Nuclear fission — AQA GCSE Physics

A uranium-235 nucleus can split entirely on its own, but spontaneous fission is rare. For fission to happen at a useful rate, the unstable nucleus must first absorb a neutron. Absorbing the extra neutron leaves the nucleus highly unstable, causing it to split into two smaller nuclei, releasing two or three further neutrons and gamma rays. These released neutrons can be absorbed by other large nuclei, creating a chain reaction. Note the word absorb: the neutron is taken into the nucleus rather than bouncing off it. A lump of uranium does not simply explode; this is because it lacks a critical mass to sustain an uncontrolled chain reaction, not merely because spontaneous fission is rare.

The products of a single fission are worth learning as a set. The nucleus splits into two smaller nuclei that are roughly equal in size, though not identical; alongside them two or three neutrons are released; and gamma rays are emitted as the excited fragments settle. Energy is released, carried mostly as kinetic energy of the fragments and neutrons. In a fill-in-the-blank question the vocabulary must be exact: energy is released from uranium nuclei, the uranium splits and releases neutrons, and the electromagnetic radiation emitted is gamma rays. Getting 'neutrons' and 'nuclei' the right way round is what the question is actually testing, and gamma, not X-rays, is the radiation named.

Every fission event transfers energy from the nuclear store of the fuel. The energy is not created; it is released because the products together are more stable than the original heavy nucleus, so the mass of the products is very slightly less than that of the nucleus and the neutron that started it. That missing mass appears as energy. Per atom this is roughly a million times more than any chemical reaction such as burning coal, which is why a small mass of fuel runs a power station for a long time. The power output depends on the number of fissions per second, so controlling how many neutrons go on to cause further fissions is exactly how the output of a reactor is controlled.

When a nucleus splits, the two fragments carry a large positive charge each and repel violently, flying apart at high speed. The two or three neutrons are also thrown out at speed. All fission products therefore have kinetic energy, which holds most of the energy released. In a reactor, the fast-moving fission fragments collide with surrounding atoms in the fuel rods, transferring energy to the thermal store of the fuel. The neutrons travel further, colliding with atoms in the moderator to slow down. Energy is transferred to the coolant, which is pumped to a heat exchanger to boil water; the steam turns a turbine, which turns a generator. The neutrons must be slowed by the moderator before they can cause the next fission.

Each fission releases two or three neutrons, and if at least one of them is absorbed by another large nucleus it causes a further fission, which releases more neutrons again. That self-sustaining sequence is a chain reaction. Whether it grows, holds steady or dies away depends on how many of the released neutrons, on average, go on to cause another fission. Exactly one gives a steady output; more than one and the number of fissions per second multiplies rapidly; fewer than one and the reaction fades. Not every neutron succeeds: many escape through the surface of the fuel, some are absorbed without causing fission, and fast neutrons must be slowed by the moderator before uranium-235 will readily absorb them.

A reactor has to hold the chain reaction at a steady rate, which means exactly one neutron from each fission going on to cause the next. The surplus neutrons are mopped up by control rods, usually boron, which absorb neutrons without undergoing fission. Lowering the rods further into the core increases the number of neutrons absorbed, so fewer fissions happen each second and the energy released decreases; raising them absorbs fewer neutrons, so more fissions occur and the output rises. Dropping the rods fully shuts the reactor down. The moderator, a separate component, does a different job: it slows the fast neutrons so they are readily absorbed by uranium-235 and can cause the next fission.

If more than one neutron from each fission goes on to cause a further fission, the number of fissions doubles or triples with every generation. Because each generation lasts a tiny fraction of a second, a huge number of nuclei split in an extremely short time and all the energy is released at once, which is the explosion of a nuclear weapon. The same runaway behaviour in a power station, where the fuel is not suitable for a bomb-type explosion, shows itself as a meltdown, the core overheating and damaging the reactor, as at Chernobyl; at Fukushima the meltdown followed a loss of cooling after the reactors had already shut down, not a runaway chain reaction. A reactor avoids this by absorbing the surplus neutrons in control rods so that on average only one neutron per fission continues the chain.

Fission diagrams follow a fixed grammar, and the marks are for the details. Start with a neutron drawn arriving at a large nucleus and being absorbed. Show that nucleus splitting into two fragments, with two or three neutrons drawn leaving it. Then take at least one of those neutrons to another large nucleus and show it being absorbed and causing a second fission, which itself releases two or three further neutrons. Label the neutrons and the nuclei, since an examiner cannot credit an unlabelled circle. Reading a diagram works the same way in reverse: check how many neutrons each fission releases, whether they reach further nuclei, and whether the number of fissions grows, stays constant or dies out.