Experimental research on the radioactive dust in the primary loop of HTR-10
Graphical abstract
This is the first time, that the solid fission products, including I-124, I-131, Cs-137, Ba-140, La-140, Eu-152 and Hf-181, and the solid activation products, including Cr-51, Mn-54, Fe-59, Co-57, Co-58, Co-60, Se-75 and Hg-203, including short lived nuclides and long lived nuclides, have been determined experimentally in the primary loop of HTR-10.
Introduction
With the development of the high temperature gas-cooled reactors (HTGRs), the very high temperature reactor (VHTR) has been considered as a candidate for the six proposed Generation IV concept reactors by the Generation IV International Forum (GIF) (NERAC and GIF, 2002). With the use of fuel elements embedded with tristructural-isotropic (TRISO) coated particles, the inherent safety performance of the HTGRs has attracted wide attention. The high thermal efficiency and the high output temperature which can be used for hydrogen production and process heat, bring large commercial space for the future development of HTGRs (Zhang and Yu, 2002). There are, however, several safety issues that have been identified previously through operation of research reactors. These include the local high temperature in the core leading to the failure of TRISO coated particles, the release of the radioactive dust in the depressurization accident, the contamination due to the radioactive dust in the primary loop, etc. (Bäumer et al., 1990, Moormann, 2008a, Moormann, 2008b). It has been noted also that dust related problems are more important in the pebble bed type reactors than in the prismatic type reactors (Humrickhouse, 2011).
In the pebble-bed HTGRs, the dust is thought to be generated by several mechanisms, including the abrasion among fuel elements, and friction between fuel elements and other graphite structures or pipelines when the fuel elements cycled (Luo, 2004, Moormann, 2008b, Kissane, 2009). Several papers have been published regarding the behavior of dust in HTGRs, including the generation, characterization, transport, coagulation, aggregation, deposition, resuspension, etc. Humrickhouse (2011) summarized the dust safety issues in HTGRs and indicated that the dust related research was urgently needed for the development of pebble bed HTGRs on topics such as the dust distribution under normal operation, dust generation, dust-fission product interaction, etc. Cogliati et al. (2011) surveyed the available literature on graphite dust production and measurements in pebble bed reactors and concluded that there was significant uncertainty on the severity of the dust production and its consequences in pebble bed reactors. Troy et al., 2012, Troy et al., 2015 and Shen et al. (2016) investigated the characteristics of graphite dust particles experimentally. Simones et al., 2011, Simones and Loyalka, 2015 measured the charge-size distributions of graphite aerosol particles and coagulation of charged aerosols pertinent to HTGRs. Moormann, 2008a, Moormann, 2008b, Kissane (2009), and Lind et al. (2010) discussed the transport of the radioactive dust which can be an important source term in pebble bed reactors. Gutti and Loyalka (2009), Boddu et al., 2011, Barth et al., 2013, and Peng et al. (2016) experimentally and/or numerically investigated the deposition behavior of dust in HTGRs. Kazuhiro et al., 1992, Stempniewicz et al., 2008, and Kissane et al. (2012) studied the resuspension behavior of dust in HTGRs.
There have been, however, very few experimental investigations regarding the radioactive dust directly in pebble bed reactors. The only available data on such dust are from Arbeitsgemeinschaft Versuchsreaktor (AVR) and Thorium Hochtemperatur Reaktor (THTR) (Bäumer et al., 1990, Moormann, 2008b, Fachinger et al., 2008). There is a need to verify conclusions from theoretical calculations and non-radioactive dust experiments against data from the radioactive dust experiments in actual reactors. Recently, several experiments related to the source terms of the 10 MW high temperature gas-cooled reactor (HTR-10) have been conducted. These include investigations of the radioactive dust, H-3, and C-14 in the primary loop, post irradiation graphite spheres from the reactor core, etc. (Xie et al., 2015a, Xie et al., 2015b, Xu et al., 2017, Wang et al., 2014, Liu et al., 2017, Li et al., 2017). Previous measurements also indicated the existence of radioactive dust in the primary loop of HTR-10 (Xie et al., 2013, Xie et al., 2015a, Xie et al., 2015b). Accordingly, an experimental sampling loop has been designed and built in the helium purification system of HTR-10.
In the present study, the radioactive dust in the primary loop of HTR-10 was investigated with use of the above sampling loop when HTR-10 was restarted in 2015. The radioactive dust has been collected with a sampling filter. The concentration of the dust in the primary loop was estimated with use of the coolant flow data. The types of solid fission and activation products which were absorbed or present in the dust were determined with a γ spectrometer (GC3018, High-purity Germanium Detectors, from CANBERRA Industries Inc.) in the radiochemistry lab in the Institute of Nuclear and New Energy Technology (INET), Tsinghua University. The counting rates of typical nuclides, including Co-60, Cs-137, I-131, etc., were measured and compared with each other. The particle size distributions of the dust from the filter elements were obtained through imaging with an optical microscope. The data reported can shed light on the behavior of fission/activation products and radioactive dust in HTGRs, and would aid in modeling and safety studies.
Section snippets
Experimental setup
The HTR-10 is a helium cooled, graphite-moderated, and thermal neutron spectrum test reactor, which was designed and built by INET, Tsinghua University, China (Wu et al., 2002). It reached its criticality in December 2000, ran up to full power operation in January 2003, and was shut down in July 2007. It was restarted at the end of November 2014 (Wei et al., 2016).
During the shutdown stage of HTR-10, an experimental sampling loop in the primary system aiming to study the behavior of radioactive
Results
After a long term shutdown from 2007 to 2014, HTR-10 was restarted at the end of November 2014, and was operated in a power stage in March 2015. During June 1st, 2015, to August 6th, 2015, two sampling experiments about the radioactive dust in the primary loop were conducted. The operational thermal power of HTR-10 was about 2.9 MW compared to the rated thermal power of 10 MW. The primary helium temperature at the reactor inlet and outlet were respectively, ∼185 °C and ∼575 °C. The primary helium
Discussion
As stated earlier there have been very few experimental data available about the radioactive dust in the pebble bed reactors. Bäumer et al. (1990) reported that the average concentration of dust in the primary loop of AVR was 5 μg/m3STP, but it could be as high as 2000 μg/m3STP at the startup or shutdown stage of the reactor. In HTR-10, this was the first attempt to investigate the radioactive dust in the primary loop. In the first sampling experiment, though only the dust in the first filter
Conclusion
A loop for sampling the radioactive dust in the primary loop of HTR-10 has been designed, built, and used. For the first time, since the start of the HTR-10, radioactive dust was collected and characterized in two successful experiments. The types of solid fission and activation products present in the dust have been determined, including short lived nuclides, Co-58, I-131, etc. and long lived nuclides, Co-60, Cs-137, etc. The interaction between fission or activation products and dust
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 11575099), the Beijing Natural Science Foundation (No. 2163051), the Chinese National Significant Science and Technology Project (No. ZX06901), and the Tsinghua University Initiative Scientific Research Program (No. 20151080375). Fruitful discussions with Prof. Dazhi Xue, Prof. Meisheng Yao, Prof. Fu Li, Mr. Liqiang Wei and Mr. Ling Liu at INET, Tsinghua University, Beijing, China, and Dr. Terttaliisa Lind at Paul
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