Premise

Here, in Italy, we had two referendums that sanctioned the prohibition of building nuclear power plants on Italian territory. Now, during the current election campaign, the center and right parties proclaim their intention to reintroduce the construction of nuclear power plants in Italy [1].

And to do so, they adopt marketing strategies, alias advertising, used to counterbalance the aura of danger and negative environmental impact that nuclear power has been carrying with it since the Chernobyl accident and that of Fukushima disaster [2]. Hence the combination of the adjectives safe and clean with the nuclear term, designed to drastically reduce any instinctive repulsion in the listener aroused by the adjective nuclear.

Now we have decided to make public what we think. Because our university preparation, ended in 1984, was the graduation of the Degree in Nuclear Engineering. So we think we understand something about this problematic.

Index:

• Premise;
• Goal;
• General concepts;
• Security;
• Environmental impact;
• Conclusions.

Goal

In this article we will focus on the concepts of security and clean so boasted by the aforementioned parties. And we will do it by retracing the steps that led us to mature our convictions: we, senior men, often think about our past.

About nuclear plants there are many other aspects that should be considered and on which, for now, we will override (and we will not justify the following statements):

• economic (nuclear plants are so expensive that their construction, and, often, even their exercise requires huge public funding; in other words: taxpayers pay a large part of the construction, and the operator pockets the profits derived from the sale of electricity);
• of operation (thermonuclear power plants cannot breathe, that is, their power must be made to vary very slowly, and must be kept fixed as much as possible; this makes them unsuitable to follow the variable trend of consumption over the course of a day; they must provide the base load [3], typically the night load; not only that, it also makes them unsuitable to complement alternative energies [4]: wind power and photoelectric, the power of which depends on environmental conditions);
• environmental (all thermoelectric plants, including nuclear ones, need large quantity of cooling water to be able to operate; right now, from July to September 2022, part of the French nuclear power plants were put on standby because, due to drought, there is not enough water in the rivers to cool them);
• fuel supply (do you know who is the world's top manufacturer of uranium ore? The Republic of Kazakhstan [5], satellite of the Russian Federation. And which is the country with the greatest capacity of uranium enrichment [6]? The Russian Federation [7]. Let's check before talking about independence from the sources of supply [8].)

For now, let's stop here. Those interested will find extensive pro and con dissertations about various aspects of nuclear power. For example Il Fatto Quotidiano has published this article signed by Angelo Bonelli, convinced anti-nuclear and exponent of Green Europe. While on the YouTube channel L'Avvocato dell'Atomo you will find various videos and speeches in favor of nuclear power.

But let's go back to the guidelines in our article: safety and clean. By the way, we don't like the term clean. Instead we will use the term environmental impact, which we find more appropriate to the subject.

The rest of this article is divided into the following sections:

• an explanatory part of some General Concepts, which will be useful to us in the subsequent presentation; who is in a hurry, can skip it, bearing in mind that something may seem unclear in the following text: you will have to trust our exposure;
• a part dedicated to the problem of Security;
• the section in which we will talk about the Environmental impact;
• and is closed by a succinct section of Conclusions.

Pay attention to the fact that there are relationships between the different sections. Not only among the one dedicated to general concepts regard safety and environmental impact. But also between safety and environmental impact.

General concepts

In the beginning, there is the atom. Consisting of a nucleus surrounded by electrons.

The nucleus in turn is formed by an aggregate of protons and neutrons.

The identity of the atom is defined by its number of protons: the atomic number, which sets its chemical characteristics. Thais to say with what others atoms can be bound, more or less stably, forming molecules.

There are atoms with the same atomic number, but different number of neutrons. When we take into consideration, in addition to its atomic number, also its number of neutrons, we speak of isotope.

The sum of the number of protons and the number of neutrons of an atom is called atomic mass number. For example in nature uranium, with atomic number 92, shows two different isotopes:

• the most common has atomic mass number 238 (²³⁸U); about 99.3% of natural uranium is made up of this isotope;
• followed by the one with atomic mass number 235 (²³⁵U); which is about the remaining 0.7% of natural uranium.

Clarification: we speak of isotopes when we refer to a specific element. That is, atoms with the same atomic number. Another example. Let's take iodine [9], which has 53 protons. Its (stable) isotope is iodine 127 (¹²⁷I), which has a nucleus formed by 53 protons and 74 neutrons. But we can also find the (unstable) isotope ¹³¹I, whose nucleus is formed by 53 protons and 78 protons.

When we wish to analyze in general the nuclear characteristics of different atoms, possibly also varying the atomic number, then we speak of nuclides.

Two paragraphs ago we introduced the terms stable and unstable. Let's clarify them. A nuclide is stable if it remains in its state indefinitely. It is unstable if, in more or less long times, the core undergoes some kind of transformation. These transformations are said radioactive decays and are accompanied by an emission of energy. To get an idea of ​​the extent of what we are talking about: 252 stable nuclides and about 87 unstable nuclides have been observed in nature. In total, about 339 nuclides have been observed on Earth. To these we can add more than 3000 non-naturally occurring radionuclides produced by nuclear reactions carried out in laboratory, or in manufacturing facilities such as nuclear power plants.

Returning to the isotopes. The knowledge of the behavior of the different isotopes of an element in the scope of a nuclear reaction is crucial. Different isotopes can behave in different ways.

We have now introduced the term nuclear reaction With this diction we mean an interaction between nuclei of atoms or between nuclei and a subatomic particle, typically a neutron. Greatly simplifying a complex subject, these interactions can lead to:

• a fusion of light nuclei, usually resulting in an heavier nuclei [10] and the release of a fair amount of energy in the form of movement of the new nuclei: that is to say heat;
• a break (fission) of a heavy nucleus, usually caused by the impact of a neutron, which generates two smaller nuclei, in addition to the generation of one or more particles, and energy in the form of heat and/or electromagnetic radiation (gamma rays).

The following image schematizes what we have said:

We reiterate: we are simplifying to the extreme. However, in the case of fission, the impacting subatomic particle is usually a neutron, which can have:

• a high speed, in this case we speak of fast neutrons;
• a speed statistically aligned with the ambient temperature in which they move; these neutrons are much slower and are called thermal neutrons.

If we were dealing with nuclear fusion we would be in raptures. As fuels we could use isotopes of hydrogen which occur in nature, or easy to produce. And the products of the reaction would be stable isotopes or would decay in rather short times.

Unfortunately, nuclear fusion requires environmental conditions that are far beyond our technological capabilities. At the University the professors told us: "It will take another twenty years of experiments." Well it passed forty years, and we are still in the experimentation phase. Researchers in this area believe we cannot realistically think about to build real industrial prototypes before year 2050. Even if there are some optimists who swear it will come before.

For this reason in this article we will not talk about nuclear fusion. Although we believe that this is the real answer to the need for energy that mankind needs.

Instead, here we will talk about nuclear fission. This is the technology currently used in power generation plants.

These plants are almost all thermal reactors, that is, they use fissions generated by thermal neutrons.

In a nuclear plant for the production of electricity, what we do is use a nuclear reactor to generate steam. This is sent to a turbine which, by dragging an alternator, generates the current fed into the electricity grid. The exhausted steam flows into a cooling radiator, called a condenser, which brings it back to liquid phase. So we have water ready to start the cycle again.

There are several ways to obtain what is stated in the previous paragraph. One of the most popular is illustrated below.

Below is a very simplified functional block diagram of a pressurized water system (PWR [11]) which illustrates what has been said in the previous paragraphs.

Let's read the scheme together. By now we see the nuclear reactor as a kind of boiler, later we will deepen. As in any self-respecting boiler, we send a flow of water go through it to subtract the heat produced: is the primary cooling circuit.

Here we don't want the water to boil and turn into steam. For this reason we keep it at a pressure just over 150atm. Water enters (from the downw) the reactor with a temperature of about 275 ° C, and comes out (from the upper) with a temperature of about 315 ° C.

In the steam generator the water from the primary heats the secondary cooling circuit. Here, in the secondary, we have steam generation, which we send to a turbine. This is a machine equipped with vanes mounted radially with respect to an axial rotating shaft. The steam pushes the turbine blades spinning his shaft quickly. This is coupled mechanically to an alternator which generates the electrical current send to the the distribution network. Then the steam coming out of the turbine, which has been lost temperature and pressure, it is sent to the condenser. This component performs the inverse function of the steam generator: it returns the steam to the liquid phase using cooling water coming from outside the system: river or sea water, depending on the location of the plant.

Now that we have clarified how, in general, a PWR plant works to generate electricity, we have to spend a few words to understand how it works the boiler, i.e. the light water nuclear reactor which is the heart of the plant.

To simplify a complex matter, let's say that if we hit a core of: sup:235U with a thermal neutron, we obtain its fission, producing:

• a couple of nuclei of lower mass,
• and the emission of some fast neutrons.

By slowing down the fast neutrons mentioned above, we have thermal capable neutrons to split other nuclei of: sup:235U.We get the slowing down of fast neutrons by making them collide with a moderator element. In the case of a PWR the thing is simple: the moderator is the same water as the primary circuit. The collisions of fast neutrons with water molecules give their energy to water: this heats up and the neutrons slow down. Two birds with one stone: we are happy.

But we also need to check that are not produced too many thermal neutrons. Otherwise the reactor temperature would suddenly increase, getting out of hand. To prevent an excess of neutrons, we need to absorb them with an adjustable mechanism. It is the control bars, which have a function similar to that of an accelerator for a car engine. They are made of a material capable of capturing neutrons [12], without releasing them. When we put the control rods in the reactor, we decrease the reaction, like when we lift the foot from the accelerator. When we extract them we get the reverse effect. When the reactor produces the power we request, we need to adjust the control rod's position so that the fission reaction in progress remains stable: without decreasing or increasing.

The following drawing [13] explains the aforementioned concepts, illustrating the construction of a nuclear reactor of a PWR plant.

Finally, an important concept, which we will find again when talking about safety. When we fully lower the control bars we almost stop the production of heat inside the reactor.

We emphasize the almost. Because, even if minimally, the fission reactions continue. But, above all, continue the decay activities of the various nuclides that were formed during the operation of the reactor. These activities develop heat.

And the residue heat produced under these conditions, if not removed, leads slowly to a heating of the reactor such as to bring the primary water to a boil. When this happens the water level in the reactor drops, partially or completely uncovering the core. At this point the fission reaction resumes in full, despite the presence of control bars, leading to the feared partial or total core meltdown. The worst accident that can happen to a nuclear plant.

For this reason, even if the reactor is off, we cannot stop cooling it just enough to remove the residual heat produced by the core. In almost all of the current plants, this means that we need electricity, because we have to feed pumps that move the water in the primary circuit. If this essential power supply fails, meltdown is assured.

Meltdown can happen for different reasons. The one described in the previous one paragraph is what happened in the Fukushima Daiichi incident, which we will mention further on. It can also happen for other reasons that lead to one uncontrolled production of heat in the core.

We conclude these introductory notes by considering the fact that one of the ways to classify nuclear facilities [14], is to refer to constructive characteristics that place them in precise temporal intervals of design and construction. This is the taxonomy by generations [15]:

• the first were civil plants built for prototype purposes in the 1950s and 1960s; these systems are currently all off;
• the second is made up of almost all commercial plants today working; built from the late 1960s onwards, they are designed for a lifetime operational for about 40 years, although now there is a tendency to lengthen it with interventions ad hoc; construction of plants of this generation are no longer in existence or planned;
• the third currently sees few plants in operation and/or under construction; these are projects derived from the second generation in which they were introduced modifications to optimize safety and energy yield.
• the fourth is under study; are six different technologies, most of the which is still on the high seas, while one, that of fast reactors, is the most analyzed and sees the presence of some experimental plants; We'll talk about it.

Security

In this section we will talk about the safety of nuclear plants, pointing out this concept has been, over time, always affirmed with great certainty by the proponents of nuclear power, regardless of the plant technology adopted. Which did not prevent the occurrence of the disasters we will mention.

Allow us to brush up on one of our hidden memories.

Fall of anno domini 1979. An excited student of Nuclear Engineering of the University La Sapienza, approached with the heart in the throat the university classrooms located in via Scarpa, in Rome. Finally it is done seriously! After two years spent studying trite and coarse materials of the scientific high school (mathematics, physics, geometry; only pleasant ones exceptions: industrial design and programming of electronic computers), now await us nuclear physics, atomic physics, radiation protection. And, further on, nuclear plants 1 and 2, nuclear reactor physics, and so on and so forth.

Good. We set out to study these fascinating subjects in good spirits.

The professors were erudite and effective guides. And during the lessons, in addition to the basic principles, we also began to talk to them about the safety.

Radiation protection introduced us to the mechanisms of the interaction of ionizing radiation with matter and, above all, with biological tissues: damage which can cause those who are irradiated. While with the professors of nuclear power plants, we calculated under different conditions, the probabilities of failure of a plant. And we spoke generically of the evacuation plans of the population [16] in the impossible case of catastrophic reactor failure.

Impossible? No, for heaven's sake. No serious professor would have used this expression. Indeed, the keyword was: zero risk does not exist. True. But the calculations performed during the exercises, with the average failure times shown by the manufacturers of nuclear power plants, spoke of probabilities with values ​​so low that the overriding idea among us students was: practically impossible.

Ok. But by March 1979 the practically impossible had already happened. The plant of Three Mile Island it had undergone a partial meltdown of the nuclear reactor. To contain the damage the manager had had to release into the surrounding environment on several occasions radioactive gases: an ethically unacceptable behavior, but obliged by physical circumstances.

The incident in question was caused by a malfunction of the plant [17], which was aggravated by the behavior of the operators in the control room: they did not understand what was happening, and they carried out maneuvers that aggravated the situation.

Good. Three Mile Island was a case study during the various courses, dissecting what had happened physically and the work of the controllers. And digesting the idea that, after all, the probability of an accident is ... probability: even if low, it can happen, but it ends here.

As a result, we considered safe nuclear facilities in operation or in progress of construction in that period [18].

Time jump. Year 1986. After leaving the University with a degree in Nuclear Engineering, we were hired by Olivetti to work in the increasingly disruptive sector of information technology. When, a few months late, ENEA communicated to us the possibility of being hired by the Department for the safety control of nuclear plants (the famous DISP), we declined the assignment because we were tormented by the inability to see a solution in human times to the problem of the storage of spent fuels, which we will discuss later.

But, in early 1986, we were still firmly convinced of the extreme operational safety of nuclear plants.

In April, the Chernobyl disaster occurred. And here our beliefs about the safety of nuclear power crumpled irreparably. Convinced as we were that we would no longer see a serious accident in our lifetime, Chernobyl troubled us deeply.

Of course, even in this case it was an accident partially due to design errors, but above all to inexperience (or worse) management by the staff. Furthermore, the passive safety of the plant was not even remotely comparable with that of Western plants. It suffice to say that did not have a safety containment system [19].

However Chernobyl convinced us that something was wrong at the root of the processes design of the safety of nuclear plants. In particular the human factor, responsible for running the plant, was not taken into consideration with due attention.

On the one hand, it was necessary to design systems resistant to a management of operators unable to analyze what were happening (because in panic conditions, or due to misleading signals from the side of the implant instrumentation, or due to lack or insufficient evidence of the signals in question ...). Basically something like the monkey test, under conditions of critical accident in progress!

On the other hand, how many times the professors of the degree course have said us it was unthinkable to entrust the safety of a nuclear plant in condition of accident to fully automatic mechanisms (or to programs of computers). The responsibility of managing a nuclear plant in critical condition had absolutely to be entrusted to human operators!

Stall! Operators need to know how to manage accidents, but when they are in incident condition they can be wrong. Such a situation is not admissible, ergo it is not possible to say that a safe system is being planned. End of a basic conviction of our technical preparation regarding the design of nuclear plants.

Good. We let the years go by, and we arrive in March 2011. In Japan, Fukushima prefecture, the nuclear plant of Fukishima Daiichi, operated by the Tokyo Electric Power Company was hit by a tsunami caused by a violent underwater earthquake.

Due to the earthquake, the plant's reactors shut down automatically. But the primary refrigeration systems had had power problems, and they were powered by emergency generators. Emergency generators that ceased to function when a 14-meter-high tsunami wave exceeded the safety wall that separated the plant from the sea, flooding the site of the five nuclear reactors that formed the plant itself. Three of these reactors meltdowned (the other two were off for maintenance, and the plant technicians managed to continue to cool them) and there were consequent explosions of hydrogen which they released ionizing radiation in the environment. An area of ​​20 km radius was evacuated around the plant: about 110,000 people.

Not only. An enormous amount of ionizing radiation was released via sea ​​water which was used to cool the reactors damaged by the incident.

The Fukushima Daiichi incident brought us a further consideration: the warnings repeatedly generated regarding the need to equip the installation of a safeguard against tsunami waves up to 15 meters high, were systematically ignored by the management responsible of the system.

Here re-emerge the economic considerations. When a security measure is too expensive, it is ignored by the designers and/or plant managers, that put the operating economy before the safety of the plant. Another missing piece in the logic of the design of nuclear plants: safety should come before economic considerations. But it is not so. Further reason that convinces us of the fact that safe nuclear systems are not designed (and managed).

A brief observation regarding the dire situation that is underway in Ukraine when we are writing these notes. We refer to the ongoing war between Ukraine and Russian Federation. War which, unfortunately, also involved a nuclear plant: the Zaporizhzhia nuclear plant. This plant consists of six Russian-designed PWR units (VVER-1000). The alarming news concerning whether the plant is connected or not to the Ukrainian power supply network do not worry so much because the plant does not produce energy towards Ukraine, but because it can't get the energy it needs for the disposal of residual heat in its reactors, which by now they have all been turned off. Under these conditions Zaporizhzhia must use the backup generators to ensure that the cores do not go into meltdown.

Well, now we are at the political elections of 2022, and it is time for talk about the future.

Why of the future? Because the declarations, and the programs, of various parties affirms the will to introduce safe nuclear power in Italy. The League on p. 57 of his program writes (this a translation from Italian language):

And this is the future: currently in Italy we have neither design skills nor construction of entire nuclear plants. Currently, at most we can talk about components. Titanic efforts (aka money) for long periods of time are needed to get a national nuclear supply chain back on its feet. And in Italy, when industry is called to large investments, invariably entrepreneurs call state to dare its substantial contribution [20].

In any case, it isn't reasonably assume a plan of less than twenty years, indeed more likely thirty years [21].

And what kind of nuclear reactor to choose? The League supports Small Modular Reactor. In fact, at page 56 of its program states (the emphasis is ours, so as the translation from Italian languag):

Let's see in more detail the points highlighted.

It is true that several countries are studying Small modular reactor. But it comes just about this: studies.

China is currently the most advanced country in this sector, as it has a first prototype under construction since October 2021. And plans to put it into production for 2026.

For this reason we analyze this reactor: designed from China National Nuclear Corporation (CNNC) <https://en.cnnc.com.cn/> , it's called ACP100.

The ACP100 has interesting technical features regarding security [22]. We will see how it will evolve, but, even if improved, still not we can think it is a safe reactor. The reason is simple. The factor of core damage frequency declared is of the order of 10^-6. That is a meltdown of the core every million years of operation. But to have the power of a current system we need to use ten units of ACP100. It means that we need to divide the million years by 10, becoming one hundred thousand years.

And this would be reassuring. In fact, taking the table 4 on pages 14 and 15 of the IAEA document Nuclear Power Reactors in the World, we observe that as of 31 December 2019, adding up all 443 nuclear reactors installed in the world, we obtain an overall operation of only 18329 years. It takes a long time to get to a hundred thousand years. So: all right?

Unfortunately not. All that glitters is not gold. A 2003 study [23] commissioned by the European Union has indicated a core damage frequency value approximately 5 · 10^-5. That is, one damage to the core every 20,000 years of operation. Well, in less than 20,000 years of operation, we have seen six core damage [#] ​_. As we have underlined in the previous paragraphs: safety calculations do not consider either human factors, nor various environmental factors. The core damage frequency is not a reliable benchmark for measuring the safety of a nuclear installation.

If, in a rush of pessimism, we report what we have observed to date on the second generation plants towards the ACP100, we would expect 30 meltdowns of the core in one hundred thousand years of operation.

Let's now consider the easiest placement of the system thanks to its small size. The site currently under construction in China is completely orthodox: it takes 5 (Chinese ... in Italy it is a must see) years of work. And the regulations to be respected (anti-seismicity, presence of water for cooling, small population, ...) is the usual one. Nothing to gain from this point of view, indeed, we have to identify 10 sites (or put 10 reactors in one site).

Why 10 reactors? We said it before. The ACP100 has a power output of only 125MWe. Quite a few: to make an average French nuclear plant, that is about 1000MWe, we need about 10 ACP100.

Also, again due to the small size, the cost of energy production will have to be verified [24].

Also the reduction of the construction time (5 years per plant) is not relevant.

Furthermore it doesn't have the ability to break down quantity of highly radioactive spent nuclear fuel. As mentioned in a previous note, the ACP100 is a third generation PWR reactor. That is, it is a pressurized water reactor: the operating principle used by the tried and tested French reactors. But it is not thrifty in terms of radioactive waste production.

On this point the League refers to fourth generation technologies, that is (some, there are six different families, not all with the same characteristics) reactors capable of being fed with plutonium and other radiactive isotopes products produced by second and third generation plants. We will discuss this in the section on the environmental impact of the new reactors.

And we come to the flexibility of use to integrate renewable energy systems. We have a PWR on our hands, albeit a small one. And the PWRs cannot vary at will their power of use. They go brought to the desired power and left there for a period of time as long as possible. Otherwise we would run into a strong reduction of the operating life of the plant. Reduction seen as smoke in the eyes by the plant operator [25].

We would like to conclude this analysis of the ACP100 with a consideration which impressed us very positively: the fact that CNNC has exposed itself by asking the International Atom Energy Agency to certify the safety of the project. Certification obtained in 2016.

We do not analyze other Small Modular Reactors because we would be talking about mere declarations of intent. As mentioned before, studies are underway, but CNNC began designing the ACP100 in 2010, to start building the prototype in 2021. If so much gives us so much, we will talk about more SMRs in 2033.

And regarding the fourth generation reactors (not necessarily SMR)? Also in this case projects and studies in progress. We will talk about it in detail in the next section, dedicated to environmental impact of nuclear plants. Here we make a couple of observations.

It must be said that in this area there are various prototypes of the technology said of fast reactors. In particular in the Russian Federation, followed by India and China.

There was plenty of talk about fast reactors in the courses we have followed at the University. In particular we had the myth of Superphenix. A fast reactor built in France since 1976, put online in 1986.

The profs raved about this plant. We a little less. Why? Because:

• it used liquid sodium for cooling the core, and water for generate the steam to be sent to the turbines; water and sodium are two elements to be kept strictly separate; if they come into contact their chemistry reaction is so energetic that it is explosive: a nightmare;
• fast reactors are so called because the fission of the nuclei of the fuel occurs largely using high-energy neutrons; and the high energy of fission neutrons makes the nucleus extremely more sensitive than that of thermal reactors (which already do not joke about reactivity); the result is that if something disturbs the conditions of the neutron flux in the nucleus, increasing it, this in turn increases its power extremely quickly (we are talking about fractions of a second), increasing the neutron flux even more; it is called positive feeback, and it is another nightmare (of many) for those who have to design a nuclear reactor.

Bottom line: Fast reactors are very dangerous beasts to handle, and if they are cooled with liquid sodium, they are even more so. The Superphenix had various security problems. Only in 1996 it reached a satisfactory continuity operational (95% uptime) and was turned off at the end of that year, without being turned on again.

Environmental impact

Speaking of environmental impact, let's start by saying that nuclear plants produce very little CO₂. Good news. But, like all thermoelectric plants, anyway they heat the environment. Bad news.

Warming of the environment by nuclear power plants occurs when in the condenser the exhausted steam leaving the turbines is cooled from the water of the external environment. This cooling water of course increases in temperature.

How much heat does a thermoelectric power plant transfer to the environment? Without being too quirky, the Rankine cycle used by these plants to produce electricity has a maximum theoretical efficiency by just under 65%. It means that at least 35% of the heat produced by the system is lost e pass to the environment.

In practice, the situation is quite different. Typical power plants have an efficiency that is about 35%. A nuclear power plant uses more than half of its power (about 65%) to heat the river water that cools it.

A typical Westingouse PWR plant feeds 1200MWe (electric Mega Watts) of electricity to the grid with a nuclear core that produces 3400MWt (thermal Mega Watts). 2200MWt heat river (or sea) water.

For a series of technical considerations we do not go into [#] _, related to efficiency plant production, the condenser cooling water must be heated as little as possible. Typically: a few degrees. Let's say water enters the condenser with a room temperature of 22° C and exits with a temperature of 25° C. This low increased temperature imposes relevant water flows, of the order of 28m³/sec. To give the idea: a capacitor every second swallows a quantity of water equivalent contained by a square room with 3m long sides and just the same height.

Insisting: this is a characteristic of all thermoelectric plants, namely that they use heat to produce electricity. Whether using nuclear, coal or methane, there is no difference (except for the relevant fact that coal and methane when burning produce also CO₂). To avoid this problem we should use hydroelectric, wind, photovoltaic plants, ...

Now let's leave the residual heat released into the environment alone and take care of another one appearance, peculiar to nuclear plants. These plants use fuel formed by various fissile elements, that is capable of giving rise to reactions of nuclear fission.

After a certain amount of time, the fuel in question must be replaced because the products of fission are in part isotopes that absorb neutrons. When these elements are too many, the reactor is no longer able to maintain a stable nuclear reaction: its power decreases more and more.

At this point we have to replace the spent fuel with other fresh fuel. Well, spent fuel what features does it have? And how do we manage it?

Here it becomes complex. Regarding its characteristics, (elements that compose it, temperature, level of radioactivity, ...) depend on several factors: the initial composition of the fuel, how long it was used, at what powers. And from how long has it been extracted from the reactor [26].

One thing is sure: it is highly radioactive. If by mistake we got close to a fuel rod freshly extracted from a nuclear reactor and unshielded, in seconds we would absorb a lethal dose of radiation that would carry us to death in a few days. Without the possibility of treatment.

So the short answer is: to move it we shield it heavily, then we put it in a swimming pool for a few years, thus waiting to decay the elements with shorter times of halving [27].

After that we find ourselves dealing with radioactive isotopes that take a medium or a long time to get out of the way. By average times, we mean up to a few hundred years. Long times are of the order of thousands years, if not hundreds of thousands years [28].

We face two possibilities:

• arm ourselves with (a lot of) patience and look for a way to manage this exhausted fuel so that it does not disperse into the environment in the years to come;
• treat the exhausted fuel trying to eliminate all or at least part of the residual isotopes that have long half-life times.

The United States led the way for the first solution. They had to treat the spent fuel to make it inert [29], and then storing it in particularly stable deep geological repositories: the Yucca Mountain Nuclear Waste Repository.

At the time of writing, the project in question has been blocked for decades. According to nuclear advocates, for purely political reasons. The fact remains that neither the Republicans nor the Democrats worked to unblock the project and overcome the hostility of local populations.

The case of Germany is different.

East Germany adopted the site of Morsleben, a salt mine used to store high intensity radioactive waste from 1971.

West Germany, in turn, used the salt mine of Axis II.

Both of these sites proved unreliable. The Moresleben site has shown subsidence since 1997, leading to closure of the site and forecasting of the need for over 2 billion Euros for its decommissioning. Similarly, the Axis II mine gradually showed signs of instability which led to the planning of its decommissioning. With a expenditure forecast of over 3.7 billion Euros. These are going to be economic commitments paid by the state. While the German nuclear plant operators have paid a few hundred million euros to storage the exhausted fuel during the period of operation of the deposits.

At the time of writing, it has been made many test repositories and small local depots to dispose nuclear exhausted fuel. At the previous link you can find a list of the plants in question.

We observe that, for now, the only large active deep geological repository project for the storage of high-intensity radioactive waste is carried out by Finnish, with the Onkalo site. It should start operating from 2023.

A personal observation. These previous considerations convinced us to look for work in computer engineering, rather than the nuclear field.

Back to us. Still in relation to exhusted fuel, there are some other considerations to make. We need to go back considering the design and use of fast reactors.

When we attended the lectures at the University, a strong concern consisted in the amount of uranium available in known mineral deposits. Thinking of a generalized use [30] of nuclear energy for production of electricity, industry researchers had predicted a depletion of mineral resources in tens of years.

This had led to the analysis of possible strategies to make the most of the available nuclear fuel, including spent fuel. Or, even better, increase available nuclear fuel.

This aspect fits well with the second hypothesis we made for the management of spent nuclear fuel: process it to eliminate as much as possible the elements that give us problems. Let's see why.

In spent fuel we have a plethora of elements, which we can group in these areas:

• fission products, a panoply of very different elements, ranging from stable elements to radioactive elements with short, medium (tens of years) and definitely long decay times [31];
• uranium; with atomic number 92, a large part of ²³⁸U and a small part of ²³⁵U they are still present in the spent fuel;
• plutonium; with atomic number 94; it is created in the reactor starting from uranium;
• other actinides; or elements with atomic number higher than that of uranium; in particular neptunium, americium and curium.

We immediately notice the presence of Uranium and Plutonium. Both of them, with with the necessary clarifications, are good nuclear fuels.

We haven't said it yet, but the ²³⁸U it can give rise to fissions. When using thermal neutrons, its probability of fission is much lower than that of ²³⁵U; but with fast neutrons things change: the ²³⁸U gives rise to fission reactions with a good efficiency.

Similarly Plutonium. In particular the ²³⁹Pu has good odds fission from fast neutrons. While the ²⁴⁰Pu can even be done fission spontaneously, without the solicitation of a neutron. An engraved: Plutonium is a very effective element for the construction of nuclear devices, more than Uranium. Consequentially some analysts think that fuel elements with high density of plutonium could be an objective of terrorist organizations or states whose management (without ethical scruples) has the purpose of acquiring nuclear weapons. End of the engraved.

The actinides mentioned above also have good fission characteristics from neutron bombardment.

Hence the spontaneous observation of persons designing fuel cycles. By separating the aforesaid elements from the rest of the spent fuel, we could feed fast reactors. Indeed, it even is even possible to hypothesize to have fast reactors capable of returning, after a period of work, more fuel than that initially supplied. This is the concept of breeder reactor.

Exhausted fuel handled in this way would have lesser amounts of radioactive elements at the end of its life cycle, because a part would be used to power fast reactors.

Only the part of the fission products, which cannot be used as fuel, remains untouched. In any case, these products make a big contribution to the radioactivity of the spent fuel.

Fast reactors have interesting strengths:

• they allow to build nuclear plants with greater energy efficiency;
• they allow to consume part of the heavy radioisotopes that are present in the spent fuels of thermal reactors;
• a careful design would allow the cooling of the halted core using the primary in natural convection, i.e. without the need to have pumps running to move the coolant.

Some authors even go so far as to argue of greater intrinsic safety due to the use of liquid sodium to cool the core. This because it is not necessary to pressurize the core. True. But in our opinion what you earn safely working at much lower pressures than PWR, you lose it due to the increase in operating temperature: sodium is pushed over 500°C [32]. Consequently, the entire primary must be designed to withstand these temperatures.

Not to mention a possible contact of the sodium of the primary with the water of the secondary. Already at room temperature these elements (literally) make fireworks. Let alone at high temperatures.

Other disadvantages:

• very high reactivity; control systems must have times of reaction less than a second [33];
• the reactor's operating environment is opaque: sodium does not allow operators to observe the result of remotely controlled activities; it is necessary operate blindly through automatic robotic systems (prone to breakdowns);
• the contact of sodium with the air gives rise to fires; they are not as serious as contact with water, but they can still give problems;
• plant cost even higher than the already astronomical one of plants of thermal reactors.

All in all, we do not think the increased risk of the security framework about fast reactors, balance how much we earn towards a (little) better management of spent fuel.

All in all, we don't think increased risk about security of fast reactors, balance the small gain we get in spent fuel management.

Speaking of fast reactors, we started talking about fourth generation reactors.

In 2001 the office of Nuclear Energy of US Department of Energy proposed an international collaboration for the study of new generations of power nuclear plants.

Several countries joined, including China, France, Russia and United Europe (Euratom), giving rise to the Gen IV International Forum [34]

This forum publication: GIF R&D Outlook for Generation IV Nuclear Energy Systems 2018 Update, allows us to understand what we are talking about.

The planned types of systems are:

• sodium-cooled fast reactors, which we have already talked about;
• reactors at very high temperatures;
• gas-cooled fast reactors;
• molten salt reactors;
• lead-cooled fast reactors;
• supercritical water-cooled reactors.

A first aspect that strikes us is the fact that almost all of these types of plants are described as being studied in the text with which we prepared the exam of Nuclear Plants I in the year 1981 [35].

On page 204 and following of the aforesaid tome we find the situation of the (very active) research in those years, the related technical considerations and, on p. 207, the pros and cons of the different solutions: none perfect.

The only novelties with respect to what was studied in 1981 are the a molten salts and those supercritical water-cooled.

The most advanced studies, and the prototypes already in operation, concern the reactors fast sodium cooled. All the others are studies, although decades have passed, and they are still largely in their infancy.

Also, from the Gen IV International Forum, the document Technology Roadmap Update for Generation IV Nuclear Energy Systems of the year 2014, provided for two phases for each of the previously exposed types, ignoring (evidently due to extreme unreliability) any forecasts of dates for having production plants:

• feasibility study;
• performance study.

A look at the graphs on page 15 of the aforementioned document, which show the development timelines makes us observe:

• as the original forecasts of 2002 predicted in 2020 the conclusion of the activities of various performance studies, to then move on, probably, to construction of production plants;
• the 2014 forecasts introduced delays of about ten years, targeting the completion of performance studies between 2025 and 2030.

But a new revision of this planning is underway, which could introduce more delays. In any case, it is not realistic to think of seeing production plants of these types before 2050.

Considering that, if we care about the climate of our planet, for 2050 we should have already canceled the anthropogenic CO:sub:2 emission , it is clear that this path is not walkable.

Conclusions

We continue to believe that the study and construction of nuclear plants is a strategic mistake:

• both from the point of view of the impossible attempt to reach an unassailable level of security;
• both from the point of view of the environmental impact, which we are not able to sustain both as thermal pollution and as pollution from ionizing radiation.

The pursuit of these strategies, advocated by International Energy Agency, steals resources to the further development and installation of photovoltaic systems, and wind turbines. As well as the expansion and management of hydroelectric plants, and green hydrogen production plants, necessary to carry out the accumulation of any surplus energy produced by photovoltaic and wind.

Enjoy. ldfa