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What /Actually/ Happened at Chernobyl

2019-06-14 · explainer · Hank · 13:30 · watch on YouTube

Topic: Science & nature · mode: Explainer · tags: timely hook, heavy topic

Hank explains the physics behind the Chernobyl explosion: the cheap RBMK design (water coolant, graphite moderator, two-way control rods), xenon buildup and nearly all rods pulled, so reinserting the rods spiked neutron flux; the 'graphite tip' is a simplification.

What is said

The parts of the video and what is said in each (★ — how much it matters to the video).

00:00:00 — Chernobyl's explosion wasn't supposed to be possible: why, and why it happened

  • ★★★ Hank says a Chernobyl reactor explosion of a kind never seen before or since was supposed to be impossible, and sets out to explain why and why it wasn't. — 00:00:00

00:00:23 — Terms: nucleus, isotopes, radioactive decay, fission and fusion, decay heat, meltdown

  • ★★ Hank explains that the number of protons fixes the element while neutron counts vary between isotopes: carbon has six protons and usually six neutrons. — 00:00:59
  • ★★ Hank explains that unstable isotopes, with too many or too few neutrons, emit energy or particles on the way to a stable nucleus: radioactive decay. — 00:01:22
  • ★★ Hank explains that fission products are always slightly lighter than the original atom; the missing mass becomes energy, most of what a reactor turns into electricity. — 00:01:46
  • ★★ Hank says reactor heat comes mostly from fission and secondarily from the decay of unstable fission products, which keeps producing heat after the reaction stops. — 00:02:12
  • ★★ Hank explains that fuel that cannot be cooled keeps heating itself and can melt through almost anything: a meltdown. — 00:02:35

00:02:50 — Uranium-235 chain reactions, enrichment, the RBMK's cheap design, moderators versus absorbers, heavy water

  • ★★★ Hank explains that uranium's rare fissile isotope, the one natural isotope that splits when hit by a neutron and releases more neutrons, enables a chain reaction and hence nuclear power. — 00:02:56
  • ★★ Hank says U-235 is rare, about 0.7% of natural uranium, so reactors and bombs require enriching the uranium. — 00:03:12
  • ★★ Hank says a nuclear bomb is about 80% U-235, so nearly every neutron causes fission and all the uranium reacts at once, which reactors must avoid. — 00:03:27
  • ★★★ Hank says enrichment is extremely expensive, and the RBMK design's appeal was a cheap self-sustaining reaction using unenriched or very slightly enriched uranium. — 00:04:05
  • ★★★ Hank explains that reactors control neutrons by absorbing them (slowing the reaction) or moderating them; fast neutrons mostly bounce off, so slowing them speeds the reaction. — 00:04:29
  • ★★ Hank explains that heavy-water reactors are expensive but self-limiting: as the reactor heats, the water boils, moderation drops and the reaction slows, a desirable negative feedback loop. — 00:05:12
  • ★★★ Hank says the RBMK used cheaper regular water as coolant, though regular water absorbs neutrons, which becomes important later. — 00:05:39
  • ★★ Hank says that instead of heavy water, RBMK reactors used big blocks of graphite as the neutron moderator. — 00:05:48

00:06:03 — Xenon-135 buildup on the day of the test and pulling nearly all the control rods

  • ★★ Hank explains that moderators and absorbers act as gas pedal and brakes because fission byproducts like xenon build up slowly and absorb neutrons, so control rods must be pulled out. — 00:06:14
  • ★★★ Hank says that on the day of the accident the reactor had unintentionally been run all day in a way that built up xenon, so when power was lowered for the test it produced almost nothing. — 00:06:40
  • ★★★ Hank says that rather than raising power slowly as normal for an RBMK, they pulled nearly all control rods out, leaving only water and xenon absorbing neutrons. — 00:07:05

00:07:33 — The 'graphite-tipped control rods' simplification versus the RBMK's two-way control rods

  • ★★ Hank says the show and much science communication describe graphite-tipped control rods, a simplification he understands given limited time. — 00:07:33

  • ★★★ Hank says the rods weren't graphite-tipped: RBMK control rods work both ways, absorber on top, and pulling one out pulls in a graphite rod, so brake and accelerator come together. — 00:08:06

    “That's not actually a graphite tip, it's part of what makes the reactor work.”

00:09:01 — Neutron flux, positive feedback, and the reinsertion of the rods that spiked flux at the bottom

  • ★★★ Hank says the biggest problem was neutron flux: graphite moderator rods are shorter than fuel rods at both ends, leaving water there, which evens flux unless all rods move at once. — 00:09:01
  • ★★★ Hank says positive feedback then kicked in: slowed water flow let the water boil, so less water absorbed neutrons, and the rising neutrons burned off the xenon. — 00:09:49
  • ★★★ Hank says the fatal step was the prescribed one: reinserting the rods, whose graphite displaced the water at the bottom, causing a neutron flux spike beyond design. — 00:10:20
  • ★★ Hank says energy rose rapidly and something broke, perhaps several things, locking the graphite rods in the most dangerous position. (hedged) — 00:10:51

00:11:08 — Side note on explosions as phase change; how Chernobyl exploded compared with Fukushima

  • ★★ Hank explains that although fire is often part of explosions, they usually happen when a solid or liquid turns into gas: rapid expansion, a phase change. — 00:11:08
  • ★★★ Hank says the reactor water became extremely pressurized steam, hot enough to split into hydrogen and oxygen; the top popped off and the graphite caught fire. — 00:11:42
  • ★★ Hank contrasts Fukushima: there hydrogen escaped the reactor and exploded, damaging it, which is bad but very different from Chernobyl. — 00:12:05
  • ★★ Hank says the unstable fission products then either melted without cooling or burned and went into the air for people to breathe for hundreds of miles. — 00:12:19

00:12:33 — Not the worst case for people; the design failed only under the specific actions of 1986

  • ★★★ Hank says this is as bad as a reactor explosion gets but not the worst case for people; it could have been much worse without hard work, clever engineering and bravery. — 00:12:33
  • ★★★ Hank concludes that reactor design is about balance: despite its problems, the RBMK design prevented catastrophe unless the very specific actions taken in 1986 were done. — 00:12:51

00:13:06 — Recommends Scott Manley's video and the HBO miniseries; sign-off

  • ★★ Hank recommends Scott Manley's video on the topic as amazing for going deeper into the physics. — 00:13:06
  • ★★ Hank recommends the HBO miniseries as very good and enlightening on the human and political causes of the disaster, as well as the engineering and physical ones. — 00:13:11

In the description and comments

  • ★ In the description Hank says the video was great fun to research, which he feels guilty about because real people died. — video description
  • ★★★ Hank corrects a claim he has heard from smart people: RBMK reactors did not have a positive fuel temperature coefficient, and no reactor does, since fuel that reacted more as it heated would explode every time. — video description
  • ★★★ Hank explains that for a moment the whole reactor had a positive temperature coefficient, from the positive void coefficient: hotter water absorbed fewer neutrons; normally the negative fuel coefficient outweighs this, which is why it was thought unable to explode. — video description

Mentioned

Central: Chernobyl disaster · control rod · neutron · neutron flux · neutron moderator · nuclear reactor · RBMK · uranium-235 · xenon-135

Also: People: Scott Manley

Works: Chernobyl (miniseries)

Organizations: HBO

Concepts: atomic nucleus · carbon · enriched uranium · explosion · heavy water · hydrogen · isotope · mass–energy equivalence · negative feedback · nuclear chain reaction · nuclear fission · nuclear fusion · nuclear meltdown · nuclear power · nuclear reaction · nuclear weapon · proton · Quantum mechanics · radioactive decay · vicious circle

Events: Fukushima nuclear accident

Only in the description or credits: void coefficient

Also centred on nuclear reactor: Why Did Fukushima Explode?

Also centred on uranium-235: Why Did Fukushima Explode?

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