Origin and validity of graphite dosimetry units and related conversion factors

There is a significant amount of historic graphite data used in the safety cases for operating reactors and in research aimed at informing the designs of Generation IV High Temperature Gas-cooled Reactors (HTRs) and Molten Salt Reactors (MSR). Over time graphite irradiation ageing has been expressed by a number of units, including Calder Hall Equivalent Dose (CED MWd ATE−1), Equivalent DIDO Nickel Dose (EDND n cm−2), En > 0.18 MeV (n cm−2) and the now preferred, displacements per atom (dpa), although the former units are still often used. Fortunately, each of these units has a standard value and there are conversion factors between units. This paper examines the origin of each of these units and conversion factors using current nuclear data and codes, to test their validity and margin of error. This work gives the reader an understanding of the derivation and origin of the various graphite dose units and quantifies the values and tolerance of conversion factor quoted in the literature. The background to these conversion factors and the methods used to derive the units are reassessed using the latest nuclear data and modern codes. Most of the calculated and standard values were found to be in reasonable agreement supporting their continued use. However, in the case of the unit based on the energy range of En > 1.0 MeV, significant differences are seen between the calculated and accepted conversion factors as well as between reactor spectrums. This latter finding was not unexpected as a significant amount of carbon atoms are displaced at lower energies above ∼0.1 MeV.