Basis for Selecting Cement-Based Waste Forms for Immobilizing Radioactive Waste

Abstract

ABSTRACTResearch at Oak Ridge National Laboratory (ORNL) has shown that cement-based waste forms can be tailored to tolerate wide fluctuations in waste-feed compositions and still maintain properties that are compatible with standard equipment and produce a product that meets regulatory requirements. The 20-year operational history of ORNL's grouting program has demonstrated this resilience and reliability on a spectrum of waste feeds encompassing the developmental history of the nuclear fuel cycle.Cement-based materials are the most frequently used waste forms for solidifying non-high-level waste. These materials are also perhaps the most misused for immobilizing wastes. This paper discusses the scientific basis for selecting a suitable cement-based waste form to solidify and immobilize radioactive waste. For instance, when a given amount of cement (the source of calcium) is replaced with class F fly ash, the strontium leachability is reduced proportionally.This paper discusses the use of gel clays such as bentonite and attapulgite to absorb excess fluids and thus increase waste loading. Also, cement-compatible natural materials such as nonswelling clays are used to ion exchange or absorb soluble ions such as cesium and thus reduce leachability.Recently, blast furnace slag has been the subject of much interest in cement-based immobilization technology. Data are presented to show that a waste form containing blast furnace slag and other cementing materials is very effective in reducing the release rates of 99Tc.