Fresh drinking water.
- Potable water is in short supply in many parts of the world. Lack of it is set to become a constraint on development in some areas.
- Nuclear energy is already being used for desalination, and has the potential for much greater use.
- Nuclear desalination is generally very cost-competitive with using fossil fuels.
In many regions of the world, water that is safe for drinking is not readily available. As many as one-fifth of the world’s population does not have access to safe drinking water: with population growth, these figures will increase substantially. The worst-affected areas are the arid and semiarid regions of Asia and North Africa.
Fresh water is a major priority in sustainable development. Where it cannot be obtained from streams and aquifers, desalination of seawater or brackish water is required. Further demand in the longer term will come from the need to make hydrogen from water.
Most desalination today uses fossil fuels, contributing to increased levels of greenhouse gases. Total world capacity is approaching 40 million m³/day of potable water, at some 15,000 plants. Most of these are located in the Middle East and North Africa, and use distillation processes. The largest plant produces 454,000 m³/day.
There are several technologies being used in the world today for desalination: reverse osmosis (RO) and multi-stage flash distillation (MSF). A minority of plants use other technologies, such as multi-effect distillation (MED) or vapour compression (VC). All of the processes are energy intensive requiring anywhere from five to 200 kWh per cubic metre of water.
Some 10% of Israel’s water is desalinated, and one large RO plant provides water at a cost of 50 cents per cubic metre. Malta gets two-thirds of its potable water from RO. In 2005, Singapore commissioned a large RO plant supplying 136,000 m³/day — 10% of its needs, at a cost of 49 cents US per cubic metre. All three plants use fossil fuels to desalinate the water.
The Ashkelon Desalination Plant in Israel uses Seawater Reverse Osmosis (SWRO).
In reverse osmosis, high pressure salt water is forced through a water-permeable membrane. Normally water would flow through the membrane to balance the concentrations, but because of the high pressure on the salt water side, the process is reversed. The product water is at approximately atmospheric pressure, while the salt water is at a pressure of anywhere between 15 and 75 atmospheres.
The major energy requirements in this form of desalination are in the pressurization of the salt water. Reverse osmosis requires the least amount of energy of the desalination processes, requiring 4-6 kWh per cubic metre.
In multi-stage flash distillation, the salt water is heated and then placed in a lower pressure vessel, which causes a portion of the salt water to instantly vaporize (flash). This steam is then collected and condensed as desalinated water. As only a portion of the water is vaporized, the remaining brine goes through a series of stages at lower and lower pressure, causing more water to be vaporized.
The major energy requirements in multi-stage flash are in heating of the salt water and controlling the pressure of vessels. The typical energy costs are in the range of 23-27 kWh per cubic metre.
Small and mid-size nuclear power plants are appropriate for the energy needs of a desalination plant. They can provide the heat for a multi-stage flash plant or the energy requirements, including electricity for both plants.
Nuclear Desalination is being used in parts of the world today, chiefly in Kazakhstan, India and Japan.
The BN-350 fast reactor at Aktau, in Kazakhstan, successfully produced up to 135 MW of electricity and 80,000 m³/day of potable water over some 27 years. Oil/gas boilers were used in conjunction with it, and total desalination capacity through 10 MED units was 120,000 m³/day.)
In Japan, some 10 desalination facilities linked to pressurized water reactors operating for electricity production have yielded 1,000-3,000 m³/day each of potable water, and over 100 reactor-years of experience have accrued. MSF was initially employed, but MED and RO have been found more efficient there. The water is used for the reactors’ own cooling systems.
India has been engaged in desalination research since the 1970s and in 2002 set up a demonstration plant coupled to twin 170 MW nuclear power reactors (PHWR) at the Madras Atomic Power Station, Kalpakkam, in southeast India. This Nuclear Desalination Demonstration Project is a hybrid reverse osmosis/multi-stage flash plant, the RO with 1,800 m³/day capacity and the MSF 4,500 m³/day.
More than 20 countries led by the UN’s International Atomic Energy Agency (IAEA) are collaboratively involved in research to investigate large-scale deployment of nuclear desalination.
One strategy is to use power reactors which run at full capacity, but with all the electricity applied to meeting grid load when that is high and part of it to drive pumps for RO desalination when the grid demand is low.
Nuclear Desalination using Canadian Technology
CANDESAL Water Systems, a Canadian company, was formed to develop the concept of nuclear desalination, combining CANDU reactors with DESALination. Their design couples an RO desalination plant with a CANDU reactor.
Pakistan is developing a demonstration MED desalination plant coupled to its KANUPP reactor (125 MW PHWR) near Karachi to produce 1,600 m3/day (it was earlier projected to produce three times this). It has also been operating a 454 m3/day RO plant for its own use.
World Nuclear Association, Nuclear Desalination, www.world-nuclear.org/info/inf71.html.
International Atomic Energy Agency, “Non-electric applications of nuclear energy,” Proceedings of an Advisory Group meeting held in Jakarta, Indonesia, 21–23 November 1995, wwwpub.iaea.org/MTCD/publications/PDF/te_0923_scr.pdf.
Organization of American States, “Source Book of Alternate Technologies for Freshwater Augmentation in Latin America and the Caribbean,” Chapter 2.1: Desalination by reverse osmosis, www.oas.org/dsd/publications/Unit/oea59e/ch20.htm.