How efficent is nuclear energy?

Answer 1

Efficiency is measured in many ways, so in order to have an answer, we really want to know the point of the question. That having been said, however, there are some observations we could make.

The fuel for most nuclear power plants is enriched uranium. The reason it needs to be enriched is because it is really the U-235 that is needed for a reaction, and only about 0.71% of natural uranium is u-235. Enriched uranium fuel has had U-238 removed to the point that it is usually about 3-5% U-235. In waste, most of the u-238 remains, but about 70% to 80% of the U-235 has been used. Since the fission fragments are radioactive and have available energy of their own, and only about 4% of the fuel was used at all, more than 96% of the available energy of radioactive decay remains unused in the waste (I have no calculation, but it is at least this amount). This seems not very efficient. There are other ways of using radioactive materials that derive far more power, but they are not currently in use as they are only under development.

The power from a nuclear reactor is thermal - heat. It is typically used to heat water to boil, making steam, which turns a turbine, which turns a generator. So the reactor output is heat, but the plant output is electricity. Not all of the heat is turned into electricity. In fact only about 35-40% of it is. The rest is usually thrown away as waste heat. Rivers, lakes, or oceans are used to supply water to cool the plants - but another way to look at this is that 60-65% of the heat generated in nuclear plants is thrown away by dumping it into these bodies of water.

This might seem very inefficient, but compared to coal plants, it is not. They typically throw away a similar amount of their heat. Oil and natural gas do have advantages here. Natural gas can operate in a cycle where the gas is burned in a turbine, making electricity; then the waste heat is used to heat steam for a second turbine, making more electricity; and finally, the last remaining heat is used to heat buildings. In this case, natural gas can be as much as 65% efficient. Nuclear plants will probably not come to this level of efficiency because safety considerations prevent them from being sited close enough to population centers to use all the waste heat. In this sense, nuclear is not too bad, but not all that good.

Yet another way of looking at the problem is to think in terms of a carbon footprint. Nuclear energy carbon footprints have been calculated without regard to construction, decommissioning, or waste disposal, so the reported figures tend to be untrustworthy. But when these activities are accounted for, nuclear power seems to be producing something on the order of 60-240 gCO2e/kWh (there is no way to estimate accurately for disposal, so it is a guess). "gCO2e/kWh" is the "equivalent" of carbon dioxide emissions per kilowatt hour - some chemicals are worse than others, and this is a basis for comparison. Wind and hydro seem to be about 15 gCO2e/kWh. Solar thermal is a bit less than that. Solar photovoltaics seem to be about in the range of 25-45 gCO2e/kWh. Natural gas, efficiently used, is about 580 gCO2e/kWh.

So in terms of using chemicals efficiently as regards their greenhouse gas emissions, nuclear has about 400% to 1600% of the emissions of solar thermal, wind, or hydro, about 200% to 800% of solar photovoltaics, and about 11% to 60% of efficient natural gas. That is not what I would call great.

Another way of looking at things is the energy output per unit weight of fuel. If this is what we were measuring, then nuclear tops the list hands down. But it really doesn't mean much because the measurement does not address anything of meaningful value.

Answer 2

Nuclear energy is considered efficient because it produces a large amount of electricity from a small amount of fuel compared to fossil fuels. However, nuclear power plants have high upfront costs and complex waste management challenges that can affect overall efficiency.