Tuesday, February 7

The Almaraz II nuclear power plant has been completely disconnected from the electrical grid (and it is normal)

The Almaraz nuclear power plant, which is located in Cáceres, has two nuclear reactors. Almaraz I has a power of 1049.40 MW, while Almaraz II is slightly less powerful because it is close to 1044.50 MW. Just seven days ago, on September 26, this last unit was disconnected from the power grid For good reason: refueling and various maintenance tasks are required.

This operation has not been improvised at all. It is part of the natural life cycle of a nuclear reactor; In fact, this unit in particular, Almaraz II, has faced no fewer than twenty-six refueling stops since it came into operation in 1984. This is the twenty-seventh. And all of them have been carried out within the margins of action that had been foreseen by the technicians responsible for the correct operation of these nuclear facilities.

What is done when a reactor is scheduled to stop for recharging

A nuclear reactor operates for eighteen months on average non-stop, although maintenance is also carried out during this period of uptime. Most equipment is duplicated, tripled, or quadrupled, so while one of them is running, revision and preventive or corrective maintenance of one of your reservations. These processes are coordinated from the control room by operators and supervisors.

The function of these technicians is to carry out the reactor shutdown maneuvers in accordance with a series of carefully laid out procedures. The operators are responsible for operating the equipment from the panel, as well as directing the field operators and operation assistants, who are the hands, eyes and part of the brain of the control room operators. The latter operate the local valves and check the status of the equipment.

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As Alfredo García, known on Twitter for his alter ego nuclear operator, during our conversation with him last year, all this work is carried out by operators led by the head of the control room and supervised by the shift manager. When the reactor shutdown maneuver, cooling and depressurization have been completed, the work of place the downloadswhich consists of leaving the equipment ready so that maintenance can carry out its work.

If it is a pump, what you have to do is remove voltage, de-energize it, isolate the suction and discharge, drain it if it is necessary to work inside the pump body, treat the water, oil or any other fluid with which the pump works. , etc. Everything is left ready for maintenance to work on that equipment. The most curious thing is that during a refueling stop, more than 10,000 work orders are produced. In fact, in Almaraz II they are carrying out right now 13,000 scheduled tasks and 22 design modifications.

These jobs range from changing a parameter in a computer that controls a system to changing a turbine for a new unit. There are small jobs that can be done in a few hours and large jobs that can take several days, and all of them are carried out by between 1500 and 2000 peoplewhich are the ones that are involved in recharging for approximately one month.

These are the properties of the fuel used in nuclear fission reactors

Before going any further, it is worth making a stop along the way to review the characteristics of the fuel used by today’s nuclear power plants. The reason why it is used in nuclear fission reactors an atom of uranium-235and not another isotope of this element or any other chemical element, is that by bombarding its nucleus with a neutron (a process known as induced fission) uranium-235 is transformed into uranium-236, which is an element more unstable.

This simply means that uranium-236 cannot remain in its current state for long, so it splits into two nuclei, one barium-144 and one krypton-89, and also emits two or three neutrons. . And here comes the really interesting part: the sum of the masses of the nuclei of barium-144 and krypton-89 is slightly less than that of the nucleus of uranium-236 from which they came (“disappears” about 0.1% of the mass original). Where has the missing mass gone? There is only one answer: has been converted into energy.

The formula E = m c², probably the most popular in the history of physics, relates mass and energy, and what it says is simply that a certain amount of mass equals a certain amount of energy, even though the mass is at rest. In fact, the equivalence between mass and energyproposed by Albert Einstein in 1905 (as you can see, more than a century ago), tells us something else very important.

The c in the formula represents the speed of light in a vacuum, which, as we all know, is a very large number (approximately 3 x 10^8 m/s). Furthermore, it is squared, which means that even a very, very small mass, such as a portion of the nucleus of an atom, even if it is at rest contains a large amount of energy. This is what we know as rest energy.

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If the mass is in motion, its total energy is greater than its rest energy. And, if we observe the equivalence between mass and energy, it is easy to realize that the mass of a moving body is also greater than its rest mass, a phenomenon that introduces us fully to relativistic physics. In any case, the energy that we obtain when fusing or fissioning atomic nuclei comes from the force that holds them together: strong nuclear interaction.

Understanding with some precision the relationship between mass and energy is important because it helps us understand how it is possible that a mass as small as that of an atom allows us to obtain such a large amount of energy. In any case, the nuclear fission process does not end here. And it is that each one of the neutrons that we have obtained as a result of the disintegration of the uranium-236 nucleus in the nuclei of barium-144 and krypton-89 can interact with other fissile nuclei, causing a chain reaction.

nuclear fuel2

The zirconium alloy tubes used in fuel rods are about 4 meters long and are filled almost completely with uranium oxide pellets.

However, not all of the neutrons emitted during the decay of the uranium-236 nucleus will interact with a fissile nucleus. But it is not necessary for all of them to do so. It is enough that only one of those neutrons achieves it so that we obtain a stable number of fissions, and, therefore, a controlled reactionwhich is the purpose of nuclear power plant reactors.

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The uranium oxide powder that nuclear fuel rod factories receive is already enriched, although never above 5%

Interestingly, the uranium oxide dust that nuclear fuel rod factories receive is already enriched, although never above 5%, which is the proportion set by international regulations for the production of fuel for nuclear power plants. This means that at most only 5% of the total mass of uranium is the uranium-235 isotope (which, as we have seen, is the “easily” fissile one), while the rest is mostly uranium-238.

It may seem that there is very little uranium-235 compared to the total mass of uranium, but this amount is actually enough to sustain the nuclear fission reaction. The most curious thing is that not all the uranium oxide received by the factory uses the same enrichment because the different technologies used by nuclear reactors require fuel with different enrichment, which, yes, can never be greater than 5%.

1,100 additional workers and 40 days of work to get the reactor ready

Everything we have investigated so far gives us the necessary context so that we can understand the complexity involved in the process of refueling a nuclear fission reactor. In the refueling and maintenance operations that are being carried out at Almaraz II right now, they are involved, in addition to a good part of the personnel of the nuclear power plant, 1,100 additional workers belonging to 70 companies specialized in some of the specific tasks that need to be undertaken.

These works will last 40 days, so that once all the pertinent checks have been completed and satisfactorily carried out, the second nuclear reactor at the Almaraz plant will resume its activity. According to the Nuclear Forum, this facility injected into the electrical network during 2021 7% of all electricity that was consumed in Spain, so it is clear that this nuclear power plant currently plays an important role within the Spanish electricity infrastructure.

More information: Nuclear Forum

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