Heinz Nabielek

The Performance of High-Temperature Reactor Fuel Particles at Extreme Temperatures

Coated particles embedded in graphitic elements are the fuel for the High-Temperature Reactor (HTR). Experimental investigations of the performance of particles at extremely high temperatures have been conducted to achieve an understanding of coating failure mechanisms and to establish the data base for safety and risk analyses of hypothetical accidents in large- and medium-sized HTRs. The primary mechanism for coating failure and fission product release in the 1900 to 2500/sup 0/C temperature range is thermal decomposition of silicon carbide (SiC). Heating tests have provided the activation energy of this process and the correlation of SiC decomposition with coating failure and subsequent fission product release.

Fission product release from ZrC-coated fuel particles during postirradiation heating at 1600°C

Abstract Release behavior of fission products from ZrC-coated UO 2 particles was studied by a postirradiation heating test at 1600°C (1873 K) for 4500 h and subsequent postheating examinations. The fission gas release monitoring and the postheating examinations revealed that no pressure vessel failure occurred in the test. Ceramographic observations showed no palladium attack and thermal degradation of ZrC. Fission products of 137 Cs 134 Cs, 106 Ru, 144 Ce, 154 Eu and 155 Eu were released from the coated particles through the coating layers during the postirradiation heating. Diffusion coefficients of 137 Cs and 106 Ru in the ZrC coating layer were evaluated from the release curves based on a diffusion model. 137 Cs retentiveness of the ZrC coating layer was much better than that of the SiC coating layer.

COATED PARTICLE FUEL FOR HIGH TEMPERATURE GAS COOLED REACTORS

Roy Huddle, having invented the coated particle in Harwell 1957, stated in the early 1970s that we know now everything about particles and coatings and should be going over to deal with other problems. This was on the occasion of the Dragon fuel performance information meeting London 1973: How wrong a genius be! It took until 1978 that really good particles were made in Germany, then during the Japanese HTTR production in the 1990s and finally the Chinese 2000-2001 campaign for HTR-10. Here, we present a review of history and present status. Today, good fuel is measured by different standards from the seventies: where initial free heavy metal fraction was typical for early AVR carbide fuel and initial free heavy metal fraction was acceptable for oxide fuel in THTR, we insist on values more than an order of magnitude below this value today. Half a percent of particle failure at the end-of-irradiation, another ancient standard, is not even acceptable today, even for the most severe accidents. While legislation and licensing has not changed, one of the reasons we insist on these improvements is the preference for passive systems rather than active controls of earlier times. After renewed HTGR interest, we are reporting about the start of new or reactivated coated particle work in several parts of the world, considering the aspects of designs/ traditional and new materials, manufacturing technologies/ quality control quality assurance, irradiation and accident performance, modeling and performance predictions, and fuel cycle aspects and spent fuel treatment. In very general terms, the coated particle should be strong, reliable, retentive, and affordable. These properties have to be quantified and will be eventually optimized for a specific application system. Results obtained so far indicate that the same particle can be used for steam cycle applications with helium coolant gas exit, for gas turbine applications at and for process heat/hydrogen generation applications with outlet temperatures. There is a clear set of standards for modem high quality fuel in terms of low levels of heavy metal contamination, manufacture-induced particle defects during fuel body and fuel element making, irradiation/accident induced particle failures and limits on fission product release from intact particles. While gas-cooled reactor design is still open-ended with blocks for the prismatic and spherical fuel elements for the pebble-bed design, there is near worldwide agreement on high quality fuel: a diameter kernel of 10% enrichment is surrounded by a thick sacrificial buffer layer to be followed by a dense inner pyrocarbon layer, a high quality silicon carbide layer of thickness and theoretical density and another outer pyrocarbon layer. Good performance has been demonstrated both under operational and under accident conditions, i.e. to 10% FIMA and maximum afterwards. And it is the wide-ranging demonstration experience that makes this particle superior. Recommendations are made for further work: 1. Generation of data for presently manufactured materials, e.g. SiC strength and strength distribution, PyC creep and shrinkage and many more material data sets. 2. Renewed start of irradiation and accident testing of modem coated particle fuel. 3. Analysis of existing and newly created data with a view to demonstrate satisfactory performance at burnups beyond 10% FIMA and complete fission product retention even in accidents that go beyond for a short period of time. This work should proceed at both national and international level.

Passive Safety Characteristics of Fuel for a Modular High-Temperature Reactor and Fuel Performance Modeling Under Accident Conditions

AbstractThe high fission product retention potential of coated particle fuel combined with inherently passive temperature controls guarantee almost complete fission product retention during an accident in a small modular high-temperature reactor. Extensive experimental results provide the basis for this claim to inherent safety.Models and codes have been developed to (a) predict realistic, or at least conservative, overall release rates from the primary circuit, (b) reduce the large number of experimental results to a small set of characteristic coefficients, and (c) predict release beyond experimental conditions. Conservative predictions of release from the core have been done using a traditional pressure vessel model for release from fuel particles and simplified diffusion models for fission product transport. This approach is based on experimental work that has been done on nearly all possible accident conditions and is limited by the finite number of experiments. Data reduction has been achieved with ...

Fission Product Release From HTGR Fuel Under Core Heatup Accident Conditions

Various countries engaged in the development and fabrication of modern fuel for the High Temperature Gas-Cooled Reactor (HTGR) have initiated activities of modeling the fuel and fission product release behavior with the aim of predicting the fuel performance under operating and accidental conditions of future HTGRs. Within the IAEA directed Coordinated Research Project CRP6 on “Advances in HTGR Fuel Technology Development” active since 2002, the 13 participating Member States have agreed upon benchmark studies on fuel performance during normal operation and under accident conditions. While the former has been completed in the meantime, the focus is now on the extension of the national code developments to become applicable to core heatup accident conditions. These activities are supported by the fact that core heatup simulation experiments have been resumed recently providing new, highly valuable data. Work on accident performance will be — similar to the normal operation benchmark — consisting of three essential parts comprising both code verification that establishes the correspondence of code work with the underlying physical, chemical and mathematical laws, and code validation that establishes reasonable agreement with the existing experimental data base, but including also predictive calculations for future heating tests and/or reactor concepts. The paper will describe the cases to be studied and the calculational results obtained with the German computer model FRESCO. Among the benchmark cases in consideration are tests which were most recently conducted in the new heating facility KUEFA. Therefore this study will also re-open the discussion and analysis of both the validity of diffusion models and the transport data of the principal fission product species in the HTGR fuel materials as essential input data for the codes.Copyright

SOFC Worldwide — Technology Development Status and Early Applications

Solid Oxide Fuel Cells (SOFC) of various types and designs have been developed world wide through the last two decades. They offer interesting advantages over other fuel cell types, but also have inherent materials problems that have caused a slower development pace as, for instance, compared to the low temperature Polymer Electrolyte Fuel Cell (PEFC). Due to their high operating temperature in the range of 700 to 1000°C, SOFC can be used with a variety of fuels from hydrogen to hydrocarbons with a minimum of fuel processing, can be coupled with gas turbines for the highest electrical system efficiency known in power generation, deliver process heat in industrial applications or supply on-board electricity for vehicles, to name but some typical applications. This report summarizes the more prominent SOFC development strands and gives an overview of the achievements of the various R&D groups. The analysis includes a benchmark that attempts to compare cell and stack characteristics on a standardized basis.