Decanting of Neutralized H-Canyon Unirradiated Nuclear Material High Activity Waste Streams

Abstract

An option to dispose of the High Activity Waste (HAW) stream from the processing of unirradiated materials directly to Saltstone is being evaluated to conserve High Level Waste (HLW) tank farm space and to reduce the future production of HLW glass logs. To meet the Saltstone Waste Acceptance Criteria (WAC), decanting the supernate from precipitated solids was proposed to reduce mercury and radionuclide levels in the waste. Only the caustic supernate will then be sent to Saltstone. Verification that the Saltstone WAC will be met has involved a series of laboratory studies using surrogate and actual HAW solutions from H-Canyon. The initial experiment involved addition of sodium hydroxide (NaOH) to a surrogate HAW test solution and subsequent decanting of the supernate away from the precipitated solids. The chemical composition of the surrogate solution was based on a composition defined from analyses of actual HAW solutions generated during dissolution of unirradiated nuclear materials in H-Canyon [1]. Results from testing the surrogate HAW solution were reported in Reference [2]. Information obtained from the surrogate test solution study was used to define additional experiments on actual HAW solutions obtained from H-Canyon. These experiments were conducted with samples from three different batches of HAW solutions. The first and third HAW samples (HAW No.1 and HAW No.3 solutions) contained the centrifuge filter cake material from a gelatin strike that is periodically added to the waste stream. The second HAW sample (HAW No.2 solution) did not contain filter cake material. Monosodium titanate (MST) was added to the HAW No.2 and HAW No.3 solutions after addition of NaOH was complete and before the settling step. The addition of MST was to improve the decontamination of alpha and beta emitters (primarily plutonium and strontium) from the supernate. The addition of excess NaOH and the addition of MST were expected to result in sufficient alpha and beta decontamination to meet the Saltstone WAC.