Developing a steady state neutronic model for the PBR-250 HTR using Serpent 2.1.31

Abstract

The PBR-250 reactor is a Generation-IV pebble bed high-temperature reactor with a rating of 250MWth and has the ability to produce electricity and process heat of up to 1000 °C. The most desirable feature of this reactor is the safety features associated with Gen IV-specific technology. The PBR-250 fuel is said to be unable to melt under design base accident scenarios. This characteristic, together with the large negative temperature coefficient, contribute significantly to the inherent safety characteristics of the reactor type (the other factors being its low power density, large heat capacity and capacity to transfer heat to its immediate surroundings without significant release of radioactivity). The main philosophy underpinning this dissertation is methodology development and the modelling of different reactor states of the PBR-250, as described in the International Atomic Energy Agency (IAEA) Coordinated Research Project (CRP), by using the Monte Carlo code Serpent 2.1.31. The development of this reactor technology brings us closer to realising the full benefits associated with Gen IV reactors while also increasing the knowledge base in research areas such as sensitivity and uncertainty analysis. Serpent input files of the PBR-250 with different pebble configurations were constructed and different reactor states were analysed. The fresh core (4% enrichment) was found to be supercritical for all pebble configurations i.e., simple cubic (SC), body-centred cubic (BCC), face-centred cubic (FCC), and thus methods to reduce the reactivity were assessed. In view of this, a reactor state in which fuel pebbles (4% enrichment) and dummy pebbles mixed at a ratio of 50:50 was developed, furthermore, the 50:50 setup was optimised to show the effect of incorporating 10B burnable poison into the dummy pebbles (burnable poison pebbles) on the multiplication factor. Finally, the incorporation of control rods would then the reactor to a critical or subcritical state. The temperature coefficients were calculated to assess the stability and temperature effect associated with the setups. It was determined that the reactor has a very strong Doppler coefficient that renders a negative temperature coefficient for the given reactor states. The reactivity of the PBR-250 is controlled by twelve control rods and a reactor shutdown system composed of twelve shutdown elements. Control rod worth and SCRAM reactivity were assessed for the BCC and FCC setup with the integral and differential rod worth of the BCC and FCC setup plotted for the modelled control rods. The neutron flux profiles for this specific reactor were assessed for different reactor states. Overall, the results give insight into some of the important aspects of this type of reactor while also developing other key research areas such as uncertainty and sensitivity analysis and model development.