Nuclear reactor control rods and blades · neutron absorbers such as boron, cadmium and hafnium, PWR rods from the top and BWR cruciform blades from the bottom, grey rods for load following, and US design criteria for redundant reactivity control
A control rod is a rod or blade containing neutron-absorbing material that is moved into or out of a nuclear reactor core to control the rate of fission, or to shut the reaction down. Ask three things first: which reactor design and drive mechanism the rod must fit, which absorber and cladding materials the licensee has qualified, and which regulatory and quality requirements govern its manufacture and supply.
Control rods, control rod assemblies and BWR control blades for commercial power and research reactors. Fuel assemblies, control rod drive mechanisms, soluble boron systems and burnable absorbers are outside this page except where compared.
CNFX editorial research · Sources checked October 12, 2026 · 4 sources
The US Department of Energy's Office of Nuclear Energy explains that in a light-water reactor the fuel rods sit in water that acts as coolant and moderator, and that control rods can be inserted into the core to reduce the reaction rate or withdrawn to increase it. [1] The World Nuclear Association describes control rods or blades as made with neutron-absorbing material such as cadmium, hafnium or boron, inserted or withdrawn to control the rate of reaction or halt it; PWR control rods are inserted from the top and BWR cruciform blades from the bottom of the core; some PWRs use special 'grey' control rods to sustain low power efficiently or follow load; and "secondary control systems involve other neutron absorbers, usually boron in the coolant", adjusted as the fuel burns up. In CANDU reactors control rods penetrate the calandria vertically, with gadolinium in the moderator as a secondary shutdown system. [2]
The World Nuclear Association also notes that zirconium, used in nuclear-grade zircaloy, is subject to trade controls because of its main use in nuclear power, an example of how materials in this sector are controlled. [2]
Give the supplier the reactor type and fuel assembly design, rod or blade geometry and interface with the drive mechanism, absorber material and form, cladding material, design life and burn-up or exposure limits, applicable regulatory requirements and quality-assurance programme, inspection and testing, documentation, export and transport controls, and delivery schedule.
02
Know the design criteria for reactivity control
The US General Design Criteria for nuclear power plants (10 CFR Part 50, Appendix A) require, under Criterion 26, two independent reactivity control systems of different design principles: one must use control rods, preferably with a positive means of inserting them, and reliably control reactivity changes in normal operation and anticipated operational occurrences with margin for malfunctions such as stuck rods, so that fuel design limits are not exceeded; one of the systems must be able to hold the core subcritical under cold conditions. Criterion 27 requires the combined systems, with poison addition by the emergency core cooling system, to maintain core cooling under postulated accidents with margin for stuck rods, and Criterion 28 requires limits on the amount and rate of reactivity increase. [3]
Editorial note: control rods for licensed reactors are specified by the plant's design and licensing basis; a buyer outside that framework should not treat this page as design guidance, and suppliers must work under the licensee's quality-assurance programme.
03
See the control rod data of an operating PWR
Nuklearna elektrarna Krško (NEK), the operator of Slovenia's Krško nuclear plant, publishes the technical data of its two-loop pressurized water reactor. The reactor has a thermal power of 1,994 MW and a gross electrical output of 730 MW, with 121 fuel assemblies of 235 fuel rods each in a 16 × 16 array. Reactivity is controlled by 33 control rod assemblies, each with 20 absorber rods and a total assembly weight of 52.2 kg; the neutron absorber is an alloy of 80% silver, 15% indium and 5% cadmium, which NEK lists as the silver content of its rod control cluster assemblies (RCCA). The reactor coolant carries boric acid as an additive, at a pressure of 15.41 MPa and vessel inlet and outlet temperatures of 287 °C and 324 °C. [4]
The plant combines control rod assemblies with boric acid in the coolant, two different means of reactivity control; Criterion 26 above requires two independent reactivity control systems of different design principles, one of them using control rods. [3][4] It is a top-entry PWR design of the kind described in the reactor overviews above. [1][2]
Editorial calculation: 33 assemblies × 20 absorber rods is 660 absorber rods in the core; 33 × 52.2 kg is about 1.72 t of control rod assemblies; 33 assemblies over 121 fuel assemblies means about 27% of fuel positions carry a control cluster.
NEK (Krško) parameter
Value
Reactor type and thermal power
PWR, 1,994 MW
Fuel assemblies
121, 16 × 16 array, 235 rods each
Control rod assemblies
33
Absorber rods per assembly
20
Weight per control rod assembly
52.2 kg
Neutron absorber
Ag 80% – In 15% – Cd 5%
Coolant additive
Boric acid
For a replacement order, give the plant's assembly drawing, the absorber material and number of rods per assembly, the cladding material and the assembly weight, and ask for the qualification and irradiation data behind the design.
04
Compare control element types
No manufacturer record has been reviewed for this product yet; the types below come from the sources cited.
Types include top-entry PWR control rods, bottom-entry BWR cruciform blades, 'grey' rods for load following, and vertical control rods in CANDU calandrias, alongside secondary control by soluble boron or gadolinium. [2]
05
Trace the control rod elements
This is a functional reference built from the sources cited; it is not one maker's specification.
Fill the specification fields with values for your reactor
A control rod quotation needs values for your reactor. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.
Explore company information and manufacturing capabilities, then discuss the scope of your project with a relevant supplier.
Share this page
Compiled and source-reviewed by the CNFX editorial team.
Published product pages show what a company offers, not tested performance. Confirm the legal seller, factory and scope for your order. How we source and review
· Page last updated 13 October 2026
Buyer questions
What does a control rod do?
It absorbs neutrons; DOE explains it is inserted into the core to reduce the reaction rate or withdrawn to increase it. [1]
What are control rods made of?
The World Nuclear Association lists neutron-absorbing materials such as cadmium, hafnium or boron. [2]
Do PWR and BWR control rods differ?
Yes; PWR rods are inserted from the top, BWR cruciform blades from the bottom of the core. [2]
What is a PWR control rod made of?
At the Krško PWR, NEK lists an absorber alloy of 80% silver, 15% indium and 5% cadmium, with 20 absorber rods per assembly, 33 assemblies and 52.2 kg per assembly. [4]
Sources and what still needs a direct check
Sources checked 2026-10-12. DOE's Office of Nuclear Energy and the eCFR text of 10 CFR 50 Appendix A are US government sources; the World Nuclear Association is the industry's international organisation.
