Void reactivity and over/under moderation

[Homer Simpson]

refering to CANDU here,

My previous thoughts were that the positive void co-eff was mainly due to the fact that we are overmoderated, so removing the HT water, which is a moderator, pushes a bit more towads 'less over moderated' so reactivity goes up.

Apparently there is more to the story that has nothing to do with over moderation at all.

k=Epfn

E- fast fission factor goes up, as no HT moderation knocks fast n's below their 1.2 MeV threshold for fast fission of U238

p - Resonance escape probablility goes up due to no fast n's getting knocked down by heat transport moderation into resonance capture peaks for u238.

n - Reproduction factor - down due to spectrum soften, below resonance peak for Pu239 fission

f - Thermal utilization factor - up due to impurities in HT water gone, so less absorption of thermal n's.

So, my QUESTION IS: Let's say the CANDU lattice pitch was reduced so that it was undermoderated... would this even matter? Would it still be positive Void coeff? The only factor that seems to be effected at all by overmoderation is 'f', so what contribution does this even make to the total void coeff compared to the other factors?

thanks to anyone who can clear this up for me!

 

[theCandyman]

I think as long as the moderation of neutrons decreases (density of the moderator/coolant decreases) there will be a positive void coefficient. I'm in charge of the safety analysis of the fast reactor in my senior design class, so most of what I know is more applicapable to LMFBRs.

 

[Homer Simpson]

Thanks Candyman, and I don't know too much outside of CANDU, perhaps the fast fission factor and resonance escape probabilities don't have such an impact with enriched U. No clue about that though.