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Selection of Irradiation Tests
A study of all available irradiation tests has been performed [23] and applicable tests that might be used to derive silver transport models and parameters have been described. Table 1 shows all applicable irradiation tests to be considered. All values in Table 1 are as reported in the literature before any evaluation has been performed. Temperatures and fluences are often pre-irradiation targets and burn-ups are first-calculated estimates. All tests are listed from most applicable to least applicable. Applicability in this case is defined according to fuel type, irradiation conditions and availability of irradiation data and silver fractional release results.
Fuel type
Ideally, considered irradiation tests must have tested fuel containing low enriched UO2 TRISO coated particles, where coating layers are as close as possible to that of the PBMR fuel design [10]. Due to low retention of silver in kernel and graphite materials, and for the purpose of this study, the nature of the kernel and pyrocarbon layers is not considered as an inhibitive factor for evaluation of an irradiation or heat-up test. Therefore, UC2- and Thbased fuels and matrix materials other than A3-3 may be considered. For the same reason, non-spherical fuel such as compacts may be considered as well, but the geometry must be taken into account. Additionally, coated particle failure and defect fractions must be more than a factor of ten lower than the fractional release of silver in a test. This is to ensure that silver release evaluations are governed by silicon carbide retention.
Irradiation history
Irradiation history refers to the availability of data (such as time-dependent temperatures, release rates of fission gases, neutron fluxes, etc.) describing irradiation conditions which coated particle fuel in the test were exposed to. Ideally the irradiation history must be comparable with expected reactor conditions considered and detailed data sets must be available.
Unfortunately much of the detailed German data have been lost or are very difficult to find. In some cases only ‘single values’ could be found, for example, a single maximum fuel temperature rather than all the temperatures measured during irradiation. Where complete irradiation histories are unavailable, an evaluation can still be performed by assuming steady state irradiation conditions, but this is of course much less satisfactory.
Often the only option is to make a conservative assumption that adds extra uncertainty in the final result. Sometimes irradiation history data is presented as graphs in documents, typically surface temperature, burn-up (% FIMA), and release rates of gaseous isotopes. The origin of these graphs is from tabulated data, which is often unavailable. The raw data may be estimated from these graphs using opto-digital software. Thermal fluxes can be back-calculated quite accurately from the burn-up history (% FIMA) of the fuel, and this can be done quite precisely if final inventories of some isotopes in the fuel element are available. The most important irradiation history parameter required is the fuel temperature, as evaluations matching fractional releases do not require accurate estimates of the inventory. Fast neutron dose (irradiation damage) has recently come under consideration as a factor affecting silver retention in silicon carbide and influencing thermal conductivity in fuel materials. In most cases the history of this dose is relatively linear in nature, and easily estimated once final values are known.
Fractional release data
If the fractional release of silver could not be found in the literature, there is little point in carrying out the irradiation evaluation. In some cases, silver released from a sphere is given as an absolute amount, and it is necessary to calculate the total silver inventory from burn-up. Other times, silver profiles are given, either in the complete sphere, or just in the fuel-free zone. Here an evaluation can still be performed for some models, but deriving transport parameters becomes questionable.
The availability of fractional release rates for other radionuclides is very valuable in evaluating silver fractional release data. Release over birth ratio (R/B) for krypton fission gases are valuable indicators of coated particle performance and caesium fractional release rates indicate the quality of SiC layers.
1. Introduction
1.1 Fuel Design
1.2 Silver Fission and Activation Products
1.3 Fission Production Sources
2. Modelling Options
2.1 Diffusion Calculation Model
2.2 Molecular Vapour Transport Release Calculation Model
3. Evaluation of Material Tests
3.1 Sorption Isotherms
3.2 Matrix Material Transport
3.3 Coated Particle Transport
3.4 Material Test Evaluation Discussion
4. Evaluation of Irradiation tests
4.1 Selection of Irradiation Tests
4.2 Molecular Vapour Transport Release Model
4.3 Diffusion Model
5. Detailed Evaluation
5.1 HFR-K3
5.2 FRJ2-K13
5.3 FRJ2-K15
5.4 R2-K12
5.5 R2-K13
5.6 FRJ2-K11
5.7 Discussion of results
6. Evaluation of Post irradiation Heat-up Tests
6.1 The KÜFA Instrument
6.2 Heat-up Tests
6.3 Discussion of Heat-up Tests
7. Application in PBMR Core Analyses
7.1 PBMR Core Model and Analyses
7.2 Silver Release from a PBMR Core
7.3 Effect on PBMR Core Analyses Discussion
8. Conclusions
9. Recommendations
10. References
