Experimental Measurement and MCNP5 Model of the Gamma Dose Distribution in the University of Utah TRIGA Reactor Pool

Gamma doses for the University of Utah TRIGA Reactor (UUTR) are estimated based on the MCNP5 model, and by measurements based on the thermo luminescent detectors (TLDs). In the experiment the gamma dose rate above the UUTR core was obtained by placing TLD-400s every 10 cm from the core top up to 150 cm measured from the surface of the UUTR core, and then every 50 cm up to 300 cm. The dose rate was also obtained at the pool surface level using a Ludlum model 19-survey meter. The measurements are obtained at a steady state thermal power of 90 kW. The MCNP5 model closely followed the experimental setup. The MCNP5 values of the dose immediately above the core were just under 30 percent difference compared to the measured values, which is within the TLD response of ± 30 percent. The TLD error is observed to remain fairly constant at around 5% of the mean. As the gamma dose approaches the lower limit for the TLDs (1 rem), the error rises almost exponentially reaching 80%. The difference between the simulated and measured gamma doses increases as the distance from the core increases. This difference at the surface of the UUTR pool is around 90 percent (the experimental gamma dose being 90% higher). The main source of the difference is due to statistical error of the MCNP5 calculation. The larger the size of the geometry, the more computational time is required to reduce the statistical error. Another source of error is the contribution of nitrogen-16 that is generated from (n, p) reaction with oxygen in the reactor pool water. The (n, p) reaction was not considered in our current MCNP5 model. Therefore, the MCNP5 model can be improved as follows: taking into account the fuel burn up that will correlate more accurately dose estimates near the surface of the pool due to the gamma emission from fission fragments, and taking into a consideration a production and transport of nitrogen-16 that will improve the dose estimates especially at the surface of the UUTR core. (author)