Axial Flux Wire Measurements at the McMaster Nuclear Reactor

Abstract

In a 2008 report on safety analysis for research reactors, the International Atomic Energy Agency (IAEA) identified experimentation as the preferred method of code validation [1]. However, many experiments currently used for code validation are performed under conditions that are not representative of real nuclear systems. Furthermore, the predominant uncertainties reported for reactor systems parameters are typically those associated with evaluated nuclear data libraries however, the significance of spatial uncertainties remains generally unknown. The magnitude of local flux measurement experimental uncertainties have not be investigated at length in the McMaster Nuclear Reactor (MNR). Such results can be used for validation of MNR models with both Monte Carlo N Particle (MCNP) and Serpent code packages. Flux wire measurements have previously been conducted at the center of an irradiation site (via the technique of neutron activation analysis), where a locally uniform flux distribution has been assumed. Early stage results show good agreement with three-dimensional neutron diffusion theory and demonstrate the viability of such measurements for continued analysis. However, the magnitude of the effects of Xenon buildup, control rod positions, and spatial sample positioning on the data remain unknown, and so a series of experiments is ongoing to address these areas of experimental variability. Full length flux wire irradiations at several high-power levels (500 kW, 800 kW, and 1 MW) are being conducted to quantify these effects. At each operating power level, several NiCr wires are irradiated, and the decay of 51Cr examined to determine the total neutron flux in the irradiation site. The use of multiple wires per irradiation provides insight into the spatial gradient of the neutron flux across one reactor site (approximately 8 × 8 cm).