Nuclear plants

The “heart” of the reactor of a fission nuclear power plant is called “core” and generally has the shape of a cylinder. The core is made of a liquid, for example water, into which cylindrical uranium bars are dipped, a couple of metres long and with a diameter of a few centimetres.

At regular intervals there are control bars capable of absorbing many neutrons. Thus, the chain reaction is kept under control and stopped, if necessary. In the most common type of reactors, the water contained in the core is warmed by the fission of uranium and is circulated by means of a pump until it reaches a heat exchanger, into which it cools down producing steam which, in its turn, rotates the turbine of the plant.

A reactor is classified according to the type of fuel, the type of coolant and the core’s inner architecture. For example, a common distinction is made between light water and heavy water reactors.

Light water reactors

In light water reactors the fuel is made of cylinders of uranium oxide enriched with uranium 235. The water circulates among the cylinders and acts both as a controlling element and as coolant.
The core is hosted in a pressurised steel container provided with the coolant intake and outlet holes.

Heavy water reactors

The fuel of heavy water reactors is made of non-enriched natural uranium. There are more modern reactors called “fast” reactors, cooled by means of liquid metal and working with highly enriched fuel by converting uranium 238 into plutonium.

The past and the future of reactors

Nuclear reactors can be classified into four generations, depending on some common characteristics and depending on the period in which they were designed and built. Currently, 440 reactors, mainly of the first and second generation, and some units of the third generation are operating.

Fusion reactor

The fusion reactor works according to the opposite principle to the fission reactor. The fission reactor divides the nuclei of heavy atoms and the resulting heat is released in order to heat water and activate, through the water vapour, a turbine that produces electricity. Instead in the fusion reactor, light atoms (hydrogen isotopes deuterium and tritium) are united into a helium atom (fusion).

The nuclear energy is considered as one of the most environmentally friendly resources: it does not generate polluting emissions such as sulphides, dust or greenhouse gases. Moreover its implementation allows a considerable reduction in the exploitation of the fossil fuel reserves.

The advantages of nuclear fusion

One of the advantages of fusion is that the functioning of the reactor excludes any loss of control since the quantity of fuel used for the reaction and present inside the reactor is reduced (only a few grams) and the combustion only lasts a few seconds. Any minimum disturbance inside the reactor makes the plasma cool down, with subsequent spontaneous interruption of the fusion reaction. Fission reactors, instead, contain a large quantity of nuclear fuel (uranium) and, should an accident occur, the chain reaction stops, the heat produced inside can melt the core of the reactor and free the radioactive products with serious consequences. For this reason today they are extremely safe, always protected by the most sophisticated security systems and by at least two water-tightness shells.

Another advantage of fusion is that it can be done almost anywhere; it does not depend on the availability of raw materials: its fuel, that is deuterium, is extracted from water. Moreover, the product of fusion is helium, i.e. that gas used to inflate balloons, and its waste looses its radioactivity in almost a hundred years (against the thousands of years needed for fission waste).

The risk of accidents

The environmental problem of nuclear power plants includes the danger involved in the process (which is dangerous in case of an accident or leak of radioactive material).

The INES scale

The INES scale (International Nuclear and radiological Event Scale) was developed, starting in 1989, by IAEA, the International Atomic Energy Agency, with the scope of classifying nuclear and radiological accidents and to make the public immediately aware of the gravity of each accident.

The problem of nuclear waste

Another risk is connected to the disposal of radioactive waste. With reference to radioactive isotopes, it is necessary to plan a controlled storage of 500-700 years, while in the case of plutonium, it takes thousands of years.

Disposal of nuclear waste

The general considerations to be made about waste classification are:

• how long will the waste keep at a dangerous level?
• what is the concentration of radioactive material in waste?
• does the waste generate heat?

The persistence of radioactivity determines for how long the waste will have to be managed.

Decommissioning

Beginning from the initial planning and design stages and the identification of the site, a nuclear power plant is a very complex plant to deal with. Each stage of its realization and each successive life-cycle of the power plant must be carefully controlled in order to guarantee maximum safety.