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PostPosted: Aug 16, 2014 9:07 am 
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On the plus side it appears the NRC may actually issue the approval for the ESBWR in September.

They claim the steam dryer issues have all been cleared up.

That would dispose of the pressurisers and so on.
ESBWR also holds out the possibility of matching the CANDUs uranium efficiency by transitioning to a faster spectrum reactor.


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PostPosted: Aug 16, 2014 4:42 pm 
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Back to the topic...

The advantage of a single large forged reactor vessel vs. plenty of pressure pipes as in the Candu or the RBMK design is that it requires hundreds of weldings each of it is a risk itself. The other disadvantage is that the thin pipes are placed in the highest neutron flux. At the end a Candu is more expensive and the Candu reactor needs a complete overhaul after 30 years while a PWR pressure vessel can be used for perhaps 60 years + if the size of the downer (neutron absorber) is sufficient.

The carbon steel forged vessels with liner used in LWR were close to the available technology in the 70ies and 80ies. There were a couple of manufacturers available in the US...Germany and other countries that produced shafts for turbines.


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PostPosted: Aug 16, 2014 5:48 pm 
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@Cyril,
More impressive is your high signal-to-noise ratio in your posts! I for one have learned a lot


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PostPosted: Aug 17, 2014 1:53 am 
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HolgerNarrog wrote:
Back to the topic...

The advantage of a single large forged reactor vessel vs. plenty of pressure pipes as in the Candu or the RBMK design is that it requires hundreds of weldings each of it is a risk itself.


This is theoretically true, though I must say, I've never heard of a LWR pressure vessel weld failure LOCA. Even the older vessels at Fukushima-Daiichi, which I think are (mostly) welded plate, do not appear to have suffered any weld damage before the core melted down completely. The gap has closed mostly with modern welding techniques plus x-ray testing. If sections are much thinner, welding becomes more attractive. BWRs often use welded plate for the upper sections of the reactor vessel. Strictly speaking you only really want a bottom head forging and the first ring forging on top of that. Above that is above the top of the core, so a LOCA there would be easily recoverable. BWRs have thinner sections than PWRs, so weld quality control is easier, and also BWRs have the advantage that they can work on just level (inventory) control. The ESBWR has taken full advantage of that.

Still have one circumferential weld... between the bottom head and the first ring section. Seems very difficult to get around this, unless we can use liquid-cast vessels of high quality.

In terms of smaller leaks below the active core, for BWRs this potential will always be there - control rod penetrations, reactor water cleanup nozzles and piping, reactor drain lines, reactor instrument lines... all welded structures.


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PostPosted: Aug 18, 2014 12:14 pm 
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Posted this in the wrong thread.

Just found this excellent overview of forging RPVs from the center for strategic and international studies:

http://csis.org/files/media/csis/events ... r_sato.pdf

Looks like the bottom head is actually made of two forgings: a bottom "petal" forging (guessing only a Frenchman would call it that) and a transition piece.

The bottom petal is only 80 tons (assume metric) when forged from A508 pressure vessel steel. So it would be only 27 tons if from Inconel 718. The first ring forging then is 127 tonnes A508, that's 43 tons Inconel 718. The transition piece is probably in between those numbers.

So our max forging need is 43 tons.

This does not appear to be a big deal in terms of forging itself. The limit for Inconel is apparently the ingot weight; over 100 tonnes gets hard because of quality control (loss of alloy homogeneity). Other than that Inconel is easy to forge, before ageing (annealed only). The pieces are also small enough that they can be aged in one piece (oven), a significant advantage.


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PostPosted: Aug 18, 2014 11:17 pm 
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Construction cost and limitations in pressure vessel construction are hurdles in development of nuclear power. It can be seen that there is good development in Russia, China and S.Korea where these problems are under control.
Some helpful ideas for others would be:-
1. Limit the reactor size to local manufacturing capacity and go for modular construction to further reduce costs and time.
2. Avoid high pressure fluids in the reactor vessel and reduce pressure requirements. Various MSR designs (LFTR, DMSR or TAP) might help.
3. If at all high pressure water/steam is desired to be produced inside the reactor vessel, use a number of smaller water tubes like the easily replaceable boiler tubes for water as moderator-coolant.
Combined with fluorex reprocessing, these ideas may enable burning of used LWR fuel in reactors.


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PostPosted: Aug 19, 2014 2:27 am 
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jagdish wrote:
Construction cost and limitations in pressure vessel construction are hurdles in development of nuclear power. It can be seen that there is good development in Russia, China and S.Korea where these problems are under control.
Some helpful ideas for others would be:-
1. Limit the reactor size to local manufacturing capacity and go for modular construction to further reduce costs and time.
2. Avoid high pressure fluids in the reactor vessel and reduce pressure requirements. Various MSR designs (LFTR, DMSR or TAP) might help.
3. If at all high pressure water/steam is desired to be produced inside the reactor vessel, use a number of smaller water tubes like the easily replaceable boiler tubes for water as moderator-coolant.
Combined with fluorex reprocessing, these ideas may enable burning of used LWR fuel in reactors.


Disagree. Problem looks quite manageable. The ingots are small, less than 45 tonnes for the heaviest one. There is enormous simplicity advantage in having just a single vessel and then operate at natural circulation like ESBWR. CANDU forced circulation and its insane number of welds in and near the reactor core is a nightmare of joints and embrittlement of your pressure vessel (zirconium alloy tubing) by full core flux neutron streaming. Then you have massive steam generators which will require heavier forgings than in my Inconel proposal. Plus tubes and tubesheets that are going to vibrate and damage themselves and then be very costly to repair. With BWR you replace the steam generator along with the fuel, so short SG lifetime is a non-issue.


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PostPosted: Aug 21, 2014 8:53 am 
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Just ran some more numbers on the thinner superalloy vessel.

Another advantage is the space inside the vessel. With Inconel 718 vessel, for the same outer diameter vessel size of the ESBWR, we save 120 mm inner radius (62 mm in stead of 182 mm) for a total of 240 mm extra inner diameter.

That space could be used to fit another say 125 fuel assemblies. That would be some 400 MWt extra, another 140 MWe of output. With the same containment, building size etc. So this is very cheap extra capacity.


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PostPosted: Aug 24, 2014 8:24 pm 
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The part of your argument that I like best is that In718 is already used in RPVs, in the head bolts. It's difficult to think of failure modes in the RPV that wouldn't happen in the head bolts already.

Your In718 RPV must stay in heat-treated form for 60 years. Will it see more neutron flux than the bolts that hold on the head in current RPVs? Will that neutron radiation anneal it?

What's the critical crack length in your In718 RPV, as opposed to a steel RPV? Can today's reactors tolerate a single head bolt failure?


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PostPosted: Aug 25, 2014 3:51 am 
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You could always have an internal neutron shield like naval reactors.


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PostPosted: Aug 25, 2014 4:55 am 
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Hi Iain, great to hear from you again.

iain wrote:
The part of your argument that I like best is that In718 is already used in RPVs, in the head bolts. It's difficult to think of failure modes in the RPV that wouldn't happen in the head bolts already.


That's true, at or near the design temperature, yielding of the head bolts is usually the weak spot in the strength of a LWR pressure vessel. In case of severe heatup combined with overpressure, the seal is sometimes the part that yields first (especially if older organic seals are still used).

Quote:
Your In718 RPV must stay in heat-treated form for 60 years. Will it see more neutron flux than the bolts that hold on the head in current RPVs? Will that neutron radiation anneal it?


The neutron dose is considerably greater than the vessel head; the vessel head has a huge water chimney on top of the core, not to mention steam separators and dryers for further shielding, plus distance (neutrons travel randomly in all directions). The neutron dose on the vessel head for BWRs is in fact negligible unless significant fuel failure has occured.

Inconel 718's gamma prime heat treatment is exceptionally stable below 600C for continuous service. The BWR has a design temperature of less than 320C. No problem at all.

Radiation annealing is not expected at BWR temperatures. Quite the opposite should occur for any nickel alloy at this temperature - radiation hardening.

BWR vessel wall sees much lower flux than PWR vessels, so there is no problem; it is of note that Inconel 718 is used in some fuel assembly structural designs, such as for high performance grid spacers. Those are under quite a bit of stress, lasting fine for years inside the nuclear reactor core.

Quote:
What's the critical crack length in your In718 RPV, as opposed to a steel RPV??


Difficult to say. Fracture toughness is not an easily defined inherent material property. Geometry plays a major role. Large relatively thin cylindrical vessels generally do very well. Inconel 718 is known for exceptional toughness, though to be fair it is slightly less ductile than A508 PV steel. Keep in mind the design stress is quite low, for a 62 mm thick vessel wall the normal operating pressure produces less than 50% of the minimum guaranteed 0.2% yield stress value of Inconel 718.

Based on various literature sources, it appears than Inconel 718 only has crack issues with >>600C service temperatures and high stresses. This is when the gamma prime phase starts to change. But this is not relevant for LWRs and in fact, the A508 steel will fail at the design pressure if held at 600C whereas Inconel vessels can do this just about forever.

Quote:
Can today's reactors tolerate a single head bolt failure


Yes, if the bolts are designed as they should, for low stress. Worst case, assume a complete shearing of a bolt. Assume all stress is divided over the two adjacent bolts (worst case) with no credit for the other bolts. This means the stress on the adjacent bolts increases by (2/3) = 50%. If they are designed for 50% of the yield strength, then the stress goes up to 75% of the yield strength.


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PostPosted: Aug 03, 2015 1:39 pm 
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This study suggests a no cobalt maraging steel is suitable for bwr vessel:

http://www.osti.gov/scitech/biblio/4807312/

The touhgness is better than 718 alloy. It is cheaper too.


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PostPosted: Aug 03, 2015 7:28 pm 
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Could these 'exotic' pressure vessels be forged by existing capacity?
Would it be possible to have fewer forgings in each vessel or would it require the same amount of force to deform these smaller forgings because the material is stronger?


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PostPosted: Aug 04, 2015 6:16 am 
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LWR with a heavy pressure vessel was appropriate for the time of its invention. It is now getting a beating in economy from shale gas in the country of its origin.A far better idea now would be a beehive of mutually supporting hexagonal channels with required number allotted to water moderator, molten fuel and molten salt/molten lead coolant. Water moderator channels could be insulated and run at low pressure through a cooling arrangement.


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PostPosted: Aug 04, 2015 11:35 am 
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E Ireland wrote:
Could these 'exotic' pressure vessels be forged by existing capacity?
Would it be possible to have fewer forgings in each vessel or would it require the same amount of force to deform these smaller forgings because the material is stronger?


Should be fine. Forging would be done in the annealed state. It is possible to increase the annealing temperature - comes at some loss of tensile strength after ageing, but should increase ductility in the aged condition. Increasing annealing temperature will make the maraging steel "super annealled" and very ductile. The actual yield strength is not much higher than regular pressure vessel steels, in the annealed state. I think forging the annealed maraging steel is easier (lower kinetic input) than forging the 508 class steels.

The big increase in yield and tensile strength in maraging steels comes from the ageing heat treatment, near 480C. This is low enough that large sections could be aged after forging. Just put the forging in a big oven at 480C for a few hours. Then ship the forged parts to the site. Then all you do is weld the forgings together onsite. The welds are typically re-aged afterwards, but the properties of the weld, when using overalloyed filler in the as welded condition look very good to me. Be quite nice to reduce onsite post weld heat treatments. I think we can use maraging steel for the piping too, this will further reduce weight and thicknesses there.


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