A Reactor Vessel Level Monitoring System, an Aid to the Operators in Assessing an Approach to Inadequate Core Cooling

The Reactor Vessel Level Monitoring System is a heated junction thermocouple (HJTC) and microcomputer based system for reactor vessel coolant level monitoring. This system, motivated by lessons learned from the TMI incident, can be an effective operator´s aid in situations where there is a reduced water level in the reactor vessel and, hence, provides an incremental improvement in plant operation and safety. The system helps the operator to assess the adequacy of core cooling by providing simple and direct information on: 1) coolant inventory and 2) coolant temperature in the reactor vessel region above the core. The measurement device (probe assembly) consists of HJTCs, splash shields, and separator tube enclosure. The HJTC is a dual thermocouple (a single heated junction and an unheated junction adjacent to it) in a common sheath. The junctions are electrically connected such that a differential temperature output is developed which is directly related to the heat transfer coefficient of the surrounding cooling medium. In addition to the differential output (used for level measurement), each of the two individual junction temperatures are also provided. The unheated junction provides a measure of fluid temperature in that region of the reactor vessel. Discussed herein are the design basis, system design development, and test results. This paper is a condensed version; for a more detailed version, please contact the author.