Core Design Experience to Adopt Long Term (18-Month) Cycle Operation Strategy From Initial Core Available to Purchase

SKN(Shin Kori Nuclear Power Plant) Unit 3, the first unit of APR1400 reactor in Republic of Korea, which is under construction, is scheduled to realize 18-month cycle operation strategy from its initial core. SKN Unit 3 is a pressurized water reactor with a thermal output of 4000 MW_th having 3-batch 241 fuel assemblies. Its target cycle length will be realized as long term(about 455 EFPD) directly from initial core, although most of previous other plants had been started according to their annual or 15-month cycle operation schedule at their initial core and gone to 18-month after 3^rd ∼ 4^th cycle. SKN Unit 3 has also been designed by meeting the following requirements as well as the above cycle length requirement; 1) peaking factor (F_xy) ≤ 1.60, 2) Negative MTC(Moderator Temperature Coefficient), 3) thermal margin ≥ 10%, 4) sufficient shutdown margin. 5) Xe stability, 6) CEA(Control Element Assembly) Re-arrangement, etc,. Higher enriched assemblies used to accomplish long term cycle at the initial core make higher radial and axial peaking factor than those of OPR1000. APR1400 introduces axial cutback region in a gadolinia fuel rod(6^″ in each of top and bottom) and axial blanket region in a normal fuel rod(6^″ in each of top and bottom) with low enriched uranium to enhance the neutron economy. It is also expected for these optimized cutback and blanket length to provide an advantage to reduce or avoid xenon oscillation phenomenon in power ascension test(PAT). In order to enhance core operability (RPCS requirement, Xe oscillation control and performance related design requirements, etc,.) as well as meeting shutdown margin requirement, CEA re-arrangement were also made. In this paper, APR1400 core design experience including all other design endeavors will be introduced.