Materials Used
Chemical elements with a sufficiently high capture cross section for neutrons include silver, indium and cadmium. Other elements that can be used include boron, cobalt, hafnium, dysprosium, gadolinium, samarium, erbium, and europium, or their alloys and compounds, e.g. high-boron steel, silver-indium-cadmium alloy, boron carbide, zirconium diboride, titanium diboride, hafnium diboride, gadolinium titanate, and dysprosium titanate. The choice of materials is influenced by the energy of neutrons in the reactor, their resistance to neutron-induced swelling, and the required mechanical and lifetime properties. The rods may have the form of stainless steel tubes filled with neutron absorbing pellets or powder. The swelling of the material in the neutron flux can cause deformation of the rod, leading to its premature replacement. The burn up of the absorbing isotopes is another limiting lifetime factor.
Silver-indium-cadmium alloys, generally 80% Ag, 15% In, and 5% Cd, are a common control rod material for pressurized water reactors. The somewhat different energy absorption regions of the materials make the alloy an excellent neutron absorber. It has good mechanical strength and can be easily fabricated. It has to be encased in stainless steel to prevent corrosion in hot water.
Boron is another common neutron absorber. Due to different cross sections of 10B and 11B, boron containing materials enriched in 10B by isotopic separation are frequently used. The wide absorption spectrum of boron makes it suitable also as a neutron shield. Mechanical properties of boron in its elementary form are unfavourable, therefore alloys or compounds have to be used instead. Common choices are high-boron steel and boron carbide. Boron carbide is used as a control rod material in both pressurized water reactors and boiling water reactors.
Hafnium has excellent properties for reactors using water for both moderation and cooling. It has good mechanical strength, can be easily fabricated, and is resistant to corrosion in hot water. Hafnium can be alloyed with small amounts of other elements; e.g. tin and oxygen to increase tensile and creep strength, iron, chromium and niobium for corrosion resistance, and molybdenum for wear resistance, hardness, and machineability. Some such alloys are designated as Hafaloy, Hafaloy-M, Hafaloy-N, and Hafaloy-NM. Its high cost and low availability limit its use in civilian reactors, though it is used in some US Navy reactors.
Dysprosium titanate is a new material currently undergoing evaluation for pressurized water control rods. Dysprosium titanate is a promising replacement for Ag-In-Cd alloys because it has a much higher melting point, does not tend to react with cladding materials, is easy to produce, does not produce radioactive waste, does not swell, and does not outgas. It was developed in Russia, and is recommended by some for VVER and RBMK reactors.
Hafnium diboride is another such new material. It can be used standalone or prepared in a sintered mixture of hafnium and boron carbide powders.
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