Nuclear power plants for energy generation use the solid ceramic material uranium oxide UO2 as “fuel”. If the uranium isotope 235U in the nuclear reactor is bombarded with slow neutrons, this produces an unstable uranium isotope 236U that very quickly decomposes into large nuclei with a mass number around 142 and around 92, such as barium and krypton. This stimulated nuclear fission releases fast neutrons that, after a slow-down, can split uranium nuclei again – a controlled chain reaction. Water in the nuclear reactor causes the deceleration of fast neutrons and thereby takes over part of the kinetic energy of these neutrons, which heats up the water. The ‘fragments’ of barium and krypton also move apart at high speed, and this kinetic energy is transferred to the neighbouring atoms of the nuclear fuel and converted into heat. Through a heat exchanger, all this heat is transferred to another water circuit that converts water into steam. This steam drives a turbine, and a generator converts the rotating movement of the shaft of this turbine into electricity. That is nuclear energy in a nutshell.
The energy released by nuclear fission is many orders of magnitude greater than the energy released by chemical reactions such as the combustion of coal, oil or natural gas. We are dealing here with very strong nuclear forces, and not with the much weaker electron forces that play a major role in chemical reactions. By way of comparison: one gram of the uranium isotope 235U produces as much energy in nuclear fission as several thousand kilograms of coal in combustion – a difference by a factor of a million. This makes nuclear energy a compact form of energy with a very high energy density – you need very little ‘fuel’. Or, considered in another way: nuclear power plants are much more compact per amount of energy generated than other power plants, and they also require less land area than other non-fossil energy generators such as solar cells and wind turbines.
From uranium ore to energy
The element uranium occurs naturally in the earth’s crust as a constituent of ores. More than 99% is in the form of the uranium isotope 238U, about 0.7% as the desirable isotope 235U and traces of 234U. After extraction, the ore is stripped of undesirable components using physical and chemical processes in order to obtain a purer raw material (U3O8).
For a common type of nuclear power plant, where ‘ordinary’ water is used for heat transfer, it is necessary to enrich the isotope 235U content from 0.7% to about 4%. To do this, the U3O8 is first converted into gaseous UF6. Because the 235U isotope of UF6 is slightly lighter than the 238U isotope, these isotopes can be separated by mass. In the past, robust ceramic membranes were developed for the separation of this corrosive and highly reactive UF6 mixture. These membranes allowed the light isotope to pass through just a little faster than the heavy, causing the 235U content to increase slightly. In this way, a whole battery of membranes in succession could be used to obtain the desired concentration. Today, ultracentrifuges are used for enrichment, in which the heavier isotope is ‘slung’ to the outside and the desired lighter 235U-rich fraction is extracted from the inside of the centrifuges.
After enrichment, UF6 is converted into UO2 which is manufactured into fuel elements for the nuclear reactor as rods or pellets. This fuel is encased in a zirconium alloy cladding that is corrosion resistant and absorbs hardly any neutrons. In addition to water for neutron and heat absorption, there are also control rods made of cadmium or boron that can be inserted into the reactor to absorb neutrons, as an (additional) control mechanism of the chain reaction. For reasons of mechanical robustness and radiation resistance, important parts of the reactor and the power plant are made of steel and concrete.
After several years of operation in the power generation reactor, three quarters of the 235U have been converted into fission products and are therefore highly radioactive. The spent fuel is removed from the reactor, and reusable fissile materials such as uranium can be recycled and reprocessed for new use as fuel. The remaining material must be packaged and stored in a proper shielding environment until the radiation has lowered sufficiently. Because nuclear energy uses relatively little fuel due to its high energy density, the volumes of material involved are small. Most of the resulting fission products have a half-life of 30 years or less, so that after a few hundred years their radioactivity has been diminished to the same level as the raw material uranium ore. A small part of the fission products has a much longer half-life; they disappear very slowly, but they also emit very little radioactivity. Actually, this radioactivity is distributed over the entire decay time.
Nuclear energy and the (near) future?
If electrification and hydrogen will become the basis of the non-fossil energy economy of the future, a combination of solar panels, wind turbines and nuclear power plants with SOEC electrolysers for hydrogen production might be a good solution. As mentioned above, the heat and steam generated by a nuclear power plant can be used to drive a generator to produce electricity. This electricity can then be used in a high-temperature SOEC electrolyser to generate hydrogen – for storage, or to add to the infrastructure of (formerly natural) gas pipelines throughout the Netherlands. Some energy is lost in this conversion of water to hydrogen, so preferably use surplus electricity for this – on days with lots of sunshine and lots of wind, with nuclear power as the stable base load.
Nuclear fission for generating energy in this way is an existing technology that is already being used today. In nuclear power plants of the further future, it is envisaged to dissolve the nuclear fuel in molten salts in order to let the fissile uranium (whether or not made by bombarding the raw material thorium with neutrons) give off its heat directly to its solvent. The big challenge: because molten salt is highly corrosive, the construction materials used must be chemically and thermally resistant.
If the necessary technical hurdles are taken, artificial nuclear fusion may be an option later in this century. This is the same natural energy generation that the sun has been using for billions of years to keep shining. In this process, heavy nuclei are not split, but light nuclei such as the hydrogen isotopes deuterium (2H) and tritium (3H) are fused with each other, releasing a lot of energy. This is an application of Einstein’s famous equation ‘E=mc2‘: light nuclei (much lighter than iron) that fuse together have a slightly higher mass than the fused product, and the mass difference is released as energy. Millions of degrees Celsius are needed for such a fusion to take place, with particles in the form of a plasma that is confined within a fusion reactor in strong magnetic fields, so that it does not come into direct contact with the inner reactor wall. Material challenges lie primarily in the resistance of the internal reactor wall to high temperatures and neutron radiation.