Bruce Nuclear Power Development (BNPD)
The inefficiency of a nuclear reactor
There is an obvious difference between fossil fuelled and nuclear power plants which is the source of heat. You will see another difference if you compare the size of the steam equipment.
A good conventional plant produces hot, high pressure (dry) steam. A CANDU reactor like the one at the BNPD (Bruce Nuclear Power Development operated by Bruce Power) delivers steam at lower temperature and pressure (that is, saturated almost wet, steam). This requires larger volumes of steam flow to transfer the same amount of energy. As a result, the steam piping and the steam turbine are physically larger than similar equipment in an equivalent fossil fuelled plant.
To increase steam temperature and pressure in the CANDU, the design would have to use thicker material. The tubing that carries the coolant past the fuel interferes with the fission process by absorbing neutrons that cause fission. It cannot be thicker. Also, the fuel would need to be redesigned to operate at a higher temperature.
Using saturated steam affects the thermal efficiency of the nuclear plant.
Equipment that turns heat energy into mechanical energy is never very efficient. In round numbers, a conventional plant throws away about 60% of the heat energy generated by burning oil or coal. A nuclear plant wastes as much as 70%. (Your automobile engine is even less efficient!)
You can see the source of this inefficiency for a steam turbine. The turbine, to operate, must have high pressure at one end and low pressure at the other. The condenser condenses the exhaust steam with cold water to maintain the pressure difference. Heat removed by the water is wasted.
The efficiency of the energy transfer depends almost completely on the temperature difference between the hot steam and the cold condensing water. With cooler steam, a higher percentage of the heat is wasted.
The following describes the energy flow path more carefully, using typical numerical values.
The CANDU operating license limits the total heat output from the fuel at full power. The smallest units have a license limit of almost 1700 MWth. The limit for the largest units is over 2800 MWth. The megawatt is a measurement of power, the rate of producing and transferring energy. A unit producing 2800 MWth must get rid of 2800 megajoules of energy each second. To see where this energy goes, refer to the Figure below.
The steam condensers reject about 65% of the total heat energy produced. This waste heat passes to the lake.
The turbine converts about 30% of the total heat energy produced into mechanical energy. The generator converts the mechanical energy to electric energy. The electrical grid distributes most of this energy to customers. The station uses some of this electricity to operate equipment.
About 5% of the total energy produced shows up as heat in the moderator system. This percentage includes a small amount of heat that escapes into the shielding around the core. Heat exchangers cool the equipment and transfer the heat to the lake.
© BRUCE ENERGY CENTRE - Copyright 2011