Nuclear energy

Can you imagine living in today's information, mobile and service-oriented society after a few hours or days without electricity? It is quite possible that after a few days or weeks something would happen what our descendants would describe in history textbooks as a collapse of civilization.

For the successful continuation of our civilization the electricity is at least as important as the sustainable management of the environment. Electricity obtained from nuclear energy represents 15% of all electricity consumed on the planet. In the countries of the European Community its contribution is twice in size and is equal to 30%. In Slovenia nuclear energy covers as much as 40% of electricity consumption. This share ranks Slovenia high on the 8th place among 30 nuclear countries in the world (IAEA 2011).

Natural nuclear reactor

Before we focus on nuclear energy, let us look at the province of Oklo in Gabon (Africa) about two billion years ago. The proportion of fissile isotope U-235 was then approximately 3.5% of all isotopes of uranium (the vast majority being U-238). Water in the lake above deposits of uranium ore was slowly seeping between uranium. Approximately 15 natural reactors in the area of today's uranium mine were created in this way. They have operated with an average output of some 100 kW for hundreds of thousands of years and then spontaneously shut down.

It is interesting to note, that we know about the natural reactors in Oklo only since 1972. French colleagues discovered at first sight insignificant difference in the proportion of U-235, when a routine surveillance of quantities of nuclear materials was performed at the time. In the ore from Oklo the proportion was only 0.717% instead of the expected 0.72%. The phenomenon was investigated further and they finally confirmed the "deficit" of U-235 and the presence of typical long-lived descendants of nuclear chain reaction. This was a proof of the existence and activity of the natural reactor. The proportion of U-235 in uranium over the past two billion years decreased from 3.5% to 0.72% due to the spontaneous decay with half-life of 700 million years.

Nuclear reactor

Fissile material in a nuclear reactor is called the nuclear fuel. In today's nuclear reactors we utilize the heat generated by the chain reaction of fissile nuclei. Uranium U-235 and plutonium Pu-239 are used primarily. Uranium U-235 isotope is found in the nature: its proportion in the nuclear fuel is increased with enrichment, which is relatively time consuming and difficult process. Isotope Pu-239 is not found in the nature and is produced in nuclear reactors. Uranium U-238 captures a neutron at first. Then the change of neutron to proton is repeated twice to produce Pu-239. There is enough Pu-239 produced so that before the retirement of a nuclear reactor, the heat produced from Pu-239 and U-235 reactions are about the same.

In nuclear reactors mostly isotopes of U-235 undergo fission. Fission is caused by a neutron that is absorbed by fissile nucleus. In this way, an unstable compound nucleus is created. Such fissioning nucleus usually splits into two smaller nuclei which are called fission fragments. 

This process produces heat and a few neutrons, which again cause splitting of other fissile nuclei.

Neutrons released in fission, have a relatively high kinetic energy. To achieve a stable chain reaction in a nuclear reactor they must be slowed down significantly before the collision with the fissile nuclei. We do this with the help of moderator, in which fast neutrons transfer part of their kinetic energy as they collide with smaller nuclei. Common moderators are water, heavy water (with hydrogen isotope deuterium) and graphite. If water is used as a moderator, about 3.5% U-235 enrichment is needed for a stable chain reaction. Heavy water is much more efficient moderator and allows for a chain reaction with natural uranium mixture (0.72% U-235).

Nuclear explosion can only be achieved in very different circumstances. We need fast neutrons and fissile material condensed as much as possible, e.g. 90% or more U-235 enrichment. Therefore, in nuclear reactors a nuclear explosion cannot occur.

Nuclear power plants

A nuclear power plant is largely similar to a conventional thermal power plant. In somewhat simplified view one has to replace a coal, mazut or gas powered steam boiler with a nuclear reactor. Nuclear chain reaction gives off heat to the water in the reactor. This heat is then converted to electricity in the non-nuclear (or the classical) part of the power plant using gas or steam turbines and generators.

Pressurized water reactor power plant in Krško uses enriched uranium (about 5%) as a fuel. Demineralized water is used as a moderator and reactor coolant. Krško NPP produces about 5 TWh of electricity annually. The power plant has to be refuelled with more than 15 tons of nuclear fuel every 18 months to produce such amount of electricity. More information about Krško NPP can be found on their website. For comparison, the thermal power plant Šoštanj consumes 4 million tons of lignite for the slightly lower annual production of close to 4 TWh.

According to the International Atomic Energy Agency there are 443 nuclear power plants in the world today. There are 64 in construction and 125 permanently stopped nuclear power plants. Geographically speaking, today’s nuclear power plants are placed in 30 world countries. More than half of operating nuclear power plants are older than 25 years. They were designed for a service life of around 40 years. In the U.S. and partly in the European Community the service life is being extended to 60 years, taking into account the latest scientific knowledge and after replacing some key components of a power plant. Of course, a possible extension must be justified and authorized for each plant separately.

Nuclear power plants in the world (source: Wikipedia CC BY 3.0)

Nuclear fuel and radioactive waste

In general nuclear waste is classified as low-level, intermediate-level and high-level radioactive waste.

Typical low-level radioactive waste is contaminated personal equipment for workers in the medical and nuclear power plants (e.g., gloves, overalls, tools ...). Among intermediate-level radioactive waste we mainly find materials that were necessary in the operation of nuclear power plants (e.g., various resins for the extraction of boric acid from the primary coolant) and pieces of medical equipment. Spontaneous decays will reduce the radioactivity of low and intermediate level radioactive wastes to harmlessness in less than 300 years. They are therefore excluded from our environment for a period of 300 years. In Slovenia we annually produce less than a liter of low and intermediate radioactive waste per person.

In Slovenia, we have identified the location and the basic characteristics of the low-and intermediate-active waste. Built and managed by the Agency for Radioactive Waste it will be located in Vrbina near Krško, i.e. in the immediate vicinity of the nuclear power plant.

High level radioactive waste contains primarily spent nuclear fuel. In the fuel we spent primarily U-235, but still very useful Pu-289 and U-238 remain inside. With a relatively complex technology, which is dominated by only a few countries, plutonium can be used as fuel in existing nuclear reactors. In fast breeder reactors of generation IV U-238 will be transformed into Pu-239, which can be used as a nuclear fuel even today. In this way, known reserves of uranium, which with today's technology are sufficient for more than 200 years, can be utilized for more than 1,000 years. Thus, nuclear power could become sustainable.

If we decide to dispose of spent nuclear fuel, we have to remove it from the biosphere for around 100 000 years. Examples of stable geological formations that make this possible are clay, granite and rock salt. This time can be reduced significantly with the separation and transmutation processes. Research in this field is relatively young, but very promising. It suggests that with appropriate processing, radioactivity of spent fuel can be reduced to a few 100 years. In Slovenia we annually produce 5 g of such waste per person.

All activities associated with the decommissioning of the Krško nuclear power plant and disposal of radioactive waste are financed in Slovenia by the fund for decommissioning and disposal of radioactive waste from NEK. Necessary funds are collected from electricity sold from NEK. The contribution to the fund, which currently amounts to slightly less than 4 EUR/MWh, is set by the decommissioning program, which ARAO renews every 5 years.

 

Page editor: Matej Tekavčič