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How nuclear fission works

An atom is made of a nucleus (composed of neutrons and protons) and electrons. If a neutron collides with a 235/92 U atom, and if the 2 or more resulting hight speed neutrons are slowed down by a moderator, a chain reactions stars, which releases energy. Used for military (nuclear bomb) or civil purposes (nuclear plant).

I. Inside a nuclear plant

Nuclear and fossil fuels plant

Differences : fuel source, fossil fuels release CO2 whereas Uranium doesn't

Similaires : steam (vapeur) is produced and turns turbine connected to an alternator

Moderator in the plant

The moderator is heavy water (2/0 H2O), which is 10% heavier than ordinary water (1/1H2O).

Reactor in the plant

The reaction can be stopped with control rods (barre de contrôle), which can absorb neutrons.

II. Pros and cons

Advantages

-Creates most of our electricity

-It doesn't produce CO2

-It provides a relative energetic independence

Drawbacks

-Very dangerous in case of a accident (Fukushima, Chernobyl)

-It produces dangerous radioactive waste (déchets)

III. Nuclear waste classification

Low-level waste

Property : contains enough radioactive material to require action for the protection of people, but not so much that it requires shielding (blindage) in handling (manipulation) or storage.

Origin : comprises paper, tools, clothing, filters... which contain small amounts of mostly short lived radioactivity.

Intermediate-level waste

Property : sufficiently radioactive to require both shielding (blindage) and cooling (refroidissement), generates > 2kW/m3 of heat and has a high level of long lived alpha-emitting isotopes.

Origin : includes resins, chemical sludge (boue chimique) and metal reactor fuel cladding (gaines métalliques du combustible des réacteurs), as well as contaminated materials form reactor decommissioning (démantèlement des réacteurs).

High-level waste

Property : requires shielding. If it has more than 4000 Bq/g of long-lived (over 30y half-life) alpha emitters it is categorised as long-lived and requires more sophisticated handling and disposal (elimination).

Origin : contains fission products generated in the reactor core.

IV. Next generation : EPR (European pressurised reactor)

Qualities

-Offers a great environmental protection.

-Has a collecting area in case the reactor would melt (fondrait) and protecting walls (murs) against an aircraft crash.

-There is a cooling area in the generator and a system that prevents the building form collapsing (effondrement) bc of an earthquake (tremblement de terre).

-There are 4 subsystem working independently in order to avoid a global failure (éviter une panne totale).

EPR and currents nuclear reactors

-The electricity costs 10% lessh

-The maintenance is easier

-Requires 17% less uranium.