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.

6 thoughts on “The worst hot spot in Fukushima

  1. Pingback: (Eng- trad) Le pire hot spot à Fukushima | FUKUSHIMA INFORMATIONS |

  2. @nuclear: Please provide some verifiable evidence before you libel people as liars.

    The emergency diesels are reported to have started up and provided backup power for almost one hour. According to the official report, the station blackout occurred at 15:41, some 55 minutes after the quake and about 10 minutes after the tsunami struck.

    The diesels failed due to a combination of factors:
    – loss of cooling of the water-cooled diesels when the cooling water intake pumps were swamped..
    – loss of fuel supply when the fuel tanks were dislocated by the flood.
    – shorting of output when the electric switch boards in the turbine hall basements were flooded by sea water.

    I am not saying there was no quake damage, but the subject of this particular blog post was a different issue.

  3. Thanks. So, if I understood correctly, you are saying that these high readings are caused by radioactive fission products that were making their way through the pipes in order to be released through the stacks, but condensed on the way and settled down in the pipes.

    This raises two thoughts:

    1) Only a fraction of the total quantity is left in the pipes – but how much?

    I guess that, when you release a large quantity of hot steam containing radioactive particles, only a fraction of those particles condense and remain in the pipe. And that fraction is now, months after the event, still emitting 10 Sv per hour. At this point in time, the radiation from I-131 is negligible, because it has a half life of eight days, so that radiation must be caused by Cesium and possibly other radioactive elements. But at the time of the release, the steam must have been replete with I-131, causing, in addition to the other elements. That would indicate that a pretty large amount of radioactivity was released. Or is that a conclusion that can’t be drawn?

    2) the stacks

    One of the arguments I often heard in the initial stages was that this accident could in no way be compared to Chernobyl, because in Chernobyl, the reactor exploded, resulting in radioactive materials being blown high into the air. This allowed them to be carried away by upper-level air currents, all the way to Europe. In Japan, contamination was unlikely to reach that far, because the radioactivity was not being released into the upper air currents. There were of course explosions, but these were hydrogen explosions, not explosions of the reactor like in Chernobyl.

    But, based on your account, it seems the emergency venting was done through the stacks. These stacks are exactly designed to release steam into higher air currents.

  4. @Jeehie:

    1) Most of the radiation emanating from those pipes is likely to be from cesium right now. At the time of venting there would have been an even larger amount of radiation from iodine 131 present, as well as noble gases such as krypton and xenon, which are normally held inside the end tips of the fuel rods and are released when their zirconium cladding melts.

    At the peak of drinking water contamination in Tokyo between March 21-28, radioactivity from iodine detected in the water was some 10-30 times higher than combined Cs-134/137 activity. That activity may also have been present inside the venting pipes in March, but has since decayed. We can only guess how hot those pipes radiated those first couple of weeks…

    2) This is not a simply yes/no question. Unlike what nuclear propagandists initially proclaimed, the presence of containments in Fukushima (vs. their absence in Chernobyl) did not prevent massive radioactive releases, because once a station blackout of more than a couple of hours occurs the relatively small BWR containments either have to be emergency-vented directly into the atmosphere or let explode from over-pressure, because of large amounts of uncondensed steam and uncondensible hydrogen.

    As you correctly point out, the high stacks in that case serve to spread the radiation wide and far, which is why large parts of eastern Japan now have to worry about their agricultural future. The reason the emergency venting happens via the stacks is to prevent the direct vicinity of the station from becoming a no-go area, preventing further measures to regain control of the accident. In that situation you’re stuck between a rock and a hard place. Imagine if all of the area around the 4 units was a tens of Sv/h zone now, like the foot of the stack. Any efforts to cool down the melted reactor cores using temporary pumps (and, more importantly, the spent fuel pools under open skies) would have had to be abandoned there and then. The situation would have totally spiraled out of control.

    Where the situation in Fukushima was still different from Chernobyl was that the reactor core was not directly exposed to the atmosphere. The venting path from steam that had been in contact with melting fuel goes through water in the suppression chamber (which holds 2100 t in the case of unit 1) and only from there into the atmosphere. A lot of vaporized fission products would have condensed either inside pipes or in the suppression chamber.

    Most of the radioactivity probably ended up either in the torus or in cooling water now gathered in the basements of the reactor and turbine buildings, instead of directly in jet streams. That’s why the atmospheric release from three melting cores was not 300% of Chernobyl, but still less than from the one block at Chernobyl.

  5. Pingback: Tepco and the dirty secret of the radioactive stack

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