Effects of radiation inside the damaged Fukushima power plants on electronics? - Page 1
Forum section: electronics / general

author: cdev
author_rank: Super Contributor
timestamp: December 31, 2020, 12:40:11 am
content: I am reading about how the estimates of the amount oif radiation inside the damaged reactor vessels have turned out to be way too low, and this has made me curious about the failure of technology in being able to somehow compensate adequately for that high level of radiation. (Edit: Wikipedia on Radiation hardeninghttps://en.wikipedia.org/wiki/Radiation_hardening)I'm wondering, from a technical perspective, whats going on with that?EDIT: It looks like they are doing much better now with more recent robots than they were previously.Here is video from inside the reactorsThis is a discussion of the radiation and one new design..Mapping the radiation

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 31, 2020, 02:56:30 am
content: Sure.Electronics, on an operational basis:- Ionizing radiation causes leakage currents to flow. This is noisy over both time and space: e.g., any given event (impact of high-energy particle) might affect some transistors or gates here or there, to varying severity. The increased leakage makes "off"s less "off". This affects analog circuits, dynamic logic, etc.- The leakage currents can effectively get trapped or rectified by PN junctions. This is much the same as light causing a voltage drop across a solar cell, but with streaks (paths of high-energy particles) of charge carriers being displaced, instead of lazily dissociated by absorbed light. These charges and voltages, in turn, cause unexpected error voltages in analog and digital circuitry. (Probably, this is mostly responsible for bit-flips in practice? I don't know the literature in depth on this.)So, CCDs will record mostly normal images, plus noise -- random pixels reading high or low (which way depends on architecture, I suppose), by an amount that roughly corresponds to the amount of energy that hit that pixel. (Or maybe it saturates quite quickly, so that for average [nuclear fission products] sources, noise mostly manifests as full-scale saturation, I don't know.)Electronics, on a reliability basis:- Freed charges get trapped in insulators (hot carriers), causing shifts in offset voltage, "on" or "off"-ness (digital thresholds), erroneous data (e.g. EPROM, Flash).For an image sensor, some of these may manifest as stuck pixels. Others as hardware broken in various ways (er, maybe the clocks or counters get corrupted, and the readout ends up with some degenerate interleaved or blocky patterns??). Or, going up a level, plain old software failures (erroneous address or data R/W, illegal instructions, etc.).- Ions can get displaced, causing defects in the crystal lattice itself.- Whole ions can get implanted, if going fast enough.Probably nothing flying through the air is hot enough to do the latter, or at least by today; but operating cores are a pretty nasty place to be, with particle energies of some MeV being common (for particles from protons, neutrons, and alphas, up to heavy atomic fragments, though the latter should be well contained by the fuel itself). I'm... not sure that that's enough energy for ions to penetrate into a packaged semiconductor, but eh, if we're considering anything currently available -- bare-die CSPs would be pretty vulnerable to this sort of exposure.(Ion implantation is even used as a manufacturing process, for doping, SOI and cleavage processes. When done with a controlled energy and deposition pattern, the ions can actually be deposited largely at some depth (~um). Doping of course is just B, P, etc. ions, in low concentration to form useful semiconducting structures. O is used to form SiO2 at a controlled depth, turning a thick wafer single-crystal into a thin-film single-crystal surface floating on glass, without having to try and saw such a thing apart. H is used to cause a break in the crystal so that very thin layers can be lifted off; this works partly by mechanical strain, partly by forming Si-H bonds which therefore break the Si-Si bonds that otherwise hold the crystal together.)Anyway, the big problem for reactors is neutrons -- being neutral, they careen straight through matter with little care for all those electrons in the way. Protons might have a little penetration depths at these energies, but heavier and heavier particles have less and less (alpha radiation already is easily stopped by thin foils). Neutrons mainly interact with nuclei, elastically (e.g., knocking hydrogens out of place in water, or carbons in graphite), or by absorption (hence neutron activation: tritium, 14C, etc.).And, by ion displacement, I mean Si atoms in the lattice itself. If knocked into a hole in the lattice, the defect acts as a doping site, where recombination can occur: this increases leakage current across PN junctions, and reduces recombination time. (This is intentionally used in high speed rectifiers, typically done through electron bombardment, and the use of transition metal dopants like Au and Pt. The downside is the lower breakdown voltage and higher voltage drop -- nothing's free.) Hm, I don't know that dislocations can act as P/N doping sites, but maybe that's something too. In which case, random defects should still just act as random doping (which doesn't usually help much). At high enough concentration, defects will dominate over intentional impurities and the semiconductor will simply not function in any way at all.At very high doses (like exposed-core levels), this failure mode extends even to bulk materials. The steel, zirconium and so on used in reactor hardware has to be, hmm I forget offhand if this applies to these metals exactly, actually -- if so, it'll be part of their composition and operation, otherwise not. But I recall specifically, at least graphite -- when used in this application -- needs to be operated above a certain temperature, otherwise these defects will accumulate and actually cause a hazard. Graphite isn't used in (western) PWR/BWRs, so this is maybe a bit irrelevant.In any case, defects can accumulate to such a degree that whole mechanical properties are affected, typically in terms of embrittlement. Mitigation for this effect can be as simple as operating above some temperature, where diffusion can take place -- moving atoms back into their proper places; annealing. Often, this is accompanied by poor mechanical properties (creep), so it depends on metal, alloy and etc.I do recall, specifically the case for graphite: at low temperatures, below the annealing point (< 200°C), it holds onto all those defects, yet the amount of damage that can accumulate is so severe that the material mightcatch firewhen heated!Clearly, if you need to send vehicles into such an environment (like an exposed core), at the very least they need to be made of extremely basic, robust materials (like steel, and ceramic or glass). Organic materials perform poorly, rapidly deteriorating by scission of molecular bonds, formation of new ones (cross-linking), decomposition (e.g. stripping hydrogens from hydrocarbon polymers), etc. The electronic state of such a material might be okay for a while, then it fails as some weird conductive or semiconducting mess. If it fails too conductive, obviously it's likely to cause a runaway failure.Perhaps vehicles could be made out of especially rad-hard conductors (suitable copper and steel alloys) and insulators (polyimide (one of the few organic polymers stable enough to resist some damage by radiation), glass fiber?); perhaps bearings can use ceramic or metal balls and no lubrication, or graphite or other dry lubricants; and mostly just motors and gearing, with everything coming up the umbilical to a control system in a safer location. I don't know exactly how rad-hard they got with the vehicles they tried at Chernobyl; what the had, all failed, though (as I recall). (Alas, I know even less about the vehicles exploring Fukushima... I have seen some of the pictures, so they at least seem to be doing something right, or at least have been keeping to the cooler parts, away from the core remains.)It is possible to shield high-tech electronics from fairly strong radiation; it's expensive though, and needs to be designed in from the start. The Juno probe (seehttps://en.wikipedia.org/wiki/Juno_Radiation_Vault) experiences, hmm, I can't find good numbers for comparison -- it may be a modest fraction of a reactor core's flux?I see a few figures on the order of 10^7 particles/cm^2/s for Jupiter, versus ~10^12 for a reactor. So, heh, given that the one is spread out in space over about the volume of the Sun, I guess one might arguably still call such a fraction "modest"...Mind, even the mission's orbital design takes radiation into account -- a highly elliptical orbit, minimizing exposure to the deepest radiation belts.So, I suppose months of operation around Jupiter might correspond to minutes inside a fission core, in terms of exposure alone. The acute symptoms (ionization of semiconductor junctions) would dominate at such levels, making operation impossible, at least without a corresponding amount of shielding (> 80dB in terms of neutron flux? I wonder how much thickness of boron would be needed for that..).Tim

author: cdev
author_rank: Super Contributor
timestamp: December 31, 2020, 03:20:51 am
content: The effect of radiation on electronics, seems to me to perhaps be similar to the effect of COVID-19 on the brain, which is terrifying. Random errors and losses of functionality might be said to occur.