Understanding Nuclear Core: Fuel Rod Composition and Control Rod Function

[biferi]

I know the Fuel Rods are 12 Feet long Tubes made of Zirconium Alloy.

And they fill them with Uranium Oxide Pellets.

And the Control Rods are inserted inbetween them to slow down the reaction.
And the Control Rods are made of Boron.

But is the Control Rods Tubes filled with Boron Pelletes or are the Control Rods just Solid Tubes made of Boron?

 

[mathman]

It looks like the rods are solid material.
from:
http://en.wikipedia.org/wiki/Control_rod

Boron is another common neutron absorber. Due to different cross sections of 10B and 11B, boron containing materials enriched in 10B by isotopic separation are frequently used. The wide absorption spectrum of boron makes it suitable also as a neutron shield. Mechanical properties of boron in its elementary form are unfavourable, therefore alloys or compounds have to be used instead. Common choices are high-boron steel and boron carbide. Boron carbide is used as a control rod material in both pressurized water reactors and boiling water reactors.

 

[Astronuc]

I know the Fuel Rods are 12 Feet long Tubes made of Zirconium Alloy.

And they fill them with Uranium Oxide Pellets.

And the Control Rods are inserted inbetween them to slow down the reaction.
And the Control Rods are made of Boron.

But is the Control Rods Tubes filled with Boron Pelletes or are the Control Rods just Solid Tubes made of Boron?

Most PWR fuel rods have an active fuel (e.g., ceramic UO₂ pellets) of 12 ft. The cladding tubes are slightly longer in order to provide a plenum or open volume to accommodate fission gas released during operation, and to accumulate the He fill gas as the cladding creeps down on the fuel and as the fuel swells during operation.

Some PWRs have longer fuel rods, e.g., 12.5 ft active fuel or 14 ft active fuel. BWR fuel rods may have fuel zones of 12.2 to 12.5 inches, and some BWR fuel assemblies incorporate shorter or part-length fuel rods.

PWR control elements are comprised of stainless steel (304L or 316L) tubes which are suspended from a steel hub and spider. The tubes slide along the guide tubes in the assembly. Each tube is seal-welded with stainless steel endplugs, and the upper endplug shank is then attached to the spider-hub assembly. The absorber material within the tubes is in solid form, which could be pellet of B₄C (with the B enriched in B-10), silver-indium-cadmium (AIG), Hf or Hf-alloy, or Dy titanate. Dy titanate has been developed in Russia and is relatively new. AIG is the most common absorber in PWR fuel. Hf was used for a while, but the Hf absorbs hydrogen which comes from the coolant and over time, the Hf swells with the accumulation of H.

BWR control blades have a cruciform structure and actually sit between fuel assemblies rather than within an assembly. The absorber material is either B₄C or Hf, or a combination.

 

[Hiddencamper]

The control rods are solid tubes with material on the inside. I don't know if it is pellets or what not, I do know that if we crack a rod it will leak all cycle and is pretty messy to deal with from both a water chemistry and reactivity perspective.

 

[Astronuc]

For metals like silver-indium-cadmium alloy or Hf, the absorber is in the form of a cylindrical bar or rod. It may be hollow. Ceramics such as B₄C are in the form of pellets or slugs.