Investigation of void effects in boiling water reactor fuels using neutron tomography

Abstract: In a boiling water reactor (BWR), the void is correlated to dry out and the power level of the reactor. However, measuring the void is very difficult so it is therefore calculated with an accuracy that leaves room for improvements. Typically the uncertainty is ± 3% for 40% average void in the reactor. If the void could be determined with improved accuracy, both safety and economical features could be improved. X-ray tomography has previously been done on BWR fuel models in order to determine the void but with unsatisfying results. In this report, tomography has been modeled on BWR fuel with neutrons instead since neutrons are much more sensitive to the hydrogen in the water compared to x-rays. The results show that even with a fairly rough resolution of 1 mm per pixel, the method can replicate a simulated area with 60% of the density of water, i.e. 40% void, to a result of 58.70% ± 0.83%. There is both a statistical error and a distortion error. The distortion error can be reduced with improved image analysis while comparisons of different reconstructions show that the statistical error is about 0.1%. This means that with improved image reconstruction, neutron tomography has potential to measure local void with an uncertainty of 0.1%. With an uncertainty this low the method could potentially be used to detect thin water films in annular flow.