Measurements and Characterization of Neutron and Gamma Dose Quantities in the Vicinity of an Independent Spent Fuel Storage Installation

As part of the decommissioning of the Maine Yankee Atomic Power Company (MYAPCo) nuclear power plant, the spent nuclear fuel is being temporarily stored in a dry cask storage facility on a portion of the original licensed property. Each of the spent nuclear fuel (SNF) storage casks hold approximately 25 spent fuel assemblies. Additional storage casks for the greater-than-Class C waste (GTCC) are also used. This waste is contained in 64 casks (60 SNF, 4 GTCC), each of which contain a substantial amount of concrete for shielding and structural purposes. The vertical concrete casks (VCCs) are typically separated by a distance of 4 and 6 feet. The storage casks are effective personnel radiation shields for most of the gamma and neutron radiation emitted from the fuel. However measurable gamma and neutron radiation levels are present in the vicinity of the casks. In order to establish a controlled area boundary around the facility such that a member of the public annual dose level of 0.25-mSv could be demonstrated, measurements of gamma and neutron dose equivalents were conducted. External gamma exposure rates were measured with a Pressurized Ion Chamber (PIC). Neutron absorbed dose and dose equivalent rates were measured with a Rossi-typemore » tissue equivalent proportional counter (TEPC). Both gamma and neutron measurements were made at increasing distances from the facility as well as at a background location. The results of the measurements show that the distance to the 0.25-mSv per year boundary for 100% occupancy conditions varies from 321 feet to 441 feet from the geometric center of the storage pads, depending on the direction from the pad. For the TEPC neutron measurements, the average quality factor from the facilities was approximately 7.4. This quality factor compares well with the average quality factor of 7.6 that was measured during a calibration performed with a bare Cf-252 source. (authors)« less