positive scram effect
양의 스크램 효과
A design flaw in RBMK reactors whereby, when control rods were fully withdrawn and a scram (emergency shutdown) was triggered, the graphite displacer attached beneath each rod's boron absorber would first push neutron-absorbing water out of the lower core, transiently increasing reactivity instead of reducing it. First identified at the Ignalina plant in 1983 but left uncorrected, the effect was identified by INSAG-7 as a decisive contributory factor in the Chernobyl disaster: pressing the AZ-5 emergency shutdown button caused the reactor to surge rather than stop. All operating RBMK control rods were subsequently retrofitted to eliminate the water column at the bottom of the core.
Sources
- Wikipedia (EN) RBMK reactor design overview noting the positive scram effect as a major shortcoming contributing to the Chernobyl disaster.
- Wikipedia (EN) Chernobyl disaster article describing the control rod graphite displacer and the positive scram mechanism that caused a power spike on AZ-5 activation.
- www-pub.iaea.org INSAG-7 (IAEA Safety Series No. 75-INSAG-7, 1992), Section 2.2 and 4.1: design of control and safety rods, discovery of the positive scram effect at Ignalina in 1983, and the conclusion that 'the scram just before the sharp rise in power that destroyed the reactor may well have been the decisive contributory factor.'
- world-nuclear.org World Nuclear Association appendix on RBMK reactors: description of the graphite displacer dimensions, the 1.25 m water column, the positive scram mechanism, and post-Chernobyl retrofitting.
- Wikipedia (RU) Russian Wikipedia article on концевой эффект, the standard Russian term for the positive scram effect, detailing the physical mechanism and the 1983 Ignalina discovery.