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Burn-up determination and accident testing of HTR-PM fuel elements irradiated in the HFR Petten
Nuclear Engineering and Design 357 (2020) 110414
Contents lists available at ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Burn-up determination and accident testing of HTR-PM fuel elements irradiated in the HFR Petten
T
Daniel Freis , Abdel El Abjani, Dragan Coric, Ramil Nasyrow, Joseph Somers, Chunhe Tang1, Rongzheng Liu1, Bing Liu1, Malin Liu1 ⁎
European Commission, Joint Research Centre, Directorate G – Nuclear Safety and Security, Hermann-von-Helmholtz-Platz1, 76344 Eggenstein-Leopoldshafen, Germany
GRAPHICAL ABSTRACT
ABSTRACT
The current paper reports the results of gamma spectroscopic burn-up determination and KüFA safety testing at JRC Karlsruhe on spherical high-temperature reactor fuel elements, which were fabricated by the Institute of Nuclear and New Energy Technology of the Tsinghua University, Beijing. The fuel elements were irradiated in the High Flux Reactor, Petten, in the frame of the HFR-EU1 and HTR-PM campaigns and transported to JRC Karlsruhe for post-irradiation examination and accident testing in the KüFA device in the frame of a bilateral safety research study. Burn-up determination was performed on seven fuel elements based on the quantitative measurement of their Cs-137 inventories using an established gamma spectroscopy set-up in the JRC hot cell facilities. Accident testing was conducted on three HTRPM fuel elements in several phases at simulated accident temperatures between 1620 °C and 1770 °C for 150 h to simulate hypothetical depressurization and loss-offorced circulation accidents. The release of Kr-85 was measured during the tests in a cold trap and solid fission product release was determined by gamma spectroscopic analyses of exchangeable cold plates in a low background environment. After successful completion of the KüFA tests the fuel elements are currently undergoing further post-irradiation examinations including profile disintegration as well as ceramography and scanning electron microscopy of individual coated particles.
1. Introduction The performance of the fuel elements is a crucial component of the safety concept of modern modular High Temperature Reactors (HTR), such as the Chinese HTR-PM which is currently under construction in the Shandong province in China (Zhang et al., 2016). To assess the safety behavior under normal operating conditions, seven spherical HTR fuel elements of Chinese production have been irradiated in the High Flux Reactor (HFR) in Petten in the frame of the HFR-EU1 (Laurie et al., 2012) and HTR-PM (Knol et al., 2018) irradiation experiments.
The fuel elements were produced by the Institute of Nuclear and New Energy Technology (INET) of the Tsinghua University, Beijing, in the frame of the Chinese HTR research and development programme; two were of the type HTR-10 (Tang et al., 2012) and five of the type HTRPM (Xiangwen et al., 2013). The main characteristics of the fuel elements are provided in Table 1. 2. Burn-up determination The burn-up (BU) of the seven fuel elements was determined by
Corresponding author. E-mail address: [email protected] (D. Freis). 1 Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China. ⁎
https://doi.org/10.1016/j.nucengdes.2019.110414 Received 22 March 2019; Received in revised form 18 July 2019; Accepted 28 October 2019 0029-5493/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Nuclear Engineering and Design 357 (2020) 110414
D. Freis, et al.
Table 1 Fuel element (FE) characteristics (Tang et al., 2012; Xiangwen et al., 2013).
Matrix graphite Matrix density Kernel Heavy metal in FE Enrichment Particle number Kernel diameter Buffer PyC Inner PyC SiC Outer PyC
Table 2 Fuel element characteristics.
HTR-10
HTR-PM
Type
Heavy metal content
Correction factor
A3-3 1.76 g/cm3 UO2 5.02 g 17.08 wt% 8500 490.3 µm 97.7 µm 42.0 µm 37.8 µm 40.8 µm
A3-3 1.72 g/cm3 UO2 7.04 g 17.08 wt% 12,000 498.5 µm 93.7 µm 43.9 µm 39.9 µm 41.0 µm
GLE 4.1 HTR-10 HTR-PM
6g 5.02 g 7.04 g
1 1.0034 0.9966
Table 3 Cs-137 activity at EOI & calculated burn-up.
measuring their Cs-137 inventory using a well-established gamma spectroscopy arrangement in the Hot Cells of JRC (Freis, 2010). The spherical fuel elements were placed in front of a collimator located in the wall of the hot cell (see Fig. 1) and rotated continuously during the measurement. A High Purity Germanium gamma detector was placed at the other end of the collimator, inside the operator area. It recorded the data, with the software determining the intensity of the Cs-137 signal at 661.66 keV. Background measurements were performed always to correct for the unavoidable gamma background emanating from the other irradiated materials in the hot cell. An AVR fuel element (Type GLE 4.1) with well-known Cs-137 inventory and the same geometry as the measured fuel elements was used for efficiency calibration of the equipment at 661.66 keV. A small efficiency correction was required to allow for the different γ self-absorption due to the slightly different heavy metal loadings of the different types of fuel elements (see Table 2). Based on the Cs-137 inventory of each fuel element at the End Of Irradiation (EOI), the burn-up (BU) in FIMA (Fissions per Initial Metal Atoms) was then calculated using the following correlation (see Equation (1)).
BU =
NCs
137, EOI
Cs 137·NHM
Ccorr (tirr ) =
1
Ccorr (tirr )
Fuel Element
Cs-137 activity at EOI (Bq)
Calculated BU (%FIMA)
HTR-10 1 HTR-10 2 HTR-PM 1 HTR-PM 2 HTR-PM 3 HTR-PM 4 HTR-PM 5
5.70E+10 7.16E+10 9.27E+10 9.97E+10 1.01E+11 9.47E+10 8.05E+10
10.17 12.77 11.64 12.53 12.66 11.89 10.11
γCs-137: Fission yield of Cs-137 (6.27% for thermal fission of U-235) λ Cs-137: Decay constant of Cs-137 tirr: Irradiation time (s) Ccorr: Correction factor The correction factor, Ccorr, accounts for the decay of Cs-137 during the irradiation itself. Based on the above expressions, the burn-up as shown in Table 3 was obtained. The burn-up values determined in the JRC Hot Cells based on Cs137 inventories are slightly higher than the derived data generated by neutronic calculations of the irradiation test in the HFR (Knol et al., 2018), but the difference between both sets of data is well within the uncertainty of the measurements. The expanded uncertainty (95% confidence limit) for the burn-up determination here has been calculated as ± 1.4 %FIMA using the GUM (Guide to the expression of Uncertainty in Measurement) Workbench error propagation analysis tool (G.U.M. Workbench, 2003). Major contributors to the uncertainty are the fission yield of Cs-137 and the Cs-137 activity of the AVR fuel element used as standard.
(1)
Cs 137 · tirr Cs 137·tirr )
e(
3. KüFA safety testing
NCs-137 EOI: Cs-137 atoms at EOI NHM: Heavy metal atoms at BOI
The heating tests were performed in the KüFA device in the hot cell installation at JRC Karlsruhe. The KüFA is displayed schematically in
Fig. 1. Experimental setup for burn-up determination by γ-spectroscopy. 2
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collected and recorded. At the lower end of the cold finger, facing the fuel element, an exchangeable stainless-steel plate is positioned. Thanks to efficient water cooling, the surface temperature of the plate itself is kept at less than 100 °C even at maximum furnace temperatures. In this way, volatile fission products such as Cs-137, Cs-134 and Ag-110 m plate out on the condensation plate with high efficiency. The cold finger can be pulled up through a system of interlocks isolated by a water cooled gate valve, and the condensation plate can be exchanged semi-automatically using a tele-manipulator. The used plates are then transported to a laboratory with low radiation background, where gamma spectra are recorded using a high-purity germanium detector to quantify the gamma-emitting isotopes (Freis, 2010). During the experiment the device is flushed with helium, which enters the gas conduction cylinder below the fuel element, flows around it, and disperses in the bell. From there it is evacuated by a membrane pump and reaches an activated-carbon cold trap. The latter, located outside the cell is cooled with liquid nitrogen to −196 °C, where released fission gases, like Krypton, condense on the activated carbon. Their activity is then measured by a sodium iodide (NaI) gamma detector mounted below the cold trap (Freis, 2010; Stradal, 1970). The NaI-detector is connected to a data acquisition system, which continuously records gamma spectra with durations of several minutes each. The gamma spectra are analyzed automatically and the evaluated activities are saved into a scanning file. In this way a continuous online gamma scan on the Kr collected in the trap is performed, which reveals time and intensity of fission gas release from the fuel element. The calibration and measurement procedures for the cold finger and the cold trap are described in detail in reference (Freis, 2010). The furnace temperature is measured and controlled by two W/Rh thermocouples, which are placed circa 3 mm below the fuel element. In the past it was observed that degradation of the thermocouples can lead to systematic deviations between the indicated and real furnace temperature with time (Seeger, 2016). To ensure correct temperature adjustment, a fresh set of W/Rh thermocouples were installed in preparation of the HTR-PM test campaign, and a series of melt wire tests was performed using the same furnace set-up and tantalum parts as later for the heating tests. The results of the tests indicated that the real temperature, even for the new set-up, was between 10 and 30 °C lower than the nominal temperature for the relevant temperature range. In order to guarantee that the required peak temperature was reached during the test, the programmed temperature (nominal) was always set 30 °C higher than the targeted temperature (real). In addition, the furnace power was monitored continuously during the tests, and no derivations from the expected values were found during the test campaign, indicating that no deterioration of the thermocouple occurred.
Fig. 2. Cold Finger Apparatus (KüFA).
Fig. 2. KüFA is the German abbreviation for Cold Finger Apparatus. It is an experimental apparatus within which spherical HTR fuel elements can be heated to a temperature close to 1800 °C in a flowing helium atmosphere at ambient pressure, while the release of fission gases and volatile fission products are measured. The original KüFA device was conceived and taken into operation at KFA Jülich in 1984 (Schenk et al., 1988). After the end of the German HTR programme the KüFA in Jülich was decommissioned, and parts of the KüFA as well as drawings and technical documents were transferred to JRC Karlsruhe, in the frame of the HTR-F European collaborative research project. The new KüFA was taken into operation at JRC in 2005 (Freis, 2010). A detailed description of the device and operating procedures can be found in references (Freis, 2010; Fries et al., 2011; Kostecka et al., 2004; Schenk et al., 1988). In the following paragraphs, its main features will be summarized briefly for the convenience of the reader. The device is designed to examine experimentally the effects of Depressurization and LOss of Forced Circulation (D-LOFC) accident scenarios on irradiated spherical HTR fuel elements. While remaining under an inert helium atmosphere, a HTR fuel element is subjected to heating schedules according to the predicted accident progression for several hundred hours. Fission gas release is monitored online during the experiment. Volatile fission products are collected on water-cooled (less than100 °C) condensation plates made of stainless steel positioned above the fuel sample on a “cold finger”. These plates can be extracted from the cell for analysis of the material deposited on them by means of gamma spectroscopy to provide information on the fission product release as a function of time and temperature. The main components of the furnace are a bell-shaped outer stainless-steel vessel, a gas conduction cylinder encompassed by a heating element, and an extractable cold finger at the top. The heater is mounted in a water-cooled copper block and surrounded by heat shields. In order to minimize the absorption of fission products in the furnace, the KüFA was designed as a metallic resistive furnace. A highcurrent transformer installed next to the hot cell delivers the necessary current. All high-temperature parts of the KüFA are manufactured from tantalum because this material is the least prone to carbonization at the contact point with the fuel element (Schenk et al., 1988). The fuel element is placed on a three-point holder on the axis of the gas conduction cylinder, about a third of the way up the length of the heating element. A thermocouple measures the temperature about 3 mm below the fuel element and enables the regulation of the furnace power. The furnace temperature as well as all operating processes and parameters are centrally controlled by a programmable logic controller and an associated computer program. All operating parameters are
4. KüFA heating test HTR-PM 1 The first KüFA test was performed on fuel element HTR-PM 1, which was irradiated in the HFR Petten from 8th of September 2012 to 30th of December 2014, amassing a total of 355 Equivalent Full Power Days (EFPD) during which time it reached a burn-up of 11.64 %FIMA (see Table 3). The most relevant data of fuel element HTR-PM 1 are summarized in Table 4 below. The KüFA heating experiment was divided into three individual heating tests, denoted hereafter as test phases, at 1620 °C for 150 h each. This block wise procedure was chosen in order to fit the experiment into the working schedule of the hot cell facility, e.g. to avoid continuous furnace operation during several weeks and long weekends. Before each test phase the furnace was heated to 300 °C for 3 h, followed by heating plateau at 1050 °C for 10 h, after which the temperature was increased slowly to the target temperature with a heating rate of 47.5 °C/h. The maximum temperature was held for 150 h before the furnace was cooled down slowly in a controlled way to room 3
Nuclear Engineering and Design 357 (2020) 110414
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Table 4 Main data of HTR-PM 1. Burn up
11.64 %FIMA
Thermal fluence (0.625 eV > tf* > 0) Fast fluence (> 0.1 MeV) Irradiation time End of irradiation Irradiation temperature
2.68 ∙ 1021 cm−2** 4.72 ∙ 1021 cm−2** 355 EFPD 30.12.2014 ~1023 °C
* neutron energy, ** (Knol et al., 2018) corrected for burn-up.
Fig. 4. HTR-PM 1 heating program (red) and fractional release of Ag-110 m (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. HTR-PM 1 heating program (red) and fractional release of Cs-137 (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
temperature within 24 h. The heating conditions are shown in Fig. 3. The release data are provided in detail in Table 5 and displayed in Figs. 3–5 together with the determined uncertainty band (Std. Dev.). Cs137 release is observed in the first phase at 1620 °C with a fractional release corresponding to about 1.63·10−7. By the end of the second phase, the fractional release reaches about 5.78·10−7, while at the end of the third phase, the fractional release reaches 1.97·10−5. Given that there are 12,000 coated particles in the spherical fuel element, even at the end of the third phase the release corresponds to about one fifth of the inventory of a single coated particle, while after the first phase, it corresponds to about two thousandths thereof. Concerning silver, the data in Table 5 and Fig. 4 show that the fractional release of Ag-110 m was about 1.16·10−5 during the first 150 h heating phase, and rose to 1.37·10−3 in phases two and three. The fractional release of Kr-85 was below the detection limit in phases one and two, but at the beginning of phase three Kr-85 (equivalent to about 2.02·10−5) was detected immediately during the initial heat-up to 300 °C, which is more likely due to an artefact of the circuit than a true release.
Fig. 5. HTR-PM 1 Kr-85 fractional release plus uncertainty. Table 6 Main data of HTR-PM 4.
Table 5 Fractional release (FR) data of HTR-PM 1. Phase-2 (1620 °C)
Phase-3 (1620 °C)
Cs-134 Cs-137 Ag-110m Kr-85
1.63E−07 1.63E−07 1.16E−05 bdl*
5.78E−07 5.78E−07 9.46E−04 bdl
1.97E−05 1.97E−05 1.37E−03 2.02E-05
Thermal fluence (0.625 eV > tf > 0) Fast fluence (> 0.1 MeV) Irradiation time End of irradiation Irradiation temperature
2.76 ∙ 1021 cm−2 * 4.88 ∙ 1021 cm−2 * 355 EFPD 30.12.2014 ~1017 °C
Before each test phase the furnace was heated to 300 °C for 3 h, followed by a heating plateau at 1050 °C for 10 h, after which the temperature was slowly increased to the target temperature with a heating rate of 47.5 °C/h. The maximum temperature was held for 150 h before the furnace was slowly cooled down in a controlled way to room temperature within 24 h. All release data are provided in Table 7. The Cs-137 release was about 4.42·10−6 in the first phase but increasing the temperature to 1650 °C and 1700 °C in phases 2 and 3 resulted in fractional releases of 1.24·10−4 and 1.69·10−2, respectively. Ag-110 m fractional release increased from 2.95·10−5 to 1.27·10−3 and 3.21·10−2 during the heating phases at 1620 °C, 1650 °C and 1700 °C, respectively. At all three
The second KüFA test was performed on fuel element HTR-PM 4 which reached a burn-up of 11.89 %FIMA (see Table 3). The most relevant data of fuel element HTR-PM 4 are summarized in Table 6. The heating experiment was divided into three individual heating tests, called test phases, at 1620 °C, 1650 °C and 1700 °C for 150 h each.
Phase-1 (1620 °C)
11.89 %FIMA
* (Knol et al., 2018) corrected for determined burn-up.
5. KüFA heating test HTR-PM 4
Isotope
Burn up
Table 7 Fractional release (FR) data of HTR-PM 4.
* below detection limit. 4
Isotope
Phase-1 (1620 °C)
Phase-2 (1650 °C)
Phase-3 (1700 °C)
Cs-134 Cs-137 Ag-110m Kr-85
4.42E−06 4.42E−06 2.95E−05 bdl
1.24E−04 1.27E−04 1.27E−03 bdl
1.69E−02 1.82E−02 3.21E−02 bdl
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Table 8 Main data of HTR-PM 2. Burn up
12.53 %FIMA
Thermal fluence (0.625 eV > tf > 0) Fast fluence (> 0.1 MeV) Irradiation time End of irradiation Irradiation temperature
2.95 ∙ 1021 cm−2 * 5.20 ∙ 1021 cm−2 * 355 efpd 30.12.2014 ~1040 °C
* (Knol et al., 2018) corrected for determined burn-up. Table 9 Fractional release (FR) data of HTR-PM 2. Fig. 6. HTR-PM 4 heating program (red) and fractional release of Cs-137 (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Isotope
Phase-1 (1620 °C)
Phase-2 (1700 °C)
Phase-3 (1770 °C)
Cs-134 Cs-137 Ag-110m Kr-85
2.23E−05 2.23E−05 6.23E−05 bdl
1.13E−03 1.10E−03 4.13E−03 bdl
5.24E−02 5.13E−02 2.23E−01 2.85E−05
All release data are provided in Table 9. The Cs-137 release was about 2.23·10−5 in the first phase, but increasing the temperature to 1700 °C and 1770 °C in Phases 2 and 3 resulted in fractional releases of 1.13·10−3 and 5.24·10−2, respectively. Ag-110 m fractional release increased from 6.23·10−5 to 4.13·10−3 and 2.23·10−1 during the heating phases at 1620 °C, 1700 °C and 1770 °C, respectively. During the 1620 °C and 1700 °C phases, no Kr-85 release was detected, whereas during the 1770 °C phase the Kr-85 fractional release reached 2.85·10−5. The primary role of the Kr-85 measurement is the identification of coated particle failure. There are about 12,000 coated particles in one spherical fuel element. The Kr-85 fractional release of 2.85·10−5 corresponds to about one third of the inventory of a single particle (Figs. 9–11).
Fig. 7. HTR-PM 4 heating program (red) and fractional release of Ag-110 m (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
7. Conclusions The burn-up of 7 spherical HTR fuel elements produced by INET have been determined using γ spectroscopy. The results are within experimental error of those derived based on neutronic calculations of the in pile irradiation conditions. Depressurisation and Loss of Forced Circulation (D-LOFC) incidents were simulated in the KÜFA device located in the JRC Hot Cells at Karlsruhe. After the actual accident simulation at 1620 °C for 150 h, the fuel elements were heated additionally two times for 150 h each, partly at significantly elevated temperatures. Typical fractional releases of Cs-137 after the first test phase at 1620 °C for 150 h were less than 2.5·10–5 for all tested fuel elements,
Fig. 8. HTR-PM 4 Kr-85 fractional release plus uncertainty.
temperatures no Kr-85 release was detected (Figs. 6–8). 6. KüFA heating test HTR-PM 2 The third KüFA test was performed on fuel element HTR-PM 2, which reached a burn-up of 12.53 %FIMA (see Table 3). The most relevant data of fuel element HTR-PM 2 are summarized in Table 8 below. The heating experiment was divided into three individual heating tests, called test phases, at 1620 °C, 1700 °C and 1770 °C for 150 h each. Before each test phase the furnace was heated to 300 °C for 3 h, followed by a heating plateau at 1050 °C for 10 h, after which the temperature was slowly increased to the target temperature with a heating rate of 47.5 °C/h. The maximum temperature was held for 150 h before the furnace was slowly cooled down in a controlled way to room temperature within 24 h.
Fig. 9. HTR-PM 2 heating program (red) and fractional release of Cs-134 (blue) and Cs-137 (grey). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 5
Nuclear Engineering and Design 357 (2020) 110414
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two heating phases for all fuel elements. In case of HTR-PM 1 and HTRPM 2 low amounts of Kr-85 (equivalent to a fractional release of 2.02·10−5 and 2.85·10−5, respectively) were detected, either directly at the beginning of phase 3 before heat-up (HTR-PM 1) or during phase 3 after circa 40 h at 1770 °C (HTR-PM 2). No sudden release resulting in a sharp Kr increase (as would be typical for pressure vessel failure of a whole particle) was observed. Acknowledgements The authors would like to thank China S&T major project (No. ZX06901) for the financial support provided. References
Fig. 10. HTR-PM 2 heating program (red) and fractional release of Ag-110 m (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Freis, D., 2010. Thesis: 'Störfallsimulationen und Nachbestrahlungsuntersuchungen an kugelförmigen Brennelementen für Hochtemperatur-reaktoren' (Accident simulation of spherical fuel elements for high temperature reactors). PhD. RWTH Aachen. Freis, D., et al., 2011. Accident testing of high-temperature reactor fuel elements from the HFR-EU1bis irradiation. Nucl. Eng. Des. 241 (8), 2813–2821. Knol, S., et al., 2018. HTR-PM fuel pebble irradiation qualification in the high flux reactor in Petten. Nucl. Eng. Des. 329, 82–88. Kostecka, H., Ejton, J., de Weerd, W., Toscano, E.H. “Post-Irradiation Testing of HTR-Fuel Elements under Accident Conditions. Part 2”, Conference Contribution, 2nd International Topical Meeting on High Temperature Reactor Technology Beijing, China, 2004. Laurie, M., et al., 2012. Results of the HFR-EU1 fuel irradiation of INET and AVR pebbles in the HFR Petten. Nucl. Eng. Des. 251, 117–123. Schenk, W., Pietzer, D., Nabielek, H., 1988, “Fission Product Release Profiles from Spherical HTR Fuel Elements at Accident Temperatures”, Jülich Report 2234. Seeger, O., et al., 2016. KüFA safety testing of HTR fuel pebbles irradiated in the High Flux Reactor in Petten. Nucl. Eng. Des. 306, 59–68. Stradal, K.A., 1970. Apparatur zur Messung der Freisetzung von Spaltgasen und anderer Spaltprodukte durch Ausheizen. Jülich Report 707-RW, Research Center Jülich, Germany. Tang, Chunhe, et al., 2012. Comparison of two irradiation testing results of HTR-10 fuel spheres. Nucl. Eng. Des. 251, 453–458. G.U.M. Workbench, 2003. The software tool for the expression of uncertainty in measurement. Metrodata GmbH D-79639. Xiangwen, Zhou, et al., 2013. Preparation of spherical fuel elements for HTR-PM in INET. Nucl. Eng. Des. 263, 456–461. Zhang, Zuoyi, et al. “The Shandong Shidao Bay 200 MWe high-temperature gas-cooled reactor pebble-bed module (HTR-PM) demonstration power plant: an engineering and technological innovation.” Engineering 2.1 (2016): 112–118.
Fig. 11. HTR-PM 2 Kr-85 fractional release plus uncertainty.
and up to 5.2·10−2 at the end of all test phases where very extreme heating conditions were applied (i.e. after three times heating for 150 h at 1620 °C, 1700 °C and 1770 °C). The release of Kr-85 was below the detection limits during the first
6
Contents lists available at ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Burn-up determination and accident testing of HTR-PM fuel elements irradiated in the HFR Petten
T
Daniel Freis , Abdel El Abjani, Dragan Coric, Ramil Nasyrow, Joseph Somers, Chunhe Tang1, Rongzheng Liu1, Bing Liu1, Malin Liu1 ⁎
European Commission, Joint Research Centre, Directorate G – Nuclear Safety and Security, Hermann-von-Helmholtz-Platz1, 76344 Eggenstein-Leopoldshafen, Germany
GRAPHICAL ABSTRACT
ABSTRACT
The current paper reports the results of gamma spectroscopic burn-up determination and KüFA safety testing at JRC Karlsruhe on spherical high-temperature reactor fuel elements, which were fabricated by the Institute of Nuclear and New Energy Technology of the Tsinghua University, Beijing. The fuel elements were irradiated in the High Flux Reactor, Petten, in the frame of the HFR-EU1 and HTR-PM campaigns and transported to JRC Karlsruhe for post-irradiation examination and accident testing in the KüFA device in the frame of a bilateral safety research study. Burn-up determination was performed on seven fuel elements based on the quantitative measurement of their Cs-137 inventories using an established gamma spectroscopy set-up in the JRC hot cell facilities. Accident testing was conducted on three HTRPM fuel elements in several phases at simulated accident temperatures between 1620 °C and 1770 °C for 150 h to simulate hypothetical depressurization and loss-offorced circulation accidents. The release of Kr-85 was measured during the tests in a cold trap and solid fission product release was determined by gamma spectroscopic analyses of exchangeable cold plates in a low background environment. After successful completion of the KüFA tests the fuel elements are currently undergoing further post-irradiation examinations including profile disintegration as well as ceramography and scanning electron microscopy of individual coated particles.
1. Introduction The performance of the fuel elements is a crucial component of the safety concept of modern modular High Temperature Reactors (HTR), such as the Chinese HTR-PM which is currently under construction in the Shandong province in China (Zhang et al., 2016). To assess the safety behavior under normal operating conditions, seven spherical HTR fuel elements of Chinese production have been irradiated in the High Flux Reactor (HFR) in Petten in the frame of the HFR-EU1 (Laurie et al., 2012) and HTR-PM (Knol et al., 2018) irradiation experiments.
The fuel elements were produced by the Institute of Nuclear and New Energy Technology (INET) of the Tsinghua University, Beijing, in the frame of the Chinese HTR research and development programme; two were of the type HTR-10 (Tang et al., 2012) and five of the type HTRPM (Xiangwen et al., 2013). The main characteristics of the fuel elements are provided in Table 1. 2. Burn-up determination The burn-up (BU) of the seven fuel elements was determined by
Corresponding author. E-mail address: [email protected] (D. Freis). 1 Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China. ⁎
https://doi.org/10.1016/j.nucengdes.2019.110414 Received 22 March 2019; Received in revised form 18 July 2019; Accepted 28 October 2019 0029-5493/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Nuclear Engineering and Design 357 (2020) 110414
D. Freis, et al.
Table 1 Fuel element (FE) characteristics (Tang et al., 2012; Xiangwen et al., 2013).
Matrix graphite Matrix density Kernel Heavy metal in FE Enrichment Particle number Kernel diameter Buffer PyC Inner PyC SiC Outer PyC
Table 2 Fuel element characteristics.
HTR-10
HTR-PM
Type
Heavy metal content
Correction factor
A3-3 1.76 g/cm3 UO2 5.02 g 17.08 wt% 8500 490.3 µm 97.7 µm 42.0 µm 37.8 µm 40.8 µm
A3-3 1.72 g/cm3 UO2 7.04 g 17.08 wt% 12,000 498.5 µm 93.7 µm 43.9 µm 39.9 µm 41.0 µm
GLE 4.1 HTR-10 HTR-PM
6g 5.02 g 7.04 g
1 1.0034 0.9966
Table 3 Cs-137 activity at EOI & calculated burn-up.
measuring their Cs-137 inventory using a well-established gamma spectroscopy arrangement in the Hot Cells of JRC (Freis, 2010). The spherical fuel elements were placed in front of a collimator located in the wall of the hot cell (see Fig. 1) and rotated continuously during the measurement. A High Purity Germanium gamma detector was placed at the other end of the collimator, inside the operator area. It recorded the data, with the software determining the intensity of the Cs-137 signal at 661.66 keV. Background measurements were performed always to correct for the unavoidable gamma background emanating from the other irradiated materials in the hot cell. An AVR fuel element (Type GLE 4.1) with well-known Cs-137 inventory and the same geometry as the measured fuel elements was used for efficiency calibration of the equipment at 661.66 keV. A small efficiency correction was required to allow for the different γ self-absorption due to the slightly different heavy metal loadings of the different types of fuel elements (see Table 2). Based on the Cs-137 inventory of each fuel element at the End Of Irradiation (EOI), the burn-up (BU) in FIMA (Fissions per Initial Metal Atoms) was then calculated using the following correlation (see Equation (1)).
BU =
NCs
137, EOI
Cs 137·NHM
Ccorr (tirr ) =
1
Ccorr (tirr )
Fuel Element
Cs-137 activity at EOI (Bq)
Calculated BU (%FIMA)
HTR-10 1 HTR-10 2 HTR-PM 1 HTR-PM 2 HTR-PM 3 HTR-PM 4 HTR-PM 5
5.70E+10 7.16E+10 9.27E+10 9.97E+10 1.01E+11 9.47E+10 8.05E+10
10.17 12.77 11.64 12.53 12.66 11.89 10.11
γCs-137: Fission yield of Cs-137 (6.27% for thermal fission of U-235) λ Cs-137: Decay constant of Cs-137 tirr: Irradiation time (s) Ccorr: Correction factor The correction factor, Ccorr, accounts for the decay of Cs-137 during the irradiation itself. Based on the above expressions, the burn-up as shown in Table 3 was obtained. The burn-up values determined in the JRC Hot Cells based on Cs137 inventories are slightly higher than the derived data generated by neutronic calculations of the irradiation test in the HFR (Knol et al., 2018), but the difference between both sets of data is well within the uncertainty of the measurements. The expanded uncertainty (95% confidence limit) for the burn-up determination here has been calculated as ± 1.4 %FIMA using the GUM (Guide to the expression of Uncertainty in Measurement) Workbench error propagation analysis tool (G.U.M. Workbench, 2003). Major contributors to the uncertainty are the fission yield of Cs-137 and the Cs-137 activity of the AVR fuel element used as standard.
(1)
Cs 137 · tirr Cs 137·tirr )
e(
3. KüFA safety testing
NCs-137 EOI: Cs-137 atoms at EOI NHM: Heavy metal atoms at BOI
The heating tests were performed in the KüFA device in the hot cell installation at JRC Karlsruhe. The KüFA is displayed schematically in
Fig. 1. Experimental setup for burn-up determination by γ-spectroscopy. 2
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collected and recorded. At the lower end of the cold finger, facing the fuel element, an exchangeable stainless-steel plate is positioned. Thanks to efficient water cooling, the surface temperature of the plate itself is kept at less than 100 °C even at maximum furnace temperatures. In this way, volatile fission products such as Cs-137, Cs-134 and Ag-110 m plate out on the condensation plate with high efficiency. The cold finger can be pulled up through a system of interlocks isolated by a water cooled gate valve, and the condensation plate can be exchanged semi-automatically using a tele-manipulator. The used plates are then transported to a laboratory with low radiation background, where gamma spectra are recorded using a high-purity germanium detector to quantify the gamma-emitting isotopes (Freis, 2010). During the experiment the device is flushed with helium, which enters the gas conduction cylinder below the fuel element, flows around it, and disperses in the bell. From there it is evacuated by a membrane pump and reaches an activated-carbon cold trap. The latter, located outside the cell is cooled with liquid nitrogen to −196 °C, where released fission gases, like Krypton, condense on the activated carbon. Their activity is then measured by a sodium iodide (NaI) gamma detector mounted below the cold trap (Freis, 2010; Stradal, 1970). The NaI-detector is connected to a data acquisition system, which continuously records gamma spectra with durations of several minutes each. The gamma spectra are analyzed automatically and the evaluated activities are saved into a scanning file. In this way a continuous online gamma scan on the Kr collected in the trap is performed, which reveals time and intensity of fission gas release from the fuel element. The calibration and measurement procedures for the cold finger and the cold trap are described in detail in reference (Freis, 2010). The furnace temperature is measured and controlled by two W/Rh thermocouples, which are placed circa 3 mm below the fuel element. In the past it was observed that degradation of the thermocouples can lead to systematic deviations between the indicated and real furnace temperature with time (Seeger, 2016). To ensure correct temperature adjustment, a fresh set of W/Rh thermocouples were installed in preparation of the HTR-PM test campaign, and a series of melt wire tests was performed using the same furnace set-up and tantalum parts as later for the heating tests. The results of the tests indicated that the real temperature, even for the new set-up, was between 10 and 30 °C lower than the nominal temperature for the relevant temperature range. In order to guarantee that the required peak temperature was reached during the test, the programmed temperature (nominal) was always set 30 °C higher than the targeted temperature (real). In addition, the furnace power was monitored continuously during the tests, and no derivations from the expected values were found during the test campaign, indicating that no deterioration of the thermocouple occurred.
Fig. 2. Cold Finger Apparatus (KüFA).
Fig. 2. KüFA is the German abbreviation for Cold Finger Apparatus. It is an experimental apparatus within which spherical HTR fuel elements can be heated to a temperature close to 1800 °C in a flowing helium atmosphere at ambient pressure, while the release of fission gases and volatile fission products are measured. The original KüFA device was conceived and taken into operation at KFA Jülich in 1984 (Schenk et al., 1988). After the end of the German HTR programme the KüFA in Jülich was decommissioned, and parts of the KüFA as well as drawings and technical documents were transferred to JRC Karlsruhe, in the frame of the HTR-F European collaborative research project. The new KüFA was taken into operation at JRC in 2005 (Freis, 2010). A detailed description of the device and operating procedures can be found in references (Freis, 2010; Fries et al., 2011; Kostecka et al., 2004; Schenk et al., 1988). In the following paragraphs, its main features will be summarized briefly for the convenience of the reader. The device is designed to examine experimentally the effects of Depressurization and LOss of Forced Circulation (D-LOFC) accident scenarios on irradiated spherical HTR fuel elements. While remaining under an inert helium atmosphere, a HTR fuel element is subjected to heating schedules according to the predicted accident progression for several hundred hours. Fission gas release is monitored online during the experiment. Volatile fission products are collected on water-cooled (less than100 °C) condensation plates made of stainless steel positioned above the fuel sample on a “cold finger”. These plates can be extracted from the cell for analysis of the material deposited on them by means of gamma spectroscopy to provide information on the fission product release as a function of time and temperature. The main components of the furnace are a bell-shaped outer stainless-steel vessel, a gas conduction cylinder encompassed by a heating element, and an extractable cold finger at the top. The heater is mounted in a water-cooled copper block and surrounded by heat shields. In order to minimize the absorption of fission products in the furnace, the KüFA was designed as a metallic resistive furnace. A highcurrent transformer installed next to the hot cell delivers the necessary current. All high-temperature parts of the KüFA are manufactured from tantalum because this material is the least prone to carbonization at the contact point with the fuel element (Schenk et al., 1988). The fuel element is placed on a three-point holder on the axis of the gas conduction cylinder, about a third of the way up the length of the heating element. A thermocouple measures the temperature about 3 mm below the fuel element and enables the regulation of the furnace power. The furnace temperature as well as all operating processes and parameters are centrally controlled by a programmable logic controller and an associated computer program. All operating parameters are
4. KüFA heating test HTR-PM 1 The first KüFA test was performed on fuel element HTR-PM 1, which was irradiated in the HFR Petten from 8th of September 2012 to 30th of December 2014, amassing a total of 355 Equivalent Full Power Days (EFPD) during which time it reached a burn-up of 11.64 %FIMA (see Table 3). The most relevant data of fuel element HTR-PM 1 are summarized in Table 4 below. The KüFA heating experiment was divided into three individual heating tests, denoted hereafter as test phases, at 1620 °C for 150 h each. This block wise procedure was chosen in order to fit the experiment into the working schedule of the hot cell facility, e.g. to avoid continuous furnace operation during several weeks and long weekends. Before each test phase the furnace was heated to 300 °C for 3 h, followed by heating plateau at 1050 °C for 10 h, after which the temperature was increased slowly to the target temperature with a heating rate of 47.5 °C/h. The maximum temperature was held for 150 h before the furnace was cooled down slowly in a controlled way to room 3
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Table 4 Main data of HTR-PM 1. Burn up
11.64 %FIMA
Thermal fluence (0.625 eV > tf* > 0) Fast fluence (> 0.1 MeV) Irradiation time End of irradiation Irradiation temperature
2.68 ∙ 1021 cm−2** 4.72 ∙ 1021 cm−2** 355 EFPD 30.12.2014 ~1023 °C
* neutron energy, ** (Knol et al., 2018) corrected for burn-up.
Fig. 4. HTR-PM 1 heating program (red) and fractional release of Ag-110 m (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. HTR-PM 1 heating program (red) and fractional release of Cs-137 (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
temperature within 24 h. The heating conditions are shown in Fig. 3. The release data are provided in detail in Table 5 and displayed in Figs. 3–5 together with the determined uncertainty band (Std. Dev.). Cs137 release is observed in the first phase at 1620 °C with a fractional release corresponding to about 1.63·10−7. By the end of the second phase, the fractional release reaches about 5.78·10−7, while at the end of the third phase, the fractional release reaches 1.97·10−5. Given that there are 12,000 coated particles in the spherical fuel element, even at the end of the third phase the release corresponds to about one fifth of the inventory of a single coated particle, while after the first phase, it corresponds to about two thousandths thereof. Concerning silver, the data in Table 5 and Fig. 4 show that the fractional release of Ag-110 m was about 1.16·10−5 during the first 150 h heating phase, and rose to 1.37·10−3 in phases two and three. The fractional release of Kr-85 was below the detection limit in phases one and two, but at the beginning of phase three Kr-85 (equivalent to about 2.02·10−5) was detected immediately during the initial heat-up to 300 °C, which is more likely due to an artefact of the circuit than a true release.
Fig. 5. HTR-PM 1 Kr-85 fractional release plus uncertainty. Table 6 Main data of HTR-PM 4.
Table 5 Fractional release (FR) data of HTR-PM 1. Phase-2 (1620 °C)
Phase-3 (1620 °C)
Cs-134 Cs-137 Ag-110m Kr-85
1.63E−07 1.63E−07 1.16E−05 bdl*
5.78E−07 5.78E−07 9.46E−04 bdl
1.97E−05 1.97E−05 1.37E−03 2.02E-05
Thermal fluence (0.625 eV > tf > 0) Fast fluence (> 0.1 MeV) Irradiation time End of irradiation Irradiation temperature
2.76 ∙ 1021 cm−2 * 4.88 ∙ 1021 cm−2 * 355 EFPD 30.12.2014 ~1017 °C
Before each test phase the furnace was heated to 300 °C for 3 h, followed by a heating plateau at 1050 °C for 10 h, after which the temperature was slowly increased to the target temperature with a heating rate of 47.5 °C/h. The maximum temperature was held for 150 h before the furnace was slowly cooled down in a controlled way to room temperature within 24 h. All release data are provided in Table 7. The Cs-137 release was about 4.42·10−6 in the first phase but increasing the temperature to 1650 °C and 1700 °C in phases 2 and 3 resulted in fractional releases of 1.24·10−4 and 1.69·10−2, respectively. Ag-110 m fractional release increased from 2.95·10−5 to 1.27·10−3 and 3.21·10−2 during the heating phases at 1620 °C, 1650 °C and 1700 °C, respectively. At all three
The second KüFA test was performed on fuel element HTR-PM 4 which reached a burn-up of 11.89 %FIMA (see Table 3). The most relevant data of fuel element HTR-PM 4 are summarized in Table 6. The heating experiment was divided into three individual heating tests, called test phases, at 1620 °C, 1650 °C and 1700 °C for 150 h each.
Phase-1 (1620 °C)
11.89 %FIMA
* (Knol et al., 2018) corrected for determined burn-up.
5. KüFA heating test HTR-PM 4
Isotope
Burn up
Table 7 Fractional release (FR) data of HTR-PM 4.
* below detection limit. 4
Isotope
Phase-1 (1620 °C)
Phase-2 (1650 °C)
Phase-3 (1700 °C)
Cs-134 Cs-137 Ag-110m Kr-85
4.42E−06 4.42E−06 2.95E−05 bdl
1.24E−04 1.27E−04 1.27E−03 bdl
1.69E−02 1.82E−02 3.21E−02 bdl
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Table 8 Main data of HTR-PM 2. Burn up
12.53 %FIMA
Thermal fluence (0.625 eV > tf > 0) Fast fluence (> 0.1 MeV) Irradiation time End of irradiation Irradiation temperature
2.95 ∙ 1021 cm−2 * 5.20 ∙ 1021 cm−2 * 355 efpd 30.12.2014 ~1040 °C
* (Knol et al., 2018) corrected for determined burn-up. Table 9 Fractional release (FR) data of HTR-PM 2. Fig. 6. HTR-PM 4 heating program (red) and fractional release of Cs-137 (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Isotope
Phase-1 (1620 °C)
Phase-2 (1700 °C)
Phase-3 (1770 °C)
Cs-134 Cs-137 Ag-110m Kr-85
2.23E−05 2.23E−05 6.23E−05 bdl
1.13E−03 1.10E−03 4.13E−03 bdl
5.24E−02 5.13E−02 2.23E−01 2.85E−05
All release data are provided in Table 9. The Cs-137 release was about 2.23·10−5 in the first phase, but increasing the temperature to 1700 °C and 1770 °C in Phases 2 and 3 resulted in fractional releases of 1.13·10−3 and 5.24·10−2, respectively. Ag-110 m fractional release increased from 6.23·10−5 to 4.13·10−3 and 2.23·10−1 during the heating phases at 1620 °C, 1700 °C and 1770 °C, respectively. During the 1620 °C and 1700 °C phases, no Kr-85 release was detected, whereas during the 1770 °C phase the Kr-85 fractional release reached 2.85·10−5. The primary role of the Kr-85 measurement is the identification of coated particle failure. There are about 12,000 coated particles in one spherical fuel element. The Kr-85 fractional release of 2.85·10−5 corresponds to about one third of the inventory of a single particle (Figs. 9–11).
Fig. 7. HTR-PM 4 heating program (red) and fractional release of Ag-110 m (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
7. Conclusions The burn-up of 7 spherical HTR fuel elements produced by INET have been determined using γ spectroscopy. The results are within experimental error of those derived based on neutronic calculations of the in pile irradiation conditions. Depressurisation and Loss of Forced Circulation (D-LOFC) incidents were simulated in the KÜFA device located in the JRC Hot Cells at Karlsruhe. After the actual accident simulation at 1620 °C for 150 h, the fuel elements were heated additionally two times for 150 h each, partly at significantly elevated temperatures. Typical fractional releases of Cs-137 after the first test phase at 1620 °C for 150 h were less than 2.5·10–5 for all tested fuel elements,
Fig. 8. HTR-PM 4 Kr-85 fractional release plus uncertainty.
temperatures no Kr-85 release was detected (Figs. 6–8). 6. KüFA heating test HTR-PM 2 The third KüFA test was performed on fuel element HTR-PM 2, which reached a burn-up of 12.53 %FIMA (see Table 3). The most relevant data of fuel element HTR-PM 2 are summarized in Table 8 below. The heating experiment was divided into three individual heating tests, called test phases, at 1620 °C, 1700 °C and 1770 °C for 150 h each. Before each test phase the furnace was heated to 300 °C for 3 h, followed by a heating plateau at 1050 °C for 10 h, after which the temperature was slowly increased to the target temperature with a heating rate of 47.5 °C/h. The maximum temperature was held for 150 h before the furnace was slowly cooled down in a controlled way to room temperature within 24 h.
Fig. 9. HTR-PM 2 heating program (red) and fractional release of Cs-134 (blue) and Cs-137 (grey). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 5
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two heating phases for all fuel elements. In case of HTR-PM 1 and HTRPM 2 low amounts of Kr-85 (equivalent to a fractional release of 2.02·10−5 and 2.85·10−5, respectively) were detected, either directly at the beginning of phase 3 before heat-up (HTR-PM 1) or during phase 3 after circa 40 h at 1770 °C (HTR-PM 2). No sudden release resulting in a sharp Kr increase (as would be typical for pressure vessel failure of a whole particle) was observed. Acknowledgements The authors would like to thank China S&T major project (No. ZX06901) for the financial support provided. References
Fig. 10. HTR-PM 2 heating program (red) and fractional release of Ag-110 m (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Freis, D., 2010. Thesis: 'Störfallsimulationen und Nachbestrahlungsuntersuchungen an kugelförmigen Brennelementen für Hochtemperatur-reaktoren' (Accident simulation of spherical fuel elements for high temperature reactors). PhD. RWTH Aachen. Freis, D., et al., 2011. Accident testing of high-temperature reactor fuel elements from the HFR-EU1bis irradiation. Nucl. Eng. Des. 241 (8), 2813–2821. Knol, S., et al., 2018. HTR-PM fuel pebble irradiation qualification in the high flux reactor in Petten. Nucl. Eng. Des. 329, 82–88. Kostecka, H., Ejton, J., de Weerd, W., Toscano, E.H. “Post-Irradiation Testing of HTR-Fuel Elements under Accident Conditions. Part 2”, Conference Contribution, 2nd International Topical Meeting on High Temperature Reactor Technology Beijing, China, 2004. Laurie, M., et al., 2012. Results of the HFR-EU1 fuel irradiation of INET and AVR pebbles in the HFR Petten. Nucl. Eng. Des. 251, 117–123. Schenk, W., Pietzer, D., Nabielek, H., 1988, “Fission Product Release Profiles from Spherical HTR Fuel Elements at Accident Temperatures”, Jülich Report 2234. Seeger, O., et al., 2016. KüFA safety testing of HTR fuel pebbles irradiated in the High Flux Reactor in Petten. Nucl. Eng. Des. 306, 59–68. Stradal, K.A., 1970. Apparatur zur Messung der Freisetzung von Spaltgasen und anderer Spaltprodukte durch Ausheizen. Jülich Report 707-RW, Research Center Jülich, Germany. Tang, Chunhe, et al., 2012. Comparison of two irradiation testing results of HTR-10 fuel spheres. Nucl. Eng. Des. 251, 453–458. G.U.M. Workbench, 2003. The software tool for the expression of uncertainty in measurement. Metrodata GmbH D-79639. Xiangwen, Zhou, et al., 2013. Preparation of spherical fuel elements for HTR-PM in INET. Nucl. Eng. Des. 263, 456–461. Zhang, Zuoyi, et al. “The Shandong Shidao Bay 200 MWe high-temperature gas-cooled reactor pebble-bed module (HTR-PM) demonstration power plant: an engineering and technological innovation.” Engineering 2.1 (2016): 112–118.
Fig. 11. HTR-PM 2 Kr-85 fractional release plus uncertainty.
and up to 5.2·10−2 at the end of all test phases where very extreme heating conditions were applied (i.e. after three times heating for 150 h at 1620 °C, 1700 °C and 1770 °C). The release of Kr-85 was below the detection limits during the first
6