Some Thermodynamics

by Peter Bursztyn

Most nuclear power reactors have triple heat transfer loops. One heat transfer loop takes heat from the reactor core and transfer this to a heat exchanger. There a 2nd heat transfer fluid contacts the first one, picking up heat from it. Typically, the 2nd fluid boils on contact with the hot fluid in the 1st loop. This steam is used to spin steam turbines. to generate electricity. A 3rd heat transfer fluid is employed to condense the cool steam from the turbine back to liquid water in another heat exchanger. The water in the 2nd loop returns to the 1st heat exchanger for reheating. The warm water in the 3rd circuit is either discharged to a nearby body of water (lake, sea, river), or sprayed through a cooling tower where some evaporates, cooling the rest.

Exposed to the reactor core, the water in the 1st heat transfer loop gradually becomes radioactive so must be kept within the reactor’s containment building. That is the reason for using a 2nd heat transfer loop to take the heat out to the turbines.

There are various reactor configurations which refer to the way heat is removed from the reactor core. The main ones are boiling water reactors, pressurized water, and gas cooled.

In a Pressurised Water Reactor, the 1st loop is kept under such high pressure that the water cannot boil. Typically, the pressure is ~150bar, and the water leaves the reactor at 350oC. Steam in the 2nd loop leaving the heat exchanger is now at 235oC and 70bar. This steam spins the turbines. Efficiency is around 34%, with 66% of the energy from the reactor wasted to the environment as heat.

In a Gas Cooled Reactor, a gas (often carbon dioxide) is used to carry heat from the core to the heat exchanger. The temperature of the outgoing gas can be far higher than that of water. In the British reactor designs, it is 640oiC at a pressure of 40bar. This gas circulates through a steam generator where water in the 2nd loop boils, and produces steam to spin the turbines. The high gas temperature increases reactor efficiency, and 41% electrical efficiency is usual.

In a Boiling Water Reactor, the water around the reactor core is allowed to boil at 285oC, under 75bar pressure. In a BWR, this steam is sent directly to the turbine, creating a simpler design and equaling the PWR efficiency.

The efficiency of all “heat engines” – and a nuclear reactor is one – is governed by the ratio of input and outlet temperatures, as shown by Sadi Carnot in 1824:

Efficiency = (T1 – T2)/T1, where T1 = input temp. T2 = outlet temp.

Temperatures must be given in oK (0oC = 273oK). This equation simply states that raising the input temperature and lowering the outlet temperature improve efficiency. That’s why thermal power stations employ a cooling device; either a large body of water, or a cooling tower where evaporation helps the outlet steam condense back to water.

All of these reactors – as well as other thermal power stations fuelled by natural gas, oil or coal – lose more of the energy in their fuel to the environment than they capture in the form of electric power. The key to improving efficiency is to increase the operating temperature of the reactor. Because a Gas Cooled Reactor doesn’t increase pressure as temperature rises, designers are working on reactors to operate at above 850oC.

There is even a plan to use the high temperature of a gas cooled reactor to help split water into oxygen and hydrogen. As temperature rises, less electrical energy is required to separate the water molecule’s hydrogen from oxygen.

Some Thermodynamics