Evaluation of analytical and Monte Carlo approaches for in situ ¹³⁷Cs inventory assessment

Analytical and Monte Carlo approaches for quantitative in situ ¹³⁷Cs inventory assessment were evaluated at a single field site using experimentally determined depth-profile data obtained from volcanic ash soils on Jeju Island, Korea. A relatively uniform test site was selected based on the measured horizontal distribution of ¹³⁷Cs. The measured ¹³⁷Cs depth profile exhibited a distinct subsurface activity maximum at a depth of approximately 10-15 cm, rather than the monotonic exponential decrease commonly assumed for fallout radionuclides. Three approaches for in situ inventory assessment were evaluated: a conventional analytical method based on an equivalent exponential distribution, a modified analytical method incorporating the measured layer-by-layer activity and bulk-density profiles with an infinite-plane source assumption, and a Monte Carlo simulation incorporating the same measured profiles within a finite source geometry. The conventional analytical method overestimated the inventory by approximately 34%, whereas the modified analytical and Monte Carlo estimates were approximately 8-9% higher than the laboratory-derived inventory and differed from each other by 1%. For the investigated site, these results indicate that the modified analytical approach can overcome limitations of the conventional analytical method in representing non-exponential depth distributions and depth-dependent bulk density while retaining the simplicity of the conventional framework for conversion-factor calculations and providing results comparable to those obtained using MCNP.