The Colorado reactor was abandoned after 10 years of commercial operation because of numerous shutdowns and poor performance. It reached only about 14 per cent of its full generation capacity of 330 megawatts, compared to the US national average of about 60 per cent at that time.

HTGRs were once seen as the safest type of nuclear-based electricity generation, with features capable of avoiding fuel meltdowns and large radioactive releases.

However, the four commercial HTGRs – built in Germany and the US from the 1960s to the 1980s – all experienced problems before they were permanently shut down.

Construction of the two small reactors with single 200MW turbine in Shandong began in 2012 and was expected to take about five years to complete. It was delayed until this year because of design and manufacturing problems.

The country’s largest government-owned power generation company China Huaneng Group is the lead organisation in the consortium building the HTGR, and said in October that it aimed to “ensure the first grid-connected power generation in 2021”.

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“The target is to connect to the power grid by the end of this year,” said Wang Yingsu, former general manager of its subsidiary China Huaneng Group Nuclear Development. The company did not immediately respond to a request for comment.

Despite the long delay, the project is a breakthrough for China’s nuclear industry and represents the country’s leading role in the field, according to industry insiders.

The reactor, based on the German design, was developed domestically with Chinese intellectual property and achieved a localisation rate of 93.4 per cent for its equipment, the company has said.

China Huaneng Group chairman Shu Yinbiao said in March that China had become one of the leading countries in nuclear power, with a complete system from R&D through to construction, operation, maintenance and manufacturing.

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The company’s interest in nuclear power goes back to the early 2000s, when Beijing’s Tsinghua University designed a 10MW HTGR which was connected to the grid in 2003.

A year later, China Huaneng Group signed a cooperation agreement with the university to jointly build the demonstration project.

“There are three reasons why Huaneng wants to construct the high-temperature gas-cooled reactor. One is that the company believes clean energy is the way of our future and Tsinghua University has this technology,” Wang said.

“When I visited the International Atomic Energy Agency (IAEA), they felt HTGR may be the closest to 4th-generation nuclear technology.”

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According to the Generation IV International Forum – an initiative that developed the criteria for the next generation of nuclear technology in the early 2000s – reactors and related fuel cycles need to be sustainable, with improved safety, economic and proliferation resistance, if nuclear is to make a contribution towards meeting the world’s future energy needs.

The high-temperature gas-cooled reactor is believed by its proponents to have “inherent safety features” and can avoid some of the safety challenges of earlier reactor designs.

Its low-power-density and high-heat-capacity core allows longer response times. The fuel – with graphite-coated particles – can withstand the core’s very high temperatures and, together with its passive cooling systems, should eliminate the possibility of a core meltdown.

Despite the failures of the four HTGRs of the last century, some other countries have also expressed an interest in developing the technology.

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British energy minister Greg Hands told the Nuclear 2021 conference in early December that Britain would build a high-temperature gas reactor for its advanced modular reactor demonstration programme.

And in 2016, the US Department of Energy awarded nuclear reactor and fuel design engineering company X-energy US$53 million over five years to develop an HTGR design. Two years later, the company received a second contract for US$10 million.

While HTGRs may be immune from certain kinds of failures, some nuclear experts warned this did not mean accidents were impossible.

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“People are saying that certain kinds of accidents are not possible, but it does not mean no accident is possible,” said M.V. Ramana, a professor at the University of British Columbia in Canada.

“If the question is, is there an accident that is possible which results in radioactive materials coming out of the reactor and escaping into the atmosphere, the answer is yes.”

In a paper published by peer-reviewed journal Joule in 2018, researchers from Germany and the US advised caution as the Chinese high-temperature gas-cooled reactors moved closer to becoming operational.

Lead author of the paper Rainer Moormann, a Germany-based nuclear safety researcher, said there was no reason for any kind of panic, “but nuclear technology has risks in any case”, in an interview with Science Daily.

“A realistic understanding of those risks is essential, especially for operators, and so we urge caution and a spirit of scientific inquiry in the operation.”

For China’s reactors, the biggest challenge will come after they begin commercial operation.

Wang Yingsu, secretary general of the nuclear power branch of the China Electric Power Promotion Council, said a major task was to see if they could work at full generation capacity.

“The design can be commercialised if it can achieve that target and the next goal is to improve economic competitiveness,” he said, adding that the HTGR would never be as cheap as light water reactors.

“If you look at the US and Germany, they did not have trouble building the first reactors,” Ramana said. “They had trouble in how the reactor operated after it was built.”