Quantitative analysis of uncertainty from pebble flow in HTR

In pebble bed HTR, along the deterministic average flow lines, randomness exists in the flow of pebbles, which is not possible to simulate with the current reactor design codes for HTR, such as VSOP, due to the limitation of current computer capability. In order to study how the randomness of pebble flow will affect the key parameters in HTR, a new pebble flow model was set up, which has been successfully transplanted into the VSOP code. In the new pebble flow model, mixing coefficients were introduced into the fixed flow line to simulate the randomness of pebble flow. Numerical simulation and pebble flow experiments were facilitated to determine the mixing coefficients. Sensitivity analysis was conducted to achieve the conclusion that the key parameters of pebble bed HTR are not sensitive to the randomness in pebble flow. The uncertainty of maximum power density and power distribution caused by the randomness in pebble flow is very small, especially for the “multi-pass” scheme of fuel circulation adopted in the pebble bed HTR.