In 1932, the English physicist James Chadwick discovered the neutron, a particle in the atomic nucleus that lacks an electrical charge. In combination with other research into the relationship between mass and energy, this discovery laid the theoretical foundations for nuclear power. In simple terms the latter could be described as the splitting of large atoms, usually uranium, through their bombardment by neutrons. When a uranium nucleus is split in two, new atomic nuclei are created, together with two or three neutrons. These neutrons are able in turn to collide with, penetrate and split additional uranium atoms. A chain reaction ensues.
But what is it that makes uranium atoms work quite so well as nuclear fuel? First of all it’s important to note that uranium is a relatively big atom. There are two opposing forces in an atom’s nucleus, the first of which is electromagnetic. It means that protons – positively charged particles in the nucleus – repel each other. This is the same type of force that makes it difficult to bring two like magnetic poles together. So what is it that holds an atom together, instead of letting it fly apart? This is where the “strong nuclear force” comes into the picture.
The strong nuclear force is 1038 stronger than gravity, and it holds the protons together. This strong force falls off rapidly as the distance between the particles increases. When a uranium 235 atom – the uranium isotope most commonly used as nuclear fuel – is penetrated by a neutron its geometry is changed. It gets bigger, and the strong nuclear force is weakened as the distance between the particles increases. In four out of five cases the changes are sufficiently large for the electromagnetic force to overcome the strong nuclear force. The protons repel each other and the atom splits.
So how do split atoms provide us with electricity? Well, the total mass of the particles that are created when the uranium is split is lower than the mass of the original uranium atom and the neutron, and as Einstein proved, this mass is transformed into energy – energy that can then be used to heat up water, make steam and drive turbines to generate electricity.
This is the basic, underlying principle that links the first nuclear reactor in 1942 with the fourth-generation reactors which are the subject of much current research. But just because the physical principles in today’s nuclear reactors are the same as yesterday’s does not mean that progress has stood still. A great deal of research is being carried out in three areas; energy efficiency, safety and waste management. And today research is also under way into the so-called fourth generation of nuclear reactors – reactors that have three main advantages over their predecessors:
- their nuclear waste lasts only for decades rather than millennia;
- one hundred times more energy can be produced from the same amount of nuclear fuel,
- it is possible to use today’s nuclear waste as fuel in the new reactors.
To be more precise research is focused on two types of reactor – thermal reactors and fast reactors, and within each of these categories there are three specific reactor designs that are the subject of studies. The big difference between thermal and fast reactors is the speed at which the neutrons that penetrate the uranium are moving. Neutrons must be slowed down in thermal reactors, but in return the chances of the neutron penetrating the uranium are higher.
Fuel can be used more efficiently in fast reactors. This means there is less waste after use, and it is even possible to use the waste from older reactor models as fuel in fast reactors. However, fast reactors are more difficult to control and more demanding from a design perspective.
Travelling wave reactor
One particular type of reactor that has received much attention recently is the so-called travelling wave reactor. Among others Bill Gates is involved in the development of this type of reactor in which uranium 238 – which is available in vastly greater amounts than the uranium 235 that most reactors use today – is used as fuel. In actual fact only a very small amount of new nuclear fuel is necessary to start a reaction, after which old nuclear waste can be used. One of the reactor’s main advantages – that it can be kept in operation for a very long time – is also one of its weaknesses, and building a plant that can handle the kind of stresses involved is no easy matter.
It is always difficult to predict how a new technology will look once it becomes commercialized. Many countries are involved in the ongoing research, and it is still too early to foresee what type or types of reactor will end up in tomorrow’s nuclear power plants, but it is highly likely they will represent a great step forward for nuclear technology and energy generation.
Nuclear power has always been the subject of passionate debate and will probably remain so, but we cannot ignore the fact that over its life cycle today’s nuclear power generates energy with lower greenhouse gas emissions than even wind power. The enormous potential locked up in the nucleus of an atom is quite simply too big to disregard. Perhaps tomorrow’s reactors will mean that this potential can be exploited to a much greater extent.
Article first published February 2011