Characteristics of Nicrosil versus Nisil thermocouples for the in-core temperature monitoring system in HTTR

Saito, Kenji ; Shimizu, Atsushi ; Hirato, Yoji; Kondo, Makoto; Kawamata, Takanori ; Nemoto, Masumi; Motegi, Toshihiro

As In-core temperature monitoring system, Type N thermocouples arranged at hot plenum block measures the primary coolant temperature at each hot plenum block in order to monitor the condition of the reactor core during the reactor operation. Type N thermocouples should have a long lifetime with high reliability under the high temperature environment of about 1000C, because they are used in HTTR reactor pressure vessel. This report shows that the characteristic change of Type N thermocouples was confirmed from operation and maintenance data of current HTTR.