Determination of the Gamma-Ray Skyshine Dose Contribution in a Loss Of Shielding Accident

The goal of this research is to determine the gamma-ray dose contribution from skyshine. In a transportation accident involving the loss of lead gamma shielding, first responders to the accident will be exposed to both direct gamma radiation streaming from the exposed spent nuclear fuel and atmospherically reflected gamma radiation. The reflected radiation is referred to as skyshine and should contribute minimally to the overall dose; however, when there is minimal shielding above the exposed source, skyshine at large distances from the source must be considered. The program SKYDOSE developed by Shultis and Faw evaluates the gamma-ray skyshine dose from a point, isotropic, polyenergetic, gamma-photon source. Assuming an infinite black wall shielding all direct radiation, the model assumes a first responder is located at varying distances from the wall. Skyshine doses are calculated both through SKYDOSE’s integral line-beam method and an approximate approach prescribed by the National Council of Radiation Protection and Measurements. Initial results from SKYDOSE indicate nearly equivalent dose rates from either direct or skyshine radiation at nine meters from the wall, which seemed unusual and not readily explained. NCRP methodology, however, yields skyshine dose rates which are drastically smaller than direct dose rates at the same distance. Further investigation using the program MicroSkyshine®, which allows a variety of source configurations, suggests skyshine contributes minimally to dose in a loss-of-shielding accident. INTRODUCTION and OBJECTIVE In order to estimate the risks and possible consequences of the transportation of radioactivematerial, a computer code called RADTRAN was developed in 1977 [1]. RADTRAN is used to assess risks from a transportation accident as well as the risks from incident-free transportation. In the near future, RADTRAN 6.0 will incorporate the capability to calculate doses received by both first responders and the general public for an incident where the transportation cask lead gamma shielding is compromised. Various devastating worst-case accident scenarios could involve the spent nuclear fuel (SNF) cask impacting a stationary object or being fully engulfed in an extremely high temperature long duration fire. Either scenario could lead to degradation in the lead gamma shielding. In a severe WM’07 Conference, February 25 March 1, 2007, Tucson, AZ impact, the lead would deform due to the force of the impact. In an engulfing fire, the lead could yield and flow away due to the low melting point. In either case, the void can potentially expose the fuel and is referred to as slump. Figure 1 represents an example of lead slump as a result of a high speed impact. Fig. 1. Finite Element Analysis for a Lead-Shielded Truck Cask involved in 144.8 km/hr corner impact without Impact Limiters [2]. To evaluate the potential radiological impact of a Loss-of-Shielding (LOS) accident, an LOS model was developed and incorporated into RADTRAN [2]. Previous work has explored the direct dose received in a LOS accident at varying radial and axial positions from a spent fuel cask [2, 3]. However, the dose contribution from gamma-ray skyshine had not been considered. The goal of this research is to determine the gamma-ray dose contribution from skyshine. In a transportation accident involving the loss of lead gamma shielding, first responders to the accident will be exposed to both direct gamma radiation streaming from the exposed SNF and atmospherically reflected gamma radiation. The reflected radiation is referred to as skyshine and should contribute minimally to the overall dose; however, when there is minimal shielding above the exposed source, skyshine at large distances from the source must be considered [4]. Three separate methods were employed to determine the skyshine dose in a hypothetical accident scenario. For a first approximation, the computer code SKYDOSE developed by Shultis and Faw was used. Secondly, an analytical approximation using a recommendation from the National Council of Radiation Protection and Measurements (NCRP) was used to compare the SKYDOSE estimates. Finally, Grove Software’s computer code MicroSkyshine® 2.0 was used to evaluate a more refined model and provided results more consistent with expected values.