Fast reactor core

A fast reactor core comprising:

• an inner zone of a single enrichment having a heterogeneous structure with a vertical axis about which is arranged a distribution of fissile material and fertile material alternating in a radial direction from said vertical axis, said distribution and enrichment providing an internal breeding gain within said inner zone that at most results in a total reactivity change that is slightly positive;
• and
• an outer zone of fertile material surrounding said inner zone, said core having the following characteristics:
• ______________________________________ Volumetric percentage - fuel (fissile material) :
• steel (cladding and structure) 22.3 % sodium (coolant) 40.6 % fuel (UO.sub.2 + PuO.sub.2) 32.9 % Volumetric percentage (fertile material) :
• steel 19.2 % sodium 27.8 % fertile material 50.5 % Height of fuel 100 cm Linear power in fuel 500 W/cm Maximum burn-up 115,000 MWd/T Thickness of axial and radial blankets in the outer zone 50 cm Diameter of fuel pellets in fuel pins 5.5 mm ##STR2## 33.5 % Number of control rods 12 Variation in reactivity after 308 days of irradiation + 250 pcm Total breeding gain (TBG) 0.47 Internal breeding gain (IGB)g 0.031 Compound doubling time 12 years Core sodium voiding effect + 700 pcm Total flattening factor 1.42 Integrated dose in displacements per atom (DPA) 70 ______________________________________