Neutrons at the ILL are delivered by its reactor.

Refurbished between 1993 and 1995, it produces the most intense continuous neutron flux in the world in the moderator region: 1.5x10¹⁵ neutrons per second per cm², with a thermal power of 58.3 MW.

The reactor operates continuously for a 50 day cycle, followed by a shutdown to change the fuel element. In addition, there is a longer shutdown to enable necessary maintenance work to be carried out. Normally there are 4 cycles a year, providing 200 days for science.