Why is U235 less stable than U238

Nuclear fission

Neutron-induced fission of uranium-235

U-236 is unstable and therefore gives about 10-14 Seconds its excitation energy mainly by splitting at the two medium-weight nuclei again. These fission products are positively charged. They therefore repel each other due to the Coulomb force and become like a cavalier start within 10-20 Seconds to full speed. Their kinetic energy, which is converted into heat, makes up around eighty to ninety percent of the energy that is released during nuclear fission. The remaining ten to twenty percent is in the radioactivity of the newly created medium-weight nuclei.

The nuclei of the fission products are not always the same, but have statistically different numbers of charges, i.e. they belong to different chemical elements. If the amount of the elements produced during the fission is represented graphically against their number of charges, a saddle-shaped curve with two maxima results. In the first maximum one finds elements such as strontium, krypton or yttrium, in the second maximum for example xenon, cesium or barium. Most of these fission products are radioactive because of an excess of neutrons and only change into stable end products after more or less long series of decays. A total of around 200 different fission products are known.

In addition to the two fission nuclei, 2 to 3 neutrons are also produced, which can be used to split other U-235 nuclei and thus release more energy and neutrons. One then speaks of a chain reaction. It is crucial for maintaining the splitting process and thus for the use of nuclear energy.

The probability that a neutron will attach to U-235 depends on its speed. It grows the smaller it gets. Since the neutrons that are released during the fission are too fast to accumulate, they have to be slowed down to so-called thermal speed by collision processes with the atomic nuclei of a moderator. To make braking through impacts effective, the moderator's atomic nuclei should have the same mass as the neutron or come as close as possible. That is why water is the ideal moderator because the nuclei of the two hydrogen atoms of the H2O-molecule consist of protons and thus have practically the same mass as the neutrons to be decelerated.

Construction of a nuclear reactor

In a nuclear reactor, uranium and moderator are arranged in such a way that a continuous fission process is maintained with the help of control devices and nuclear energy is released as heat in a controlled manner. The ratio of the two uranium isotopes 235 and 238 is a critical variable. In light water reactors, which are mainly operated worldwide today, the 0.7 percent uranium-235 in natural uranium is not sufficient to maintain a continuous fission process. For this reason, uranium-235 is enriched to around three percent before it is used in the reactors, which requires special uranium enrichment plants.

The released neutrons not only split the uranium-235 but they can also be captured by the dominant uranium-238. This creates the so-called transuranic elements, such as the subsequent radioactive decay, the new isotope plutonium-239, which in turn is a fission material and can therefore also release energy. Depending on the type of reactor, it even contributes around 30 percent to the energy-generating cracking process. The transuranic elements also include chemical elements such as neptunium, americium or curium. The transuranic elements contain isotopes, some of whose radioactivity is very long-lived. As a result, all of the radioactivity in the spent fuel elements will only have largely subsided after a few hundred thousand years. Overall, after about three years of operation (pressurized water reactor type), the spent fuel elements of the light water reactors contain just under one percent uranium-235, just under half a percent uranium-236, around 95 percent uranium-238 and just under one percent plutonium isotopes. The rest is made up of fission products (three percent) and other transuranium elements (less than 0.05 percent), the so-called actinides. The proportion of U-235 thus almost corresponds again to the percentage of natural uranium.

The concept of the closed fuel cycle, as it is pursued in France, for example, aims to chemically separate the uranium and plutonium by reprocessing the spent fuel elements in order to recover them for energy generation and to store the radioactive residue safely for a very long time . This means that around 97 percent of the spent fuel can be reused. Further concepts in the state of research and development include the additional separation of the long-lived radioactive substances, for example in order to convert them into much shorter-lived chemical elements for final disposal through interaction with fast neutrons, also known as transmutation.

In Germany it was decided to implement the concept of the open fuel cycle, i.e. to directly dispose of the spent fuel elements and other highly radioactive waste from nuclear power plants. According to the J├╝lich Research Center, this initially involves 7,790 tonnes of spent fuel elements that were produced from German nuclear power plants by the end of 2011. After all nuclear power plants are no longer in operation in 2022, the Federal Office for Radiation Protection predicts that another 2,760 tonnes will be added. Together with storage containers and the highly radioactive waste from previous reprocessing in France and Great Britain, the spent fuel elements will have a total volume of 28,100 cubic meters or 10 Olympic swimming pools. The long-term radioactivity is generated by substances that make up less than one percent of the fuel elements. It is planned to move all spent fuel elements and the waste from reprocessing to an underground repository.

In addition to uranium, the chemical element thorium is also important for the production of nuclear energy.

Energy from nuclear fission

Energy is obtained because mass is converted into energy during nuclear fission. So there is no chemical reaction, as is the case with burning fossil fuels, and therefore there is no CO2 released into the atmosphere.

The nuclear fission process is very efficient. For example, when one kilogram of U-235 is split, only about one gram of mass (one per thousand) is lost, which is converted into thermal energy. Using the Einsteinian relation E = mc2 this results in a value of around 25 million kilowatt hours. This corresponds to combustion energy of around 2,500,000 kilograms of hard coal with an energy content of 7,000 kilocalories per kilogram. The energy yield per kg of fuel is around 2.5 million times higher than when burning hard coal.

The reason for these enormous differences is ultimately that two natural forces with differently large interactions are used. During combustion, the underlying chemical processes take place in the shell of the atoms involved. Here the electromagnetic interaction rules. In nuclear energy, in which the nuclei of the atoms play the decisive role, the much larger strong interaction that binds the nucleons together is decisive.

The decisive factor here is the size of the binding energy per nucleon in the nucleus. It is not constant for the elements, but increases from the lightest element, hydrogen, at first very steeply and then more slowly to the medium-heavy elements, such as krypton. After that, it falls slightly down to the heavy elements. When heavy nuclei are split into two medium-sized ones, the difference in binding energies is released in the form of heat through the movement of the fission products.

The differences in the strength of the interactions are also expressed in another number: The breakdown of a heavy atomic nucleus into two medium-weight nuclei results in an amount of energy that is around 400,000 times greater than chemical reactions between whole atoms. These huge differences may explain why, on the one hand, nuclear energy is extremely attractive from an energy point of view, but on the other hand, because of the enormous energy density that has to be mastered, it requires a particularly high degree of responsibility and care with regard to the safety of nuclear power plants.