The World of Nuclear Science

Nuclear Reactors

In a fission reaction, an atom such as uranium-235 is bombarded with a neutron. The uranium-235 atom then absorbs the neutron to form uranium-236. However, this is unstable, and it breaks up into two roughly equal sized atoms, releasing some neutrons and a lot of energy. The overall process can be drawn in a diagram as

although the products may be different on each occasion. (The products could theoretically be anything provided that mass number is conserved.)

Regardless of what the products may be, the neutrons released can continue on to break up more uranium-235 atoms. Thus, if a chain reaction can be created and sustained, there will be a lot of energy released. In 1942, scientists at the University of Chicago in USA achieved such a self-sustaining chain reaction. They did this in the operation of the world's first ever nuclear reactor. However, there are no photos of this reactor due to military secrecy.

Uranium-235 atoms will only absorb slow moving neutrons. However, the neutrons produced from the breaking up of a uranium-235 atom move at very high speed. Thus it is difficult to achieve a sustained chain reaction unless the produced neutrons can be slowed down. The slowing down is done by a moderator. Laws of physics stipulate that the best moderator would be one where the atoms are of similar masses to the neutrons. Thus hydrogen-1 would be a good theoretical moderator because its mass is very close to that of a neutron. However, hydrogen-1 is likely to absorb neutrons. The next-best option, hydrogen-2, does not absorb neutrons, so that is often used instead. In practice, either isotope is used, in the form of water. (See below.) Heavy water is water containing predominantly hydrogen-2 atoms, light water contains mostly hydrogen-1 atoms (normal water). Another alternative to hydrogen as a moderator is carbon-12.

Light water is readily available - almost all of the world's water is "light". However, since hydrogen-1 in the light water will absorb neutrons easily, the uranium used in the reactor needs to be enriched to uranium-235, so as to increase the probability that the neutron produced from the fission of one uranium atom will go on to split another uranium-235. Less than 0.7% of all uranium is this isotope. Reactors that operate on uranium-235 with light water moderators are called light water reactors.

Enrichment is usually not necessary for heavy water reactors since hydrogen-2 will not easily absorb neutrons. Therefore, it can fission uranium-238 atoms. However, the probability of a sustained uranium-238 fission reaction is small because the neutrons produced will be of too low speed. Uranium-238 fissions only with high speed neutrons.

Produced neutrons can escape through the surface area of the fuel sample, affecting the possibility of a chain reaction being sustained. If the surface area of the sample is too large compared to its mass, a chain reaction will not be sustained. A sample of uranium-235 the size of a golf ball will have a large surface area compared to its mass, and neutrons released from one fission reaction may simply "leak out" from the surface, preventing a chain reaction. However, a larger sample the size of a baseball will be able to sustain a chain reaction because the neutrons lost through the surface are compensated for by those created from other fission reactions within the larger sphere. This minimum mass required to sustain a fission chain reaction is called the critical mass. Note that this is dependent on the shape of the sample because different shapes have different surface areas, even if they have the same mass. The mass also depends on what moderator is used, and the purity of the fuel.

The average number of neutrons released per fission that go on to produce further fissions is called the multiplication factor and is given the symbol f. A sustaining chain reaction can be achieved only when the fission of an atom releases at least one neutron (which then continues the chain reaction). Thus for a chain reaction to be sustained, f must be greater than or equal to 1. An average of 2 to 3 neutrons are released for the fission of a uranium-235 atom, so a sample of pure uranium-235 will be very likely to sustain a nuclear chain reaction. Such materials that can sustain fission chain reactions are called fissile materials. When f is greater than 1, the mass of the sample is supercritical.

Nuclear reactors have control rods that absorb some of the neutrons, to keep f just above 1 so that the chain reaction does not occur to fast. These control rods are often mde of cadmium or boron - materials that absorb neutrons easily.

Radioactive nuclear wastes can be stored in ponds of water that absorb any emitted radiation.

a photo of a water pond used for storing radioactive materials. The water contains mainly hydrogen-1 atoms which can absorb neutrons produced from radioactive decay.

a schematic of a reactor. The control rods (red) can be moved in and out of the structure to control the rate of the chain reaction. (They are interposed between fuel rods to absorb some of the produced neutrons.) The fuel and control rods are together placed in the moderator, and the whole lot is encased in concrete/lead shielding.

A nuclear reactor is a device that is capable of achieveing and sustaining a nuclear reaction. Nuclear reactors are also known as atomic piles.

The essential parts to a nuclear reactor are

• a moderator. This is used to slow down the neutrons that are produced in fission reactions.
• enriched fuel in the case of light water reactors, to increase the probability that produced neutrons will fission other uranium-235 atoms instead of being absorbed by the hydrogen-1 in the light water.
• fuel above the critical mass so that a chain reaction can be sustained.
• control rods. These are used to absorb neutrons so that the speed of the chain reaction can be controlled.

A light water reactor uses light water (water containing hydrogen-1) as its moderator.
A heavy water reactor uses heavy water (water containing hydrogen-2) as its moderator.

Some images on this page were used with permission from another organisation. They own the respective copyright. See the Acknowledgements page for more info.