An Engineer's Explanation of the Fukushima Incident • Hacker News Board

Re: An Engineer's Explanation of the Fukushima Incident

#21

Post by Tuna-Fish » Wed, Apr 06, 2011, 11:18 PM UTC

Tobias42 wrote: post #17

bdonlan wrote: post #11

Earlier quoted context omitted.

There's a difference between sub/super critical (producing energy via fission) and "hot" (producing energy _at all_). Chernobyl became more reactive (= more fission) as the water boiled away, leading to a runaway criticality, and explosive disassembly of the core. Fukushima, however, has a negative void coefficient. Moreover, the fission reaction was stopped immediately after the earthquake successfully by the automa…

Do you know if new plants are generally built with containments that work without cooling?

No. The new reactors under construction are mostly very conservative and traditional Gen III+ PWR designs, with some added safety features. The reactors that can survive complete loss of cooling are all Gen IV designs, that are currently under discussion, not construction.

Re: An Engineer's Explanation of the Fukushima Incident

#22

Post by nitrogen » Wed, Apr 06, 2011, 11:40 PM UTC

klbarry wrote: post #9

nitrogen wrote: post #8

It seems that it might be a good idea to design future nuclear reactors with pneumatic-powered robots in mind; purpose-built shafts and ramps could allow robots with minimal electronics into areas unsafe for humans. I say pneumatic instead of hydraulic, as based on my very limited understanding of nuclear physics, lightweight gases are less likely to become neutron activated for extended periods of time.

I believe radiation messes with machines a bit too, though.

I could see neutrons activating the parts of a robot, but what physical mechanism would prevent a pneumatic motor from driving some wheels or moving an arm in the presence of radiation, other than the parts heating up and expanding?

Re: An Engineer's Explanation of the Fukushima Incident

#23

Post by nitrogen » Thu, Apr 07, 2011, 12:19 AM UTC

bdonlan wrote: post #12

nitrogen wrote: post #8

It seems that it might be a good idea to design future nuclear reactors with pneumatic-powered robots in mind; purpose-built shafts and ramps could allow robots with minimal electronics into areas unsafe for humans. I say pneumatic instead of hydraulic, as based on my very limited understanding of nuclear physics, lightweight gases are less likely to become neutron activated for extended periods of time.

Getting robots in wouldn't help with the Fukushima incident much; the real problem there is getting energy out, not getting robots in.

They could have been used for monitoring inaccessible locations earlier in the disaster, possibly by using fiber optic cameras and lighting.

Re: An Engineer's Explanation of the Fukushima Incident

#24

Post by nathanhammond » Thu, Apr 07, 2011, 12:22 AM UTC

Tuna-Fish wrote: post #19

nathanhammond wrote: post #13

Earlier quoted context omitted.

Control rod insertion is designed to be an entirely passive operation, effectively making the default state one where the fission reaction is not active. Your proposals are all different ways to stop the fission reaction but do not address residual heat. What you're really asking for is a system wherein the cooling of the fuel after fission has stopped (this is where the problems occurred at the Fukushima plants) is…

> What you're really asking for is a system wherein the cooling of the fuel after fission has stopped (this is where the problems occurred at the Fukushima plants) is entirely passive. This can be achieved by designing the safe core temperatures to be much higher, which makes cooling easier. The very interesting small molten salt breeder design FUJI MSR http://en.wikipedia.org/wiki/Fuji_MSR is designed to survive com…

And that was a fun way to spend an evening: new reactor designs are quite cool :). It seems like a lot of the innovation is occurring in parallel with both fuel types and containment approaches.

I too look forward to having completely passive shutdown of reactors, but from what I can tell (outside of a few small-scale test reactors) we're just not there yet. Someday...

Re: An Engineer's Explanation of the Fukushima Incident

#25

Post by VladRussian » Thu, Apr 07, 2011, 8:01 PM UTC

bdonlan wrote: post #11

VladRussian wrote: post #10

i'm wondering about the main design principle of the reactor cores - by default they are "hot", ie. producing heat/energy, so one needs to make effort to keep them "cold", ie. non-producing heat/energy. This hot-by-default principle was the main reason of Chernobyl catastrophe and here as well. Why not reverse the principle and design the cold-by-default core? For example, instead of inserting graphite rods into the…

There's a difference between sub/super critical (producing energy via fission) and "hot" (producing energy _at all_). Chernobyl became more reactive (= more fission) as the water boiled away, leading to a runaway criticality, and explosive disassembly of the core. Fukushima, however, has a negative void coefficient. Moreover, the fission reaction was stopped immediately after the earthquake successfully by the automa…

>You can only remove the energy actively until these short-lived products decay to the point where air cooling is sufficient to keep them at a safe temperature.

this is my point - necessity for active removal of energy.

The closest thing to what i was talking about seems to be molten salt reactors where energy producing reaction goes only when salt is pumped through the core/moderator. If something goes wrong - the valves open and the salt is dumped [gravitationally] into the tanks which can be large enough to allow for passive air cooling of the secondary decay energy.

Re: An Engineer's Explanation of the Fukushima Incident

#26

Post by hga » Fri, Apr 08, 2011, 9:05 PM UTC

nitrogen wrote: post #22

klbarry wrote: post #9

Earlier quoted context omitted.

I believe radiation messes with machines a bit too, though.

I could see neutrons activating the parts of a robot, but what physical mechanism would prevent a pneumatic motor from driving some wheels or moving an arm in the presence of radiation, other than the parts heating up and expanding?

A robot that can't see isn't very useful....

One problem I've read in these sorts of situations is the robot eventually or unexpectedly gets killed by the ambient radiation ... and now you have to get humans to dash in there to pull it out of the way.

Re: An Engineer's Explanation of the Fukushima Incident

#27

Post by hga » Fri, Apr 08, 2011, 9:09 PM UTC

derrickpetzold wrote: post #16

I am certainty no expert on nuclear engineering or containment design but it seems odd to me that the spent fuel rods are stored in above ground pools. I wouldn't even let my kids play in one much less store radioactive materiel that requires active cooling in one.

I've read It's an issue with manipulating the fuel: to do so, you unbolt the containment cap on the top of the reactor pressure vessel and fill the entire path from on top of it to the initial spent fuel pool with water. That allows you to safely transfer hot (thermally as well as radiatively) rods and it's pretty hard to get around if you're using this sort of reactor geometry (I understand it holds for both PWRs as well as these BWRs).