Forgot your password?
typodupeerror
Japan Power

Chain Reactions Reignited At Fukushima 234

Posted by CmdrTaco
from the someone-explain-the-science dept.
mdsolar writes "Radioactive byproducts indicate that nuclear chain reactions must have been burning at the damaged nuclear reactors long after the disaster unfolded. Tetsuo Matsui at the University of Tokyo, says the limited data from Fukushima indicates that nuclear chain reactions must have reignited at Fuksuhima up to 12 days after the accident. Matsui says the evidence comes from measurements of the ratio of cesium-137 and iodine-131 at several points around the facility and in the seawater nearby."
This discussion has been archived. No new comments can be posted.

Chain Reactions Reignited At Fukushima

Comments Filter:
  • by Anonymous Coward

    That is all.

  • Not surprising: (Score:4, Insightful)

    by Hartree (191324) on Monday May 09, 2011 @09:45AM (#36071298)

    If you melt the fuel, you can get localized criticalities.

    • Without a moderator? (Score:4, Interesting)

      by JSBiff (87824) on Monday May 09, 2011 @09:51AM (#36071360) Journal

      How, without a moderator?

      My understanding is that LEU (low-enriched uranium) cannot achieve criticality without a moderator to slow down the neutrons?

      Can anyone with a nuclear physics/engineering background give any explanation of how you can get a chain reaction without moderator?

      Ok, they were cooling the reactor with water, and water is a moderator, but the water was also boronated, which should cancel the moderation property of water, shouldn't it?

      • by Pumpkin Tuna (1033058) on Monday May 09, 2011 @09:55AM (#36071388)

        In the first-12-day timeframe, the water wasn't boronated, it was just seawater.

        • by JSBiff (87824)

          I thought the news reports all said they boronated the seawater? Maybe I'm misremembering, but it seemed like that had been the case.

          • by Hartree (191324) on Monday May 09, 2011 @10:40AM (#36071828)

            They didn't initially use seawater. They still had normal water in the pile and as far as I know hadn't triggered the systems to release boron in it.

            These would be tiny little areas that would have an accelerated fission rate over just the fuel sitting in the elements. I'm not even sure you could truly call it a criticality in that it wouldn't be self sustaining. You'd get a momentary spike that would tail off. It's pretty insignificant as far as a source of heat or radiation compared to the decay heat and radiation from the fission products.

            Thing is, using a mass spectrometer, you can measure truly tiny amounts of isotopes. You could expect some of the shorter life isotopes from just from occasionaly fissions without criticality. What this study was saying was that the observed ratio of isotopes was such that the particular researcher felt that it would require more than just the expected rate of fissions to get to that ratio.

            That really doesn't surprise me. Nor is it terribly significant.

        • by Vreejack (68778)

          It isn't clear to me how sea water would affect a neutron flux, especially after it had boiled a bit. I don't think it is clear to anyone else, either, but certainly the lack of boron absorb stray neutrons and keep them out of the chain reaction makes criticality more likely.

      • by Dunbal (464142) * on Monday May 09, 2011 @10:15AM (#36071580)

        The scientific method in general terms consists of observation, then hypothesis, then designing an experiment to prove the hypothesis.

        You are arguing "shouldn't it" and closing your mind to the understanding of the observed results - it doesn't matter what it "should" and "shouldn't" do under current models - what is important is what it actually did. Which means that either a) there were conditions that we don't know about that enabled the reaction or b) there are additional underlying scientific principles that we don't fully understand yet. My money would be on the former. However that the data do not agree with what you expected does not necessarily mean the data are wrong. It means you are wrong. Especially in a situation like this where I am sure that the data have been double and triple-checked.

        If you stop trying to fit a square peg into a round hole, this will help you understand the universe better.

        • by JSBiff (87824) on Monday May 09, 2011 @10:30AM (#36071714) Journal

          I'm not arguing anything. I asked a question. If (and that still hasn't been conclusively proven, but there is evidence to indicate a good possibility) that re-criticality occured, then the natural next question becomes *how* did this happen? How is my model flawed? There's absolutely nothing wrong with that.

          I never, ever said in my post that the data is wrong, nor even implied that. I simply asked how this happened without a moderator. So, please climb down off that horse and join the rest of us.

          • by JSBiff (87824)

            I think another poster, perhaps, has a good explanation, btw - I thought the seawater they injected was boronated from the very start, but that may have been the result of either reading inaccurate media reports, or perhaps just confusion on my part as they apparently *did* boronate the water later, but perhaps not right from the start.

            So, it seems the answer to my question may be as simple as, they injected "moderator" (in the form of non-boronated water) into the reactor, creating the conditions necessary

        • The scientific method in general terms consists of observation, then hypothesis, then designing an experiment to prove the hypothesis.

          No! You never design an experiment to prove the hypothesis, you design an experiment to disprove it. If people try for a bit and fail, then the theory is accepted (which is not the same as being true).

          • by camperdave (969942) on Monday May 09, 2011 @03:13PM (#36074754) Journal

            No! You never design an experiment to prove the hypothesis, you design an experiment to disprove it.

            Hypothesis:
            Paper is combustible in air.

            Method:
            Obtain a piece of paper from the photocopier.
            Attempt to ignite paper by exposing it to flame from a lighter.

            Observations:
            Flame appeared to grow in size.
            Paper turned black at flame edge, and appeared to be consumed.
            Flame continued to spread even after removal of ignition source (lighter).
            Much heat was produced necessitating that the sample be dropped into the recycle bin.
            After a short interval, tall flames and smoke were observed issuing from inside the recycle bin
            After a period of a few minutes, flames had reached nearly to the ceiling. At this point alarms started sounding and the sprinkler system began spraying water.
            Approximately eight minutes later, fire trucks arrived and fire crews evacuated the building. No further observations were possible.

            Conclusion
            Paper is combustible in air.

      • by camperslo (704715) on Monday May 09, 2011 @10:21AM (#36071648)

        Perhaps it has something to do with more fuel clumped more closely, like in a pile at the bottom of containment.

        I believe it was unit 1 that had temperatures shoot up after a magnitude 7 aftershock. Given that the cooling situation hadn't changed, is there anything else but fuel shifting that would account for that?

        Fuel that's piled up on the bottom may also get less of the inhibiting effects from either the boron control rods, or boron in solution.

        Some believe that has has been some level of criticality in the unit 4 fuel pond based on the nature of the radiation coming off of that. Between some fuel damage from previous loss of coolant, possible use of coolant without boric acid for a time, and the world-wide industry practice of re-racking, it isn't surprising to have an issue with that. Re-racking is the practice of placing fuel assemblies at a closer spacing than original safety standards called for in or to be able to store more spent fuel.

        Unit 3 has mixed oxide (MOX) fuel which includes plutonium. Since it gives off more neutrons when hit by them, it is harder to control. Reactors may need additional control rods and more boric acid in the coolant during normal operation to stay in control, and more yet when shut down. Unit 3 is potentially more troublesome to control if too much damaged fuel piles up on the the bottom. The environmental damage is also more apt to be longer term. As plutonium breaks down, the material produced actually gives off more radiation..

        This blog has a fairly in depth look at MOX fuel

        http://abundanthope.net/pages/Environment_Science_69/MOX-Fuel---Insanity-Part-1.shtml [abundanthope.net]

      • by KenSeymour (81018)

        These reactors have fuel rods, moderator rods, and control rods. A sub-critical reactor still generates heat.
        Subtract the cooling water, melt some fuel and moderator, the geometry changes, then who knows.
        I can't find what material is in the moderator rods, probably graphite.

        • by JSBiff (87824)

          Are there moderator rods? I was under the impression that Boiling Water Reactors just used the coolant water as the moderator, not rods?

          I think the poster who said the emergency seawater coolant wasn't boronated probably has the answer. I had been (perhaps wrongly) under the impression that boron was being added to the seawater before injection specifically to keep the seawater from acting as a moderator.

          If that was not the case, then there would have been moderator present, and if there were any holes/chan

        • by camperslo (704715) on Monday May 09, 2011 @11:08AM (#36072110)

          Chernobyl had grapite rods which added to the problems since they burned.

          The Fukushima reactors have boron control rods.

          Hopefully there won't be additional fuel damage. There apparently was some in unit 1 a week ago. Although they reported things as stable, they interruptted cooling for an hour or two to set up more permanent power connections. Later the temperature at the bottom of the reactor went from 110C to 143C. They increased the rate of adding water some. I think they're in a hurry to get better cooling with actual recycling, finned radiators, filtering, and good control of the boron levels going. They got air filtering going recently and made the building safe to enter. Last I heard they were about to remove some contaminated material and start checking the original circulating pump. It's good to see them finally making some progress. For a while it seemed like they were hopelessly kept away by the highly contominated water all over. Hopefully they'll get whatever cleans/processes that working well before they run out of space to put the water. Starting to recycle would really help that mess. It sounded like much of the water being pumped out was from turbine areas or tunnels nearby. Without actually sealing up the leak, whatever water does come out will tend to build up more and more contamination.
          I believe they concluded that that mess is all coming from the unit 2 suppression tank. In the drawing it looks like a tire around the bottom (old GE Mark I design). But it's huge. A during-construction photo I saw with someone standing nearby made that suppression pool look maybe 30 feet tall. They'd have to pump in an awful lot of concrete or something to seal that leak...don't know if that;d work while wet and many tons of water and hour going through.

          • by camperslo (704715) on Monday May 09, 2011 @12:48PM (#36073186)

            update:
            more radiation than they hoped in unit 1, 700 ms/hr on the first floor. It won't be easy to work in there unless they can bring that down somehow.

            http://www3.nhk.or.jp/daily/english/08_18.html [nhk.or.jp]

            the unit 4 fuel pond is less damaged than expected, so some good news.

            http://www3.nhk.or.jp/daily/english/08_18.html [nhk.or.jp]

            Diablo Canyon Unit 2 is shut down for refueling and maintenance. Since it was shut down about a month ago and they didn't decide to start refueling then, I suspect there is more to this. They're likely giving it some extra attention. They recently had a motor with the rotor slipping on the shaft. I wondered if they could have had a control system issue (PLC?) instead of mis-calibrated micro-switches and shaft tolerance issues as given for the reason the backup cooling was down for 18 months. Any modifications or even rebooting of a critical control system are potentially dangerous, so those things are best not done with a plant running. It's probably not totally risk free even when shut down since cooling is still essential, but no-doubt they have extra people that know exactly what to watch for and have prepared. It's important that all plants be completely on top of any software vulnerabilities as well as normal issues. There may be a few hot-headed people in some other places about now.

            Some huge military helicopters were seen headed the general direction of Diablo Canyon late last week.. The same type were seen when boric acid was picked up for use in Japan. Foreign news sources had also mentioned Japan dealing with France and South Korea as sources of boric acid.
            They must be going through quite a bit of it and will until they can recycle coolant. Hopefully the 20 mule-team people or whoever are keeping adequate supplies available...

            Hmmm... I bet radioactive coolant with boric acid in it would work great for getting rid of termites... or would they mutate? Someone should make more 50's style movies. Mutants from the sea raising sunken fishing boats...

      • by vlm (69642)

        How, without a moderator?

        My understanding is that LEU (low-enriched uranium) cannot achieve criticality without a moderator to slow down the neutrons?

        Can anyone with a nuclear physics/engineering background give any explanation of how you can get a chain reaction without moderator?

        I did about one year of nuke eng, after saying F Chem-Eng, then said F nuke eng and went EE. And then I never did any EE other than ham radio at home and have been a programmer / sysadmin since then. Yeah I was indecisive as a kid.

        Anyway read the paragraph under the table at:

        http://en.wikipedia.org/wiki/Critical_mass#Critical_mass_of_a_bare_sphere [wikipedia.org]

        I cannot get a straight answer on how enriched the fuel was in the U plants. I believe the one reactor with MOX Pu was running about 5%.

        I cannot get a straight

      • without a moderator?

        Is the moderator something that would go away when everything melts, or is it something that would fall down in the pool of unevenly mixed material we're talking about? Because we're not talking about pure uranium, we're talking about a mess.

        Ok, they were cooling the reactor with water, and water is a moderator, but the water was also boronated

        They were dumping sea water scooped up from helicopters on the damn things, and you think they were stopping in mid-air to add some boron?

        • by JSBiff (87824)

          As I mentioned in one of my previous replies, I do remember seeing news reports which specifically mentioned the use of boronated seawater. However, it's possible that the boronation effort didn't start right away. I think you're right that during the timeframe that they were dropping it from choppers, they probably weren't bothering to boronate it.

          Which raises the question, if you are going to put unboronated water on a melted reactor, do you risk results that would be *worse* than just leaving it to melt,

  • Monsters! (Score:5, Funny)

    by CarsonChittom (2025388) on Monday May 09, 2011 @09:45AM (#36071304) Homepage

    I feel bad for it, but I can't help but wish—just a little bit—that we'll get Godzilla out of this.

  • by Anonymous Coward

    http://www.energyfromthorium.com

    We have no one to blame but ourselves for any accident that happens when a safer, cleaner, more efficient, and cheaper nuclear fuel is readily available and already has most of the hard problems with its implementation worked out through several running prototypes.

  • by sribe (304414) on Monday May 09, 2011 @09:49AM (#36071344)

    If the reactors had been successfully scram'd completely, heat from decay of by-products would have burned out in a very few days. As became obvious, that didn't happen.

    • Re:Well, duh. (Score:5, Interesting)

      by Anonymous Coward on Monday May 09, 2011 @09:58AM (#36071408)

      It did scram completely. The decay heat, which is 7% of 1000 MW boiled away all the water they lost the ability to pump, and then melted the zircalloy fuel rods into a pile of molten slag in places. That slag then has the geometrical configuration to do some more fission. Ironically, they may have had no problems if they didn't scram, as the reactor could then drive power to the cooling pumps, as opposed to relying on diesel generators.

      • Ironically, they may have had no problems if they didn't scram, as the reactor could then drive power to the cooling pumps, as opposed to relying on diesel generators.

        Could be, but you are assuming that all the other stuff was intact after the tsunami: generators, pumps, cooling systems for the generators, etc. I'm guessing they were not, since they've had such huge issues getting water circulating after the tsunami.

        • No, they were. The problem was that the diesel pumps were swamped by the tsunami, and therefore unavailable; The reactor itself was functioning despite a quake in excess of its designed tolerance. The SCRAM shut down the reactor, the diesel generators were unavailable... "Hilarity" ensued (with the portable generators having different connectors to those required for the pumps). With a short period of time before the coolant remaining boiled off, they had to get drastic and pumped in sea water. As water is
        • by JSBiff (87824)

          Most of the newer reactor designs actually use the energy of decay heat to drive some physics that move the heat out of the reactor (mostly by creating convection loops to move coolant up to some heat exchange surfaces which dump the thermal energy into the local air), without requiring any external power, so you're not far off in the idea that the best source of energy to cool a hot reactor is the energy of the hot reactor.

      • by JSBiff (87824) on Monday May 09, 2011 @10:23AM (#36071668) Journal

        "The decay heat, which is 7% of 1000 MW"

        IIRC, the reactors were 1000MW *electrical* output. Because of thermal efficiencies of steam generators of around 35%, I believe that means the thermal output of each reactor would have been about 1000/.35 ~= 2800 MW thermal energy.

        So, instead of 7% of 1000MW = 70MW, I think you're looking at 7% of 2800 = 196MW.

        That's a LOT of heat to get rid of, even if it is a small percentage of the 2800MW full output.

        • by Grendol (583881)

          "The decay heat, which is 7% of 1000 MW"

          IIRC, the reactors were 1000MW *electrical* output. Because of thermal efficiencies of steam generators of around 35%, I believe that means the thermal output of each reactor would have been about 1000/.35 ~= 2800 MW thermal energy.

          So, instead of 7% of 1000MW = 70MW, I think you're looking at 7% of 2800 = 196MW.

          That's a LOT of heat to get rid of, even if it is a small percentage of the 2800MW full output.

          http://en.wikipedia.org/wiki/Fukushima_Daiichi_Nuclear_Power_Plant [wikipedia.org] shows the plant #2 at 784MW for electrical power out.

          Assuming 30% thermal efficiency (35% seems high for a 1973 reactor, but I am guessing honestly), then the full thermal load would be ~2600 MW. 7% of that would be 183MW. So, you aren't too far off.

          Not sure what the water volume of the reactor would be, but if you ever have a hard time falling asleep the NRC has the standards for a BWR/4 reactor (plant #2) at this site http://www.nrc. [nrc.gov]

  • by toygeek (473120) on Monday May 09, 2011 @09:50AM (#36071354) Homepage Journal

    I saw that movie. Not only does it end well but its got Neo in it. Don't worry. There is no spoon.

    Seriously though... that's scary. It might not be Chernobyl but this has got to be the worst nuclear disaster of its type. Although since they're in Japan wouldn't it be called the South America Syndrome? (polar opposite of Fukushima is Chile)

  • I've looked through the paper this report is based on http://arxiv.org/PS_cache/arxiv/pdf/1105/1105.0242v1.pdf [arxiv.org] and I don't see much discussion of the amount of power generated by the proposed post-shutdown criticality. It seems to me that standard operating power is assumed but I don't see how that could work without other signs such as a glowing reactor building.
  • Alternatives... (Score:2, Interesting)

    by ibsteve2u (1184603)
    Join the World Community Grid/Harvard Clean Energy Project [harvard.edu].

    And don't say you don't have a computer.
  • As well as that there has been some speculation that the explosion in unit 3 was more than just a hydrogen explosion. If you compare the unit 1 and unit 3 explosions, you see the unit 3 was far larger in magintude, plus there is a flash right where the spent fuel pool is located. Also pieces of nuclear fuel rods were found 2 km from the site. Arnie Gundersen speculates that this was caused by a "prompt criticality" in the fuel pool, triggered by the hydrogen explosion. http://fairewinds.com/updates [fairewinds.com]

    • by mdsolar (1045926)
      This seems like a stronger argument than the current paper.
    • I very much doubt this, since the both the steel and concrete containment of block 3 are still intact. I think the difference may have been due to the difference of the outer shell, which was not made out of reinforced concrete in the case of block 1. Also, the power of block 3 was around double the one of block 1, so it is possible that more hydrogen was produced. On top of that, the hydrogen accumulated for two days longer in block 3 than in block 1.

      Ah, just saw that a criticality in the spent fuel poo

  • One big issue I see is that the assumption is 7 to 9 months of fuel usage. In block 4, the fuel in the pond was probably significantly older.
    • by mdsolar (1045926)
      According to the paper, that is where the highest ratio I/Cs was found so this is the strongest case for a reaction.
  • There are reactor designs that currently exist that are more resilient to meltdowns.
  • i have to say that the article is interesting, but as far as i understand the fuel in the different reactors is different and has undergone a quite different history.

    The data and evaluation seems a little weak to me in that respect.

The meat is rotten, but the booze is holding out. Computer translation of "The spirit is willing, but the flesh is weak."

Working...