In pictures: Final module in place at Haiyang 3

The component - measuring 26 metres in diameter, 10 metres in height and weighing about 419 tonnes - was lifted into place on top of the containment vessel and shielding building roof on 4 August, State Power Investment Corporation subsidiary Shandong Nuclear Power Group announced. The hoisting of the module took one hour and 52 minutes to complete.

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The CB-20 module will store more than 3,000 tonnes of water, which can be used to help cool the reactor in an emergency. The water can also be directed into the reactor's used fuel pool, while the tank itself can be refilled from water stored elsewhere on site.

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The tank is part of the plant's passive safety systems which require no operator actions to mitigate potential emergency situations, using natural forces such as gravity, natural circulation and compressed gas to achieve their safety function. In conjunction with other passive safety features, the CB-20 module can maintain unit safety for 72 hours without requiring human intervention.

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The construction of two CAP1000 reactors - the Chinese version of the Westinghouse AP1000 - at each of the Haiyang, Sanmen and Lufeng nuclear power plant sites in China was approved by the country's State Council on 20 April 2022. The approvals were for Haiyang 3 and 4, Sanmen units 3 and 4 and units 5 and 6 of the Lufeng plant. The Sanmen and Haiyang plants are already home to two AP1000 units each.

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Unit 1 of the Haiyang plant entered commercial operation in October 2018, with unit 2 following in January 2019. Together, Haiyang units 1 and 2 can provide some 20 TWh of electricity to the grid annually, sufficient to meet one-third of household demand in Shandong province.

The first safety-related concrete was poured for the nuclear island of Haiyang unit 3 in July 2022, and in March the outer steel dome of the nuclear island containment building was hoisted into place. Construction of Haiyang 4 began in April last year. The planned construction period for Haiyang 3 and 4 was 56 months, with the two units scheduled to be fully operational in 2027.

Installation of the CB-20 module marks "the full entry of unit 3 into the installation and commissioning stage", Shandong Nuclear Power Group said.