The Fermi Chronicles - Part 26: Control Rods

As I discussed last time, fission poisons are both a necessary component of reactor design as well as a byproduct of the fission reaction. This has a large implication on control rod design, which is particular to the reactor type. Control rods must precisely control the reactivity of a nuclear reactor core. Boron and halfnium are common elements in control rods, but there may also be silver, indium, and cadmium. As a general rule, however, control rods should have a large absorption cross section, and should not burn up quickly. That is, they should last for a long time even though as the element absorbs neutrons it changes into something else. Control rods come in two basic varieties - gray and black. Black rods refer to perfect absorbers. they will absorb essentially all incident neutrons. Gray rods have a lower absorption cross-section and burn-up slower as a result.

Black rods are also known as safety rods as they will shut down all fission when inserted into the core. Gray rods are either shim rods that are used for course reactivity control, or regulating rods for fine control.

The most important function of control rods, however, is in an emergency shutdown procedure known as a reactor SCRAM, or trip. When a reactor is tripped, all fission must cease. In this case, control rods are inserted into the core immediately, and these safety rods tend to be black - quickly absorbing all neutrons.

It is interesting that the control rod design is significantly different between the pressurized water reactor (PWRs) and boiling water reactors (BWR). For the PWR, the control rods come in from the top of the fuel assembly as such:

Here's another that clearly shows the cradle assembled from the top down:

In boiling water reactors, the control rods are inserted into the core from the bottom. There's a good reason for that difference and it is that in BWRs the boiling water bubbles need space to rise into. In PWRs, since there is no bubble formation, the rods coming down from above won't affect the flow so much. There is no reactor design to my knowledge in the PWR or BWR designs where the rods are mounted sideways.

Previously:
The Fermi Chronicles - Part 25: Fission Poisons
The Fermi Chronicles - Part 24: Reaction k-Factor
The Fermi Chronicles - Part 23: Davis Besse, Ohio, 2002
The Fermi Chronicles - Part 22: Nuclear Events - Chernobyl, 1986
The Fermi Chronicles - Part 21: Nuclear Events - Three Mile Island, 1979
The Fermi Chronicles - Part 20: Nuclear Events - Browns Ferry, Alabama, 1975
The Fermi Chronicles - Part 19: Nuclear Events - Fermi 1, 1966
The Fermi Chronicles - Part 18: Nuclear Events - SL-1 Event, Idaho, 1961
The Fermi Chronicles - Part 17: Nuclear Events - Windscale, UK, 1957
The Fermi Chronicles - Part 16: Nuclear Events - Chalk River, CAN, 1952
The Fermi Chronicles - Part 15: The Nuclear Business Model
The Fermi Chronicles - Part 14: Neutron Moderation
The Fermi Chronicles - Part 13: Nuclear Reactor Types
The Fermi Chronicles - Part 12: Generating Electricity
The Fermi Chronicles - Part 11: Worldwide Uranium Availability
The Fermi Chronicles - Part 10: Utilizing Nuclear Reactions To "Breed" More Fuel
The Fermi Chronicles - Part 9: Nuclear Fission
The Fermi Chronicles - Part 8: Neutron Interaction
The Fermi Chronicles - Part 7: Radioactive Decay and Half-Life
The Fermi Chronicles - Part 6: Atomic Structures
The Fermi Chronicles - Part 5: Nuclear Waste Storage
The Fermi Chronicles - Part 4: Radiation Types and Radiation "Dose"
The Fermi Chronicles - Part 3: Radiation Types
The Fermi Chronicles - Part 2: A week of training
The Fermi Chronicles - Part 1: The alpha post