In all LWRs, control rods function to control the fission rate, or reactivity, by inserting or withdrawing neutron-adsorbing material from the reactor fuel core. Current large PWRs typically use 17 X 17 fuel assemblies that include guide tubes for 24 control rod fingers which are operated together through a spider assembly. All fuel assemblies are capable of hosting a control rod assembly, but not all fuel assemblies will be fitted with a control rod assembly in any given fuel cycle. Control rod assemblies are generally split into two groups; control groups and shutdown groups. The shutdown control rod groups are completely withdrawn from the core to provide a large source of negative reactivity to shut down the reactor in the event of an accident. The control groups are generally partially inserted into the core and are slowly withdrawn over the fuel cycle to compensate for fuel burn up.

Current large PWRs also use a second method to control reactivity by adding soluble boric acid to the reactor coolant. Boric acid is a strong neutron absorber and is referred to as chemical shim. Current large PWRs heavily borate the RCS at the beginning of a fuel cycle and slowly dilute the boron concentration over the fuel cycle in conjunction with control rod motion to maintain operating temperature.

Many iPWR designs plan to use a half-height version of proven 17 X 17 array fuel assemblies. This will allow similar spider assembly control rods to be used. The iPWR cores contain fewer fuel assemblies than the current large PWRs and will typically have a higher percentage of fuel assemblies fitted with a control rod spider assembly. In addition, some iPWR designs may fit a very high percentage of the fuel assemblies with a control rod spider assembly and opt not to implement a chemical shim for normal reactor operation. However, it is anticipated that most iPWR designs will use concentrated boron as an emergency backup to shut down the reactor in the event not all the control rods are able to be inserted into the core. Therefore, support systems for batching boric acid will be required on most iPWR designs.

5.2.2 Control rod drive mechanisms

In all current large PWR designs, the control rod drive mechanisms (CRDMs) are external to the reactor vessel above the reactor vessel head. Prior to removing the PWR head for refueling, the CRDMs are decoupled from the control rod spider apparatus. The control rods are then left in the respective fuel assemblies while the head is removed.

Some iPWR designs plan to continue the practice of using CRDMs that are external to the reactor vessel. However, because the iPWR reactor pressure vessel is much taller than current PWR pressure vessels and the iPWR pressure vessel flange is not located at the top of the pressure vessel, some design consideration will be necessary regarding how the shaft of the control rod spider is decoupled from the CRDM and protected when the upper reactor pressure vessel is removed from the lower reactor pressure vessel for refueling. Integral PWR designs planning on the use of external control rods include the SMART reactor and the NuScale reactor (Lee, 2010; NuScale, 2012).

Other iPWR designs intend to use CRDMs that are internal to the reactor pressure vessel. This will require some materials and reliability testing given the high temperature, pressure, and radiation environment inside the reactor pressure vessel. In addition, electrical cabling will be necessary that passes through the reactor pressure vessel flange to operate the control rod drive mechanisms and provide control rod position indication. Current designs planning on the use of internal CRDMs include the Generation mPowerTM reactor and the Westinghouse SMR reactor (Kim, 2010; Memmott et al., 2012). In addition, the IRIS design had planned to use internal

CRDMs. (Carelli et al, 2004). The CAREM reactor uses internal hydraulic CRDMs, which will not require electrical cabling for operation; just position indication (Mazzi, 2011).