a significant amount of added water (+72% water would explain the figures). That is to be hoped, because if the water level was in fact the same and the figures were correct then a lot of the pool water radioactivity would have had to escape into the atmosphere with steam or leaked with water to some other part of the reactor building.

Arnold Gundersen (YouTube, starting from 4:28) raised an interesting point about the isotope testing in the unit 4 pool: There’s too much iodine-131 for this to be from leaking fuel rods. The unit 4 reactor was shut down for maintenance on 2010-11-29, about 5 months ago. 151 days from 2010-11-29 to 2011-04-29 is close to 19 half lives for iodine-131, meaning only 2 millionths of the original iodine-131 levels should be left in the freshest nuclear waste in that pool, even less in older fuel rods. If 220,000 Bq/l came from atmospheric contamination (e.g. fallout from overheated unit 3 which is right next to it), then that is 220,000,000 Bq/m3 of water, left over after 4 half lives, so the original amount would have been 3,520,000,000. The pool is about 13m deep. To get it that radioactive, some 45,760,000,000 Bq/m2 had to land on the pool, which is a staggering amount.

Tepco revises core damage estimates

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.

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Fukushima watch 2011-04-27

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”.

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From Chernobyl to Fukushima

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.

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