Session: 02-10: Physics and Transport Theory - II

Paper Number: 132764

132764 - Research on the Dependency Between Transuranic Isotopes and Burnup in Pebble Bed High-Temperature Reactors 

Abstract: 

The analysis and measurement of transuranic isotopes in nuclear fuel constitute a crucial aspect of nuclear material accountability, holding significant implications for nuclear security and the prevention of nuclear proliferation. In traditional reactors, the analysis of nuclear fuel during operation can be conducted by inputting initial nuclide information and irradiation history into burnup simulation software for forward burnup calculation. However, in pebble bed high-temperature reactors, characterized by the use of numerous fuel spheres to form the core, the movement of these fuel spheres introduces uncertainties, making it impractical to track and record every individual fuel sphere. This characteristic renders the irradiation history of fuel spheres unknown, preventing the application of traditional nuclear material analysis methods.

Currently, the estimation of transuranic isotopes of fuel spheres in pebble bed high-temperature reactors is based on the correlation between transuranic isotopes and burnup. However, the accumulation of major transuranic isotopes has a strong dependency on irradiation history of spheres, resulting in that this correlation is not entirely accurate. Therefore, this paper undertakes work in the following three aspects. 1) Through an analysis of the flow characteristics of fuel spheres within the core, the paper summarizes the variation patterns in the irradiation history of fuel spheres. Utilizing the burnup chain of major heavy metal elements and burnup equations, it theoretically calculates the uncertainty of characterizing major transuranic isotopes using burnup indicators (such as Cs-137). The paper also analyzes the range of this uncertainty. 2) Based on a continuous diffusion model, the core was divided into channels and layers in conjunction with the neutron flux distribution in the pebble bed high-temperature reactor core. Using a random sampling method, a large number of irradiation histories for fuel spheres were generated. The study further investigated the correlation between major transuranic isotopes and burnup indicators using nuclear inventory calculation software. 3) By utilizing gamma spectroscopy measurements to obtain nuclide activity, an inversion of the irradiation history of fuel spheres was performed. Subsequently, burnup extrapolation was conducted based on the obtained irradiation history to calculate the major transuranic isotopes. This method was employed to enhance calculation precision.

The research findings of this paper demonstrate that the current method for estimating transuranic isotopes in pebble bed high-temperature reactors, based on burnup depth, exhibits a certain level of systematic error. To further enhance the precision of nuclear material accountability, it is imperative to establish new methods for nuclear material analysis. In addition to improving accountability precision, the outcomes of this research also contribute to establishing a fundamental physical model for the irradiation history of fuel spheres. Furthermore, they can serve as an analytical foundation for benchmarking studies related to nuclear materials in pebble bed high-temperature reactors.

Presenting Author: Zhang Hongjian Tsinghua University

Presenting Author Biography: The presenting author is a Ph.D. candidate in Nuclear Science and Technology at Tsinghua University, specializing in pebble bed high-temperature reactor burnup measurements and nuclear material accountability research. The current focus includes analyzing fuel sphere irradiation history, addressing burnup calculation uncertainties, and contributing to the development of advanced methodologies in nuclear material analysis.

Authors:

Zhang Hongjian Tsinghua University
Zhu Qing Tsinghua University
Zhang Liguo Tsinghua University
Ma Tao Tsinghua University