Energy From Thorium Discussion Forum

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PostPosted: Dec 17, 2013 2:12 am 
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Joined: Mar 16, 2010 1:48 am
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Location: Guangdong, China
As we know, there is ~10% fission energy with gamma and beta ray emit. But for graphite moderated reactor, part of those rays will deposite into graphite. How could we get the energy deposite in graphite?

As for PWR, why couldn't the gamma energy deposite in water?


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PostPosted: Dec 17, 2013 6:14 am 
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Joined: Jul 14, 2008 3:12 pm
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Most likely we're having a Babylonic moment here, but the issue with graphite heating is to get that heat out. It is why you need a lot of surface area and short heat transfer paths, so no solid thick logs of graphite without cooling channels. Graphite isn't actually bothered by the heat much, but its life under irradiation is temperature dependant, so there is an indirect damage effect from overheating.

PWRs do have gamma deposition in water. But there's not much water in there - tight pitch - and water is less dense than graphite. As a result the PWR has much less heating from gamma rays in the water. There is also neutron heating however, H2O is a good neutron stopper.

More importantly, the moderator in PWRs is the coolant, so more heating is not a problem at all - in fact it helps because the energy that is deposited directly in the coolant needn't be transferred through the fuel-clad, reducing peak fuel/clad temperatures.


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PostPosted: Dec 19, 2013 5:04 am 
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Cyril R wrote:
PWRs do have gamma deposition in water. But there's not much water in there - tight pitch - and water is less dense than graphite. As a result the PWR has much less heating from gamma rays in the water. There is also neutron heating however, H2O is a good neutron stopper.
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I understand now. As you know, is it necessary for the pipe water heating for first loop? supposing 5% power deposite in water, there is more than 2% power in pipe, so the temperature of core outlet will lower than inlet of SG(primary side), and the SG (primary side)outlet will hotter than core inlet. I'm not sure what is the range of temperature change between two sides of first loop. I have not seen such calculation.


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PostPosted: Dec 25, 2013 12:26 am 
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Joined: Dec 31, 2008 12:09 am
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Location: Berkeley, CA
About 2.5% of the energy from a fission reaction is in the neutrons, about 3.5% gamma photons, and 6% neutrinos. The neutrinos are not absorbed in any significant quantity anywhere in the reactor. The energy from the neutrons and gamma photons are absorbed in the coolant, moderator, structures and fuel. The normal practice in predicting peak fuel and clad (if used) temperatures is to assume that all of the fission energy except neutrinos is deposited in the fuel, since this is the worst case.

However, for structures it is important to also assess the gamma and neutron heating, to assure that adequate cooling is provided to keep temperatures and stresses reasonable. Graphite has high thermal conductivity and good thermal shock resistance, so this analysis is normally deferred until other more difficult design problems have been addressed.


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