In 2011, an earthquake at a nuclear power plant in Fukushima, Japan, disrupted the electricity supply. The nuclear reactors shut down, and diesel-powered backup generators started automatically to power the pumps that circulated coolant to remove residual heat. Then a tsunami flooded the plant, and the diesel generators failed, heat built up and there were three nuclear meltdowns.

More than a decade later, intense levels of radiation in and around the Fukushima plant have forced workers to use robots and remote-controlled diggers, backhoes and other equipment to assess damage and remediate the site. The lubricants in these devices are exposed to excessive amounts of radiation.

Radiation is a form of energy. The energy in gasoline is contained in hydrocarbon molecules and released when the molecules burn in internal combustion engines. The energy in radiation is contained in electromagnetic waves (ultraviolet, X-ray, beta and gamma waves) and subatomic particles (electrons, protons, neutrons, alpha particles). Radiation from the Sun impacts solar panels to generate electricity, drive photosynthesis in plants and darken human skin. The dose of radiation—the amount or concentration of radiation that is absorbed—influences whether solar panels produce a trickle or a surge of electricity, whether plants thrive or wither and whether skin tans, burns or becomes cancerous. 

The response of a lubricant to radiation depends on its composition, the type and intensity of radiation, the length of time of exposure and the dose.

Such radiation-resistant products as hydraulic fluids, gearbox lubricants and greases were developed to meet engineering standards for use under normal operating conditions at nuclear power plants and for aviation and aerospace, medical imaging, nuclear-powered submarines, food sterilization and research applications. 

Effects of Radiation

Heat adds thermal energy to all the molecules in a lubricant. Small amounts cause molecules to move and vibrate more vigorously and change their shape; viscosity decreases, and lubricating films become more prone to failure. Large amounts destabilize molecules and cause their bonds to break. For hydrocarbons, the consequences include oxidation, changes in color and odor, and formation of reactive ions and free radicals that polymerize, corrode metals and form sludge and varnish. 

Like heat, radiation adds energy to molecules. However, radiation is more molecule-specific than heat. When the frequency (or energy) of electromagnetic radiation is just right, it enhances absorption by specific bonds or atoms. When a bond absorbs more energy, it vibrates more vigorously and may break.

Some forms of radiation—including beta particles, gamma radiation and fast neutrons—tend to match the energies of electrons in organic molecules. That is, organic molecules absorb the energy from these types of radiation, which excites some of their electrons. If the excitation is large enough, then an excited electron escapes from its orbit around the nucleus of its atom. This produces a reactive free electron and a reactive molecule with a positive charge (a cation).

Subatomic particles such as neutrons are absorbed by the nuclei of atoms. This alters a nucleus and its stability. The atom may then emit secondary radiation that affects other molecules. 

When lubricants are exposed to radiation, the molecules of base oil and additives can absorb energy. In hydrocarbon base oils, if hydrogen-carbon bonds break, then the viscosity can decrease. If reactive fragments combine to form larger molecules (polymerize), then the viscosity increases. Reactive fragments of molecules also cause corrosion, sludge and varnish.

Small amounts of radiation cause soap-thickened greases to soften and lose consistency as thickener particles disintegrate. More radiation causes the base oil to polymerize, which raises the consistency—and may even solidify—the grease.

Until now, radiation-resistant lubricants have been formulated from base oils with bonds that are stronger than those used in conventional lubricants and antioxidants that are believed to be particularly effective against radiation. Lubricating greases have been formulated with thickeners that appear to have good resistance to radiation. But the results of recent research suggest that this approach may overlook other factors that contribute to radiation resistance.

Measuring Radiation Resistance

Radiation consists of electromagnetic waves (X-rays, gamma waves) and high-energy subatomic particles (neutrons, cosmic ray particles). Lubricants are exposed to radiation that is produced by X-ray machines and other medical scanning devices, equipment that uses ultraviolet light to sterilize food and medical items, processes to generate electricity in nuclear power plants and submarines, and equipment at high-energy particle accelerators at laboratories, like Argonne and Brookhaven National Laboratories in the United States and CERN (the European Organization for Nuclear Research) in Europe.  

Additionally, in the Sun and other stars and on Earth, unstable atoms spontaneously release or emit radiation, known as ionizing radiation. Some elements, like uranium, have isotopes in which atoms have the same numbers of protons and electrons but different numbers of neutrons in their nucleus. Unstable atoms have more energy and/or mass than their stable counterparts. Atoms of unstable isotopes decay to more stable isotopes by releasing subatomic particles and emitting radiation.

For example, uranium (atomic number 92) has three naturally occurring isotopes: uranium-238, uranium-235 and uranium-234 with atomic weights of 238, 235 and 234, and neutron counts of 146, 143 and 142, respectively. Uranium-235 is the least stable of the three isotopes and can undergo fission. 

In soil on Earth, uranium-238 decays to other isotopes of uranium, then to isotopes of radium and eventually to lead. Radium decays to form radon, a colorless, odorless gas. Radon has 39 isotopes, all of which are radioactive and pose health hazards.

Radioactivity refers to the amount of ionizing radiation released by the radioactive decay of a material, according to the U.S. Environmental Protection Agency. Geiger counters are used to measure radioactivity (as the number of atoms that decay per unit time) in units of bequerel (Be) or curie (Ci). 

The amount of radiation absorbed by a material is measured in units of gray (Gy) or rad (1/100 of 1 Gy) by absorption spectroscopy. One Gy corresponds to the absorption of 1 joule (energy) per kilogram of matter, which corresponds approximately to the amount of energy required to lift a vegetable or piece of fruit a distance of one meter (a little more than a yard).

For lubricants, radiation resistance is measured by exposing samples to controlled amounts of radiation and comparing their responses (e.g., change in color, mass, viscosity or consistency).

Radiation-Resistant Formulations

It is widely believed that the radiation resistance of the base oil is the primary factor that determines the radiation resistance of a lubricant.

Base oils that consist of linear hydrocarbon molecules are particularly vulnerable to damage by radiation because their hydrogen-carbon bonds are relatively weak. In contrast, stable aromatic rings are more resistant to radiation. Benzene is more stable than cyclohexane, and hydrocarbons with phenol groups are more stable than aliphatic hydrocarbons because of the way the aromatic rings absorb energy from radiation, store and release it. 

Commercially available radiation-resistant lubricants and greases are formulated from base oils with aromatic (phenol-type) rings, like alkylated naphthenics, which are Group V base oils. Other radiation-resistant lubricants are formulated with polyphenyl ethers, which are long-chain polymers of aromatic ethers.

Other radiation-resistant Group V base oils are perfluoropolyethers, which are long chain (polymeric) ethers of fluorocarbon units. The carbon-fluorine bond is much stronger than the carbon-hydrogen bond. Thus, carbon-fluorine bonds can absorb more energy (from radiation or heat) without breaking than carbon-hydrogen bonds.

Silica base oils and synthetic hydrocarbons are used in some radiation-resistant lubricants.

Radiation-resistant lubricants are formulated with phenolic antioxidants, anti-foam additives, metal deactivators and corrosion inhibitors.

For radiation-resistant greases, thickeners include polytetrafluoroethylene, zinc oxide and silica for more demanding applications and soaps (sodium, lithium) for less demanding situations.

Comparison of Greases

In 2019, Matteo Ferrari with CERN conducted a comparison study of nine commercially available greases, most of which were marketed as radiation-resistant products. Samples were radiated with a mixture of gamma rays and neutrons in the TRIGA (Training, Research, Isotopes, General Atomic) Mark II reactor at the University of Pavia in Pavia, Italy. The study was designed to model the effects of secondary radiation on lubricants during future use at new particle accelerators.

Radiation resistance was evaluated in terms of the relative consistency (i.e., Crel(D) = C(D)/C0 where C0 is the consistency of grease as supplied and C(D) is the consistency after dose D of radiation, both measured by cone penetration (mm/10) according to ASTM D217 and D1403). 

Ferrari reported that seven greases softened and became fluid after receiving relatively small radiation doses—four for doses less than 1 MGy and three for doses between 1.5 and 5 MGy. These seven greases were prepared from various thickeners and base stocks, including fluorinated chemistries. Spectroscopy data showed that cleavage of the bonds in the thickeners was the primary damage mechanism.

The best radiation resistance was observed for Kluber Lubrication’s Petamo GHY 133N (formulated with a polyurea thickener in mineral oil) and MORESCO Corp.’s RD-42R-1 (formulated with a polycarbonate thickener in PPE), which underwent less than 5% change in consistency for doses as large as 8.9 and 7.5 MGy, respectively. The compositions of these two greases were completely different.

Ferrari concluded that radiation resistance of lubricants cannot be predicted from base oil and thickener chemistry and that radiation damage depends on the form of radiation, dose rate, oxygen diffusion in the lubricant and temperature. 

These results demonstrate the importance of measuring the radiation resistance of lubricants under controlled conditions that model new and future applications.  

Mary Moon, Ph.D.,  has experience formulating, testing and manufacturing lubricating oils and greases and polymers. She has served as Chair of the Philadelphia Section of STLE and Technical Editor of The NLGI Spokesman and received the Clarence E. Earle Memorial Award (2018) and the Golden Grease Gun Award (2022) from NLGI. She is currently working as a professional writer and editor. Contact her at mmmoon@ix.netcom.com