CANDU reactor, heat loss to D2O moderator

Gregorian Chanter
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CANDU reactor, heat loss to D2O moderator

Post by Gregorian Chanter » Oct 05, 2010 4:11 pm

Not thorium, I know, and perhaps answered before here, but my question is: What prevents massive heat loss from the pressure tubes with the fuel and primary coolant to the surrounding moderator heavy water in a CANDU reactor. I realize there is some heat loss, and so the heavy water is cooled, but the reactor looks a lot like a tube-in-shell heat exchanger (or a boiler) where one wants to have massive heat flow out of the tubes into the surrounding water.

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jaro
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Re: CANDU reactor, heat loss to D2O moderator

Post by jaro » Oct 05, 2010 4:48 pm

The pressure tubes are NOT surrounded by the moderator.
They are surrounded by a gas annulus, which is contained within calandria tubes -- only the latter are surrounded by the moderator.

However, you are quite right that "the reactor looks a lot like a tube-in-shell heat exchanger (or a boiler) where one wants to have massive heat flow out of the tubes into the surrounding water": this is an inevitable consequence of the use of solid fuel, and the limited heat exchange capacity permissible, if fuel damage during long-term irradiation is to be avoided, and the spent fuel removed intact to storage.

By contrast, liquid fuel offers enormously larger heat transfer potential, so the same plant electric output can be had with ten times fewer fuel channels -- taking also into account much higher operating temperature and thermodynamic conversion efficiency for electricity generation.

So despite the very high liquid fuel temperature, the much reduced heat transfer area across the gas annulus (less fuel channels) can actually reduce heat loss to the moderator.

One part of the gross heat loss to the moderator can NOT however be changed: that is the heating by gamma and neutron radiation deposited in the moderator by the nuclear reactions in the fuel.
I provided some numerical examples in old posts dealing with the HW-MSR or CANDU-MSR.....

Gregorian Chanter
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Re: CANDU reactor, heat loss to D2O moderator

Post by Gregorian Chanter » Oct 06, 2010 10:20 am

Jaro, thanks for the info. I knew there had to be something missing in what I was looking at. All the drawings I had seen were simplified, not showing the gas annulus, making it appear that the pressure tubes were in direct contact with the moderator.

Cyril R
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Re: CANDU reactor, heat loss to D2O moderator

Post by Cyril R » Oct 06, 2010 11:36 am

I'm not sure why people keep worrying about the moderator heat loss. Its not even lost, you can easily use the moderator heat as feedwater preheater before the main steam bleed reheater. So you get to keep the 3-5% or so of moderator heat as useful heat. Emergency heat exchangers to dump into the environment during power outage etc. can still be used for added safety measures. Not that this is actually needed, unless one is a regulator ratcheteer...

jagdish
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Re: CANDU reactor, heat loss to D2O moderator

Post by jagdish » Oct 08, 2010 3:35 am

jaro wrote:The pressure tubes are NOT surrounded by the moderator.
They are surrounded by a gas annulus, which is contained within calandria tubes -- only the latter are surrounded by the moderator.

However, you are quite right that "the reactor looks a lot like a tube-in-shell heat exchanger (or a boiler) where one wants to have massive heat flow out of the tubes into the surrounding water": this is an inevitable consequence of the use of solid fuel, and the limited heat exchange capacity permissible, if fuel damage during long-term irradiation is to be avoided, and the spent fuel removed intact to storage.

By contrast, liquid fuel offers enormously larger heat transfer potential, so the same plant electric output can be had with ten times fewer fuel channels -- taking also into account much higher operating temperature and thermodynamic conversion efficiency for electricity generation.

So despite the very high liquid fuel temperature, the much reduced heat transfer area across the gas annulus (less fuel channels) can actually reduce heat loss to the moderator.

One part of the gross heat loss to the moderator can NOT however be changed: that is the heating by gamma and neutron radiation deposited in the moderator by the nuclear reactions in the fuel.
I provided some numerical examples in old posts dealing with the HW-MSR or CANDU-MSR.....
If we provide Mg65Al35 eutectic as coolant with solid fuel, it will create some of the beneficial conditions of HW-MSR like a high working temperature and low temperature. Ease of disposal of Xe will not be achieved. I wonder if the temperature of HW moderator can still be achieved?
If all the benefits of a liquid fuel cannot be done without, UCl3-UCl4 eutectic in a fuel tube in a coolant tube within a calandria tube could still be possible! Chlorides have lower melting points and Cl37 could be used.

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