Introduction of impurity atom due to the transmutation of the absorbing nucleus.

Atomic displacement due to recoil atoms or knock-ons

Thermal neutrons cannot produce displacements directly, but can indirectly as a result of radiative capture and other neutron reactions or elastic scattering.

RADIATION EFFECTS IN ORGANIC COMPOUNDS

As described previously, the effects of gamma and beta radiation on metal are not permanent. On the other hand, organic material will suffer permanent damage as its chemical bonds are broken by incident gamma and beta radiation. This chapter discusses how radiation effects organic compounds.

EO 1.23 STATE how gamma and beta radiation effect organic materials.

EO 1.24 IDENTIFY the change in organic compounds due to radiation.

a. Nylon

b. High-density polyethylene marlex 50

c. Rubber

EO 1.25 IDENTIFY the chemical bond with the least resistance to radiation.

EO 1.26 DEFINE the term polymerization.

Radiation Effects

Incident gamma and beta radiation causes very little damage in metals, but will break the chemical bonds and prevent bond recombination of organic compounds and cause permanent damage. Ionization is the major damage mechanism in organic compounds. Ionization effects are caused by the passage through a material of gamma rays or charged particles such as beta and alpha particles. Even fast neutrons, producing fast protons on collision, lead to ionization as a major damage mechanism. For thermal neutrons the major effect is through (n,gamma) reactions with hydrogen, with the 2.2 MeV gamma producing energetic electrons and ionization. Ionization is particularly important with materials that have either ionic or covalent bonding.

Ion production within a chemical compound is accomplished by the breaking of chemical bonds. This radiation-induced decomposition prevents the use of many compounds in a reactor environment. Materials such as insulators, dielectrics, plastics, lubricants, hydraulic fluids, and rubber are among those that are sensitive to ionization. Plastics with long-chain-type molecules having varying amounts of cross-linking may have sharp changes in properties due to irradiation. In general, plastics suffer varying degrees of loss in their properties after exposure to high radiation fields. Nylon begins to suffer degradation of its toughness at relatively low doses, but suffers little loss in strength.

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.

Figure 7 Effect of Gamma Radiation on Different Types of Hydrocarbon

The stability of organic (and other covalent) compounds to radiation is frequently expressed by means of the "G" value, which is equal to the number of molecules decomposed, or of product formed, per 100 eV of energy dissipated in the material. As an example of the use of G values, the data in Table 3 are for a number of polyphenyls exposed to the radiation in a thermal reactor.

The table shows the number of gas molecules produced, G(gas), and the number of polyphenyl molecules, G(polymer), used to produce higher polymers per 100 eV of energy deposited in the material. Note that this adds up to approximately 1000 atoms of gas and 10,000 atoms forming higher polymers per each 1 MeV particle. It is also of interest to note that the terphenyls are even more resistant to radiation than diphenyl and, since they have a higher boiling point, a mixture of terphenyls with a relatively low melting temperature was chosen as the moderatorcoolant in organic-moderated reactors.

* A mixture of the three terphenyls plus a small amount of diphenyl.

An effect similar to that described above occurs in water molecules that are decomposed by radiation into hydrogen and oxygen in a reactor. Control of oxygen produced by this process is an important part of reactor chemistry.

Summary

The important information in this chapter is summarized below.

Radiation Effects in Organic Compounds Summary

Gamma and beta radiation have little effect on metals, but break the chemical bonds and prevent bond recombination of organic compounds and cause permanent damage.

Radiation causes changes in organic materials.

Nylon has a degradation of its toughness at relatively low doses and little loss of strength.

High-density (linear) polyethylene marlex 50 loses both strength and ductility at relatively low doses.

Typically rubber increases in hardness when irradiated. Butyl or Thiokol rubber soften or become liquid with high radiation doses.

The chemical bond with the least amount of resistance to radiation is the covalent bond.

Polymerization is the recombining of broken hydrocarbon chains into longer ones.