Nine years under construction, the Grohnde nuclear power plant in Lower Saxony, Germany, finally came online in 1985 and quickly set about breaking records. In six of the next dozen years, the single pressurised water reactor produced more net electricity than any other nuclear facility in the world.

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Grohnde recently passed a new milestone, achieving 350 billion kWh of energy output during its 32-year operational life, making it the world's most productive commercial nuclear plant. With just one reactor on a single square mile of land, it is the most productive power plant of its size, full stop.

Given its stellar performance and the fact that it provides ten billion kWh of baseload capacity annually, the decision to shutdown Grohnde decades ahead of its useful life – as part of the phased closure of Germany's nuclear plants by 2022 – has perplexed many energy industry analysts and reignited the heated debate over the economic viability of Chancellor Andrea Merkel's anti-nuclear policy shift.

A report by the German federal network agency and grid authority Bundesnetzagentur (BNetzA) cited grid stability as the major concern, along with generation and transmission capacity, as the country continues its transition to renewables, such as solar and wind, known as Energiewende.

"Germany has made unusually big mistakes… abolishing nuclear power so quickly is crazy."

"The historically singular simultaneous shutdown of power plants amounting to 5,000MW capacity and the long-term lack of some 8,500MW capacity brings the transmission grids to the edge of their resilience," stated BNetzA after the 2011 Fukushima disaster. "Consequently, there are many hours in which secure network operation is impossible, meaning that it is vulnerable to a single failure."

A special report published in November 2015 by The Economist pointed out that French households pay roughly half as much as German ones for electricity, and questioned the speed of Germany's clean energy revolution. "Germany has made unusually big mistakes. Handing out enormous long-term subsidies to solar farms was unwise − abolishing nuclear power so quickly is crazy," It states.

Given these criticisms, is the decision to shutdown Grohnde premature and what lessons can the energy industry in Germany and beyond learn from the long-term success of this nuclear facility?

A pressurised water reactor

Grohnde's single Siemens/KWU reactor utilises both enriched uranium and MOX fuel, and produces in the region of 1,400MW of electric power, around 40% higher than the average nuclear plant.

The facility uses a pressurised water system whereby heat released by nuclear fission is absorbed by water within the primary circuit, which is pressurised to prevent the water from boiling. The primary cooling circuit transfers the heat generated in the main reactor to steam generators connected to the water-steam or secondary circuit. The steam generators form a barrier between the primary and secondary circuit, thereby preventing radioactive material from escaping from the primary circuit.

According to owner E.On Kernkraft , Grohnde has achieved an average annual output of just below 11TWh over its 32-year operational life and topped the global annual output table for eight years during that period. The environmental stats are also impressive; Grohnde has, to date, saved around 350 million tonnes of CO2 emissions, equivalent to twice the annual CO2 from Germany's entire road traffic.

In addition to operational efficiency and a minimal environmental footprint, Grohnde is also home to interim storage facilities for the fuel assemblies irradiated at the site. Decommissioned fuel elements are kept in the plant's water-based fuel cooling station before they are placed in final storage. After approximately five years, they are packaged into hermetically sealed containers for interim storage.

From baseload to grid stability: safety and the switch to load-following

Grohnde's efficiency and reliability over three decades can be attributed to more than technological prowess, however. As E.On Kernkraft management board member Erwin Fischer states, a robust safety and maintenance regime at the site in the district of Hamelin-Pyrmont has also helped the power plant punch above its weight in the worldwide energy market for more than three decades.

"Good results in terms of performance, a highly motivated and qualified labour force, problem-free operation and continuous improvement in occupational safety are our contributions to ensuring a secure and reliable electricity supply in Germany," he said of the record-breaking facility in March.

"Grohnde's efficiency and reliability over three decades can be attributed to more than technological prowess."

Grohnde's enviable safety record and availability can be attributed to a combination of passive and active features. Passive safety barriers encapsulate the radioactive material contained in the reactor core, providing a reliable shield to the surroundings, and include everything from gastight, pressure-resistant casings for fuel assemblies to the nuclear reactor building's reinforced concrete outer shell.

The NPP's active safety features multiple redundancies, operate independently of each other and are physically separated. The reactor protection systems monitor and compare all the plant's key operating parameters and trigger automatic safety measures if threshold values are exceeded.

Thanks to high fixed and low variable costs, nuclear power plants are primarily viewed as baseload sources of electricity. Grohnde churns out baseload power 90% of the time, but in the future will lead the way in supporting Germany's transition to renewables by load-following, the term given to power plants that provide grid stability by adjusting their output as demand for electricity fluctuates.

"While the plant was used exclusively in baseload [around the clock operation] in the past, it is now used up to 600 hours each month to regulate load," E.ON Kernkraft said in March, adding that it has increased the capability of the reactor four-fold, to ramp up or down to 40MW per minute.

The reduced baseload operation of the single reactor was also apparent in Grohnde's 2015 output statistic, which remained below 10TWh, slightly below its annual average, despite 92.4% availability.

Bridge to the future: German's nuclear legacy

Until March 2011, Germany obtained a quarter of its electricity from 17 nuclear reactors and before they are decommissioned for good by 2022, Germany's remaining eight nuclear power plants have a key role to play in providing power and supplementing the nation's developing renewables industry.

According to the World Nuclear Association, the cost of replacing nuclear power with renewables is estimated to be in the region of €1,000bn, with an increasing reliance on coal, particularly lignite.

In a recent Forbes editorial, geochemist James Conca claims that replacing Grohnde will take 3,000 wind turbines and cost over $12bn, not including the back-up capacity likely to come from brown coal.

"Shutdown of eight nuclear power plants has already led to postponements of scheduled service and maintenance in the transmission grid, because much of it can only be done when there is little or no load," Conca writes.

"Coupled with policies supporting the expansion of renewable power, which erode the viability of conventional generation, the entire generating system in northern Europe is reeling under an uncertain and fabricated future," he adds.

With public opinion in Germany remaining broadly opposed to nuclear power, and with little support for building new nuclear plants, Grohnde's days are numbered. In the interim, however, the public will continue to rely on the world's most productive nuclear power plant and its contemporaries.