1. Reactor Physics

How the Reactor Works

At the heart of your reactor are uranium fuel rods. Uranium atoms are unstable — when a tiny particle called a neutron strikes one, it splits apart. This releases heat and, crucially, more neutrons, which go on to split more atoms. This is the chain reaction, and it is the source of all your power.

Two other materials shape how your reactor behaves, and you'll feel both of them constantly:

• Graphite blocks surround the fuel. Fresh neutrons come out of fission moving too fast to reliably split the next atom, so the graphite slows them down to a speed that keeps the chain going. This is called moderation.
• Water flows up through the core to carry away heat. But water also quietly soaks up some neutrons. This detail seems minor — it is not. It's the reason the RBMK is so twitchy, and it's covered under Positive Void Coefficient below.

Your job is to control the chain reaction. Control rods are your main tool: they're made of a material that greedily absorbs neutrons. Push them in to starve the reaction and slow it down; pull them out to let it speed up.

Neutron Multiplication (the number k)

Every fission releases, on average, about 2.5 neutrons. Not all of them go on to split another atom — some are absorbed by the water or structure, some escape the core entirely, and some are swallowed by your control rods. What matters is how many useful neutrons survive to continue the chain.

This is captured by a single number called k, the multiplication factor. It tells you how many neutrons in the next generation exist for every neutron in this one.

• k = 1.000 — every neutron produces exactly one replacement. The reaction holds steady. This is called critical, and it's where you want to sit when running at a stable power.
• k below 1.000 — each generation is smaller than the last. The reaction is subcritical and dies out on its own.
• k above 1.000 — each generation is bigger than the last. The reactor is supercritical, and power is climbing.

Supercritical is not automatically dangerous — you pass through it briefly every time you raise power on purpose. The danger is in how far above 1.000 you push, and for how long.

Reactor Period

Reactor period is the practical way to feel what k is doing. It's the time it takes for reactor power to multiply by about 2.7 (roughly triple).

• Long period (60 seconds or more) — power is creeping up slowly. You have plenty of time to react. This is how you should normally raise power.
• Short period (a few seconds) — power is doubling over and over in quick succession. Act now.
• Period under a second — the reactor is climbing faster than any human can respond. At this point automatic protection is your only hope.

Rule of thumb: watch the period meter, not just the power meter. Power can look fine one moment and be out of control the next — the period warns you first.

Reactivity

Reactivity is the "gas pedal" for the reactor. It's really just a measure of which way k is leaning: it tells you whether the neutron population is growing, shrinking, or holding steady.

• Positive reactivity (+) — stepping on the gas. The chain reaction is speeding up; the reactor is getting hotter and more powerful.
• Negative reactivity (−) — hitting the brakes. The chain reaction is slowing; the reactor is cooling off or shutting down.
• Zero reactivity — cruise control. Power is steady. (This is the same thing as being critical.)

Nearly everything you do — moving rods, changing coolant flow, letting the reactor heat up — adds or removes a little reactivity. Managing that balance is the job.

βeff — the Speed Limit

To drive the reactor safely you need to know about βeff ("beta-effective"). Think of it as the speed limit on your gas pedal.

Here's why it exists. Most fission neutrons appear instantly ("prompt" neutrons), but a small fraction — about 0.65% — trickle out a few seconds late ("delayed" neutrons). That tiny delayed fraction is what makes the reactor controllable at human speed. βeff is the size of that fraction, and reactivity is often measured in units of it (one "beta," sometimes called one "dollar," equals βeff).

• Reactivity below βeff — the safe zone. The chain reaction can only grow as fast as those slow, delayed neutrons allow. Power rises on a timescale of seconds — slow enough for you to adjust. Stay here.
• Reactivity at or above βeff — prompt critical. The reaction no longer needs the delayed neutrons; the prompt ones alone sustain it. Power now doubles in fractions of a second. This is the meltdown zone — the reactor can destroy itself before you finish reaching for a button. Never let reactivity reach βeff.

Reactivity Margin — Your Rod Reserve

At any moment, some of your control rods are inserted in the core, holding the reaction down. The operational reactivity margin is simply how many rods' worth of braking power you have left in reserve — how much "brake" you could still apply if you needed to.

• Normal running: about 26–30 rods. Comfortable margin. Plenty of authority to shut down fast.
• Down to 15 rods: shut the reactor down immediately. Below this you no longer have enough braking power to guarantee control, and — because of the void coefficient — the reactor becomes dangerously unstable.

The trap is that pulling rods out to chase power spends this reserve. It feels productive — power goes up — but you're quietly draining your ability to stop. Keep an eye on your margin, not just your power.

Xenon Poisoning — the Reactor Fights Back

When you run the reactor, it slowly builds up a neutron-absorbing waste product called xenon. At steady power this is harmless — it builds and burns off at a constant rate, and you simply account for it.

The problem comes when you drop power sharply. Xenon keeps being produced for a while but is no longer being burned off fast, so it piles up over the next few hours. This surge of extra absorber pushes the reactor toward shutdown all on its own — operators call it the "xenon pit."

To keep the reactor alive through the pit, you have to pull out more and more control rods to counter the xenon — which drains your reactivity margin exactly as described above. Fighting the pit with the rods is how a reactor ends up dangerously stripped of its brakes. The safe move after a big power drop is often to wait for the xenon to decay, not to force power back up.

Positive Void Coefficient

This is the RBMK's signature flaw, and the mechanism behind its worst behavior.

Remember that the water in the core soaks up neutrons. Now suppose the reactor gets a little too hot and some of that water flashes to steam. Steam is far thinner than water, so it absorbs almost nothing — the neutron "sponge" vanishes wherever a bubble forms. Those freed neutrons go on to split more atoms, so power rises. More power makes more heat, which makes more steam, which frees still more neutrons...

That's a vicious cycle: heat → steam → more power → more heat. Left unchecked it runs away — the reactor speeds up because it's overheating, the opposite of what you'd want. This is why low-power, unstable conditions are so dangerous in an RBMK: a small wobble can snowball before you catch it.

The Scram Trap

Your emergency shutdown (the scram / АЗ-5 button) slams all the control rods into the core to kill the reaction. It is your ultimate safety net — but on the RBMK it hides a cruel quirk.

Each control rod has a graphite displacer on its end. As the rod first enters the core, that displacer pushes neutron-absorbing water out of the channel ahead of it — briefly adding reactivity right at the bottom of the core before the absorber arrives to remove it. Under normal conditions this is a harmless blip. But if the reactor is already in a bad state — power skewed toward the bottom, margin nearly gone — hitting scram can give a short, vicious power spike instead of an instant stop. This is precisely what turned a shutdown into an explosion at Chernobyl.

The lesson for the operator is the same one every section here keeps pointing to: don't let the reactor reach the state where the scram trap can bite. Keep your period long, your margin healthy, and your power stable, and the safety net works the way it should.