论文信息 - Effects of random pebble distribution on the multiplication factor in HTR pebble bed reactors

Effects of random pebble distribution on the multiplication factor in HTR pebble bed reactors

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 k eff of three phenomena in random pebble distributions: non-uniform packing density, neutron streaming in between the pebbles, and variations in Dancoff factor. For a 100 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. k eff calculations were performed with three codes: Monte Carlo, diffusion, and finite element transport. By comparing k eff of the homogenized and zones model the effect of including density fluctuations in the pebble bed was found to increase k eff by 71 pcm for the infinite cylinder and 649 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 k eff by 606 pcm for the infinite cylinder, and by 1240 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 Δ streaming values of 340 and 1085 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 k eff , showing that variations in Dancoff factor in pebble bed reactors can be ignored.

[1] Effects of Spatial Variations in Packing Fraction on Reactor Physics Parameters in Pebble-Bed Reactors , 2003 .

[3] M. Ding,et al. Evaluation of experiments in the AVR with the DALTON-THERMIX coupled code system , 2009 .

[4] William W. Schertz,et al. Thermal and material transport in nonisothermal packed beds , 1969 .

[5] T. Williams,et al. Definition and Analysis of an Experimental Benchmark on Shutdown Rod Worths in LEU-HTR Configurations , 2002 .

[6] Behrens. THE EFFECT OF HOLES IN A REACTING MATERIAL ON THE PASSAGE OF NEUTRONS, WITH SPECIAL REFERENCE TO THE CRITICAL DIMENSIONS OF A REACTOR , 1949 .

[7] B. Boer,et al. In-core fuel management optimization of pebble-bed reactors , 2009 .

[8] W. Scherer,et al. Treatment of the upper cavity in a pebble-bed high-temperature gas-cooled reactor by diffusion theory , 1987 .

[9] J. Groen,et al. Comparison of Experiments and Calculations of Void Fraction Distributions in Randomly Stacked Pebble Beds , 2010 .

[10] P. Mrafko. HOMOGENEOUS AND ISOTROPIC HARD SPHERE MODEL OF AMORPHOUS METALS , 1980 .

[11] J. Lieberoth,et al. Neutron Streaming in Pebble Beds , 1980 .

[12] R. F. Benenati,et al. Void fraction distribution in beds of spheres , 1962 .

[13] Jan Leen Kloosterman,et al. Analytical Calculation of the Average Dancoff Factor for a Fuel Kernel in a Pebble Bed High-Temperature Reactor , 1999 .