4.18.3.1 Graphite Irradiation Damage

Gross physical property changes can occur in the graphite PFMs through two generic routes: (1) near­surface damage caused by interaction with plasma ions and, to a lesser extent, electrons, and, (2) bulk dis­placements caused by neutrons emanating from the plasma or back scattered by the surrounding struc­ture. Of the tokamaks, only TFTR had significant D+T fusion reactions and, therefore, experienced a
significant flux of fusion neutrons (see eqn [1]). Even so, the dose from that TFTR was not high enough for the structural materials to experience appreciable neutron effects. However, machines such as the ITER will see a significant neutron dose from both D+D and D+T reactions.

As energetic particles travel through matter, they can interact with their surroundings, losing energy (per unit path length) in three ways: elastic collisions, electron excitations, and nuclear inter­actions. The interaction of primary interest from the materials property evolution point of view results from the particle elastic collisions with the graphite crystal. This has also been discussed in Chapter 4.10, Radiation Effects in Graphite (Section 4.10.4). If an ion or a neutron can provide sufficient energy to overcome an atom’s binding energy (Ed carbon«20-30 eV), the carbon can be displaced from its original lattice position. If the energy transferred to the displaced atom is sufficient to displace further atoms, a series of displacement events or a ‘cascade’ occurs. In the simplest interpre­tation, the Kinchin-Pease5 model is used to calculate the total number of atoms displaced. For example, if a carbon atom were ejected by the plasma and

reimpacted onto the carbon tile with a kinetic energy (Ecarbon) of 1 keV, the estimated number of atoms displaced (n) would be estimated as follows:

n = — ar—£^ = ^20 atoms [5]

The interaction of high-energy neutrons with matter is very similar to that of high-energy ions. The pri­mary difference between the two is the amount of energy transferred in a single collision and the dis­tance over which the interactions take place. An ion, which has a relatively large coulombic interaction radius, loses its energy over a short path length (typi­cally less than a micron). In contrast, the compara­tively small uncharged 14.1 MeV fusion neutrons undergo only simple elastic or ‘billiard ball’ collisions with a mean free path between collisions of ~10 cm. So, on average, a fusion neutron will have an elastic collision with a carbon atom once in 10 cm of graph­ite. The amount of energy transferred to the carbon in this first collision (—c) is calculated by simple elastic theory as: 4mcmn

(«c + mn)2

4 x 12 x 1 (12 + 1)2

where mc and mn are the carbon and neutron mass (in amu), respectively, —o is the neutron energy, and a is the angle between the neutron path before and after the collision. For a totally back scattered neu­tron (the maximum imparted energy), the energy transferred to the displaced carbon is ^4.7 MeV. Again, from eqn [5], the number of displaced carbon atoms in this 14.1 MeV neutron collision event is nearly 100 000. The vast majority of these atoms do not stay ‘displaced,’ but condense back into the gra­phitic structure within a few picoseconds. To assess the effects such collision events will have on a mate­rial, a convention has been adopted to compare irra­diation doses. The displacement per atom (dpa) gives the average number of times an atom has been knocked from its original lattice position. The dpa is an integrated average quantity, and takes into account the atomic density, the interaction cross-section, and the neutron energy spectrum. For the next-genera­tion fusion reactors such as ITER, peak end of life values for PFMs due to neutrons will be on the order of tenths of a dpa, while power fusion reactors could potentially be subjected to greater than 10 dpayear-1.

4.18.3.2 Surface Effects

While fast neutrons will produce relatively uniform atomic displacements, ions will produce very high near-surface damage. This damage can be on the level of hundreds of dpa, even for the experimental machines in use today and certainly for machines such as ITER and beyond. However, the damage is typically limited to much less than a micron in depth. The effect of this high damage level will be the reduction of a well-graphitized structure into a struc­ture that appears amorphous. However, these near­surface regions are subjected to erosion either by physical sputtering (caused by elastic collisions), or by chemical interactions. Both these effects are addressed in Section 4.18.4. A second surface radia­tion damage issue, that is, the ability of the thin damaged surface layer to retain and transport hydro­gen, is discussed in Section 4.18.5.