The role of ex-vessel neutron dosimetry in reactor vessel surveillance in South Korea

Korea Hydro and Nuclear Power (KHNP) is the sole nuclear utility in South Korea and is among the four largest nuclear power generation companies in the world. KHNP operates 20 nuclear power plants which produce approximately 40% of the electricity in Korea. In addition, KHNP has six new reactors under construction at the Shin-Kori and Shin-Wolsong sites. The KHNP operating fleet consists of 16 PWRs and four PHWRs. Of the PWRs, two are Westinghouse-design two-loop plants, four are Westinghouse-design three-loop plants, two are Framatome-design three-loop plants, and the remainder are the Korea Standard Nuclear Plant (KSNP), similar to CE-design plants where the reactor was supplied by CE and DOOSAN. Each operating reactor currently has a reactor vessel surveillance program usually consisting of from six to eight surveillance capsules located between the core and the reactor vessel in the downcomer region near the reactor vessel wall. The neutron sensors contained in these capsules provide an infrequently obtained measurement at a single location within the reactor geometry that is rarely near critical reactor vessel locations. By themselves, therefore, they cannot provide information regarding the azimuthal, axial, and radial gradients of neutron exposure within the reactor vessel. In some pressurized water reactor designs the neutron exposure rate at the surveillance capsule locations is much greater than that at the peak location on the reactor vessel thus dictating that the capsules be removed fairly early in plant life to prevent over exposure of the material specimens. Finally, there are significant problems reading the dosimetry from some of the surveillance capsule designs. These issues have led KHNP to work with the Korea Atomic Energy Research Institute (KAERI) to develop ex-vessel neutron dosimetry programs for all of the KHNP PWRs. Comprehensive sensor sets including Cu, Ti, Fe, Ni, Nb, Co, ²³⁸U and ²³⁷Np radiometric monitors are employed at discrete locations within the reactor cavity to characterize the neutron energy spectrum variations axially and azimuthally over the beltline region of the reactor vessel. Stainless steel gradient chains are used to complete the mapping of the neutron environment between the discrete locations chosen for spectrum determinations. In choosing sensor set locations advantage is taken of the octant symmetry typical of pressurized water reactors. Cycle-specific neutron transport calculations employ both 3D synthesis and full-3D calculations. The current plan is to install ex-vessel neutron dosimetry in all of the Korean PWRs. To date dosimetry has been installed in seven of the 20 PWRs. Since the use of ex-vessel neutron dosimetry is new to Korea, the regulator (KINS) has requested that a Korea-specific database of measurements be established as quickly as possible. (author)