In a nuclear reactor, high energy, fast neutron flux leads to the displacement of carbon atoms in the graphite crystallites via a ‘cascade.’ Many of these atoms will find vacant positions, while others will form small interstitial clusters that may diffuse to form larger clusters (loops in the case of graphite) depending upon the irradiation temperature. Con­versely, vacancy loops will be formed causing the lattice structure to collapse. These vacancy loops will only become mobile at relatively high tempera­tures. The production of transmutation gas from impurities is not an issue for highly pure nuclear graphite, as the quantities of gas involved will be negligible and the graphitic structure is porous.

The change in graphite properties is a function of the displacement of carbon atoms. The nature and amount of damage to graphite depends on the partic­ular reactor flux spectrum, which is dependent on the reactor design and position, as illustrated in Figure 6.

It is impractical to relate a spectrum of neutron energies to a dimensional or property change at a

single point in a material such as graphite. Therefore, an ‘integrated flux’ is used and is discussed later.

4.11.5.1 Early Activation Measurements on Foils

Although one cannot directly measure the damage to graphite itself, it is possible to measure the activation of another material, because of nuclear impacts adja­cent to the position of interest. This activation may then be related to changes in graphite properties.

This was done in early experiments using cobalt foils and by measuring the activation arising from the 59Co(n, g)60Co reaction. This reaction has a cross-section of 38 barns and 60Co has a half-life of 5.72 years, which need to be accounted for in the fluence calculations. Such foils were included in graphite experiments in BEPO and the Windscale Piles, and are still used today for irradiation rig validation and calibration purposes.

In these early experiments, after removal from the reactor, cobalt foils were dissolved in acid, diluted, and the decay rate measured. A measure of fluence could then be calculated from knowledge of the following:

• the solution concentration

• the time in the reactor

• the decay rate

• the activation cross-section

Source: Simmons, J. H. W. The Effects of Irradiation on Graphite; AERE R R 1954; Atomic Energy Research Establishment, 1956.

Unfortunately the 59Co(n, g)60Co reaction is mainly a measure of thermal flux and atomic displa­cements in graphite are due to fast neutrons. An improvement was the use of cobalt/cadmium foils, but this was not really satisfactory. Measurements made in this way are often given the unit, neutron velocity time (nvt).

Table 3 gives an example of thermal flux deter­mined from cobalt foils defined at a standard posi­tion in the center of a lattice cell in BEPO. Graphite damage at other positions in other reac­tors could then be related to the standard position in BEPO.