Designs heat up in Asia

JAPAN’S 30MWT HIGH TEMPERATURE ENGINEERING Test Reactor (HTTR), in Oarai, Ibaraki Prefecture is generally viewed as the best performing high temperature gas- cooled reactor (HTGR) in the world in terms of both stability and safety. Japan is also the first country to establish a commercial fuel fabrication facility for HTGR fuel.

HTTR, a small prototype graphite-moderated helium gas- cooled reactor, was constructed by the Japan Atomic Energy Research Institute (JAERI) – now the Japan Atomic Energy Agency (JAEA) – to develop HTGR technology and nuclear heat utilisation technology as well as for the irradiation of materials under high temperatures.

JAERI began research and development of HTGRs in 1969 and construction of the reactor building began in March 1991. All the major components inside the containment vessel, including the reactor pressure vessel and primary cooling system, were installed by March 1996.

It achieved first criticality in November 1998 and full power of 30MWt and a reactor outlet coolant temperature of 850°C were achieved in December 2001. Rated power operation and safety demonstration tests started in fiscal year 2002. A reactor outlet coolant temperature of 950°C was achieved in April 2004 – a world first.

From January to March 2010, HTTR was operated successfully for 50 days under high temperature and full power conditions. However, in February 2011 it was taken offline for planned inspections and it has remained shut down since then, pending safety upgrades in the wake of the Fukushima Daiichi accident in March 2011.

Revised nuclear safety regulations were announced by Japan’s Nuclear Regulation Authority (NRA) in July 2013, which must be met before reactors can be approved to restart. JAEA applied to the NRA in November 2014 for inspections to verify whether measures taken at the HTTR met the new safety standards. In June 2020, NRA said changes to the reactor were in conformity with requirements. NRA also confirmed that no fuel damage would occur even in the event of a ‘beyond design basis’ accident, such as multiple losses of reactor shutdown functions.

JAEA is now developing an international safety standard based on the safety characteristics of the HTTR, which it believes is important for HTGR commercialisation and will contribute to strengthening international competitiveness of Japan’s HTGR technology.

Once HTTR restarts, planned activities include: safety demonstration tests for the OECD/NEA Loss of Forces Cooling project, which aims to simulate an accident scenario with reduced or even no availability of the cooling systems; discussion on technology demonstration tests of heat utilisation systems consisting of helium gas turbine and hydrogen production facilities; fuel performance tests; and international cooperation and human-resource development utilising the HTTR.

The 950°C heat produced by the HTTR is expected to have a broad range of uses, including hydrogen production, power generation, and the desalination of seawater. JAEA plans to construct a hydrogen production system linked to the HTTR.

Mitsubishi Heavy Industries (MHI), which was involved in the design and construction of the HTTR and has developed software technologies to verify the performance and integrity of the reactor core, is now developing hydrogen production technology for HTGRs.

TRISO fuel development in Japan

HTGRs, including the HTTR, use tri-isotropic (Triso)-coated fuel particles with diameter of around 1mm. Triso fuel consists of a micro spherical kernel of oxide or oxycarbide fuel with coating layers of porous pyrolytic carbon (buffer), inner dense pyrolytic carbon, silicon carbide (SiC) and outer dense pyrolytic carbon. The principal function of these coatings is to retain fission products within the particle. The SiC coating layer acts as a barrier against the diffusive release of metallic fission products and provides mechanical strength.

Japan does not use the pebble bed design for its HTGR fuel but instead places the coated fuel particles into fuel assemblies.

JAEA began developing HTGR fuel fabrication technologies in the 1960s with the collaboration of Nuclear Fuel Industries Ltd (NFI). NFI has fabricated many campaigns of irradiation test samples for the Japan Material Testing Reactor (JMTR), as well as the first and second loading of fuels for the HTTR.

Japan is now working on advanced HTGR fuel and new SiC Triso fuel for extended burnup has been fabricated based on the HTTR fuel technologies.

One option being studied is replacing the SiC layer with a zirconium carbide (ZrC) layer. JAEA has performed ZrC coating tests on surrogate oxide fuel kernels, to investigate the influence of coating parameters and material properties such as stoichiometry and density. These surrogate particles have been irradiated at the High Flux Isotope Reactor (HFIR) in Oak Ridge National Laboratory in the USA in order to clarify the fundamental irradiation response of ZrC.

Fuel fabrication started at NFI’s HTR fuel facility in 1995. Some 4770 fuel rods were fabricated involving the fuel kernel, coated fuel particle and a fuel compaction process, and 150 fuel elements were assembled in the reactor building in December 1997.

In May 2019, US-based X-energy signed a memorandum of understanding with NFI of Japan establishing a partnership for the supply of Triso fuel fabrication equipment. X-energy and NFI are planning to transfer commercial-scale equipment from NFI’s Tokai facility to X-energy’s Oak Ridge Triso fuel fabrication facility.

In July 2020, X-energy teamed with NFI to be the exclusive counterparty to supply fuel to Japan’s HTGR.

China: Progress with HTR-PM

China’s pebble bed-type reactors are based on German technology, and differ considerably from Japan’s block-type HTGRs. There are no fuel assemblies and the spherical fuel particles (made from a mixture of uranium particles and graphite powder) are dropped directly into the reactor core, one after another.

China began developing HTGR technology in the 1970s, mainly at the Institute of Nuclear and New Energy Technology (INET) of Tsinghua University in Beijing. INET undertook fundamental research into reactor design and fuel fabrication. In the late 1980s, a national R&D programme designed, constructed, commissioned and operated a 10MWt high temperature helium-cooled test reactor (HTR-10), which achieved first criticality in December 2000 and was connected to the grid in January 2003. Between April 2003 and September 2006, the inherent safety features of the modular HTGR were proven in four experiments that imposed extreme scenarios on the HTR- 10 without counter-measures, supervised by the Chinese National Nuclear Safety Administration (NNSA).

During this period China also began manufacturing the spherical fuel elements, and developed technologies for fuel handling and transport, helium process technologies, domestic manufacture of key equipment for HTGRs and fully digital reactor-protection systems. Based on the HTR-10, INET then began development of a demonstration commercial modular HTGR plant.

The high-temperature gas-cooled pebble-bed modular reactor (HTR-PM) project was launched in 2001 to demonstrate the inherent safety and economic competitiveness of HTGR technologies, including standardisation and modularisation.

The demonstration plant’s twin HTR-PM reactors will drive a single 210MWe turbine. Helium gas will be used as the primary circuit coolant with a steam generator transferring heat from helium coolant to a water/steam loop. The design temperature of the HTR-PM reaches 750°C. In January 2006, the HTR-PM became one of China’s 16 National Science and Technology ‘major projects’, given top priority and strong support. In January 2008, the State Council of China approved a plan for the HTR-PM project with a detailed R&D technology roadmap. After a construction licence was issued by the NNSA and government approvals received, first concrete was poured in December 2012. Civil work on the nuclear island was finished by June 2015. Main component installation started with the first reactor pressure vessel in March 2016, followed by the metallic internals, water cooling panels for the reactor cavity cooling systems, ceramic internals, graphite pebbles, etc. The upper head of the first RPV was installed in December 2017, marking completion of the first reactor.

CNNC said in March 2020 that the “pairing of the key nodes” of the second reactor had been completed. The pressure vessel, steam generator and hot gas duct were “connected in the form of a flange to form a primary circuit system for the thermal energy transmission of the reactor, which constitutes a second barrier to prevent the leakage of radioactive materials.”

As a commercial-scale demonstration power plant, HTR- PM requires a full scope simulator to train and certify the operators, and this was delivered in December 2015.

A further 18 such HTR-PM units are proposed at Shidaowan.

Beyond HTR-PM, China proposes a scaled-up version called HTR-PM600, with one 650MWe turbine driven by six HTR-PM reactor units. Feasibility studies on HTR-PM600 deployment are under way for Sanmen (Zhejiang province), Ruijin (Jiangxi), Xiapu and Wan’an (Fujian) and Bai’an (Guangdong).

China Huaneng is the lead organisation in the consortium building the demonstration units together with CNNC subsidiary China Nuclear Engineering Corporation (CNEC) and INET, which is the research and development leader. Chinergy, a joint venture of INET and CNEC, is the main contractor for the nuclear island.

Fuel development for HTR-PM

Fuel for the HTR-PM comprises thousands of six-centimetre graphite spheres containing uranium enriched to 8.9% uranium-235. Instead of cooling water, the reactor’s graphite core is bathed in helium gas with an outlet temperature of up to 750°C.

In 2005, a prototyping fuel-production facility was constructed at INET with an annual capacity of 100,000 fuel elements. In 2013 construction began on an HTGR fuel-production factory in Baotou, Inner Mongolia. Commissioning and trial production began at the plant in 2015, and in July 2017, China began mass production of the fuel. The production line, with capacity to produce 300,000 spherical fuel elements annually, has already delivered the 200,000th spherical fuel element, marking the transition from test production to industrial production.

Each graphite sphere is 60mm in diameter and weighs about 0.192kg. Every fuel element contains 7g of heavy metal. The enriched uranium kernels — about 0.5mm in diameter — are coated with three layers of pyro-carbon and one layer of silicon carbon. The coated fuel particles are dispersed in a graphite matrix which is 5cm in diameter. Surrounding the fuel-containing graphite matrix is a 5mm thick graphite layer. Each HTR-PM reactor cavity will be filled with a total of 245,318 fuel elements, to a depth of over 11m.

An irradiation test of five fuel spheres for the HTR-PM started in October 2012 in the High Flux Reactor in Petten, Netherlands, and was completed in December 2014. INET requires qualification of its fuel to support licensing of the HTR-PM reactor systems.

South Korea looks at VHTRs

South Korea has a programme for large-scale hydrogen production using very high temperature reactors (VHTRs) in the 2020s as part of its long term energy policy, and an operating licence for a Nuclear Hydrogen Development and Demonstration (NHDD) is planned.

The NHDD is a 200MWt VHTR reactor concept, operating at 950°C and using a sulphur-iodine thermo-chemical cycle. Current R&D efforts focus on developing design tools, assessing high temperature anti-corrosion materials and components, and developing a small-scale gas loop, a bench-scale Triso fuel fabrication facility, a bench-scale sulphur-iodine (SI) thermo-chemical cycle closed loop and high pressure sections of the SI thermo-chemical process.

In 2004, the Korea Atomic Energy Research Institute (KAERI) launched a nuclear hydrogen programme in collaboration with the Korea Institute of Energy Research and the Korea Institute for Science and Technology. KAERI has R&D cooperation with JAEA in Japan, General Atomics in the USA and INET in China for collaborative HTR research. KAERI has been developing Triso-coated particle fuel technology as a part of the VHTR project, and in 2014 completed its first irradiation test of Triso fuels in its research reactor, Hanaro.

In 2006, the Korean government started a project called “Development of Key Technologies for Nuclear Hydrogen.” The main focus of the project was to:

• Develop the computer codes and the databases to support design of the VHTR.
• Develop a manufacturing and qualification process for the Triso coated fuel particles.
• Select and modify the materials for the very high temperatures and coupled thermo-chemical process.
• Verify the performance of the materials in the small gas loop to be constructed.
• Develop the SI thermo-chemical process for pressurised conditions.

Another test facility for verifying the passive cooling capability of VHTR was designed at a 1/4 scale and construction started in 2013. KAERI has also been working with Argonne National Laboratory and the University of Wisconsin to continue developing the reactor cavity cooling system. Korea aims to complete design of demonstration reactor by 2026. KEPCO E&C is developing the technology to demonstrate and commercialise the VHTR.

Korea has a high temperature and pressure small scale- gas loop capable of performance tests for laboratory-scale components and code validation.

KAERI has also submitted a VHTR design to the Generation IV International Forum with a view to using it for hydrogen production. This is envisaged as 300MWt modules operating at 900-950oC, each producing 30,000t of hydrogen per year. The engineering design was completed in 2014.

Earlier, KAERI had signed a memorandum of understanding with steel-maker Posco to conduct joint research and development on technologies VHTRs. However, it is unclear how KAERI’s plans will progress now that the Korean government has adopted a nuclear phaseout plan.

Indonesia hopes for HTRs

Indonesia has no HTGR facilities but has ambitious plans. In December 2013, on the 55th anniversary of the founding of Indonesia’s National Atomic Energy Agency (Batan) it was announced that a 30MWt experimental nuclear power reactor, Reaktor Daya Eksperimental (RDE) and a gamma irradiation facility would be built at Serpong, the site of Indonesia’s largest operating research reactor. In March 2015 Batan said the RDE would be a small fourth generation nuclear reactor, which, as well as producing electricity, could be used for desalination, hydrogen production and coal liquefaction. Batan noted that several countries were interested in becoming partners in the RDE project including Japan, China, South Africa and Russia.

In August 2014, JAEA had said it had agreed to extend a cooperation agreement with Batan to include research and development of HTGRs. In April 2015 Rusatom Overseas (part of Russian state nuclear corporation Rosatom) announced that a consortium of Russian and Indonesian companies led by NUKEM Technologies had won a contract for the preliminary design of the multi-purpose 10MWt HTR in Indonesia, which would be “a flagship project in the future of Indonesia’s nuclear programme”. The contract included a feasibility study on the conceptual design and the basic design documentation, which were completed by Russia’s OKBM Afrikantov in December 2015.

In August 2016 Batan signed a cooperation agreement with China Nuclear Engineering Corporation (CNEC) to develop HTRs in Indonesia.

Two years later, Batan signed an agreement with China’s Tsinghua University to cooperate on human resources capacity building. Batan said this was an extension of existing research cooperation between China and Indonesia and included an agreement to implement the HTGR joint laboratory programme. Tsinghua University was selected as a partner in the construction of the RDE because the university has experience in HTGRs.

Batan also signed an agreement with power utility Perusahaan Listrik Negara for a further feasibility study in August 2019. Plans envisaged a number of 100MWe units in Kalimantan, Sulawesi and other islands. Construction of the demonstration unit was expected to take four years.

All these partnerships are certain to be confirmed and strengthened in when Indonesia hosts an international thematic meeting on high-temperature reactor technology. in the second quarter of 2021.

Author information: Judith Perera, Contributing Editor, Nuclear Engineering International