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 Barseb├Ąck nuclear power station was in addition to any filtration provided by the wetwell pool, not in place of it. Barseb├Ąck 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.

3 thoughts on “Tepco’s unfiltered vent path

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  3. If you have any questions about the filter systems, you can ask the Swedish Radiation Safety Authority.

    http://www.stralsakerhetsmyndigheten.se/In-English/About-the-Swedish-Radiation-Safety-Authority1/

    Recent stress tests:
    http://www.stralsakerhetsmyndigheten.se/In-English/About-the-Swedish-Radiation-Safety-Authority1/News1/Stress-tests-The-Authoritys-initial-assessment/

    Small correction: The filter installed at the now decommissioned Barseback plant were

    The 10 reactors currently operating (when they are not stopped due to overeager upgrade efforts…) are based venturi / scrubber system.

    As to the cost, I dimly recall that the Barseback plant got the FILTRA system due to its closeness to the Danish capital. It was not that cheap, since it was quite big. On the plus side, every technology involved in it was quite “dumb”, and unlikely to fail.

    The other reactors got their containment venting filters later, since there was more work to verify their functions.

    According to a telephone call to the Rad Safe Authority, the expected probability of the filters not working is around 1%.

    I sincerely hope all reactor operators quickly put some serious funds into studying filtered containment venting systems, and then quickly get them into place.

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