WSRC-MS-2002-00146

Novel Method for Removing Gadolinium from
Used Heavy Water Reactor Moderator

E. Wilde and C. Berry
Westinghouse Savannah River Company
Aiken, SC 29808

M. Goli
Mississippi Valley State University
14000 Highway 82 West
Itta Bena, MS 38941

This document was prepared in conjunction with work accomplished under Contract No. DE-AC09-96SR18500 with the U.S. Department of Energy.

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Keywords: Gadolinium, Nitrate Heavy Water

Abstract

A novel process to treat used heavy water moderator (D2O) contaminated with high concentrations of the neutron poison, gadolinium nitrate,Gd(NO3)3, is described. Gadolinium is removed by precipitation. The resultant precipitate (GdPO4. 6H2O) represents an extremely rare compound of considerable potential value. The resultant supernatant consisting of residual nitrate (NaNO3 or KNO3) is less toxic and easier to process than the original waste. Thus, the alkali metal waste handling can be done with considerably less environmental concern. This waste can potentially be treated by a combination of electrochemical and biological methods.

I. Introduction

A potentially cost-effective process is described for purifying spent heavy water moderator (D2O) contaminated with high concentrations of gadolinium nitrate, a chemical used as a neutron poison in heavy water nuclear reactor operations. The chemical is used for this purpose because the high neutron absorption cross sections of some gadolinium isotopes make gadolinium salts such as GdNO3 effective in controlling nuclear activity in aqueous systems (1, 2, 3). The proposed clean-up process was developed for the clean-up of numerous drums containing spent moderator water at the Savannah) River Site (SRS) near Aiken, SC. The process has the potential to lower the cost of heavy water purification by allowing the use of smaller equipment with less product loss and a reduction in the quantity of secondary waste materials produced in comparison with currently known alternative processes such as ion exchange. Furthermore, the recovery and re-use of the rare metal gadolinium has increasing promise because this element is being used with increasing frequency in medical treatment (MRI techniques) (4, 5).

Microbiological studies (6) revealed that growth of microbes is inhibited at gadolinium concentrations between 100 mg/l and 1000 mg/l, but growth can occur at much higher concentrations of other cations such as sodium and potassium. Since gadolinium levels in drums containing spent moderator at SRS were as high as 280,000 mg/l and averaged 80,000 mg/l, the use of algae or other microbes to sequester gadolinium and or nitrate in the absence of other treatment did not seem feasible. This led to an evaluation of the removal of the gadolinium by precipitation prior to nitrate removal.

II. Precipitation Experiments

The basic chemical reactions for precipitation with sodium phosphate and potassium phosphate were as follows:

Formula

This precipitation step was tested several times with quantities ranging from 100 mg to 2.0 g of Gd(NO3)3. The reactions were tested with both NaPO4 and KH2PO4. The general experimental process was to place 2 g Gd(NO3) 3. 6 H20 in a 125 ml Erlenmeyer flask and dissolved it in 50 ml DI water. An equimolar Na3PO4.12 H2O (s) or aqueous solution prepared by dissolving the solid in 20 ml of DI water was then added to the Gadolinium solution. A magnetic stirrer facilitated the precipitation. In numerous tests, precipitation was always complete in less than five minutes. To evaluate the exact chemical composition of the precipitate, the reaction mixture was filtered trough a Corning 25934-200 filter system using a vacuum (0.1 torr). Then, 5.0 ml of DI water was added in to the reaction flask and the washings were added on to the precipitate to wash off all of the soluble inorganic material. This step was repeated second time to make sure the precipitate transfer was complete. Finally the precipitate was washed with 5 ml of DI water. The filtrate was transferred to a volumetric flask and diluted appropriately. The sample was then analyzed for Gd and Nitrate concentrations using Ion Chromatography. The white gel type precipitate was transferred onto a previously weighed watch glass. The watch glass with the precipitate was heated to dryness in a vacuum oven over night at 300oF (148oC). A yield of >99.9% gadolinium was achieved. The compound is hygroscopic. To determine the extent of water captured in the precipitation process, the precipitate was sent to EAC Elemental Analysis Corporation, Lexington, KY and analyzed for Gadolinium, phosphorus, and oxygen using Proton Induced X-ray Emission (PIXIE). Table 1. Shows the experimental values compared to the theoretical values for the hexahydrate form of the compound. The experimental and theoretical numbers based on GdPO4. 6H2O are, % Gd 48.24 (43.65), P 8.29 (8.38), and O 43.47 (44.41). The experiments using potassium phosphate or sodium phosphate as the precipitating agent demonstrated that virtually all dissolved gadolinium can be precipitated in a single precipitation step that takes minutes and can be done by a solid or liquid (solution) form of the precipitating agent. The yield was close to 100% in all cases. Analyses of the precipitate indicated that it resembles a compound like GdPO4.6H2O. Although it appears that a precipitation step results in a small loss of heavy water due to hydrate formation within the precipitate, that heavy water can be recovered by heating the hydrate to 150C and condensing the vapor.

III. Conceptual Process for Reprocessing Used Moderator Water

During the course of the developmental work involving potential reuse of spent moderator fluid at SRS, a novel conceptual process for the purification of heavy water containing large concentrations of gadolinium nitrate was developed (Figure 1). The process contains the following steps:

  1. Chemical precipitation to remove gadolinium from the drums
  2. Removal of the cationic precipitating agent (e.g. K) and much of the NO3 using electrochemical techniques
  3. Complete (nitrate removal (e.g. to ppb level) by the growth and subsequent removal of select microbes
  4. Upgrading D2O purity as necessary for reuse/resale of the purified heavy water
  5. Recovery and reprocessing, if necessary, to provide gadolinium for reuse or resale as a high value product.

Step 1 has been demonstrated experimentally. Step 2 appears to be achievable using conventional electrochemical procedures. Step 3 has been partially demonstrated experimentally (6) and step 4 is common practice at SRS. Step 5 has not been tested at this time. However, it involves recovery of a potentially highly profitable process byproduct that could greatly improve the overall economics of spent moderator purification and reuse.

The proposed process has three significant advantages over the "baseline" technology, ion-exchange: (1) Gadolinium can be recovered for re-use or potentially sold as a rare, potentially valuable new commercial product, rather than being included in the secondary waste stream, (2) less dilution of heavy water occurs in the process resulting in improved economics when the purified heavy water is reused in nuclear reactors, and (3) secondary waste is reduced more than three-hundred-fold (from approximately 300% of the original volume to <1% of the original volume) in comparison with ion exchange. Step 1 Additional process development work is needed to put this conceptual process into practice.

Acknowledgements

The authors are grateful to Marilyn Frank, Mary Anne Johnson, Fatina Washburn, Sherold Johnson, Mike Polochko, Robert Ray and Charlie Varner for providing technical assistance on this project. Funding for the work was provided under Contract DE-AC09-96SR18500 with the U.S. Department of Energy.

References

  1. Baumann, E.W. Preparation and properties of nitrate-deficient gadolinium nitrate solutions. J. Inorg. Nucl. Chem. 42:1705-1709 (1980).
  2. Gilbert, R. Lepine, L., Laurin, M., Ouellet, L., and R. Gauthier. Study of complexing reactions causing the depletion of gadolinium ions from the moderator of CANDU-PHW Nuclear Reactors. Can. J. Chem. Eng. 63:978-986 (1985).
  3. Rodenas, L.G., Prini, R.F., and S.J. Liberman. Radiolysis of aqueous solutions of gadolinium nitrate. J. Radioanalytical and Nuclear Chem. Articles. 139: 277-286 (1989).
  4. Engström M, Thuomas KA, Naeser P, Stålberg E, Jonsson L. Facial nerve enhancement in Bell's palsy demonstrated by different gadolinium-enhanced magnetic resonance imaging techniques. Arch Otolaryngol Head Neck Surg 119:221-225 (1993)
  5. Embry A, Snowdon, Vieth L. Gadolinium-Enhancing Magnetic Resonance Imaging Lesions in Multiple Sclerosis. Annals of Neurology 48:271-272 (2000)
  6. Wilde, E.W., Berry, C.J. and Goli, M.B. Bull. Environ. Contam. Toxicol. 68:420-427 (2002).

Table I. Elemental Analysis of the Precipitate from the
Reaction of Gadolinium Nitrate with Potassium Phosphate


Element

Measured Value (%)

GdPO4.6H2O
Theoretical Value %

Gadolinium

48.24

43.65

Phosphorus

8.29

8.38

Oxygen

43.47

44.41

 

Figure 1. Conceptual Process for Purifying Drums of Heavy Water Containing Gd(NO3)3

Figure 1. Conceptual Process for Purifying
Drums of Heavy Water Containing Gd(NO3)3