A cold neutron source is a secondary moderator which locally reinforces the neutron flux by supplying cold neutrons. This is achieved by slowing down both fast neutrons coming from the core and thermic neutrons coming from the primary moderator through successive collision of incoming neutrons with atomic nuclei of a cold medium. The cold moderator technique of producing cold neutrons was experimented as early as 1957 at Harwell.

In the case of the ILL reactor, the cold medium is liquid deuterium (25 K = -248°C) in order to maximize efficiency and minimize absorption of neutrons.

The Vertical Cold neutron Source

The Vertical Cold neutron Source - an aluminium sphere of 38 cm diameter filled with boiling liquid deuterium at 25 K - was part of the ILL reactor since the beguining and was modernized in 1985.