Development of a real-time in-core neutron monitoring system for neutron flux variations during neutron activation analysis

Abstract

The pneumatic NAA facility at The University of Texas at Austin is widely used for the measurement of trace elemental concentrations. It has been shown for extremely short duration NAA irradiations, sample activation measurements can vary by up to 12% from normalized values. The typical university research reactor’s external neutron monitoring instruments are unable to detect small-localized variations in neutron population likely resulting in the observed error. The primary method to reduce this uncertainty is to irradiate traceable flux monitor samples in addition to the experimental samples to be analyzed. This method doubles the required sample prep, measurement, and analysis effort. An in-core neutron monitoring system was designed and installed adjacent to the pneumatic NAA sample system terminus to track these localized fluctuations. This design consists of a miniature in-core fission chamber fitted through the upper grid plate of the reactor core. The system is controlled by a NI LabVIEW data logging application. This system provides the capability of monitoring sample irradiance in real-time. In preliminary testing, the system was able to track short irradiation NAA sample variance to within 3% of normalized values. Thus, a significant reduction in the uncertainty of NAA measurements for trace elemental concentrations is achievable.