Key differences in the fabrication, irradiation and high temperature accident testing of US and German TRISO-coated particle fuel, and their implications on fuel performance

doi:10.1016/S0029-5493(03)00033-5

Nuclear Engineering and Design

Volume 222, Issues 2–3, June 2003, Pages 281-297

https://doi.org/10.1016/S0029-5493(03)00033-5 Get rights and content

Abstract

Historically, the irradiation performance of TRISO-coated gas reactor particle fuel in Germany has been superior to that in the US. German fuel generally has displayed gas release values during irradiation three orders of magnitude lower than US fuel. Thus, we have critically examined the TRISO-coated fuel fabrication processes in the US and Germany and the associated irradiation database with a goal of understanding why the German fuel behaves acceptably, why the US fuel has not faired as well, and what process/production parameters impart the reliable performance to this fuel form. The postirradiation examination results are also reviewed to identify failure mechanisms that may be the cause of the poorer US irradiation performance. This comparison will help determine the roles that particle fuel process/product attributes and irradiation conditions (burnup, fast neutron fluence, temperature, degree of acceleration) have on the behavior of the fuel during irradiation and provide a more quantitative linkage between acceptable processing parameters, as-fabricated fuel properties and subsequent in-reactor performance.

Introduction

High temperature gas reactor technology is achieving a renaissance around the world. Without a conventional containment, this technology relies on high quality production and performance of coated particle fuel. The behavior of this fuel over the past three decades has been mixed. The Germans have demonstrated high quality production of TRISO-coated fuel and excellent irradiation and safety test behavior under reactor relevant conditions. By contrast, for nominally the same fuel under very similar service conditions, the US fuel has been much less satisfactory. Our goal is to critically compare the German and US fuel fabrication processes and the corresponding irradiation databases to identify the technical reasons for the differences in in-reactor behavior and to identify those specific fuel attributes and/or fabrication process conditions that impart superior in-reactor performance to TRISO-coated particle fuel.

Section snippets

Fabrication processes

A review of the fabrication processes used in Germany and the US to make coated particle fuel indicates that the scale of fuel fabrication and development efforts in the last 25 years were quite different (Petti et al., 2002). German fabrication of modern TRISO fuel was industrial/production scale incorporating improvements from fuel production for the German AVR reactor. Strict process control was used to adhere to a process specification that produced high quality fuel. Only ∼100 defects were

Irradiation performance

Numerous in-pile irradiation experiments have been conducted in both the US and Europe as part of the US and German TRISO-coated particle fuel development efforts (Gontard and Nabielek, 1990, Petti et al., 2002). These irradiations were conducted at a variety of burnups, temperatures, and fluences. The rate of accumulation of burnup and fast fluence (i.e. the degree of acceleration) in the irradiation relative to that expected in the reactor may also be an important difference. For most of

Impact on in-reactor performance

A comparison of the microstructures of the layers of the TRISO coatings in German and US fuel and a detailed review of the fabrication processes has revealed many differences. There are three specific technical differences in the coating layers produced by the respective fabrication processes that have important impacts in terms of performance under irradiation and accident conditions: pyrocarbon anisotropy and density, IPyC/SiC interface structure, and SiC microstructure.

Failure mechanisms

A review of the irradiation and safety testing of coated particle fuel reveals a number of potential failure mechanisms. These failure mechanisms are functions of temperature, burnup, fluence, and temperature gradient across the particle. Mechanisms that may result in particle failure, which ultimately leads to fission product release, are:

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Pressure vessel failure caused by internal gas pressure.
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Pyrocarbon layer cracking and/or debonding due to irradiation-induced shrinkage which ultimately

Summary and conclusions

Our review has concluded that there have historically been differences in the quality of US and German fuel as evidenced by the level of initial as-manufactured defects and the fuel performance results from many US and German irradiations. These differences in as-manufactured defects appear to be related to differences in the manufacture of the fuel body (pebble versus compact). The differences in irradiation performance have in part been traced to technical differences in the microstructures

Acknowledgements

This work was supported through the INEEL Bechtel Corporate Funded R&D (CFRD) Program under DOE Idaho Operations Office Contract DE-AC07-99ID13727.

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