[en] A major mission of the Hanford Defense Waste Program is to achieve permanent disposal of Hanford defense wastes with safe, environmentally acceptable, and cost-effective disposal methods which meet applicable regulations. To achieve this mission, the Defense Waste Management Plan (DWMP) establishes, as an objective, that the high-level waste be immobilized prior to shipment to a geologic repository. The Hanford Waste Vitrification Plant (HWVP) Program has been established to accomplish this objective. The goal of the HWVP is to vitrify pretreated waste in borosilicate glass, cast the glass into stainless steel canisters, and store the canisters at Hanford until they are shipped to a Federal Geologic Repository. This document contains the results of research activities conducted by Pacific Northwest Laboratory (PNL) during fiscal year 1986 that pertain to the preparation of feed for the vitrification process. The ultimate goal of this work is to establish a feed preparation procedure that will result in melter feeds with acceptable chemical and rheological properties. The high-level waste slurry received at the HWVP facility will undergo several processing steps before it can be fed to the ceramic melter. These steps include slurry concentration, treatment with formic acid (to improve slurry rheology), and the addition of glass frit. Additional feed concentration may also be required following addition of the frit. The laboratory experiments that were conducted during FY 1986 included three single-variable scoping studies and three larger, multivariable tests. These tests are described. 40 refs., 59 figs., 41 tabs