Shimpei Hamamoto

Helium Chemistry for Very High Temperature Reactors

Chemical impurity contained in the coolant helium of Very High Temperature Reactors (VHTRs) affects not only the deterioration of mechanical strength caused by decarburization of the high-temperature materials utilized at the heat transfer tubes of the Intermediate Heat Exchanger (IHX) but also the deterioration of heat transfer efficiency caused by carbon deposition, which could occur at the surface of the heat transfer tubes. Such deterioration results in the shortening of the lifetime of high-temperature equipment. Since the helium purification technology applied in past high-temperature gas-cooled reactors can only maintain the core integrity by limiting the oxidation of graphite, it is necessary to establish a control technology in order to maintain the mechanical strength as well as heat transfer efficiency of hightemperature equipment. In this study, carbon deposition that could occur at the surface of the heat transfer tubes of theIHX and decarburization of Hastelloy XR used at the heat transfer tubes were evaluated by referring to the actual chemistry data obtained by the HTTR. Also, the chemical composition to be maintained during a reactor operation was proposed by evaluating not only the core graphite oxidation but also carbon deposition and decarburization. It was identified that when the chemical composition could not be maintained adequately, the injection of 10 ppm carbon monoxide could effectively control the chemical composition towards the designated area at the chromium stability diagram where the stable oxidized layer would be generated sufficiently. The proposed chemistry control technology is expected to contribute to the economy of the purification systems of future VHTRs.

Active Chemistry Control for Coolant Helium Applying High-Temperature Gas-Cooled Reactors

Lifetime extension of high-temperature equipment such as the intermediate heat exchanger of high-temperature gas-cooled reactors (HTGRs) is important from the economical point of view. Since the replacing cost will cause the increasing of the running cost, it is important to reduce replacing times of the high-cost primary equipment during assumed reactor lifetime. In the past, helium chemistry has been controlled by the passive chemistry control technology in which chemical impurity in the coolant helium is removed as low concentration as possible, as does Japan’s HTTR. Although the lifetime of high-temperature equipment almost depends upon the chemistry conditions in the coolant helium, it is necessary to establish an active chemistry control technology to maintain adequate chemical conditions. In this study, carbon deposition which could occur at the surface of the heat transfer tubes of the intermediate heat exchanger and decarburization of the high-temperature material of Hastelloy XR used at the heat transfer tubes were evaluated by referring the actual chemistry data obtained by the HTTR. The chemical equilibrium study contributed to clarify the algorism of the chemistry behaviours to be controlled. The created algorism is planned to be added to the instrumentation system of the helium purification systems. In addition, the chemical composition to be maintained during the reactor operation was proposed by evaluating not only core graphite oxidation but also carbon deposition and decarburization. It was identified when the chemical composition could not keep adequately, injection of 10 ppm carbon monoxide could effectively control the chemical composition to the designated stable area where the high-temperature materials could keep their structural integrity beyond the assumed duration. The proposed active chemistry control technology is expected to contribute economically to the purification systems of the future very high-temperature reactors.© 2008 ASME

Feasibility study of large-scale production of iodine-125 at the high temperature engineering test reactor

The feasibility of a large-scale iodine-125 production from natural xenon gas at high-temperature gas-cooled reactors (HTGRs) was investigated. A high-temperature engineering test reactor (HTTR), which is located in Japan at Oarai-machi Research and Development Center, was used as a reference HTGR reactor in this study. First, a computer code based on a Runge-Kutta method was developed to calculate the quantities of isotopes arising from the neutron irradiation of natural xenon gas target. This code was verified with a good agreement with a reference result. Next, optimization of irradiation planning was carried out. As results, with 4 days of irradiation and 8 days of decay, the 125I production could be maximized and the 126I contamination was within an acceptable level. The preliminary design of irradiation channels at the HTTR was also optimized. The case with 3 irradiation channels and 20-cm diameter was determined as the optimal design, which could produce approximately 1.8 × 105GBq/y of 125I production.

Helium leak and chemical impurities control technology in HTTR

Japan Atomic Energy Agency (JAEA) has designed and developed high-temperature gas-cooled reactor (HTGR) hydrogen cogeneration system named gas turbine high-temperature reactor (GTHTR300C) as a commercial HTGR. Helium gas is used as the primary coolant in HTGR. Helium gas is easy to leak, and the primary helium leakage should be controlled tightly from the viewpoint of preventing the release of radioactive materials to the environment. Moreover from the viewpoint of preventing the oxidization of graphite and metallic material, the helium coolant chemistry should be controlled tightly. The primary helium leakage and the helium coolant chemistry during the operation is the major factor in the HTGR for commercialization of HTGR system. This paper shows the design concept and the obtained operational experience on the primary helium leakage control and primary helium impurity control in the high-temperature engineering test reactor (HTTR) of JAEA. Moreover, the future plan to obtain operational experience of these controls for commercialization of HTGR system is shown.

Establishment of floating support technology applied to high-temperature components and piping in HTTR

In the primary cooling system of the High Temperature Engineering Test Reactor (HTTR) with an outlet coolant temperature of 950°C, high-temperature components and piping such as an intermediate heat exchanger and coaxial double piping reach very high temperature, and large and complex thermal displacements arise in them. In order not only to absorb the thermal displacements but also to withstand earthquakes, the HTTR has adopted a new three-dimensional floating support system. In the limited space of the containment vessel, the support system can support the components’ and pipings own weights and follow the thermal displacements and have seismic capacity. On the other hand, the adoption of the support system was unprecedented in nuclear plants. Thus, the effectiveness of the support system was demonstrated through the HTTR operation. In this paper, by using the HTTR operation data, the thermal displacement behavior of the high-temperature components and piping is investigated, and the behavior and characteristics are simulated numerically. In addition, the aftermath of the Great East Japan Earthquake on the HTTR is confirmed. As a result, the effectiveness of the three-dimensional floating support system adopted by the HTTR is verified.

Investigation on dust captured by quintuple filters installed upstream of primary gas circulators in HTTR

A main objective to install filters upstream of primary gas circulators in the high temperature engineering test reactor (HTTR), besides having a primary helium purification system, is the reduction and removal of circulating dust in the primary circuit. A problem encountered with the filters during the initial operations of the HTTR was that the differential pressure across the filters had increased excessively over the duration of the operations so that the differential pressure would be expected to exceed the limit value regulated in the HTTR operation manual. It was speculated that either the carbon traced back chemical reactions, the debris from mechanical contacts or both of these sources might be captured by the filters. Then, the filters were replaced and inspected to identify the cause of the increase of the filter differential pressure. As a result, it was found that the increase is caused by clogging of the filters by the dust traced back to the physical contact of the piston rings of the gas circulators equipped in the primary helium purification system. Hence, prismatic block-type very high temperature reactors (VHTRs) do not continuously supply carbon dust from the core during its operation.

Dust Generation and Transport Behavior in the Primary Circuit of HTTR

Dust is to be limited in the primary circuit of the high temperature gas-cooled reactors (HTGRs) with regard to the reactor operational stability and structural integrity of the heat exchanger because the dust in the coolant adheres to the heat transfer pipe surface, and it is lowered with the performance of the heat exchanger. Furthermore, the dust, including the fission products (FPs) which adhered to the piping, must be reduced in order to be discharged in the depressurization accident with coolant helium.In High Temperature Engineering Test Reactor (HTTR), the rise of filter differential pressure of primary helium gas circulator (HGC) showed that the dust was arising in the coolant. The increase of differential pressure at the filter of the HGC for the primary pressurized water cooler (PPWC) in HTTR was observed from the beginning of the HGC operation. The differential pressure rose with the increase in operating time of the HGC, and the filter had to be exchanged the filter approximately in about every 8400 hours.Therefore, the source of the dust should be investigated using The Scanning Electron Microscope (SEM) and The X-ray Fluorescence (XRF) analyzer. SEM was used to observe the appearance of the captured dust in detail and to identify its shape and its size. This result clearly indicated that the dusts mainly composed of carbon. XRF was used to estimate the sources of the carbonaceous dust particles. It was clarified that the source of dust is the slide member of the compressor which is not of a primary cooling system.© 2012 ASME