AbstractBreeding ratios for clean near-thermal systems containing only U233and moderator have been presented by Chernick and Moore (1). The present work treats Pu239systems also and extends the results in both systems to take account of the effects of the higher isotopes and fission products. The extra absorption by these higher isotopes tends to depress the breeding ratio, but fission in U235or Pu241compensate for this, and the net effect, as will be seen, is an increase in breeding ratio for plutonium-fuelled systems.