Radionuclides in Irradiated Graphite of Uranium–Graphite Reactors: Decontamination by Thermochemical Methods

Abstract

Graphite samples from blocks and sleeves are characterized by extremely heterogeneous spatial distribution of radionuclides on the scale of tens and hundreds of micrometers. The graphite decontamination occurs in three steps: Up to 25% of ¹⁴C is removed in the course of low-temperature annealing or treatment of graphite with liquid reagents (total weight loss of graphite ≤10%); up to 70–80% of ¹⁴C is removed in the course of 20-h heat treatment under oxidizing conditions; and in the final step the degree of removal of ¹⁴C exceeds 90%, with a proportional decrease in the graphite weight. Oxidation of graphite in the first steps of thermochemical treatment mainly occurs along (micro)cracks and other defects, which favors the removal of relatively weakly bound ¹⁴C, e.g., from the pore space. The removal of ¹⁴C from the crystal lattice of graphite requires its breakdown, e.g., by high-temperature heat treatment.