South African power utility Eskom has reported “a major milestone” towards completion of planned steam generator replacements at unit 1 of the Koeberg NPP. The first of the ageing steam generators has been removed from the containment building and placed in the specially-built storage building.
This is part of the programme to extend the operating life of the two-unit NPP by 20 years. Replacement of the steam generators at unit 1 had originally been planned for the first half of 2021, with those of unit 2 to be replaced between January and May 2022. However, this was delayed mainly because of concerns about possible power shortages while the units are offline.
The current licence expires in 2024-2025 and in 2021 Eskom applied to South Africa's National Nuclear Regulator to extend the operating licence. Eskom submitted the safety case for long-term operation in support of the application in July 2022. The regulator has two years to conclude the review and provide an outcome. No safety concerns have been identified that would preclude long term operation.
The two 930 MWe (net) pressurised water reactors at Koeberg, built by Framatome, began commercial operation in 1984 and 1985. They generate about 5% of the South Africa’s electricity. In 2014, Eskom signed a ZAR4.4bn ($240m) contract with Areva (now Orano) to design, manufacture and install the replacement steam generators. They were made in China under subcontract by Shanghai Electric Power Equipment Company.
Eskom said removal of the first steam generator was a significant accomplishment for the Koeberg team, the contractor and the numerous local and international subcontractors involved in the project. “It is a great relief to have reached this milestone as the steam generator replacement project has experienced numerous false starts in previous outages and some unexpected challenges during the execution in the current outage to get to this point in the project,” Eskom noted.
“The logistics of moving the steam generators from their installed position (vertical), out of containment (horizontal at an elevation of 20 metres) to placing them on a flatbed transporter (horizontal) can only be appreciated if one understands the size and weight.”
Each steam generator is 22 metres in height with a diameter of 4.5 metres (top half) and 3.5 metres (bottom half) and weighs more than 320 tonnes.
The next step is to remove the other two steam generators and install three new ones. Eskom says this will involve the following sequence of activities:
- Rigging the remaining two steam generators out from their positions inside containment.
- Rigging the three new steam generators into their exact position inside the containment building.
- Performing the six critical welds (two for each steam generator) that joins the steam generators to the primary system piping.
- Performing radiography on the welds to ensure they meet the code requirements.
- Re-installing all the access platforms that enable people to work all the way up the steam generators (seven stories of permanent structures and temporary scaffolding).
- Reinstalling all the other piping connections (steam pipe at the top, feedwater pipe, and all the other smaller sampling and instrumentation connections) once access is available. In all cases meeting the required level of quality control.
- Installing new thermal insulation over the whole steam generator surface and all the pipes that were worked on (around 120 tonnes was removed).
- Removing all the temporary equipment that was required to be installed to allow the work to be done safely (scaffolding - 85 tons; lead shielding - 70 tonnes; electrical supplies - 22 distribution boards and 1 km of cabling.
After this it will be necessary to complete the maintenance activities scheduled for the outage, commission all the systems, refuel the reactor, and return the unit to service. “Due to the delays that have already been experienced, the original return to service date for the unit is no longer achievable,” Eskom notes. “Although every effort is being made to reduce the impact, we are currently running a few weeks late. The Generation production plan is being optimised to minimise as far as possible the impact of the projected delay on the system.”
Unit 2 will continue to operate safely while unit 1 is in this extended outage. It will then undergo a similar long outage to replace its three steam generators starting later this year. This is expected to last 180-200 days.
“In accordance with the safety analysis that was performed and submitted to the National Nuclear Regulator in support of the application to extend the plant life by 20 years, the steam generators are the last large component replacements that are needed to ensure Koeberg can operate safely for the requested additional period of operation.”
Image: The steam generator being manoeuvred into the specially-built storage building (courtesy of Eskom)