Startup of “CANDLE” burnup in a Gas-cooled Fast Reactor using Monte Carlo method

During the past decade, the CANDLE burnup strategy has been proposed as an innovative fuel cycle and reactor design for complete utilization of uranium resources. In this strategy the shapes of neutron flux, nuclide densities and power density distribution remain constant but the burning region moves in axial direction. The feasibility of this strategy has been demonstrated widely by using the diffusion technique in conjunction with nuclide transmutation equations. On the other hand since the Monte Carlo method provides the exact solution to the neutron transport, the Monte Carlo technique is becoming more widely used in routine burnup calculations. The main objective of this work is startup of CANDLE burnup in a Gas cooled Fast Reactor using a Monte Carlo burnup scheme. In this case only natural or depleted uranium is required for fresh fuel region. However, the construction of the first CANDLE core is faced with a big problem. In equilibrium state the burning region contains a spectrum of fission products as well as higher actinides. These isotopes are not easily available for constructing the initial CANDLE core. The solution is startup of a special reactor using the enriched uranium in starter zone. At the end of core life the fuel for the next core is produced with the composition close to the equilibrium state. An originally MCNP–ORIGEN linkage program named MOBC has been used for criticality and isotopic evaluation of the core. The results of analysis showed that the use of burnable absorber rods for positive reactivity swing offset is necessary. In this regard the multiplication factor changes are limited in acceptable margin where the maximum reactivity swing is 655 pcm during a transient and 180 pcm during equilibrium state. On the other hand, although the power shape changes drastically in the earlier stage of core operation, it remains constant during the equilibrium state. In summary, by using the enriched uranium as a starter fuel the achievement of an equilibrium state is possible.

► In equilibrium state of a CANDLE core, the burning region contains fission products and actinides.
► These isotopes are not available. The solution is startup of a reactor using easily available materials.
► At the end of core life the fuel for the equilibrium core is produced.
► In this work the startup of a CANDLE-GFR has been evaluated using Monte Carlo technique.
► The results show that the equilibrium state could be achieved after some minor transients.