An individual water molecule passes through the process in fifteen minutes
Did you know that it takes an individual water molecule about fifteen minutes to travel through the process at Olkiluoto plant unit 1 or 2? The water molecule experiences speed, different pressures as well as several variations in temperature and state as it rides on this enclosed rollercoaster.
The total water volume circulating in the process of a nuclear power plant is one million one hundred and fifteen thousand litres, or one thousand 115 cubic metres. About 20 cubic metres of new water is added to the process every day, as part of the process water is lost in sampling and clean-up sequences. This means than an individual water molecule stays in the system for an average of 56 days, repeating its fifteen-minute cycles through the process.
Journey starts from the condenser
The adventure of the water molecule starts from the condenser, where the temperature is a comfortable +30 degrees Celsius and speed close to zero.
From the condenser, the journey continues to the condensate pumps and both speed and temperature start to rise. The water molecule passes through the condensate clean-up filters to three low-pressure preheaters. The temperature is in each preheater increased by 30 degrees, i.e. to a total of +120 degrees. The pressure is 22 bar.
The heated water molecule next hits the feedwater pumps, where conditions are even more uncomfortable. The pressure of the water is there increased to 82 bar to make it higher than pressure in the reactor.
The condensate is now converted into feedwater. The water molecule travels on to the feedwater preheaters where the temperature rises to 183-185 degrees. After the preheaters, the heated and pressurised water rushes along the feedwater pipeline to the reactor. At this point the flow rate of the water is 1 250 kg per second.
The toughest part of the cycle in the reactor core
The toughest part of the water molecule’s journey starts now, with the recirculation pumps pumping the water through the reactor core, between the fuel rods. Water becomes steam at an over-pressure of ca. 70 bar.
This is the culmination of the journey for the little molecule. The temperature and pressure are both at the maximum value. Sometimes this can also be the end of the journey for the water molecule. It can burst into atoms i.e. dissociate into hydrogen and oxygen.
Next the journey continues through the steam separator and the steam drier into the main steam pipes which carry the water molecule to the high-pressure turbine. The speed is now 50 metres per second.
Miracle of power generation
This is where the miracle of power generation takes place. A vast amount of the minuscule water molecules collides with the turbine blades. If we imagine that the water molecule can feel pain, this is where that happens. The collision makes the turbine blades move. They rotate the eight-metre long shaft and the generator starts to generate electricity. Mass is power.
The water molecule converted into steam rushes from the high-pressure turbine through the moisture separators to the reheaters, where the steam is dried and temperature increased to 240 degrees. This converts the wet steam into superheated steam, which is led to four low-pressure turbines. The shafts of these turbines are six metres long.
Right after the turbine blades, the speed of the water molecule reaches its highest value: more than 200 metres per second.
The water molecule converted into steam returns from the low-pressure turbines to the condenser, where it is condensed back into water with seawater. As the water molecule descends on the surface of the condenser tubes, its speed has fallen to almost zero.
The cycle is complete and the next one starts.
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