Japan without nuclear power

Since last weekend, Japan is without a single nuclear power station feeding power into the grid, the first time in 42 years. All 50 nuclear power stations are currently off-line (this count does not include the 4 wrecked reactors in Fukushima I, which are no longer officially counted — it used to be 54 nuclear power stations).

Some of these power stations were shut down because of problems after the March 11, 2011 earthquake and tsunami. Others were taken offline one by one for routine inspections and maintenance but have not been started up again, which would only happen with the consent of nearby local governments. That consent has not been forthcoming.

Electrical utilities and the government are raising concerns about a power shortage when the summer heat sets in, which usually results in peak usage for air conditioners. Critics of nuclear power see an opportunity for a quick exit from nuclear power. Others are concerned that if the government rushes to bring power stations back online before the summer without safety upgrades and a change in the regulatory regime, a unique chance to prevent the next nuclear disaster will be squandered. If upgrades and reforms don’t happen when the memory of Fukushima is still relatively fresh, what’s the chance of it happening a few years down the road?

The utility companies are facing high costs from buying more fossil fuels for gas and oil fired thermal power stations to cover the demand; restarting the nuclear power stations would keep those costs in check. But that is only part of the reason they are keen on a restart. The sooner they can return to the pre-Fukushima state of power generation, the less leverage governments and the public have for making them accept new rules, such as retrofitting filters for emergency venting systems or a permanent shutdown of the oldest and seismically most vulnerable stations. Because of this it’s in the interest of the utilities to paint as bleak a picture of the situation as possible. Japan would be smart to proceed cautiously and not miss a unique chance to fix the problems that are the root cause of the Fukushima disaster and of disasters still waiting to happen.

Using Sanyo Eneloop Ni-MH AA batteries to power your mobile phone

About two years ago I started using Sanyo’s rechargeable eneloop batteries. These relatively inexpensive Nickel-Metal Hydride (Ni-MH) cells are available in both AA (単3形) and AAA (単4形) sizes. They are low self-discharge cells that keep their charge for months when not in use. I’ve bought boxes of 8 cells of either type, for use in flash lights, bike blinkies, helmet lights and Bluetooth keyboards.

They are initially more expensive to buy than regular alkaline (primary) cells, but you only need to re-use them about three times before they work out much cheaper than primary cells, while you can actually recharge them hundreds of times before they start losing significant capacity.

Here are some nice gadgets that will take them, which I found sold in convenience stores here Japan.

These little cases (by alicty.co.jp) take power from two or three regular alkaline AA or Ni-MH AA cells and provide a USB port for powering mobile phones and other small gadgets with a USB power cable. As you would expect, the three cell version is slightly more powerful, looking to my Google Samsung Nexus S as an AC charger (i.e. it provides more than 500 mA). For the two cell version, the phone shows “charging (USB)” as the status, i.e. it can draw up to 500 mA. The two cell version has a USB-A socket (female) for generic USB cables while the three cell version comes with an integrated micro USB (male) cable. A very similar concept has been around for a while as the MintyBoost.

The nice thing is, if you carry enough pre-charged eneloop cells with you, you can swap cells as needed and have virtually unlimited power. You could even buy primary cells to top up if desperate (one set came bundled with each device), but they would end up costing you more than re-usable eneloop cells in the long term. I’ll carry some Ni-MH cells as spares on long bike trips or hikes, which could come in handy with these little cases.

UPDATE 2012-04-04: I also tried using this adapter with alkaline (primary = non-rechargable) AA cells and it goes through them quite rapidly. Alkaline AA batteries have a notoriously poor performance in high drain applications because of their high internal resistance. You’re much better off sticking with Ni-MH batteries such as Sanyo Eneloop!

It says on the pack that a set of 3 AAs will boost the charge state of a smartphone battery by 30-40%, i.e. it would take you about 3 sets (9 cells) to fully recharge an empty battery. Or put another way, if the phone lasts 5 hours on one charge doing whatever you’re doing, you will consume a set of fresh AAs every 100 minutes to keep it topped up. To provide 500 mA at 5 V (2.5 W) on the USB connector at 80% efficiency would draw 3 W from the batteries, or 700 mA at 4.5 V (3 x 1.5 V). At that kind of load, an alkaline battery might only supply a quarter of its rated capacity, which is normally measured at a much smaller load (which is OK for alarm clocks, TV remote controls, etc. but not high powered electronics like digital cameras or smart phones).

Fukushima “cold shutdown” announcement up to 25 years too soon

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.

GPS-logging my bike rides

Five weeks ago I started logging my road bike rides, runs and mountain hikes using the GPS in my Google Nexus S Android phone. I use the iMapMyRide app which requires Android 2.1 and later (of course there’s also an iPhone version).

Start the app, a few taps on the screen and it starts recording. You can pause the recording any time, say if you stop for food or rest. When you’re done you can easily upload the complete route with GPS coordinates and timing to the MapMyRIDE.com website. Besides bike rides you can also use the app for hiking, running or walking.

As it records it displays basic map information, so it can be used for simple navigation too, but most of the time I relied on Google Maps for that.

Afterwards you can view the workout on your PC. It will show altitudes along the route, including total gain. It shows average speeds for each km of progress. It calculates how many kcals you used based on the route, your weight and your age.

A calendar view shows all days on which you exercised, with distances for each workout, weekly totals and monthly totals. This can be a powerful tool to keep up a certain level of exercise on a regular basis.

Battery usage

As with many mobile applications, battery life is of concern to users. So far my longest recorded hike was 5 1/2 hours and my longest bike ride was 4 1/2 hours. I have not run out of power yet, but I’ve had the battery low warning pop up on occasion.

There are a few things you can do to optimize power usage. I make sure to disable WiFi and Bluetooth to minimize power usage. If I am in areas without mobile data coverage, such as high on a mountain I switch the phone into “airplane mode”, which will still let it receive GPS data but it won’t download map data (which it can’t anyway without a nearby cell phone tower). Disabling these wireless connections prevents the phone from wasting energy on trying to reconnect.

It makes a big difference how much you use the LCD screen. If you often turn it on to consult the map for a new or unknown route that will eat battery life.

In order not to have to worry too much about that and to be able to record longer and further rides and hikes, I got myself a cheap external Li-ion battery on Amazon Japan, into which I can plug the USB cable of my Android phone for extra power. I paid JPY 2,380 (about $30) including shipping. Its capacity is listed as 5000 mAh and it has two USB output ports, plus one mini-USB input port for recharging. It comes with a USB cable for charging, a short spiral USB output cable and 10 adapters to connect it to different phone models (including the iPhone and iPod). Because of the standard USB ports you can use any existing USB cable that works with your phone. It’s like running your smartphone off power from your computer.

The device is about the weight and size of my phone. It came charged to about 60%. It should take a couple of hours to fully recharge it from empty.

If fully charged it should theoretically provide three complete charges for my mobile phone, which has a 1500 mAh battery inside, thereby quadrupling the length of rides I can record. Most likely, I will run out of energy long before my battery does 🙂

My longest distances with MapMyRIDE so far:

  • Bike ride: 71 km, 560 m elevation gain
  • Mountain hike: 14 km, 1020 m elevation gain
  • Run: 10 km in Tokyo

A few rough edges

While the iMapMyRIDE+ app feels fairly solid, it will need fixes for a few problems.

The major issue for me is that the app and the website don’t see eye to eye on time zones. For example, if I record a ride at 17:00 (5pm) to 18:00 (6pm) on a Sunday, the recorded workout title will include the correct time. However, if I view that workout on the computer’s web browser, it is shown on the calendar as having been recorded on the following day (Monday). If I check the details, the start and end times are listed as 8am (08:00) and 9am (09:00): Wrong day and off by 9 hours.

Probably not by coincidence my time zone (Japan Standard Time) is 9 hours ahead of UTC. It’s like the app sends up the start and end time in UTC but the website thinks the data is local time. Yet for determining the date it seems to add those 9 hours again, which takes it beyond midnight and Sunday gets turned into Monday.

I can manually correct every single workout from the website, which also fixes that date on the calendar, but then the app displays the wrong time, which I am prepared to simply ignore.

When I enter my height on the website and then view my details on the app, I am 2 cm shorter than I entered, perhaps as the result of my height having been converted from metric to imperial and back to metric with numeric truncation.

I wish the site would support 24 hour clocks, not just AM/PM. I also wish the site would let the metric size to be entered as cm, not just m and cm separately (probably a hangover from code written for feet and inches).

Note to the app developers out there: The world is much bigger than the US and most of it is metric.

UPDATE 2011-12-13:

I have used the Li-ion battery on two weekend bike rides now. One was 93 km, the other 101 km in length. In both cases I first used the phone normally until the remaining charge level was heading towards 20% (after maybe 4 hours), then I hooked it up to the 5000 mAh battery and continued the ride. The longer of the two rides was about 8 hours, including lunch and other breaks. At the end of the 101 km ride the phone battery was back up to 75% charged, while the external battery was down to 1 of 5 LEDs, i.e. close to empty.

I wasn’t as careful to conserve power with the external battery hooked up. My phone is configured to not go into sleep mode while hooked up to a USB cable, unless I manually push the power button. That’s because I also use it for Android application development, where it’s controlled from a PC via the cable. I should really turn that developer mode off on rides to have the screen blank after a minute as usual even when getting external power. Total capacity with the external battery probably at least 10 or 11 hours, more if I put the phone into “airplane mode”, which disables map updates and hence navigation.

My headlight currently consists of a twin white LED light using a pair of CR2032 batteries that I need to replace every now and then. It’s not very bright, especially where there are no street lights. Probably next year I’ll upgrade the front wheel using a Shimano DH-3N72 dynamo hub,

which can provide up to 3W of power while adding very little drag. A 6V AC to USB adapter will allow me to power USB devices like my phone and the headlights from this without ever having to buy disposable batteries or connecting anything to a mains charger.

UPDATE 2012-01-02:

I have had the front wheel of my Bike Friday rebuilt with a Shimano DH-3N80 dynamo hub. The old 105 hub is now a spare while the rim with tube and tyre were reused. Here is the bike in our entrance hall:

Closeup view of the hub with AC power contacts:

I purchased a USB power adapter made by Kuhn Elektronik GmbH in Germany. It weighs 40 g and measures 8 cm by 2.5 cm. It provides a standard USB-A socket which fits standard USB cables such as the one that came with my Google Nexus S:

USB power adapter with Google Nexus S:

UPDATE 2012-03-14:

At the end of January I started using Strava for tracking rides, in addition to MapMyRides (MMR). I stopped using the MMR app because there is no way in MMR to export GPX files with time stamps, so you can not track your speed or performance on any sites besides MMR. They lock in your data. Instead I either record with Strava on my Android 4 Nexus S or with Endomondo on my Android 1.6 Google Ion. That way I can generate GPX files that will upload to Strava, Endomondo, MapMyRide or just about any other site. The automatic competition feature of Strava is superb. MMR’s best features are its calendar view with weekly and monthly statistics and its mapping feature for planning rides. If those were merged with what Strava can do, it would be a terrific GPS cycling app and site.

Radiation maps for Eastern Japan

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.

How (not) to decontaminate Japan

An article in Japan Times (2011-11-09, “Scrub homes, denude trees to wash cesium fears away”) provided advice on how to decontaminate areas affected by nuclear fallout, such as in Fukushima, Tochigi and northern Chiba prefecture. Most of the advice is sound, but some is downright alarming:

As for trees, it’s best to remove all their leaves because of the likelyhood they contain large amounts of cesium, Higaki [of University of Tokyo] said.
(…)
What should you do with the soil and leaves?
(…)
Leaves and weeds can be disposed of as burnable garbage, a Fukushima official said.

So let me get this right: you should collect all those leaves because they contain so much radioactive cesium (cesium 134 has a half life of 2 years and cesium 137 of 29 years). And then, when you have all that cesium in plastic garbage bags, you have it sent to the local garbage incinerator, so the carefully collected cesium gets spread over the whole neighbourhood again via the incinerator smokestack. That makes no sense at all.

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.

See also:

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.

See also:

Solar energy, USA vs. Germany

Recently I came across an article that quoted a Forbes commentary (“Sue OPEC? Congress Should Sue Itself”, 2008-07-09) comparing solar energy development in the USA (or lack thereof) with the situation Germany, where 2010 was a veritable boom year for photo-voltaic panels.

Two maps and one quote underneath caught my attention:

Check out the map above. With the exception of Seattle, the entire continental U.S. is much sunnier than Germany. Yet Germany has 17 times the installed solar base per capita.

According to the map, Germany received amounts of sunlight comparable to the region around notoriously cloudy Seattle and arctic Alaska, while most of the states along the Canadian border got 50% more sun than the Southern half of Germany.

This did not seem plausible to me. While it’s true that most of Germany lies further North than the 49th parallel that marks most of the US-Canadian border and should therefore receive less sun than most of the US, most of Germany’s climate is far sunnier than Seattle, which lies about as far North as Mannheim or Nuremberg (Nürnberg) in Southern Germany. Based on latitudes and annual rainfall, solar insolation (the amount of solar energy radiated onto a given area) should be largely comparable between at least southern Germany and the northern US outside the Pacific Northwest. I’ll give you some data to verify this theory.

Here is a map of insolation for the entire US, showing kWh per square metre per day at latitude tilt (multiply by 365 for annual figures like in the Forbes map):

As you can see, most of the US gets between 4 and 5.5 kWh/day (yellow-grey to dark yellow), or 1450-2000 kWh per year.

And here is a map of insolation in Germany, showing horizontal irradiation in kWh per square metre per year:

Note that the colour scale is not the same. The southern part of Germany gets 1200 kWh and more per year, the northern part less than that.

But that’s not the whole picture. If you you paid attention, you noticed the “at latitude tilt” (US) versus “horizontal irradiation” (Germany). It makes a big difference, because without taking it into account, the comparison of the raw numbers become an apples to oranges comparison: The numbers in both maps don’t actually measure the same thing!

The further you move north, the lower the sun stands at midday to the South. Consequently, when you install solar panels anywhere but in a tropical country, you don’t install them horizontally but make sure to tilt them at the right angle to catch the most sun per square metre of expensive panel, based on the average position of the sun at noon throughout the year, which depends on your distance from the equator. It will be more tilted at a more northern location than somewhere further south.

The US solar data is measured per square metre of panel. The German data however is per square metre of shadow the panels cast on the ground, which is not the same. The two ways of measuring insolation only match at the equator.

Let’s look at an example: In Chicago, at around 42 degrees North, a solar panel tilted 42 degrees towards south is exposed to 37% more sun than a flat piece of lawn of the same size (1/cos(42 degrees) = 1.37). So a one square metre panel’s 1500 kWh/year in total solar irradiation in Chicago is basically the same as 1100 kWh of horizontal exposure, which is the same or less than you catch just about anywhere in Germany. Far from being comparable only to rain-swept Seattle, Germany’s annual exposure to the sun is actually not too different from the US east of the Mississippi, except for the Southern sunbelt from Texas to Florida, which does get more sun.

So what do we learn from this?

1) As our physics teacher always used to tell us: Watch your units! Numbers don’t tell you anything unless they go with the proper unit.

2) The German solar subsidy program is no crazy boondoggle at taxpayer’s expenses. Solar energy does make sense in Germany, as it does in most other countries in temperate climates.

Of course it makes even more sense in dry climates like Spain, Turkey, North Africa and the arid US Southwest, where it’s ideal. On the other hand, Seattle or Iceland may not be the greatest places for it.

Ideally, solar energy investment should start where it brings the highest returns, accompanied by sufficient investment into a power grid that can carry power wherever it’s needed.

In 2010, Germany produced about 1.9% of its electricity usage from solar panels, almost double what it was the year before. By 2020 this is expected to more than triple again, to 40 GWh or 6%. As mass production brings costs down while oil prices keep going up, German solar electricity is expected to become cost-competitive with conventional power by about 2015, which will be great for cutting dependence on dwindling oil reserves.

Also, let’s not forget that solar energy is not the only renewable game in town: Wind for electric power is already much more competitive on a cost per kWh basis than photovoltaics and deserves more attention. Many regions that have relatively poor solar prospects have excellent wind opportunities, for example Scotland. A mix of solar and wind often works better than just one or the other.

Germany has great potential for wind power, both land based and off-shore, and so do the US. What is holding wind power back particularly in the US (but to some extent also in Germany), is an under-sized grid that can not move enough power over the necessary distances from areas with plentiful wind to where the power is needed. Investment in a 21st century power grid will be essential for a low-carbon future.

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.