More delightful news from Fukushima; and a question


I just read that a second exploratory robot sent into the #2 unit’s primary containment vessel had to be abandoned after losing mobility(unknown if it was radiation or just tread crud; but they had to cut the tether and leave it).

A prior attempt with(I think) the same model robot had to be aborted early after the cameras started failing more rapidly than expected.

So far, so not good; but not terribly surprising. I looked up Toshiba’s press piece for these robots; and then I became confused. They are supposed to be good for a dose of 1000 sieverts during the course of an operation; but everything I read about sieverts tells me that those are the units for ‘equivalent dose’ or ‘effective dose’; both of which are based on weightings of relative biological effectiveness of different sorts of radiation; and weightings of the risks of exposure to different parts of the human body.

I can quite readily understand how an analogous metric for a given robot would be useful; and quite possible to develop; but I don’t understand what it means to quote a robot’s expected survivable dose in sieverts.

Are they saying that it is expected to survive radiation levels that would produce a 1000 sievert exposure in an equivalently placed human? Are the biological effectiveness weightings for tissue sufficiently similar to those for hardware that it is considered negligible for stating an absorbed dose?

I can see why just just the absorbed dose in grays wouldn’t be enough to be meaningful for a robot(just as it isn’t for humans, which is why sieverts are a thing); but what does it mean when a unit developed for human exposure modeling shows up without further comment on a robot’s spec sheet?


I look forward to an answer to your question, which I cannot provide. In my limited understanding of sieverts, their use for robots makes little sense to me, except perhaps as a marketing tool, i.e. “our robots can withstand 1000 times as much radiation as a human.”


I can understand the need for a direct analog of the sievert, for a robot; if anything, electronics are probably markedly more design-dependent in the details of their reaction to different sorts of radiation(eg. a robot with dense metallic shielding would potentially hold up well against gamma rays, if the x-ray fluorescence isn’t intense enough; but be vastly less resistant to neutron sources or charged particles producing bremsstrahlung in the shielding; while humans don’t come in ‘lead shielded’ vs. ‘neutron moderators added’); and since these sorts of robots are usually being judged on their ability to survive for a relatively short time in the face of intense bombardment, not their odds of getting cancer a decade from now, different sorts of damage would be of concern.

What I don’t understand is how a human-weighted sievert could be applicable to a robot.


Sievert weighting has nothing to do with cancer induction… The weighting is just to adjust for the “killing ability” of a given radiation type. The “standard” is typically 250 kVp x-ray rays (if my memory serves correct). They will expose a cell culture to 1 Gy of x-rays and another culture to 1 Gy of 5 MeV neutrons and then compare how well the cells survive (in this case, the neutrons will probably kill off 10 times more cells – implying that 1 Gy of 5 MeV neutrons =10 Sv). Since the killing potential is generally related to the ability of a given radiation type to deliver a lot of energy to a small target (DNA), it should be relatively proportional for a target like chip “traces”. Sievert has no real meaning for electronics, but is likely directly proportional to damage induction.
Edit: forgot to mention that silicon diode detectors are used to measure dose and are relatively tissue-equivalent (at least for electrons/x-rays over a fairly broad energy range)


more delightful news from Fukushima


Poor Toshiba, even with fat stacks of taxpayer Yen their robotics are still crap.


My guess, and it is a guess–I’m no robonuclear engineer, is that most radiation sources are bio-shielded and rated. So if you have a robot poking around where humans dare not tread, it’s likely useful to know the dose in sieverts because all of your shielding’s documentation tells you how many sieverts there are at X distance from Y and at what angle.


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