Question: what is the most interesting concept you have come accross in your job

Tina-Jaine Haigh answered on 24 Jan 2025:

Ooh, there are lots.
One concept I like is that the rector is not the hottest place in a PWR nuclear power station like Hinkley Point C. The pressuriser is the hottest place.
A pressurised water reactor (PWR) uses pressurised water as the “primary coolant”, running through the reactor core around the fuel. That water has 3 jobs – move the heat to where it can be used, and cool the fuel to prevent it melting, and help with controlling the nuclear reaction. The water is kept pressurised to prevent it boiling and turning into steam even though the temperature is way above 100 degrees C (the boiling point of water at atmospheric pressure). If you apply enough pressure to water or any other liquid, you can stop it turning to a gas even if it is hotter than it’s boiling point.
We want to keep the water liquid, because otherwise it would boil inside the reactor on the surface of the fuel, and the fuel would be in contact with steam instead of liquid water. Steam is not very good at transferring heat, so it wouldn’t do the job of cooling the fuel.
But there is 1 place where this water is allowed to boil and turn into steam – the pressuriser. This is a big container linked to the reactor and primary circuit by pipes. It’s partly full of liquid water and partly full of steam, but no air. It is used to control the pressure.
The Pressuriser makes use of the fact that steam can be compressed (i.e. change it’s size/volume) very easily, but liquid water doesn’t change it’s size/volume very much. Allowing the steam to compress or expand with small differences in pressure helps to keep the overall pressure steady. This is a natural stabilising effect.
Because it’s connected to the reactor, the pressure is always the same in the pressuriser as the reactor because pressure transmits throughout the system. But the temperature can be higher in the pressuriser if the water doesn’t mix much between the pressuriser and the reactor. Imagine pouring hot water into 1 end of your bath if it’s gone luke warm. Initially there are temperature differences until you mix the water. The connection between the pressuriser and the reactor is along quite a narrow pipe, so there isn’t much mixing between the 2 and you can keep the pressuriser hotter than the reactor. It has to be hotter, to let the water to boil in there and keep some steam in there. So it has electric heaters.
The pressuriser also has a shower head inside it at the top. Primary coolant water can be sprayed from the shower head, and it will fall though the steam. The droplets of water from the spray encourage the steam to condense and stick to them and form bigger water droplets, like the way a cloud forms. This results in overall less steam in the pressuriser so it brings the pressure down. The spray is only used occasionally, when you need to reduce the pressure by more than the natural stabilising effect of the steam bubble. Afterwards, some of the water droplets will evaporate again, and it will go back to it’s steady state.
If you need to increase the pressure in the system, you turn up the electric heaters in the pressuriser, and more water will evaporate to make more steam, which pushes down on the liquid water more.
There is some nice physics of pressure and temperature, and water changing state between liquid and gas, going on in the pressuriser. It can all be explained using the universal gas law PV = nRT and the latent heat of evaporation. The pressuriser is my 2nd favourite bit of a nuclear power station.