Why does a control rod work in a nuclear reactor?

[RickVS]

Can I get in on this thread? I'd like to know if there is a limit to how long a control rod can be effective. Do they at some point become unable to absorb more electrons? Is this a theoretical limitation (like maybe taking 10,000 years) or a practical one (like not lasting the life of the plant it is operating in requiring new rods to replaced the worn out ones)? Thanks!

 

[Astronuc]

Can I get in on this thread? I'd like to know if there is a limit to how long a control rod can be effective. Do they at some point become unable to absorb more electrons? Is this a theoretical limitation (like maybe taking 10,000 years) or a practical one (like not lasting the life of the plant it is operating in requiring new rods to replaced the worn out ones)? Thanks!

Yes - there is both a nuclear limit and a mechanical limit.

The nuclear limit is based on the depletion of boron 10 (B¹⁰). When the control blade looses so much worth (a measure of the ability of the rod to absorb neutrons) it is moved (shuffled or relocated) to a new less critical position, and a fresh rod is put in the place, which is usually the 'shutdown' bank.

Control blades are also subject to mechanical limits due to irradiation effects, e.g., irradation-assisted stress corrosion cracking, or mechanical deterioration of the handle and structural parts, and also the internal pressure due to He production from the neutron capture by B¹⁰, which produces an (n, alpha) reaction.

At some point the control rod is removed from the core and placed in the spent fuel pool. Ultimately it is cut up and sent to a disposal repository.

 

[M. Bachmeier]

Yes - there is both a nuclear limit and a mechanical limit.

The nuclear limit is based on the depletion of boron 10 (B¹⁰). When the control blade looses so much worth (a measure of the ability of the rod to absorb neutrons) it is moved (shuffled or relocated) to a new less critical position, and a fresh rod is put in the place, which is usually the 'shutdown' bank.

If I understand correctly, you are saying that control rods (shutdown) are the newest, but that there may be variability in their age (life-cycle as (shutdown) control rods)? Older rods would be used to control the arrangement or geometry of (power generation) normal reactivity?

 

[daumphys]

How about making small RTG(radioactive thermoelectric generator) assembled nuclear power plant? First make small RTGs in the battery plant, and power station only assemble them.
And used battery is retreated, and use it again. At the present high voltage technology is not developed, but we can make high voltage by using solar energy collection method.