Nuclear Power Plant criticallity question?

  • #1

Main Question or Discussion Point

Hi,

I needed to know some things about reactivity and how it is changed by different factors while making a reactor critical? Control Rods, Booster Rods, Boron, Moderator Level, Xenon etc are affecting the reactivity of the reactor, but how to find out their contribution at certain point and how are they balancing out each other at different operation conditions. For example, if moderator level is changed lets say by 1 inch then how will it affect the reactivity of others and how will they change to counter this change so that they reactor may remain critical. If someone here can give me the detail about reactivity changes with different changing parameters and operation conditions then I will be thankful
 

Answers and Replies

  • #2
Astronuc
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There is no simple answer because it depends in the type of reactor, size, fuel lattice, enrichment (mean and distribution), neutron spectrum, power density, cycle length and batch size.

Type of reactor: LWR (PWR/VVER, BWR), SCWR, HWR, Graphite moderated, Liquid metal fast reactor, Breeder reactor. All use different enrichments, and some use moderators, while others do not.

Control Rods: Most PWRs have control rods withdrawn during operation, although some may use grey rods for power shaping and quick maneuvering (e.g., load following and frequency control). BWRs use control rods (control blades) during operation to control reactivity. In either case, the control rod system are designed to be able to shutdown the reactor with the highest worth rod(s) stuck out of the core.

PWR control rods usually contain Ag-In-Cd, but some may use B4C, Hf, Dy. BWR control rods contain B4C and Hf. One must be careful with Hf since it absorbs hydrogen and may swell thus compromising the ability to use the control rod.

BWRs use 'deep' and 'shallow' rods, i.e., the deep rods are more inserted than shallow rods. During the cycle, the deep and shallow rods are swapped to balance the exposure in different fuel assemblies. Also, different groups (sequences) of control rods are used to balance exposure in the different groups of assemblies in the core (Think of quadrant (and even octant) symmetry)

Booster Rods: In general commercial rods do not use 'booster' rods. It's not a term used in commercial plants. What does one mean by booster rod. Some research reactors, e.g., Halden, can use booster rods to enhance the local flux/power. In general, using systems that insert positive reactivity when inserted into the core is discouraged.

Boron: Soluble boron? PWRs use soluble boron buffered with LiOH (or KOH in VVERs) to control reactivity during the cycle. It may be roughly constant or slightly increasing early in the cycle, but then gradually (nearly linearly) decreasing throughout the cycle as enrichment is depleted. The concentration of B10 in the coolant depends on the fuel enrichment and excess reactivity in the core.

Moderator Level: PWRs have a constant moderator level, whereas BWRs, which experience boiling in the core, can adjust the liquid moderator level using flow control. Using flow control, BWRs can do 'spectral shift' in which more voiding takes place in the upper part of the core, and with a harder neutron spectrum, more Pu-239 is produced. Later during the cycle, the flow increased to reduce the voiding and the power is shifted upward to use the Pu-239. Cooler (lower power) assemblies have less voiding than higher power assemblies.

Xenon: Xe-135 is the most important isotope because of its high thermal neutron absorption coefficient. Xe-135 is a factor when reducing power or shutting down the reactor since as power is reduced, its concentration increases since the reduction in neutron flux changes the equilibrium.

Generally reactivity is gradually increased in order to prevent loss of control of a reactor. Negative reactivity may be abruptly added to the core, but in most applications, it too is done gradually, unless shutdown is the intent. Most changes are done with cents-worth of reactivity, and nothing approaching 1 dollar (Δ k = 0.0065 to 0.007, or one dollar of reactivity is equivalent to the effective delayed neutron fraction). Prompt criticality must be avioded in a reactor.
 
  • #3
Thank you Astronuc. That information was help. I am doing this for PHWR (CANDU) Reactor. I am working on how much reactivity is introduced by all these factor. Like I said if I am changing the moderator level then it is affecting the reactivity by some factor which is countered by other parameters. I want to understand how these parameters are depending on each other and affecting the overall reactivity of the reactor?

You said:
"Xe-135 is a factor when reducing power or shutting down the reactor since as power is reduced, its concentration increases since the reduction in neutron flux changes the equilibrium."

Can you explain it with some more detail or may be suggest me some book or article where I can study it in detail?

Thanks again!
 
  • #4
Astronuc
Staff Emeritus
Science Advisor
18,824
2,051
http://canteach.candu.org/library/20041310.pdf [Broken]

Interestingly, the document mentions booster rods - highly enriched rods which are inserted into the core when override capability is needed. This is not the case for LWRs, but seems unique to CANDU.

CHAPTER 5: REACTOR CONTROL AND PROTECTION - CANTEACH
canteach.candu.org/library/20053334.pdf

CANDU Fundamentals - Xenon: A Fission Product Poison
canteach.candu.org/library/20040720.pdf
 
Last edited by a moderator:
  • #5
http://canteach.candu.org/library/20041310.pdf [Broken]

Interestingly, the document mentions booster rods - highly enriched rods which are inserted into the core when override capability is needed. This is not the case for LWRs, but seems unique to CANDU.

CHAPTER 5: REACTOR CONTROL AND PROTECTION - CANTEACH
canteach.candu.org/library/20053334.pdf

CANDU Fundamentals - Xenon: A Fission Product Poison
canteach.candu.org/library/20040720.pdf
Thanks for the help.
 
Last edited by a moderator:

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