In-Core Instrumentation, an Important Tool for Achieving High Specific Powers in Heavy-Water Reactors

An extensive line of in-core instruments has been developed and applied at HBWR, Norway. The instruments have proved to be reliable and are considered to be an important tool for achieving maximum power density in a core, which can only be done when the real limits have been determined and means found to safely operate the fuel assemblies close to those limits. The burn-out power limit, the channel instability limit and the centre fuel temperature limits are the main factors which determine the maximum power of a natural circulation fuel channel of a boiling heavy- water reactor. At the present stage theoretical calculations are not sufficiently accurate to determine these limits. In-core measurements are therefore required. To determine limiting fuel and channel characteristics the following instruments are used: Turbine flow meters at channel inlet and outlet to determine flow rate and exit void fraction; An impedance void gauge to measure dynamic void fraction; Gamma thermometers or beta current neutron detectors to indicate channel power. Exit steam quality and slip ratio can be computed when power and inlet and outlet velocities are known; Coolant thermocouples at inlet and outlet of channel; A calibration or throttle valve, to enable calibration of channel power detectors and to obtain critical heat transfer conditions; Thermocouples to determine central fuel temperatures; Burn-out detector to determine critical heat transfer conditions. An in-core detector system to determine the power distribution in the core will be used in order to operate the fuel assemblies close to their power limits. This system can be with thermal neutron flux detectors, gamma flux detectors or by material activation method (i.e. wire scanning, aeroball system). The number and interpretation of signals from these systems require the use of a computer. The in-core instrumentation programme on the Halden Reactor has already involved 40 instrumented fuel assemblies and more than 60 new assemblies will be used over the next two years. Together with a power detector system and the use of an on-line process computer, methods for power distribution determinations and optimal fuel usage will be sought and developed. (author)