7.4 Reactor Basics & Radiation Safety
Key Takeaways
- A pressurized water reactor (PWR) has four essential components: fuel (enriched uranium pellets), moderator (water, slows neutrons), coolant (water, removes heat via separate primary/secondary loops), and control rods (neutron absorbers that raise or lower power)
- 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
- The inverse-square law means doubling distance from a radiation source cuts dose to one-quarter, and tripling distance cuts dose to one-ninth
Everything in this chapter builds toward this section: candidates pursuing the Navy's Nuclear Field program need a working, public-level understanding of how a reactor actually uses fission to generate usable power, plus the radiation safety principles that protect anyone working around radioactive material. This section covers general pressurized water reactor (PWR) concepts — the same level of detail published in public encyclopedias, textbooks, and government educational materials — not operational or design-specific engineering detail.
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 — slows fast neutrons down through repeated collisions, without absorbing them, so more of them go on to cause additional fission events. 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 — increasing distance from a source reduces dose rapidly, according to the inverse-square law: dose rate is proportional to 1/distance². Doubling your distance from a source cuts your exposure to one-quarter; tripling distance cuts it to one-ninth.
- Shielding — placing appropriate absorbing material, matched to the radiation type per the table above, between yourself and the source.
Worked Inverse-Square Law Example
If the dose rate measured 2 meters from a radiation source is 40 millirem per hour, what is the 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
This is the same inverse-square logic used throughout radiation safety planning: relatively small increases in distance produce large reductions in exposure.
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?
A worker's dose rate at 3 meters from a radiation source is 90 millirem/hour. Approximately what would the dose rate be at 9 meters?