My Terra-P dosimeter (MKS-05) by Ecotest

Yesterday my geiger counter arrived here in Japan. It is a Terra-P dosimeter made by Ecotest, a company based in L’viv/Ukraine, about 300 km west of Chernobyl.

I bought the device on eBay from a supplier in Australia for US$399 including shipping. It arrived within 9 days and seems to work well. Although the buttons on my Terra-P are labelled in Cyrillic (either Russian or Ukrainian) and so is the manual, English manuals for it are easy to find online, so that’s not really a problem.

The Terra-P is a consumer grade dosimeter, so it’s not quite as versatile or as precise as professional devices costing $1000 or more, but it covers the basics very well. Its power source are two AAA-batteries, accessible via a lid at the back of the unit, which are easy to replace. It measures gamma rays and is suitable for checking for caesium contamination.

The user interface consists of an LCD, two buttons and speaker. One push of the right hand button (“режим” = mode) switches the dosimeter on and puts it into the measuring mode. The display switches to a microsievert per hour (µSv/h) readout. For the first 70 seconds the resulting number blinks, as it averages the dose over that period and the number gradually becomes more meaningful. After the initial sampling period, the number displayed will always be the average of the last 70 seconds, so you can move it from location to location and will get a decent result provided you wait for about a minute.

After several minutes the device enters power save mode, in which it continues counting radioactive decays, but the LCD is off and less power is used. To turn it off completely when it’s active, push the mode button once more and then push and hold it for four seconds, until the LCD blanks.

The Terra-P also has a user-settable alarm threshold (default: 0.30 µSv/h) and a clock mode. The built-in speaker usually makes one click for every gamma photon detected and sounds an alarm if the radiation exceeds the alarm threshold.

Checking my home after unpacking the device, I found the radiation level was a little higher than the 0.055 µSv/h reported for Tokyo by the local government, but still somewhat lower than the 0.10 µSv/h in my home town in Germany. On the other hand, I was relieved to see the wooden deck outside our living room was no more radioactive than inside the house. As expected, the gutters at the edge of the road, where rain water drains into the sewers, was more radioactive, with about 0.20 µSv/h, which is still far from alarming.

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Radiation map of Japan

The Japanese government has published online map data about radiation levels in Eastern Japan. You can zoom in and out, scroll around and select data from:

  • Background radiation in microsievert per hour
  • Contamination by caesium 134 and 137 combined (Cs-134+Cs-137) in becquerel per square metre
  • Contamination by caesium 134 (Cs-134) in becquerel per square metre
  • Contamination by caesium 137 (Cs-137) in becquerel per square metre

The data was collected via helicopter flights carrying instruments that detect gamma radiation of different energy spectrums, allowing a breakdown by isotopes causing it.

There are the following data sets:

  • April 29
  • May 26
  • July 2
  • Miyagi prefecture, July 2
  • Tochigi prefecture, July 16
  • Ibaraki prefecture, August 2
  • Chiba and Saitama prefecture, September 12
  • Tokyo and Kanagawa prefecture, September 18

Click on this link:

either the online maps or download PDF files of the maps and click on “同意する” (“I do agree”, the left button) to get access.

The government is planning to extend the radiation survey to the whole of Japan, not just within about 250 km of the wrecked reactors as is currently the case.

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Tepco’s unfiltered vent path

On August 13, 2011 I wrote about TEPCO not adding filters to their reactors even decades after Sweden did. Since the, on August 16, 2011, Tepco defended not having any kind of filters in the so-called “hardended vent” path between the containment and the exhaust stack:

Corrections and Clarification of a news report program, “ETV Special” by NHK, broadcasted on August 14

August 16, 2011
Tokyo Electric Power Company
NHK TV program regarding Fukushima Daiichi Nuclear Power Station reported contents that are incorrect and could cause misunderstandings. We hereby provide facts below.
3. Claim on the PCV ventilation has no filtration
In the program, it was mentioned several times that there were no filters in the primary containment vessel ventilation line. However, boiling water reactors that we operate use “wetwell vent”, which has scrubbing effect to mitigate emission of radioactive materials at the comparative level to the filters. That is to say, in principle, our venting procedure uses the water in the suppression chamber as filteration and we have prepared and added the necessary equipment and procedures for accident management measures.

Tepco’s reactors have a “Standby Gas Treatment System” for filtering gases released into the atmosphere. This system is claimed to be at least 97% effective in unit 1 and at lest 99.9% effective in units 2 and 3. However, it can’t be used for venting high pressure gas from the drywell or wetwell (the containment) in emergencies. In the above press release Tepco defends its decision by claiming that the water pool in the suppression chamber (wetwell) is as effective as some other kind of filter system that it could have had installed when adding the hardended vent path in 1999-2001.

This claim is disingenuous. The FILTRA system installed at the Swedish Barsebäck nuclear power station was in addition to any filtration provided by the wetwell pool, not in place of it. Barsebäck had boiling water reactors like in Fukushima (the plant has since been decommissioned).

Furthermore, it’s not clear how effective the filter effect of the wetwell on its own really is. A US report from 1988 entitled “Filtered venting considerations in the United States” writes:

Within the United States, the only commercial reactors approved to vent during severe accidents are boiling water reactors having water suppression pools. The pool serves to scrub and retain radionuclides. The degree of effectiveness has generated some debate within the technical comnunity. The decontaminatlon factor (DF) associated with suppression pool scrubbing can range anywhere from one (no scrubbing) to well over 1000 (99.9 % effective). This wide band is a function of the accident scenario and composition of the fission products, the pathway to the pool (through spargers, downcomers, etc.), and the conditions in the pool itself. Conservative DF values of five for scrubbing in MARK I suppression pools, and 10 for MARK II and MARK III suppression pools have recently been proposed for licensing review purposes. These factors, of course, exclude considerations of noble gases, which would not be retained in the pool.

The decontamination factor of 5 for the Mark I containment (as used in units 1 through 5 of Fukushima Daiichi) means that 80% of the radioactive substances (excluding noble gases) is retained, while 20% is released. The FILTRA system installed at 10 Swedish nuclear power plants and one in Switzerland is designed to ensure that in a severe accident 99.9% of core inventory is retained in the containment or the filters. The difference between releasing up to 20% versus 0.1% is huge, it means up to 200 times more radioactivity is released in the system defended by Tepco versus the enhanced system used in Europe and commercially available worldwide.

Fukushima: Keep out indefinitely

The government of Japanese prime minister Kan is finally telling evacuees from within the 20 km exclusion zone around the wrecked Fukushima-I nuclear power plant that they won’t be able to return any time soon.

For months the official line had been that residents could not return until the reactors had achieved “cold shutdown”, which normally means that the chain reaction had stopped and the decay heat in the core had subsided enough for the coolant to be below 100 degrees C (below boiling point). This line ignores the fact that three of the reactors no longer have a cooling system in the usual sense and more importantly, that the area around the power station was severely contaminated by radioactive fallout from the broken reactors.

That reality has finally been accepted, explains the New York Times:

The government was apparently forced to alter its plans after the survey by the Ministry of Science and Education, released over the weekend, which showed even higher than expected radiation levels within the 12-mile evacuation zone around the plant. The most heavily contaminated spot was in the town of Okuma about two miles southwest of the plant, where someone living for a year would be exposed to 508.1 millisieverts of radiation — far above the level of 20 millisieverts per year that the government considers safe.

The survey found radiation above the safe level at three dozen spots up to 12 miles from the plant. That has called into question how many residents will actually be able to return to their homes even after the plant is stabilized.

Many of the evacuees had been led to believe that most of them might be able to return to their homes some time after the turn of the year, based on Tepco’s promises of establishing some sort of a cooling system and generally stabilizing the situation at the plant. Probably over 100,000 people will have to rebuild their lives from scratch elsewhere now, with no prospect of returning for decades, just like in the exclusion zone in Ukraine and Belarus near the Chernobyl plant, that has been off-limits for over a quarter of a century already. Besides a semicircle of 20 km radius around Fukushima-I there is also a strip of land to the North West, towards Fukushima City, that includes the village of Iitate mura, which is just as badly contaminated as the vicinity of the reactor itself.

People from the area should have been told much sooner, but presumably the prospect of having to pay compensation played a part in holding off giving people the facts. If somebody other than Kan was prime minister, they may well have waited for another 6 months. Kan will probably step down shortly and frankly I don’t hold much hope for how his successor, whoever he may be, will deal with the situation.

Kan famously took on ministerial bureaucracies and vested interests as health minister before and he tried his best in this situation. Many others would try to avoid confrontation with the nuclear industry, following the path of least resistance instead. Kan’s support ratings are extremely low now, but perhaps his efforts, however imperfect they were, may not be fully appreciated until we have his successors to compare with…

Tepco and the dirty secret of the radioactive stack

On July 31, 2011 Tepco detected a radioactive hotspot at the base of an exhaust stack serving units 1 and 2 of Fukushima Daiichi. The following day, a measurement showed the activity to be more than 10 Sv/h (10,000 mSv/h), the highest measured in any accessible part of the wrecked nuclear power station so far (see “The worst hot spot in Fukushima“).

The high activity is assumed to date from attempts at venting the containment of unit 1 on March 12, after it had suffered core damage the previous evening. The venting, necessary to prevent the containment from bursting from excessive pressure, used a so called “hardened vent path” installed by Tepco between 1999 and 2001, following similar upgrades in the US after the Three Mile Island accident.

The reactors have filter system known as the “Standby Gas Treatment System” (SGTS), located on the second floor of the reactor building right next to the turbine hall, but the SGTS doesn’t work without electricity, i.e. the condition that triggered core damage in Fukushima Daiichi. Also, gas from the containment is released at high pressure, which would overwhelm the SGTS.

Therefore the gas is released untreated. Via a strong steel pipe it flows directly into the base of an over 100 m tall exhaust stack. High altitude winds carry it away from the plant. The only filtering that has taken place before than is by water in the pressure suppression pool at the bottom of the containment, through which any gas releases from the reactor pressure vessel are forced to bubble. As the high radioactivity level around the stack demonstrates some 5 months later, there is still plenty of activity left in the gas after that “wet scrubbing”.

It didn’t have to be that way. First of all, if Tepco had built the reactors at a higher location or surrounded them with a sea wall to protect them from a 14 meter high tsunami, there probably would not have been a total station blackout and hence a failure of the residual heat removal system (RHR). The cores wouldn’t have melted. It has long been known that the relatively small Mark 1 containment of these Boiler Water Reactors (BWRs) have problems dealing with vast volumes of hydrogen gas produced when core damage occurs. There was no way to make the containment bigger, short of decommissioning the old reactors and building new ones, which would have cost a lot of money. But that wasn’t the only option.

In 1981, Sweden came up with the FILTRA system for their Barsebäck nuclear power station, which consisted of two BWRs similar in size and functionality to the units in Fukushima. Construction of Barsebäck had started in 1969, only two years after the first unit in Fukushima. It was located about 500 km south of the Swedish capital of Stockholm, but only 20 km from the Danish capital of Copenhagen. The Copenhagen metropolitan region is home to about 1.9 million people in Denmark and South Sweden is also one of the most densely populated parts of the country.

The FILTRA system consists of concrete pipes or rock tunnels filled with gravel, through which the released gas must flow. It gives off heat to some 10,000 m3 of rock, which condenses steam and radioactive fission products, which collect at the bottom of the gravel chambers. After the gas has cooled it is sent through sand or water for further filtration. Only then is it released in the stack. The system is simple and robust enough to cope with high pressure and temperatures. Since it doesn’t use any high technology it was relatively cheap to build.

There was plenty of space available at Fukushima. The Swedish system was no secret. Here is the 30 year old report that discussed its details. Why did Tepco not build a filter system like that, to be shared among its 6 units at Fukushima Daichi? By building the hardened vent path, it already admitted to the possibility of a core meltdown, so why not make sure the venting could be cleaned up? Perhaps Tepco didn’t want to spend more money on safety than it absolutely had to. Perhaps it was also because the Swedish reactors were designed by Asea-Atom AB, whereas the Fukushima design was by GE. Whatever the reason, the technology to protect the people of Japan in a nuclear accident has been available for three decades and Tepco chose not to use it.

The worst hot spot in Fukushima

A week ago, Tepco detected radiation exceeding 10,000 mSv/h (i.e. 10 Sv/h) at an outdoor location near unit 1. This is the highest dose so far recorded outside a reactor core at the site. The previous record was 4,000 mSv/h in unit 2. To put this in perspective, about 7 Sv is already a fatal dose. Therefore 40 minutes at that location would mean certain death from radiation sickness. The local hourly dose of 10 Sv is 100 times as much as the 100 mSv a nuclear worker may normally be exposed to over a total of five years. It is 40 as much as the 250 mSv the government has permitted workers to be exposed to during the Fukushima disaster efforts.

While it’s alarming to hear about record radiation levels being found months after three of the reactor buildings exploded, most likely this radiation is not something new but has been there since the early days of the disaster. It was detected at two locations, an outdoor stack serving units 1 and 2 for venting purposes, and an indoor filter room on the second floor of the turbine hall of unit 1. Both locations are connected using a high pressure steel pipe used for emergency venting. Tepco recently started some construction work near the stack, erecting a steel and plastic cover around unit 1 and perhaps that increased human activity around there led to the discovery.

When Fukushima-I was hit by a station blackout (total electric power failure) after being flooded by a 14 meter high tsunami on March 11, 4 of its reactors and their spent fuel pools were left without cooling. As a result, the cores of units 1, 2 and 3 overheated and their fuel rods melted. During the overheating, steam was vented from the reactor pressure vessel into the suppression chamber of the containment vessel, until the relatively low design limit of the containment was exceeded. In order to not risk an explosion, the operators at that point decided to release radioactive gasses from the containment through the emergency venting system, which is connected to the exhaust stacks.

If during normal operation of the power plant any gases have to be released, they are treated via the Standby Gas Treatment System (SGTS), which comprises HEPA filters followed by active charcoal filters. The unit 1 SGTS is claimed to capture better than 97% of iodine, while units 2 and 3 have a claimed retention rate of better than 99.9%. Treated gas is sent by electrically operated blowers to the exhaust stack, where winds would usually disperse it over the sea. If you look at aerial pictures of the station (see last picture), you see large pipes running from the units to the stacks. Unit 1 and 2 share one stack, units 3 and 4 another (unit 4 was shut down for repairs at the time). These are low pressure pipes, designed for relatively slow releases. Each unit has two systems capable of handling 0.5 m3/s (unit 1) or 0.75 m3/s (units 2/3) each. Because the SGTS relies on electric blowers it is not usable during a total station blackout.

Next to the fat pipes are much thinner high pressure pipes, which are part of the “hardened venting system” installed by Tepco between 1999 and 2001. They were added specifically for major disasters, when the pressure inside the containment reaches dangerous levels, requiring an urgent release of pressure. The hardened venting system skips the charcoal filters. Its only filtering mechanism is the water pool in the suppression chamber, through which steam vented from the reactor core first bubbles. Its scrubbing effect is much more crude than the normal gas filters. The efficient filtering system can not be used during high pressure containment venting because it would offer too much resistance and could not stand the heat or pressure of an emergency release. Yet I still remember when a government spokesman announced that Tepco was going to vent the unit 1 containment into the atmosphere on March 12, we were told that the venting would be filtered, which wasn’t really true.

radiation measurement at bottom of unit 1/2 vent stack

A June 2011 report by the Japanese government to the IAEA explains on page IV-13:

TEPCO built new vent pipes extending from the S/C and D/W [suppression chamber and dry well, the two parts of the containment vessel, JW] to the stacks from 1999 to 2001 as PCV [primary containment vessel, JW] vent facilities during severe accidents as shown in Figs. IV-2-13 and IV-2-14. These facilities were installed to bypass the standby gas treatment system (hereinafter referred to as SGTS) so that they can vent the PCV when the pressure is high.
The facilities are also provided with a rupture disk in order to prevent malfunction.

As the pressure dropped in the hot, compressed gases from the containment and they came into contact with the cooler pipes leading to the stack, the gas cooled off and there was condensation inside the pipes. Some radioactive fission products from the melted fuel rods must have been deposited on the pipe walls. Radiation from these deposits is penetrating the pipe walls, causing high levels at the base of the exhaust stack and inside the filter room through which the venting pipe runs.

What we eat after Fukushima

When people ask me how things are here in Japan after the Fukushima nuclear disaster, I tell them life is mostly normal for my family down here in Tokyo, except that we are very careful what we eat. Given the relatively high food prices in Japan I used to pay more attention to how much things cost, but now I watch more where everything is from. I do it not just for me and my wife but also for our kids (two teenagers).

I really do feel sorry for the farmers in areas affected by the radioactive plumes released after the cores of units 1, 2 and 3 melted down and the buildings of units 1, 3 and 4 blew up in hydrogen explosions, but I largely avoid several prefectures as places of origin. I do not have faith in the government or the food distributors to protect us. The sad fact is, there is very limited capacity for inspections and the surprises just keep on coming. Consumer level geiger counters are not suitable for food safety inspections. That takes high end hardware that costs more than US$100,000 apiece and it’s very time consuming. There are not many of these machines around.

Much of the early radioactive scares were about iodine 131, which decays with a half life of only 8 days. It showed up in Tokyo drinking water and in leafy vegetables as far south as Chiba, in Tokyo’s commuter belt. Within 2 months more than 99% of that I-131 had decayed. By now it’s no longer an issue.

Then attention turned to cesium, which is a more long term problem. It will be with us for much longer, for the rest of our lives in the case of Cs-137 (half life: 30 years). If ingested, about half of radioactive cesium is removed again from the body every 3 months, so it’s not as severe as strontium, which stays in the bones forever, but any internal contamination must be taken seriously. There are parts of my native Bavaria, 1200 km from Chernobyl, where 25 years later most wild pigs shot by hunters still have to be disposed of because they exceed government limits for radioactive cesium. They tell us the radioactive release from Fukushima was much smaller than from Chernobyl, but we’re also much closer to it than Germany was to Ukraine and much of our food is grown even closer to it.

Japanese tea as far away as Shizuoka, some 300 km from Fukushima-I, has exceeded government limits for cesium. Rice straw from northern Miyagi prefecture, some 150 km north of Fukushima-I was too contaminated to be fed to cattle.

Rice prices in shops have increased 20-30% recently, from under 1500 yen per 5 kg bag to 1800-2000 yen per bag. This is the result of consumers stocking up on 2010 rice ahead of the next harvest, which is less than 2 months away. People appear to be concerned about what levels of cesium will be measured in 2011 rice. Beef exceeding government limits had already made it to supermarket shelves and dinner tables before the problem was detected, so people are naturally concerned if this won’t also happen with rice. It’s not an easy problem. By mixing rice from different areas, perhaps no single bag of blended rice exceeds government limits, but that is not the answer. According to current scientific theory, a given amount of radioactivity does not cause fewer cases of cancer by spreading it over more people. The proper answer would be to test rice from every field that is potentially affected and exclude rice from contaminated production areas. Naturally farmers will want compensation for food that can’t be sold, which ultimately will be paid by the government. This sets up a direct conflict of interest: The more testing the government does and the more it does to not dilute contaminated rice among uncontaminated rice, the more money it will have to pay to farmers. It is hard to have confidence that consumer safety will take priority under these circumstances.

A lot of the vegetables for stores in Tokyo are grown in Ibaraki, Fukushima prefecture’s southern neighbour, but whenever I can, I buy produce grown either further north (Hokkaido, Aomori) or further west or south (Gunma, Nagano, Shikoku, Kyushu) or imported (e.g. South Korea). Hokkaido in the far north is about as far from Fukushima-I as Kansai (Osaka, Kyoto) is to the west. Most of the dairy products on my shopping list are now from Hokkaido. Our sea food consumption has gone way down compared to pre-3/11 levels.

Domestically produced (koku-san) foods have been near-religiously venerated in Japan for many years. Consumers have been paying huge markups to eat domestically grown food instead of imports and expected the price to reflect higher quality. For example, I could buy twice was much Chinese eel or three times as much Chinese garlic as their Japanese equivalents for the same money. The Japanese government has maintained domestic rices prices above world market levels. People here always had some suspicion about pesticides or other contaminants in imported food, especially from China, but also from the US. With the nuclear disaster, the tables have turned. Gone is the assumption of safety of “koku-san” food, which will make it hard to maintain the price premium that came with it. The radioactive contamination problem is not just a health worry for millions of Japanese, it is also a devastating blow for the future of farmers and fishermen across much of Japan.

Fukushima: Of cows, water and steel

TEPCO is starting nitrogen injection in unit 3 of the Fukushima-I nuclear power plant to guard against the risk of hydrogen explosions, but it initially faced the obstacle of high radiation levels on the first floor of the reactor, where it wanted to connect the nitrogen pipes. Levels as high as 180 mSv/h between the truck bay entrance on the south-west and the containment at the center made this a no-go area. Efforts by a robot to vacuum radioactive dust off the floor on June 2 were ineffective. TEPCO finally solved the problem by laying 1 cm thick steel sheets on the floor around where workers needed to access.

The ex-skf blog reports that this solution was addressing intense gamma radiation coming up from the basement, penetrating the reinforced concrete floor. The unit 3 reactor building basement was estimated by TEPCO to be flooded with 6400 tons of water, containing 1.5 million Becquerels of Cs-134 per cm3 (Bq/cm3) and 1.6 million Becquerels of Cs-137 per cm3 (Bq/cm3). That amounts to 9,600 Terabecquerels (TBq) of Cs-134 and 10,200 Tbq of Cs-137 or 19,800 TBq in total. Besides that there are other radioisotopes such as Iodine and Strontium.

Because the 1 cm of steel (and below that probably more than 10 cm of concrete) still leave too much radiation through, TEPCO is considering another layer of steel sheets now.

Meanwhile, NHK reported yesterday (July 10, 2011 07:33 JST):

The Tokyo Metropolitan government has begun tracing beef from 6 cows shipped from a Fukushima farm where 11 other cows were found contaminated with high levels of radioactive cesium.

On Friday, tests detected 1,530 to 3,200 becquerels per kilogram of cesium in beef from the 11 cows raised in Minami Soma city, about 20 kilometers from the crippled Fukushima Daiichi nuclear power plant. The national safety limit is 500 becquerels. Tokyo ordered the beef to be removed from distribution.

But beef from 6 cows shipped from the farm to
Tokyo and Tochigi in May and June are believed to have already made it to market without radiation testing.

How did that beef end up with 3 to 6 times the legal limit of radioactive cesium? According to the farmer, the cows were raised on hay from last year, before the reactor catastrophe, but were drinking water from a local well.

The ex-skf blog translates a Yomiuri Shimbun article:

According to the investigation by Fukushima Prefecture, 2924 meat cows have been shipped from the same area since the end of April.

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UPDATE (2011-07-13):

It has been reported that the cattle on the farm in Minamisoma had been fed straw that was not covered by a roof and therefore could have been exposed to fallout in rain.

The 105,000 ton cleanup

On Friday, 17 Jun 2011 TEPCO started up the water cleanup plant built for it by Areva SA in France, initially using only two of the four processing lines. Its objective is to decontaminate an estimated 105,100 t of radioactive water to make it safe to use it for reactor cooling or for shipping it to a nuclear waste processing site. Each processing line is designed to handle 300 t of water per day. Within five hours the processing had to be halted, as radioactivity built up much more quickly inside the system than expected.

TEPCO is under tremendous time pressure. Another 500 t of cooling water are pumped into the reactors from the Sakashita dam about 8 km to the wets of the plant every day. After it has cooled what’s left of the reactor cores it has nowhere to go.

Based on numbers published by the company on June 3, there were already 16,200 t of water in unit 1, 24,600 t in unit 2, 28,100 t in unit 3, 22,900 t in unit 4 and 13,300 t in the central radiation waste treatment building, which had mostly pumped been pumped there for from unit 2. In each unit, the basement of the turbine hall accounts for about half the water, with about another quarter in the reactor building and the rest split between the unit’s adjacent radioactive waste treatment building and a underground trench.

Radiation levels near the accumulated water in the basements are as high as 1000 mSv/h. The current legal limit any emergency worker can be exposed to is 250 mSv in total, which they would get in 15 minutes. It’s not safe to work anywhere near this water. If enough accumulated radioactive water can not be decontaminated far enough to be able to reuse it for cooling then TEPCO needs to keep pumping fresh water while bringing online ever increasing storage capacities to prevent the radioactive water from flooding the plant and spilling into the Pacific ocean.

Between them the buildings hold about 142,000 Terabecquerels (TBq) of Cesium-134 (half life 2 years) and 141,000 TBq of Cesium-137 (half life 30 years). Unit 2 holds about half of the total, and almost all of the rest evenly split between unit 3 and the central radiation waste building. That leaves the water in the basements of unit 1 and unit 4, which taken together only contribute a little over 1 percent of the total radioactivity according to TEPCO data. Units 2 and 3 each not only contain 50-60% more water than unit 1, it also is on average about 30 times more radioactive than the unit 1 water, which in turn is 10 times more radioactive than the water in unit 4. The latter had been shut down for maintenance since late November last year and had no fuel in the core at the time of the accident.

Judging solely by the water in the basements, unit 2 was the source of roughly 3/4 of the radioactive release in Fukushima, with most of the rest coming from unit 3. Even though on photographs unit 2 looks the least damaged of the four blocks, internally it is assumed to be in the worst shape, as it suffered an explosion in its pressure suppression chamber (torus) and it may also be damaged elsewhere.

An even greater percentage of the contamination of land is linked to unit 2 versus unit 3 than indicated by their basement radioactivity levels: When unit 3 was being vented and subsequently suffered the worst hydrogen explosion of all four units, the wind was blowing from the west, carrying most of the released radioactivity out over the Pacific. When things went badly wrong at unit 2 however, the wind was blowing from the South-East, carrying huge amounts of contamination over land to Namie and Iitate in the North-West. This is where contamination levels as high as in the most polluted portions of the Chernobyl exclusion zone have been measured.

The nuclear soup in the reactor basements contains about 3 kg of Cs-134 and 44 kg of Cs-137. If the water decontamination system works, most of this will eventually end up in sludge and filter elements that will be stored as highly radioactive waste.

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Fukushima surprises

On April 12, one month after the devastating earthquake and tsunami that took TEPCO by surprise, leading to the meltdown of three reactor cores in Fukushima 1, the Japanese government raised its accident rating for the event. It moved it from a level 5 on the INES scale to a level 7 (same as Chernobyl), based on the amount of radioactivity released by then. At the time the Nuclear and Industrial Safety Agency (NISA) estimated that 370,000 terabecquerels (TBq) had been released into the environment, while the Nuclear Safety Commission (NSC) calculated the release as 630,000 TBq, with all nuclides converted to an equivalent amount of I-131 for comparison purposes.

Yes, we were told, Fukushima technically now ranked on the same scale as Chernobyl, but it had released “only” 10% of the radioactivity of the worst nuclear accident in history. The total release from Chernobyl is estimated at 5,200,000 TBq.

Now NISA has revised its estimate for Fukushima. “NISA on Monday more than doubled its estimate of the radioactive material ejected into the air in the early days of the Fukushima nuclear crisis to 770,000 terabecquerels,” reports the Japan Times. Apparently, the revision was based on the realisation that unit 2 not only leaked through the ruptured suppression chamber as previously known, but also leaked radioactive substances through a damaged containment.

That containment, if you remember the early days of the accident, was going to be why Fukushima was not going to be “another Chernobyl”. Or so we were promised. Now we know it leaked in all three units and even if it had worked, it was so weak that it would have ruptured if some of its content wasn’t intentionally leaked (“vented”) anyway.

Hot water in unit 2

TEPCO has installed a heat exchanger in the spent fuel pool of unit 2, in the only building amongst units 1-4 that still has a roof on it. They were hoping this would allow them to start repairing other parts of the reactor as soon as possible. Their theory was that the high humidity (greater than 99%) in the building was caused by evaporation from hot water in the spent fuel pool under the roof, with the moisture getting trapped inside the building. They managed to bring down the pool temperature much more quickly than anticipated, but to their surprise the humidity didn’t budge much: Unit 2 is still as moist as a greenhouse. This high humidity prevents air filters from being used for bringing down radioactivity levels in the air inside the building before sending in repair crews.

The humidity is probably rising off hot water in the basement. The radioactive decay of what’s left of the reactor core currently still produces about 6 MW of heat inside the containment, which is conducted through the concrete, pipework and any water and steam leaks. 6 MW of heat is roughly the amount of heat that would be produced burning 600 liters of kerosene every hour.

Nobody is really sure where the cores are now. They could still mostly be inside the reactor pressure vessel, with only a small amount leaked into the containment. Or it could be mostly on the concrete floor of the containment. Nobody really knows for sure yet.

Assuming the cores in all three units have melted, the melted core (“corium”) probably has much lower heat output than it originally did, because some 70% of the decay heat in nuclear fuel are from the more volatile elements. Once the uranium oxide heats up high enough to liquify, the volatile elements trapped inside the uranium ceramics can boil off and escape. Later they condensate inside the walls of the pressure vessel or containment. There they mostly get dissolved in water when cooling gets reestablished. After that the lump of uranium and plutonium oxide will only give off some 30% of its original decay heat because so much of the radioactivity will now be elsewhere in the reactor pressure vessel, the containment or other locations.

No pressure in unit 1 RPV and they knew

TEPCO sent workers into unit 1 to install new manometers to measure pressure inside the reactor. Doing their work they were exposed to about 4 mSv each, more than an ordinary person would receive during a whole year, but their risk has enabled us to receive proper data about the reactor pressure in unit 1.

As it turned out, the reactor pressure vessel (RPV) of unit 1 is at atmospheric pressure, which suggests the RPV is connected to the containment (i.e. has holes) and the containment is connected to the outside too. For weeks TEPCO had been pumping nitrogen gas into the RPV to dilute any existing hydrogen, in order to guard against the risk of explosions. No evidence of this nitrogen can be found now, or at least no gas pressure from its pressence.

In parallel with the nitrogen injection, a pressure gage for unit 1 had been showing increasing pressures in the unit 1 RPV, climbing as high as 1,6 MPa (about 16 bar) over the last couple of weeks. However, even before TEPCO installed a new gage they knew that value was wrong, without them telling the public. How do we know that they knew? As member “elektrownik” noticed, the new gage that they had those workers install has an instrument range of only 0.0 to 0.3 MPa…

TEPCO makes space for a lot of water

The Daily Yomiuri has a picture of some of the water tanks that TEPCO is buying for Fukushima 1. It has ordered 200 tanks holding 100 t each and 170 tanks holding 120 t each, for a total of 40,000 t. Several of these will be brought in each night by truck.

At the roughly 500 t of water that TEPCO pumps in for cooling purposes per day the tanks would last for less than three months. From June 15 TEPCO is planning to recycle 1200 t of highly radioactive water per day, pumped from the reactor buildings. It wants to either reuse it for cooling or to send it to the reprocessing plant in Rokkasho village for final cleanup.

If the water treatment plant doesn’t work as expected, that would be one surprise that TEPCO could definitely do without. The tanks may be one form of insurance against that possibility.