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 Post subject: core moderation
PostPosted: Mar 17, 2014 3:44 pm 
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How much moderation is too much moderation?
this question is in regard to a 200 kW / L thermal design.
I am aiming to a ratio of 90% moderator 10% fuel salt with 1% U235 / U233 ( molar ), but a physicist told me 90% is way to much moderation, which comes into contradiction with what I read on the forum.

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 Post subject: Re: core moderation
PostPosted: Mar 17, 2014 5:25 pm 
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It will depend of what you want ( breeder or not, single fluid or two fluids, the geometry of the core, the rate of the salt reprocessing, lifetime of graphite, power density ...)

You can maybe compare with the DMSR and the MSBR. For example the DMSR :

The core's volume of the ORNL's DMSR is 88.3 % graphite. If you take into account the radial gap, the plenum and the reflectors it is 90.3 % of graphite.

It has a thermal power of 21.6 kW per liter of salt and the thermal power density is 5 MW per cubic meter of core.

The molar concentration of 235UF4 at beginning is roughly 0.25 % if I am correct.


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 Post subject: Re: core moderation
PostPosted: May 03, 2014 10:49 am 
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I'm no expert, but browsing a nuclear engineering text the other day I did see something there that I thought addressed whether a design is over moderated or under moderated or just right. I'd like to think that for anyone in this forum with formal training could explain it.

I think that it was a balance between moderation enhancing the reactivity of nuclear material and neutron absorption in the moderator reducing the reactivity of the nuclear material as the drag of absorption overcomes the benefit of moderation, something like that. This is vague recollection, don't rely on it, but it may spur a confirmation or correction from someone more knowledgeable on the physics.


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 Post subject: Re: core moderation
PostPosted: May 03, 2014 11:07 am 
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The Wiki entry suggests (but doesn't explicitly state) that "overmoderated" is something that can only occur in those reactors where the fuel coolant (which can form voids at above-normal temperatures) is separate from the moderator.
If the fuel serves as its own coolant (as in MSRs) and has a separate moderator, then it can also be overmoderated, if a reduced density (either expansion or boiling) doesn't decrease reactivity through neutron leakage out of the reactor, more than the increase in reactivity due to an effective increase in moderator-to-fuel ratio.

http://en.wikipedia.org/wiki/Nuclear_re ... tor_design


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 Post subject: Re: core moderation
PostPosted: May 03, 2014 1:13 pm 
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Quote:
If the fuel serves as its own coolant (as in MSRs) and has a separate moderator, then it can also be overmoderated, if a reduced density (either expansion or boiling) doesn't decrease reactivity through neutron leakage out of the reactor, more than the increase in reactivity due to an effective increase in moderator-to-fuel ratio.


If I understand well, you said that an "overmoderated" MSR has a positive void, but "overmoderated" is maybe not the good word here. If the reactor has a lot of moderator in it, the neutron spectrum is already well thermalized and the increase of moderator-to-fuel ratio does not increase the reactivity. The effects of neutron capture in the moderator, and less fuel in the core, become predominant. The void effect is negative :

http://arxiv.org/pdf/nucl-ex/0506004v1.pdf

(see page 8 , the void effect is negative with a lot of moderator (overmoderated), negative with very little moderator, and positive between the two)


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 Post subject: Re: core moderation
PostPosted: May 03, 2014 2:24 pm 
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fab wrote:
Quote:
If the fuel serves as its own coolant (as in MSRs) and has a separate moderator, then it can also be overmoderated, if a reduced density (either expansion or boiling) doesn't decrease reactivity through neutron leakage out of the reactor, more than the increase in reactivity due to an effective increase in moderator-to-fuel ratio.


If I understand well, you said that an "overmoderated" MSR has a positive void, but "overmoderated" is maybe not the good word here. If the reactor has a lot of moderator in it, the neutron spectrum is already well thermalized and the increase of moderator-to-fuel ratio does not increase the reactivity. The effects of neutron capture in the moderator, and less fuel in the core, become predominant. The void effect is negative :

http://arxiv.org/pdf/nucl-ex/0506004v1.pdf

(see page 8 , the void effect is negative with a lot of moderator (overmoderated), negative with very little moderator, and positive between the two)

Yes, except I'm not sure I follow their argument about moderator (graphite) heating effects as a function of fuel channel radius.

With small fuel channel radius the graphite blocks are thicker, so they will be less well cooled, which means they will be hotter on the inside (due to neutron & gamma heating). This will shift the neutron spectrum in the reactor, as they say, even though the moderator-to-fuel ratio is higher.

I find it's easier to imagine what happens when the moderator temperature is UNAFFECTED -- for example like in the case of a liquid moderator that is circulated through a heat exchanger (can't do that with a solid like graphite or BeO).


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 Post subject: Re: core moderation
PostPosted: May 03, 2014 3:48 pm 
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Quote:
With small fuel channel radius the graphite blocks are thicker, so they will be less well cooled, which means they will be hotter on the inside (due to neutron & gamma heating). This will shift the neutron spectrum in the reactor, as they say, even though the moderator-to-fuel ratio is higher.


Good catch. It is not clear if the French group have considered this as it is thermal hydraulics related and that is not their forte.

Since the the graphite coefficient is always positive in the French work this is clearly a bad effect to have. It shouldn't be a major problem as long as total coefficient is still negative, like for the really overmoderated designs.

For me the upshot is, make the core either very well moderated or severely undermoderated/fast. Not something in between.


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 Post subject: Re: core moderation
PostPosted: May 03, 2014 5:03 pm 
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Quote:
Quote:
With small fuel channel radius the graphite blocks are thicker, so they will be less well cooled, which means they will be hotter on the inside (due to neutron & gamma heating). This will shift the neutron spectrum in the reactor, as they say, even though the moderator-to-fuel ratio is higher.



Good catch. It is not clear if the French group have considered this as it is thermal hydraulics related and that is not their forte.


I don't really understand what you want mean. I guess the effects are calculated from a steady state where the mean temperature of graphite is higher with small fuel channels. The density effect and the graphite's temperature effect are both taken into condideration. At first order these effects are additive, I guess the coupling between these two effects is negligible ( lower order).

What I missed ?


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 Post subject: Re: core moderation
PostPosted: May 04, 2014 1:33 am 
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85-90% of volume filled with graphite moderator sounds over-moderated to me. An under-moderated core with water/heavy water as moderator in tubes should be optimum. The moderator should only constantly produce thermal neutrons to start fission which could cool off away from it. Parts of core away from tubes should be acting as a neutron sink. A liquid fuel should enable sufficient mixing to keep fuel near the moderator critical in thermal mode.


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 Post subject: Re: core moderation
PostPosted: May 04, 2014 4:20 pm 
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fab wrote:
Quote:
Quote:
With small fuel channel radius the graphite blocks are thicker, so they will be less well cooled, which means they will be hotter on the inside (due to neutron & gamma heating). This will shift the neutron spectrum in the reactor, as they say, even though the moderator-to-fuel ratio is higher.



Good catch. It is not clear if the French group have considered this as it is thermal hydraulics related and that is not their forte.


I don't really understand what you want mean. I guess the effects are calculated from a steady state where the mean temperature of graphite is higher with small fuel channels. The density effect and the graphite's temperature effect are both taken into condideration. At first order these effects are additive, I guess the coupling between these two effects is negligible ( lower order).

What I missed ?


The transient effects are much more interesting than steady state. During startup for example the graphite will be at salt temperature. Then with power increasing the graphite heats up from the gamma and neutron radiation. If the graphite has a positive effect on feedback (which it should have with Th cycle) then power would increase where the graphite is hotter. And the graphite would be hotter where there is more power. If the reactor is to be load following and such this would be important to know even if the total feeback is still negative. Graphite temperature is very important for graphite life.


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 Post subject: Re: core moderation
PostPosted: May 04, 2014 4:23 pm 
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jagdish wrote:
85-90% of volume filled with graphite moderator sounds over-moderated to me. An under-moderated core with water/heavy water as moderator in tubes should be optimum. The moderator should only constantly produce thermal neutrons to start fission which could cool off away from it. Parts of core away from tubes should be acting as a neutron sink. A liquid fuel should enable sufficient mixing to keep fuel near the moderator critical in thermal mode.


You have not been paying attention. The French work indicates that the reactor must be very overmoderated or quite fast, to gain negative total coefficient.

Though the French design is quite specific and nuclear reactor core design is a very flexible work area. There may be other geometrical solutions that might allow epithermal spectra.


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 Post subject:
PostPosted: May 04, 2014 5:52 pm 
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Quote:
The transient effects are much more interesting than steady state. During startup for example the graphite will be at salt temperature. Then with power increasing the graphite heats up from the gamma and neutron radiation. If the graphite has a positive effect on feedback (which it should have with Th cycle) then power would increase where the graphite is hotter. And the graphite would be hotter where there is more power. If the reactor is to be load following and such this would be important to know even if the total feeback is still negative. Graphite temperature is very important for graphite life.


What I wanted to mean with "steady state" is that maybe the coefficients of reactivity are calculated with some kind of "perturbation technic", they imagine the reactor from a steady state (where the graphite is hotter with small fuel channels) and then they look what happens with a little increase in temperature.
I agree with the rest of your post.


By the way, I wanted to talk about something else relating to this topic (core moderation) : I looked at the ORNL's DMSR coefficients of reactivity and it seems that the void effect is positive ! Or maybe I am wrong and I missed something. I looked at the ORNL-TM-7207 :

http://www.energyfromthorium.com/pdf/ORNL-TM-7207.pdf

(at page 35 of the document, page 41 of the PDF)

You see : (in 10^-6 K^(-1) )

fuel-salt doppler : 57 ( but I guess it is -57 )
fuel-salt density : 30
fuel-salt thermal spectrum : -60

total fuel salt : -87

...

total reactor coefficient : -72

The total coefficient is still negative but I wonder if it would be acceptable now to have a positive void.

Does that mean that the DMSR is still undermoderated even with near 90 % graphite in the core and a power density of 5 MWth per cubic meter ?


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 Post subject: Re: core moderation
PostPosted: May 07, 2014 11:37 pm 
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Yes, just about every graphite MSR ever designed has been under moderated and only small cores tend to give a negative density coefficient (the temperature coefficient of the salt is so strongly negative though it is not a safety concern). If you try to increase moderation to be not so "under moderated" (less salt fraction or lower fissile/fertile ratio) then neutrons spend so long looking for fissile that graphite and salt absorptions start to go way up which drives down your conversion and/or breeding ratio.

David L.


Last edited by David on May 09, 2014 8:29 am, edited 3 times in total.

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 Post subject: Re: core moderation
PostPosted: May 08, 2014 2:57 am 
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If the graphite moderated reactor, with 90% core volume is still under-moderated, it is worthwhile providing a water-in-tube moderator to molten salt reactors or to keep it un-moderated. You will invest more fissile and convert more, in a smaller structure. The fissile matter in used fuel, partitioned as in the UK, France or Russia and stored as waste in the US is best put inside a reactor.
For long term economy, a simpler pyro-processing, such as IFR reprocessing should be developed. It is also synergetic with MSR.
If your aim is to produce steam for an industrial or mining process, just put molten salt nuclear fuel in a water tube boiler!


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 Post subject: Re: core moderation
PostPosted: May 08, 2014 1:09 pm 
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Ok thanks for the response Dr. LeBlanc.


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