Those constraints set a limit on fuel enrichment, average and maximal burn-up, power peaking factor and boron concentration during operation. Safety concerns require negative fuel and coolant coefficients and a minimal anti-reactivity margin throughout reactor operation. For example, in a 3-batch strategy only 1/3 of the assemblies are replaced at each refuelling, thus each fuel assembly stays in the core for 3 cycles. On the other hand, a higher enrichment results in a burden on the fuel fabrication plant, higher fabrication cost and additional technical-economic constraints for fuel transport and storage.