How is nuclear energy harnessed?

[trini]

I understand how it is that nuclear processes release energy during fusion reactions, but i don't know how that energy is used to create positive work. I know radiation is produced as well as heat, but what techniques are used to convert these to usable and storable(not a word, i know) forms of energy?

 

[theCandyman]

All that energy released in fission heats up the nuclear fuel in the core. A cooling fluid is pumped through the core. Depending on the plant design, the working fluid powers a turbine or transfers its energy through a heat exchanger to another loop that runs a turbine.

 

[russ_watters]

In other words, a nuclear plant works fundamentally the same as any steam power plant, just with the heat being generated in a different way.

Note that due to the lower temperature of steam pluss losses due to the extra piping, and heat exchanger, the thermodynamic efficiency of a nucler plant is lower than a most other types of steam engines.

 

[gmax137]

Note that due to the lower temperature of steam pluss losses due to the extra piping, and heat exchanger, the thermodynamic efficiency of a nucler plant is lower than a most other types of steam engines.

Well that's true enough. The plant's thermo efficiency is a measure of the electrical power provided as a fraction of the thermal power produced in 'burning' the fuel. Other measures of 'efficiency' might try to quantify the 'amount' or 'value' of the fuel being burned to produce a unit of electric power. A 1000 MWe nuclear station, with its lowly 34% thermodynamic efficiency, burns up about 100 tons of fuel in 2 years. The super efficient coal station, is burning over 200 tons an hour to produce the same electrical output. Over the same two years that would be 3.5 million tons of coal used up compared to 100 tons of uranium oxide.

And remember, the uranium isn't really useful for anything else, other than WMD.

So the statement that "the thermodynamic efficiency is lower" is true, but it isn't very meaningful in the context of comparing different power stations.

 

[QuantumPion]

Well that's true enough. The plant's thermo efficiency is a measure of the electrical power provided as a fraction of the thermal power produced in 'burning' the fuel. Other measures of 'efficiency' might try to quantify the 'amount' or 'value' of the fuel being burned to produce a unit of electric power. A 1000 MWe nuclear station, with its lowly 34% thermodynamic efficiency, burns up about 100 tons of fuel in 2 years. The super efficient coal station, is burning over 200 tons an hour to produce the same electrical output. Over the same two years that would be 3.5 million tons of coal used up compared to 100 tons of uranium oxide.

And remember, the uranium isn't really useful for anything else, other than WMD.

So the statement that "the thermodynamic efficiency is lower" is true, but it isn't very meaningful in the context of comparing different power stations.

LWR's don't burn 100 tons of fuel in 2 years. 100 tons is along the lines of the total uranium loading of the core, most of which is U-238. If the uranium started at 5% enrichment, after 2 years you would have used up ~5 tons of U-235 (plus ~3 tons of Pu on top of that by conversion).

 

[Hologram0110]

LWR's don't burn 100 tons of fuel in 2 years. 100 tons is along the lines of the total uranium loading of the core, most of which is U-238. If the uranium started at 5% enrichment, after 2 years you would have used up ~5 tons of U-235 (plus ~3 tons of Pu on top of that by conversion).

I'm sure he is talking about a once through cycle since reprocessing has yet to become wide spread for economic reasons. Although you are right to point out that even 'spent' fuel still contains a lot of potential energy which could still be extracted.

A simpler answer to the original question:
Nuclear plants work they same way as any other thermal (heat) generation station. The only difference is how the heat is produced. In a nuclear plant that comes from nuclear reactions in the fuel, in a coal plant it comes from burning coal. In both cases the heat is used to boil water creating high pressure steam. This steam goes to a turbine which spins a generator.

Nuclear plants tend to an extra heat exchanger that other types of plants don't have. Basically hot water from the reactor is used to heat water in a different set of pipes. This keeps the water in the reactor separate from the water on the turbine side. Unfortunately, this extra heat-exchanger wastes some energy but is needed for other reasons. That is what they are talking about when they say lower thermodynamic efficiency.

 

[gmax137]

LWR's don't burn 100 tons of fuel in 2 years. 100 tons is along the lines of the total uranium loading of the core, most of which is U-238. If the uranium started at 5% enrichment, after 2 years you would have used up ~5 tons of U-235 (plus ~3 tons of Pu on top of that by conversion).

well OK, I was going on the notion that the spent fuel is "burned up" and not then separated / re-used for anything. If you ran a 3000 MWth reactor continuously for two years, that would be 53 billion kW-hr, or 5.9 x 10^21 fissions (at 200 MeV), or 2300 kg (just over two metric tons) U-235. As you said, some of the power really comes from Pu, so the U235 used is even less.

But the point is, the fuel 'usage' in a reactor is a few tons per year, vs. more than 200 tons per *hour* in a coal burner.