trailNuclear probabilistic risk assessments predicted core-damage frequencies well below the observed historical accident rate
Complex risk analyses historically exhibit error rates exceeding their claimed risk bounds
Actuarial comparison· for
Published probabilistic risk assessments for nuclear power plants typically estimated core-damage frequencies of about one in 10,000 to one in 100,000 reactor-years or lower
atomic
Core-damage accidents in commercial nuclear reactors have occurred historically at a rate on the order of one per few thousand reactor-years
At least five commercial nuclear power reactor units have experienced core-damage accidents
Worldwide commercial nuclear power reactors have accumulated roughly 20,000 reactor-years of operating experience
Reference-class mismatch· against
The historical core-damage record is dominated by early-generation and Soviet reactor designs outside the scope of most probabilistic risk assessments
Core-damage events before 1980 occurred almost entirely in experimental or first-generation reactor designs
The Chernobyl RBMK reactor design had no probabilistic risk assessment before the 1986 accident
The Fukushima Daiichi reactors had plant-specific probabilistic risk assessments predicting core-damage frequencies around one in a million reactor-years before the 2011 accident
Japanese nuclear plant probabilistic safety assessments before 2011 addressed internal events only, excluding tsunami hazards
claim · empirical · derivedA factual claim that rests on inference from other evidence rather than direct observation.constitution →claim page ↗︎
Nuclear probabilistic risk assessments predicted core-damage frequencies well below the observed historical accident rate
↑SupportedThe evidence favors the claim, but the examination is incomplete or the evidence is indirect.constitution →credence 0.85Credence, from 0 to 1: the Steward's probability that the claim, as stated, is true. Stated only where a single number is an honest summary; normative and evaluative claims usually carry none.constitution →
1depended on by
this rests on ↓
claimA box is a claim: a single proposition the graph assesses, with its own page and map. Click any claim to centre the map on it.constitution →quantityA box with a rule beneath its text is a quantity: a value the graph estimates rather than a proposition it affirms or denies. Its estimate stands where a claim's status would.constitution →questionA rounded box is a question: answered rather than affirmed or denied, with its candidate answers, subquestions and presuppositions beneath it. Its mark says how well answered it is.constitution →argumentA pill is an argument: one line of reasoning stating how the claims beneath it combine to bear on the claim above it, for or against. Click one to see its reasoning and open its page.constitution →✓verifiedThe evidence, examined directly, establishes the claim.constitution →↑supportedThe evidence favors the claim, but the examination is incomplete or the evidence is indirect.constitution →⇄inconclusiveThe evidence and argument have been examined and do not come down on either side.constitution →↓doubtfulThe evidence weighs against the claim, but the examination is incomplete or the evidence is indirect.constitution →✕contradictedThe evidence, examined directly, weighs against the claim.constitution →?unknownInsufficient information to assess.constitution →◌unassessedNo current assessment. Attention goes where its expected value is highest and someone funds it; nothing has funded an assessment of this claim yet, and anyone can.constitution →verified factunsettledvalue premisetheoremprizeA double ring marks a claim with a live prize: an amount in owls offered for a machine-checked proof or disproof of its published formal statement. The amount is in the preview and on the claim page; a prize changes nothing about how the claim is assessed or how important it is judged to be.prize rules →⊢machine-checked⊢ marks a claim whose published formal statement has a machine-checked proof or disproof. The checker confirms the proof; the verdict beside it is still the steward's judgment of the claim as worded.prize rules →supportsthis provides evidence for the parentsteward instructions →contradictsthis argues against the parentsteward instructions →assumesbackground the parent's framing takes as givensteward instructions →requiresa load-bearing premise: the parent is false without itsteward instructions →answersa candidate answer to the questionconstitution →Fig. Detail falls off with distance; every claim is an address.