Pressurized water reactors turned out to be extremely stable power producers. Because of the fact that water is used to moderate the energy level of neutrons, making them more effective in causing fission, the concentration of water in the core is an important part of determining the reactivity of the core.
An increase in core temperature leads to a decrease in coolant density. This tends to reduce the reactivity of the core, which lowers the core power level and tends to force temperature back to the temperature that existed before the transient started.
Under normal operating conditions, this tendency makes it easy to control the reactor. If more power is needed, the steam throttle valve can be opened to allow more steam into the turbine. This will cause the primary coolant to cool down, eventually causing the reactor power level to increase to maintain a stable temperature.
If the power level of a pressurized water core rapidly increases, the water temperature will increase tending to slow down the rate of power level increase. A loss of water from a core can be dangerous because of the loss in heat removal capacity. However, if water pressure drops below the saturation pressure because of a large leak, the water will boil. Since steam is a very poor neutron moderator, the core will tend to shut itself down.
These characteristics tend to make PWRs respond in a safe direction under accident conditions.
Temperature control of reactivity is not sufficient for complete reactor control. Shutdowns are often accompanied by cooling down the reactor to allow maintenance. Also, there must be a method of adjusting for the effects of fuel depletion and fission product poisons. Because of this, neutron absorbing control rods are used.
Several different materials have a high enough affinity for neutron absorption that they can be used for reactor control. Like cladding, however, control rods in a water cooled reactor must be corrosion resistent, dimensionally stable, and strong.
One potential control material for the submarine program was a silver cadmium alloy with a high temperature steel cladding. The other alternative was hafnium, a material with a high neutron affinity normally found in nature mixed with zirconium.
Rickover never liked the idea of having a low temperature material like cadmium in his reactors, even if it was coated with a material with higher temperature capability. It also seems that hafnium might have appealed to Rickover’s poetic nature.
Since Rickover had already decided to use a zirconium alloy as the fuel cladding, a large amount of hafnium would be produced as a waste product of fuel rod production. Because hafnium is naturally quite compatible with zirconium it must have seemed like a God inspired choice to put the newly created surplus material to good use in control rods.
Like zirconium, hafnium has a high resistance to corrosion in a hot water system. It needs no cladding. This particular material choice has not been as popular with the commercial reactor program as most of Rickover’s other technical decisions, largely due to the fact that hafnium is still quite expensive compared to other alternatives.
For long lived Naval cores, however, hafnium continues to be the material of choice because of its long term stability and continued absorption capability.