Emergency Core Cooling Systems are designed to deliver cooling water to the reactor core when the normal cooling path is unavailable. The reactor continues to generate decay heat even after shutdown, so ECCS provides water to keep the fuel rods covered and remove that heat, preventing overheating and potential fuel damage. This safety function is distinct from chemistry control of the coolant, environmental radiation monitoring, or boosting reactor power. ECCS uses redundant systems to ensure cooling is available under different accident conditions, maintaining core integrity when the regular cooling system is compromised.

Penetration describes how well radiation can pass through a material. It refers to the portion of the radiation that continues on without being stopped or absorbed, which depends on the radiation’s energy and the material’s properties (like density and composition). The higher the energy and the lower the material’s shielding effect, the greater the penetration. Absorption, by contrast, is about the radiation’s energy being taken up by the material itself, reducing the beam’s intensity. Scattering involves the radiation changing direction, which can still allow some radiation to emerge but not define the beam’s ability to pass through. Shielding is the use of material to block or greatly reduce exposure, rather than the intrinsic property of passing through. So, to describe the ability of radiation to pass through materials, the appropriate term is penetration. For context, gamma rays can penetrate concrete, while alpha particles are stopped by a sheet of paper.

Fusion in stars requires nuclei to collide with enough energy to overcome their electrostatic repulsion, and the reaction rate increases sharply with temperature. In stellar cores, temperatures reach a few tens of millions of kelvin, which is enough for protons to fuse mainly through the proton-proton chain (for Sun-like stars) or the CNO cycle (in more massive stars). The proton-proton chain becomes efficient around about ten million kelvin, and the Sun’s core is even hotter, around 15 million kelvin, sustaining the fusion that powers the star. So the minimum core temperature needed for fusion to occur at an appreciable rate is about ten million kelvin. The other options are far too low to overcome the Coulomb barrier, or higher than the threshold needed.

PWR fuel uses low-enriched uranium so that a thermal reactor with water as a moderator can sustain a controlled fission chain reaction. Natural uranium has only a tiny fraction of U-235, which isn’t enough to maintain a steady reaction in a light-water environment. By increasing the U-235 content to a modest level, the fuel becomes capable of sustaining the reaction over the fuel burnup while still allowing for safe reactor operation, fuel management, and licensing. That enrichment level is chosen to balance several factors: it provides enough fissionable material to achieve and maintain criticality with the reactor’s neutron spectrum, it supports efficient fuel burnup over the fuel cycle, and it avoids the higher proliferation and safety considerations associated with much higher enrichments. In short, PWRs use a relatively small, practical enrichment—more than natural uranium but well below what’s used for highly enriched fuel.