Control rods

Nuclear Reactor Control Rods

Updated 1/5/2019

Sometimes people ask me what the control rods in nuclear power plants are. Well, here’s the answer:

PWR_control_rod_assemby

Uses of control rods

Nuclear reactor control rods have three primary types

  • Safety rods – These rods have a high overall “rod worth.” These are used for emergency shutdowns. They must be capable of reducing reactor power below a certain amount within a certain time frame. The rod worth is the stopping power of the control rod in slowing a fission reaction.
  • Flux peaking rods – These control rods are used for controlling local neutron flux levels. They control fuel usage and maintain it within a specific band for localized fuel burnout. By raising and lowering on these rods that have a relatively low rod worth, the flux profile can be shaped to match the engineered ideal profile. Think of it like braking and accelerating your car in the most efficient way to save gas.
  • Power control rods – This type of rod is used for an overall control of reactor power during transient power changes. They may have a faster rod speed than other rods in use. These are the rods that are used to control coolant temperatures when operating at power. It bears mentioning that borated water may be used in some circumstances instead of control rods to control reactor power levels. This is called a “chemical shim” and provides a more even neutron flux change.  Also, variation of recirculation pump speeds in BWR designs will change the coolant to steam ratio inside of a fuel channel and, based on the void coefficient, change reactor power levels. These are alternative methods of changing power levels without using control rods.
Chernobyl

Principles of Operation 
All control rods must operate on two basic principles

  • They must operate at a speed fast enough to overcome a peak end of life xenon transient following a reactor shutdown to prevent a xenon precluded startup. This allows the reactor to reach design end of core life without core life being limited by fission product poisons. What does that mean? Well, sometimes nuclear poisons (not people poisons, these prevent fission from happening by absorbing neutrons) build up in the core following certain power transients. To overcome that poisoning effect, your control rods must be fast enough to change your neutron flux rate fast enough to keep up with the negative effects of the poisons.
  • They must operate slow enough to allow operator action to prevent a power excursion in the event of a rod withdrawal casualty. If rod speed is too great, a rod withdrawal casualty will cause an uncontrollably high startup rate that will cause a peak reactor power level in excess of thermal limits.

This is the Rod Position Indication used at Chernobyl power plan. Each individual dial shows the position of the control rod. Spread throughout the panel are local power level indicators. Due to the size of the Chernobyl power plants, it was possible for one part of the core to be operating critical while another part remained subcritical.
Design of control rod drive mechanisms
Control rods may be inserted either via the top of the reactor vessel through the flange or in through the bottom of the vessel such as in boiling water reactors.  CRDMs may either function off of an electric motor or hydraulically. The electric motor type system will rotate causing insertion or withdrawal of the control rod via a lead screw attached to roller nuts. The hydraulic system will cause the differential pressure across a drive piston to force the rod inward or outward depending on the position of directional control valves. This is an example of hydraulically operated control rod drive mechanisms.

CRDM

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