How safe are helium cooled pebble bed reactors?

jagdish
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Re: How safe are helium cooled pebble bed reactors?

Post by jagdish » May 06, 2014 11:12 pm

It appears that the Chinese are starting the construction of a pebble reactor
http://en.wikipedia.org/wiki/HTR-10
I wonder what coolant they are using. Is it helium?

E Ireland
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Re: How safe are helium cooled pebble bed reactors?

Post by E Ireland » May 06, 2014 11:20 pm

Is there any way you could build a gas cooled reactor that could scram and then cool by water spray injection followed by water flooding?
If the reactor is already 'overmoderated' I think the water injection would be a net reactivity reduction and it could solve the cooling issue in a blackout scenario if the thermal shock issues can be overcome.

Using distilled water (if possible in criticality terms) would allow it to be easily boiled off at reactor restart prior to recommencing conventional gas cooled operation.

jagdish
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Re: How safe are helium cooled pebble bed reactors?

Post by jagdish » May 06, 2014 11:58 pm

The only way water could quench the heat as well a nuclear activity in a gas cooled reactor, in my opinion, would be by a solution containing a neutron poison like borates or cadmium salts.

E Ireland
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Re: How safe are helium cooled pebble bed reactors?

Post by E Ireland » May 07, 2014 12:40 am

Graphite moderated water cooled reactors have had positive void coefficients (like the RBMK) which implies that you could build a gas cooled core that would have its reactivity reduced by introducing water.

Soluble salts would make this more efficient but would have all sorts of other issues like removing the salts after the blackout has ended.

Cyril R
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Re: How safe are helium cooled pebble bed reactors?

Post by Cyril R » May 07, 2014 1:14 am

E Ireland wrote:Graphite moderated water cooled reactors have had positive void coefficients (like the RBMK) which implies that you could build a gas cooled core that would have its reactivity reduced by introducing water.
Water reacts with hot graphite to make flammable noncondensable gas.

Its like trying to put out a fire with diesel. It can work, if you inject enough diesel quickly enough... otherwise you will make things worse.

The idea with helium cooled pebble beds is they have passive safety. No water injection. Just heat losses to some air cooled cavity system.

Because helium is a poor coolant, the temperature differential across the fuel and graphite is huge. Since they have negative temperature coefficient, adding water is a very bad idea. It will add reactivity. But again, adding water is silly anyway, since it will combust the graphite to a flammable gas.

In any case a reactor with positive power coeffient will not get a license just about everywhere these days.

E Ireland
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Re: How safe are helium cooled pebble bed reactors?

Post by E Ireland » May 07, 2014 1:22 am

Could just dump liquid nitrogen into the reactor vessel I suppose.... have a dewar of liquid nitrogen on site to provide it.......

I am sure you could rig some kind of failsafe system that would start injection of the LN2 into the reactor on depressurisation, then have an temperature sensitive rupture disk to depressurise the reactor if natural circulation/emergeancy circulation fails for whatever reason.

If fuel damage has not occured on a massive scale (and it is hard to imagine how such damage could occur in a Pebble Bed System as the fuel is meant to be almost indestructable) couldn't you just vent the overpressure gas to atmosphere through filters to trap any radioactive particles.
Overcoming the temperature coefficient of reactivity problem would just require careful design of the injection system to prevent shock cooling of any particular part of the assembly.

And I would hope the shutdown system would be able to keep the reactor subcritical at room temperature.

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Re: How safe are helium cooled pebble bed reactors?

Post by Cyril R » May 07, 2014 4:12 am

Not sure if nitrogen is enough poison to guard against Xe and Np/Pa decay reactivity insertion. Likely not, since gaseous nitrogen has low density and is not that absorptive.

Analysis from Syd Ball suggests even TRISO fuel spheres are not safe in the PBMR long term ATWS-SBO. Temperatures in excess of 2000C are seen. At that temperature even modern TRISO fuel is not guaranteed containment.

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Re: How safe are helium cooled pebble bed reactors?

Post by E Ireland » May 07, 2014 9:24 am

If we could intervene quickly enough the massive cooling sink introduced by the liquid nitrogen supply which could be easily over a thousand tonnes (using a commercially available storage tank from Linde) might be able to prevent such enormous temperatures from occuring - at least for several days to weeks.
That would give time for a portable nitrogen production plant to arrive on site.

Shouldn't the shutdown system normally be designed to hold up against those reactivity insertions otherwise it would never fbe able to control a fully fueled fresh core?

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Re: How safe are helium cooled pebble bed reactors?

Post by Boris H » May 07, 2014 10:22 am

jagdish wrote:It appears that the Chinese are starting the construction of a pebble reactor
http://en.wikipedia.org/wiki/HTR-10
I wonder what coolant they are using. Is it helium?
It's Helium, yes.
jagdish wrote: Could just dump liquid nitrogen into the reactor vessel I suppose.... have a dewar of liquid nitrogen on site to provide it.......
The thermal shock of such a rapid cooling will break vessel and piping. Something you may want to avoid...
E Ireland wrote: Shouldn't the shutdown system normally be designed to hold up against those reactivity insertions otherwise it would never fbe able to control a fully fueled fresh core?
In an ATWS scenario, you assume the the normal shutdown system is not available---however reliable it may be.

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Re: How safe are helium cooled pebble bed reactors?

Post by Cyril R » May 07, 2014 12:00 pm

Yes, the idea here is the control system fails and the power is out.

Having liquid nitrogen injection may not help much. The decay heat can be cooled away by the passive cavity cooling type system. It is the added fission heat that we are concerned with, especially when Xe135 decays and adds reactivity. The helium cooled pebble bed reactor has negative power coefficient, so cooling it down with nitrogen may have the opposite effect. The reactor will want to run at a higher temperature. That is the issue here.

It will help more to improve the thermal conductivity of the core so that the peak temperature is closer to average core temperature. This is evidenced in the much lower peak temperatures of the GT-MHR. Turns out pebbles aren't very good in conducting heat away...

jagdish
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Re: How safe are helium cooled pebble bed reactors?

Post by jagdish » May 12, 2014 12:18 am

Helium is an exotic material used for experimental reactors only. The gas cooled power reactors are AGR in the UK, using carbon dioxide.

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bensoon
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Re: How safe are helium cooled pebble bed reactors?

Post by bensoon » May 19, 2014 12:50 am

The Chinese HTR-PM, which is a scale up of the HTR-10 from Tsinghua University uses Helium.

Anyway the question of how safe helium cooled PBRs are depends entirely on who you are asking. If run competently by competent engineers and operators, it has the potential to be very safe. That's if you totally disregard N.I.S. (Negligence, Incompetence & Sabotage).

Another question you should ask is about the TRISO fuel elements. The demise of the SA PBMR project was in large part due to the fact that they could not satisfactorily answer the question of graphite dust (formed from inevitable intra-fuel element friction) within the primary loop. This would not be so serious were it not for the fact that it is an established fact that even modern fuel elements 'insufficiently retain' metallic fission products (in particular Sr-90 and Cs-137) over the course of normal operation in a PBR which features many excursions into different temperature ranges. These fission products readily attach themselves to the graphite dust.

This completely devalues Helium's low cross section (low activation) properties in the worst case scenario of a containment and primary loop breach. The ultra high pressure required to effectively operate a HTGR coupled by Helium's lighter than air properties create the perfect conditions for rapidly dispersing the graphite dust and it's cargo across tremendous areas. Heck you don't even need Helium to cause the dispersion, just the pressure alone would do it, but Helium just adds icing to the cake.

This is my main concern for the Shandong Reactor in China. But so far, according to everything I've heard, there have been no reports of ANY incidents minor or major during the HTR-10 program at Tsinghua, compared to the utter catastrophes the Germans had with the AVR (which HTR-MODUL is based off of and subsequently which HTR-10 & HTR-PM is based off of).

This leads me to the only conclusion that the Chinese are way better engineers than the Germans ;)

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Re: How safe are helium cooled pebble bed reactors?

Post by jon » May 19, 2014 2:46 am

' This leads me to the only conclusion that the Chinese are way better engineers than the Germans '
Maybe they are - or at least they're profitting from the Germans' mistakes. According to Rod Adams, who spent a lot of time promoting a nitrogen cooled PBR, a big factor in the creation of dust and pebble breakage was that the Germans were dropping their control rods straight into the pebbles. Putting the control rods in the hole of the pebble torus, or outside the edge, fixes that.
Filtration should normally prevent dust from getting into the environment; the HTHR only leaked dust after a duct was inadvertently left open while they were trying to unjam a stuck pebble.
I thought the most problematic fission product in silicon carbide coated fuel was silver, but this paper claims it is manageable.
http://web.mit.edu/pebble-bed/papers1_f ... arbide.pdf

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bensoon
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Re: How safe are helium cooled pebble bed reactors?

Post by bensoon » May 20, 2014 1:12 am

That's good to know, I certainly hope they've corrected all the German mistakes. I'm personally not a big fan of PBRs, as great as they appear to be, I still have a hard time getting around the idea that we should adopt such an approach when the LFTR presents the best engineering solution to the question of nuclear energy generation. i.e. why go for second best if you can go for the best solution in with the same investment in time and money?

But in anycase, they may have other considerations like existing industry pressure, political considerations etc etc whatnot so I won't dispute that for now.

Still the movement of pebbles within the core still cause intra-pebble friction, the Pyrolytic carbon outer layer are resistant to impact damage but I have yet to see any studies dispute the fact that friction between the pebbles WILL NOT cause the formation of graphite dust. The only way to mitigate that problem if it cannot be definitively solved is to use a molten salt as the coolant in place of gas since that will chemically oxidize the fission products even if they should migrate outside the pebbles.

I see the problem of perception again dogging the public consciousness of reactor safety; even if non-harmful levels of Sr-90 should escape the reactor confinement boundary, you are going to get HUGE public backlash regardless of whether it's warranted or not. Sad but true current state of affairs.....

jagdish
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Re: How safe are helium cooled pebble bed reactors?

Post by jagdish » May 24, 2014 5:51 am

Volume of carbon moderator is generally nearly 10 times the fuel, as stated in other threads. It would be true of fuel pebbles too. This would increase the size and cost of reactor building. Moving the pebbles in the core would increase the dust. Only a fast spectrum pebble fuel may make sense. It could a use molten salt coolant not needing ultra-pure Li-7.
The Chinese Helium cooled reactor may be only an experimental one. T in the HTR may stand for temperature or test like the Indian FBTR reactor.

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