Control Rods, Chemical Shim, and Reactivity Control Systems

Control Rods, Chemical Shim, and Reactivity Control Systems

Reactivity control is at the heart of safe nuclear plant operation. Reactivity (symbol ρ) is the measure of how far a reactor is from a critical, self-sustaining chain reaction. When reactivity is positive the chain reaction is accelerating; when negative it is slowing down; at zero the reactor is exactly critical and power is steady. Two primary tools keep reactivity under precise control: control rods and, in pressurized water reactors (PWRs), chemical shim (boric acid dissolved in the coolant).

Control Rods

Control rods are assemblies of neutron-absorbing material — commonly hafnium, silver-indium-cadmium alloy, or boron carbide — that can be inserted into or withdrawn from the reactor core. When inserted, they absorb neutrons that would otherwise sustain fission, reducing reactivity. When withdrawn, more neutrons are available for fission and reactivity rises.

The reactivity worth of a control rod describes how much reactivity it adds or removes. Total worth is the total reactivity change from fully withdrawn to fully inserted. Differential worth is the reactivity change per unit of rod travel (e.g., per inch or per step). Because neutron flux is not uniform across the core — it peaks at the center and falls off toward the edges — a rod absorbs more neutrons when it is near the core midplane than when it is near the top or bottom. This produces the characteristic S-shaped (sigmoidal) differential worth curve: differential worth is low when the rod tip is at the top or bottom of the core and reaches its maximum when the rod tip is at the core midplane.

This shape has a direct safety implication. If an operator withdraws a rod rapidly while its tip is at the midplane, the reactivity insertion rate is at its highest. Plant procedures and automatic rod withdrawal limiters are designed to prevent uncontrolled positive reactivity additions — a condition that, if unchecked, can lead to a reactivity accident.

Integral worth is the cumulative reactivity added from full insertion to any given position. It is the area under the differential worth curve up to that point.

Chemical Shim — PWR Only

In a PWR, boric acid (H₃BO₃) is dissolved in the reactor coolant (the primary loop water). Boron-10 is a strong neutron absorber. Increasing boron concentration (measured in parts per million, ppm) adds negative reactivity; decreasing it (dilution with unborated water) adds positive reactivity. This is called chemical shim because it trims reactivity over the long term rather than providing rapid control.

Chemical shim is used to compensate for slow reactivity changes such as:

• Fuel burnup — as fuel is consumed over months, negative reactivity builds up; boron is diluted gradually to keep the reactor critical.
• Xenon transients — xenon-135, a fission product, is a powerful neutron absorber whose concentration changes with power history.
• Temperature and power-level changes during planned maneuvers.

Because adjusting boron concentration is a deliberate, relatively slow process involving pumps and mixing time, it is not used for rapid power maneuvers or emergency shutdown. Control rods fulfill those roles.

Shutdown Margin

Shutdown margin (SDM) is the amount of negative reactivity that would exist in the reactor — with all control rods fully inserted and credited absorbers in place — beyond what is needed to reach and stay subcritical. Regulations require a minimum SDM at all times. In a PWR, the boron concentration must be high enough to ensure SDM is met even if the most reactive rod is assumed to be stuck out (the "stuck rod" criterion). Verifying adequate SDM before any mode change is a fundamental operator responsibility and a key exam topic.

BWR Reactivity Control

Boiling water reactors (BWRs) do not use soluble boron for normal power control. Instead, BWRs rely on control rods (entering from the bottom of the core) and on coolant void fraction — the amount of steam bubbles in the core. More steam means fewer moderating water molecules, which reduces reactivity (a strong negative feedback). BWR operators adjust power by moving control rods and by changing recirculation flow rate, which alters the void fraction. Emergency boration systems exist in BWRs but are not part of normal operation.

Safety Considerations

All work near reactor controls falls under strict procedural and regulatory controls:

• Reactivity manipulations must follow approved procedures. Any deviation is a potential reportable event under NRC regulations.
• Boron dilution is one of the most safety-significant evolutions in a PWR. Uncontrolled dilution can raise reactivity unexpectedly. Verification steps and independent checks are mandatory.
• Simulator training allows operators to experience transients safely. The IAEA PCTRAN simulator and similar tools replicate real plant responses without risk.
• OSHA general industry standards apply in plant areas outside the reactor building; inside radiologically controlled areas, radiation worker training and dosimetry requirements add another layer of protection. Always follow the site radiation protection program and posted RCA (Radiologically Controlled Area) requirements before entering.
• Lockout/tagout (LOTO) procedures apply to any maintenance on control rod drive mechanisms or chemical injection pumps — energy isolation must be verified before any hands-on work.

Key Takeaway

Control rods provide rapid, precise reactivity control in both PWRs and BWRs. Chemical shim provides long-term reactivity management in PWRs by adjusting boron concentration. Together — and within the bounds of strict procedural controls — they keep the reactor operating safely, maintain adequate shutdown margin, and allow operators to respond to changing plant conditions.