Thorium cycle in unmodified PWRs

For more than ten years the Commissariat a l'Energie Atomique and the Electricite de France have jointly carried out experimental and design studies in different fields: HTRs, MSRs, PWRs. With regard to the PWRs, the studies first concerned the use of Plutonium with Thorium to start the cycle. A satisfactory solution has been obtained by recycling the Plutonium with Thorium as from the first core loading. The solution proposed consists of loading the whole reactor with Th-Pu assemblies from the first core and no longer using the standard checkerboard loading; the assemblies are loaded in rings. Two positive points appear: burnable poisons are no longer necessary and the assemblies are no longer divided into several zones (as with Plutonium recycle with Uranium in reloadings). There are two rather negative characteristics: the second generation Plutonium may be used only in FBRs and the conversion factor is rather small. Nevertheless the proposed solution gives acceptable pin power peak, temperature coefficients, etc... Satisfactory solutions have also been obtained for cores loaded with Th-U233 assemblies. The main problem is, in this case, the moderator temperature coefficient. Different solutions may be found to solve this problem: we have chosen the one which gives simultaneously an acceptable moderator coefficient and a conversion factor improvement (the use of spectrum shift control). (author)