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.

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.

See also:

• Fukushima gov’t inspects farm over beef radiation concerns (Mainichi, 2011-07-11)

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.

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.

See also:

• Flow of Water Treatment Facilities (TEPCO, PDF)
• Temporary storage tanks
• Chernobyl in the basement

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 physicsforums.com 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.

TEPCO has released radiation figures for the 2700 t of water in the basement of unit 1 of the Fukushima 1 nuclear power plant:

Tokyo Electric Power Co. <9501> said Monday that the amounts of radioactive materials in water at a reactor building of the Fukushima No. 1 nuclear power plant were about 10,000 times the normal levels for water inside a nuclear reactor.
The water, recently found in the basement of the No. 1 reactor building of the nuclear power plant, contained 30,000 becquerels of iodine-131 per cubic centimeter, 2.5 million becquerels of cesium-134 and 2.9 million becquerels of cesium-137.

(Jiji Press, 2011-05-30)

A total of 2700 t of water are estimated to be in the basement. That works out as 6.75E15 Bq = 6.75 PBq of Cs-134 and 7.83 PBq of Cs-137. For comparison, the total radioactive release from Chernobyl is estimated at 48 PBq of Cs-134 and 89 PBq of Cs-137. That means the water in the basement of unit 1 alone contains about one tenth of all the radioactivity released into the environment by the Chernobyl disaster. When the accident started, we were told that “Fukushima is no Chernobyl” because unlike the Soviet reactor its reactors had containments. Unfortunately, they didn’t work. All that radioactivity mentioned above is already outside of the containment, in a place that was never designed to be flooded with thousands of tons of radioactive water.

Units 2 and 3 are assumed to be in equally bad shape as unit 1, as both suffered a meltdown and neither of those units’ containments still hold any pressure. All three units have flooded basements. Water in a trench near unit 2 was measured at 2 million becquerels of cesium-137 per cubic centimeters on March 30, quite similar in contamination to the unit 1 basement.

When people first entered unit 1 again on May 13, the water level in the basement of unit was reported to be 4.2 m. Two weeks later it is 4.6 m and the storage tanks at the reactor site are almost full. Very soon something needs to happen about that water. To complicate things even more, June is the rainy season in Japan, when typhoons can dump huge amounts of water onto Japan.

The water treatment plant built by Areva is expected to start operating around the middle of this month. TEPCO quoted the cost of decontaminating water with this system at 210,000 yen per ton (about US$2600). It intends to decontaminate 250,000 tons of water by the middle of January 2012, for a total cost of 53.1 billion yen (about US$650 million).

Currently 5 cubic meters per hour (m3/h) are being pumped into unit 1 for cooling, 4.9 m3/h into unit 2 and 12.5 m3/h into unit 3 (which is still overheating). That’s about 550 tons of water per day, which is assumed to leak into the basements of the three reactors, loaded with dissolved radioactive waste from damaged fuel rods. If cooling water feeds continue at current rates, there will be another 8,000 tons of highly radioactive water to be taken care of by the time AREVA’s treatment plant starts up — that is, if all goes according to plan…

A “mega-float” that has been towed to the site for storing and transporting radioactive water can probably only be used for decontaminated water (i.e. with 99.9% of the radioactivity removed), otherwise radiation would be too high to handle. Contaminated water being pumped from the basement of the turbine buildings to storage tanks gives off so much radiation that the piping is covered up with lead wool wherever people have to walk over it and is cordoned off with security tape everywhere else.

If a water pump fails and it has to be replaced, someone will have to disconnect and reconnect pipes to pumps that carry billions of becquerels of radioactivity per liter of water. There will be leaks and puddles and spills. This is not how things would be done if there was any choice.

Fukushima 1 may not make the headlines of the world media much any more, but the situation there is still nightmarish. It’s not the TEPCO managers who are paying for their company’s gross negligence, but unnamed workers of hired subcontractors who are risking their lives and health.

The cleanup and compensation for thousands who lost their homes and incomes through the fallout far exceeds TEPCO’s ability to pay, so tax payers will be forced to pay the bill, yet the government chose not to force TEPCO into bankruptcy. Its share price may have fallen, but its stock did not become worthless as say General Motors in the US car industry bail-out, or perhaps the crops grown by farmers nearby or the now deserted homes owned by families around the plant. TEPCO’s well-connected shareholders were effectively bailed out by the government at our expense.

See also:

• Backroom deal saves Tepco from meltdow (BusinessDay, 2011-05-23)

A failure of a sea water pump at unit 5 of the Fukushima 1 Nuclear Power Plant demonstrated the still volatile situation at the reactors. It allowed water temperatures inside the reactor (which had officially been in “cold shutdown” since March 20) to climb as high as 93 degrees C again before a replacement pump restored the water flow and allowed temperatures to drop again. Unlike the heavily damaged units 1 through 3, units 5 and 6 (along with unit 4) had been shut down for maintenance when the quake and tsunami hit. Due to their slightly higher elevation they escaped the worst of the tsunami. A single backup diesel generator for units 5 and 6 survived. So far TEPCO has not announced any plans yet to permanently decommissioning the two units, although the government of Prime Minister Kan pushed for that.

The failed temporary pump provides cool sea water to the heat exchanger of the Residual Heat Removal System (RHRS). TEPCO reports:

At 9:14 pm on May 28th, we found that one temporary residual heat removal system seawater pump of Unit 5 stopped. At 8:12 am on May 29th, replacement work to the spare pump started. After finishing the replacement work, we started the pump at 12:31 pm, and restarted cooling from 12:49 pm.

The RHRS is a cooling system that is used whenever the reactor does not drive a steam turbine, where heat is removed via the attached condenser unit. It is also used to cool the water in the spent fuel pools. Both a loaded reactor core and fuel elements in the pool produce decay heat that needs to be removed for months and years after a shutdown. Both the condenser and the RHRS require a steady flow of seawater to carry away heat.

According to a diagram at the NISA website Units 5 and 6 had been using a temporary pump near their regular cooling water intake channels, which suggests that the normal pumps of the RHRS had been damaged in the tsunami. TEPCO has now set up a new pump (as well as a spare next to it) halfway between the cooling water intake channel and the RHRS and cooling water circulation pumps. For the last two months there had been no spare in place.

Unit 1 dry well radiation levels

There have been some online discussions about spiking radiation level figures in the dry well (primary containment) of unit 1. Here is a graph from atmc.jp:

The original data for this graph are figures published by TEPCO/NISA of the CAMS radiation monitor readings for the three units. Until May 25 this data was available in daily updates on the NISA website under “Seismic Damage Information（the nnnth Release）(As of hh:mm May dd, 2011″, document “Fukushima Dai-ichi Nuclear Power Station Major Parameters of the Plant (As of h:mm, May dd)”. Since then NISA no longer seems to publish those figures. It can still be found vi the “Status of Fukushima Daiichi and Fukushima Daini Nuclear Power Stations after Great East Japan Earthquake” page on the TEPCO website though: Look for “The parameters related to the plants in Fukushima Daiichi Nuclear Power Station”, which has a link to the latest document and a ling to an archive page with previous daily data sets. “CAMS radiation monitor” has entries for D/W A and B, S/C aA and B. The D/W B sensor is the one in the above chart. Strangely, the numbers on the atmc.jp do not always match the data on the NISA/TEPCO sites:

5/18: NISA: 25.4 Sv/h / atmc.jp: 45.4
5/19: NISA: 36.3 Sv/h / atmc.jp: 36.3
5/20: NISA: 46.5 Sv/h / atmc.jp: 46.5
5/21: NISA: 36.2 Sv/h / atmc.jp: 36.2
5/22: NISA: 196 Sv/h / atmc.jp: 196
5/23: NISA: 33.1 Sv/h / atmc.jp: 201
5/24: NISA: 30.5 Sv/h / atmc.jp: 192
5/25: NISA: 204 Sv/h / atmc.jp: 215
5/26: TEPCO: 39.3 Sv/h / atmc.jp: 43.7
5/27: TEPCO: 53.5 Sv/h / atmc.jp: 63.8
5/28: TEPCO: 215 Sv/h / atmc.jp: 215
5/29: TEPCO: 225 Sv/h / atmc.jp: 225
5/30: TEPCO: 41.8 Sv/h / atmc.jp: n/a

The source of the discrepancy between the two sources is unclear as ultimately TEPCO must be the only source of data on the reactor.

Some people have speculated that the sudden increase in the graph indicates that a large amount of fuel melted through the pressure vessel and leaked onto the floor of the dry well. However, that would not explain why the values significantly dropped again and then shot back up again.

TEPCO states that they think the radiation sensors are malfunctioning. I suspect their explanation is the correct one, but how reassuring is that?

Trying to manage a badly damaged nuclear reactor (actually, 3 of them!) and the 4 loaded spent fuel pools next to them with broken instruments whose readings can not be trusted any more is a bit like trying to drive a car without a speedometer and with a shattered windscreen that prevents you from seeing the road, going only by the directions of your passenger who sticks his head out of the window to keep you on the road.

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

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

The executives did not give the order to begin venting until Saturday — more than 17 hours after the tsunami struck and six hours after the government order to vent.

As workers scrambled to comply with their new directive, they faced a cascading series of complications.

The venting system is designed to be operated from the control room, but operators’ attempts to turn it on failed, most likely because the power to open a critical valve was out. The valves are designed so they can also be opened manually, but by that time, workers found radiation levels near the venting system at Reactor No. 1 were already too high to approach, according to Tokyo Electric’s records from the accident’s early days.

At Reactor No. 2, workers tried to manually open the safety valves, but pressure did not fall inside the reactor, making it unclear whether venting was successful, the records show. At Reactor No. 3, workers tried seven times to manually open the valve, but it kept closing, the records say.

The results of the failed venting were disastrous.

Reactor No. 1 exploded first, on Saturday, the day after the earthquake. Reactor No. 3 came next, on Monday. And No. 2 exploded early Tuesday morning.

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

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

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

See also:

• Reactor Vents That Failed in Japan Are Used in U.S. (New York Times, 2011-05-17)