www.lesswrong.com/posts/qFzWTTxW37mqnE6CA/iabied-book-review-core-arguments-and-...
1 correction found
In a nuclear reactor the neutron multiplication factor needs to be around 100% and it would fizzle out or explode if it were slightly lower or higher.
Nuclear reactors can be (slightly) supercritical (k>1) during normal, controlled power increases; they do not “explode” merely because k is slightly above 1. What must be avoided is *prompt* supercriticality / prompt criticality, a specific threshold beyond mere k>1.
Full reasoning
Why this is incorrect
The post implies that a reactor would "explode" if the neutron multiplication factor were even slightly above the steady-state value (i.e., slightly supercritical). In reactor operations, k>1 (supercritical) is explicitly a condition used for increasing power, and is not synonymous with an explosion.
What the evidence shows
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The U.S. Nuclear Regulatory Commission (NRC) glossary defines a "Supercritical reactor" as "a reactor in which the power level is increasing with time" and "Supercriticality" as "the condition for increasing the level of operation of a reactor"—i.e., supercriticality is an operating condition for controlled power increases, not an automatic explosion. (nrc.gov)
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A reactor start-up / kinetics protocol hosted on the IAEA site distinguishes between:
- supercritical condition (k > 1) as part of normal start-up/control, and
- prompt supercritical as the dangerous regime where power rises too fast for control rods to reasonably control and "must never occur." This directly contradicts the post’s claim that being even slightly above the target would "explode." (nucleus.iaea.org)
Bottom line
A reactor going slightly above critical (k just above 1) corresponds to a controlled increase in power (often relying on delayed neutrons and control systems). Explosion risk is associated with crossing into prompt critical/supercritical regimes, not merely being “slightly” supercritical.
2 sources
- Full-Text Glossary | U.S. Nuclear Regulatory Commission
Defines “Supercritical reactor” as “A reactor in which the power level is increasing with time” and “Supercriticality” as “The condition for increasing the level of operation of a reactor...” (i.e., an operational condition, not automatic explosion).
- Reactor Start-up (PDF hosted on IAEA site)
Explains that reactor state can be converted to “critical (k=1) or to a supercritical condition (k>1)” and separately describes “Prompt Supercritical Reactor” as an uncontrollable accident regime that must not occur; distinguishes safe delayed-supercritical from prompt-supercritical.