Nuclear Fission Explained | GCSE & IGCSE Physics Revision

Nuclear reactors require several key components: Moderator (slows down fast neutrons to increase fission probability, using heavy water D2O, graphite, or beryllium), Control Rods (regulate reaction rate using neutron-absorbing materials like boron, cadmium, or hafnium), and Coolant (transfers heat from core to heat exchanger). Three coolant types exist: Gas coolants (helium, hydrogen), Liquid coolants (light water H2O, heavy water D2O), and Molten metal coolants (sodium, lithium). The choice depends on reactor design and operating conditions.

In a nuclear reactor: (1) Moderator is used to slow down neutrons produced by nuclear fission. (2) Coolant is used to absorb heat generated in the chain reaction. (3) Control rods (made of cadmium and boron) are used to control the chain reaction by absorbing neutrons.

Nuclear reactors use control rods and moderators to control the chain reaction. Control rods (made of boron or cadmium) absorb neutrons to slow down or stop the reaction. Moderators (made of water or graphite) slow down neutrons by colliding with them, reducing their kinetic energy. This is necessary because fast neutrons tend to pass through nuclei without being absorbed, while slow (thermal) neutrons are more likely to be absorbed and cause fission. Together, these components allow precise control of the reaction rate.

In nuclear reactors, the moderator slows down fast neutrons to thermal energy through collisions, enabling efficient fission reactions; control rods made of neutron-absorbing materials like cadmium regulate the reaction rate by absorbing excess neutrons; and coolant removes excess heat generated during fission, similar to how coolant systems work in vehicle engines.

Nuclear reactors contain fuel rods (uranium-235, plutonium, or thorium), moderators (water or graphite) that slow neutrons, control rods (boron or cadmium) that absorb neutrons to control reaction rate, coolant (usually water) that removes heat, and radiation shields. In boiling water reactors, water moderator is allowed to boil off as steam to drive turbines. The moderator slows fast neutrons so they can be captured by uranium-235 nuclei, sustaining the chain reaction. Control rods can be inserted to stop the reaction in emergencies.

Nuclear power presents three major challenges: environmental contamination from radioactive waste that persists for millions of years (e.g., iodine-129 with a 15.7 million year half-life), safety risks including catastrophic accidents like Chernobyl and vulnerability of waste storage facilities to terrorist attacks, and high costs requiring taxpayer subsidies (approximately $10 billion per reactor) while the industry generates significant profits without bearing the full costs or risks.

Nuclear power produces two types of radioactive waste: low-level waste (tools, clothing) with short half-lives (100-1000 years) stored on-site in barrels, and high-level waste (spent fuel rods) with extremely long half-lives (10,000+ years) requiring cooling pools for 5 years followed by dry cask storage on reinforced concrete pads. The Yucca Mountain repository proposal (1987) aimed to consolidate waste but faced concerns about seismic activity, hydrology, and transportation risks. Historical accidents like Three Mile Island (1979) and Chernobyl (1986) demonstrated the severe health consequences of nuclear accidents, including radiation exposure causing cancer, thyroid issues, and mutations, with high-level waste remaining lethal within 3 minutes at 1 meter distance.

Nuclear power represents a fundamental failure across three dimensions: technical, economic, and ethical. Technically, it produces deadly radioactive waste (strontium-90) that accumulates in human tissues, as proven by the Baby Tooth Study showing accumulation in children's teeth, mammary glands, and thyroids. Economically, nuclear plants are financially burdensome—Sacramento lost $575 million initially but gained $46 million within three years after closing Rancho Seco. The NRC chairman acknowledged all 104 US reactors have unfixable safety problems. Ethically, operating nuclear facilities kills people, yet regulators claim they're doing their job while cancer rates increase. Fukushima continues leaking 100,000 gallons of contaminated water daily, with radiation detected in bluefin tuna off California and increased hyperthyroidism in West Coast newborns. Despite proponents claiming meltdowns occur only once in millions of reactor-years, three major accidents occurred within 50 years: Three Mile Island, Chernobyl, and Fukushima. Diablo Canyon sits on active fault lines and is the only plant among five seismic failures still operating illegally. Edison charges $90 million monthly for no nuclear power since 2012, while California ISO analysis shows excess grid capacity exists even after closing half nuclear plants, proving nuclear is unnecessary for reliability.

Nuclear energy, initially hailed as non-polluting, has two serious inherent problems: accidental leakage and safe disposal of radioactive waste. Nuclear radiation causes DNA mutations at high rates, with high doses being lethal and lower doses causing disorders including cancer. The effects are transmitted to subsequent generations through genetic mutations. Major accidents include Three Mile Island (1979, 2.5 mega curies released), Chernobyl (1986, 137 acute radiation syndrome cases, 38 deaths), and Fukushima Daiichi (largest since Chernobyl). Nuclear fuel remains dangerously radioactive for thousands of years. Low-level waste can be disposed of in landfills, while higher-level waste can be reprocessed or encased in glass and stored 500 meters underground.

Nuclear power has experienced 11 accidents between Richter scale 4 and 7 over 70 years, including two super-gaus (Tchernobyl and Fukushima), translating to approximately one severe accident every 6.4 years. An ethics commission has stated that nuclear power is ethically unacceptable due to potential catastrophic damage and the practice of passing waste management to future generations. Current contamination issues, such as cesium contamination in German forests from Tchernobyl, demonstrate long-term environmental consequences. The debate also considers potential terrorist attacks and internal sabotage risks.

The mass defect is the difference between the mass of a nucleus and the combined mass of its individual protons and neutrons, which can be converted to nuclear binding energy using Einstein's equation ΔE = Δm × c²; the binding energy per nucleon is calculated by dividing the total binding energy by the number of nucleons (protons + neutrons), and this value can be expressed in joules or converted to mega electron volts (MeV) using the conversion factor 1 MeV = 1.6 × 10⁻¹³ joules.

When protons and neutrons combine to form a nucleus, the total mass is less than the sum of individual particle masses. This mass defect is converted into energy according to Einstein's equation E = mc², where c is the speed of light (3 × 10⁸ m/s). This released energy is called binding energy, which holds the nucleus together. The mass defect represents the energy that must be supplied to break the nucleus apart. Converting just 1 gram of mass completely to energy would release approximately 9 × 10¹³ joules, enough to power a city for a year. For nuclear calculations, binding energy is expressed in mega electron volts (MeV) using the formula: Binding Energy (MeV) = Mass Defect (amu) × 931.5 MeV/amu.

To find mass defect from binding energy, rearrange Einstein's equation: Δm = ΔE / c². When binding energy is in MeV and c² = 931 MeV/u, the mass defect will be in atomic mass units. This reverse calculation is useful when binding energy is given and mass defect is required.

Mass defect (Δm) is the difference between the sum of the masses of individual protons and neutrons and the actual mass of the nucleus. Binding energy (BE) is the energy equivalent of this mass defect, calculated using Einstein's equation: BE = Δm × c². The mass defect is calculated as: Δm = (Z × m_H + N × m_n) - m_nucleus, where Z is the atomic number, m_H is the mass of hydrogen, N is the number of neutrons, and m_nucleus is the actual mass of the nucleus.