Send your question to the site Webmaster
The CEA is open to the public. To visit its research laboratories free of charge, register : CEA open to the public (,
Q01 : The global inventory of tritium present in the installation is limited so that a major accident would not require evacuation of neighbouring populations." => definition of a major accident? consequences of a major accident?
The term " major accident " is a technical expression meaning " the most serious accident " likely to happen on an installation. The consequences of all accidents are determined by fault trees. In practical terms, these trees are made up of a succession of questions/answers such as: " if such and such pipe breaks at such and such a point, what wouldl be the consequences on such and such other component? If this component fails in turn, what will be the consequences on the other parts of the system? etc. " At the end of the fault trees, we arrive at relevant questions for the general public. Should the public be evacuated from the areas surrounding the installation? The accident with the greatest consequences is thus treated as a major accident.
The overall conclusion of the safety studies carried out by European teams state that fusion reactors offer attractive advantages in terms of safety and, in particular, that a runaway reaction is intrinsically impossible and no accident would lead to melting of structural materials. The physical integrity of the reactor is thus never endangered. It is to be noted as well that, despite the presence of tritium, no accident on a fusion installation should ever result in the evacuation of the general public, an operation which is compulsory when a significant amount of toxic (SEVESO) or radioactive (CHERNOBYL) product is released.
The electrical power of a fusion reactor is similar to that delivered by a large fission reactor, around 1,500 electric MW (eMW). The use of the electrical power produced by fusion will be the same as for a fission reactor and it will be in particular possible to use it at any time of the day (here we talk in terms of "base load operation"), which is impossible with most renewable energy sources (solar and wind power in particular). The average power delivered to a typical French household is around 10 kW (cf EDF billing), which means that a 1,500 eMW fusion reactor can supply 150,000 households, all taking their full energy allowance at the same time. For the first of January 1999, the electrical power in France was 114,500 eMW with 63,000 e MW supplied by nuclear power stations. If we suppose that all this power were used for Frances power requirements alone and that the ratio of non-nuclear and nuclear power was maintained, 42 fusion reactors producing 1,500 eMW would be enough.
The surface area taken up by this type of reactor is similar to that taken up by a "conventional" reactor with all auxiliary parts. As an example, a "conventional " EDF nuclear power station site takes up a surface area of around 1 km2. On this site there are always two to four nuclear reactors, or 1,800 to 5,200 eMW. To give an idea of scale, around 150 km2 of photovoltaic cells would be needed or 2,500,600 kW wind-turbines (50m rotor) operating continuously on a particularly favourable site in order to produce the same power as a fusion reactor of 1,500 eMW.
Q03 : The structural materials will become radioactive due to the production of neutrons, reducing the time that they can be used. What is the lifetime of a fusion reactor? What reuse of these materials have you allowed for?
To be economically viable, the total lifespan of a fusion reactor should be comparable to or even greater than the lifespans of other energy producing installations (at least 30 years). A certain number of components have to be replaced periodically (for example the components which are nearest to the plasma with a lifetime of between 2 and 5 years). Before going on to talk about the reuse of materials in a fusion reactor, it must be remembered that the fusion reaction does not produce any radioactive waste. On the other hand, the structural parts will be become radioactive as in any other installation where structures receive neutron irradiation. According to what is currently known, 40% could be declassified, i.e. reused outside the nuclear industry, while the rest is recycled within the nuclear industry, mainly in places or in devices subject to radiation.
It must not be forgotten that up to now the development of fusion energy is at an experimental stage. The notion of profitability applies rather to electro-generating fusion reactors (i.e. installations connected to the electric grid), which will not be ready before 2050.
Energy balance : if we just look at the plasma, this balance has the name of Q factor in plasma scientific jargon. It represents the ratio between energy supplied by the plasma (fusion power) and the external energy supplied to the plasma (to heat it). It is to be noted that JET has to date achieved plasmas close to Q=1 (the plasma supplied almost as much energy as it was provided with). The ITER machine was designed to achieve plasmas of Q=10. A reactor should achieve plasmas with Q>50. Seen from this point of view, a fusion reactor is a power amplifier supplying 50 times as much energy as it uses up. It is also worth mentioning that it is possible to achieve self-maintained plasmas, in other words plasmas with enough energy to maintain the temperatures necessary to achieve fusion reactions without input power. In this state, the external energy can be switched off ; this state is called "ignition" (Q=infinite).
Efficiency : Although meaningless for an experimental installation, efficiency is crucial for the reactor, whose goal is to become a means of production of energy that is economically viable. Fusion reactors have been under study for a long time. Recent fusion reactor designs show that overall yield (from the energy source to the grid) between 33% and 45% are reachable, which is comparable to the productivity of currently operational fission reactors. It has been shown that ignition is unnecessary and that plasmas with an energy balance of >50 (Q>50) are enough.
Studies on thermonuclear fusion are still at experimental stage and connection to the grid of the first fusion power station is highly unlikely before the year 2050. This may seem far away, but it must be remembered that the fusion reactor concept currently considered was only thought up in 1958 by two Russian physicists. A hundred years from the original idea to the final development is a quite common period (the principle of solar cells dates back to 1839, A. Becquerel).
The remarkable breakthroughs achieved in the last few years have brought research on controlled fusion to a very important stage in its history: the design and then the building of a large scale experimental installation (ITER) showing feasibility of controlled fusion. This project, currently at the final stage of engineering, is the result of intense international collaboration between Europe, Japan and Russia (the United States participated in the first phase of the project).
Many results were obtained over the period 1995-2000 in various experimental installations worldwide. It is worth pointing out that the European installation JET is to this day the largest experimental fusion machine in the world. As plasma performance is closely related to plasma size, the most significant results have been obtained with this machine. A notable example was in 1997 the achievement of 16MW of fusion power in the installation. Although smaller in size, the recently obtained results on Tore Supra are just as important for the future of fusion. Tore Supra is the largest tokamak in the world with superconducting toroidal magnets (the magnets in JET are made of copper). This enables Tore Supra to achieve plasmas with long duration (2 minutes or more) and permits the study of physics and technology relevant to mastering long pulses. Pulses of 2 minutes were achieved as early as 1996. To oversimplify, it can be said that JET studies high performance plasmas for short durations (<10 s) while Tore Supra is concerned with mastering less performance plasma, but for much longer periods (2 minutes or more). To be complete, the study of high performance plasma with long duration will be the main objective of the next experimental ITER machine, which is currently being designed.
A runaway reaction is intrinsically impossible. In controlled magnetic fusion, a very hot plasma is confined in a " magnetic bottle " inside a confinement area. As soon as the " magnetic bottle " is imperfect, the hot plasma reaches the walls, cools and the reaction stops automatically.
The most serious accident that can occur is breach of the confinement wall and the blanket (which generates the tritium) causing possible release of some radioactive solids and some tritium. As the fuel present inside the confinement area is only sufficient for a few seconds of operation, the radioactivity released from the reactor never reaches a level requiring evacuation of the population. Detailed studies have shown that the probability of such an event occurring is practically nil, given the internal energy in the system. Even if the likelihood is nil, this major accident is nevertheless taken into account in the extremely strict safety rules of the International Atomic Energy Agency (IAEA ) which are used in the design of fusion reactors.
In the easiest fusion reaction to achieve, the two basic fuels, deuterium (a natural element in heavy water) and lithium, are not radioactive. The product of the reaction is helium (a noble gas), which is also not radioactive.