Control Rods

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.

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

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.

How to control the reactor power?

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.

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.