The Japanese government has announced that the wrecked Fukushima Daiichi power station has reached a “cold shutdown”. The BBC quotes Prime Minister Noda:

“The nuclear reactors have reached a state of cold shutdown and therefore we can now confirm that we have come to the end of the accident phase of the actual reactors.”

It is meaningless to still use the term “cold shutdown” for a reactor in which the fuel rods and containment vessel have lost their integrity. It’s like saying the bleeding has been stopped in an injured patient who had actually bled to death.

The normal definition of “cold shutdown” is when, after the chain reaction has been stopped, decay heat inside the fuel rods has been reduced enough that the cooling water temperature finally drops below 100 C. This means the cooling water no longer boils at atmospheric pressure, making it possible to open the pressure vessel cap and remove the fuel rods from the reactor core into the spent fuel pool. After that the reactor core no longer needs to be cooled.

Only units 4, 5 and 6 have reached a genuine cold shutdown. Unit 4 had been shut down for repairs in 2010 and did not contain any fuel at the time of the accident. In units 5 and 6 a single emergency diesel survived the tsunami and prevented a meltdown there.

In units 1, 2 and 3 of Fukushima Daiichi the fuel melted, dropped to the bottom of the reactor pressure vessel and penetrated it. The melted rods then dripped down onto the concrete floor of the containment vessel and are assumed to have partly melted into the concrete up to an unknown depth.

While in a regular cold shutdown fuel can be unloaded within weeks, the Japanese government estimates it may take as much as 25 years before all fuel will have been removed. The technology to remove fuel in the state it’s in now does not even exist yet and will have to be developed from scratch. Even the most optimistic schedule puts it at 5 years, during which time the reactors will have to be cooled 24 hours a day, with no new earthquakes damaging them or knocking out cooling again, no major corrosion problems, no clogged water pipes, etc.

In my opinion, the announcement of a “cold shutdown” at Fukushima Daiichi is greatly exaggerated and was made mainly for political purposes. More than anything, it is meant to provide political cover for restarting other idled nuclear power stations during the coming year.

The Japanese government has released updated radiation maps for Eastern Japan from its helicopter survey. The maps now cover prefectures as far west as Gifu and as far north as Iwate and Akita. Previously there was map data only for Tokohoku (excluding Aomori) and the Kanto area. The PDF can be downloaded here.

The previous set of maps documented caesium contamination and background radiation levels in Fukushima, Tochigi, Miyagi, Ibaraki, Chiba, Saitama, Tokyo and Kanagawa. The latest set adds maps for Iwate, Shizuoka, Nagano, Yamanashi, Gifu and Toyama. Akita, Yamagata and Niigata have also been surveyed and are shown on the overview map.

The most heavily contaminated areas are in the eastern half of Fukushima prefecture, within about 80 km of the wrecked nuclear power stations. The southern part of Miyagi to the north and the northern part of Ibaraki to the south also took a hit.

A major radioactive plume moved south-west from Fukushima, polluting the northern half of Tochigi and the northern and western part of Gunma. A separate plume reached the southern part of Ibaraki, the north-west of Chiba and the eastern part of Tokyo.

There is also some caesium in the mountainous far west of Tokyo and Saitama that extended from Tochigi, but most of Saitama, Tokyo and Kanagawa seem relatively OK, as are Shizuoka, Yamanashi, Nagano, Gifu, Tokyama, Niigata, Yamagata and Akita. There is some fallout in a strip from southern Iwate to northern Miyagi, while central Miyagi and the rest of Iwate look clean. There is no published data for Aomori and Hokkaido yet, but based on the distance and the absence of significant pollution in Akita and adjacent parts of Iwate they will probably be fine.

The maps only give the overall picture, as there may be local hotspots in areas that are relatively clean overall, based on rainfall and wind patterns as well as soil and vegetation that can retain more or less fallout.

Update 2011-12-06:
The ministry has also published radiation maps for Aichi, Aomori, Ishikawa and Fukui prefecture.

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:

• http://ramap.jaea.go.jp/map/

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.

See also:

• Tepco’s unfiltered vent path (2011-08-25)
• “Okutama is no Chernobyl” (LTE in Japan Times, 2011-10-20)

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.

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.

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.