Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
1729344 | Annals of Nuclear Energy | 2010 | 11 Pages |
Abstract
In pebble bed reactors the pebbles have a random distribution within the core. The usual approach in modeling the bed is homogenizing the entire bed. To quantify the errors arising in such a model, this article investigates the effect on keff of three phenomena in random pebble distributions: non-uniform packing density, neutron streaming in between the pebbles, and variations in Dancoff factor. For a 100Â cm high cylinder with reflective top and bottom boundary conditions 25 pebble beds were generated. Of each bed three core models were made: a homogeneous model, a zones model including density fluctuations, and an exact model with all pebbles modeled individually. The same was done for a model of the PROTEUS facility. keff calculations were performed with three codes: Monte Carlo, diffusion, and finite element transport. By comparing keff of the homogenized and zones model the effect of including density fluctuations in the pebble bed was found to increase keff by 71Â pcm for the infinite cylinder and 649Â pcm for PROTEUS. The large value for PROTEUS is due to the low packing fraction near the top of the pebble bed, causing a significant lower packing fraction for the bulk of the pebble bed in the homogenized model. The effect of neutron streaming was calculated by comparing the zones model with the exact model, and was found to decrease keff by 606Â pcm for the infinite cylinder, and by 1240Â pcm for PROTEUS. This was compared with the effect of using a streaming correction factor on the diffusion coefficient in the zones model, which resulted in Îstreaming values of 340 and 1085Â pcm. From this we conclude neutron streaming is an important effect in pebble bed reactors, and is not accurately described by the correction factor on the diffusion coefficient. Changing the Dancoff factor in the outer part of the pebble bed to compensate for the lower probability of neutrons to enter other fuel pebbles caused no significant changes in keff, showing that variations in Dancoff factor in pebble bed reactors can be ignored.
Related Topics
Physical Sciences and Engineering
Energy
Energy Engineering and Power Technology
Authors
G.J. Auwerda, J.L. Kloosterman, D. Lathouwers, T.H.J.J. van der Hagen,