Radioactivity Revision Notes

Understanding Nuclear Energy Sources

Nuclear reactions are processes that change the structure of atomic nuclei. These reactions, unlike chemical reactions that only involve electrons, can release immense amounts of energy. The main types of nuclear reactions that act as energy sources are fission, fusion, and radioactive decay.

Types of Nuclear Reactions as Energy Sources

Nuclear fission is the splitting of a large atomic nucleus into smaller nuclei, producing energy, as used in nuclear power reactors. Nuclear fusion is the joining of two small atomic nuclei to form a larger one, releasing even greater energy, as seen in stars. Radioactive decay is the spontaneous transformation of unstable nuclei, emitting radiation and energy, which can be used in specialized applications like space probes.

Initiating Fission

Fission in Uranium-235 (U-235) is triggered when its nucleus absorbs a slow-moving neutron. This absorption transforms the U-235 into an extremely unstable isotope, U-236, which is short-lived.

Products of Fission

The unstable U-236 nucleus rapidly splits into two smaller 'daughter' nuclei. Common examples of daughter nuclei are barium and krypton. As well as these nuclei, two or three high-energy neutrons are also emitted, and a substantial amount of energy is released.

Energy Release

The total mass of the products (daughter nuclei and emitted neutrons) is slightly less than the initial mass of the U-235 nucleus plus the incident neutron. This 'missing' mass is converted into a large amount of energy, primarily in the form of kinetic energy of the fission products and gamma radiation, in accordance with E=mc².

Chain Reaction Potential

The neutrons released during fission can go on to strike other U-235 nuclei, causing further fission events. This creates a chain reaction, which is the basis for both nuclear power generation and nuclear weapons.

Understanding Nuclear Fission

Nuclear fission is the process where a heavy, unstable nucleus splits into two or more smaller nuclei, along with a few neutrons and a significant amount of energy. This process can be induced by striking the heavy nucleus with a neutron. For instance, Uranium-235 atoms readily undergo fission when struck by a slow-moving neutron.

The Chain Reaction

The neutrons released during a fission event can subsequently cause other nuclei to undergo fission. If, on average, more than one neutron from each fission event causes another fission, the reaction will accelerate, leading to an uncontrolled chain reaction. In a nuclear reactor, the goal is to maintain a steady rate, where, on average, exactly one neutron from each fission causes another fission, allowing for a controlled energy release.

Controlling the Reaction Rate

To control the rate of fission in a nuclear reactor, control rods are employed. These rods are made from materials specifically designed to absorb neutrons, such as boron or cadmium. By adjusting the depth of these control rods within the reactor core, the number of neutrons available to cause further fission reactions can be regulated, thereby controlling the overall power output of the reactor and preventing overheating.

The Role of Moderators

Moderators are essential materials in a nuclear reactor, such as graphite or heavy water. Their primary function is to slow down fast neutrons released during nuclear fission. Fast neutrons are less likely to cause further fission in uranium-235 nuclei. By reducing their kinetic energy to 'thermal' levels, moderators increase the probability of these neutrons being captured by other fissile nuclei, thus sustaining the chain reaction efficiently.

The Function of Control Rods

Control rods are crucial for regulating the rate of the chain reaction. These rods are made from neutron-absorbing materials, typically boron or cadmium. They are moved into and out of the reactor core to absorb excess neutrons. Inserting control rods absorbs more neutrons, reducing the rate of fission and decreasing the reactor's power output. Withdrawing them allows more neutrons to continue the chain reaction, thereby increasing power. This precise adjustment prevents the reaction from overheating or going critical.

Maintaining a Controlled Chain Reaction

A nuclear reactor works by carefully balancing the production and absorption of neutrons. Moderators slow neutrons to make them effective for fission, while control rods absorb excess neutrons to prevent the reaction from accelerating uncontrollably. This fine-tuning ensures a stable and safe power generation, allowing the reactor to produce electricity without the risk of a runaway reaction.