The inside of a nuclear reactor is a tightly packed, precisely engineered space dominated by hundreds of metal fuel assemblies standing upright in a pool of water, all enclosed within a massive steel pressure vessel with walls up to 10 inches thick. When the reactor is operating and you could somehow peer inside, the most striking feature would be an intense blue glow radiating from the core, caused by a phenomenon called Cherenkov radiation.
The Blue Glow
That eerie blue light is the signature visual of a nuclear reactor, and it has a fascinating explanation. When a reactor is running, it emits charged particles (mostly electrons) that travel through the surrounding water faster than light itself moves through water. Light slows to about 75 percent of its normal speed in water, which means these particles can actually outpace it. As they do, they disrupt water molecules along their path, releasing photons in a visible “shockwave” of blue or violet light. It’s the optical equivalent of a sonic boom. The effect is called Cherenkov radiation, and it’s why every photo you’ve seen of an active reactor core has that distinctive blue-violet aura.
You can see this glow most clearly in research reactors, which sit at the bottom of open pools of water. Commercial power reactors are sealed inside thick pressure vessels, so the glow isn’t visible during normal operation, but it’s happening inside all the same.
The Fuel at the Center
The heart of any reactor is the fuel. Nuclear fuel starts as small ceramic cylinders of uranium oxide, each about the size of a thimble: roughly 3/8 of an inch in diameter and 5/8 of an inch long. These pellets are dark gray or black, dense, and smooth. Hundreds of them are stacked end to end inside long, thin metal tubes called fuel rods, which are typically made of a zirconium alloy chosen because it resists corrosion and doesn’t absorb too many neutrons.
Those individual fuel rods are bundled together into fuel assemblies, square-shaped clusters that look like tall metal grids when viewed from above. A typical pressurized water reactor core holds around 200 of these assemblies standing vertically, each one roughly 12 feet tall. Looking down at the top of the core, you’d see a tight grid pattern of these square assemblies filling a roughly cylindrical space.
The Core Barrel and Pressure Vessel
All of those fuel assemblies sit inside a structure called the core barrel, a large steel cylinder that holds the core in position and directs the flow of cooling water. The core barrel itself sits inside the reactor pressure vessel, a massive container of carbon steel with walls up to 10 inches (25 cm) thick. The interior surface of the pressure vessel is lined with a thin layer of stainless steel cladding to resist corrosion from the hot, pressurized water flowing through it.
The pressure vessel is roughly cylindrical with a domed top (the “head”) that can be unbolted and lifted off for refueling. When the head is removed during a refueling outage, you can look straight down into the open vessel and see the tops of the fuel assemblies submerged under about 20 feet of crystal-clear water.
Control Rods and Drive Mechanisms
Threaded among the fuel assemblies are control rods, long metal rods made of materials that absorb neutrons and slow or stop the nuclear chain reaction. In a pressurized water reactor, these rods enter the core from above. The drive mechanisms that raise and lower them are mounted on top of the reactor vessel head, forming a forest of vertical cylinders and electrical connectors that gives the top of the vessel its distinctive bristling appearance.
These mechanisms use an electromagnetic jack design. Magnetic coils grip and lift the control rod one small step at a time. In an emergency, the electromagnets simply lose power and the rods drop into the core by gravity, shutting down the reaction in seconds. Inside the core, guide tubes run vertically through the fuel assemblies, providing channels that keep the control rods aligned as they slide in and out.
How Water Moves Through the Core
Cooling water doesn’t just sit around the fuel. It follows a precise path. Cold water enters the pressure vessel through large inlet pipes (called “cold legs”) near the top of the vessel, then flows downward through a narrow gap between the core barrel and the pressure vessel wall. This downward channel is called the downcomer. The water collects in a lower chamber beneath the core, reverses direction, and flows upward through the fuel assemblies, picking up heat as it passes along the fuel rods. The heated water then exits through outlet pipes near the top of the vessel.
This upward flow through the core is carefully managed. A core support plate at the bottom positions each fuel assembly and controls how much water enters each one. Flow distribution plates and orifices in the lower chamber help ensure that every fuel assembly receives adequate cooling.
Sensors Inside the Core
Buried within the fuel assemblies are monitoring instruments that give operators a real-time picture of what’s happening inside the core. Thin detector assemblies are threaded through guide tubes and into narrow metal thimbles built into individual fuel assemblies. A typical detector assembly contains four neutron-sensing detectors spaced at 20, 40, 60, and 80 percent of the core’s height, giving operators a vertical profile of the nuclear reaction at that location. A thermocouple at the top of each assembly measures the temperature of the water leaving the core at that spot. In one common design, 45 of these thermocouples are distributed across the core, each sitting about a foot above the top of the fuel.
These instruments are invisible from outside, but they’re threaded through the vessel via small nozzles and have to be bent at various angles to reach their assigned positions. They’re one reason the inside of a reactor is more complex than it appears in simplified diagrams.
How BWRs Look Different From PWRs
Not all reactors look the same inside. The two most common types of commercial reactors, pressurized water reactors (PWRs) and boiling water reactors (BWRs), have noticeably different internal layouts.
In a PWR, the water stays under enough pressure that it never boils inside the vessel. The interior is relatively clean in design: fuel assemblies, control rods entering from above, and flowing pressurized water. Steam is generated in a separate piece of equipment outside the reactor vessel.
A BWR, by contrast, allows the water to boil directly inside the reactor vessel. That means the vessel has to be taller to accommodate large steam separators and dryers positioned above the core. These components look like stacked metal cylinders and plates sitting on top of the fuel assemblies, and they take up a significant portion of the vessel’s interior volume. Their job is to separate water droplets from the steam before it leaves the vessel and heads to the turbines. Control rods in a BWR also enter from the bottom rather than the top, which changes the look of the lower vessel and eliminates the cluster of drive mechanisms on the vessel head.
From a visual standpoint, looking down into an open BWR during refueling reveals the same grid of fuel assemblies and blue-tinged water as a PWR, but with the bulky steam separation equipment removed and set aside.