Infinite refuelling equilibrium burnup analysis in pebble-bed HTR

Pebble-bed High Temperature Reactor (HTR) adopts multi-pass refuelling fuel management or MEDUL cycle, where fuel pebbles would run through the core multiple times before reaching their burnup value limits and being discharged. This cycle ultimately leads reactor to an equilibrium state whose characteristics are closely related to the allowed refuelling times in the cycle. Typically, increasing refuelling times permits a more homogeneous equilibrium state with lower maximum power density. Also, atomic density uncertainty, e.g. contributed from nuclear data, inside this equilibrium core would be reduced with increased refuelling times. Although it is beneficial for reactor operation safety, increasing refuelling times also burdens fuel handling system and shortens their service life. Analysing equilibrium state under hypothetical infinite refuelling times will reveal the limiting effect of refuelling times on the characteristics of equilibrium state in pebble-bed HTR, and it could be used to justify fuel management design. This work proposes a Lagrangian burnup framework based infinite refuelling burnup model. A burnup model constructed from HTR-PM (High-Temperature gas-cooled Reactor Pebble-bed Module) is used to verify the proposed infinite refuelling model, and results are compared with finite refuelling calculations. It is found that infinite refuelling highlights the limiting effects of refuelling times on equilibrium state in terms of neutron flux, regional power density, actinides and fission products’ batch averaged atomic density. Increasing refuelling times makes equilibrium state approaching infinite refuelling equilibrium state nonlinearly, and equilibrium state obtained from fifteen times refuelling is quite close to that obtained from infinite times refuelling. As a hypothetical model, the infinite refuelling equilibrium burnup model developed in this work could balance out the randomness as well as uncertainty associated with fuel pebbles’ movement inside pebble-bed HTR. It is expected to be a reference for multiple design and analysis of pebble-bed HTR.