Modeling Erosion and Transport of Depleted Uranium, Yuma Proving Ground
Uranium, chemical symbol U, is a naturally occurring silver-white metal which belongs to the actinide group of elements. Uranium is found as a mixture of three isotopes, 238U, 235U and 234U. Uranium rich in the isotope 235U is preferred for atomic weapons and nuclear reactor fuel. To obtain this isotope, uranium is processed, or enriched, to increase the proportion of 235U. The portion which is deficient in 235U, but rich in 238U, is referred to as depleted uranium or DU. The DU is a workable, malleable metal with a specific gravity of about 19 (about the same as gold), and a melting point of 1132øC. Its high density, relative abundance, and metallic properties have made DU an obvious choice for high energy ballistic projectiles. The U.S. Army’s Yuma Proving Ground (YPG) in southwest Arizona tests DU projectiles. When DU projectiles strike a target or the downrange ground surface, they fragment. Most of the fragments are collected after the test firings, but some are either too small or too dispersed to be easily found. Unfortunately, DU has some undesirable characteristics, it is a radioactive material (alpha particle emitter) and a toxic metal. Therefore, the fragments not found pose a potential environmental hazard at the YPG and to surrounding areas. In order to ascertain what this hazard might be, the U.S. Army has contracted with the Los Alamos National Laboratory (LANL) to study the fate of DU at the YPG. In turn, LANL has contracted with New Mexico State University (NMSU) to investigate water transport of DU during rainfall-runoff events. Evaluating DU movement with surface water runoff and associated erosion requires an understanding of transport mechanics at the site. In this study, field measurements were taken on site and rainfall/overland flow simulation was used on a 1m x 3m soil box physical model in the laboratory. Nine simulation runs were made on YPG soil containing DU fragments from the impact areas. The nine simulations consisted of three groups of three: rainfall simulation only, overland flow only, and rainfall/overland flow combination. Infiltration rates in the DU ballistic impact areas at YPG were very close to zero. Larger concentrations of DU were contained in the impact area soil which was retained by the 1 mm and 0.5 mm sieves when grain-size analysis was performed. After all nine simulations were completed, a mass balance of the uranium showed most DU remained in the soil box even under extreme hydraulic conditions, though about 7% (by weight) did exit the box. No DU was found in the suspended sediment samples and this suggests that most transport of DU is restricted to contact or bed-load movement during overland flow. Because the experiments were conducted to simulate extreme rainfall-runoff conditions, the probability of significant transport of DU from land surface appears to be low.