[HN Gopher] Chinese pebble-bed nuclear reactor passes "meltdown"...
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Chinese pebble-bed nuclear reactor passes "meltdown" test
Author : bilekas
Score : 91 points
Date : 2024-11-29 18:21 UTC (4 hours ago)
(HTM) web link (www.ans.org)
(TXT) w3m dump (www.ans.org)
| thebeardisred wrote:
| 20+ years later - https://www.wired.com/2004/09/china-5/
| dbcooper wrote:
| I remember that article. An older friend had worked on
| fluidised-bed reactors at Tsinghua in the 1990s, and I sent it
| to him.
|
| Using helium indicates a problem with kinematic viscosity of
| cooling gases? Nitrogen would be non-reactive enough, I assume?
| semi-extrinsic wrote:
| Helium has excellent heat transfer properties (low Prandtl
| number) and does not undergo nuclear reactions when subjected
| to a neutron flux.
|
| Nitrogen will undergo an (n-p) reaction to produce carbon-14
| which has a half-life of 5700 years.
| cyberax wrote:
| Yup. That's why regular PWRs take care not to nitrogen to
| pressurize or flush the primary coolant loop after
| maintenance.
| pfdietz wrote:
| And why reactors that use nitride fuels use fuel made
| with the isotope nitrogen-15.
|
| The front runner steel for use in fusion reactors,
| EUROFER-97, contains a necessary small amount of
| nitrogen. This is enough under some nations' rules to
| render it into intermediate level radioactive waste after
| use, due to the carbon-14 content.
| cyberax wrote:
| Oh yeah. I have a friend who was working on researching
| fusion-safe steels. Solving it fully is going to be a
| real engineering challenge.
|
| Apparently, even a small natural niobium contamination
| would make it a low-grade waste.
| pfdietz wrote:
| Yes, niobium is the other problem. I think steel makers
| are used to _adding_ alloying elements, but not set up
| for _excluding_ them to ppm levels. I saw someone
| bemoaning that the steel alone for DEMO would cost 3
| billion euros (or dollars?), and I 've wondered if this
| is the reason.
|
| https://scipub.euro-fusion.org/wp-
| content/uploads/eurofusion...
|
| "[...] in the first layer, nearest the plasma, the rate
| of production of 94Nb - via neutron capture (n,g)
| reactions on the stable 93Nb of niobium - is so high that
| Eurofer in this region is predicted to exceed the France-
| LLW limit within the first year of operation, and
| consequently would not be disposable as LLW under French
| regulations for more than 1000 years."
|
| It's not just the steel. Beryllium typically contains
| about 100 ppm U, and an estimate of the cost of purifying
| it enough to avoid excessive fission products was another
| billion.
| HPsquared wrote:
| Nitrogen is a big neutron absorber in gas-cooled reactors.
| It's actually used as a secondary shutdown mechanism in the
| UK's AGR reactors. So if it was to leak out (lose pressure)
| you'd see an increase in power at the same time as a loss of
| cooling... Not a great idea!
| pineaux wrote:
| Can someone explain to me how this reactor is meltdown proof?
| DennisL123 wrote:
| It's meltdown proof in principle since the process goes into an
| equilibrium rather than into a runaway process.
| cosmotic wrote:
| Wikipedia has an explanation:
| https://en.wikipedia.org/wiki/Pebble-bed_reactor#Safety
|
| Summary: As the temperature rises, neutron absorption
| increases, reducing fission and thus temperature.
| chickenbig wrote:
| > As the temperature rises, neutron absorption increases,
| reducing fission and thus temperature.
|
| Negative fuel temperature coefficient is not an unusual
| feature.
|
| The real question is whether the heat removal system of the
| reactor as a whole is sufficient to remove the decay heat to
| keep the fuel within the limits.
| Gibbon1 wrote:
| I remember talking to an engineer at the old GE nuclear
| research facility in San Jose. He said you can design
| reactors to be cooled by natural convection.
| pfdietz wrote:
| It just makes them larger. And it makes the building
| containing them larger. And this makes them more
| expensive.
|
| NuScale's reactor was originally motivated by the desire
| to make it safer by using natural convection. But it ends
| up requiring 1/3rd more labor hours to build a NPP using
| their reactors than it does to build a conventional large
| reactor power plant.
| euroderf wrote:
| IIRC there is a question about graphite fires.
| AtlasBarfed wrote:
| Can pebble beds have a cooldown pan similar to a LFTR, where a
| plug melts and the "pebbles" fall and spread into a pan where
| they won't stay critical because they are too separated /
| unconcentrated?
|
| Because the real problem with solid rods is that they ... are
| solid rods, and if they start "overreacting" you can't split up
| the rods, unlike a pile of pebbles/spheres.
| cyberax wrote:
| The unique "melt plug" safety story of LFTRs is mostly a
| fairy tale.
|
| Modern PWRs also have this safety feature, if a core melts
| down, the molten mass will be contained in a core catcher.
| Where it'll be mixed with inert material that can provide
| enough surface area and thermal mass to prevent further fuel
| mass migration.
|
| The biggest problem in the core catcher design was to make
| sure that the molten fuel lava spreads out enough for the
| passive cooling to stop it from melting through concrete.
|
| Pebble bed reactors will have a similar problem. You can
| "drain" pebble beds somewhere, but then you need to make sure
| that this "somewhere" can conduct away the decay heat without
| melting.
| cyberax wrote:
| The reactor vessel is humongous, so the natural convective
| cooling can carry away the decay heat. The pebbles themselves
| can tolerate extremely high temperatures (literally glowing
| white-hot) without burning.
| kevin_thibedeau wrote:
| It's not, just statistically unlikely assuming no fuel pebbles
| crack and coalesce their fragments.
| muditmudit wrote:
| Tangentially related:
| https://youtu.be/0gskQJE6lxU?si=nztv5C0Et7pBJeMj
|
| This video explores an incident with a reactor of a similar
| design, and very rudimentarily explains the way pebbles and the
| helium gas is used.
| petre wrote:
| They should also test with cracked pebbles.
| cyberax wrote:
| Pebble bed reactors are a bad idea in general.
|
| They will be HUMONGOUS because they need a large surface to
| radiate away the heat for the passive safety, so they can't be
| easily put into a containment building.
|
| A core of a PWR plant is _tiny_ for the amount of power it
| produces (around 3GWt!), just around 5 meters in diameter and 15
| meters in height.
|
| The pebble bed reactor in the article (HTR-PM) is around the same
| size, but it produces a mere 0.25 GWt.
|
| Pebbles themselves are also problematic, they tend to swell,
| crack, and they can't be reprocessed using the current
| technologies. They MASSIVELY increase the amount of waste.
| Grosvenor wrote:
| Seems like that would be fine for places where you have <
| 0.25GWt energy needs, and need a safe power source. Like remote
| installations/towns. Antarctic research stations, etc.
| cyberax wrote:
| Pebble bed reactors are indeed researched as a source for
| process heat (e.g. for steel or concrete production). But I
| really dislike that.
|
| If you just need 250MW of power, then just use electricity
| sourced from a regular PWR for heating. It'll be cheaper.
| shepherdjerred wrote:
| Is GWt a common abbreviation for gigawatt? I first read that as
| gigawatt-tons which is a... confusing unit
| howenterprisey wrote:
| Gigawatt thermal, as opposed to gigawatt electric. Gigawatt
| thermal is the heat your power plant makes, whereas gigawatt
| electric is the electricity that the heat is used to
| generate. They're not the same because not all the heat can
| be converted into electricity, and the percent of heat that
| gets converted varies from power plant to power plant.
| shepherdjerred wrote:
| Oh that makes total sense! Thanks for explaining
| freeone3000 wrote:
| So for a 3GW pebble bed reactor, we're looking at a core the
| size of small house instead of a master bedroom? I don't see a
| huge difference here; it's the same amount of everything else
| (cooling, pumps, turbines, security) since it produces the same
| amount of heat/power.
| cyberax wrote:
| > So for a 3GW pebble bed reactor, we're looking at a core
| the size of small house instead of a master bedroom?
|
| No, we're looking at a core the size of a small residential
| tower. Probably around 30 meters in height.
| JojoFatsani wrote:
| That's not too big man
| pfdietz wrote:
| The other problem is mechanical abrasion of the pebbles,
| creating radioactive dust.
|
| Germany's PBR had to filled with concrete after it was
| defueled, they couldn't decontaminate it enough to dismantle
| it.
| Animats wrote:
| But is it jam-proof? [1][2]
|
| [1] https://en.wikipedia.org/wiki/AVR_reactor
|
| [2] https://en.wikipedia.org/wiki/THTR-300
| consumer451 wrote:
| Thanks for sharing those links. That was a very interesting
| read.
| cynicalsecurity wrote:
| The obsession with "green" energy wouldn't do us any good, would
| it.
| yk wrote:
| > Other pebble beds: The pebble bed technology and design has
| previously been used in prototype reactors in China and Germany,
| but not a larger-scale plant like Shidaowan.
|
| That's wrong, Hamm-Uentrop was a full scale commercial reactor.
| It did run in total for a week or so between 1985 and 1989 and
| was then shut down. The fundamental problem is, that the pebbles
| grind against each other, and being of the same material as
| pebbles they can grind each other down. (Now if you wonder why
| this wasn't discovered at the experimental reactor in Juellich,
| those guys just never mentioned that they lost fuel.)
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