Neutron flux monitoring system for ITER-FEAT (abstract)

Abstract

The concept of the neutron flux measurements for International Thermonuclear Experimental Reactor ITER-FEAT is discussed. In spite of the fact that ITER-FEAT has reduced fusion power with respect to ITER-FDR, the requirements for neutron flux monitors are similar—wide dynamic range (seven orders), good temporal resolution (1 ms), and high accuracy (10%). It is clear that fission chambers are the most suitable detectors for this application. However high neutron intensity of the fusion plasma and hard requirements lead to a more sophisticated detection system than the ordinary fission chamber. Another problem is an absolute calibration of the detectors. We propose a neutron flux monitoring system, which consist of microfission chambers placed inside the ITER vacuum chamber, three wide range fission chambers placed outside the vacuum chamber, natural diamond detector based compact neutron monitors placed inside the channels of the neutron cameras, and a compact neutron generator for calibration. Microfission chambers could be installed in the standard plugs with other detectors (vacuum x-ray diode, magnetic probe). 235U could be used as well as threshold fission materials (238U, 237Np, 232Th). In the last case the fission chamber will be covered by a boron shield to reduce the changes in the sensitivity. Wide range fission chambers will operate in both pulse count mode and Campbell mode. High linearity is provided by count mode. Temporal resolution of 1 ms is provided by the count mode at low neutron flux and by the Campbell mode at high flux. The nonlinearity of the fission chamber during the switch from count mode to Campbell mode will be corrected by another fission chamber with low sensitivity operating in count mode. Compact neutron flux monitors placed inside neutron cameras will consist of up to ten natural diamond neutron counters with sensitivity to DT neutrons doubled by properly installed poliethilen radiators. Such monitors provide DT neutron flux profile measurements with dynamic range (three orders), temporal resolution (1 ms), and accuracy (10%). The full system could be calibrated by compact moveable neutron generator with neutron yield 1011 neutron/s, which operate in continuous mode. All elements of the system are commercially available, except for the neutron generator and diamond detector based monitors. The neutron generator and multidiamond detector based DT neutron monitors are now under development. An existing prototype of the neutron generator operates with a yield of one order less. Performance and major characteristics of the proposed systems of neutron flux monitoring will be discussed from the point of view of their application for neutron flux measurements and control of deuterium and deuterium–tritium phases of the ITER-FEAT operation.