Dose Rate Analysis of Upgraded Storage Drums for Disused Sealed Radioactive Sources by a Multivariate Interpolation Program Developed Using MCNP5

Abstract

The DOST-PNRI maintains a centralized radioactive waste management facility (RWMF) that performs conditioning of disused sealed radioactive sources (DSRS). These radioactive wastes are encapsulated in stainless steel and deposited into storage drums, which are in turn piled within the facility. This work describes the enhancement of the storage drum design using a dual-layer shielding of lead and concrete. This enables the drums to contain a total of three stainless steel encapsulations, therefore minimizing radioactive waste volumes within the facility. For this design, the MCNP5 code was used to verify that the surface contact dose rate limits will not be surpassed, for all drums to be fabricated. Each drum will be designed with a lead thickness that depends on the strength of radioactive waste to be contained. In order to account for different volumes, activities, and radionuclide types of the encapsulated waste as well as the required lead shielding, series of simulations were prepared to create a program in Microsoft Excel that will interpolate results based on these variable parameters. This program was subsequently applied to three existing source capsules. Results were verified with good agreement by PHITS Monte Carlo (MC) models. Using the program, design parameters were also optimized by acquiring the optimum source volume fractions per capsule, activity insertion ratios, and the maximum insertable radioactivity per drum. The degree of dose rate reduction within the old single-layer design and the enhanced dual-layer design was shown by mesh diagram. The interpolation program can be used as a tool for rapid fabrication of enhanced storage drums that can increase the capacity of the facility due to minimization of radioactive waste volumes.