Utilization of high temperature heat from nuclear reactor using inorganic chemical heat pump

A new chemical heat pump designed to utilize high-temperature heat generated from high-temperature gas reactor (HTGR) is discussed. The calcium oxide / lead oxide / carbon dioxide reaction system was found to be a suitable reaction system for the desired heat pump from experimental survey of inorganic oxide / carbon dioxide reaction systems. The proposed heat pump using the reaction system was expected to be heat storage and heat transform system for HTGR. To demonstrate the validity of the heat pump, equilibrium relationship and kinetics of the reaction system was studied experimentally. The practical operation conditions of the heat pump were revealed from the experiment. This system was available to store heat above 800°C and transform it to higher temperature under a thermal driving condition. The heat output of the heat pump was valued enough compared to a common system. The applied system of the heat pump combined with HTGR was proposed to show the way of effective utilization of HTGR.     

Carbon dioxide zero-emission hydrogen system based on nuclear power

The possibility of a hydrogen production system for Fuel Cell (FC) vehicles, which was zero carbon dioxide emission based on nuclear power, was investigated. The reactivity of calcium oxide to carbon dioxide was used for the carbon dioxide fixation and also for heat source of fuel reforming in experimental discussion. Methane was chosen as the first candidate reactant for steam reforming. Simultaneous reaction of methane reforming and carbon dioxide fixation by calcium oxide was demonstrated in a reactor packed with a reforming catalyst and calcium oxide. High-purity hydrogen, of which the concentration was higher than one at reaction equilibrium of conventional reforming, was generated from the reactor under mild operation conditions at temperature of 500–600°C and under pressure of 101 MPa. The efficiency of the fuel reforming system was estimated from the experimental results. The proposed system was expected to be applicable as a hydrogen carrier system in carbon dioxide zero-emission FC vehicles based on nuclear power.     

Medium temperature carbon dioxide gas turbine reactor

A carbon dioxide (CO₂) gas turbine reactor with a partial pre-cooling cycle attains comparable cycle efficiencies of 45.8% at medium temperature of 650 °C and pressure of 7 MPa with a typical helium (He) gas turbine reactor of GT-MHR (47.7%) at high temperature of 850 °C. This higher efficiency is ascribed to: reduced compression work around the critical point of CO₂; and consideration of variation in CO₂ specific heat at constant pressure, C_p, with pressure and temperature into cycle configuration. Lowering temperature to 650 °C provides flexibility in choosing materials and eases maintenance through the lower diffusion leak rate of fission products from coated particle fuel by about two orders of magnitude. At medium temperature of 650 °C, less expensive corrosion resistant materials such as type 316 stainless steel are applicable and their performance in CO₂ have been proven during extensive operation in AGRs. In the previous study, the CO₂ cycle gas turbomachinery weight was estimated to be about one-fifth compared with He cycles. The proposed medium temperature CO₂ gas turbine reactor is expected to be an alternative solution to current high-temperature He gas turbine reactors.     

Thermal response of plasma sprayed tungsten coating to high heat flux

In order to investigate the thermal response of tungsten coating on carbon and copper substrates by vacuum plasma spray (VPS) or inert gas plasma spray (IPS), annealing and cyclic heat load experiments of these coatings were conducted. It is indicated that the multi-layered tungsten and rhenium interface of VPS-W/CFC failed to act as a diffusion barrier at elevated temperature and tungsten carbides were developed after 1 h incubation time when annealing temperature was higher than 1600 °C. IPS-W/Cu and W/C without an intermediate bonding layer were failed by the detachment of the tungsten coating at 900 and 1200 °C annealing for several hours, respectively. Cyclic heat load of electron beam with 35 MW/m² and 3-s pulse duration indicated that IPS-W/Cu samples failed with local detachment of the tungsten coating within 200 cycles and IPS-W/C showed local cracks by 300 cycles, but VPS-W/CFC withstood 1000 cycles without visible damages. However, crack creation and propagation in VPS-W/CFC were also observed under higher heat load.     

Thermal conductivity of neptunium dioxide

The thermal diffusivity of neptunium dioxide was measured in the temperature range from 473 to 1473 K by using a laser flash method. The thermal diffusivity slightly decreased with increasing temperature in the temperature range investigated. The heat capacity of NpO₂ was measured in the temperature range from 334 to 1071 K by using a drop calorimetry method. The heat capacity of NpO₂ determined in this study was slightly larger than that of UO₂ and about 7% smaller than that of PuO₂. The thermal conductivity of NpO₂ was determined from the thermal diffusivity, the heat capacity and the bulk density. It was found that the thermal conductivity of NpO₂ from 873 to 1473 K lay between those of UO₂ and PuO₂.     

Low exergy reactor for decentralized energy utilization

A new conceptual reactor named ‘low exergy nuclear reactor (LER)’ was proposed and its possibility was discussed qualitatively. The LER is defined by the reactor that generates low-exergy energy at a temperature near environmental temperature between about 100°C and 300 °C, which is lower than the output temperature of the energy generated by the conventional light water reactors (LWR). The necessity of the LER was discussed based on the present energy technology trend. Cogeneration systems based on the LER combined with absorption heat pump, thermoelectric device and steam turbine were proposed for decentralized energy utilization. The enthalphy balance of those decentralized energy supply systems was evaluated using practical data. The thermal efficiency of the LER system was compared with that of conventional LWR system. The LER system was expected to be suitable for a decentralized heating and cooling system.     

Study of steam, helium and supercritical CO2 turbine power generations in prototype fusion power reactor

Power generation systems such as steam turbine cycle, helium turbine cycle and supercritical CO₂ (S-CO₂) turbine cycle are examined for the prototype nuclear fusion reactor. Their achievable cycle thermal efficiencies are revealed to be 40%, 34% and 42% levels for the heat source outlet coolant temperature of 480 °C, respectively, if no other restriction is imposed. In the current technology, however, low temperature divertor heat source is included. In this actual case, the steam turbine system and the S-CO₂ turbine system were compared in the light of cycle efficiency and plant cost. The values of cycle efficiency were 37.7% and 36.4% for the steam cycle and S-CO₂ cycle, respectively. The construction cost was estimated by means of component volume. The volume became 16,590 m³ and 7240 m³ for the steam turbine system and S-CO₂ turbine system, respectively. In addition, separation of permeated tritium from the coolant is much easier in S-CO₂ than in H₂O. Therefore, the S-CO₂ turbine system is recommended to the fusion reactor system than the steam turbine system.     

高温气冷堆非纯氦气环境下高温合金碳迁移的化学动力学研究

高温气冷堆（HTGR）一回路冷却剂中的低含量杂质在HTGR超高温运行时会对堆内高温合金产生严重的腐蚀。其中高温合金与非纯氦气之间的碳迁移对于材料性能影响很大。本文探究了高温合金在非纯氦气环境中碳迁移的化学动力学原理,并基于此获得了判断材料脱碳和渗碳的理论判据。根据化学热力学和化学动力学原理,对非纯氦气环境的氧分压和碳活度进行了计算,指明CH₄与H₂O的分压比值较高时可能导致合金严重渗碳。在此基础上介绍了一种应用广泛的碳迁移模型——“铬的稳定相图”,分析了铬活度计算方法并归纳出取值建议。本文根据铬的稳定相图计算得到了清华大学设计的10 MW高温气冷堆（HTR-10）的实际运行工况下的腐蚀行为。      

模块式高温气冷堆非能动余热排出系统分析与研究

非能动的余热排出系统是高温气冷堆固有安全性的重要体现之一。本文介绍了模块式高温气冷堆余热排出系统热工水力计算方法,并给出了不同工况、不同环境温度下余热排出系统的运行参数,为余热排出系统的设计和运行提供了参考。对事故工况下舱室混凝土温度分布进行了数值分析,结果表明混凝土最高温度低于安全限值。     

Feasibility study on the applicability of a diffusion-welded compact intermediate heat exchanger to next-generation high temperature gas-cooled reactor

The development of an intermediate heat exchanger (IHX) transferring high temperature heat to a process heat application is of prime importance for a next-generation high temperature gas-cooled reactor (HTGR). The IHX needs high structural integrity and reliability over 900°C for a long duration. A plate fin type compact heat exchanger (PFCHX) has a large heat transfer area per heat exchanger volume and is expected to be used as the IHX. However, the brazing for connecting fins and plate is not reliable when existing PFCHXs are used in a high temperature condition for a long time. We have proposed a concavo-convex plate type compact heat exchanger (CPCHX) which consists of concavo-convex plates (CPs) welded by solid state diffusion and made of nickel-based superalloy Hastelloy XR. In our study, first, an optimized condition for the solid state diffusion welding between the CPs of the CPCHX was found by experiments using test pieces made of Hastelloy XR. Second, small-scale diffusion-welded CPCHXs were designed, manufactured and installed in a test loop to investigate the reliability of the diffusion welding. As a result of leakage tests, it was confirmed that the reliability of the solid state diffusion welding between the CPs of the small-scale CPCHX is sufficient. A thermal performance test revealed that the thermal conductance of the small-scale CPCHX was better than calculated. In addition, a design study for the CPCHX was performed to investigate the feasibility of the diffusion-welded CPCHX to the IHX in a next-generation HTGR.     

Size dependent hcp-to-fcc transition temperature in Co nanoclusters obtained by ion implantation in silica

In this work we present in situ investigations on the increase of the hcp-to-fcc transition temperature for Co with respect to the bulk value (420 °C) when nanoclusters are considered. Starting from Co:SiO₂ composites obtained by ion implantation with average Co cluster size of about 5 nm, a transition temperature between 800 °C and 900 °C is found upon thermal annealing in vacuum by in situ transmission electron microscopy. Preliminary results on electron irradiation to promote the transition at lower temperatures are presented.     

Core concept of a passive-safety fast reactor “METAL-KAMADO” and reactivity coefficients

New concept of a passive-safety simple fast reactor “METAL-KAMADO” with metallic fuels is presented, which has same concept as a passive-safety thermal reactor “KAMADO”. A fuel element of the “METAL-KAMADO” consists of metallic fuel (U–10%Zr) and cooling holes of He gas flow. These fuel elements are located in a reactor water pool of atmospheric pressure (0.1 MPa) and low temperature (<60 °C). In case of LOF, decay heats of fuel elements are removed by natural heat transfer from surfaces of the fuel elements to the reactor water pool.

Preliminary neutronic calculations of the “METAL-KAMADO” show possibility of high burn-up of more than 120 GWd/t with 10% enriched U–Zr fuel. Reactivity coefficients of the core are also discussed.     

高温气冷堆用碳毡材料导热系数测量及反问题计算

碳纤维材料已成为核能、航天等领域不可或缺的重要功能材料,在高温气冷堆及其相关实验中需要使用大量碳纤维保温材料。但由于目前测试方法的限制,相关材料物性参数测量数据严重不足,尤其是缺乏高温1000℃以上的热物性参数,致其使用受到限制。为此,清华大学核能与新能源技术研究院研制了模拟高温气冷堆温度、环境氛围的材料测试装置,可提供1600℃以下的材料性能测试。根据该装置一次典型实验过程的测量数据,详细介绍了采用非线性导热反问题方法确定材料温度相关导热系数的完整过程和具体算法。提出了一种依据稳态、非稳态热传导原理求解反问题的简明算法,该方法既可单独使用,也可为其他反问题算法提供良好的迭代初值。实验确定了高温气冷堆用碳毡保温材料在1600℃以下的导热系数,将为高温气冷堆相关实验和其他特高温条件下的应用提供重要参考。     

Effect of annealing on the optical absorption of Ni nanoparticles in MgO single crystals

Ni⁺ ion implantation with an energy of 64 keV in MgO single crystals was conducted at room temperature up to a fluence of 1 × 10¹⁷ ion/cm². The as-implanted crystals were annealed isochronally at temperatures up to 900 °C. Optical absorption spectroscopy, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) have been utilized to characterize the changes of optical properties and the microstructure of the annealed samples. XPS results showed that the charge state of implanted Ni was still mainly in metallic Ni⁰ after annealing at 900 °C. TEM analysis revealed metallic Ni nanoparticles with depth-dependant dimensions of 1–10 nm in the annealed sample. Optical absorption spectroscopy indicated that the Ni nanoparticles exhibited a broad surface plasmon resonance absorption band in annealed samples and the band shifted to a longer wavelength with the increasing annealing temperature.     

Thermal conductivity degradation of ceramic materials due to low temperature, low dose neutron irradiation

The thermal conductivity degradation due to low-temperature neutron irradiation is studied and quantified in terms of thermal resistance terms. Neutron irradiation is assumed to have no effect on umklapp scattering. A theoretical model is presented to quantify the relative phonon-scattering effectiveness of the three dominant defect types produced by neutron irradiation: point defects, dislocation loops and voids. Several commercial ceramics have been irradiated with fission reactor fast neutrons at low temperatures to produce defects. Materials include silicon carbide, sapphire, polycrystalline alumina, aluminum nitride, silicon nitride, beryllium oxide, and a carbon fiber composite. The neutron dose corresponded to 0.001 and 0.01 displacements per atom (dpa) for a 60 °C irradiation and 0.01 and 0.1 dpa for a 300 °C irradiation. Substantial thermal conductivity degradation occurred in all of the materials except BeO following irradiation at 60 °C to a dose of only 0.001 dpa. The data are discussed in terms of the effective increase in thermal resistance caused by the different irradiation conditions. Evidence for significant point defect mobility during irradiation at 60 and 300 °C was obtained for all of the ceramics. The thermal stability of the radiation defects was investigated by isochronal annealing up to 1050 °C.     

高温气冷堆螺旋管式直流蒸汽发生器热工水力学

高温气冷堆蒸汽发生器具有一次侧氦气工质、二次侧直流、螺旋管结构、工作温度高等特点,其热工水力特性与传统压水堆自然循环蒸汽发生器存在很大区别。针对高温气冷堆蒸汽发生器的特点,对其基础热工水力及特有热工水力学问题进行了阐述,主要包括螺旋管内单相及两相流阻及换热计算、横掠螺旋管束流阻及换热计算、温度均匀性及两相流不稳定性等。同时介绍了清华大学核能与新能源技术研究院针对高温气冷堆蒸汽发生器热工设计、温度均匀性及两相流不稳定性等热工水力学问题所开发的一维稳态程序、一维瞬态程序、二维分析程序和方法,并对分析结果和结论进行了讨论。相关研究方法、程序和结论对其他相似参数螺旋管和直管式直流蒸汽发生器具有参考和借鉴意义。     

Enthalpies of formation of U-, Th-, Ce-brannerite: implications for plutonium immobilization

Brannerite, ideally MTi₂O₆, (M=actinides, lanthanides and Ca) occurs in titanate-based ceramics proposed for the immobilization of plutonium. Standard enthalpies of formation, ΔH⁰_f at 298 K, for three brannerite compositions (kJ/mol): CeTi₂O₆ (−2948.8 ± 4.3), U_0.97Ti_2.03O₆ (−2977.9 ± 3.5) and ThTi₂O₆ (−3096.5 ± 4.3) were determined by high temperature oxide melt drop solution calorimetry at 975 K using 3Na₂O · 4MoO₃ solvent. The enthalpies of formation were also calculated from an oxide phase assemblage (ΔH⁰_f-ox at 298 K): MO₂ + 2TiO₂=MTi₂O₆. Only UTi₂O₆ is energetically stable with respect to an oxide assemblage: U_0.97Ti_2.03O₆ (ΔH⁰_f-ox=−7.7±2.8 kJ/mol). Both CeTi₂O₆ and ThTi₂O₆ are higher in enthalpy with respect to their oxide assemblages with (ΔH⁰_f-ox=+29.4±3.6 kJ/mol) and (ΔH⁰_f-ox=+19.4±1.6 kJ/mol) respectively. Thus, Ce- and Th-brannerite are entropy stabilized and are thermodynamically stable only at high temperature.     

Research and development on nuclear hydrogen production using HTGR at JAERI

JAERI has been conducting R&D on HTGR and on hydrogen production using HTGR. The reactor technology has been developed using HTTR installed at Oarai site of JAERI. HTTR reached its full power operation of 30MW in 2001 and demonstrated reactor outlet helium temperature of 950°C in April 2004. As for the hydrogen production technology, the thermo-chemical IS process is under study. The process control method for continuous hydrogen production has been examined using a bench-scale apparatus. Also, studies are underway on process improvement and on materials of construction to be used in the corrosive environment. As for the system integration of HTGR and the hydrogen production plant, R&D is underway aiming to develop technologies for safe and economical connection. It covers safety technology against explosion, safety technology against radioactive materials release, control technology to prevent the thermal disturbance from hydrogen production plant to reactor, etc.     

XPS and optical studies of Xe-implanted and annealed YSZ single crystals

Xe⁺ ion implantation with 200 keV was completed at room temperature up to a fluence of 1 × 10¹⁷ ion/cm² in yttria-stabilized zirconia (YSZ) single crystals. Optical absorption and X-ray photoelectron spectroscopy (XPS) were used to characterize the changes of optical properties and charge state in the as-implanted and annealed crystals. A broad absorption band centered at 522 or 497 nm was observed in the optical absorption spectra of samples implanted with fluences of 1 × 10¹⁶ ion/cm² and 1 × 10¹⁷ ion/cm², respectively. These two absorption bands both disappeared due to recombination of color centers after annealing at 250 °C. XPS measurements showed two Gaussian components of O_1s spectrum assigned to Zr–O and Y–O, respectively, in YSZ single crystals. After ion implantation, these two peaks merged into a single peak with the increasing etching depth. However, this single peak split into two Gaussian components again after annealing at 250 °C. The concentration of Xe decreased drastically after annealing at 900 °C. And the XPS measurement barely detected the Xe. There was no change in the photoluminescence of YSZ single crystals with a fluence of 1 × 10¹⁷ ion/cm² after annealing up to 900 °C.     

Thermal and hydraulic tests of test sections in the helium engineering demonstration loop

The helium engineering demonstration loop (HENDEL) has been constructed and operated to test the large-scale components of the high temperature engineering test reactor (HTTR) under simulated reactor operating conditions. The fuel stack test section (T₁) of HENDEL simulates the fuel stack of the HTTR core and is used to investigate thermal and hydraulic performance. Hot tests with 1000°C helium gas have been conducted using simulated fuel rods having uniform, exponential and cosine axial heat flux distributions. The test results agreed with previously proposed correlations, although the simulated fuel rods had various heat flux distributions and high heat flux rates.

The in-core structure test section (T₂) also was installed in the HENDEL to verify the performance of the core bottom structure of the HTTR. The tests show that good performance was obtained. Examination of the thermal mixing characteristics indicated that mixing started at the location where the hot helium gas flowed into the hot plenum and that complete mixing was achieved during the downward flow in the outlet hot gas duct. The seal performance testing indicated no change of the leakage flow rate after 4000 hours of operation. The temperature of the metal portion of the structure was below 500°C and uniform around circumferential cross-sections due to the good performance of the thermal insulation blocks.