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Title: Phase 2 Report--Mercury Behavior In The Defense Waste Processing Facility

Abstract

The purpose of this report is to provide a summary of the DWPF processing history in regards to mercury, document the mercury results obtained on the product and condensate samples, and provide further recommendations based on the data obtained.

Authors:
 [1];  [1]
  1. Savannah River Site (SRS), Aiken, SC (United States)
Publication Date:
Research Org.:
Savannah River Site (SRS), Aiken, SC (United States)
Sponsoring Org.:
USDOE Office of Environmental Management (EM)
OSTI Identifier:
1326336
Report Number(s):
X-ESR-S-00279
TRN: US1700026
DOE Contract Number:
AC09-08SR22470
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES; 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; SAVANNAH RIVER PLANT; MERCURY; RADIOACTIVE WASTE PROCESSING; CONDENSATES; RECOMMENDATIONS; RADIOACTIVE WASTE FACILITIES; QUANTITATIVE CHEMICAL ANALYSIS; CPC; Melter; Eurofins; DWPF

Citation Formats

Bannochie, C., and Fellinger, T.. Phase 2 Report--Mercury Behavior In The Defense Waste Processing Facility. United States: N. p., 2016. Web. doi:10.2172/1326336.
Bannochie, C., & Fellinger, T.. Phase 2 Report--Mercury Behavior In The Defense Waste Processing Facility. United States. doi:10.2172/1326336.
Bannochie, C., and Fellinger, T.. 2016. "Phase 2 Report--Mercury Behavior In The Defense Waste Processing Facility". United States. doi:10.2172/1326336. https://www.osti.gov/servlets/purl/1326336.
@article{osti_1326336,
title = {Phase 2 Report--Mercury Behavior In The Defense Waste Processing Facility},
author = {Bannochie, C. and Fellinger, T.},
abstractNote = {The purpose of this report is to provide a summary of the DWPF processing history in regards to mercury, document the mercury results obtained on the product and condensate samples, and provide further recommendations based on the data obtained.},
doi = {10.2172/1326336},
journal = {},
number = ,
volume = ,
place = {United States},
year = 2016,
month = 7
}

Technical Report:

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  • The Defense Waste Processing Facility (DWPF) is currently evaluating an alternative Chemical Process Cell (CPC) flowsheet to increase throughput. It includes removal of the steam-stripping step, which would significantly reduce the CPC processing time and lessen the sampling needs. However, its downside would be to send 100% of the mercury that come in with the sludge straight to the melter. For example, the new mercury content in the Sludge Batch 5 (SB5) melter feed is projected to be 25 times higher than that in the SB4 with nominal steam stripping of mercury. This task was initiated to study the impactmore » of the worst-case scenario of zero-mercury-removal in the CPC on the DWPF melter off-gas system. It is stressed that this study is intended to be scoping in nature, so the results presented in this report are preliminary. In order to study the impact of elevated mercury levels in the feed, it is necessary to be able to predict how mercury would speciate in the melter exhaust under varying melter operating conditions. A homogeneous gas-phase oxidation model of mercury by chloride was developed to do just that. The model contains two critical parameters pertaining to the partitioning of chloride among HCl, Cl, Cl{sub 2}, and chloride salts in the melter vapor space. The values for these parameters were determined at two different melter vapor space temperatures by matching the calculated molar ratio of HgCl (or Hg{sub 2}Cl{sub 2}) to HgCl{sub 2} with those measured during the Experimental-Scale Ceramic Melter (ESCM) tests run at the Pacific Northwest National Laboratory (PNNL). The calibrated model was then applied to the SB5 simulant used in the earlier flowsheet study with an assumed mercury stripping efficiency of zero; the molar ratio of Cl-to-Hg in the resulting melter feed was only 0.4, compared to 12 for the ESCM feeds. The results of the model run at the indicated melter vapor space temperature of 650 C (TI4085D) showed that due to excessive shortage of chloride, only 6% of the mercury fed is expected to get oxidized, mostly as HgCl, while the remaining mercury would exist either as elemental mercury vapor (90%) or HgO (4%). Noting that the measured chloride level in the SB5 qualification sample was an order of magnitude lower than that used in the SB5 simulant, the degree of chloride shortage will be even greater. As a result, the projected level of HgCl in the actual SB5 melter exhaust will be even lower than 6% of the total mercury fed, while that of elemental mercury is likely to be greater than 90%. The homogeneous oxidation of mercury in the off-gas was deemed to be of primary importance based on the postulation that mercury and other volatile salts form submicron sized aerosols upon condensation and thus remain largely in the gas stream downstream of the quencher where they can deposit in the off-gas lines, Steam-Atomized Scrubbers (SAS), and High-Efficiency Mist Eliminator (HEME). Formation of these submicron semi-volatile salts in the condensate liquid is considered to be unlikely, so the liquid phase reactions were considered to be less important. However, subsequent oxidation of mercury in the liquid phase in the off-gas system was examined in a simplified model of the off-gas condensate. It was found that the condensate chemistry was consistent with further oxidation of elemental mercury to Hg{sub 2}Cl{sub 2} and conversion of HgO to chlorides. The results were consistent with the available experimental data. It should also be noted that the model predictions presented in this report do not include any physically entrained solids, which typically account for much of the off-gas carryover on a mass basis. The high elemental mercury vapor content predicted at the DWPF Quencher inlet means that physically entrained solids could provide the necessary surface onto which elemental mercury vapor could condense, thereby coating the solids as well as the internal surfaces of the off-gas system with mercury. Clearly, there are many process benefits to be gained by removing the steam-stripping step from the CPC cycle. The goal of this task was to study what adverse impact the zero-mercury-removal scenario would have on the DWPF melter off-gas system operation. It is stressed again that this study was intended to be scoping in nature, so the results presented in this report are preliminary. Any further substantiation of these results for actual implementation into the DWPF flowsheet would require an in-depth modeling study of all three reaction zones, including the aqueous-phase reactions in the quencher, OGCT, Steam Atomized Scrubber (SAS), and off-gas condenser with recirculated condensate, and the proof-of-principle experiments.« less
  • The Defense Waste Processing Facility (DWPF) is currently evaluating an alternative Chemical Process Cell (CPC) flowsheet to increase throughput. It includes removal of the steam-stripping step, which would significantly reduce the CPC processing time and lessen the sampling needs. However, its downside would be to send 100% of the mercury that comes in with the sludge straight to the melter. For example, the new mercury content in the Sludge Batch 5 (SB5) melter feed is projected to be 25 times higher than that in the SB4 with nominal steam stripping of mercury. This task was initiated to study the impactmore » of the worst-case scenario of zero-mercury-removal in the CPC on the DWPF melter offgas system. It is stressed that this study is intended to be scoping in nature, so the results presented in this report are preliminary. In order to study the impact of elevated mercury levels in the feed, it is necessary to be able to predict how mercury would speciate in the melter exhaust under varying melter operating conditions. A homogeneous gas-phase oxidation model of mercury by chloride was developed to do just that. The model contains two critical parameters pertaining to the partitioning of chloride among HCl, Cl, Cl{sub 2}, and chloride salts in the melter vapor space. The values for these parameters were determined at two different melter vapor space temperatures by matching the calculated molar ratio of HgCl (or Hg{sub 2}Cl{sub 2}) to HgCl{sub 2} with those measured during the Experimental-Scale Ceramic Melter (ESCM) tests run at the Pacific Northwest National Laboratory (PNNL). The calibrated model was then applied to the SB5 simulant used in the earlier flowsheet study with an assumed mercury stripping efficiency of zero; the molar ratio of Cl-to-Hg in the resulting melter feed was only 0.4, compared to 12 for the ESCM feeds. The results of the model run at the indicated melter vapor space temperature of 650 C (TI4085D) showed that due to excessive shortage of chloride, only 6% of the mercury fed is expected to get oxidized, mostly as HgCl, while the remaining mercury would exist either as elemental mercury vapor (90%) or HgO (4%). Noting that the measured chloride level in the SB5 qualification sample was an order of magnitude lower than that used in the SB5 simulant, the degree of chloride shortage will be even greater. As a result, the projected level of HgCl in the actual SB5 melter exhaust will be even lower than 6% of the total mercury fed, while that of elemental mercury is likely to be greater than 90%. The homogeneous oxidation of mercury in the off-gas was deemed to be of primary importance based on the postulation that mercury and other volatile salts form submicron sized aerosols upon condensation and thus remain largely in the gas stream downstream of the quencher where they can deposit in the off-gas lines, Steam-Atomized Scrubbers (SAS), and High-Efficiency Mist Eliminator (HEME). Formation of these submicron semi-volatile salts in the condensate liquid is considered to be unlikely, so the liquid phase reactions were considered to be less important. However, subsequent oxidation of mercury in the liquid phase in the off-gas system was examined in a simplified model of the off-gas condensate. It was found that the condensate chemistry was consistent with further oxidation of elemental mercury to Hg{sub 2}Cl{sub 2} and conversion of HgO to chlorides. The results were consistent with the available experimental data. It should also be noted that the model predictions presented in this report do not include any physically entrained solids, which typically account for much of the off-gas carryover on a mass basis. The high elemental mercury vapor content predicted at the DWPF Quencher inlet means that physically entrained solids could provide the necessary surface onto which elemental mercury vapor could condense, thereby coating the solids as well as the internal surfaces of the off-gas system with mercury. Clearly, there are many process benefits to be gained by removing the steam-stripping step from the CPC cycle. The goal of this task was to study what adverse impact the zero-mercury-removal scenario would have on the DWPF melter off-gas system operation. It is stressed again that this study was intended to be scoping in nature, so the results presented in this report are preliminary. Any further substantiation of these results for actual implementation into the DWPF flowsheet would require an in-depth modeling study of all three reaction zones, including the aqueous-phase reactions in the quencher, OGCT, Steam Atomized Scrubber (SAS), and off-gas condenser with recirculated condensate, and the proof-of-principle experiments.« less
  • The Nippon Mercury/RA-3000 system installed in 221-S M-14 has been qualified for use. The qualification was a side-by-side comparison of the Nippon Mercury/RA-3000 system with the currently used Bacharach Mercury Analyzer. The side-by-side testing included standards for instrument calibration verifications, spiked samples and unspiked samples. The standards were traceable back to the National Institute of Standards and Technology (NIST). The side-by-side work included the analysis of Sludge Receipt and Adjustment Tank (SRAT) Receipt, SRAT Product, and Slurry Mix Evaporator (SME) samples. With the qualification of the Nippon Mercury/RA-3000 system in M-14, the DWPF lab will be able to perform amore » head to head comparison of a second Nippon Mercury/RA-3000 system once the system is installed. The Defense Waste Processing Facility (DWPF) analyzes receipt and product samples from the Sludge Receipt and Adjustment Tank (SRAT) to determine the mercury (Hg) concentration in the sludge slurry. The SRAT receipt is typically sampled and analyzed for the first ten SRAT batches of a new sludge batch to obtain an average Hg concentration. This average Hg concentration is then used to determine the amount of steam stripping required during the concentration/reflux step of the SRAT cycle to achieve a less than 0.6 wt% Hg in the SRAT product solids. After processing is complete, the SRAT product is sampled and analyzed for mercury to ensure that the mercury concentration does not exceed the 0.45 wt% limit in the Slurry Mix Evaporator (SME). The DWPF Laboratory utilizes Bacharach Analyzers to support these Hg analyses at this facility. These analyzers are more than 10 years old, and they are no longer supported by the manufacturer. Due to these difficulties, the Bacharach Analyzers are to be replaced by new Nippon Mercury/RA-3000 systems. DWPF issued a Technical Task Request (TTR) for the Savannah River National Laboratory (SRNL) to assist in the qualification of the new systems. SRNL prepared a task technical and quality assurance (TT&QA) plan that outlined the activities that are necessary and sufficient to meet the objectives of the TTR. In addition, TT&QA plan also included a test plan that provided guidance to the DWPF Lab in collecting the data needed to qualify the new Nippon Mercury/RA-3000 systems.« less
  • The waste acceptance specifications presented in this document represent the first stage of the Nevada Nuclear Waste Storage Investigations Project effort to establish specifications for the acceptance of waste forms for disposal at a nuclear waste repository in Yucca Mountain tuff. The only waste forms that will be dealt with in this document are the reprocessed waste forms resulting from solidification of the Savannah River Plant defense high level waste and the West Valley high level wastes. Specifications for acceptance of spent fuel will be covered in a separate document.
  • Testing was completed to develop a chemical processing flowsheet for the Defense Waste Processing Facility (DWPF), designed to vitrify and stabilize high level radioactive waste. DWPF processing uses a reducing acid (formic acid) and an oxidizing acid (nitric acid) to rheologically thin the slurry and complete the necessary acid base and reduction reactions (primarily mercury and manganese). Formic acid reduces mercuric oxide to elemental mercury, allowing the mercury to be removed during the boiling phase of processing through steam stripping. In runs with active catalysts, formic acid can decompose to hydrogen and nitrate can be reduced to ammonia, both flammablemore » gases, due to rhodium and ruthenium catalysis. Replacement of formic acid with glycolic acid eliminates the generation of rhodium- and ruthenium-catalyzed hydrogen and ammonia. In addition, mercury reduction is still effective with glycolic acid. Hydrogen, ammonia and mercury are discussed in the body of the report. Ten abbreviated tests were completed to develop the operating window for implementation of the flowsheet and determine the impact of changes in acid stoichiometry and the blend of nitric and glycolic acid as it impacts various processing variables over a wide processing region. Three full-length 4-L lab-scale simulations demonstrated the viability of the flowsheet under planned operating conditions. The flowsheet is planned for implementation in early 2017.« less