Void reactivity and over/under moderation

  • Thread starter Homer Simpson
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In summary, the positive void coefficient in CANDU reactors is not solely due to overmoderation, as removing the heavy water moderator also affects other factors such as fast fission factor, resonance escape probability, reproduction factor, and thermal utilization factor. If the lattice pitch is reduced to undermoderated levels, the void coefficient would still be positive due to decreased neutron moderation. However, the impact of overmoderation on the total void coefficient may be lower compared to other factors such as impurities in the heavy water. The speaker's expertise is mainly focused on CANDU reactors, but they acknowledge that the role of factors like fast fission and resonance escape may differ in reactors with enriched uranium.
  • #1
Homer Simpson
184
1
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 absorbtion 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!
 
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  • #2
I think as long as the moderation of neutrons decreases (density of the moderator/coolant decreases) there will be a positive void coefficent. 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.
 
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  • #3
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.
 

Related to Void reactivity and over/under moderation

1. What is void reactivity and why is it important?

Void reactivity is a measure of the change in reactor power caused by the presence of voids, or empty spaces, in the reactor core. This is important because it affects the stability and safety of the reactor, as well as its overall performance.

2. How does over-moderation affect void reactivity?

Over-moderation occurs when there is an excess of moderator, which slows down neutrons and increases the likelihood of fission. This leads to a decrease in void reactivity, as more neutrons are absorbed by the moderator instead of fuel, resulting in a decrease in reactor power.

3. How does under-moderation affect void reactivity?

Under-moderation occurs when there is not enough moderator, which allows neutrons to move faster and decreases the likelihood of fission. This leads to an increase in void reactivity, as more neutrons are available to cause fission, resulting in an increase in reactor power.

4. How do engineers account for void reactivity in the design of nuclear reactors?

Engineers use various methods to control void reactivity, such as adjusting the amount and type of moderator, using control rods to absorb excess neutrons, and designing the reactor core to minimize void formation. They also conduct extensive modeling and testing to ensure the reactor operates safely and efficiently.

5. What are some potential consequences of not properly managing void reactivity?

If void reactivity is not properly managed, it can lead to unstable reactor power, which can result in a loss of control and potentially cause a nuclear accident. It can also affect the efficiency and longevity of the reactor, as well as increase the risk of radiation exposure for workers and the surrounding environment.

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