Control Rods

Control Rods are rods that are used to control the rate of reaction inside and within a Nuclear ReactorNuclear ReactorIn it's most simplest form, a nuclear reactor uses Uranium and other radioactive materials and the fission from uranium to create heat, and transfer that heat into steam to create power. Nuclear reactors are one of the biggest sources of energy, although not renewable, uranium has a very high energy density resulting in massive power transmissions. There are many different types of nuclear reactors, and this term serves as a broad hub/introduction for each type. After uranium is used in reacto. They have a specific design that allows for this control.

BWR reactor parameters

In the GE Marathon control rod, it contains stainless steel tubes filled with boron carbide poison. This isotope contains a high cross-section for the absorption of neutrons, acting as a control rod neutron poison. These control rods are uniformly placed into a reactor core according to its design parameters.

Some control rods might also have hafnium, that has isotopes with very good cross sections for neutron absorption. This results in greater control rod service lifetime and avoids poison tube cracking that may occur in boron powder tubes. However, this does result in a higher cost and weight, and also a redesign of the control rod velocity limiter

Each control rod also has two sets of rollers that help it scram quickly, one at the top end casting, and one at the bottom on the velocity limiter.

Control Rod Velocity Limiter

The control rod velocity limiter is a simple yet integral part of the control rod end casting. It is engineering for the emergency of a rod drop accident - as rods are inserted through the bottom. When rods are inserted, water flows through the cone shaped limiter and into the annulus between the tube at the limiter. If there is a dropout, water is trapped by the lower cone, discharging through the annulus and causing a sever turbulence, slowing the descent of the control rod to limit increased reactor reactivity.

Why are control rods entered from the bottom?

Specifically? Bottom entry control rods are used for several reasons: • Less time is required during refueling outages to remove and reinstall the reactor vessel head • Control rods remain operable when the reactor vessel head is removed • Internal moisture removal and steam separation can be more easily accomplished without interference from top mounted control rods • There is a large percentage of voids in the upper part of the BWR core . The voids significantly reduce the power in the upper core. If control rods were partially inserted from the top of a BWR core they would severely depress the upper core flux further • The control rods are used for axial power shaping. Specifically leaving some control rods partially inserted in the lower portion of the core helps to control local flux peaking and yields a more optimum fuel burnup • Bottom entry control rods allow maximum use of water as a neutron shield for control rod drive mechanism components (this is the direct quote from 2.2.9.3 of 2.2-14 GE BWR_4)