Why is boron so good at absorbing neutrons? Why does it have such a large target area compared to the size of its nucleus?
Nuclear Physics – Why Is Boron So Good at Neutron Absorption?
neutronsnuclear-physics
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I was wondering today, how long boron control rods remains in a nuclear power plant?
- As low as 4 years, as high as 30 years.
- Depends on the CR location in the core, since power varies with radial distance from the center the of core and sometimes with polar coordinate in the core (BWRs?). Also, depends on operating conditions, time spent inserted at power, vibration, etc.
- The location of each CR is rotated cycle-to-cycle to spread the fluence (neutron absorption) on the rod over many years.
So, how long does it take until a control rod is completely depleted (no absorbing boron atoms left), that they are useless and have to be exchanged?
- Control rods rarely absorb so much that they have lost their effectiveness. This is especially true in PWRs since power control is performed by soluble boron in the coolant and very little of the operating cycle is spent with CRs inserted.
- Some plants will still measure CR lifetime in B-10 % remaining, but this is for ease of comparing simulation results since B4C pellet swelling has been correlated to a certain B-10 %.
- The reason both BWR and PWR control rods are retired are due to structural issues related to:
- absorber-cladding interaction due to absorber fluence (main PWR reason)(see 'why boron' below)
- irradiation-assisted stress corrosion cracking
- cladding neutron embrittlement
- vibration induced wear on the fingertips.
And I also don't know why they use boron for control rods? What is the special with boron?
- Boron-10, which is about 19.8 percent of natural boron, has a high neutron absorption cross-section. It is relatively inexpensive. These qualities make it a good CR material.
- Boron is usually used in the form B4C. This material has a tendency to swell once it has absorbed a lot of fluence, which makes the material interact with the CR cladding. This is usually the end of life for PWR control rods.
Why can't you use any other stable element? Other isotopes in control rods are used.
- Element stability is not of concern in CR isotope selection. Cost, swelling, and mainly absorption cross-section are of interest.
- AgInCd - Silver, Indium, Cadmium rods are used. For most CRs using this material, only the tips contain it since the tip sees the highest fluence and it swells less than boron. These have good absorption qualities, though, as it might sound, are expensive.
- Hafnium is a significantly better absorber because most Hf isotopes absorb a neutron and become another high absorption cross-section Hf isotope. It is also disproportionately more expensive, some argue because of the nuclear navy hoarding all the Hf.
Best Answer
It's boron-10 that is the good neutron absorber. Boron-11 has a low cross section for neutron absorption.
The size of the nucleus isn't terribly relevant because neutrons are quantum objects and don't have a precise position. The incident neutron will be delocalised and some part of it will almost always overlap the nucleus. What matters is the energy of the reaction:
$$ ^{10}\text{B} + n \rightarrow ^{11}\text{B} $$
and the activation energy for the reaction.
I'm not sure we understand nuclear structure well enough to give a quantitative answer to this. However neutrons, like all fermions, like to be paired and $^{10}$B has 5 neutrons while $^{11}$B has 6 neutrons. So by adding a neutron we are pairing up the neutrons and completing a neutron shell. We would expect this to be energetically favourable.
This argument would apply to any nucleus with an odd number of neutrons, but $^{10}$B is a light nucleus so we expect the effect to be particularly big. The lightest such nucleus is $^{3}$He, with one neutron, and that has has an even bigger neutron absorption cross section. However practical considerations rule out the use of $^{3}$He as a neutron absorber. $^{6}$Li, with three neutrons, also has a reasonably high cross section, though it is less than boron and helium.