7.4 Reactor Basics & Radiation Safety
Key Takeaways
- A conceptual PWR overview includes fuel, water acting as moderator and coolant, and neutron-absorbing control rods; water primarily slows neutrons by scattering, though some neutron absorption occurs.
- Control rods directly manage the neutron multiplication factor (k) introduced in section 7.3: inserting them lowers k and slows or stops the reaction, withdrawing them raises k and increases power
- Alpha radiation is stopped by paper or skin, beta by thin metal or plastic, gamma/X-ray by dense material like lead or concrete, and neutron radiation by hydrogen-rich material like water or polyethylene
- ALARA (As Low As Reasonably Achievable) radiation safety rests on three controllable factors: time, distance, and shielding
- For an unshielded ideal point source at distances large relative to source size, inverse-square scaling predicts double distance gives one-quarter dose rate and triple gives one-ninth.
This section provides a public, introductory overview of reactor components and general radiation-safety concepts. It is optional editorial enrichment, not a claim about NAPT scope, Navy training requirements, or operational reactor procedures.
The Four Basic Components of a PWR
A pressurized water reactor (PWR) is the most common type of reactor design used for both commercial power generation and naval propulsion worldwide. At a conceptual level, it has four essential components:
| Component | Typical Material | Function |
|---|---|---|
| Fuel | Enriched uranium formed into ceramic pellets, stacked in metal fuel rods | Provides the fissile material (U-235) that undergoes fission and releases energy |
| Moderator | Ordinary water | Slows fast neutrons released by fission down to slower, thermal neutrons, which are far more likely to cause further fission |
| Coolant | Water, kept under high pressure to stay liquid at high temperature | Carries heat away from the reactor core to generate steam and drive a turbine |
| Control rods | Neutron-absorbing material (e.g., boron or cadmium) | Absorb neutrons to raise or lower the reaction rate, or shut the reactor down |
Fuel
The fuel is uranium enriched so that it contains a higher concentration of fissile U-235 than natural uranium ore. It is formed into small ceramic pellets, stacked inside long metal tubes called fuel rods, which are bundled together into fuel assemblies. This is where fission (section 7.3) actually takes place.
Moderator
Fission releases fast neutrons, but fast neutrons are relatively unlikely to cause further fission in U-235 — slower, thermal neutrons are much more effective. The moderator—ordinary water in a PWR—primarily slows fast neutrons through repeated scattering collisions. Some neutrons are absorbed by water, so moderation is not absorption-free; the useful effect is that many surviving neutrons reach lower energies where U-235 fission is more likely. This directly supports the chain reaction discussed in section 7.3.
Coolant
Fission releases enormous heat, and the coolant is the fluid that carries that heat out of the reactor core so it can be turned into useful power. In a PWR specifically, the coolant is water kept under high pressure so it stays liquid even at very high temperature (this is what gives the pressurized water reactor its name). Critically, a PWR's primary coolant loop — which circulates through the reactor core and can pick up some radioactivity — is kept physically separate from a secondary loop, which uses the primary loop's heat, through a heat exchanger, to boil separate water into steam that spins a turbine. Keeping the loops separate means the water that turns the turbine never directly contacts the reactor core.
Control Rods
Control rods are made of material that readily absorbs neutrons — commonly boron or cadmium, at the general public-knowledge level — without itself undergoing fission. Inserting control rods further into the reactor core absorbs more neutrons, lowering the multiplication factor k from section 7.3 and slowing or stopping the chain reaction. Withdrawing control rods absorbs fewer neutrons, allowing more of them to cause additional fission and increasing reactor power. Control rods are therefore the primary way operators adjust power level and shut a reactor down.
Radiation Types and Shielding
Section 7.2 introduced alpha, beta, and gamma decay. For radiation safety purposes, what matters most is each type's penetrating power — how much material is needed to stop it — plus neutron radiation, which is released directly during fission and matters specifically around a reactor core.
| Radiation Type | What It Is | Penetrating Power | Typical Shielding Needed |
|---|---|---|---|
| Alpha (α) | Helium-4 nucleus (2 protons + 2 neutrons) | Lowest | A sheet of paper or the outer, dead layer of skin; dangerous mainly if inhaled or ingested |
| Beta (β) | Fast-moving electron | Moderate | A thin sheet of metal (a few millimeters of aluminum) or thick plastic |
| Gamma (γ) / X-ray | High-energy photon (no mass or charge) | Highest | Dense, thick material — lead, thick concrete, or steel |
| Neutron | Uncharged particle released directly by fission | High, but stopped differently | Hydrogen-rich material — water, concrete, or polyethylene — which slows and captures neutrons |
Notice that neutron radiation is not stopped effectively by the same dense metals that block gamma rays; it takes hydrogen-rich material (the same general principle as the moderator above) to slow neutrons down enough to be absorbed.
Radiation Safety Principles: Time, Distance, Shielding
Radiation safety programs are built around a guiding principle called ALARA — As Low As Reasonably Achievable — which holds that radiation exposure should always be minimized as far as practically possible, even when a dose is already within a legal limit. ALARA rests on three controllable factors:
- Time — dose accumulates with exposure time, so minimizing time spent near a radiation source directly minimizes dose.
- Distance — for an unshielded ideal point source, and at distances sufficiently large compared with the source dimensions, dose rate approximately follows inverse-square scaling (1/distance²). Under those conditions, doubling distance gives about one-quarter the dose rate and tripling gives about one-ninth.
- Shielding — placing appropriate absorbing material, matched to the radiation type per the table above, between yourself and the source.
Worked Inverse-Square Approximation
Treat the source as an unshielded ideal point source, with both distances sufficiently large relative to its dimensions. If the dose rate at 2 meters is 40 millirem per hour, what is the approximate dose rate at 6 meters?
- Distance increases by a factor of 6 ÷ 2 = 3
- Dose rate decreases by a factor of 3² = 9
- New dose rate: 40 mrem/hr ÷ 9 ≈ 4.4 mrem/hr
The result is an idealized geometric approximation. Real dose rates can differ because sources have finite size, shielding, scattering, absorption, and geometry.
In a pressurized water reactor (PWR), what is the primary role of the moderator?
Which radiation type requires hydrogen-rich material such as water or polyethylene for effective shielding, rather than dense metal?
Treat a radiation source as an unshielded ideal point source, with 3 m and 9 m both large relative to its size. If dose rate is 90 millirem/hour at 3 m, what is the approximate rate at 9 m?