The reactor Orphee (I) is the third modern reactor built after those of Brookhaven (HFBR, 196.5) and Grenoble (HFR, 197 1), especially dedicated to provide neutron beams for fundamental research. The reactor design is similar to the HFR of ILL. A core of very small size, which provides a high neutron density, surrounded by a heavy water tank to give a good thermal flux, is cooled by a downward circulation of light water. The biological shielding is achieved by a light water pool and a heavy concrete wall. Twenty neutron beams are issued from the reflector tank by nine horizontal beam ports in tangential position. The therma1 power is 14 MW and the maximum thermal neutron flux is 3 × 1014n/cm2/s. The reactor operates for ∼250 days/year in 2½ cycles of ∼100 days each.