Direct nuclear heating measurements in fusion neutron environment and analysis

Experimental measurement of nuclear heating rates has been carried out in a simulated D-T fusion neutron environment from 1989 through 1990 under the USDOE/JAERI collaborative program at the Fusion Neutronics Source Facility. The microcalorimetric technique has been employed for online measurements. Small probes of materials have been irradiated in close vicinity of a rotating target. A typical probe contains a core measuring 2 cm in diameter by 2 cm in length. Probes of leading candidates, for different applications, have been investigated: Molybdenum, tungsten, titanium, graphite (plasma facing components), copper (magnet coils,), iron, stainless steel 304, nickel (structural material components) and aluminum. The measured temperature-change rates range from 30 μK/s (iron) to 330 μK/s (graphite). The corresponding nuclear heating rates range from ∝35 μW/g (tungsten) to ∝225 μW/g (graphite). The measurements have been analyzed using three dimensional Monte Carlo code MCNP and various heating number/kerma factor libraries. The ratio of computed to measured heating rates shows large deviation from 1 for all the materials. In addition, there is a large spread for different libraries; for example, this ratio varies from 1.03 to 1.81 for aluminum. Also, there have been three experiments with each having a host medium of iron, graphite or copper, that measures 85 mm in diameter by 100 mm in length. Small single probes of the host medium graphite and tungsten are placed inside to measure the spatial profile of heat deposition. Analysis of the measurements shows that the ratio of computed to measured rates varies widely, e.g., in iron host, it goes from 0.5 to 1.1. Further effort is to be invested to locate the sources of this discrepancy. (orig.)