Despite all the advantages of nuclear energy, not many people will think of nuclear as a sustainable energy source and not many people will prefer nuclear energy more than the renewable energy sources like wind and solar. However, the path towards a society with only renewable energy is long and uncertain. By using nuclear energy for at least another 50 years, we buy time to develop economically attractive renewable energy without excessive production of greenhouse gases. In the mean time nuclear energy can develop further and become so attractive that society will decide to continue with nuclear energy in parallel with the alternatives developed meanwhile. This trust in nuclear energy is based on the progress made in the last two decades in two important fields, which I would like to share with you, namely:
There are two kinds of uranium (symbol U) in nature: one heavy isotope named U-238 and one light isotope U-235. Because U-238 has an even number of protons and an even number of neutrons in its nucleus, it is more stable then the light isotope. That’s why 99.3% of all uranium left over in the earth’s crust is made of this heavy isotope. The U-235 has a smaller half live of ‘only’ 700 millions of years and has almost comp3ely decayed. Only 0.7% of all uranium is made of U-235. However, because of the odd number of neutrons in its nucleus, the binding energy that is released when U-235 absorbs a neutron is large enough to fission the nucleus. That’s why U-235 fissions easily when it absorbs a neutron and that’s why it makes a good fuel in nuclear reactors.
There are two ways to extract energy from uranium. One is to use geo-thermal energy for electricity production and heating purposes. Geo-thermal energy is due to the heat flow from the molten core at the center of the earth to the surface, supplemented with the energy released by the decay of uranium and thorium, which are the only two actinides that can still be found in nature. In the long term, uranium and thorium contribute about 40MeV per atom by decaying in multiple steps to stable lead.
A faster way to extract energy from uranium is to let U-235 capture a neutron, after which it fissions in two fragments, the so called fission products. Because the actinides are neutron rich, also two to three new neutrons are released. The fission products have a high kinetic energy, which is eventually converted to heat. Most fission products are radioactive, which means that they emit ionizing radiation when decaying to stable nuclides. However, already after one year, three out of four fission products are stable.
The forces in a nucleus are dominated by the attractive strong force that works between nucleons, neutrons and protons, and the weaker Coulomb force that works repulsive between protons. Because of the strong force, fission of a nucleus releases much more energy than in chemical reactions, where only the electrons in electron shells are redistributed. Fissioning of one gram of uranium releases the same amount of energy as the burning of 2500 liters of gasoline or 3000 kg of coal.
The binding energy for neutron capture in U-235 is larger (6.6 MeV) then for neutron capture in U-238. That’s why U-235 can easily be fissioned by a neutron at virtually zero energy, a so-called thermal neutron. U-238 can only be fissioned when the incoming neutron has a high energy of more than 1 MeV. The fission cross section of a nuclide is the effective cross section per nucleus for a certain reaction to occur. Its unit is barn, which equals 10-24 cm2. For U-235 and other fissile nuclides like Pu-239, this fission cross section is very high at low energy for the incoming neutron, while for fissionable nuclides like U-238, this cross section only has a non-zero value at high energies. To favor fission above other nuclear reactions, it is beneficial to slow down or moderate the neutron to thermal energies below 1 eV. This is usually done by letting the neutron collide with other particles, like hydrogen (protons) in water.
Another means to favor nuclear fission, is to enrich the uranium in the fissile isotope U-235. A value of 4 to 5% of U-235 in the fresh fuel is usually sufficient to sustain the fission chain reaction for a long time and to extract enough energy from the nuclear fuel. Enrichment is usually done by centrifuge of UF6 which separates the light and heavy molecules by the centrifugal force. In the Netherlands the enrichment process is adopted by the URENCO in Almelo. Some countries still use the gas diffusion enrichment process to separate the light and heavy molecules, but because of the high energy consumption in this process, new factories all adopt the centrifugal method.
Let’s summarize the processes in a nuclear reactor core. If a U-235 nucleus absorbs a neutron, enough energy is released to overcome the Coulomb barrier and to fission the nucleus in two fission products. These are usually radioactive and will decay to stable products in a few steps. Besides that, two or three neutrons are released with high energy of about 2 MeV on average. These neutrons are moderated to thermal energy by means of collisions on hydrogen atoms (protons) or graphite. One of the thermalized neutrons is needed to sustain the fission chain reaction. One of the other neutrons might be captured by the U-238 in the fuel, because 96% of all uranium is still the non-fissile isotope U-238, and the remaining neutron fraction may be lost by leakage or parasitic capture in structural materials or control rods. The neutron captured by U-238 is not lost, because after a few intermediate steps, the U-239 nucleus decays to Pu-239, which is also a fissile isotope. After some time, this Pu-239 may be fissioned as well releasing the same amount of energy as fissioning of U-235. In this way, several tens of percents of all the energy production in a nuclear power plant may be produced by fissioning of Pu-239.
In practice, the enriched uranium in the chemical form of UO2 is pressed into pellets of about 8 mm diameter and 1 cm height. These pellets are stacked upon each other and encapsulated in a metal cladding to form a fuel pin, and several hundreds of these fuel pins are assembled into a fuel element. A modern PWR contains several hundreds of these fuel elements. The nuclear power plant in Borssele for example, which is a rather small one, contains 121 of these fuel elements. Two fuel pellets would be sufficient to generate enough electricity for a Dutch family during one year (about 5000 KWh).
The NPPs in the world are mainly of the Pressurized Water Reactor (PWR) and Boiling Water Reactor (BWR) type. In the first, the fuel elements are positioned in a reactor vessel to form the nuclear core. The cooling water flows from bottom to top through the reactor core to take away the fission heat and to moderate the fission neutrons. The temperature of the water increases only from 300 till 330 °C, but because of the high pressure, usually around 150 bar, the water does not boil in the primary circuit. The water in the secondary circuit takes up the heat in the steam generator and will start to boil. The steam is fed into a series of high pressure, medium pressure and low pressure turbines to expand, and, after condensation, fed to the steam generator again. The second type, the Boiling Water Reactor, operates at a lower pressure in the primary circuit; usually around 75 bar, and has therefore no need for a separate steam generator. In principle this may save some costs and may lead to a slightly higher efficiency. However, because of the larger reactor vessel containing more components, like a steam/water separator and a steam dryer and control rods which have to be inserted from the bottom instead of from the top, the total costs for a BWR do not differ much from that of a PWR.
Nuclear safety can be subdivided into two important topics:
The first topic is assured by means of inherently safe feedback mechanisms. If the coolant temperature suddenly increases, the coolant density will decrease leading to reduced moderation and a slightly harder neutron spectrum. This means the fission process stops due to the disturbed neutron balance in the reactor core. Furthermore, if the fuel temperature increases, the neutron absorption by the U-238 increases due to the nuclear Doppler effect. This effect is due to the abundant resonances in the capture cross section of U-238. When the uranium atoms are moving faster, this effectively leads to a broadening of each resonance and a higher probability for neutron capture.
The decay heat emitted by the fission products amounts up to 6% of the initial fission power and should always be transferred to the secondary circuit or the environment. If not, fuel damage may occur with large economic and environmental consequences. This is actually what happened in Harrisburg at TMI-2 in 1979. Due to a loss of coolant, the fuel became uncovered with water, which lead to a molten fuel 3 hours after the start of the accident. After 3-4 hours of decay heat production the decay heat amounts up to 160 full power seconds, which is about the amount needed to heat up the primary circuit of a Boiling Water Reactor from 1000C to operating temperature.
Besides the inherently safe feedback coefficients, nuclear safety is focused on having multiple core cooling systems with as much redundancy built in as possible. To prevent any release of radioactive fission products, multiple barriers are designed to keep the radioactive core inventory inside and to protect the core from the outside. These barriers consist of the fuel pellet and cladding, the primary circuit, and a steel liner and one or two concrete containments. The European Pressurized-water Reactor (EPR), for example, one of the modern generation III reactors, has several independent safety buildings and can withstand an internal overpressure of 6.5 bar.
The resulting risk of a nuclear power plant is extremely small. The group risk expresses the probability for a certain accident as a function of the expected number of casualties. For a ten times larger number of casualties the risk should be a hundred times smaller. Many accepted industrial activities have a group risk higher than the reference value OWI. The Borssele NPP has a value much lower than the reference value: of the order of 10-10 per accident with 10 casualties.
With regard to safety, the High Temperature gas-cooled Reactor currently under development excels. The fuel is made of small particles dispersed in a graphite moderator. The particles contain a small kernel with diameter of 0.5 mm made of enriched-uranium-dioxide coated with a porous buffer and 3 other layers. These so-called TRISO particles are designed to withstand a temperature of 1600 °C without any significant failure. By designing the reactor core as a long slim cylinder, the decay heat of the fission products can always be transferred to the environment by radiation, convection and conduction without exceeding the maximum fuel temperature. In this way, a loss of coolant cannot damage the fuel and no fission products will be released from the fuel. This reactor is called inherently safe. In China, a small test reactor of this kind is currently in operation and a larger demonstration prototype is being built.
Spent fuel consists of two components. At the one side hundreds of different fission products are produced in the fission process, while at the other side actinides heavier than uranium are produced due to neutron capture in U-238. Besides the useful Pu-239 and Pu-241, which are fissile like U-235, these are the non-fissile plutonium isotopes Pu-240 and Pu-242 and the higher actinides like americium and curium. These latter are produced in minor quantities and are therefore called the minor actinides.
During irradiation in a nuclear reactor, the fuel composition changes due to the fission process and neutron capture reactions. The fresh fuel contains about 96% U-238 and 4% U-235. After typical irradiation time of four years, the spent fuel contains about 94% of U-238, slightly less than one percent of U-235, 4% of fission products, a mix of plutonium isotopes summing to one percent of the mass, and the minor actinides like neptunium, americium and curium, each amounting up to 0.1%. Note that the spent fuel usually contains uranium with fissile contents of about 0.9%, which is still higher than that of natural uranium.
To assess the potential danger of spent fuel to the environment, the radioactivity of the spent fuel is not a good measure because it does not weight the nuclides with their decay mode and the energy of the radiation emitted, and with their biological half life. All these elements are accounted for in the radiotoxicity of nuclides, which has the unit of Sievert instead of Becqeruel. The radiotoxicity of spent fuel is dominated by the actinides already after a few decades of storage. The contribution of the fission products becomes comparable to the radiotoxicity of the uranium ore originally used to manufacture the fresh fuel already after 250 years, while the contribution of the actinides remains dominant until 200,000 years of storage.
Of the actinides, it is mainly the plutonium that gives a large contribution. If we would recycle all the plutonium in the spent fuel, the required storage time would decrease to somewhere between 5,000 and 10,000 years instead of 200,000 years. Full recycling of all the plutonium and americium would give a required storage time dominated by the fission products.
Of course, separating the plutonium and americium without further treatment does not really solve the problem. Plutonium contains a few good fissile isotopes, in total about 65% of the plutonium mass, which can be re-used as a nuclear fuel. Also the uranium in spent fuel contains more U-235 than natural uranium and can be re-used. Plutonium can be recycled in Light Water Reactors like the Borssele nuclear power plant as so-called MOX fuel, which is a mix of plutonium-dioxide mixed in uranium-dioxide made of depleted or recycled uranium. This way, plutonium can be recycled two times, after which it contains a too large fraction of the even non-fissile plutonium isotopes.
The non-fissile plutonium isotopes and the americium isotopes cannot fission in a thermal reactor. To also transmute these isotopes, so-called fast reactors are needed, which do not contain a neutron moderator to slow down the fission neutrons. To sustain a critical fission chain reaction in a fast reactor, the fuel usually contains MOX fuel with much higher plutonium contents of about 20 to 25 percent. To avoid neutron slowing down, the coolant must be non-moderating. Liquid metals like sodium are perfect coolants in this reactor type. Several fast reactors have been in operation, like the French Phenix and Super-Phenix reactors and the Japanese Monju reactor. The latter is expected to restart soon, while in France a new proto-type reactor is being designed and planned to be built in the next decade. Also China made a lot of progress in the last decade and is now ready to fuel the China Experimental Fast Reactor. In India the Prototype Fast Breeder Reactor (PFBR) is still under construction.
In the current fuel cycle, it is possible to recycle plutonium twice as MOX fuel in LWRs. The remaining High Level Waste contains the fission products and minor actinides, like americium, and is usually dissolved in a glass matrix. This vitrified waste is encapsulated in canisters and stored at a temporary storage site like COVRA in Vlissingen. After intermediate storage of about 100 hundred years, these canisters should be disposed of in a geological storage facility for another 5,000 to 10,000 years. Within several decades, it is probably possible to recycle the remaining plutonium as well as the americium in fast reactors. In this way the required storage time for High Level Waste can be further reduced to values between 500 and 1,000 years. However, fast reactors can also be operated in ‘breeding mode’ to convert more U-238 to fissile plutonium isotopes than are consumed. In this way, about 60 times more energy can be extracted from uranium.
The earth’s crust contains 40 times more uranium than silver and almost as much uranium as tin. Uranium is not an uncommon material, despite the fact that since the creation of the earth, 50% of all uranium has already decayed. According to the Nuclear Energy Agency, cheap uranium with a price of 130 USD/kg is available to fuel all existing nuclear power plants for another 80 years. If the price doubles, extended exploration will most probably lead to a tenfold increase of uranium stocks (40-50 MT), which would be enough for many hundreds of years of electricity production. Uranium from phosphate deposits amount up to 22 MT. If we can afford a uranium price of 450 USD/kg, which would ‘only’ double the costs of nuclear electricity production, we can extract uranium from seawater. Then the uranium stocks would amount up to 4,000 MT of uranium. Finally, all these numbers can be raised with a factor of 100 if we would use fast breeder reactors to breed fissile plutonium isotopes from U-238.
The earth’s crust contains 10,000 times more carbon than uranium. This would suggest that the energy contents of fossil fuels are much larger than that of uranium. The opposite is true! If each carbon atom would form a chemical bound with two hydrogen atoms, we can extract about 5 eV per CH2 molecule. In a nuclear reactor, we can extract approximately 2 MeV per uranium atom. This means we can extract 200 times more energy from uranium than from carbon. If we would use fast breeder reactors to fully exploit the uranium resources, this number increases with another factor of 100. In some nuclear reactors like the High Temperature Reactor and the Molten Salt Reactor it is possible to exploit the thorium reserves, by converting the fertile Th-232 to the fissile U-233 nuclide. The thorium resources are expected to be five times more abundant than uranium.The slides can be downloaded here.
|For more information, please contact j.l.kloosterman (at) tudelft.nl|