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PostPosted: Feb 01, 2014 12:50 pm 
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Cyril R wrote:
The graphite has a different expansion coefficient than zircalloy tubing, so you either get bad thermal contact (insulating gaps) or thermal stress cracking your tubes. You'd want some sort of flexible thermally conducive binder in between (possibly graphite powder) to prevent this problem but it further complicates things. Bad contact will result in overheating.

In the RBMK they have alternating split rings made of graphite around the zircalloy pressure tubes.
One set was in contact with the bulk moderator blocks, and the other set was in contact with the pressure tube.
They sit on top of either on in the channel, something liket hat might work, although one set of rings might have to be made of Zircalloy or something.

Cyril R wrote:
There's a far more serious problem in the buildup of Wigner energy in graphite. It is negligible at molten salt reactor temperatures (>600C graphite) but if you're cooling with water tubes you get a lot of the graphite very cold.

As I understand it the annealing temperature for graphite is roughly ~250C.
The feedwater temperature in a supercritical CANDU concept is roughly the same, which implies that even the graphite in the coldest possible position (near the bottom of the cooling tube) will be above the annealing temperature.
You can carefully control the graphite cooling flow during startup to allow annealing temperature to be reachd as fast as possible.

Cyril R wrote:
Inspection is another issue. With good thermal contact, the cooling jacket for the pressure tube and cooling tubing for the graphite, are essentially not inspectable inservice, and would be tough to inspect out of service. Big graphite cores are opaque across the EM spectrum, and not practical for ultrasound inspections.

The inside of a PCHE is also not inspectable, but with proper engineering it is likely that the tubes can be cleared for substantial operating lives without inspections, additionally you can perform pressure tests of the plenums during refueling outages by charging the fuel tubes with some marker gas and then inspecting the containment gas system and the cooling plenum system to determine if the gas is leaking.

Cyril R wrote:
To retain neutron economy advantage for graphite you want heavy water in the graphite cooling tubing. So you will have tritium. Why not just have heavy water all over the place.

AIUI Tritium production should be approximately proportional to the number of scattering events by heavy water - by replacing the Heavy water bulk moderator (which is 60% of the Heavy water inventory of a CANDU 6) with graphite Tritium production can be reduced.
Additionally, by adopting supercritical heavy water as the primary coolant the amount of heavy water in the Heat Transfer system can be drastically reduced per megawatt as a result of the enormously increased enthalpy change across teh core.

The primary reason to use heavy water in the HTS is not really neutron economy, it is primarily a result of a desire to reduce the positive void coefficient, the lower absorption cross section of heavy water (which is almost negligible compared to light water) should reduce this figure since if its not absorbing any neutrons it can't stop absorbing any neutrons.

Cyril R wrote:
If we have just light water we eliminate all the tritium equipment and even the calandria itself. Visual inspection inservice becomes possible just like a research pool type reactor. We also get fully compatible materials and inherent pressure suppression from a completely submerged reactor.


Although I really like the CANDU design I am not entirely sure a light water moderator design is entirely practical as you will still get large scale moderation from the water in your heat tube system, you may have to put the tubes so close as to be almost touching which makes drastically reduces the amount of water available for cooling it.
It might also cause problems during high pressure leakage events since you could get local overpressures that will potentially damage the tubes.


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PostPosted: Feb 01, 2014 1:08 pm 
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Cyril R wrote:
moderating ratio) makes light water look much better, and graphite much worse...
So how come graphite and HW reactors can run on NU, but LWRs can't ? .....maybe instead of "much better" the correct term is "slightly better" when moderating ratio is factored in ?


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PostPosted: Feb 01, 2014 2:01 pm 
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jaro wrote:
Cyril R wrote:
moderating ratio) makes light water look much better, and graphite much worse...
So how come graphite and HW reactors can run on NU, but LWRs can't ? .....maybe instead of "much better" the correct term is "slightly better" when moderating ratio is factored in ?


The fuel efficiency isn't much better. In fact most can be explained by fuel shuffeling, so a light water pressure tube reactor with fuel shuffeling would do almost as well in uranium use/kWh as a CANDU.

Natural uranium fuel isn't that attractive: the burnup is poor.


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PostPosted: Feb 01, 2014 2:46 pm 
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Cyril R wrote:
jaro wrote:
Cyril R wrote:
moderating ratio) makes light water look much better, and graphite much worse...
So how come graphite and HW reactors can run on NU, but LWRs can't ? .....maybe instead of "much better" the correct term is "slightly better" when moderating ratio is factored in ?


The fuel efficiency isn't much better. In fact most can be explained by fuel shuffeling, so a light water pressure tube reactor with fuel shuffeling would do almost as well in uranium use/kWh as a CANDU.

Natural uranium fuel isn't that attractive: the burnup is poor.

Even with burnup factored in, the Advanced Candu Reactor design - which included shuffling and LW PHT loop, combined with LEU fuel - got considerably poorer overall fuel utilization, in terms of total fuel requirements prior to enrichment.

Rather than making fuel utilization worse, why not extend NU burnup drastically by changing to fluid fuel ?

As the TAP publication says,

Quote:
burnup in CANDUs is limited by the accumulation of fission products that are trapped in the fuel rods. The TAP reactor circumvents this limitation by continuously removing fission products from its liquid fuel.


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PostPosted: Feb 01, 2014 5:36 pm 
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Now find me a solvent with a high enough uranium content. It is not as easy as it sounds. Fluid fuelled reactors need systems solid fuel reactors certainly do not.


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PostPosted: Feb 01, 2014 6:16 pm 
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E Ireland wrote:
As I understand it the annealing temperature for graphite is roughly ~250C


There doesn't appear to be a fixed annealing temperature, just a lower and lower (but never zero) energy buildup with increasing temperatures. Its negligible at 700C but not negligible at 250C. It would be hard to prove that there are no local buildups without having intermittent annealing runs, which complicates design of the reactor.

E Ireland wrote:
Although I really like the CANDU design I am not entirely sure a light water moderator design is entirely practical as you will still get large scale moderation from the water in your heat tube system, you may have to put the tubes so close as to be almost touching which makes drastically reduces the amount of water available for cooling it.
It might also cause problems during high pressure leakage events since you could get local overpressures that will potentially damage the tubes.


It works well with a tubesheet vertical reactor. These can be welded on a very tight pitch. Like a CANDU-SCWR layout, big high pressure low temperature (feedwater temperature) plenum at the top. Headers and drums at the bottom. Compact reactor is an advantage in economics. Not just the reactor is smaller, also the supporting systems will be smaller. There wouldn't be any high pressure leakage problems because all leaks are quenched by the large body of water. Having a flimsy layer of cold water is sufficient for very good cooling. But most of the energy deposited bypasses the pressure tube altogether; the neutron and gamma heating directly into the moderator water. Any locally boiled water would be quenched again by the huge water volume of the pool around it, which would be maintained subcooled by coolers mounted in the top of the pool. None of these advantages would be had in a graphite moderated reactor, in stead you get a lot of graphite to chemically react with leaked steam and a big pressure rise in containment.


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PostPosted: Feb 01, 2014 7:44 pm 
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Magnox operated with a graphite moderator at inlet temperatures as low as 145C and outlets as low as 345C.
Decomissioning studies have not detected significant Wigner energy buildups in those cores.

It seems reasonable that 250C would be sufficient.

As to an ultra tight pitch, if your pressure tubes are almost touching you will have almost no water actually in the Calandria and you, in my opinion, risk pressurising your containment since there will simply not be enough water to quench everything.
You would require steam to move sideways through the very restrictive gaps in the plenums.

This could easily overheat nearby fuel channels and increase venting unless you can very very rapidly depressurise the entire system.
I am going to do some more research to that proposed RBMK successor reactor (MKER?) to see how it obtained its lower coefficients and similar.
It may yet be feasible to use light water in such a graphite-water design.

Also who says that pressure suppression has to be inside the containment with the graphite?
You can have frangible disc blocked vents in the primary containment connecting it to a secondary containment that can be arranged however is convenient and can even be filled with water for pressure suppression if required.
I believe some iterations of CANDU do something similar.

EDIT:

Additionally if you have the supercritical steam venting into the containment you can scram the reactor immediately and use squib valves to vent the effected channels into said pressure suppression system, completely depressurising the channel in seconds and stopping the leak. Another squib valve would connect it to the light water header.
(Can you have squib-closed valves? This would simplify the above)


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PostPosted: Feb 02, 2014 4:47 am 
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MAGNOX reactors build up considerable amounts of Wigner energy - at 155C the design documents specify over 500 calories/gram graphite which is a stored energy of heating up 1 gram of water by 500 degrees. This is hardly negligible. At 255C it still exceeds 100 cal/g over time. Not negligible at all.

The magnox reactors simply assumed that cooling would be available for emergencies. This is not a very modern way to sell a nuclear reactor to the public (quite an archaic one in fact that even a nuclear advocate such as me would reject).

It will be very difficult to sell RBMK type reactors anywhere. Even in Russia.

Regarding your comments on tight pitch not allowing leaked steam to vent and be quenched, I can't see this at all. The pressure tubes are open to the pool water on all sides. They are quite long so even though you have a tight pitch the surface area for steam escape is large.

Sure you can have pressure suppression with graphite moderator, but it requires a higher pressure drywell or rupture disc/vacuum chamber (CANDU) system. This adds cost and complexity, especially because of the added CO and H2 noncondensables that can form from graphite-steam reaction.

For me the simplicity and operational advantages of a giant pool of water are hard to ignore.

The safety advantage is also clearly on the water pool side. You can use the water pool as always present, always on, decay heat removal. Squibs can fail in a number of ways (wet explosive charge, damaged circuitry, fire signal generation failure).


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PostPosted: Feb 02, 2014 5:11 am 
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I checked some burnup figures per % enrichment for RBMK. The RBMK is typically in the 20-30 GWd/t on the post Chernobyl 2.4% reload fuel enrichment. Lets call that 25 GWd/t on 2.4%, 10.4 GWd/t/%.

ESBWR does 50 GWd/t on 4.2% reload fuel enrichment. 11.9 GWd/t/%. Surprisingly its slightly better for BWR. Meaning fuel efficiency of RBMK is not improved over BWR.


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PostPosted: Feb 02, 2014 6:46 am 
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For reference the CANDU can apparently make 20GWd/t on 1.2% SEU.
That is 16.7GWd/t.%


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PostPosted: Feb 02, 2014 7:57 am 
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E Ireland wrote:
For reference the CANDU can apparently make 20GWd/t on 1.2% SEU.
That is 16.7GWd/t.%


Yes, heavy water clearly does much better.

It was a bit of a surprise for me though that a boiling water, graphite moderated reactor is not better than a BWR. Since graphite is a more efficient moderator (though not more effective than H2O), and the RBMK can refuel online with more shuffeling options than the BWR.

Even the British Magnox reactors do poorly on uranium utilization, despite a lower absorptive coolant. Though poor net station electrical efficiency is a factor there.

By the way... CANDU 6 has only 230 tonnes D2O in the calandria, at $300/kg making only $69 million. This is only $100/kWe or so. Still the simplicity of all H2O and elimination of the calandria, tritium removal and containment systems, is attractive.


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PostPosted: Feb 02, 2014 9:31 am 
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I don't suppose there is a way to just keep the Calandria entirely sealed throughout the reactor life, avoiding the need for all the Tritium handling systems?


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PostPosted: Feb 02, 2014 9:46 am 
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E Ireland wrote:
I don't suppose there is a way to just keep the Calandria entirely sealed throughout the reactor life, avoiding the need for all the Tritium handling systems?


It might well be feasible, yes. The tritium source term is almost nothing compared to the fission products so having all of it around may be acceptable. Tritium has a 12 year half life so over a 40-60 year reactor life most of the inventory would decay anyway. Jaro has mentioned before that the reactor coolant loop is the biggest issue because of the high pressure. But the calandria could well be held under slight subatmospheric pressure. Though we do have a coolant pump and heat exchanger where a leak could occur. Those could probably be operated at lower pressure than the other side too.

Since the tritium half life so acceptably short, it may be acceptable to just keep the calandria sealed after the useful reactor life with a defuelled reactor, for a number of decades.


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PostPosted: Feb 02, 2014 12:44 pm 
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Cyril R wrote:
Since the tritium half life so acceptably short, it may be acceptable to just keep the calandria sealed after the useful reactor life with a defuelled reactor, for a number of decades.
There are usually HW storage tanks in the basement of the service building.
Some are used for new HW storage, some are used for contaminated HW storage. Some plants are looking at increasing contaminated HW storage capacity.

Tritium leakage from the PHT system (mainly) into the containment building, gets too high (for collection by the de-humidifiers) after about 25 years.

A simple strategy is to put the contaminated HW into storage and replace with fresh HW.
The stored HW may then be slowly processed using a small, low-capacity de-tritiation system, or simply left to decay for a few decades.

In the long run - over several reactor generations - the same HW keeps getting re-used, with nearly zero extra cost (on per-kWh basis) -- except of course in case of fleet expansion.
If a new reactor design can get the same power from half the amount of HW or less (far higher power density with fluid fuel than solid), then even fleet expansion is not impacted for some time.....


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PostPosted: Feb 02, 2014 12:58 pm 
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So perhaps it would be cheaper to have a larger inventory of Heavy Water and simply rotate water through 'decay storage' to keep tritium decay in balance with production at a far lower production value?

Or install a compact detritiation system.

Either way CANDU's economics seem better than I thought.
I can see how those reactors in China made the $2000/kW cost.

Although I rather like the idea of the reactor at Winfrith with the BWR loop.


Last edited by E Ireland on Feb 02, 2014 1:00 pm, edited 1 time in total.

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