Estimating Dose Rates from Activated Groundwater at Accelerator Sites

At sites with powerful particle accelerators, the problem of groundwater activation by direct neutron radiation arises. Licensing of particle accelerators requires evidence that groundwater activation is within the legal limits and thus will not endanger workers, the public, or the environment.

In this study we focus on the following radionuclides: ¹⁴C, ⁴¹Ca, ⁴⁵Ca, ³⁶Cl, ⁵⁵Co, ⁵⁷Co, ⁶⁰Co, ³H, ⁵⁴Mn, ²⁴Na, ³²P, ³⁵S, ³²Si, and ⁵⁰V. The conventional approach for calculating activation of soil and groundwater is described and utilized for a fictive 5-MW proton accelerator at Jülich, Germany, with a beam loss of 1 Wm^-1. An updated overview of partition coefficients for relevant radionuclides in sand, clay, loam, and organic soils is presented. Based on the two aforementioned methods, groundwater activation is estimated with a simplified homogeneous groundwater transport model. The results indicate ³H, ¹⁴C, and ³⁶Cl as the most relevant radionuclides concerning the resultant activity concentrations and estimated dose rates at the site boundary. For this fictive test case, the site boundary is located a distance 250 m downstream of the accelerator, which leads to acceptable risk for the public, given the legal standards.