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 Post subject: UK AGRs
PostPosted: Feb 16, 2014 2:24 am 
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http://www.world-nuclear-news.org/C-UK- ... 02148.html
Compared to new EPR's, the AGR would be an economical source of power. The UK should develop Th-Pu MOX for their AGR's like the Norwegians are trying with PWR's. Thorium based fuel will
1. Utilize more Pu than U-Pu MOX in initial loading.
2. Have a higher burn up. It will burn more Th in situ to get more power from same Pu as compared to U-Pu MOX fuel.
3. Produce U-233 in the used fuel. It could be used in LFTR or fast thorium reactors if and when developed. It could be extracted more easily than Pu in the PUREX by chloride volatility or leaching out as uranyl salts.
Not only the Pu stocks available be usefully employed as fuel but gas turbines could be developed for higher thermal efficiency. If CO2 at higher temperature is corrosive, Argon could be tried as alternative. It is premature to give up on AGR.


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 Post subject: Re: UK AGRs
PostPosted: Feb 16, 2014 10:08 am 
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Jagdish, agree with most of what you said except for the economics and argon.

AGRs were reasonably cheap in the time that all reactors were still cheap. But PWRs and BWRs being built at the time of AGR build were cheaper than AGR. If you build an AGR today, the cost would almost certainly be higher than EPR on a $/kWe basis.

CO2 is not very corrosive to the right materials. Just don't be cheap with urinal quality steel. Be sensitive. Use higher alloy steels and you're fine. Generally all materials used in supercritical steam systems will be applicable to supercritical CO2 with slightly lower corrosion rates.

Argon is a horrible coolant. Stay away from it, unless your purpose is to insulate (reduce heat losses). In addition to the worst thermo-hydraulic properties, it also has unacceptable thermal neutron activation.


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 Post subject: Re: UK AGRs
PostPosted: Feb 16, 2014 12:32 pm 
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Sizewell B was completed for a capital cost of about £1.70/W all up - and that was the first ever civilian PWR in Britain.
Which is so much less than the EPR that its not even funny.

But thats just because EPR is a disaster.


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 Post subject: Re: UK AGRs
PostPosted: Feb 16, 2014 4:23 pm 
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E Ireland wrote:
Sizewell B was completed for a capital cost of about £1.70/W all up - and that was the first ever civilian PWR in Britain.
Which is so much less than the EPR that its not even funny.

But thats just because EPR is a disaster.


In the early days of LWR buildout in the USA typical costs were in the $200-400/kWe range.

That's not for old unsafe equipment, its for systems with all the backups, containments etc. installed.


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 Post subject: Re: UK AGRs
PostPosted: Feb 16, 2014 10:33 pm 
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There were gas reactors in France too, they were built in the 1950's, the UNGGs (Uranium Naturel Graphite Gaz), they used natural uranium and were moderated by graphite and cooled by CO2.

This design was abandoned in 1969 and we switched for the PWRs with the Westinghouse's license if I am correct.

If I remember well, the main reasons for this abandon was economics :

  • It was difficult to have more power than 700 MWe
  • The PWRs were more economical, thanks to the higher power density and economy of scale I guess

But now it seems to me that I read that the chinese's HTR will be competitive with second generation LWRs even with their low power density and low power output. I guess this is due to the design's simplicity.

So maybe a low power density reactor can be competitive if it is well designed.


Last edited by fab on Feb 17, 2014 11:37 am, edited 1 time in total.

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 Post subject: Re: UK AGRs
PostPosted: Feb 17, 2014 5:26 am 
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One reason EPR is costly is that first of a kind problems are not yet over. A standardized design repeated a number of times becomes more economical. It can be built in a shorter time and costs less.
An existing design is in any case easier to convert to new fuel for using the plutonium stocks profitably. The news item indicates that AGR has matured giving higher availability. Do not tar it with the brush of early difficulties.
Nuclear costs have been inflated by safety concerns topped by weapon proliferation phobia. Latest CANDU designs have been given up in Canada. EPR design has been termed a disaster in earlier posts. AP1000 had to go to China to succeed. It is time to work on change of fuels for old designs. It can be done in a shorter period.
Thorium is cheap now as the low hanging fruit which stage has passed for uranium. Now is the right time to use plutonium from nuclear 'waste' with thorium, even in established designs of reactors. UK has a head start in plutonium stocks. AGR is their established design still running.


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 Post subject: Re: UK AGRs
PostPosted: Feb 17, 2014 6:47 pm 
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From descriptions in Wikipedia, it appears that the EPR design is sound being based on the old Westinghouse PWR, but the craftsmanship is poor. The one being built in Finland had poor concrete and poor welds. It is odd that for such a high profile project being viewed with beaucoup eyeballs that basic construction could be the undoing of this project. What happened to European craftsmanship?


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 Post subject: Re: UK AGRs
PostPosted: Feb 17, 2014 8:21 pm 
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EPR is a Gen II PWR with yet more redundant equipment piled on it.
It has no passive features at all as far as I can see.


And as to 'European Craftmanship' - it fell to the internationalisation of the construction trade and the breakup of integrated construction firms a long time ago.


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 Post subject: Re: UK AGRs
PostPosted: Feb 18, 2014 11:09 am 
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Quote:
It has no passive features at all as far as I can see.


The mitigation of severe accidents is passive if I am correct. The EPR is designed to deal passively for 3 days with a core meltdown, there is a core catcher with an internal reserve of water to flood the molten core.

Despite that the concept is conservative unfortunately, there are a lot of active sytems with a lot of redundancy.


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 Post subject: Re: UK AGRs
PostPosted: Feb 18, 2014 11:21 am 
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fab wrote:
Quote:
It has no passive features at all as far as I can see.


The mitigation of severe accidents is passive if I am correct. The EPR is designed to deal passively for 3 days with a core meltdown, there is a core catcher with an internal reserve of water to flood the molten core.

Despite that the concept is conservative unfortunately, there are a lot of active sytems with a lot of redundancy.


IIUC valve actuation for the core catcher flooder is active. As is the containment spray system operation, which also needs AC power from the diesel generators to run the sprayer pumps. The EPR needs at minimum DC power for valves and a turbopump.

The approach though is to harden the electricity divisions so they can withstand floods and such.


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 Post subject: Re: UK AGRs
PostPosted: Feb 18, 2014 2:51 pm 
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I am sorry, I read several times that the openning of the flooding line was passive but I look closely and it seems that only the actuation is passive :

Page 14 of this NRC document :

http://pbadupws.nrc.gov/docs/ML1322/ML13220B012.pdf

Quote:
Prior to core melt, the normally closed, de-energized motor operated isolation valves of the passive flooding lines will be manually opened by the operator when core outlet temperature reaches 1,200°F. The arrival of the melt into the spreading compartment triggers the opening of spring-loaded valves that initiate the gravity-driven flow of water from the IRWST into the spreading compartment. Initially, a cable holds each spring-loaded valve closed. Within the spreading compartment the cable is attached to a thermally sensitive initiator, consisting of a material of low melting point. When the initiator is destroyed during contact with molten debris, the cable will allow the spring-loaded actuator to open the flooding valve and allow water to flow from the IRWST.


There is also some troubles with the grace period before the activation of the active containment spray system, in some documents I read 3 days but in the NRC document it's written "several hours" (page 16).


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 Post subject: Re: UK AGRs
PostPosted: Feb 18, 2014 3:22 pm 
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Ok so passive actuation, but still mechanical valves that could stay stuck in the closed position (considering they're always closed normally).

I never understood the excessive complexity and elaboration of the EPR core catcher scheme. Invessel retention is so much simpler and lower in uncertainty and reduces the amount of volatile FPs going into the containment atmosphere.

Regarding grace time for containment spray. I've been wondering if the EPR containment annulus filtering system would work without power. If it does then containment overpressure failure (no spray) would result in only a small amount of FP release. But to answer your question. I will refer you to this very interesting and worthwhile South Korean study.

http://www.kns.org/jknsfile/v44/JK04403 ... 53e16fe3d7

The 1000 MWe PWR case can be used as a worst case since the EPR has increased margins and a core catcher limiting MCCI. Still the containment for the older 1000 MWe PWR holds for an impressive 113 hours in a SBO. So a good deal more than the overconservative NRC.


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 Post subject: Re: UK AGRs
PostPosted: Feb 18, 2014 4:12 pm 
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Quote:
I never understood the excessive complexity and elaboration of the EPR core catcher scheme. Invessel retention is so much simpler and lower in uncertainty and reduces the amount of volatile FPs going into the containment atmosphere.


It seems that the surface area of the RPV is maybe too small for this amount of power, so they decided that the margins to failure was too small and adopted a conservative approach.

The AP-1000's vessel has maybe a better surface-to-power ratio, I will look at it if I find the figures. Or maybe the AP-1000 designers have less constraints.


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 Post subject: Re: UK AGRs
PostPosted: Feb 18, 2014 4:33 pm 
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fab wrote:
Quote:
I never understood the excessive complexity and elaboration of the EPR core catcher scheme. Invessel retention is so much simpler and lower in uncertainty and reduces the amount of volatile FPs going into the containment atmosphere.


It seems that the surface area of the RPV is maybe too small for this amount of power, so they decided that the margins to failure was too small and adopted a conservative approach.

The AP-1000's vessel has maybe a better surface-to-power ratio, I will look at it if I find the figures. Or maybe the AP-1000 designers have less constraints.


Its kind of strange, the AP1000 designers mentioned a very large margin (hundreds of %) to the critical heat flux where failure occurs. The EPR has a heavy neutron reflector around the core that the AP1000 does not have, it acts like a heat sink and thermal shield for the vessel wall, plus more material to melt and dilute the corium. The EPR should just as well be able to get invessel retention, maybe with smaller margin than the AP1000 but still very large.

There could be something to do with chemical explosions where molten zirconium gets into contact with water (so called steam explosions). But I don't see why the AP1000 would have better performance here, the NRC certainly is a tight regulator that looks at things like steam explosions in great detail.


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 Post subject: Re: UK AGRs
PostPosted: Feb 18, 2014 6:28 pm 
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The EPR has a thermal power of 4500 MWth and the pressure vessel internal diameter is 4.88 m
The AP1000 has a thermal power of 3400 MWth and the pressure vessel internal diameter is 3.99 m

I don't know the exact form of the bottom of the RPVs but in order to have an order of magnitude let's say that the corium will be in contact with an hemisphere that has the same diameter than the RPV. That gives us for power to surface ratio :

120 MWth/m^2 for the EPR

134 MWth/m^2 for the AP1000

So the situation seems more favorable for the EPR but I have over-simplified the problem ( so I apologize if it's ridiculous).

Speaking of that, if I take a decay heat of 1% of the full power ( between 2 and 3 hours after shutdown ), I find a heat flux of 1.2 MWth/m^2 for the EPR and 1.34 MWth/m^2 for the AP1000, of course there is heat conduction in the vessel so the surface of heat exchange is greater and the heat flux must be lower but 1.2 MW/M^2 is on the same order of magnitude of the critical heat flux of most smooth metallic surfaces (around 1.2 MW/m^2 for the steel of the EPR vessel). If we take into account the whole surface of the vessel, the heat flux may be divided by something between 3 and 6 but if the meltdown occurs rapidly after shutdown, the maximum heat flux may be not much lower than the critical heat flux. We should look at the analyses of the NRC on the AP1000 to see true simulations and explanations.


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