1 – Some history, some physics.

Let’s start the tour !

We started the tour in the bus while the guide was explaining us the different parts of the center before to go to the « communication’s building ». Here he made us a brief explaining of the history of the nuclear energy.

The communication’s building :

A bit of history (about radioactivity)

  • The natural radioactivity has been discovered in 1896 by Henri Becquerel.
  • In less than 40 years, the industrials owned the nuclear energy for its powerful thermic caracteristics.
  • In the early 1900, radioactive materials were used in creams or clothings before it’s been forbidden in 1937 (they sold creams for the face (to burn dead skins) and clothings compounded of it to keep the owner warm before they discovered a pic of skin cancers) (It was tense).

A bit of physics :

  • To stabilize, the radioactive core free energy.
  • The studies on nuclear fusion and fission are based on a special property of atoms : isotops.
    Radioactive isotops are those unstable.
    For exemple, the Tritium, isotop of the hydrogen is unstable whereas the Deutrium (another isotop of the hydrogen) is stable.

Well, it’s dangerous :

The dangerosity of radioactivity is measured with 3 units:

  • The Becquerel :
    Is the number of desintegrations per second. It’s the unity of activity.
  • The Gray :
    The energy recieved per unity of irradiated mass. It’s the unity of the absorbed dose.
  • The Sievert :
    The biological damages on irradiated living tissues. The unity is a dose equivalence (gray processing).
    Example : On absorbed radiations, how important were the damages on my body.

To better understand what the units stand for, he made a metaphore about rocks :

  • The Becquerel is the number of rocks.
  • The Gray is the number of impacts.
  • The Sievert is the damages per impact.

What are the limitations of exposure on irradiated materials when you’re working on the CEA :

There are 3 categories (three, again) :

  • Non-exposed employees : 1 mili sievert per year.
  • Grade B employees : 6 mili sievert / year (sometimes present on laboratories)
  • Grade A employees : 20 mili sievert / year (actively present on laboratories)

To give you a scale of the care they take in France : in Brasil, the limit is fixed at 50 mili sievert/year (9 times more).
The health status in France are set far from the risk of impact on humans.

When does it become risky ?

If you reach 500 mili sievert in a single absorbtion, you risk to get a sunstroke.

The lethal dose is between 10 and 20 sieverts in a single absorbtion.

To remember that we’re here talking about risks if you’re exposed for a single absorbtion. The limits are absorbtion per year !

So, how to protect ?

Three ways, again (incredible) !

  • The lead held, the concrete: category of screens. Anything that can absorb radiation from the issued elements.
  • The distance. The more you’re far, the less you receive radations.
  • The exposure time.

What are the « issued elements » ?

Those elements are issued when a neutron break an heavy core : creating two lighter cores and emitting depending on the atom divised : an element and a neutron (this last will cause the chain reaction, what they try to master in nuclear reactors).

You know how much element there exists before to read ! 3 !

  • Alpha emissions : an atom of Helium, massive.
  • Beta emissions : or a positron, or an electron.
  • Gamma emissions : massive energy liberated (photons).

The alpha emissions are stopped with a sheet of paper. Uranium and plutonium only emit Alpha particles.
Those who are working on it only have to use an isolated room and gloves. It’s a natural radioactivity.

The gamma emission is stopped by 4 meters of concrete. The handling of matiers emitting this kind of particles is only done by tele-operation.
Operators are isolated by a wall of lead and everything’s handled by a mechanic arm. It’s artificial radioactivity.
For instance, a battery (pile) having been passed in a nuclear reactor.

Nuclear reactors

The guide explained us how does a fission nuclear reactor work and how they control the reaction.

How does a nuclear reactor works ?

Afficher l'image d'origine

First it about to lunch a neutron on a heavy atom (such as uranium).

Uranium when cracked releases energy in thermal form and in the form of neutrons which  continue the reaction.
The chain reaction is controlled continualy.
We fill mechanical pencil with diuranium which will be bombarded by neutrons.
This reaction proceeds in the reactor core. This happens in the « battery (pile) » that is four meters high by five in diameter.
The battery is immersed in a tank of fifteen meters filled with water.
It is covered by a building of forty meters high.
Water pumped from the river cools the core and circulates the heat.

To control the reaction, some pencils are filled with a neutron-absorbing liquid to let pass only one neutron out of three. The material of the liquid neutron absorber is compounded of boron or graphite.
If there is an earthquake, the Antineutron structure is fully released and the reaction stops.
The temperature of the water in the primary circuit reached 300 ° C.
The pressurizer allows keeps the water in the liquid state to avoid thermolise and prevent explosions.
The fuel pellets are very small (7cm wide).

The reaction freeing energy creates thermical power which will drives a turbine : converting mecanical energy in electricity.

Once passed, the water is realeased in the air or in rivers at a cool temperature to protect the wild life.

History of the reactors over the world

We have now 4 generations of reactors.

  • From 1960 to 1999, reactors were used to create plutonium for a military use to make atomic bombs.
  • Since the 80’s, 58 centrals create 73% of the energy in France. It’s the most nuclearized country after the United States.
  • From 2003, exist highly pressurized reactors with big security systems.
  • For 6 years, has been built reactors which are cooled by liquid sodium. The advantage is to use an element today considerated as a waste.

Every 10 years is proceeded a total verification of centrals with potential renovations decided.

Three actors, 3.

Today in France exists three actors on the nuclear domain. Each for a special aim.

  • The CEA : researchers. Here they imagine and design the reactors of tomorrow. They register patents they sell to Areva.
  • Areva : they owns mines of fuels (Uranium…) they gather and build the reactors.
  • EDF (Electricity of France) : They buy the reactors of Areva and their fuel to produce energy.