Korean Candu restarts after refurbishment

Unit 1 at the Wolsong nuclear power plant in South Korea has been restarted following the completion of a refurbishment of the pressurized heavy water reactor. It marks the first time that a Candu-6 reactor has been successfully dismantled, retubed and restarted.

Unit 1 at the Wolsong nuclear power plant in South Korea has been restarted following the completion of a refurbishment of the pressurized heavy water reactor (PHWR). It marks the first time that a Candu-6 reactor has been successfully dismantled, retubed and restarted. 

Atomic Energy of Canada Ltd (AECL), which supplied the reactor, was contracted by Korea Hydro and Nuclear Power (KHNP) in 2006 to retube the unit and the work began in 2009. All 380 calandria tubes, which house the reactor's fuel channels and through which heavy water coolant circulates, have been removed and replaced in work which should enable the 679 MWe reactor to operate for a further 25 years.

The four Candu 6 units at Wolsong (Image: Konicof)

Each calandria tube is made of zirconium alloy and is approximately 6 metres long and 13 centimetres in diameter. Heavy water coolant is circulated between the reactor and the steam generators through the end fittings on the reactor's fuel channel assemblies. The first-of-a-kind refurbishment project has involved the development of hundreds of specialised tools and systems, said AECL.

AECL had completed the removal and replacement of calandria tubes, pressure tubes and end fittings by late 2010 and since then heavy tooling systems have been removed as well as the work platforms supporting them, under work managed jointly with KHNP. Remotely-controlled tools and massive, highly-shielded machines are required to conduct the work safely inside the reactor due to the highly radioactive environment.

Wolsong 1 was reconnected to the electrical grid on 18 July. The entire retubing outage took 839 days to complete, during which time KHNP undertook various other refurbishment activities.

Candu reactors are designed to undergo refurbishment after approximately 25 years of operation, requiring a major outage but allowing reactor life to be extended by up to 30 years. For the more modern Candu-6s, this involves complete retubing. AECL is currently also working on a similar refurbishment project at Canada's Point Lepreau, although that project has been running well over budget and behind schedule. Lessons learned from Point Lepreau have been invaluable for the refurbishment of Wolsong, however, and with the design also in operation in Argentina, Romania and China, the experience will benefit future projects.

At the Bruce A units in Canada - which features earlier, larger (769 MWe) Candu models - AECL has completed the retubing of unit 2 and received regulatory approval on 30 June 2011 to start reloading fuel into the reactor. It is expected to be reconnected to the grid by the end of the year. Refurbishment work of unit 1 is also nearing completion and that unit is expected to be reconnected to the grid in early 2012.

Researched and written

by World Nuclear News