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.

Tepco is talking about plans to cool units 1 and 3 of Fukushima 1 via the containment vessel. The water level inside the usually dry portions of the containment (called the dry well, D/W) will be raised high enough to flood the exterior of the reactor pressure vessel (RPV) up to the top of the fuel rods. Normally only the inside of the suppression chamber (S/C, a circular pipe running around the bottom of the RPV) is filled with water. Tepco thinks the reinforced concrete of the dry well is sturdy enough to take the weight of the water and is now seeking the approval of the nuclear regulators.

The US NRC has previously voiced concerns about the safety of the containment in earthquakes when filled with large amounts of water:

When flooding containment, consider the implications of water weight on seismic capabilities of containment.

Unit 2 is not included in the plan because its suppression chamber appears to have suffered damage in an explosion on March 15 and has probably been leaking since then. Most of the highly radioactive water in a trench under the turbine hall is assumed to have leaked there from unit 2. It is not clear if or how those leaks can be plugged.

Tepco has requested French nuclear company AREVA (which is also its supplier for nuclear fuel) to set up a plant for decontaminating radioactive water at the reactor site. This would make it less risky to move the water to the nuclear waste plant in Rokkasho village, Aomori prefecture or for recycling it as cooling water.

Yesterday I experienced my first earthquake since returning to Tokyo. It was a M6.1 quake centered under Chiba, some 60 km from Tokyo. During the night I was woken up by a M5.6 quake that hit Fukushima.

UPDATE 1: Tepco will be short of 8500 MW this summer

Tepco’s maximum supply capacity today is 38.4 GW (one Gigawatt is 1,000 Megawatt or 1,000,000 Kilowatt). The forecast maximum demand today is 34 GW. Hence there will be no rolling blackouts today. For comparison, last year’s peak demand around this time was about 44 GW.

Tepco expects power demand to stay moderate in the near future, due to mild temperatures and the Golden Week holidays in May, rising to about 38 GW by the end of May. By then it hopes to have restored enough supply capacity for a celing of 39 to 42 GW. Tepco expects to have 46.5 GW online this summer versus a demand peak of 55 GW. Last year’s exceptionally hot summer even saw a peak load of 59.9 GW.

Tepco’s customers will largely have to give up on air-conditioning this summer or periodically lose all electric power. Perhaps after Cool Biz it is time to go one better and recommend Bermuda shorts.

UPDATE 2: Rising pressure in unit

Pressure in the reactor pressure vessel (RPV) of Fukushima 1 unit 1 has gradually increasing since March 24, when it was below 0.5 MPa (about 5 bar). On April 19 according to a NISA report it was at 1.141 MPa (about 11 bar) according to sensor B, while sensor A indicates 0.524 MPa. It is not clear how much the nitrogen being injected is responsible for the rise in pressure and how much may be due to a buildup of hydrogen, but the pressure buildup started two weeks before the nitrogen injection, which was started in response to the rising pressure.

NISA status reports:

• 2011-03-18 14:00
• 2011-03-19 12:00
• 2011-03-21 17:00
• 2011-03-22 11:00
• 2011-03-23 06:00
• 2011-03-23 12:00
• 2011-03-23 18:00
• 2011-03-24 05:00
• 2011-03-24 18:05
• 2011-03-25 18:00
• 2011-03-26 11:00
• 2011-03-27 06:00
• 2011-03-27 14:00
• 2011-03-28 06:00
• 2011-03-28 14:00
• 2011-03-29 12:00
• 2011-03-30 14:00
• 2011-04-02 06:00
• 2011-04-03 06:00
• 2011-04-03 13:00
• 2011-04-05 07:00
• 2011-04-07 06:00
• 2011-04-10 12:00
• 2011-04-13 14:00
• 2011-04-16 07:00
• 2011-04-19 07:00

Update 3: Spent fuel pools comparison – unit 2 and unit 4

A week ago I wrote here about the results of radioisotope testing on water in the spent fuel pool of unit 4 of Fukushima 1.

There results for the unit 4 spent fuel pool were:

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

For comparison, here are the figures from an analysis of water in the skimmer surge tank of unit 2, taken on April 16. The skimmer surge tank is an overflow container adjacent to the spent fuel pool into which water can splash or overflow from the main pool. Tepco tested it to get ideas for installing a heat exchanger and decontamination unit for the pool.

Here are the unit 2 skimmer surge tank figures:

• Caesium 137: 150,000,000 Bq/l (half life: 30 years)
• Caesium 134: 160,000,000 Bq/l (half life: 2 years)
• Caesium 136: 4,000,000 Bq/l (half life: 13 days)
• Iodine 131: 4,100,000 Bq/l (half life: 8 days)

The small container of sampled water radiated at 3.5 mSv/h at the surface, which is roughly 100,000 times the natural background radiation level.

The caesium radioactivity of the skimmer surge tank water in unit 2 is about 1000 times higher than water in the spent fuel pool of unit 4. However the shortlived iodine is only about 20 times more active. This could suggest the radioactivity is from damaged fuel rods in the spent fuel pool and not the adjacent damaged unit 2 reactor core. Spent fuel was unloaded into the unit 4 spent fuel pool more recently than into the unit 2 spent fuel pool, hence it will have a higher Iodine-131 to caesium ratio. The higher caesium activity in unit 2 suggests more damage to fuel rods, but in the case of iodine this is partly counterbalanced by a longer nuclear decay since the last unload from the core into the pool compared to unit 4.

After a bit over a month in Germany we’re back in Tokyo again. This doesn’t mean I think it’s safe here: Bad things could still happen at Fukushima 1 or elsewhere in Japan. Radioactivity may still be leaking for months from the wrecked nuclear power plant. However we have two kids enrolled at schools in Tokyo whose continued education was at risk if we stayed away longer. Meanwhile unlike up in Fukushima, background radiation levels in Tokyo are supposed to be no higher than in Bavaria, if you trust published figures. As long as no more hydrogen explosions happen, some form cooling is maintained for the reactor cores for the next couple of years and the spent fuel pools get topped up regularly it seems likely that the worst is behind us. The situation at Fukushima 1 will remain severe for a long time, but everywhere else it may stabilize.

There is still a lot of concern about contamination of food from areas closer to the nuclear ruins, or anywhere where rain fell soon after Tepco was forced to vent the uncooled reactors when it couldn’t get cooling pumps restarted. Background radiation spiked on the first weekend, then dropped again around the Kanto area, then picked up again when rains fell during winds from Fukushima around March 21-24. Radiation in rain and dust (in Bq/m2) peaked during those days in the Kanto area:

Until consumers are reassured via broad and thorough testing of food, I think a lot of buyers will avoid food from the whole region (Fukushima and adjacent prefectures), even if radiation in food near or exceeding legal limits was measured mostly inside the evacuation zone (where everybody is forced to leave now) and an area Northwest of it that may also get evacuacted. The government has dragged its feet too much to be able to maintain confidence, for example in Iitate, where both Greenpeace and the IAEA (as unlikely a couple as any) drew attention to radioactive contamination levels warranting evacuation before the government finally asked inhabitants to leave within one month.

Bottled water seems in demand in Tokyo even though, according to the Tokyo water board, caesium-131 and other radioisotopes are below detection levels now.

Nuclear, wind and sockets

Businesses and households are trying as hard as they can to save electricity, after Tepco lost 15 GW of generating capacity, forcing it to impose rolling blackouts. It hopes to restore 5 GW of capacity before the summer by installing gas turbines at existing thermal power plant sites. The situation reminds me of a tongue in cheek advertising slogan used by the nuclear industry in Germany in the 1970s: “Why nuclear power? My electricity comes from the socket!” The nuclear lobby was trying to paint its opponents as ignorant people who had no idea how to secure supplies. It is this arrogant we-know-it-all attitude that has led to disaster and consequently to a disruptive power shortage. “Why alternative energy? My electricity is supplied by Tepco!” is how people were led to think. Millions of sockets in Eastern Japan have been without power because of this, not to speak of tens of thousands evacuated from a nuclear waste land.

Japan has ample potential for geothermal and wind power. It has thousands of km of coastline that could be used for offshore wind farms, yet in 2010 a mere 2.3 GW of wind power was installed, compared to 27.2 GW in Germany. Four of the German states (Sachsen-Anhalt, Mecklenburg-Vorpommern, Schleswig-Holstein, Brandenburg) already get between 47 and 38 percent of their annual electricity production from wind power, far bigger than Tepco’s pre-Fukushima share of nuclear power.

Those junior nuclear engineers

If you thought things were getting back to normal here, think again: In a surprise move, the Japanese Ministry of Education has set a limit of 3.8 microsievert/hour for children in kindergartens, elementary schools and junior high schools. Multiplied by 8 hours a day, 6 days a week for one year it is 10 millisievert per year. This is in fact half the annual limit of radiation exposure that applies to workers in the nuclear industry in Germany, even though children are more sensitive to radiation damage than adults. No, this is not a late April Fool’s joke. Just like nuclear workers, teachers in Fukushima will be issued portable dosimeters to be able to verify that kids stay under the limit. 3.8 microsieverts per hour is about 40 times the current background radiation level in Tokyo. I must admit, that is one of the weirdest developments I have come across in the whole Fukushima disaster so far.

See also:

• Reading of environmental radioactivity level by prefecture
• Offshore windmills weather crisis (Japan Times, 2011-03-12)

The New York Times reports that Tepco has announced a timeline for securing the wrecked Fukushima 1 nuclear power plant:

The first part of the plan would take about three months and include installing a cooling system to lower the temperature in the reactors and spent fuel pools, as well as reducing radiation in the surrounding area, said Tsunehisa Katsumata, the chairman of Tokyo Electric.

The second part, which would take an additional six months, would include more pumping of water, the introduction of a new heat removal system and reducing the amount of contaminated water. The wreckage from three of the four most severely damaged reactor buildings would then be removed and the reactors inside would be covered.

While this vague outline is the most concrete recovery plan to date, it is of course subject to what is physically possible. Intentions, even with an approximate time scale, do not in any way prove that the plan is workable.

Let us remember that on the weekend following the quake and tsunami, Tepco moved mobile diesel generators onto the reactor site hoping to restart the cooling pumps using mobile emergency power, but found that the power connections were incompatible and the electrical switches had been destroyed by sea water. That plan had to be abandoned.

Then Tepco installed a fresh external power line and a new switch panel to restart pumps with restored grid power, but no further progress has been made in getting the original cooling pumps going again. That plan has been quietly abandoned and Tepco is still using temporary electrical pumps that inject fresh water from a nearby dam. There is no closed circulation system in place. Unlike the purified fresh water normally used for cooling, the dam water can contain small amounts of calcium that can clog pipes and valves.

Tepco no longer seems to publish how much water is being injected per hour into the three units. It also seems to have stopped publishing radiation figures for the drywell and suppression chamber, perhaps to avoid questions about a sudden major spike of radiation in unit 1 after the April 7 quake.

No explanation is being given about what is happening to all the injected water, how much re-condenses from steam into the reactor drywell (primary containment) or suppression chamber or leaks in liquid form into these areas and how much may be leaking outside the reactor containment or is released as steam (the containments of unit 2 and 3 are at atmospheric pressure). The NRC suspects that the circulation pump seals at units 1 through 3 are damaged, allowing water to leak. Probably Tepco doesn’t tell us where the water goes because nobody (including Tepco) really has any data.

It’s easy to say it will take three months to install a new cooling system, but if even one of the units still leaks large amounts of water too radioactive to go near it, that’s going to be a tough plan to implement. It’s not enough to get water into the reactor and to cool water or steam coming out of the reactor with a heat exchanger, all of this also has to happen without massive leaks that continually carry out radioactivity from damaged fuel rods. Many more afterquakes expected in the region for the rest of the year, some of them at magnitude 7 and greater, will complicate the recovery.

The recovery plan for Fukushima 1 will remain an uphill struggle for many months to come.

See also:

• Tepco presentation (2011-04-18)
• Plan Set for Crippled Japanese Nuclear Plant (New York Times, 2011-04-17)

Tepco has released the results of isotope testing in the water of the spent fuel pool of unit 4, which holds 1331 nuclear fuel assemblies:

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

The figures suggests the fuel assemblies are damaged, but Tepco believes the damage is minor and think some of it may have been caused by fragments of the building collapsing into the pool after a hydrogen explosion and fire.

The pressure reading of one of the sensors in the reactor pressure vessel (RPV) of unit 1 has been steadily rising since March 22 even though the temperature reading has been fluctuating (rising and falling) over the same period. This suggests a buildup of hydrogen from a reaction between steam and zirconium or a buildup of hydrogen and oxygen from radiolysis (breakdown of water into its elements by gamma radiation). Unit one is supposed to have suffered the most severe core damage of the three units in operation when the quake struck.

The RPV is designed for a maximum pressure of 8.7 MPa. The pressure reading on April 15 was about 0.953 MPa. If too much hydrogen accumulates, the operators may be forced to vent the reactor pressure vessel and/or the reactor containment. To minimize the risks of a renewed hydrogen explosion, nitrogen has been pumped into the RPV since April 6, but venting from the containment could also release considerable amounts of radioactivity into the atmosphere, as during he first days of the disaster.

Mobile power generators and fire fighting equipment is being relocated to a higher location to be able to cope better with the possibility of a tsunami triggered by another strong aftershock.

Tepco reports that when a concrete pump truck was used to take a water sample from the spent fuel pool of Fukushima 1 unit 4 on 2011-04-12, the water temperature there was 90C, i.e. close to boiling.

The water level in the 13 meter deep pool was 5 meter below normal and only 2 meters above the upper end of the fuel rod assemblies. 195 tons of water were added, raising the water level about 1 meter. No results have been announced yet for the analysis of the water sample.

Pumping of highly radioactive water from the basement of unit 2 was interrupted for 4 hours yesterday for a leak check at the condenser. No leaks were detected. Pumping was completed later that day. The spent fuel pool of unit 2 was topped up using an electric pump for 1 1/2 hours.

According to reports in Japanese media, the Japanese government is discussing setting up a backup site for itself at Itami airport in Osaka in case of a major disaster striking Tokyo.

Since yesterday evening Tepco (or its subcontractors or sub-subcontractors) has been pumping highly radioactive water from the pipe and cable ducts under the turbine hall of unit 2 to the condenser storage tank. The presence of this water has been a major obstacle to reactivating cooling pumps or installing a new cooling system. The water has been in contact with damaged fuel rods and is loaded with fission products, with radiation levels exceeding 1000 mSv/hour.

A crack in a concrete wall near the cooling water intake building for unit 2 is being blocked with a steel sheet to stop radioactive water from leaking into the sea.

Yesterday a water sample was taken from the spent fuel pool of unit 4, which will be quantitatively analyzed for various radioactive isotopes to gain information about the state of the more than 1300 fuel rod assemblies in that pool. The spent fuel pool of unit 4 holds the biggest amount of long lived radioactive isotopes such as caesium 137 of all reactor cores and spent fuel pools at Fukushima 1. It has gaping holes in its walls and its roof was blown off in a hydrogen blast early in the nuclear disaster. When the cooling system failed due to a lack of external power after the March 11 quake, the fuel rods overheated and caught fire (it was that fire that prompted me and my family to head out of Japan the week after the quake…).

The last of five Putzmeister concrete pumps took off from Stuttgart/Germany for Japan in a Russian transport plane on Tuesday night. The remote controlled pumps have highly mobile arms 58 to 70 meters in length in 5 or 6 segments that are typically used for pouring concrete in dam or bridge construction, but can also be used for fire fighting. Between May and November 1986, 11 of the pump trucks were used to entomb the stricken unit 4 reactor in Chernobyl.

See also:

• Radiation and earthquake graphs for Japan

NISA, the Japanese Nuclear and Industrial Safety Agency has finally raised the Fukushima accident to level 7 (the highest) on the INES scale. Previously, the Chernobyl accident was the only one worldwide ever rated at that level. At the same time NISA stated that the amount of radioactivity released so far was about 10% of the amount released in Chernobyl. Most of the leaked radioactivity is assumed to have come from unit 2, which leaked highly radioactive water into its basement and from there into the sea.

The rating probably does not indicate a recent surge of danger to the public, but reflects a more realistic assessment of the existing data than the previous rating of 5, which would have put it on a level with the Three Mile Island accident in 1979: Only one reactor block suffered a melt down in TMI and the containment remained intact, versus 3 damaged reactor blocks and 4 overheating spent fuel pools in Fukushima and a damaged pressure vessel and containment in at least one unit. According to Gesellschaft für Anlage- und Reaktorsicherheit the revised rating primarily reflects the fact that a lot more than 10,000 terabecquerel (TBq) of Iodine-131 have been released into the atmosphere so far. The release of Iodine-131 and Cesium-137 lies somewhere between 370,000 TBq and 630,000 TBq.

US NRC assessment of Fukushima 1

The March 26 NRC assessment of the situation in the six units at Fukushima one makes for interesting reading. It suggests that water flow inside the base of the reactor pressure vessels is severely restricted by melted fuel and salt buildup. Water injected into unit 1 apparently does not contain any boron to guard against criticality. The seals in the recirculation pumps of units 1, 2 and 3 are assumed to have failed, effectively preventing the reactor cores from being filled high enough to cover all fuel, even if sufficient water could get past the melted fuel and salt crust: If the water rises any higher, it will simply leak out of the pumps. The primary containments of units 2 and 3 are assumed to be damaged. Spent fuel may have been ejected “up to 1 mile” from the spent fuel pools of unit 4 or unit 3 in a hydrogen explosion

See also:

• US NRC assessment of Fukushima 1 (2011-03-26)

NHK reports that Tepco is using remote-controlled bulldozers and power shovels for clearing rubble around the wrecked reactor. While it’s good news that Tepco finally can perform some cleanup work without putting workers at risk, the bad news is how necessary such equipment may be. The New York Times reported on April 5 about a confidential document prepared by the US Nuclear Regulatory Commission:

The document also suggests that fragments or particles of nuclear fuel from spent fuel pools above the reactors were blown “up to one mile from the units,” and that pieces of highly radioactive material fell between two units and had to be “bulldozed over,” presumably to protect workers at the site. The ejection of nuclear material, which may have occurred during one of the earlier hydrogen explosions, may indicate more extensive damage to the extremely radioactive pools than previously disclosed.

There was more nuclear fuel in the spent fuel pools of units 1-4 than there is in the reactor cores of these units.

Today I read the article “What Caused the High Cl-38 Radioactivity in the Fukushima Daiichi Reactor #1?” by F. Dalnoki-Veress, which discusses the possibility that a chain reaction could have happened in Fukushima 1 unit 1 even after the reactor had been shut down, based on the presence of chlorine isotope Cl-38.

Another quake knocks out pumps at Fukushima 1

Today at 17:16 JST a magnitude 7.1 quake with its epicenter close to the Fukushima reactor sites caused another loss of grid power, like during 9.0 quake one month earlier. Since the backup diesel generators had been destroyed by the tsunami on 2011-03-11 the electric pumps used for water injection for core cooling in the wrecked reactors stopped immediately this time. They didn’t start up again until 50 minutes later, during which time no water was being pumped into the reactor.

This raises the question of what would have happened if the reactor had stayed off the grid for days. After the big quake it took about two weeks before electric pumps could be used for cooling agin. Are there no mobile diesel generators available on site yet or if there are, is there no way to connect them to the temporary electric pumps used for water injection?

Tsunami was 15 meter high at Fukushima 1

More information on the tsunami on 2011-03-11: According to Tepco the devastating wave that hit the power plant was 15 meter high, flooding the surroundings of the reactors including outside the turbine halls up to a height of 5 meters.

Diesel problems at Higashidori nuclear power plant

After the 7.1 quake on 2011-04-07 at 23:23 JST that knocked out the grid supply to a shut down reactor at Higashidori, one of the emergency diesels was stopped due to an oil leak at 2011-04-08 at 14:06 JST. By that time only one of the three grid connections had been restored (at 03:30 JST). The other two connections came back online at 14:59 JST. The emergency diesel was repaired by the following morning, 07:00 JST.

What’s the recovery plan?

Currently 6000 liters of fresh water per hour are being injected into unit 1. Units 2 and 3 which contain more fuel elements are receiving 7000 liters per hour. There is no working cooling cycle. All this water must either be released somewhere as steam or leaking as liquid or building up inside the containment. If it escapes as steam or leaks out through cracks in the containment or from pipes it will contain highly radioactive fission products from the damaged fuel rods. While you can read about how much water is being injected, nobody seems to want to talk about where and how this water is coming out again and in what quantities.

As mentioned before in this blog, Sankei Shimbun has reported about a plan to build new cooling circuits for units 1, 2 and 3 using new pumps and heat exchanges located far enough away from the reactor blocks and their spent fuel pools to not expose operators to high levels of radiation. Tepco expects this work to take about a month, but it probably hinges on first clearing highly radioactive water from the basement of the turbine hall and associated tunnels. Considering the ongoing major afterquakes and the high levels of radioactivity in the reactor core to be cooled, this project is going to be extremely challenging.

There has been talk about covering the reactors in sheets with filters for emissions, which could happen around June. One would expect radioactivity levels under those sheets to be very high, considering that it will probably trap steam from the spent fuel pools, some of which hold damaged spent fuel rods. Basically, most of the radioactivity now being carried out over the Pacific by winds (or spread across Northeast Japan or the Kanto area, depending on the direction of the winds) should then stay behind, right outside the building. I suspect the sheets or tents won’t be put up until there is some kind of closed circuit cooling system in place, to minimize water vapor trapped under the sheets.

Toshiba has submitted a plan for decommissioning and cleaning up the reactors. The likely price tab will run into billions of dollars, considering experience from Three Mile Island, Chernobyl and the Vermont Yankee reactor.

See also:

• Japanese Workers Braved Radiation for a Temp Job (New York Times)
• “End to crisis is years, fortune away