(528a) Evaporation Tests Using Redc Lllw Simulant Solutions

EVAPORATION TESTS USING REDC LLLW SIMULANT SOLUTIONS

P. A. Taylor

Oak Ridge National Laboratory

The Radiochemical Engineering Development Center (REDC) at Oak Ridge National Laboratory (ORNL) is a hot cell facility that recovers heavy elements from irradiated targets and produces targets for irradiation. Since the mid-1960s, REDC has been the production, storage, and distribution center for transuranium elements used for the heavy-element research program of the US Department of Energy and other users. REDC discharges about 45,000 L/year of liquid low-level waste (LLLW), mostly aqueous with 2–3% organics, into the ORNL central LLLW system. The total LLLW generation rate has been about 700,000 L/year, with most of the volume generated by environmental management (EM)-related activities. REDC is the primary source of radionuclides discharged to the LLLW system. LLLW is evaporated, and the concentrate—a mixture of sludge and supernate—is stored in underground tanks. The Transuranic Waste Processing Center (TWPC) has solidified and disposed of most of the supernate and will treat the sludge in the future. Remediation work eventually will eliminate most of the EM-generated LLLW. The existing LLLW treatment system and TWPC are planned for decontamination and decommissioning after the LLLW generation rate is reduced. The remaining LLLW will contain a much higher radionuclide concentration, and the chemical composition will change. New evaporation and solidification facilities will be needed to treat future LLLW.

A series of laboratory-scale evaporation tests have been conducted using a simulant of the LLLW generated at REDC. The purpose of these tests was to determine the volume reduction factor (VRF = initial volume / final volume) that can be achieved before solids start to form, the VRF where the evaporator concentrate solidifies into a salt cake, and the distribution of the organics between the condensate and concentrate solutions. The tests also were intended to identify potential operational issues with evaporating this solution. A simulant solution was formulated from 11 years (2001–12) of data on the composition of LLLW discharges from REDC (see Table 1).

Table 1. Aqueous LLLW Simulant Composition

The sodium aluminate (NaAlO₂) concentration has averaged 0.02 M, but future programs may increase the concentration, so concentrations of 0.1 and 0.6 M also were tested. The organic portion of LLLW includes extractants, such as 1,4-diethylbenzene, tributyl phosphate, and Bis(2‑ethylhexyl) hydrogen phosphate, as well as petroleum naphtha diluents.

Solids started to form in the boiling simulants at a VRF above 5.2 for the simulant with 0.02 M NaAlO₂, at a VRF = 2.9 for the 0.1 M NaAlO₂ simulant, and at a VRF = 2.1 for the 0.6 M NaAlO₂ simulant. Solids formed at lower VRFs when the solution was cooled. Most of the organic diluent evaporated, but the extractants stayed with the concentrated simulant and started to solidify after being boiled for many hours.