The effects of annealing frequency on the properties of irradiated graphite have been investigated.
Samples of virgin AGOT graphite were irradiated in the Brookhaven Graphite Research Reactor at 30-50Β°C for about 200 MWd (N 1.5 x lo'* nvt for neutrons above 0.6 MeV). At regular exposure intervals of 15, 30, 50 or 100 MWd, samples were removed from the reactor and were heated to 350Β°C. Changes in sample dimensions, c-axis values, Hall coefficient and electrical resistivity were measured. It was established from these measurements that the radiation damage decreased with increasing annealing frequency. In the case of the samples annealed every 15 or 30 MWd, dimensional stability was achieved after a number of cycles. Additional irradiation and annealing cycles produced no further growth. Electrical resistivity and Hall coefficient measurements were made on the diiensionally stabilized samples. Electrical damage occuring during each irradiation period was almost completely recovered during the subsequent anneal. The annealing of the dimensional and electrical damage was followed in detail up to 350Β°C. For the samples annealed every 15 MWd, dimensional recovery occurred between 100 and 110Β°C. No further dimensional changes occurred up to 350Β°C. For the samples annealed every 30 MWd, dimensional recovery occurred over a wider temperature range. Changes in electrical properties occurred continuously from about 100 to 350Β°C for both sets of samples. Some "reverse annealing" phenomena were observed in the electrical resistivity and dimensional measurements.
The results from these groups of samples are compared to information obtained on highly damaged graphite samples which were also annealed at regular intervals. Possible mechanisms of damage and recovery are considered in terms of the atomic and electronic defects produced. The effects of interstitial complexes, lattice vacancies and atomic and electronic traps are discussed.