Experimental quantification of the background neutron flux in the adanced test reactor fuel storage canal

The Advanced Test Reactor at the Idaho National Laboratory is one of only a few high-power, versatile, research reactors of its type in the world. The ATR fuel storage canal features a gamma irradiation facility that has various uses as well. It is composed of a simple dry tube that extends down into one of the fuel storage rack positions. This tube is surrounded by used fuel elements that can be selected from the inventory to produce the desired gamma radiation level and, to some extent, spectrum within the dry tube. The gamma field within the dry tube is accompanied by a small background neutron flux, produced primarily by photoneutron production in the small fraction of heavy water (D₂O) that is naturally present in light water. Neutron activation spectrometry was employed to quantify the background neutron flux in the dry tube in a configuration where the gamma exposure rate was approximately 5x10⁶ Roentgens per hour. The measurement technique used is very sensitive and provided self-consistent results in terms of raw activation data quality. The background neutron fluxes in the gamma facility were very low, but still detectable. The total neutron flux in the energy region above 0.414 eV and the thermal neutron flux (below 0.414 eV) were estimated from the activation measurements to be 7.5x10² and 1.0x10² n/cm²-s, respectively. High uncertainties (factors of approximately 20-30% in the above-thermal range and 100% in the thermal range) apply to these results. These high uncertainties are primarily due to the limited number of activation interactions that could be used to detect such small fluxes and the resulting propagation of uncertainties associated with the various input parameters involved in the spectral unfolding process.