In a recent post, we (Emma Bateman and myself) talked about mains water usage at Sizewell nuclear power plant¹. This has created some confusing (not least amongst people working from EDF) which this post is intended to clear up.

Below is a very simplified diagram of the water flow in a pressurised water reactor (PWR).

The heat source (nuclear fuel rods) heats up water, which does not turn into steam since it is a very high pressure. This water then passes through a steam generator which heats up water which is not highly pressurised, and it turns to steam. This steam then turns a turbine to generate the electricity. On the other side of the turbine, the steam is cooled to turn it back into liquid water using a third loop of water.

The first two water loops in the diagram – the red and light blue – are ‘closed’. That means that the water is ideally recycled continuously. The third loop could be ‘closed’ or, as in the case of a power station such as Sizewell it is ‘open’. That means that the water is taken from the heat sink (at Sizewell this is the sea) and then returned rather than being recycled.

Water in the first two loops is ideally recycled, although this is not the case in practice (see below). In the third loop, seawater is withdrawn but then returned at a higher temperate. We therefore have a difference between the water withdrawn – taken and then replaced, and the water consumed – taken from the mains supply but not replaced.

The maximum amount of seawater which can be withdrawn is 5,011,200m³/day². This is considerably higher than the 800m³ of mains water which is consumed given for Sizewell B¹.

What Is Mains Water Used For?

The primary and secondary cooling loops are rather like a central heating system. As you may know with time central heating systems clog up and cold spots can appear in radiators. The cooling system is a lot bigger than a domestic central heating system, being equivalent to nearly 4 million central heating systems³ and clogged pipes can cause serious problems.

To avoid erosion and clogging in the pipework, the water is filtered, monitored and topped up when necessary. These filters and others have to be flushed and cleaned using deionized water.

Water is also needed for waste treatment and topping up the cooling ponds as well as standard washing, showering and laundry.

The exact figures for these vary for different plants. For more detail, see the discussion by the IAEA⁴.

Overall Water Use

This post is very limited in its scope, and I have not attempted to discuss the subject of the overall water use for nuclear plants. However, I feel that it is important just to make a few comments at this stage, although I may go into the subject further in future posts.

Nuclear power plants require slightly more water than other thermal plants such as coal or gas (because they are thermally less efficient since they run at lower temperatures) although the difference is significant it is not that much greater.

If we want to properly analyse their overall use then we have to include the water used in mining, extraction of uranium from the ore, enrichment etc.

In the discussion so far I have assumed that the heat sink shown in the diagram at the top of the post is the sea. However, it can be other things such as a river, lake or a wet or dry cooling tower. These will have quite different water requirements.

¹ Sizewell C 2nd Consultation: Mains Water, Peter Lux’s Blog (http://www.plux.co.uk/sizewell-c-2nd-consultation-mains-water/). A further write-up with additional information was published in The Ecologist: Sizewell C consultation: EDF ‘forgets’ to mention 600,000 m3 a year of mains water, The Ecologist, 23rd January 2017 (http://www.theecologist.org/campaigning/2988533/sizewell_c_consultation_edf_forgets_to_mention_600000_m3_a_year_of_mains_water.html)

2 Environmental Monitoring Around Sizewell, Environment Agency (UK), September 2014 (http://www.magnoxsites.co.uk/wp-content/uploads/2014/07/EA-Environmental-Monitoring-Around-Sizewell.pdf)

3 Waste power is about 2175 MW which is 5.63 x 10¹⁶J assuming an 82% capacity factor. Average household gas consumption is 3,938KWh which is 1.42 x 10¹⁰J. Therefore the waste heat is enough to heat about 4 million homes.

4 Efficient Water Management in Water Cooled Reactors, International Atomic Energy Authority, 2012 (http://www-pub.iaea.org/MTCD/Publications/PDF/P1569_web.pdf)