Just amazing about Fukushima

The Japan Times reported on 29 March that radiation inside the vessel has reached 73 sieverts per hour – high enough to administer a lethal dose to a human in a matter of minutes, even to disable the robotic devices which are sent regularly into the reactor to monitor what is happening there.

High radiation levels inside a nuclear reactor.

Just fancy that, eh?

7 thoughts on “Just amazing about Fukushima”

  1. That is appears to be going back up is not good news. It might not be particularly bad news, of course. The radiation level inside a sealed pressure vessel being essentially unimportant (compared to the radiation level outside.)

  2. Except most of that radiation is from fission products, isn’t it? The uranium fuel has a halflife in the hundreds of millions of years, so it has a very low radioactivity per unit weight. The fission products (iodine, cesium, all sorts of other nasty crap) have much shorter half-lives, so an increase in radioactivity would suggest to this layman that there is still some fission reaction going on in there. Or it could be something to do with decay-chain stuff.

    And I guess fission going on inside an uncontrolled and probably inadequately cooled reactor is not good.

  3. Or it could be something to do with decay-chain stuff.

    Yup. If you’ve got a gamma emitter decaying to an alpha emitter, you are likely to get an increase in the effective dose (Seiverts) even if there is no increase in the absolute dose (Grays).

    But, frankly, as some decay chains vary wildly in the half-lives of successive decay products and, given the likely mix of fission remnants, it’s not, of itself, indicative of anything dodgy.

    Remember that low levels of sustained reaction can be maintained even in natural reactors such as that posited at Oklo. You’d be able to tell the difference between fission and decay by monitoring the neutron radiation dose (0 for decays.)

  4. So Much For Subtlety

    Surreptitious Evil – “The radiation level inside a sealed pressure vessel being essentially unimportant (compared to the radiation level outside.)”

    Yes but are we dealing with a sealed vessel? The problem has been leakage from the start. Not that I think people need to worry about what is inside the pressure vessel.

    3 Surreptitious Evil – “Sorry, that should have been “indistinguishable from 0 for decays”.”

    Presumably there will be some measurable level of spontaneous fission of either the uranium fuel or of the plutonium produced in the reactor. Surely that would be very measurable?

    I love the second comment to that article:

    It is disappointed that there have been no direct environmental or human effects offsite, but has professional liars on the job of diverting attention from this to the supposed unknowability of future effects.

    Yes, isn’t it a tragedy that there have been no adverse outcomes? Isn’t it a tragedy no one has died yet?

  5. SMFS,

    Yes but are we dealing with a sealed vessel?

    Hence the importance of measurements outside rather than inside and my qualification “it might not be bad news.”

    Presumably there will be some measurable level of spontaneous fission of either the uranium fuel or of the plutonium produced in the reactor. Surely that would be very measurable?

    But that is the critical question. Is there any spontaneous fission going on (there will be natural decay of the various fuels)? To determine this, you need to measure the level of neutron radiation – as that is actually emitted from fission but not from alpha or beta decays. Measuring the overall radiation level, whether in Sieverts or Grays simply does not give us this information.

    I’m sure you don’t want a lecture on “critical neutron density”. For there to be more than a trivial amount of fission going on, there either needs to be a lot of fuel in close proximity, so that the very small number of fast neutrons have a statistical chance of being absorbed and causing a fission or there needs to be a moderator to slow them down and increase the absorbtion to reflection ratio. There is water in there, which acts as a moderator – the question is the geometry.

    If you asked me to guess, I would suggest that there would be some fission going on but it is unlikely to be a significant contributor to the overall heat output, when compared to the various decay chains. This is based on the stuff having been there, quiescent in a macroscopic context, for over a year. It is not impossible for material to settle or otherwise move into a position to significantly increase the (fission) reaction rate, it is just unlikely.

    I would note that, as we now have a water moderator, if the internal temperature does increase because of any putative fission reaction, then the water will become a less efficient moderator and therefore fission will slow. If we have water trapped in some small pocket which is acting as an effective moderator because of some topological accident, then that is bound to be very close to the fuel and will therefore be heated much more than the rest and its moderating effect will be even more damped.

  6. I’ve just spotted the appalling pun in my comment #6. My apologies to any of the linguistically sensitive who have have suffered psychological trauma from the above, wholly accidental, language abuse.

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