How a BWR reactor works

A BWR vessel is larger than that of a PWR of the same power, about 6 meters in diameter and 22 tall, because besides the core it houses the steam separators and dryers. The core has 500 to 800 fuel assemblies, each a bundle of 8×8 to 10×10 rods inside a Zircaloy channel box that guides the flow, with uranium enriched to 2 to 4%.

Water enters at the top of the annular downcomer, flows down through the jet pumps, crosses the core from bottom to top and leaves boiling. Two external recirculation loops, each with a pump, drive the jet pumps: that is how the flow, and with it the power, is regulated without moving a single rod.

It is the essential difference from the PWR: the water boils inside the reactor. At about 70 bar it boils at 285 °C, less than half the primary pressure of a PWR, which allows a thinner-walled vessel. At the core outlet about 13% of the mass is steam; the rest is still liquid water.

Bubbles moderate worse than water, so the more steam there is, the fewer neutrons are slowed and power drops. That negative void coefficient stabilizes the reactor on its own, and it is what turns recirculation flow into a power control: more flow, less steam, more power.

The water-steam mixture rises through the vertical tubes of the separators, where vanes set it spinning: the denser water clings to the wall and falls back to the downcomer; the steam keeps rising through the center. It is a cyclone with no moving parts.

Above them, the dryers are zigzag sheet-metal panels that catch the last droplets. Steam leaves the vessel with less than 0.1% moisture, through four large-diameter lines, straight to the turbine hall. There is no steam generator and no pressurizer: a single circuit.

That steam passed through the core and carries nitrogen-16, an isotope formed when neutrons strike the oxygen in the water. It emits very energetic gamma rays, but its half-life is 7 seconds: once the reactor is shut down, it is gone within a minute.

That is why in a BWR the turbine hall is a controlled area: during operation nobody can enter without shielding or stay near the lines, and the steam is isolated by fast-closing valves. In exchange the cycle is simpler, with fewer components and fewer losses, and the turbine is like that of any plant: high pressure, low pressure and generator.

The exhausted steam condenses below the turbine at about 0.05 bar on tubes carrying river or sea water. The condensate is filtered and demineralized, preheated to about 215 °C with steam bled from the turbine, and the feedwater pumps return it to the vessel.

It enters through a ring of spargers above the core and mixes with the water falling from the separators. Since it is the same circuit, the feedwater must also be kept very pure: any impurity becomes activated as it passes through the core.

Because the top of the vessel is taken up by separators, dryers and steam lines, the control rods enter from below, pushed by hydraulic drives. They are cruciform, of boron carbide, and slide between the boxes of four fuel assemblies. In an emergency shutdown, pressurized water inserts them in under 4 seconds; they do not fall by gravity.

The primary containment is compact: a steel drywell wrapping the vessel, light-bulb-shaped in the Mark I, connected by vent pipes to a ring holding thousands of tonnes of water, the suppression pool. If a pipe breaks, the steam discharges into that water and condenses, which limits the pressure. Everything sits inside the reactor building, the secondary containment. At Fukushima Daiichi, six GE BWRs, the total loss of cooling after the 2011 tsunami melted three cores; later designs, such as the ABWR and the ESBWR, add passive cooling and filtered venting.