*Why* were the Chernobyl control rods tipped with graphite?

"Tipped" is a somewhat simplistic term that has been repeated so often it has become fact. In fact the use of graphite for part of the control rods is more involved that "tip" might suggest.

The report :

• INSAG-7 The Chernobyl Accident : Updating of INSAG-1 A REPORT BY THE INTERNATIONAL NUCLEAR SAFETY ADVISORY GROUP
  • Link to download page
  • ISBN 92-0-104692-8
  • Safety report No. 75

goes into some detail on the design of the rods.

2.2. DESIGN OF CONTROL AND SAFETY RODS

The control rods and the safety rods of an RBMK reactor are inserted into the reactor core from above, except for 24 shortened rods which are inserted upwards and which are used for flattening the power distribution. A graphite rod termed a displacer is attached to each end of the length of absorber of each rod, except for twelve rods that are used in automatic control.

The lower displacer prevents coolant water from entering the space vacated as the rod is withdrawn, thus augmenting the reactivity worth of the rod. The graphite displacer of each rod of all RBMK reactors was, at the time of the accident, connected to its rod via a 'telescope', with a water filled space of 1.25 m separating the displacer and the absorbing rod (see Fig. 1).

The dimensions of rod and displacer were such that when the rod was fully extracted the displacer sat centrally within the fuelled region of the core with 1.25 m of water at either end. On receipt of a scram signal causing a fully withdrawn rod to fall, the displacement of water from the lower part of the channel as the rod moved down-wards from its upper limit stop position caused a local insertion of positive reactivity in the lower part of the core. The magnitude of this 'positive scram' effect depended on the spatial distribution of the power density and the operating regime of the reactor.

So the use of graphite had more than one purpose.

The graphite displacer improved the efficiency of the cycle by improving the geometry and 'coupling' between the upper and lower active sections of the core when the control rod was fully withdrawn. At the 'full out' position the displacer improves the flux pattern and reduces the amount of heat absorbed by the Control Rod Channel Cooling System. Allowing this space to be full of water would be a 'small' but continuous thermal loss for each withdrawn rod. The displacers would save significant operating cost over the life of the unit.