Ce cours vous introduit à la physique subatomique, c'est à dire à la physique du noyau et à celle des particules élémentaires. Plus spécifiquement les questions adressées sont les suivantes : - Quels sont les concepts de la physique des particules et comment sont-ils implémentés? - Quelles sont les propriétés du noyau atomique et comment peut on les utiliser? - Comment accélérer et détecter des particules et mesurer leurs propriétés? - Qu’est-ce qu’on apprend à partir des réactions de particules à haute énergie et leurs désintégrations? - Comment fonctionnent les interactions électromagnétiques et comment peut-on les mettre à contribution? - Comment fonctionnent les interactions fortes et pourquoi sont-elles difficiles à comprendre? - Comment fonctionnent les interactions faibles et pourquoi sont-elles spéciales? - Quelle est la masse des objets au niveau subatomique, et comment y intervient le Higgs? - Que peut-on apprendre de la physique des particules concernant l’astrophysique et l’Univers tout entier? Le cours est structuré en sept modules. Suivant le premier module qui introduit notre sujet, les modules 2 (Physique nucléaire) et 3 (Accélérateurs et détecteurs) dépendent peu du reste du cours et peuvent être étudiés séparément. Les modules 4 à 7 approfondissent les notions de la matière et des forces élémentaires.
2.12 Centrale nucléaire de Beznau
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來自 University of Geneva 的課程
Physique des particules - une introduction
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從本節課中
Physique nucléaire
Pendant ce deuxième module on parle de la physique du noyau atomique et de ses applications. On visitera les travaux pratiques nucléaire de l’UniGe pour voir les expériences sur la radioactivité. A la fin du module, on visitera un réacteur expérimental à fusion et une centrale nucléaire.
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Martin PohlProfesseur ordinaire
Département de physique nucléaire et corpusculaire
Mercedes PanicciaCollaboratrice scientifique
Département de Physique Nucléaire et Corpusculaire
[MUSIC] >> During this second module,
we are discussing the physics of the atomic nucleus.
In video 2.8, the physical principles of nuclear fission were discussed
and in video 2.9, it was explained
how these principles are implemented to build a nuclear reactor.
Today, we visit the Beznau nuclear power plant, which has been
in production since 1969, making it the oldest nuclear power plant
that is still being commercially exploited.
The power plant welcomes us in its showroom
called the Axporama, which you see around me.
After watching this video, you will know how to do three things.
The first is to describe the operation of a
pressurized water nuclear power plant such as Beznau.
The second is to describe the regulation mechanisms
of the chain reaction and the
security mechanisms that monitor the safety of this process.
And the third, appreciate the role of nuclear energy
in the mix of energy production in Switzerland,
in Europe and in the world.
Today we are invited to the Beznau nuclear power station, which belongs to Axpo SA.
We are in the Axporama exhibition, which is part of the plant,
and which informs about 15'000 visitors each year about energy in general and about nuclear energy and its production by nuclear fission in particular.
Our host today is Dr. Urs Weidmann.
He is a particle physicist like me.
He is a graduate of the University of Zürich with an experimental work on the violation of muon number,
still a topical field of research, which is still pursued at the Paul Scherrer Institute not far from here.
He obtained his PhD at the University of Bern, about the influence of CO2 emissions on the climate,
still a topic of high interest 30 years after his PhD. In particular, he made a model of ocean currents
and their influence on the adsorption of CO2 by seawater, and the delay this causes on the climate effect.
The use of nuclear power produces virtually no CO2,
so it seems logical that after his doctorate he moved to the Federal Inspectorate for Nuclear Security ENSI.
From there he changed to NAGRA, the Swiss competence center for the storage of radioactive waste.
And after 3 years to a nuclear power producer, that is to say the Beznau nuclear power plant, first in the monitoring department,
then in the electrotechnical department and finally for 8 years as director of the power plant.
Thank you very much, Dr. Weidmann, that you are available this course with your broad background of knowledge.
Could you briefly introduce us to Axpo company?
>> Axpo is a Swiss power company owned by the cantons of northeastern Switzerland.
The group produces, sells and distributes energy for more than 3 million people and several thousand companies in Switzerland.
It is the leader in the production of electricity of hydraulic and nuclear origin, as well as of renewable energy sources, in particular wind power.
Axpo is active in more than 30 countries. It employs around 4,500 people, including 4,250 in Switzerland.
>> So a major company.
Could you briefly introduce us to the Beznau nuclear power plant, the KKB, of which you are the director.
>> The KKB consists of two identical pressurized water reactors from Westinghouse.
The two blocks have a net electrical power of 365 MW each.
The production of these two blocks covers about 10% of the Swiss electricity consumption.
The Beznau nuclear power plant complies with the most modern safety requirements.
This high standard of security has been maintained by constant updates and renewal investments of more than 2.5 billion Swiss francs.
In this way it is assured that the KKB, during its operation of more than 45 years, has always fulfilled the current requirements in safety,
in several aspects it even surpassed them.
The Swiss supervisory authority, IFSN, confirmed this fact on several occasions.
Axpo plans to operate its nuclear power plants as long as they are safe and economically viable.
>> We are interested in this course less by the economic aspect than by the practical use of fission energy.
In particular, we are interested in the control of the chain reaction
that we have already presented in video 2.10.
To fission 235U, one needs slow neutrons.
The chain reaction is not a self-regulating process, it is influenced by neutron flux and energy.
Could you expand on that?
>> To some extent the process is still self-regulating,
but let me first briefly explain what is going on inside the reactor
during its operation, of course simplifying a bit.
The energy, which heats the fuel rods, is produced by the fission of heavy nuclei such as 235U.
If much heat is to be produced, many nuclei must be fissioned per unit of time;
if less heat is required, the fission rate must be reduced.
Fission is induced by neutrons of relatively modest kinetic energy,
we call them thermal neutrons.
To control fission, it is simply a matter of ensuring
that the proper amount of thermal neutrons is available at all times to generate fissions
- not too much, but not too little either.
The control parameter is therefore the neutron flux.
The systems for measuring this flux are placed outside the reactor pressure vessel,
they can measure the neutron flux
over a very large range of about 10 decades.
>> Thus the measurement of the neutron flux is carried out outside the reactor. But how is this flow controlled?
>> At each fission reaction two to three neutrons of high kinetic energy are released.
They are called fast neutrons.
If the energy produced by the reactor is to remain stable, the fission rate must be kept constant,
i.e. each fission must be capable of generating a single subsequent fission.
For this purpose, it is necessary to ensure that from two to three neutrons there remains on average only one fast neutron,
the high energy of which must then be reduced to the thermal level.
In this way one has one thermal neutron to generate the next fission.
>> This thus concerns first the neutron flux. How do we reduce this flux and how do we reduce the energy of neutrons, both of which are too high?
>> The neutron flux is regulated by a more or less strong absorption, by the capture of neutrons.
For this purpose, control rods are used which contain a material, which has a large cross-section for the absorption of neutrons.
On the other hand, the cooling water of the fuel rods contains boron.
Boron also has a high efficiency for neutron absorption.
By inserting the control rods more or less deeply, and by varying the concentration of boron
in the cooling water, the flux of neutrons is influenced.
>> So the neutron flux is regulated to the right level.
It remains to reduce their energy such that they can generate fission.
>> Fast neutrons are slowed down to thermal energy by a moderator.
In a pressurized water reactor, the boron in the cooling water also acts as moderator,
in addition to its role as a neutron absorber.
In this way, the remaining neutrons are thermalized.
>> You mentioned at the beginning the astonishing fact that the fission chain reaction has some self-regulating component.
Can you go into some detail?
>> During the operation of the reactor two feedback effects occur.
By increasing the temperature of the fuel, the relative movement of the 238U nuclei increases, which causes an increased absorption of neutrons by Doppler effect.
When neutron absorption increases, there are fewer that are available for the next fission.
At the same time, the temperature of the cooling water increases, which causes a decrease in density and a less effective moderation.
These two mechanisms eventually decrease the fission rate
as the temperature increases, such that the temperature is reduced automatically.
>> Despite this fact, a regulation of the reactor is necessary to keep under control the chain reaction, which could escape control and cause major accidents.
How is it ensured that the control of a nuclear reactor works?
>> When operating a nuclear power plant the following important safety goals must be ensured in a consistent manner:
1. One must be able to stop fission instantaneously, that is to say to stop the chain reaction at any time.
2. The reactor core, that is to say the fuel rods must be cooled at all times, including after the reactor has been switched off.
3. The enclosure of radioactivity must be ensured permanently.
4. One must avoid that radioactive material can contaminate the environment.
These principles are absolutely to be respected.
>> These are the principles.
But how does one implement these goals, and how does one ensure that they are indeed respected?
>> To ensure this, the principles and rules that are defined by the national and
international authorities must already be respected at the planning stage.
When a plant is built, compliance with the requirements
is monitored by the competent authorities in a consistent manner.
The requirements ultimately result from a consistent application
of the principle of "defence in depth"
and the concept of multiple barriers for the isolation of radioactivity.
>> "Defence in depth" has a little aftertaste of military strategy.
Could you explain that to us a bit more?
>> The comparison with the military significance is in fact justified.
It means actually implementing a defence against accidents staggered in depth.
It can be characterized as a chain of tiered security measures.
It includes, for example, choosing a suitable reactor concept,
but also redundant systems, that is to say several systems for the same purpose.
These redundant systems are chosen on purpose from different technologies,
such that the same incident cannot occur twice simultaneously.
And in particular the training and know-how of the employees are essential
elements in this accident protection concept staggered in depth.
>> Nevertheless, accidents are known to have occurred.
The last serious accident was that of Fukushima.
And as a result of these accidents a few countries, including Switzerland and Germany,
have decided to stop the use of nuclear energy in the long term,
and to rely on renewable energies such as wind-, water- and the solar energy.
Despite this fact, Axpo continues to invest in the maintenance and operation of its oldest nuclear power plant.
Can you resolve this apparent contradiction?
>> When we analyze the situation more thoroughly, we realize that few countries
in the world have decided to reverse their nuclear policy after the Fukushima accident.
Many countries have even decided to invest anew in nuclear technology to ensure their electricity supply.
By early 2014, 437 reactors were operating in 31 countries.
There are 72 new power plant projects worldwide, including 20 in China.
But, as I noted at the beginning, Axpo is not only a leader in the production of nuclear and hydropower,
but also in the exploitation of renewable energy source, especially in wind farms.
In line with the ideas of the Federal Council and parliaments, Axpo plans its future without the construction of new nuclear power plants.
But the continued operation of existing proven plants,
as long as they are safe and economical,
is for Axpo an important and even necessary prerequisite for a successful implementation of the Federal Council’s strategy.
The operation of existing nuclear power plants is also of great importance for the continuity of electricity supply in Switzerland.
A dependency on foreign countries is in my opinion, and that of Axpo, neither effective nor economically desirable.
>> This seems comprehensible to me. Another problem amply discussed is of course the storage
of highly radioactive waste, also partially highly toxic, which is produced in a nuclear power plant.
How do you see the solution to this problem?
>> There's no ambiguity. Radioactive waste must be disposed of in such a way
that the sustainable protection of man and the environment is ensured at all times.
To this end, the whole chain of waste must be scrupulously rethought, from its generation in the nuclear plant, its conditioning,
through temporary storage until a final disposal in geological formations.
Each step must be treated carefully. Ultimately, waste must be permanently isolated
from the habitat so that it cannot re-enter the biosphere and the food chain.
>> But this must be ensured over enormous periods of some 100,000 years.
This is not an easy exercise. What is your vision of final storage over such long periods?
For almost all countries that use nuclear energy, the chain of disposal of highly radioactive
waste ends with storage in deep layers of geologically stable rock.
In the international circle of experts there is also a consensus that well-designed
deep underground repositories ensure long-term safe isolation of radioactive substances.
Thus the protection of man and the environment can be assured.
In Swiss legislation, final deep underground storage is even mandatory. There are no alternatives to this.
>> In spite of this, there is a non-negligible resistance of the population against such storage facilities in their neighborhood.
How can one find a place of storage that is accepted by the public and at the same time meets the requirements of geological safety?
>> It is also unanimously accepted internationally that the establishment of such a deep underground repository imperatively requires a step-by-step approach.
The results of a preliminary phase must be taken into account in the planning and implementation of the next phase.
The constant development of science and technology must also be taken into account.
This makes the process of identifying a storage site a long one.
But this ensures that one can find the optimal alternative.
>> At any given moment. >> Correct.
>> Thank you, Dr. Weidmann, for this interesting and open discussion.
I would like to thank Dr. Urs Weidmann for this extremely informative and open discussion.
And I would also like to thank Mrs Christine Müller, the director of Axporama,
for opening the doors of this exhibition, which I highly recommend
for an instructive and didactic visit.
