Since 2008, the US nuclear regulator the NRC has been examining Mitsubishi Nuclear Energy Systems’ 1700MWe US-APWR design for licence approval in the USA. The lead customer is utility Luminant’s Comanche Peak 3&4 project, which is in the running for a US Department of Energy loan guarantee.

An innovative feature of the four-loop PWR is in its balance of plant: instead of industry-standard diesel engines for emergency backup generators, it specifies what might become the first gas turbines used for backup power of a nuclear station in the USA.

Gas turbines are used in combined heat and power stations, and also for thrust in aeroplane jet engines. A rotary compressor pushes air into a pressurised combustion chamber, where fuel is ignited. The exhaust gases drive a turbine (which also drives the compressor).

According to MNES, gas turbines are simpler than diesel engines, with fewer components, easier cooling requirements—air rather than water—and simpler support systems. The gas turbine is also potentially more reliable, and more compact.

Like any turbine, they only work when the blades are spinning, so they require another engine to fire them up. However, the US-APWR’s design offers gas turbines extra time, MNES says. “At the existing plants the emergency electric source equipment that can start up within 15 seconds must be installed in order to address events caused by the loss of offsite power…In US-APWR, the adoption of an advanced accumulator tank makes it possible to prolong this startup time of emergency electric source equipment to 100 seconds.” The accumulator passively injects each reactor loop with borated water if the reactor coolant system pressure were to drop below a pre-set pressure, as in a loss-of-coolant-accident.

Because gas turbines have not been used for nuclear power station backup in the USA before, the Nuclear Regulatory Commission has requested type testing and analysis. Following hearings in 2009, MNES says it is now conducting new verification procedures based on the US regulatory requirements. MNES says it is planning to conduct a continuous load-following operation test, continuous startup test, and other necessary tests by the end of 2010.

Once type testing is finished, other utilities would in theory be able to apply for permission to retrofit diesel engines with gas turbines, NRC spokesman Scott Burnell told NEI. “If the GTGs are approved for use on the US-APWR design, the operating fleet could apply to use them,” he said. He adds: “Gas turbine technology is not new, only the application as a qualified class 1E power source for NPPs is new. Just as new diesel generator designs are required to be qualified for Class 1E service … once qualified and installed in an NPP, they [gas turbines] will be subjected to the same ongoing reliability programmes as the current operational diesel generators.”

Although gas turbines have not been used in the US nuclear industry, NEI understands that they have been installed at nuclear facilities in Canada (at Bruce), Japan (Tokai-Daini), and Europe (Flamanville in France, Olkiluoto in Finland, and Heysham, Hartlepool, Hinkley Point, Oldbury, Wylfa and Sellafield in the UK).

According to EDF, the 1300MW-class Flamanville 1&2 PWRs, which achieved criticality in the mid-1980s, have one diesel backup generator, and one gas turbine backup. (However, the 1650MW Flamanville 3 EPR now under construction will only use diesels for backup).

By coincidence, the other European site where an EPR is being built also has units featuring gas turbines for emergency back-up power. The 680MW Olkiluto 1&2 BWRs in Finland, which achieved criticality in the late 1970s, have both diesel and gas turbines. In addition to four 10kV emergency diesel generators, there are two 690V diesel generators for decay heat removal or severe accident management, and two independent 50MW power gas turbine plants, each one with generator and two 25MW gas turbines, according to Finnish regulator STUK. Olkiluoto 3 will be connected to these systems in the future.

In the UK, one of the main gas turbine manufacturers and suppliers is Rolls-Royce. In a prepared comment, it said: “Rolls-Royce has experience of providing gas turbines for stand-by power application to support nuclear power plants, with approximately 400MW installed power experience for such applications.”

Centrax of the UK has supplied three 5MW gas turbines for nuclear power backup: two turbines at the Sellafield reprocessing plant in 1988 and 2003, and one at French nuclear power plant Paluel; it also works with Rolls-Royce. The first Sellafield unit included an integral diesel hydraulic starting system; the other two relied on an external 200kW external diesel generator set for startup power.

Although gas turbines are more expensive to buy, maintenance and repair is simpler and less expensive, says Peter Ward, Centrax sales and marketing general manager. He recommends that the units are started once a month; in addition Centrax performs an annual two-day inspection.

Author Info:

Will Dalrymple is editor of NEI