An End State Methodology for Identifying Technology Needs for Environmental Management, with an Example from the Hanford Site Tanks by Committee on Technologies for Cleanup of High-Level Waste in Tanks in the DOE Weapons Complex

Author:Committee on Technologies for Cleanup of High-Level Waste in Tanks in the DOE Weapons Complex
Language: eng
Format: epub
Tags: Environment and Environmental Studies. Engineering and Technology : Applications of Technology
Publisher: NATIONAL ACADEMY PRESS
Published: 1999-03-15T00:00:00+00:00

14C

? 8

90Sr

? 150

99Tc

? 3

137Cs

? 44

129I

? 0.08

Transuranic elements

? 100 nCi/g [<TRU (by definition)]

SOURCE: U.S. Nuclear Regulatory Commission (1982a,b).

Low-Activity Waste Immobilization. Although other immobilization processes are candidates for this function, only vitrification is discussed since it is the process currently accepted by the Hanford TPA signatories and stakeholders. In this operation the concentrated liquid product from cesium separation and water recovery is mixed with glass-making chemicals and heated to a temperature above 1000 °C. The result is first the evaporation of the residual water, then water of hydration, and then the decomposition of species such as nitrates, nitrites, carbonates, and sulfates to yield gaseous nitrogen oxides, carbon dioxide, and sulfur dioxide. What remains in the vitrifier is mostly oxides of various cations, which are incorporated into a glass matrix and poured into containers. The products of LAW vitrification are packages of LAW glass and a significant offgas stream discussed below. The end state of the vitrified LAW is on-site near-surface disposal. The performance requirement of the LAW disposal unit may dictate the maximum acceptable dissolution rate of certain radionuclides and limited concentrations of troublesome elements in the LAW glass.

On-Site Near-Surface Disposal. This type of disposal represents the physical end state for the vitrified, containerized LAW and secondary solid low-level wastes. As noted earlier in this chapter, the end state also imposes additional requirements on the long-term performance of the disposal site, very likely requiring the use of additional barriers beyond the waste form. For the purposes of the committee’s reference scenario, an appropriate basis for planning and conducting a technology development program is to assume that the LAW disposal site will require a means to fill any void spaces in the waste emplacement horizon, a multicomponent cap designed to last for centuries, barriers to intruder access, monitoring wells, and occasional surveillance to detect and limit any intrusion.

Enhanced Sludge Washing. The solids from the solid-liquid separation operation still contain large amounts of non-radioactive process chemicals that are not highly soluble in near-neutral solutions. Examples of these are aluminum, chromium, and iron compounds. It is cost effective to remove some of these chemicals to reduce the volume of the HLW, which has much higher processing and disposal costs than the LAW. This is accomplished by enhanced sludge washing, which involves contacting the solids with a concentrated aqueous solution of caustic soda. Under these conditions it is thought that many of the HLW glass volume-limiting constituents other than iron will be dissolved, leaving most of the radionuclides in the solid phase. The remaining solids are separated from the liquid by solid-liquid separation techniques similar to those described below.

The extent to which the solids should be solubilized is bounded by two competing goals. First, the extent and nature of enhanced sludge washing must not be such that too many radionuclides (except for cesium, which will be removed later) are dissolved or suspended in the liquid stream. If this occurs, the allowable concentration of radionuclides in the LAW would likely be exceeded.

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