Nuclear emergency diesels: backfitting Beznau

This year and next year should see completion of a major upgrade project of the emergency power supply system at the Beznau nuclear plant in Switzerland (2 x 365 MWe (net) Westinghouse pressurised water reactors), part of a long-running investment programme that aims to enable the plant to continue to operate for another 17 years, by which time it would be a remarkable 60 years old. Unit 1 at the site, which entered commercial operation in 1969, is already the world's oldest operating pressurised water reactor and possibly the oldest operating nuclear unit of any type.

The original motivation for the project, called AUTANOVE, was expiration of the licence of the Beznau hydro plant, which acts as one of the nuclear plant's several emergency power sources. Expiry of the licence means that the hydro plant must close for refurbishment, requiring the nuclear plant to be equipped with an alternative source of emergency power. This will consist of two diesel generator buildings, seismically qualified and flood proofed, each with two MTU 12V956TB33 engines rated at 3750 kWe each (four engines in total).

An extra complication is that the project scope also includes installation, for each unit, of an additional seismically qualified safety train for decay heat removal, a requirement of the regulator. The plant has been extensively backfitted and modernised over the years, at a cost of about 1.8 billion Swiss francs, with an estimated reduction in core damage frequency of about a factor of 100. Studies have suggested that earthquakes are now the dominant risk contributor but the actual number of seismically qualified trains at Beznau (pre AUTANOVE) is only two, dropping to one for a few days each year due to maintenance. With the additional safety train, after completion of AUTANOVE, the plant will have at least three safety trains for each envisaged hazard type (decay heat removal, loss of coolant accident, earthquake, flooding, etc).

Before the project there are six individual emergency power sources per unit for decay heat removal and three for loss of coolant accidents (two hydro, two 0.4 kV "flood" diesel generators, two 6 kV bunkerised diesel generators). Four of these sources (the "flood" diesels and the bunkerised diesels) are protected against flooding and independent of river/service water, two are fully qualified for seismic events (the bunkerised diesel generators).

After the project, the hydro sources and "flood" diesel generators are replaced with the new 6 kV diesel generators, which will be cooled by air (ie are not dependent on river or service water supply).

Each of the new AUTANOVE diesel gensets has the capacity to deliver the emergency power requirements needed to deal with a loss of coolant accident, estimated at 3380 kW. For decay heat removal duty, each of the new AUTANOVE diesel gensets has the capacity to provide the emergency power for two safety trains, in its own unit or in the other unit.

For loss of coolant accidents, each unit, after the retrofit, has three emergency power supply trains: the two new gensets of the AUTANOVE project and the existing bunkerised genset. This is the same number of trains as before the project, but using different power sources.

For decay heat removal, post backfit, there are six emergency power supply sources available per unit: the four new gensets of the AUTANOVE project, plus the two existing bunkerised diesel gensets. Again this is the same number as before the project.
In addition each unit is being provided with a new seismically qualified and spatially separated emergency bus powering a new safety train including new emergency well water pump and new reactor coolant pump seal injection pump.

The AUTANOVE project was planned well before before the Fukushima events of March 2011. As a result of Fukushima the seismic requirements were increased, resulting in a 6.5 month delay.

In terms of flooding, the design requirements already stipulated the ability to withstand an external flood of 1.65 m above plant level.

Project delivery

The AUTANOVE milestone dates can be summarised as follows:

• 2008: Initial concept
• 2009: Revision of concept (regulatory requirement). Request for proposals
• October 2010: Contract signed (with Westinghouse)
• October 2010-September 2013: Fabrication of components
• After March 2011: Project delay due to Fukushima (seismic design reappraisal)
• January 2011-November 2013: Civil construction of diesel generator buildings
• December 2012-Autumn 2014: Installation of components in diesel generator buildings
• March 2013-August 2013: Update of full-scope simulator
• August 2014-December 2014: Testing and start-up of unit 1
• March 2015-July 2015: Testing and start-up of unit 2

Overall, AUTANOVE has proved to be a particularly complex and challenging project, owing to the configuration of the new emergency power systems, the requirement to integrate new systems into an existing plant and the demanding seismic requirements. But the result will be increased safety and a plant that is well equipped for a good few more years of continued operation.

MTU and Westinghouse strengthen emergency diesel partnership

The two main suppliers on the Autanove project at Beznau, Westinghouse (main contractor) and engine maker MTU Friedrichshafen (part of Rolls Royce Power Systems AG, formerly called Tognum) have signed a five-year contract for strategic co-operation in the field of emergency diesel generators for nuclear power plants, expanding and consolidating the existing partnership.

Under the new agreement, Westinghouse will give preference to MTU for projects in Europe, the Middle East and Africa. For all its new build or repair/modernisation projects in these regions, Westinghouse will assess whether co-operation with MTU would be feasible. Westinghouse says it will act as general contractor for construction or modernisation on these projects and "call on the expertise of MTU Friedrichshafen in the field of emergency power."

Further reading

Paper by Roland Kaeser and Martin Richner of AXPO at TüV conference, Emergency Power Systems at Nuclear Power Plants, Munich, 11-12 April 2013