Cyril R wrote:
So increasing moderator temperature in an accident increases reactivity? That doesn't sound nice? You'd need to quantify this in an accident analysis. That might be part of your thesis, it's very interesting work combining various interesting fields (materials, stress analysis, chemistry, radiochemistry, thermalhydraulics, reactor physics, transport dynamics, etc.). If you can get the tools to do this it would be a great addition to the thesis and it is fascinating work to do.
I think we could stay out of the overmoderated regio giving the lower density of the moderator at 180C, but that does lead to wierd reactivity insertions when the reactor is cooling down during an ordinary shutdown that will require additional control absorbers to keep it under control.
Can't use CANDU style liquid control absorbers though due to wanting to keep gas out of the pressure vessel, last thing we need is a LOCA causing the pressure in the vessel to drop and thus creating a giant bubble that might stop coolant getting to fuel elements. Will have to be mechanical absorbers - which were planned for the ACR so its probably not a big issue.
Cyril R wrote:
Interesting stuff. For welding I was thinking more along the lines of any pipework attached to this thing. Even if it is integral it is going to need many penetrations, tendon sleeves, prestressing galleries, pipe attachments, maybe some entrance hatch.
I think it will come to many hundreds, before we even include the 1400 fuel channel closures and all that [~700 channels with one at each end].
It might be feasible to use relatively small blocks for the areas around pipe attachments and similar and then do the welding in a factory before shipping it to the site. We can move blocks up to a hundred tonnes relatively feasibly.
The access hatches are more troublesome but it appears you need forged rings or similar to carry the stressing load around the opening.
Cyril R wrote:
You'd have to run some numbers on thermal stress. 200-300 degrees over a couple meters of metal is going to be a large stress. You can get cracks if the stress exceeds the prestress substantially. Young's Modulus is really big... couple hundred billion Pascals for most metals.
One interesting idea is if the liner can bridge small gaps without support, it might be possible to taper the blocks at the inner face so that differential expansion doesn't try and break the blocks apart - they will just change shape to close the gaps.
Just an idea though, but I iamgine it would be hard to design a vessel that did that without it being much weaker.
Cyril R wrote:
That might work, you'd need a lot of these thermosyphons. I guess you can just run a normal cooling system for power heat recovery and then have any passive concept that rapidly loses heat with increasing temperature. Heat pipes can be configured to be like that too, as they work on phase change so are going to have a fairly steep cut-off point, especially wick-less ones.
I think probably a few hundred of them, but they would be small and probably easily manufacturable in a factor (only ~10cm in diameter or something). That way a leak in any one of them is a negligible issue.
But a lot of the top of the pressure vessel would likely be covered in them, would have to fit it around the reactivity mechanisms in the centre of the core. Especially since the core is only 6m long.
Cyril R wrote:
Really, that's interesting. It's not a new idea though. I actually ran numbers on this many years ago (where goes the time). For the AP1000. Turns out that (based on steam tables differences in specific enthalpy comparison), superheating that cold steam would to modern 600+ deg C superheat steam conditions adds some 50% enthalpy, and pushes up efficiency by at least 30% and maybe more with multi reheat stages. So installing the superheater is like having a twin unit AP1000 for the price of one plus a gas heater. Oversimplifying of course but that's pretty amazing and rather tempting. Reductions in turbine cost by going to full speed superheat turbine are substantial, not to mention eliminating the complicated moisture separator-reheater, the steam separator, and steam dryer (thus making containment smaller too since the nuclear SGs become much smaller). Almost too good to be true so it makes you wonder why it hasn't been done yet.
So early PWRs and BWRs did that, especially in the US and Sweden - but it seems to have fallen out of favour in the late 60s, I imagine when moisture seperators became a big thing.
Also it appears most of that work was with direct oil firing - and that fell out of favour after the oil crisis and improvements in refining technology put the price of residual fuels thorugh the roof.
Additionally it is a bit of an operational headache if your turbine plant can't function if you have no oil or gas available for whatever reason. Although if you only want 600+C superheat, you might even able to use an OCGT as the superheater, which would increase your efficiency even more.
Or even a FIRES block, although that would be interesting.
Cyril R wrote:
By the way if you are planning on having the turbine in the containment, you'd have to look at safety and operational aspects of that. Can it be maintained, what about turbing missiles, CO2 LOCAs, electrical/hydrogen generator fires etc. (hopefully you can use a helium cooled generator with your setup).
The turbines are relatively lightweight (they weigh <100t tonnes rather than 5000t or whatever for a normal turbine), although we would be dealing with full speed machines so the energy per unit turbine weight is significantly higher.
The Carbon dioxide LOCA is the big one that I have identified, as the carbon dioxide in the turbines might be pressurised all the way up to 350 bar, and there will be significant holdup in the recuperators, precoolers, intercoolers and in the gas heaters. It could easily hugely overpressure the reactor building.
The idea of a second pressure vessel around the turbine plant starts to look significantly better as it would provide protection against both those threats and would also stop generator fires from affecting the reactor building as the pressure vessel would be vented to the outside of the confinement building through suitable ducts.
Another alternative would be a CANDU style vacuum building containment, but instead of having a pure water douse it would douse with sodium hydroxide solution. That would suck up carbon dioxide with ease if anything happened.
You could even line the confinement buildings with blocks of porous activated carbon which would absorb large quantities of carbon dioxide as the building pressurised - but that sounds a little crazy and ofcourse activated carbon is quite flammable so it would be a potential fire risk.
Splitting the turbine plant in two is an obvious solution - and you would not necessarily have to have two generators or such as the shaft is much shorter than in an LWR so you could have the turbomachines all be coaxial.
Given that our recuperators and gas coolers are made of small modules thanks to the manufacturing constraints on PCHEs and H2X heat exchangers and you might not loose much plant efficiency there.
Either way, many interesting ideas.
We could also put a combined heavy water upgrader/tritium removal plant inside the confinement building which would reduce the risk of tritium leaks and would also provide more volume for a carbon dioxide overpressure to expand into.
Perhaps even water cooled buffer stores for spent fuel.