An irradiation creep law evaluated for nuclear graphite a high dose and temperature

The properties of nuclear graphite change when exposed to irradiation and high temperature in a reactor. Fortunately for reactor designers, when irradiation creep occurs it relaxes the stresses that build up within the graphite components. This report evaluates a relationship between irradiation dose and creep strain. Extensive irradiation creep data exist for low-dose irradiation. However, there are very few data at high dose, and the accompanying irradiation data required for dimensional change, coefficient of thermal expansion and Young's modulus, which are required to fully assess the form of the creep law, are often incomplete. For this reason there is some uncertainty in the form of the creep law to account for changes in the graphite structure at high dose and for high radiolytic weight loss. United Kingdom Atomic Energy Authority (UKAEA) experiments at 850°C and 1050°C have been described in a previous study. Data are taken from this study, up to a dose of 20 × 20²⁰ EDND (equivalent dido nickel dose), and creep strains are deduced. From the values of creep strain derived in this work, coefficients are determined for a creep law that relates irradiation-induced creep to dose, which is purported to apply to all nuclear graphites. The variation in Young's modulus with irradiation dose enables a structure term to be evaluated which accounts for the change in graphite microstructure with irradiation. This is performed for high-dose data obtained describing the effects of irradiation on graphite at 900°C. Using the structure term and creep law coefficients previously deduced, a comparison is made with known data. It was shown that the creep law follows the trend of high-dose data, although underestimating the creep values by 20-40%.