those constraints set a limit on fuel enrichment average and maximal burnup power peaking factor and boron concentration during operation safety concerns require negative fuel and coolant coefficients and a minimal antireactivity margin throughout reactor operation for example in a batch strategy only of the assemblies are replaced at each refuelling thus each fuel assembly stays in the core for cycles on the other hand a higher enrichment results in a burden on the fuel fabrication plant higher fabrication cost and additional technicaleconomic constraints for fuel transport and storage