Noise Thermometer at the FRM II Hot Neutron Source

The 20 MW research reactor FRM II operated by the Technische Universitaet Muenchen is equipped with a hot neutron source (HNS). The source is aimed to shift the well thermalized neutron spectrum in the heavy water moderator tank to higher energies as requested by the experimental users. The main component of the HNS is a solid graphite cylinder being heated by gamma radiation from the reactor core up to a temperature of about 2000 oC. The hot graphite cylinder is surrounded by a high-temperature insulation of carbon fiber, to achieve and maintain the high temperature. Due to the extremely harsh environment, the high temperature and the nuclear radiation, the temperature inside the graphite cylinder is measured by a purpose-built noise thermometer. It measures the white noise of an electrical resistor and determines the absolute temperature of the graphite cylinder. During nuclear commissioning of the hot neutron source, the temperature of the graphite cylinder was measured by the noise thermometer at several power steps of the reactor. The following relevant parameters of the HNS had been determined: the maximum temperature, the heating rate and the cooling rate after shut down of the reactor. The relative long time needed to reach the maximum temperature was used to measure the heat-up effect of the HNS. Since the nuclear start-up of the reactor the noise thermometer of the HNS is operated without significant problems. (author)