|Abstract inglese (opzionale):
Chlorine is present in nuclear grade graphite as a result of the purification processes during which it remains trapped within the graphite structures. Prior to irradiation, chlorine is present at natural isotopic abundance levels (as Cl-35). During the irradiation, part of it forms Cl-36, in proportion to the cumulative neutron flux generated in the core of the reactor. The graphite microstructure is characterized by open porosity (OP - pore structures connected to the surface of the graphite) and closed porosity (CP - pore structures isolated from the graphite surface). CP can be transformed into OP by oxidation of graphite and chemical species contained in it can be released after a certain time. Cl-35 in the OP is released relatively quickly at operating temperature, therefore only a very small fraction of the chlorine located in the initially open porosity can be activated; on the contrary, the Cl-35 located in the initially CP, can potentially reside in the flux for a considerable period, and thus undergo greater activation. The length of residence in the flux, and hence the quantity of Cl-36 released from the graphite, will therefore depend critically upon the rate at which CP is converted in OP. Experimental data suggests that closed porosity is accessed faster in the earlier stages of radiolytic oxidation than in the later.