Nuclear reactor neutron absorber

[Crazymechanic]

Hi could you please explain me what happens with the atomic structure and overall material structure of neutron absorber rods used in fission reactors?

As I imagine when they are used they absorb the neutrons that keep the chain reaction going so to stop the chain reaction hence shut the reactor down, but the absorber rod material then while absorbing these neutrons changes it's atomic structure I guess as every other element being bombarded with fast or thermal neutron would , so how long a absorber rod material would last and how does it changes it's structure over time , I guess the explanation should be about the overall tendencies as each reactor type uses a little different material or mix of materials like boron , boric acid , cadmium, hafnium etc.

Thanks.

 

[mfb]

Neutrons do not change the element of a nucleus - and thermal neutrons do not have enough energy to destroy chemical bonds. Therefore, the absorption of thermal neutrons itself does not change the material structure at all, it just influences the isotope composition and the total weight of the material a bit. Quick neutrons can break chemical bonds, and radioactive decays (and radiation from somewhere else) can change the material.

 

[Crazymechanic]

Well one big out of many engineering challenges for the tokamak torus structure is the material degradation due to neutron bombardment is that correct and if so then are those the fast neutrons that do cause damage to metals and other types of materials when penetrating them?

Also I read that reactors especially PWR's because the have a highly pressurized sealed reactor vessel do have to be shot down when their design lifetime is over, and one of the main reasons is that the reactor vessel outer steel has become structurally unstable for further use because of neutrons is that correct and are they the same fast neutrons?

As much as I know I believe that atleast fast neutrons do cause different materials to become radioactive like they say in the tokamak case but how much degraded they become compared to the induced radioactivity?

 

[snorkack]

Neutrons do not change the element of a nucleus - and thermal neutrons do not have enough energy to destroy chemical bonds.

They do. While most neutron capture reactions are n,γ, so element remains the same (till followup beta decay, which are common though not universal), n,p and n,α reactions also happen. And B-10 relies exactly on n,α.

Even n,γ gives the nucleus recoil energy. Deuteron formed by slow neutron capture should emit about 2,2 MeV gamma - leading to about 1200 eV recoil. But a few eV will suffice to break chemical bonds.

 

[Crazymechanic]

So the neutron absorber rod in a fission reactor does degrade with time and needs to be replaced ? Or particular materials can take a lot of "beating" for a long time and show only minor changes in the atomic structure?

Also I think that sooner or later this happens to pretty much all the metals and plastics that make up a fission or potential fusion reactor.

 

[mfb]

@snorkack: Ah right, I forgot the energy released in neutron capture, sorry.

 

[snorkack]

Some materials do take beating easily, others do not.

When a fast neutron displaces a nucleus, the chemical bonds to that nucleus are broken. What happens next depends on the structure of the bonds in materials.

In some materials, the moving ions break a large number of bonds. New bonds form which are very different from those that are broken. The new bonds can store a large amount of Wigner energy. The material may go brittle, shrink or swell... also the helium nuclei that enter the material or are formed inside by n,α reactions must be fitted somewhere.

In other materials, a large number of chemical bonds are indeed broken - but the displaced atoms rapidly choose new positions where they are indistinguishable from the ones that were displaced, and form new indistinguishable bonds. The energy spent on breaking bonds is dissipated harmlessly as heat, and the material remains unchanged if unstressed or under low stress, or perhaps undergoes a slight creep under high stress.

 

[QuantumPion]

So the neutron absorber rod in a fission reactor does degrade with time and needs to be replaced ? Or particular materials can take a lot of "beating" for a long time and show only minor changes in the atomic structure?

Also I think that sooner or later this happens to pretty much all the metals and plastics that make up a fission or potential fusion reactor.

The neutron-absorbing properties of PWR control rods do not deplete much at all during their lifetime because by design, they strongly reduce the flux in their vicinity. They are not exposed to full power flux. Radiation growth of their cladding necessitates their replacement of control rods every 10-15 years. Burnable absorber rods on the other hand, are used to control power distribution and are designed to deplete over the course of a cycle. Burnable absorbers are only used one cycle.

 

[QuantumPion]

Well one big out of many engineering challenges for the tokamak torus structure is the material degradation due to neutron bombardment is that correct and if so then are those the fast neutrons that do cause damage to metals and other types of materials when penetrating them?

Also I read that reactors especially PWR's because the have a highly pressurized sealed reactor vessel do have to be shot down when their design lifetime is over, and one of the main reasons is that the reactor vessel outer steel has become structurally unstable for further use because of neutrons is that correct and are they the same fast neutrons?

As much as I know I believe that atleast fast neutrons do cause different materials to become radioactive like they say in the tokamak case but how much degraded they become compared to the induced radioactivity?

Reactor vessels can be annealed to repair the radiation damage accumulated over time (although I'm not sure if it has been done in practice yet). Note that the vessel itself is exposed to much less neutron flux than the fuel itself, because it is shielded by the core barrel and coolant bypass flow.

A tokamak reactor would be exposed to higher flux and higher energy fast neutrons compared to a fission reactor vessel. However, the tokamak I would imagine could be made out of neutron transparent materials such as zirconium, since it does not have to have the same material strength as a fission reactor vessel - it is only a pressure vessel holding a vacuum at 15 psi, it does not have to contain water at 2200 PSI.

 

[Astronuc]

Reactor vessels can be annealed to repair the radiation damage accumulated over time (although I'm not sure if it has been done in practice yet). Note that the vessel itself is exposed to much less neutron flux than the fuel itself, because it is shielded by the core barrel and coolant bypass flow.

VVER-440 RPVs have been annealed.
http://capture.jrc.ec.europa.eu/publications/AMES/docs/old-pubs/EUR16278EN.pdf

A tokamak reactor would be exposed to higher flux and higher energy fast neutrons compared to a fission reactor vessel. However, the tokamak I would imagine could be made out of neutron transparent materials such as zirconium, since it does not have to have the same material strength as a fission reactor vessel - it is only a pressure vessel holding a vacuum at 15 psi, it does not have to contain water at 2200 PSI.[/QUOTE] Finding the optimal first wall material and supporting structure is still an active area of research. Activation, swelling and embrittlement are three factors of concern; the key concern is structural integrity.

 

[Astronuc]

Hi could you please explain me what happens with the atomic structure and overall material structure of neutron absorber rods used in fission reactors?

As I imagine when they are used they absorb the neutrons that keep the chain reaction going so to stop the chain reaction hence shut the reactor down, but the absorber rod material then while absorbing these neutrons changes it's atomic structure I guess as every other element being bombarded with fast or thermal neutron would , so how long a absorber rod material would last and how does it changes it's structure over time , I guess the explanation should be about the overall tendencies as each reactor type uses a little different material or mix of materials like boron , boric acid , cadmium, hafnium etc.

Thanks.

One has to distinguish between BWR and PWRs, and in PWRs, those designs that use control rods for shutdown only vs those that use grey rods for power shaping and quick power control (grey rods) or reactivity control.

In BWRs, groups of control rods are used intermittently of reactivity control, in addition to using the void distribution. Groups of control rods are swapped periodically to balance the reactivity in the core. After a certain service period, the active control rods are swapped into shudown banks, and those control rods in the shutdown banks are removed from service when they reach their design exposure. BWR control rods use B₄C and some Hf in peak flux locations. Boron may be enriched in B-10, and B-10 undergoes an n,α reaction that depletes the B-10. Hf absorbs neutrons, but does not disintegrate like B-10.

http://web.mit.edu/nrl/Training/Absorber/absorber.htm

One has to be careful with Hf, because within the stainless steel structure, it may absorb hydrogen and swell. The structural material is usually high purity (low S, P and other impurities) 304L or 316L.

PWRs usually do not use control rods during operation for reactivity control, but rather the control rods are withdrawn above the core and only used to shutdown the reactor. Some designs use 'grey rods' which contain a reduced absorber content or a Ni-based absorber. These are used for radial and axial power shaping, as well as fine reactivity control for load following. Some new reactor designs do use more advanced grey rods, and at least one new PWR-type design is planning to use control rods for reactivity control to eliminate the need for boric acid (chemical shim). PWRs traditionally use boric acid buffered with LiOH to ensure the pH is within certain limits during the cycle.

PWR control rods use B₄C, AIC (Silver (Ag)-Indium (In)-Cadmium (Cd), Hf, or Dy (in form of Dysprosium titanate). Use of B₄C and AIC has to take into account the swelling that occurs with exposure. Use of Hf means one has to be concerned with hydrogen absorption. In some cases, B₄C may be used with AIC tips.

Both BWR and PWR fuel use gadolinia mixed with the UO2 fuel for control of power peaking in the fuel, and in some PWR fuel, some fuel pellets are coated with ZrB₂ instead of using gadolinia. The isotopes Gd-155 and -157 are strong thermal neutron absorbers as compared to Gd-156 and Gd-158.

PWR control rods use 304L or 316L for structural material. Swelling and intergranular stress corrosion cracking are two concerns with respect to performance and lifetime. Austenitic steels are susceptible to swelling.

Neutron irradiation does cause changes to atomic microstructure in metals - mainly dislocations of atoms in the crystal structure. This has the effect of hardening and embrittling the metals. Irradation may sensitize certain alloys, i.e., make them more susceptible to cracking.

Lifetime of PWR control rods is about 15 years.

 

[Crazymechanic]

Thank you all for answers and very detailed ones.