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

Re: An Engineer's Explanation of the Fukushima Incident

#11

Post by bdonlan » Wed, Apr 06, 2011, 8:10 PM UTC

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 automatic insertion of control rods. The heat that is being generated is from short-lived fission products (ie, nuclear waste) undergoing spontaneous decay events, thus producing heat (but only a small fraction of the heat of an active reactor).

It is physically impossible to design a reactor in which such spontaneous decay can be stopped. 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. You can, however, design a reactor in which the passive containment structures can withstand the temperatures of fresh waste material with no active cooling; however such technology was not available at the time the Fukushima plants were built.

Re: An Engineer's Explanation of the Fukushima Incident

#12

Post by bdonlan » Wed, Apr 06, 2011, 8:11 PM UTC

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.

Re: An Engineer's Explanation of the Fukushima Incident

#13

Post by nathanhammond » Wed, Apr 06, 2011, 8:14 PM UTC

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…

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 entirely passive.

I'm sure somebody could get clever with siphons to make this happen but you'll need an absolutely massive amount of water to do so and/or a way to do the cooling such that the water itself is not exposed to radiation. This would allow you to, for example, safely take it from one side of the dam to the other.

An active solution instead allows recirculation of the same water (often borated) and can generally be a closed-loop (and radiation apathetic) system.

Re: An Engineer's Explanation of the Fukushima Incident

#14

Post by VladRussian » Wed, Apr 06, 2011, 8:42 PM UTC

nathanhammond wrote: post #13

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…

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…

>Control rod insertion is designed to be an entirely passive operation, effectively making the default state one where the fission reaction is not active.

well, the rods stuck at 1/3 while being dropped into Chernobyl core as the things had already started going wrong.

Re: An Engineer's Explanation of the Fukushima Incident

#15

Post by andrewcooke » Wed, Apr 06, 2011, 9:38 PM UTC

nathanhammond wrote: post #13

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…

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…

Convection driven cooling is also possible (no need for siphons).

Re: An Engineer's Explanation of the Fukushima Incident

#16

Post by derrickpetzold » Wed, Apr 06, 2011, 9:51 PM UTC

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.

Re: An Engineer's Explanation of the Fukushima Incident

#17

Post by Tobias42 » Wed, Apr 06, 2011, 10:09 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…

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

Re: An Engineer's Explanation of the Fukushima Incident

#18

Post by uvdiv » Wed, Apr 06, 2011, 10:38 PM UTC

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…

The problem here isn't criticality but waste heat: heat generated by uncontrolled radioactive decay of waste products (fission products and products of neutron activation). There's no way to turn off this heat; it continues uncontrollably for a long time, decaying super-exponentially (here's a graph: [1]).

As far as the nuclear reaction goes, the Fukushima reactors were "off" within seconds of the initial earthquake, and remained so since then. (Though, tangentially, there's been speculation of some low-power criticality events since then, but the evidence is flimsy. It's not particularly important anyway). That's the polar opposite of the Chernobyl disaster, where it was an uncontrolled nuclear reaction which destroyed the reactor building and released vast amounts of nuclear waste in an explosion (and subsequent fire). In Fukushima (and Three Mile Island, and others) there was no uncontrolled reaction; but there was decay heat and its effects. The big explosions at Fukushima were an indirect effect of decay heat: explosions of hydrogen generated by when extremely hot zirconium "burned" in steam. And the large scale fuel failure ("meltdown") was also caused by decay heat -- too much decay heat, and not enough cooling.

[1] http://en.wikipedia.org/wiki/Decay_heat#Power_reactors_in_sh...

Re: An Engineer's Explanation of the Fukushima Incident

#19

Post by Tuna-Fish » Wed, Apr 06, 2011, 10:55 PM UTC

nathanhammond wrote: post #13

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…

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 complete external cooling loss by shutting down fission (either under control, or by the heat physically destroying the neutron mirrors required for it's operation), and then dealing with the decay heat by conductive transfer to the environment and simply letting the reactor heat up.

The fact that the core is molten in normal operation, under no pressure and entirely chemically inert even at elevated temperatures makes passive safety somewhat easier.

Re: An Engineer's Explanation of the Fukushima Incident

#20

Post by nanidin » Wed, Apr 06, 2011, 11:03 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 feel bad for your kids