Hydrogen/Power Plant Design
JAERIs hot stuff30 July 2005
At Oarai, the Japan Atomic Energy Research Institute’s High Temperature Engineering Test Reactor attained an outlet temperature of 950°C in April 2004. Heat from the reactor will be used to make hydrogen in the near future. By Nariaki Sakaba, Yukio Tachibana, Kaoru Onuki, Yoshihiro Komori and Masuro Ogawa
There can be no doubt that high-temperature reactors (HTRs) will play a dominant role in the future worldwide fleet. The promises of the HTR – inherent safety, economic viability, high efficiency, very high burn-up, wide industrial application (from electricity generation to hydrogen production) – are so overwhelming.
Twenty years ago, HTRs were mainly meant to produce electricity like LWRs, albeit with a greater level of safety, achieving a higher burn-up of fuel, higher efficiency and with reduced capital costs. Ten years ago an HTR had to be coupled – under any circumstances – to a helium turbine; anything else was considered to be ‘heretic’. Today the HTR is considered primarily to be a very safe design, but also to be the producer of hydrogen for the future.
Japan Atomic Energy Research Institute’s (JAERI’s) High Temperature Engineering Test Reactor (HTTR) at Oarai Research Establishment, has an output of just 30MWt but it is not a small project. The reactor building is 48m by 50m with two floors above the ground and three below.
Construction of the reactor building began in 1991 and first criticality was attained in 1998. Reactor-outlet coolant temperatures of 850°C were reached in 2001. After receiving an operating permit from the government, safety demonstration tests were conducted to demonstrate the HTR’s inherent safety features as well as to obtain core and plant transient data not only for commercial HTRs but also the VHTR, which is a Generation IV reactor candidate. In April 2004, the HTTR achieved its maximum reactor outlet coolant temperature of 950°C – a world first.
The reactor outlet coolant temperature of 950°C makes it possible to extend HTR use beyond the field of electric power. Also, highly effective power generation with a high-temperature gas turbine becomes possible, as does hydrogen production from water. Reaching 950°C will be a major contribution to the realisation of hydrogen production from water using HTRs.
The HTTR is the first HTR in Japan and, as a test reactor, it has following purposes:
- Establishment of basic HTR technologies.
- Demonstration of HTR safety operations and inherent safety characteristics.
- Demonstration of nuclear process heat utilisation.
- Irradiation of HTR fuel and materials under HTR core conditions.
The HTTR is a graphite-moderated, helium-cooled reactor. The fuel assemblies are hexagonal graphite blocks, 360mm across the flats and 580mm in height. JAERI chose the prismatic block type design in preference to the pebble-bed design developed in Germany because it is more suitable for scaling up to a larger plant.
Primary cooling system
The primary cooling system mainly consists of an intermediate heat exchanger (IHX), a primary pressurised water cooler (PPWC) and a primary concentric hot gas duct. Primary coolant helium from the reactor at up to 950°C flows inside the inner pipe of the primary concentric hot gas duct to the IHX and PPWC.
The HTTR has two operation modes. One is the single-loaded operation mode using only the PPWC for the primary heat exchange. Almost all the basic performance of the HTTR system is confirmed by the single-loaded operation mode. The other is the parallel-loaded operation mode using the PPWC and IHX.
It is planned to use the secondary helium loop of the IHX for nuclear process heat to achieve water-splitting using the iodine-sulphur (IS) process.
During high-temperature operation, reactor characteristics and performance were confirmed, while operations were monitored to demonstrate operational safety and stability.
The reactor power was increased step-by-step while monitoring the thermal parameters and coolant impurities. For safety, the core temperature was raised within the rate of 35°C/h up to 650°C, and 15°C/h after that. The reactor power was kept at 50%, 67%, and 100% for more than two days in a steady temperature condition in order to measure the power coefficients of the reactivity. The reactor power was also kept at 82%, at which the reactor outlet coolant temperature is a little below 800°C, in order to remove chemical impurities using a helium purification system. The calibration of the neutron instrumentation system with the thermal reactor power was performed at 97% power.
Behaviour of fuel and fission product gases
Fuel and fission product gases behaviour was monitored in order to evaluate the release behaviours of the fission product gases and to confirm that the levels of the released fission product gases were within their limits. Results were that not only were all signals less than the alarm level of 10GBq/m3, which corresponds to 0.2% of fuel failure, but all signals were less than the detection limit (1GBq/m3).
The detected fission gas nuclides in the primary coolant were 85mKr, 87Kr, 88Kr, 133Xe, 135Xe, 135mXe, and 138Xe, all of which are the same isotopes as for previous tests.
The measured release-to-birth ratio, of 88Kr was 1.0x10-8 at full power operation. This was three orders lower than the limitation of 5.35x10-4, which corresponds to 1% fuel failure.
It suggests that the measured values were within the release level by diffusion of the generated fission gas from the contaminated uranium in the fuel compact matrix, and no significant failure occurred during 950°C operation.
Chemistry control is important for the helium coolant because impurities cause oxidation of graphite used in the core and corrosion of high-temperature materials used in the heat exchanger. During high-temperature operation, chemistry behaviour was monitored to confirm that each impurity was steadily removed by the helium purification system. After the temperature rose from 850°C, the impurities of hydrogen, carbon monoxide, carbon dioxide and nitrogen increased rapidly, and small amounts of methane and oxygen were detected.
There are two reasons for the increase: one is the emission of impurities from the graphite material used in the core and as an insulator in the concentric hot gas duct. The other is the chemical equilibrium in the core.
The water vapour which was emitted from the graphite converted to hydrogen and carbon monoxide in an immediate reaction in the high-temperature conditions in the core.
Maximum fuel temperature
The maximum fuel temperature was evaluated using the measured temperature data during the 950°C operation and its value was 1478°C. This does not exceed the normal operation limit of 1495°C.
HYDROGEN PRODUCTION ACTIVITY
JAERI has been conducting R&D on hydrogen production technologies using nuclear energy to obtain large-scale, economical hydrogen production capability that is environmentally friendly. Among the hydrogen production methods that split water using heat from an HTR, the iodine-sulphur (IS) process has a significant impact in most future scenarios, although predicted thermochemical cycle efficiencies have yet to be demonstrated. The IS process, one of the thermochemical water-splitting processes, was selected by JAERI as an important research priority for future technologies.
The hydrogen production activity using the IS process is developing in four stages. The initial stage is laboratory scale. In this stage, the theory was verified with the 1 litre per hour hydrogen production in 1997. The second stage is bench scale. An automatic control system was successfully developed with a hydrogen production rate of 30 normal l/h between 1999 to 2004. The third stage is the pilot plant test, in which hydrogen will be produced at 30m3/h by 0.4MW helium heating under industrial conditions (for materials, pressure, and temperature). The fourth stage is the HTTR-IS system stage. In this stage, the actual reactor and 1000m3/h class IS process chemical plant will be connected after establishing a safety theory.
Bench scale test
In the closed-cycle IS system, maintaining a balance of reacting constituents of the three chemical reactions is essential for its continuous operation. All the chemicals except water, hydrogen, and oxygen must circulate through the process without being discharged. During the bench scale test, newly developed control methods were verified by Shinji Kubo to maintain the acid production solution composition in a stable state, which was vital to obtain the balance (see Nuclear Engineering and Design volume 233, p347).
The acids produced during acid production separate into two phases, the hydriodic acid (HI) phase, and the sulphuric acid (H2SO4) phase. These acids were periodically sampled during the test and their compositions were analysed. The hydrogen production was efficiently stable at a rate of 31 normal l/h, and the production ratio of hydrogen and oxygen was 2:1. The results indicate that the water-splitting reaction was sufficiently
stable during its operation. Kaoru Onuki and Atsuhiko Terada presented the work at the Innovative Nuclear Energy Systems 2004 and International Conference on Nuclear Engineering 13 meetings, respectively.
Pilot plant test
The pilot plant test project is planned to establish technical bases for practical plant designs using HTRs. Its actual fabrication will start in Japanese FY2007. The hydrogen will be
produced at a maximum rate of 30 normal m3/h, continuously using high-temperature helium gas supplied by a helium gas loop, with a 400kW electric heater. The pilot plant will employ an advanced hydriodic acid-processing device for efficient hydrogen production, and the usefulness of the device was confirmed from mass and heat balance analysis.
The pilot plant will feature high operating pressure, practical materials for construction, use of helium gas for heating and advanced HI processing for high thermal efficiency. The pilot plant test project started from conceptual designs, flowsheets, basic engineering, trial manufacture and simulation codes in 2004.
The main objectives of the pilot tests are to demonstrate engineering feasibilities:
- Closed loop continuous operation. Improved control systems using a predictive control method will be applied to IS process conditions (flow rates of H2SO4, HI and supplied water). Also, it is necessary to harmonise the IS process operation with that of the HTTR startup and shutdown procedure.
- Fabricability of components. Since H2SO4 and HI are corrosive fluids, almost all the components must be made of corrosion-resistant materials. The H2SO4 and SO3 decomposers will be fabricated of SiC ceramics in order to operate under high-temperature conditions. Throughout the construction and operation of the pilot plant facility, fabrication techniques will be confirmed for the next steps, HTTR-IS system and commercial systems.
A hydrogen production system based on the IS system is planned to be connected to the HTTR. This will establish the hydrogen production technology with an HTR, including the system integration technology for connection of hydrogen production systems to HTRs. It will be the world’s first demonstration of hydrogen production directly using heat supplied from an HTR.
- The HTTR-IS system aims to:
- Establish procedures on safety design and evaluation.
- Add to experience of construction, operation, and maintenance.
- Establish the control technology.
- Establish the technology on key high-temperature components such as high-temperature isolation valves.
- Verify analysis codes.
Japan’s future HTR – GTHTR300C
The newly proposed system concept of GTHTR300C for hydrogen cogeneration is based on an HTR and co-produces electricity using a direct gas-turbine cycle and hydrogen using an IS process.
The GTHTR300C employs a maximum unit power of 600MW for a passively safe HTR, with a reactor-outlet coolant temperature of 950°C.
GTHTR300C’s cogeneration ratio of hydrogen to electricity should be appropriate for the projected demands for both during 2020-2030, when the first plants are intended to be deployed.
TablesHTTR main specifications