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