Water is a very important resource for the development of any nation. It is used for many domestic activities such as washing, drinking, cooking etc. It is also used for industrial activities such as manufacturing of products, cooling of machines etc. It is also used for Agricultural activities such as irrigation of farms and fish farming and drinking water for animals. The importance of water can never be over emphasized. Majority of these uses of water can only be done using fresh water and not saline water. Many countries like Ghana have seas which have lots of saline water but lack fresh water. To solve this problem some countries have adopted seawater desalination.
What is seawater Desalination
Seawater desalination can be defined as the removal of salt from the unlimited supply of seawater. Seawater desalination is an energy intensive process. The source of energy for seawater desalination is very important. The source of energy can be solar energy, electrical energy, fossil fuel, nuclear power reactors etc. Using some of these sources of energy such as fossil fuels will not be compatible with sustainable development. This is because fossil fuels reserves are finite and must be conserved for other essential uses, whereas demands for desalted water will continue to increase. Hence there is a need to consider nuclear desalination.
What is Nuclear Desalination
Nuclear desalination is the production of potable water from seawater, using a nuclear reactor as the source of energy for the desalination process.
Nuclear desalination is the only technically feasible, economically viable and sustainable solution to provide desalted water on a large scale for future water demands because:
- Nuclear reactors provide heat across a large range of temperatures, thus allowing easy adaptation to any desalination process.
- Some nuclear reactors furnish waste heat (normally evacuated to a heat sink) at ideal temperatures for desalination.
- Nuclear reactors produce energy which is less expensive than that produced by conventional power plants. This is due to the low nuclear fuel cost.
- Use of nuclear energy for desalination is environmentally friendly. Unlike fossil fuels it does not generate gaseous pollutants such as CO2 emission and green house gases.
Nuclear Desalination can be achieved by coupling a nuclear power plant to a desalination system.
Nuclear Power Plant
Most large desalination plants are sited close to nuclear power plants. These power plants supply heat for evaporating the seawater and generate electricity to drive pumps and other desalination plant equipment. Nuclear power plants can be classified as either water-cooled or gas-cooled depending on the coolants they use. Most types of water-cooled and gas-cooled nuclear power plants are used for nuclear desalination. Thermal desalination processes such as the Multi Stage Flash (MSF) and Multi Effect Distillation (MED), are often coupled with nuclear power plants so as to desalt seawater. Desalination processes require energy in the form of heat and/or electricity, which can be supplied by nuclear reactors. Nuclear power is a clean energy alternative that is already used to provide water and electricity. Nuclear reactors as an energy source for seawater desalination can supply electricity or heat, or both, as required by the desalination process.
Some examples of nuclear power reactors that can be used for nuclear desalination are :
- Pressurized Water Reactor (PWR)
- Boiling Water Reactor (BWR)
- Advanced Boiling Water Reactor (ABWR)
Desalination systems can be classified based on the method of separation used in the desalination process. Below is a diagram showing the classification of desalination processes (systems). From the diagram below you can see some examples of desalination systems. There are other systems which are not covered by the diagram. But so far these are the popular desalination systems. They have been classified using the means by which water is separated from salt. That is some use heat (thermal energy) for separation whiles some use membranes and other means for separation.
Classification of Desalination Systems
Coupling of Nuclear Reactors with Desalination Systems
There are two ways in which a nuclear power can be coupled to a desalination plant. They can be coupled as a single purpose plant or a co-generation plant. In a single purpose nuclear desalination plant, the nuclear energy form the nuclear power plant is solely used to produce desalinated water whiles in a co-generation nuclear desalination plant, part of the nuclear energy is used to for desalination and the remainder used for another purpose such as generating electricity simultaneously. Power reactors can be coupled to electrically driven desalination systems such as Reverse Osmosis (RO) whiles heating reactors can be coupled to thermal desalination systems such as MSF and MED.
Nuclear power plants that co-produce heat and electricity, produce steam that can be bled off at suitable points in the secondary circuit of the power plant for use by the desalination plant. To avoid the fresh water from being contaminated by radioactivity from the nuclear power plants protective barriers are used in all co-generation plant.
Description of the Nuclear Desalination Process
The process of nuclear desalination can be achieved by coupling a nuclear power plant such as a PWR to a thermal desalination plant such as a Multi effect distillation (MED) system. The nuclear fuel in the reactor vessel undergoes a fission chain reaction, to produce heat. The heat generated is heat the primary coolant. There is an exchange of heat between the primary coolant and the secondary coolant at the steam generator. In the process the secondary coolant is changed to steam. Part or the whole steam generated is sent to the thermal desalination system (MED). In the MED, the steam is used to evaporate the seawater. During evaporation, the salt is separated from the water vapor. The water vapor is then cooled to obtain fresh water. In the MED, seawater is separated into fresh water and brine. The brine is sent back in to the sea while the potable water is collected and stored in storage tanks for future use. The process is repeated over again. The Multi Stage Flash (MSF) desalination system is another thermal desalination system that can be coupled to a nuclear power plant to achieve a similar result. However their mode of operation is a bit different. There are other desalination systems that use electrical energy instead of thermal energy. Example is the Reverse Osmosis process.
A Nuclear Power Plant
PWR Coupled to MSF Desalination Plant
How The PWR-MSF Desalition Plant Works
The nuclear fuel in the reactor vessel undergoes a fission chain reaction, to produce heat. This heat is used to heat the primary coolant by thermal conduction through the fuel cladding. The hot primary coolant is then sent to the steam generator. Heat is then transferred from the primary coolant to the lower pressure secondary coolant located on the shell side of the steam generator where it evaporates to form pressurized steam. All the pressurized steam generated in the steam generator is then expanded in the turbine to an elevated turbine backpressure so as to turn the turbo generator to produce electricity. Then the low-grade steam exiting the turbine is passed directly to the condenser .In the condenser the latent heat of condensation of the low grade steam is used to generate the brine heater heating fluid (steam).The heating fluid is used to heat the brine recycle stream in the brine heater to the top brine temperature (TBT) which is the highest design temperature of recycled brine. The condensate of the low grade steam (saturated liquid) exiting the condenser is then returned to the steam generator through a feed water heater by an isentropic pump. And the process repeated again.