Posted by: Mark Foreman | November 4, 2013

Brayton cycle for a nuclear plant

Dear Reader,

As a result of an unforeseen event I have had to teach thermodynamics this year to the first years, while I cannot say that I normally experience the same ecstatic feelings when thinking about thermodynamics as I get when considering a decent bit of either organic or inorganic chemistry I saw something which interested me when I was reading a nuclear journal.

The problem is that after a SCRAM at a power reactor site the turbine will normally slow down within seconds and stop being a source of energy. Because of the decay heat of the used fuel the plant will need some power to operate pumps and other vital equipment to keep the core of a light water reactor covered with water.

While if one unit at a multireactor park like Ringhås was to shut down it would be possible to import electric power from another unit, if all the units were scrammed at the reactor park then electric power would need to be imported from elsewhere. If the power lines to the plant remained good then one simple method would be to take in energy from the grid.

Another option would be to use a diesel set to provide the power for vital services at the plant, some plants also have gas turbine sets as an alternative to running on diesel sets. What happened at Fukushima was that all the units which were running at the time of the earthquake scrammed (underwent an emergency shutdown which was commanded by a seismic sensor), but the earthquake and nasty big wave damaged the power lines, washed away the diesel fuel tank and soaked important electrical systems with water.

Now back to thermodynamics, a key thing in thermodynamics is heat engines which operate using the difference between a hot and a cold thing. One idea is to put a heat engine into the reactor which will use the steam from the SCRAMed reactor to heat up carbon dioxide.

The idea is to run a compressor using a turbine, the compressed gas is then heated using the steam from the reactor. The heated gas then expands through a turbine which shares a shaft with the compressor. The hot gas from the exit of the turbine then is cooled outside the reactor building before it is returned to the compressor.

The great thing is that the turbine generates more mechanical work than the compressor consumes, the difference could be used to run a generator or some other useful device. While the heat to electricity efficiency is poor this device is designed more to cool the steam coming from the core. The cooled steam will condense and then help to keep the core covered with water.


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