Burnup characteristics analyses of graphite impurities in HTGR fuel element

In the fuel element of high-temperature gas cooled reactors (HTGRs), there exist various impurities in the graphite matrix, which are mainly originated from the raw materials, such as naturally occurring graphite, artificial graphite and resin. These impurities can reduce the excess reactivity of the reactor core, both in the initial criticality and during the overall operation history. In the practical physics design of HTGR, the equivalent boron content (EBC) method is usually used to describe the graphite impurities, where the impurity isotopes are simply converted to some amounts of boron by preserving the neutron absorption. Although it is found to be effective in criticality calculations, EBC may introduce errors in burnup analyses. This paper investigates the burnup characteristics of the graphite impurities. According to their behaviors during burnup, the impurities are classified into three categories, i.e. the high-burnable isotopes, low-burnable isotopes and unburnable isotopes. Moreover, a revised equivalent boron content (REBC) is proposed to describe the absorption rates of impurities, which can improve the computational accuracy in the burnup analyses of HTGR fuel.