Control Rods

Control rods are rods, plates, or tubes containing a neutron absorbing material (material with high absorption cross-section for thermal neutron) such as boron, hafnium, cadmium, etc., used to control the power of a nuclear reactor. A control rod is removed from or inserted into the reactor core to increase or decrease the reactor’s reactivity (increase or decrease the neutron flux). This, in turn, affects the reactor’s thermal power, the amount of steam produced, and hence the electricity generated.

By absorbing neutrons, a control rod prevents the neutrons from causing further fissions. Control rods are an important safety system for nuclear reactors. Their prompt action and prompt response to the reactor are indispensable. Control rods are used for maintaining the desired state of fission reactions within a nuclear reactor (i.e., subcritical state, critical state, power changes). They constitute a key component of an emergency shutdown system (SCRAM).

Control rods cluster assembly.
Control rod assembly for VVER reactor. Absorber – boron carbide
Source: www.gidropress.podolsk.ru/files/proceedings
/mntk2011/documents/mntk2011-108.pdf

Control rods usually constitute cluster control rod assemblies (PWR) inserted into guide thimbles within a nuclear fuel assembly. The cladding protects the absorbing material (e.g., pellets of Boron Carbide), usually made of stainless steel. They are grouped into groups (banks), and the movement usually occurs by the groups (banks). The typical total number of clusters is 70. This number is limited, especially by the number of penetrations of the reactor pressure vessel head.

A control rod is removed from or inserted into the reactor core to increase or decrease the reactor’s reactivity (increase or decrease the neutron flux). By the changes of the reactivity, the changes of neutron power are performed. This, in turn, affects the reactor’s thermal power, the amount of steam produced, and hence the electricity generated.

In PWRs, they are inserted from above, with the control rod drive mechanisms being mounted on the reactor pressure vessel head. Due to the necessity of a steam dryer above the core of a boiling water reactor, this design requires the insertion of the control rods from underneath the core.

Operability of control and shutdown rods

In PWRs, the shutdown and control rods’ operability (i.e., trip ability) is an initial assumption in all safety analyses that assume rod insertion upon reactor trip.

Operability of control rods

  1. ARI-1 condition. In all the safety analyses, it is usually assumed that control rods are fully inserted except for the single control rod of highest reactivity worth, which is assumed to be fully withdrawn (i.e., stuck rod).
  2. Inoperability of the control rod. When one or more rods are inoperable (i.e., untrippable) during MODE 1 (Power Operation), there is a possibility that the required SDM may be adversely affected. Under these conditions, it is important to determine and ensure the SDM. The plant must be brought to a MODE or condition in which the LCO requirements are not applicable if the inoperable rod(s) cannot be restored to operable status. The unit must be brought to at least MODE 3 (Hot Standby) within (for example, 6 hours) to achieve this status.

Control Rods usage

  • Reactor startup.
  • Control of the reactor and power maneuvering.
  • Axial offset control.
  • Reactor shutdown.
  • Emergency shutdown – SCRAM.

Grey control rods

Some nuclear power plants use load following. These plants have the capability to make power maneuvering between 30% and 100% of rated power, with a slope up to 5% of rated power per minute. They can respond very quickly to the grid requirements. Special control rods have to be used to fulfill these requirements without introducing a large perturbation of the power distribution. These control rods are called “grey” control rods. Grey control rods use a grey neutron absorber, which absorbs fewer neutrons than a “black” absorber.  Consequently, they cause smaller depressions in the neutron flux and power in the vicinity of the rod.

Boron as the neutron absorber
Natural boron consists primarily of two stable isotopes, 11B (80.1%) and  10B (19.9%). In nuclear industry boron is commonly used as a neutron absorber due to the high neutron cross-section of isotope  10B. Its (n,alpha) reaction cross-section for thermal neutrons is about 3840 barns (for 0.025 eV neutron). Isotope  11B has absorption cross-section for thermal neutrons about 0.005 barns (for 0.025 eV neutron). Most of (n,alpha) reactions of thermal neutrons are 10B(n,alpha)7Li reactions accompanied by 0.48 MeV gamma emission.

(n,alpha) reactions of 10B

Boron 10. Comparison of total cross-section and cross-section for (n,alpha) reactions. Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data Library
Boron 10. Comparison of total cross-section and cross-section for (n,alpha) reactions.
Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data Library

Moreover, isotope 10B has a high (n, alpha) reaction cross-section along the entire neutron energy spectrum. The cross-sections of most other elements become very small at high energies, as in the case of cadmium. The cross-section of 10B decreases monotonically with energy. For fast neutrons, its cross-section is on the order of barns.

Boron, as the neutron absorber, has another positive property. The reaction products (after a neutron absorption), helium and lithium, are stable isotopes. Therefore there are minimal problems with decay heating of control rods or burnable absorbers used in the reactor core.

On the other hand production of helium may lead to a significant increase in pressure (under rod cladding) when used as the absorbing material in control rods. Moreover, 10B is the principal source of radioactive tritium in the primary circuit of all PWRs (which use boric acid as a chemical shim) because reactions with neutrons can rarely lead to the formation of radioactive tritium via:

 10B(n,2x alpha)3H                             threshold reaction (~1.2 MeV)

and

10B(n,alpha)7Li(n,n+alpha)3H     threshold reaction (~3 MeV).

Cadmium as the neutron absorber
Cadmium cut-off energy
Neutrons of kinetic energy below the cadmium cut-off energy (~0.5 eV) are strongly absorbed by 113-Cd.
Source: JANIS (Java-based nuclear information software) www.oecd-nea.org/janis/

Natural cadmium consists of eight isotopes, 106Cd (1.3%),  108Cd (0.9%), 110Cd (12.5%), 111Cd (12.8%), 112Cd (24.3%), 113Cd (12.2%), 114Cd (28.7%) and 116Cd (7.5%). Two of them are radioactive isotopes with very long half-life (113Cd – 7.7 x 1015 y and 116Cd – 2.9 x 1019 y).

Cadmium is commonly used as a thermal neutron absorber in the nuclear industry due to the very high neutron absorption cross-section of 113Cd. 113Cd has a specific absorption cross-section. There is a cadmium cut-off energy (Cadmium edge) in the absorption cross-section. Only neutrons of kinetic energy below the cadmium cut-off energy (~0.5 eV) are strongly absorbed by 113Cd. Therefore cadmium is widely used to absorb thermal neutrons in thermal neutron filters.

n+_{48}^{113}\textrm{Cd} {\rightarrow} _{48}^{114}\textrm{Cd^{\star}} \rightarrow _{48}^{114}\textrm{Cd} +\gamma

How to control the reactor power?

Reactor criticality
Reactor criticality. A – a supercritical state; B – a critical state; C – a subcritical state

The reactor’s thermal power is determined by the number of fission reactions per time unit and by remaining decay heat (~tens of MW). During the normal operation of the reactor, the thermal power from fission dominates. The neutron flux determines the number of fission reactions in the reactor. The position of control rods directly affects the criticality of the reactor. When the reactor is critical (control rods in a critical position), the reactor’s power and the neutron flux are stable at a given power level. When the reactor is subcritical (control rods below a critical position), the reactor’s power and the neutron flux exponentially decrease. When the reactor is supercritical (control rods above a critical position), the reactor’s power and the neutron flux exponentially increase. It should be noted this behavior describes “zero power criticality” (i.e., a reactor without reactivity feedbacks, 10E-8% – 1% of rated power).

Criticality of a Power Reactor

For power reactors, the reactor can behave differently at power conditions due to the presence of reactivity feedbacks. Power reactors are initially started from hot standby mode (a subcritical state at 0% of rated power) to power operation mode (100% of rated power) by withdrawing control rods and boron dilution from the primary source coolant. During the reactor startup and up to about 1% of rated power, the reactor kinetics is exponential as in a zero-power reactor. This is due to the fact all temperature reactivity effects are minimal.

On the other hand, temperature reactivity plays a very important role during further power increase from about 1% up to 100% of rated power. As the neutron population increases, the fuel and the moderator increase their temperature, which results in a decrease in reactivity of the reactor (almost all reactors are designed to have the temperature coefficients negative).

The negative reactivity coefficient acts against the initial positive reactivity insertion, and this positive reactivity is offset by negative reactivity from temperature feedbacks. Positive reactivity must be continuously inserted (via control rods or chemical shim) to keep the power increasing. After each reactivity insertion, the reactor power stabilizes itself on the power level proportionately to the reactivity inserted.

See also: Reactor Criticality.

Power increase – from 75% up to 100%
Let assume that the reactor is critical at 75% of rated power and that the plant operator wants to increase power to 100% of rated power. The reactor operator must first bring the reactor supercritical by inserting a positive reactivity (e.g., by control rod withdrawal or boron dilution). As the thermal power increases, moderator temperature and fuel temperature increase, causing a negative reactivity effect (from the power coefficient), and the reactor returns to the critical condition.

Positive reactivity must be continuously inserted (via control rods or chemical shim) to keep the power to be increasing. After each reactivity insertion, the reactor power stabilizes itself proportionately to the reactivity inserted. The total amount of feedback reactivity that must be offset by control rod withdrawal or boron dilution during the power increase (from ~1% – 100%) is known as the power defect.

Let assume:

  • the power coefficient:                 Δρ/Δ% = -20pcm/% of rated power
  • differential worth of control rods:    Δρ/Δstep = 10pcm/step
  • worth of boric acid:                                      -11pcm/ppm
  • desired trend of power decrease:              1% per minute

75% → ↑ 20 steps or ↓ 18 ppm of boric acid within 10 minutes → 85% → next ↑ 20 steps or ↓ 18 ppm within 10 minutes → 95% → final ↑ 10 steps or ↓ 9 ppm within 5 minutes → 100%

reactor power - 75 to 100 of rated power

Accident-tolerant control rods – ATCR

Control rods are an important safety system of nuclear reactors. Their prompt action and prompt response to the reactor are indispensable. Control rods are used for maintaining the desired state of fission reactions within a nuclear reactor (i.e., subcritical state, critical state, power changes). They constitute a key component of an emergency shutdown system (SCRAM).

Control rods are rods, plates, or tubes containing a neutron absorbing material (material with high absorption cross-section for thermal neutron) such as boron, hafnium, cadmium, etc., used to control the power of a nuclear reactor. By absorbing neutrons, a control rod prevents the neutrons from causing further fissions.

Control rods usually constitute cluster control rod assemblies (PWR) inserted into guide thimbles within a nuclear fuel assembly. The cladding protects the absorbing material (e.g., Boron Carbide or Ag-In-Cd alloy), usually made of stainless steel.

Nevertheless, the melting point of Ag-In-Cd alloy (~790 ̊C), the eutectic temperature of boron carbide (B4C) and Fe (~1150 ̊C), and the eutectic temperature of Fe and Zr (~950 ̊C) are lower than the temperature (≳1 200) at which Zr-alloy fuel cladding begins to be intensively oxidized under severe accident conditions. Accordingly, the control rods may melt and collapse before the reactor core is significantly damaged in the case of severe accidents.

The following inherent characteristics are required in accident tolerant control rods:

  • The reactivity worth of ATCR should be comparable to or exceed that of conventional CR.
  • The neutron-absorbing materials used in ATCR should have a sufficiently high melting point and high eutectic temperature with cladding to prevent CR breakage from extensive fuel rod failure in a severe accident, thus avoiding uncontrollable recriticality even if coolant without boron is injected for emergency cooling of the core.

The main idea is to replace the conventional neutron-absorbing materials with proper ceramic materials that satisfy the above requirements. The candidate of a new absorber material for ATC includes gadolinium (Gd2O3), samarium (Sm2O3), europium (Eu2O3), dysprosium (Dy2O3), hafnia (HfO2). The melting point of these materials and the liquefaction temperature with Fe are higher than the rapid zirconium alloy oxidation temperature.

See previous:

Nuclear Fuel

See above:

Nuclear Power Plant