Could you explain the extent to which nuclear power plants depend on water for cooling?

Water-cooled NPPs normally require more water than fossil-fuelled power plants because of the lower efficiency in the conversion of heat to electricity. Also, they require a larger amount of water during construction mainly for the concrete used to shield and protect the reactor. A typical NPP of 1000MWe may require around 100,000 cubic metres of water per day for operating. In addition, back-up water is required.

The most common nuclear reactor is a pressurised water reactor (PWR), which has three circuits. The first circuit is where heat is produced in the reactor. This heat is then transferred to a secondary circuit through a steam generator; this steam drives the turbines to produce electricity. The third circuit is then used to release to the environment (i.e., heat sink) the residual heat from the condensed steam. It should be noted that the first and secondary circuits, or loops, are closed off from one another.

What are the different methods of cooling for NPPs?

Let’s talk about cooling the NPP as a whole, and not just the reactor core cooling. Surface water, in general, has been used for cooling NPPs, especially for the once- through cooling system. Once-through cooling means water is taken from the sea, or river or lake, and passes through a condenser. In 2005, the U.S. Environmental Protection Agency (EPA) began taking action to abolish once-through cooling. Other countries have also sought to take similar measures. The reason for this is that once-through cooling can have an environmental impact on biota such as fish; as a consequence, some NPPs have adopted a different form of cooling, such as using cooling towers.

Are there other ways to cool NPPs?

Using cooling towers is another possibility. Cooling towers can have a variety of different designs. We have wet cooling towers, naturally driven cooling towers, forced cooling towers, dry cooling towers, hybrid cooling towers. There are many. 

What are the main environmental problems arising from NPP cooling?

Based on the design of the seawater intake system, heat dissipates into the marine eco-system, creating a possible impact on it. Still, there are strict regulations preventing NPPs from releasing water that is too warm. At such times, NPPs must either reduce electricity production to avoid a negative impact on the environment or even shut down.

Another possible environmental impact, also common to other non-nuclear thermal electrical power plants, is due to impingement and entrainment. Impingement occurs when marine life forms are trapped against the intake screens by the suction force and velocity of water, often with a lethal impact. Entrainment refers to organisms that pass through the openings of the seawater intake screens and into the water treatment system, killing them.

Does this have relevance to the safety of the plant?

It does not have relevance for the safety of the plant as long as the supply of the necessary amount of water is ensured.

What about plants in arid regions sited near the sea, such as Abu Dhabi?

The sea temperature is a factor. In hot regions, this can reduce the thermal efficiency of the NPP and may require additional considerations for the design of the water intake cooling system. Many different cooling systems can provide a heat sink, including dry cooling systems which use air instead of water. In areas where the air is hot, for instance, a hybrid system is a possibility, which uses water when the air is too hot and dry cooling in the winter. This can be expensive. Another alternative is to redesign the intake system deep into the surface water in order to withdraw cooler water for the plant.

What support does the Agency offer newcomer countries on water management?

In 2010, the IAEA launched the Water Management Program (WAMP) to help Member States better address this issue. WAMP is freely available to all Member States and can be used to estimate an NPP’s needs for cooling water and other essential systems. It helps in the selection process for cooling systems by evaluating three different criteria: water resources, environmental impact and economics.

What are the main features of WAMP?

WAMP helps to estimate the quantity of water withdrawn and consumed based on siting conditions and the type of cooling system as well as the capital and operational costs associated with a given cooling system. It also provides a qualitative environmental impact assessment of the selected cooling system. WAMP is designed to help both beginners and experts to make interactive comparisons of different cooling systems (open loop, wet, dry), reactors (light, heavy water) and site conditions (temperature, humidity, wind, etc.).

What are the factors that need to be taken into account?

There are many. The system you select depends on the siting parameters, the capacity of the reactors, etc. What we tried to do is to model different cooling systems for the three main outputs: the first is the water balance (how much you consume and how much you will lose), and the second is economics (the cost of operating the system, including overnight capital cost). This is a qualitative and not quantitative assessment. And the third output is the environmental impact (using a certain type of cooling system means you must expect a certain environmental impact).

Are member states using this?

Yes. Several Member States are using WAMP. They have reported that WAMP has produced results consistent with those from consulting firms they contracted to undertake specific studies. 

Dr. Ibrahim Khamis is a senior nuclear engineer and the Project Manager of the Non-electric applications of nuclear energy at International Atomic Energy Agency (IAEA), Section of Nuclear Power Technology Development. His duties involve activities related to non-electric applications of nuclear energy including nuclear seawater desalination, hydrogen production, district heating and other industrial applications.