The CRD system in 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 is crucial for its operation. The CRD is necessary for the percise control of Control RodsControl RodsControl Rods are rods that are used to control the rate of reaction inside and within a Nuclear Reactor. 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. in a reactor. In BWR reactorBWR reactorA boiling water reactor uses Light Water as both coolant and neutron moderator. The second most used reactor, next to the PWR reactors, there are approximately 75 plants in current operation. The efficiency of these reactors is about 46%, with 33-34% in practice. Enriched uranium is used as nuclear fuel, as light water absorbs too many neutrons to use Uranium that is natural. Light water is not as good of a moderator compared to Heavy Water or graphite, but it is good as in the event of a LOCA os, this is especially necessary, as control rods are inserted via the bottom penetration points in the RPVRPVReactor Pressure Vessel - contains all of the reactor heat. In BWR reactors, the RPV contains the reactor core - basically the entirety of the main reactor assembly. The RPV is designed to withstand a very large amount of force considering that in a BWR it must withstand the pressure that both it operates at and at emergency designs -- this is due to the fact that in most designs, the RPV isn't considered to be at major risk: even during a major LOCA the RPV is considered to be at healthy condi, and if there is a SCRAM, gravity cannot be relied on to insert the rods, differing from other reactor designs. Therefore, a method to SCRAM the plant and percisely control the rods in a BWR is necessary.

BWR Control Rod Drive Mechanisms

Fine Motion Control Rod Drive (FMCRD)

The FMCRD is responsible for providing electric motor driven positioning for normal insertion, withdrawal. It also provides electric motor run-in of all control rods that is separated from the hydraulically controlled SCRAM system, simultaneous with SCRAM.

The FMCRD is also responsible for the hydraulic scram system, accompanied by the HCU

Hydraulic Control Unit

The HCU contains high pressure water that has enough water pressure for the SCRAM of two FMCRDs. the HCU is also a flow path from purging the water to drive rods during normal operation

Control Rod Drive Hydraulic (CRDH)

The CRDH has high pressure demineralized water, regulated and controlled to charge each HCU scram accumulators, purge water, and provide backup water to the RPV when feedwater flow is down.

Misc Systems

The CRD also has a Scram Discharge Volume attached to it; during a scram, after water has entered into the rods and pushed them in, the scram discharge water collects here. The majority of NRC reported incidents related to SCRAMs occur due to the SDV being full for various reasons