Nuclear power plants. Once closed, what?

2000/12/01 Carton Virto, Eider - Elhuyar Zientzia Iturria: Elhuyar aldizkaria

• Ingurumena

The first nuclear power plants were built in the 50s and 60s when they did not know what to do and how to act when closing. Five decades later, the definitive closure of nuclear power plants efficiently and safely is a matter of great responsibility. In fact, the teams of then are about to overcome or overcome a full life

... Nuclear power plants are designed to function properly for about 40 years, although sometimes they close earlier for various reasons. In the next 20-30 years, some 400 nuclear facilities in Western Europe and the United States and some 160 in the European Union should be closed, without the work being done in any way.

Following the serious accident in Chernobyl in 1986, the International Atomic Energy Agency (EEA) developed the standards on atomic energy security, establishing, among others, the general procedure for the definitive closure of nuclear power plants. Since then, the EANA itself, the Nuclear Energy Agency (AEMA), which is part of the Organization for the Economy, Cooperation and Development (OECD), and the European Commission, are working on this.

A long three-step road

The EANA recommends the closure of nuclear power plants in three steps, defined according to the physical characteristics of the facilities and the measures necessary for their conservation. The first task is to remove nuclear fuel and radioactive material immediately after closing, as well as waste generated in the usual operations. The disassembly phase of the installation is then started.

In the first phase they are left when there is a barrier that protects the reactor and other mechanical opening systems are sealed, that is, the most radioactive products are removed and the rest is protected. Strict care and reviews are set out below as the next step will not be addressed immediately. In the second phase the protection barrier will be reduced to the maximum and the biological shield (usually water) surrounding it will be extended and sealed. The other buildings are dismantled or decontaminated to adapt to new uses. Finally, the non-reuse of facilities in nuclear projects involves the extraction of all materials that exceed the levels of natural radioactivity, leaving the space occupied by the nuclear power plant completely clean.

Steps can be done below or not. The second option has been carried out by most countries that must close nuclear power plants, except for Germany and Sweden. The United States and Japan have set a 10-20 year deadline, extendable to 60, to stop the reactor and begin definitive closing work; Canada and France have decided to wait several decades. In the case of the UK, the power plants will leave undismantled for over a hundred years with uranium-fueled commercial Magnox reactors.

The significant reduction of radioactivity in 135 years, according to the managers, will allow the use of manual techniques for the disassembly of the plants and greater safety, in addition to reducing the generation of waste. The Tokai-1 Japanese reactor, which stopped working in 1998, will be completely dismantled in 5-10 years and is of the same type.

More frequent strategies

Given the variety of existing nuclear reactors and facilities, there are numerous strategies for applying the steps recommended by the EANA. However, the plans announced in most countries are generally variants of the DECON, SAFSTOR and ENTOMB strategies:

In the DECON strategy all radioactive components and structures are decontaminated or disassembled from the beginning. Small activity waste (see Elhuyar. Science and Technology no. 158 p. 26) are transported or stored in the corresponding warehouses. Although it takes about 5 years to do the work, once it is finished, the space is clean. In SAFSTOR and ENTOMB strategies things get slower.

The DECON strategy is the one that shows the most respect to the following generations, but also the one that generates the most waste. Therefore, it is not usually chosen. However, the significant progress made in recent years in the field of decontamination techniques has significantly reduced radioactive waste, making it possible to strengthen the DECON strategy.

Drilling cement, sandblasting or blasting are conventional ways and microwave and laser tests have also been performed. The diamond tip knife for large-scale decontamination is very useful as it can also cut metal components articulated in cement. The area of 400 m2 used by Labana in the Eurochemic project of the European Union was three times faster than drilling and generated half of the waste. It is much more comfortable for employees. In the case of metals, decontamination is used in the crust area and is usually done by physical chemical methods. If you opt for the SAFSTOR strategy, the Nukean plant will remain the same for 20-150 years. A safety shield will be built around the plant, especially to ensure that radioactive materials will not cause accidents.

The SAFTOR strategy uses time as a decontamination agent, as atoms lose radioactivity over time. After 30 years, for example, radioactivity of cobalt isotope 60 will be quarantine of the initial, and at 50 one-thousandth of the initial. Once the level of radioactivity has been sufficiently reduced, the final closure of the plant will be carried out in accordance with the DECON strategy.

The ENTOMB strategy offers the least guarantees in all three. All components are coated with a resistant material, for example cement, until radioactivity decreases sufficiently. Subsequently, when they are no longer dangerous, the cover will be removed and the disassembly will begin. This is a long time since at 100 years it is very possible that the level of radioactivity is above the maximum allowed, so it is necessary to foresee long-term security measures. Leaving a nuclear power plant buried in cement for 100 years does not seem, a priori, an adequate solution.

How to choose the way?

Franois Chevenier, head of the French Radioactive Waste Management Organization in 1990, described as negligence the enjoyment of electricity thanks to nuclear energy and the cession of waste to future generations. But that is precisely what is done: In 1999, 94 nuclear reactors stopped working, while only a few have been dismantled.

The type of reactor, the physical state of the plant and the level of radioactivity are important elements to choose a disassembly process or another. The degree of radioactivity and its location depends on the reactor. For example, in the boiling water reactor (BWR) radioactive emissions are transported from the turbine circuit, while in the pressurized water circuit (PWR) radioactivity is found in the primary refrigerant system and does not reach the turbines.

Nor is it a question of what to do with the waste generated by the plant. 99% of the radioactivity of nuclear reactors is associated with the fuel that is removed in the first phase. If the soil is not polluted, the rest of the radioactivity is due to the "activated products", that is, to the metal components that have long suffered a neutron bombardment. Neutrons are fission agents and products, but at the same time they turn the stable atoms of the roofs surrounding the reactor into radioactive isotopes such as iron 55, cobalt-60, nickel-63 and carbon-14. The first two are very radioactive and emit gamma rays.

Nuclear power plants generate mainly short-lived radioactive nuclei, which at 5 and 30 years lose their radioactivity considerably. But that does not mean that in such a short time they cease to be harmful: half the life of nickel 59 is 80,000 years and one million years must be expected to lose all radioactivity.

Most of the waste generated in the disassembly can be stored together with others already generated by the plant, since the volume and nature of the waste resulting from disassembly is of the same order of magnitude as that generated in operation. However, the volume can be greatly reduced by decontamination, compaction, segmentation and incineration of surfaces. However, many countries lack sufficient capacity to store waste.

Accounts

The cost of closing a nuclear power plant has not yet been fully defined, nor who is responsible. Some calculations indicate that the cost ranges from 10% to 40% of the amount initially established, others 100% and that closing can be more expensive than opening. That means billions in pesetas, but who pays them? The exploiters of the plant? Consumers? Descendants? Monetary funds have been established or proposed in many countries to ensure the financing of dismantling. The idea is to calculate and extend the amount that will be needed when closing the plant. To do this, three are the most common ways: to put money to close the plant with start-up; to collect a portion of what consumers pay for electricity, around 5% annually; and to make sure that with insurance or credits there will be money to close the plant. However, there is suspicion that both waste and funding will be given to future generations.