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 Post subject: Re: High Pressure BWR
PostPosted: Sep 22, 2017 9:54 am 
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500MW is far too small.

And with a prestressed cast iron/steel vessel there is little reason to stick to small sizes such as those forced by the limited availability of very large forging capacity.

There is no reason not to shoot for 2000MWe.


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 Post subject: Re: High Pressure BWR
PostPosted: Sep 23, 2017 1:58 am 
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Large sizes is one of the reason for decline of new construction of nuclear power plants. The size has to be such that the utilities could finance them as replacements of existing thermal or nuclear plants. What is happening now in UK and other places is not healthy. There is even a thinking on SMR plants. 500MW is an optimum size. Standardisation will also help.


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 Post subject: Re: High Pressure BWR
PostPosted: Sep 23, 2017 7:00 am 
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But SMRs are a false economy - they might allow easier financing but they make the basic problem of nuclear plants, price per kilowatt is high, and make it worse deliberately.

There is a reason the trend in unit sizes has been going in precisely one direction since the Atomic age began.

And standard coal unit sizes have been climbing for years - and is now approaching 1000MWe+

We were already past 660MWe per unit from coal in the 70s.


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 Post subject: Re: High Pressure BWR
PostPosted: Sep 24, 2017 8:56 pm 
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That has to balance against grid connectivity that can handle a single point source at 1GW or greater. Grid operators are increasingly reluctant to increase large interconnects and core high transmission line capacity in the face of more distributed generation from renewables. Renewables effectively takes the wind out of centralized grid expansion, which would underwrite such large generation facilities. Though that assumes such a facility would only be generating electricity, and not taking thermal energy customers into account. If you had the industrial cooperation in place to have colocated thermal energy customers, going to larger unit sizes makes some sense even in this stunted grid climate.

Large unit sizes are in many ways predicated on economies of physical size because they currently benefit poorly from economies of production lot size. The chicken/egg issue is if you had a extensive nuclear rollout, the economics favors SMR's in the current grid situation, and can provide large concentrated generation via multi-unit facilities.


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 Post subject: Re: High Pressure BWR
PostPosted: Sep 25, 2017 5:03 am 
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With a prestressed pressure vessel however you can achieve modularity of construction of vessel components even in large units with small production runs.


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 Post subject: Re: High Pressure BWR
PostPosted: Sep 25, 2017 8:36 am 
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E Ireland wrote:
With a prestressed pressure vessel however you can achieve modularity of construction of vessel components even in large units with small production runs.


What's your thoughts about using a cast steel pressure vessel? Then you'd eliminate the prestressing tendons, galleries, possible weak points near keys. Casting A508 RPV steel seems easy enough. The supercritical water reactor vessel is maybe small enough to make it out of one piece, for a large foundry. It is also relatively thick walled which is good for casting.

Casting has lots more options on geometry - you can cast reinforcing ribs, double curvatures, etc. that are not easily available or available at all with forged rings.

Castings are already used for high pressure steam turbine casings, so it's already applied in this field.


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 Post subject: Re: High Pressure BWR
PostPosted: Sep 26, 2017 11:35 am 
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I'd be a bit worried about castings in that, unlike a prestressed pressure vessel, they are actually subject to catastrophic failure mechanisms relating to imperfections in the casting.

A crack can potentially break the vessel open, and you have enormous thickness (indeed thicker than with forgings!) welds that have to be Quality Assured, then surveilled over the life of the reactor.

With a prestressed vessel there are no welds, and with cast metal [iron or steel] cracks in blocks cannot spread across block boundaries.
In addition if your prestressing cable looks suspicious you can simply replace it during an ordinary statutary outage [at least for axial cables rather than the wire wound ones, where you would probably just have los of redundancy].


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 Post subject: Re: High Pressure BWR
PostPosted: Sep 26, 2017 12:55 pm 
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E Ireland wrote:
I'd be a bit worried about castings in that, unlike a prestressed pressure vessel, they are actually subject to catastrophic failure mechanisms relating to imperfections in the casting.

A crack can potentially break the vessel open, and you have enormous thickness (indeed thicker than with forgings!) welds that have to be Quality Assured, then surveilled over the life of the reactor.

With a prestressed vessel there are no welds, and with cast metal [iron or steel] cracks in blocks cannot spread across block boundaries.
In addition if your prestressing cable looks suspicious you can simply replace it during an ordinary statutary outage [at least for axial cables rather than the wire wound ones, where you would probably just have los of redundancy].


If the vessel is cast out of one piece there would not be any thick welds.

Not buying the "prestressed can't fail catastrophically" argument. Stress in the tendons is enormous, much bigger than the stress in a cast vessel. Yes there are multiple tendons so you can design for redundancy, but there is no guarantee that there won't be a common problem with the tendons - manufacturing defect, corrosion, etc.

If you look at the fracture mechanics studies for RPV steels, it's basically negligible even on a one in a million year level, to have a catastrophic failure. Probably similar to multiple tendons failing at the same time. The main problem is pressurized thermal shock which can be avoided by design provisions such as thermal sleeving/internal insulation.

In any case I'm planning for a design where the whole vessel is submerged in a pool of water so any sort of catastrophic failure is not a threat to containment (and would lead to rapid reflooding of pool water back into the broken vessel).


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 Post subject: Re: High Pressure BWR
PostPosted: Sep 27, 2017 7:58 pm 
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The present situation where there are just a few builders of reactor vessels is not healthy. The Canadians found the alternative of hundreds of tubes long ago. This could be further modified by using molten salt fuel and leaving only a high temperature problem and pushing the high pressures out of reactors. This, of course, is opposed to the idea of high pressure BWR. This will enable the new entrants to build their own reactors if they so choose.


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 Post subject: Re: High Pressure BWR
PostPosted: Oct 03, 2017 1:32 pm 
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An inspector for the ONR came into give a talk to my course mates and I today on the regulatory framework

Amongst other things she said that she is really not a fan of castings where alternatives exist for QC reasons [outside parts where castings are the only reasonable option].

And unfortunately in the UK compliance with the regulations does not guarantee regulatory approval.
Prestressed vessels are probably a bit better off regulations-wise as they are only under compression and defects seem less likely to have major consequences in compression, especially if the vessel is pressurised


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