Radiolysis is the dissociation of molecules by ionizing radiation. It is the cleavage of one or several chemical bonds resulting from exposure to high-energy flux. The radiation in this context is associated with ionizing radiation; radiolysis is therefore distinguished from, for example, photolysis of the Cl2 molecule into two Cl-radicals, where light is used. For example, water dissociates under alpha radiation into a hydrogen radical and a hydroxyl radical, unlike ionization of water which produces a hydrogen ion and a hydroxide ion. The chemistry of concentrated solutions under ionizing radiation is extremely complex. Radiolysis can locally modify redox conditions, and therefore the speciation and the solubility of the compounds.
Water decomposition
Of all the radiation-chemical reactions that have been studied, the most important is the decomposition of water. When exposed to radiation, water undergoes a breakdown sequence into hydrogen peroxide, hydrogen radicals, and assorted oxygen compounds, such as ozone, which when converted back into oxygen releases great amounts of energy. Some of these are explosive. This decomposition is produced mainly by the alpha particles, which can be entirely absorbed by very thin layers of water. Summarizing, the radiolysis of water can be written as:
It is believed that the enhanced concentration of hydroxyl present in irradiated water in the inner coolant loops of a light-water reactor must be taken into account when designing nuclear power plants, to prevent coolant loss resulting from corrosion.
The current interest in nontraditional methods for the generation of hydrogen has prompted a revisit of radiolytic splitting of water, where the interaction of various types of ionizing radiation with water produces molecular hydrogen. This reevaluation was further prompted by the current availability of large amounts of radiation sources contained in the fuel discharged from nuclear reactors. This spent fuel is usually stored in water pools, awaiting permanent disposal or reprocessing. The yield of hydrogen resulting from the irradiation of water with β and γ radiation is low but this is largely due to the rapid reassociation of the species arising during the initial radiolysis. If impurities are present or if physical conditions are created that prevent the establishment of a chemical equilibrium, the net production of hydrogen can be greatly enhanced. Another approach uses radioactive waste as an energy source for regeneration of spent fuel by converting sodium borate into sodium borohydride. By applying the proper combination of controls, stable borohydride compounds may be produced and used as hydrogen fuel storage medium. A study conducted in 1976 found an order-of-magnitude estimate can be made of the average hydrogen production rate that could be obtained by utilizing the energy liberated via radioactive decay. Based on the primary molecular hydrogen yield of 0.45 molecules/100 eV, we obtain 10 tons per day. Hydrogen production rates in this range are not insignificant, but are small compared with the average daily usage of hydrogen in the U.S. of about 2 x 10^4 tons. Addition of a hydrogen-atom donor could increase this about a factor of six. It was shown that the addition of a hydrogen-atom donor such as formic acid enhances the G value for hydrogen to about 2.4 molecules per 100 eV absorbed. The same study concluded that designing such a facility would likely be too unsafe to be feasible.
Gas generation by radiolytic decomposition of hydrogen-containing materials, has been an area of concern for the transport and storage of radioactive materials and waste for a number of years. Potentially combustible and corrosive gases can be generated while at the same time, chemical reactions can remove hydrogen, and these reactions can be enhanced by the presence of radiation. The balance between these competing reactions is not well known at this time.
Radiation therapy
When radiation enters the body, it will interact with the atoms and molecules of the cells to produce free radicals and molecules that are able to diffuse far enough to reach the critical target in the cell, the DNA, and damage it indirectly through some chemical reaction. This is the main damage mechanism for photons as they are used for example in external beam radiation therapy. Typically, the radiolytic events that lead to the damage of the -cell DNA are subdivided into different stages that take place on different time scales:
The physical stage, consists in the energy deposition by the ionizing particle and the consequent ionization of water.
During the physico-chemical stage numerous processes occur,e.g. the ionized water molecules may split into a hydroxyl radical and a hydrogen molecule or free electrons may undergo solvation.
During the chemical stage, the first products of radiolysis react with each other and with their surrounding, thus producing several reactive oxygen species which are able to diffuse.
During the bio-chemical stage these ROS might break the chemical bonds of the DNA, thus triggering the response of enzymes, the immune-system, etc.
Finally, during the biological stage the chemical damage may translate into biological cell death or oncogenesis when the damaged cells attempt to divide.
Earth's history
A suggestion has been made that in the early stages of the Earth's development when its radioactivity was almost two orders of magnitude higher than at present, radiolysis could have been the principal source of atmospheric oxygen, which ensured the conditions for the origin and development of life. Molecular hydrogen and oxidants produced by the radiolysis of water may also provide a continuous source of energy to subsurface microbial communities. Such speculation is supported by a discovery in the Mponeng Gold Mine in South Africa, where the researchers found a community dominated by a new phylotype of Desulfotomaculum, feeding on primarily radiolytically produced H2.
Methods
Pulse radiolysis
Pulse radiolysis is a recent method of initiating fast reactions to study reactions occurring on a timescale faster than approximately one hundred microseconds, when simple mixing of reagents is too slow and other methods of initiating reactions have to be used. The technique involves exposing a sample of material to a beam of highly accelerated electrons, where the beam is generated by a linac. It has many applications. It was developed in the late 1950s and early 1960s by John Keene in Manchester and Jack W. Boag in London.
Flash photolysis
is an alternative to pulse radiolysis that uses high-power light pulses rather than beams of electrons to initiate chemical reactions. Typically ultraviolet light is used which requires less radiation shielding than required for the X-rays emitted in pulse radiolysis.
