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 Post subject: Re: core moderation
PostPosted: May 08, 2014 7:38 pm 
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David wrote:
If you try to reduce moderation (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.

This sounds like a contradiction.


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 Post subject: Re: core moderation
PostPosted: May 09, 2014 7:10 am 
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Lindsay wrote:
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.


You are actually correct. Stricto sensu, the definition of over/undermoderation is that, neglecting heterogeneity effects, in a lattice, the k-inf (multiplication factor) varies with the moderator-to-fuel ratio. The general trend is: starting from no moderator, the reactivity first goes up, then reaches a maximum while still increasing the moderator share, and then goes down because neutrons are preferentially absorbed by the moderator (since there is so much). The typical textbook illustration that makes everything clear is this: (see attached picture)

This is a graph of kinf against the moderator-to-fuel ratio. The under-moderated region is below the peak of each curve and the over-moderated region is logically above.

The link with the void coefficient is not by definition but specific to solid fuel, coolant-moderated systems like LWRs. All LWR fuel lattice are (or should be) designed as slightly under-moderated. The reason why is clear on the graph: in case of coolant(=moderator) voiding (neglecting the temperature effect), the moderator-to-fuel ratio will decrease and you will move toward the left of the axis. That means that if you are over-moderated, your kinf will first go up (positive void coeff. !). Being slightly under-moderated guarantees this does not happen.


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 Post subject: Re: core moderation
PostPosted: May 09, 2014 8:23 am 
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jaro wrote:
David wrote:
If you try to reduce moderation (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.

This sounds like a contradiction.


Yes, sorry your right. Meant to say when you try to "increase" moderation (i.e. reduce the under-moderation). I'll change it in the original post...

David L.


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 Post subject: Re: core moderation
PostPosted: May 09, 2014 9:28 am 
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Boris H wrote:
The general trend is: starting from no moderator, the reactivity first goes up, then reaches a maximum while still increasing the moderator share, and then goes down because neutrons are preferentially absorbed by the moderator (since there is so much). The typical textbook illustration that makes everything clear is this: (see attached picture)

Your textbook illustration is incomplete: It starts at only 1 : 1 moderator to fuel ratio.

When we look at smaller moderator to fuel ratios "starting from no moderator", we find that at first the reactivity goes DOWN. That's because at low moderator fractions the moderating effect is too small to boost reactivity, while the overall density of the reactor is decreased, which increases neutron leakage out of the reactor.
This effect is more pronounced with graphite than with water, because graphite is so much less effective at neutron moderation than water, so a lot more of it is required before the beneficial effect kicks in.


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 Post subject: Re: core moderation
PostPosted: May 10, 2014 3:42 am 
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Water seems to be the most effective moderator for low moderation. Let the Transatomic power people work on it for a waste burner.


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 Post subject: Re: core moderation
PostPosted: May 14, 2014 6:14 am 
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jaro wrote:
Your textbook illustration is incomplete: It starts at only 1 : 1 moderator to fuel ratio.

When we look at smaller moderator to fuel ratios "starting from no moderator", we find that at first the reactivity goes DOWN. That's because at low moderator fractions the moderating effect is too small to boost reactivity, while the overall density of the reactor is decreased, which increases neutron leakage out of the reactor.
This effect is more pronounced with graphite than with water, because graphite is so much less effective at neutron moderation than water, so a lot more of it is required before the beneficial effect kicks in.


You make a good point! The graph does not show it because it was made to illustrate the difference between UOX and MOX fuels in power LWRs, in which as you know such a low moderator to fuel ratio is not really of interest. :)


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