US EPA

1433722 

Journal Article 

Performance Assessment for Depleted Uranium Disposal in a Near-Surface Disposal Facility 

Pinkston, KE; Esh, DW; Grossman, CJ 

2009 

1 

Materials Research Society Symposium Proceedings
ISSN: 0272-9172
EISSN: 1946-4274 

Materials Research Society Symposium Proceedings 

1124 

537-542 

WOS:000270898900071 

The U.S. Nuclear Regulatory Commission (NRC) staff has
conducted a technical analysis to assess the potential impacts of disposal of large quantities of
depleted uranium in a near-surface disposal facility. The nature of the radiological hazards
associated with depleted uranium presents challenges to the estimation of long-term effects from
its disposal - namely that its radiological hazard gradually increases over time due to the in-
growth of decay products. In addition, these decay products include a daughter in gaseous form
(Rn-222), which has significantly different mobility in the environment than the parent
radionuclides. NRC staff developed a screening performance assessment model of a reference low-
level radioactive waste (LLRW) disposal facility to evaluate the risk and uncertainties
associated with the disposal of depleted uranium as low-level waste. The model was constructed
with the dynamic simulation software package GoldSim (R), a Monte Carlo simulation software
solution for dynamically modeling complex systems. The depleted uranium source is modeled as
releasing to a backfill assumed to surround the depleted uranium in the disposal cells.
Radionuclides released to the backfill are vertically transported via advection through
unsaturated zone cells to an underlying aquifer, where they are transported to a receptor well.
Radon that emanates from radium present in the depleted uranium is modeled as diffusing through
an engineered cap into the interior of a residence placed over the disposal area or to the
external environment. The model evaluates the radiological risk to future residents and intruders
(acute or chronic exposures) near or on the land overlying the disposal facility. Calculations
were performed probabilistically to represent the impact of variability and uncertainty on the
results. Key variables evaluated included: disposal configurations, performance periods,
institutional control periods, wasteforms, site conditions, pathways, and scenarios. The impact
of these variables on projected radiological risk can be significant. For example, estimated
risks are very sensitive to the performance period, and estimated disposal facility performance
is strongly dependent on site specific hydrologic and geochemical conditions. In addition, radon
fluxes to the environment are very sensitive to the long-term moisture state of the system and to
the disposal depth.