positive scram effect

Origins of the design

An RBMK control rod consisted of a neutron-absorbing boron carbide (B4C) absorber with a 4.5 m graphite "displacer" attached at its end. The displacer kept coolant water from entering the space vacated as the rod was withdrawn, thereby augmenting the rod's reactivity worth. The problem lay in the dimensions. With a rod fully withdrawn, the displacer sat centrally within the fuelled region of the core, leaving 1.25 m of water at either end. Because water absorbs neutrons, that water was locally depressing reactivity.

How a scram raised reactivity

Under those conditions, a scram signal reversed the situation. As the rod fell from its upper limit stop, water in the lower part of the channel was replaced by the graphite displacer. Absorbing material was thus displaced before the absorber arrived, so positive reactivity was locally inserted into the lower core. A measure meant to shut the reactor down instead briefly pushed local power up.

The size of the effect was not constant. According to INSAG-7, its magnitude depended on the spatial distribution of the power density and on the operating regime of the reactor. For the positive scram actually to occur, the control and safety rods had to be in a particular configuration.

The 1983 discovery and a buried warning

The effect was known before the accident. It was identified at the Ignalina plant in 1983, and restrictions on the complete withdrawal of control and safety rods were intended to be imposed. Those restrictions were never imposed and the matter was apparently forgotten. The operators at Chernobyl unit 4 were likewise unaware of the effect.

26 April 1986

Chernobyl unit 4 was operating at low power with more manual control rods withdrawn than permitted and an operating reactivity margin below the required minimum. At 01:23:40 the operator pressed the AZ-5 emergency shutdown button, and the control rods entering the core increased reactivity at the bottom of the core. The SCSSINP commission, Annex I of INSAG-7, identified the pressing of the EPS-5 button, with the RBMK-1000 at low power and with an impermissibly large number of manual control rods withdrawn, as the event that initiated the accident. Why the button was pressed at that moment was never established.

Post-accident retrofit

The Chernobyl accident brought extensive changes to the RBMK design. Central among them was the retrofitting of the control rods so that, with the rods fully retracted, no water column would remain at the bottom of the channels. Scram rod insertion time was cut from 18 to 12 seconds, and tests at the Ignalina and Leningrad plants in 1987-88 confirmed the performance of the new fast-acting emergency protection (FAEP) system.