Application of 3D Transport Calculation to Evaluate Radiation Embrittlement of Reactor Pressure Vessel

Reactor Pressure Vessel(RPV), which is one of the multiple barriers to maintain the integrity during the EOL(End Of Life), takes an important role restraining a radioactive substance from being released. However, a long-term exposure of fast neutron(E>1.0MeV) gives a serious damage to RPV, which consists of Ferritic Low Alloy, by weakening mechanical and chemical properties called radiation embrittlement. An accurate analysis of the fast neutron fluence(n/cm²) and flux(n/cm²-sec) about RPV beltline region could be the key point when it comes to the evaluation of radiation embrittlement. Traditionally, 2 dimensional(D) and 1D synthesis method(It is called '2D/1D synthesis' below all the sentences) based on DORT transport calculation code has been widely applied to determine fast neutron exposure to RPV. Although 2D/1D synthesis could be profit in view to evaluate the RPV beltline region, 2D/1D synthesis has not provided accurate fast neutron fluence evaluation at the reactor internals and nozzle. Recently, with regard to the more accurate evaluation of RPV integrity, Westinghouse in the US and Korea Reactor Integrity Surveillance Technology(KRIST) in KOREA developed RAPTOR-M3G(RApid Parallel Transport Of Radiation-Multiple 3 dimensional Geometries) performing 3D parallel discrete ordinate calculation in order to figure out a much detailed neutron exposure of reactor internals, nozzle and etc. It was applied to Hanbit Unit 4(Korea Standard Nuclear power Plant, OPR-1000). In a way to generate 3D transport calculation model for Hanbit Unit 4, geometric model, core source distribution and cross-section were defined and 3D transport calculation by RAPTOR-M3G was performed based on input parameters from NDRs(Nuclear Design Report) for the every fuel cycle. RAPTOR-M3G used S10 quadrature set for angle discretization and 0.001 for inner iteration convergence criteria. As a result, the neutron fluence(E>1.0MeV, n/cm²) in RPV at EOL(32 EFPY(Effective Full Power Year) was shown as 1.62E+19. It is a result which is less than 1% when comparing with the 2D/1D synthesis result(1.633E+19). A possibility for applying RAPTOR-M3G to evaluate RPV integrity is confirmed from both results shown as minute difference. However, the neutron flux and fluence in 2D/1D synthesis at RPV had 6% higher than RAPTOR-M3G results. It seems like conservatism of 2D/1D synthesis method. Moreover, in order to verificate the 3D calcualtion results, a comparisons between the calculations and measurements of reaction rate were performed for the In and Ex vessel neutron dosimetor and the results show good agreements. Finally, the RPV integrity of Hanbit Unit 4 at the EOL satisfied related requirement by showing 59.1 °F value for RTNDT(Reference Temperature Nil-Ductility Transition)-the representative parameter of fracture resistance in some material. From this research using RAPTOR-M3G, the fundamental datas for the evaluation to reactor internals including baffle or barrel as well as RPV region could be obtained and 3D transport calculation would play a important role for somewhere 2D/1D synthesis could not express like nozzle. Additionally, several advantages like service renewal were expected by applying RAPTOR-M3G performing 3D transport calculation