Annual overhaul keeps KKB in shape

Normally there are around 500 people working on the Beznau island in the Lower Aare Valley – currently it’s about double that. Block 2 of KKB has been off the grid since 4 August for an overhaul and fuel change. It’s a routine job – but by no means a small one. ‘And it’s a bit like servicing a car,’ says Nicolai Braun, Head of KKB. 

For around six weeks, checks and maintenance work are carried out on Block 2 of Beznau Nuclear Power Plant, and new fuel elements are installed. The KKB teams collaborate closely with external specialists from Switzerland and abroad, who represent a wide range of different disciplines. They are essential to the smooth execution of this overhaul according to the start of the art – and the most stringent internal, national and international regulations.

As always, inspectors from the Swiss Association for Technical Inspections (SVTI) and the Swiss Federal Nuclear Safety Inspectorate (ENSI) are on hand as well. After the work is finished, Block 2 can only be powered up again to produce electricity once ENSI gives the green light.

That’s still some way off, and for now the site is a hive of activity. But you won’t find people madly rushing about – not even Nicolai Braun, Head of KKB. Since the beginning of the year the qualified mechanical engineer has been guiding operations on the island. He previously held a management function at the Philippsburg nuclear power plant in the German state of Baden-Württemberg. The plant was decommissioned in late 2019 in line with Germany’s nuclear phaseout. 

Up to half of winter electricity

There is a good reason the two blocks of KKB are taken off grid separately for fuel element replacement and overhaul in spring or summer – that’s when demand for electricity is lower. ‘We have to use this window to keep KKB fit, especially with a view to the winter months,’ says the KKB boss, adding: ‘In the colder months Switzerland’s nuclear power plants contribute up to half of domestic electricity production.’ 

An overhaul includes thousands of work steps and has to be meticulously planned months in advance. This year the focus is on maintenance work and routine tests. Also on the to-do list are a major overhaul of a generator as well as inspection of steam generators and the two main reactor pumps.

‘And we will also “fill up the tank”,’ adds Nicolai Braun. Out of 121 fuel elements, 20 are being replaced. The fuel elements are positioned in the reactor core according to a defined order to ensure efficient usage.

Three days for cooling

To ensure the work can begin at all, the plant first has to be powered down, cooled and emptied, step by step. Because a nuclear power station is really just a thermal power plant. Water is heated to a very high degree, it turns to steam which drives the turbines and in turn the generator, which produces electricity. The heat is generated by the nuclear fission in the fuel elements within the reactor core. The water is spun around and heated up to about 300 degrees under high pressure (approximately 150 bar).

So it’s hardly surprising that it takes around three days until the whole plant – both the reactor and the conventional parts – is sufficiently cooled, depressurised and emptied. Once that happens, the closure head of the reactor pressure vessel – which weights over 50 tonnes – is unsealed and lifted off with a hoist crane, and the upper reactor internals are removed. The fuel elements are now clearly visible in the shimmering blue water. 

Ready for replacement

Only now is everything ready for the replacement of the fuel elements – which are only moved about under water. Finally, qualified ultrasound, eddy current and X-ray technicians get to work, using high-tech cameras to meticulously scour every inch of the plant for irregularities. 

At the same time, components such as pumps, motors, valves and turbines, as well as the generator, are inspected and maintained, depending on the maintenance cycle of the individual component. There are also regular checks of the cooling and safety system, including the diesel generator for emergency power.

Operational safety and security of supply

‘An overhaul is a bit like servicing a car. We test and check the functioning, material aging and wear-and-tear of components, and replace them where necessary. But the difference is that our “servicing” has two objectives: operational safety and security of supply.’

Throughout the year, KBB supplies around 6 terawatt hours of electricity – a tenth of Switzerland’s annual electricity demand.