Abstract
HTR-10 is an experimental type generation IV reactor built in China, which belongs to the pebble bed design and contain spherical shape fuel elements. In particular, HTR-10 possesses the highly desirable feature of online-refueling in fuel management, which greatly reduces outage times, leads to a more efficient use of natural uranium, and results in higher burnup in spent fuels. However, such an online-refueling presents a great challenge for computer simulation and bunrup characteristics analysis. In this study, an appropriate burnup model was thus proposed to solve the problem. All the associated computations were performed by MCNP5/X Monte Carlo computer code together, with the ENDF/B-VII cross-section library. First, the model proposed in the literature was revised in this study, and the associated neutronics characteristics such as the variation of effective multiplication factor and neutron spectrum with moderator-to-fuel ratio was investigated. For burnup computations, an appropriate burnup model was proposed to closely simulate the online-refueling. In particular, this model simplified the bottom of the core from the conical-shaped to cylindrical-shaped, and the core was equally divided into five layers in the axial direction from bottom to top. At the time of refueling, the bottom layer was discharged from the core and discarded while a new layer containing only fresh fuel pebbles was added to the top layer of the core. Hence, the ratio of the fuel pebbles to total pebbles increased with greater operation time. This study further proposes that each fuel cycle attempts to initiate the refueling process for next fuel cycle whenever the effective multiplication factor (keff) lies between 1.005 to 1.01. The fuel cycle tends to reach an equilibrium cycle once the core has been refueled five times. Notably, the axial power distribution tends to change from a bottom-peaked to a top-peaked phenomenon as the fuel cycle number increases. In essence, the axial power distribution is nearly un-changed once the reactor core reaches an equilibrium cycle. This phenomenon can be also verified by the corresponding axial burnup distribution, average burnup, and mass of special nuclides as a function of operation time. Finally, an automatic process was established in this study in order to reduce the artificial process in preparing any necessary revision of the burnup model. For example, the core can be equally divided into more than five layers in the axial direction from bottom to top. Also notice that, three subroutines were built in the automatic process, in which there existed some verification-used results in order to comfirm the reliability of each subroutine.