Boron is a naturally-occurring chemical element with atomic number 5 which means there are 5 protons and 5 electrons in the atomic structure. The chemical symbol for boron is B. Significant concentrations of boron occur on the Earth in compounds known as the borate minerals. There are over 100 different borate minerals, but the most common are: borax, kernite, ulexite etc.

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

Moreover, isotope 10B has high (n,alpha) reaction cross-section along the entire neutron energy spectrum. The cross-sections of most other elements becomes 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 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 primary circuit of all PWRs (which use boric acid as a chemical shim), because reactions with neutrons can rarely lead to 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).

See also: Tritium

Boron 10. Comparison of total cross-section and cross-section for (n,alpha) reactions. JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data LibraryBoron 10. Comparison of total cross-section and cross-section for (n,alpha) reactions. JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data Library
Boron letdown curve (chemical shim) and boron 10 depletion during a 12-month fuel cycle.Boron letdown curve (chemical shim) and boron 10 depletion during a 12-month fuel cycle.

At the beginning of specific fuel cycle concentration of boric acid is highest. At the end of this cycle concentration of boric acid is almost zero and a reactor must be refueled, because there is no positive reactivity that can be inserted to compensate negative reactivity of fuel burnup (increase in reactor slagging).

  • Chemical shim. By chemical shim, we mean that boric acid is dissolved in the coolant/moderator. Chemical shim is used to long term reactivity control.
  • Control rods. Many control rods use isotope 10B as a neutron absorbing material.
  • Safety systems. The one of three primary objectives of nuclear reactor safety systems is to shutdown the reactor and maintain it in a shutdown condition. Therefore all safety systems (which must ensure subcriticality of the reactor after the transient) use high concentrations of boric acid.
  • Burnable absorbers. Isotope 10B is widely used as the integral burnable absorber. When compared to gadolinium absorber (another commonly used burnable material), 10B has an order of magnitude smaller cross-sections. Therefore 10B compensate reactivity longer, but not so heavily.
  • Converter in neutron detectors. Neutrons are not directly ionizing and they have usually to be converted into charged particles before they can be detected. Most common isotope for the neutron converter material is 10B.
By chemical shim, we mean that boric acid is dissolved in the coolant/moderator. Boric acid (molecular formula: H3BO3), is a white powder that is soluble in water. In pressurized water reactors, chemical shim (boric acid) is used to compensate an excess of reactivity of reactor core along the fuel burnup (long term reactivity control). At the beginning of specific fuel cycle concentration of boric acid is highest (see picture). At the end of this cycle concentration of boric acid is almost zero and a reactor must be refueled.

In certain cases also fine power changes can be controlled by chemical shim. If it is desired to increase power, then the boric acid concentration must be diluted, removing 10B from the reactor core and decreasing its poisoning effect. When compared with burnable absorbers (long term reactivity control) or with control rods (rapid reactivity control) the boric acid avoids the unevenness of neutron-flux density in the reactor core, because it is dissolved homogeneously in the coolant in entire reactor core. On the other hand high concentrations of boric acid may lead to positive moderator temperature coefficient and that is undesirable. In this case more burnable absorbers must be used.

Moreover this method is slow in controlling reactivity. Normally, it takes several minutes to change the concentration (dilute or borate) of the boric acid in the primary loop. For rapid changes of reactivity control rods must be used.

Since the isotope 10B has a significantly higher neutron cross-section, the 10B depletes much more faster than 11B. Without the addition of fresh boron (19,9% of 10B) into the primary coolant system the enrichment of 10B in boric acid continuously decreases. In the result the enrichment of 10B at the end of the fuel cycle can be for example below 18% of 10B. This phenomenon must be considered in all the criticality calculations (e.g. shutdown margin calculations, estimated critical conditions or general core depletion calculations).
In pressurized water reactors, chemical shim (boric acid dissolved in primary coolant) is used to compensate an excess of reactivity of reactor core along the fuel burnup (long term reactivity control). The amount of reactivity compensated by chemical shim is determined by the concentration in the primary coolant.

The concentration of boron dissolved in the primary coolant is usually expressed by following units:

  • ppm (parts-per-million, 10−6)
  • g/kg of boron atoms; one gram of boron per kilogram of coolant
  • g/kg of H3BO3; one gram of boric acid per kilogram of coolant

The conversion factor between concentration of H3BO3 and other units is 5.72. This factor is determined by the ratio of atomic masses:


mH3BO3/mB = (3*1 + 10.81 + 3*16)/10.81 = 5.72

See above: