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Why did the Chernobyl RBMK reactors go unstable at low power?

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At high power, in steady state, a fission reactor produces a mix of fission products. One of the most important products is Xenon 135. This particular isotope is interesting because it has a huge neutron absorption cross section (2M barns) and a short half-life (9.2 hours). read more

Furthermore, because steam absorbs neutrons much less readily than water, increasing the intensity of vaporization means that more neutrons are able to split uranium atoms, increasing the reactor’s power output. This makes the RBMK design very unstable at low power levels, and prone to suddenly increasing energy production to a dangerous level. read more

Under low flow/low power/low rod density conditions, any boiling plant is unstable. RBMK types are more concerning than BWRs, as RBMKs have a positive void feedback loop, while BWRs have a negative void feedback loop. What this means is an RBMK can go unstable and have a power excursion leading to core/plant damage, while a BWR can go unstable and at worst cause some localized plastic/elastic damage to the cladding. read more

The Soviet-designed RBMK (Reaktor Bolshoy Moshchnosty Kanalny) high-power channel reactor was a pressurized water-cooled reactor with individual fuel channels and using graphite as its moderator. This meant that the cooling medium and the moderation medium are separated. read more

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