RADIATION EFFECTS IN ORGANIC COMPOUNDS
High-density (linear) polyethylene marlex 50 loses both strength and ductility at relatively low
doses. In general, rubber will harden upon being irradiated. However, butyl or Thiokol rubber
will soften or become liquid with high radiation doses.
It is important that oils and greases be evaluated for their resistance to radiation if they are to be
employed in a high-radiation environment. Liquids that have the aromatic ring-type structure
show an inherent radiation resistance and are well suited to be used as lubricants or hydraulics.
For a given gamma flux, the degree of decomposition observed depends on the type of chemical
bonding present. The chemical bond with the least resistance to decomposition is the covalent
bond. In a covalent bond, the outer, or valence, electrons are shared by two atoms rather than
being firmly attached to any one atom. Organic compounds, and some inorganic compounds
such as water, exhibit this type of bonding. There is considerable variation in the strength of
covalent bonds present in compounds of different types and therefore a wide variation in their
stability under radiation. The plastics discussed above can show very sharp property changes
with radiation, whereas polyphenyls are reasonably stable.
One result of ionization is that smaller hydrocarbon chains will be formed (lighter hydrocarbons
and gases) as well as heavier hydrocarbons by recombination of broken chains into larger ones.
This recombination of broken hydrocarbon chains into longer ones is called polymerization.
Polymerization is one of the chemical reactions that takes place in organic compounds during
irradiation and is responsible for changes in the properties of this material. Some other chemical
reactions in organic compounds that can be caused by radiation are oxidation, halogenation, and
changes in isomerism. The polymerization mechanism is used in some industrial applications to
change the character of plastics after they are in place; for example, wood is impregnated with
a light plastic and then cross-bonded (polymerized) by irradiating it to make it more sturdy. This
change in properties, whether it be a lubricant, electrical insulation, or gaskets, is of concern
when choosing materials for use near nuclear reactors. One of the results of the Three Mile
Island accident is that utilities have been asked to evaluate whether instrumentation would
function in the event of radiation exposure being spread because of an accident.
Because neutrons and gamma rays (and other nuclear radiations) produce the same kind of
decomposition in organic compounds, it is common to express the effects as a function of the
energy absorbed. One way is to state the energy in terms of a unit called the rad. The rad
represents an energy absorption of 100 ergs per gram of material. As an example of the effects
of radiation, Figure 7 shows the increase in viscosity with radiation exposure (in rads) of three
organic compounds that might be considered for use as reactor moderators and coolants.
The ordinates represent the viscosity increase relative to that of the material before irradiation
(mostly at 100F), so that they give a general indication of the extent of decomposition due to
radiation exposure. This figure illustrates that aromatic hydrocarbons (n-butyl benzene) are more
resistant to radiation damage than are aliphatic compounds (hexadecane). The most resistant of
all are the polyphenyls, of which diphenyl is the simplest example.