In the near-field chemistry of a salt repository, the radiolytically-induced redox reactions in concentrated saline solution are of particular importance because the radiolysis of saline solutions results in oxidizing chlorine-containing species, which may oxidize actinide species to higher oxidation states. If the brines are irradiated, the solutions containing radiolytic species such as hypochlorite, hypochlorous acid or hydrogen peroxide, their pH and Eh may be altered. The oxidation and complexation states of actinides, which might be present in the salt brine, will change thus influencing their speciation and consequently their mobility. Furthermore, radiolytically formed oxidizing species such as ClO- or H2O2 may enhance the corrosion of the canister material. Therefore, radiation effects on salt brines must be integrated into the database, which described the chemical processes near a disposal site. Investigations in that context usually focus on the radiation chemistry of solid NaCl however our focus is on the radiolytic products, which are formed when salt brines are irradiated by a 10 MeV linear accelerator of electrons (LAE). We attempt to quantify the irradiation-induced formation of typical radiolysis by-products such as the hypochlorite ion (OCl-) by using a 13/9 MeV LAE with doses between 120 KGy to 216 KGy while monitoring the pH of the brine solutions

Dziewinski, J.

Hartmann, T.

Lu, N.

Marczak, S.

Paviet-Hartmann, P.

Rafalski, A.

Walthall, M.

Zagorski, Z. P.

UNT Digital Library

WM'02 Conference, February 24-28, 2002, Tucson, AZ   1 SPECTROSCOPIC INVESTIGATION OF THE FORMATION OF RADIOLYSIS BY-PRODUCTS BY 13/9 MeV LINEAR ACCELERATOR OF ELECTRONS (LAE) IN SALT SOLUTIONS   Patricia Paviet-Hartmann1, Jacek Dziewinski1, Thomas Hartmann1, Stanislaw Marczak1, Ningping Lu1, Mark Walthall2, Andrzej Rafalski3, Zbigniew Pawel Zagorski3 1Los Alamos National Laboratory  Environmental Science and Waste Technology Division, MS J 514 Los Alamos, NM 87545, USA 2Carlsbad Environmental Monitoring Research Center Carlsbad, NM 88220, USA 3The Institute of Nuclear Chemistry and Technology 16 Dorodna St., 03-195 Warsaw, Poland     ABSTRACT  In the near-field chemistry of a salt repository, the radiolytically-induced redox reactions in concentrated saline solution are of particular importance because the radiolysis of saline solutions results in oxidizing chlorine-containing species, which may oxidize actinide species to higher oxidation states.  If the brines are irradiated, the solutions containing radiolytic species such as hypochlorite, hypochlorous acid or hydrogen peroxide, their pH and Eh may be altered.  The oxidation and complexation states of actinides, which might be present in the salt brine, will change thus influencing their speciation and consequently their mobility.  Furthermore, radiolytically formed oxidizing species such as ClO- or H2O2 may enhance the corrosion of the canister material.  Therefore, radiation effects on salt brines must be integrated into the database, which described the chemical processes near a disposal site.  Investigations in that context usually focus on the radiation chemistry of solid NaCl however our focus is on the radiolytic products, which are formed when salt brines are irradiated by a 10 MeV linear accelerator of electrons (LAE).  We attempt to quantify the irradiation-induced formation of typical radiolysis by-products such as the hypochlorite ion (OCl-) by using a 13/9 MeV LAE with doses between 120 KGy to 216 KGy while monitoring the pH of the brine solutions   INTRODUCTION  In nuclear waste repositories, the radiation fields arising from the wastes may have an influence on the actinide speciation via the radiolysis products of water and more specifically the radiolysis products of NaCl or MgCl2.6H2O i.e. hypochlorite, hydrogen peroxide in the case of a salt repository like the Waste Isolation Pilot Plant (WIPP) in New Mexico, USA or Gorleben in Germany.  Plausible inundation scenario of salt repository will allow chloride brines to be WM'02 Conference, February 24-28, 2002, Tucson, AZ   2formed, which consequently will be exposed to radiation from the waste.  Key radioelements in Intermediate Level Waste (ILW), High Level Waste (HLW) or TRU waste have been found to be plutonium, americium, neptunium, uranium, and technetium.  These all have more than one accessible oxidation state under possible repository conditions.  There is some probability to establish radiolysis as a mechanism for causing enhanced aqueous concentration of actinide and radionuclide in nuclear waste repositories near fields.    Furthermore, these experiments will help to understand under which assumed micro-environments in a salt repository, irradiation may produce enough oxidizing agents to increase actinide solubilities.  The situation of a salt repository for actinide solubility and actinide speciation may be rather complex and reducing effects are in competition with oxidizing effects.  It is important to correlate the salt repository situation to experimental results on radiolysis on the less-dimensional HLW systems.  From the HLW experiments [1-5], we know that - in first order - the achievable hypochlorite concentrations entirely depend on the accumulated absorbed doses, while production rates depend on activity levels.  Under this guideline, small activity levels can result in the formation of oxidizing radiolysis products after accumulating high enough doses.  Our long-term goal is to demonstrate how the main oxidizing chloride species such as hypochlorite caused by radiolysis may affect the overall behavior of actinides under salt repository conditions. This paper describes our first steps towards identification and determination of these oxidizing species.  EXPERIMENTAL  The chemical reagents were purchased from Aldrich Chemicals suprapure (99.99%), and were used without further purification.  Two salts solutions 5 m NaCl and 3.7 m MgCl2.6H2O as well as two synthetic salt brines, Brine 1 and Brine 2 (see table I) were prepared.  The chloride solutions are made by dissolving reagent grade NaCl or MgCl2.6H2O in distilled water. All experiments are carried out in glass vessels at 24  2 C under normal atmosphere. PH is adjusted by adding appropriate amounts of HCl or NaOH and measured during the experiment using a combination glass electrode (“Ross type, Orion Co).  The chloride species are determined spectrophotometrically (Cary 500 and Perkin Elmer)  The linear electron accelerator, LAE 13/9 produces energies 5 to 13 MeV and has a power of 9 kW in the beam. It is equipped with a special electromagnet for the measurement of energy[6].  The four salt solutions (100 mL each) were transferred in a closed beaker and irradiated at 3 different doses: 120 kGy, 160 kGy and 216 kGy.  The initial pH was 6.87 for 5 m NaCl,  4.42 for 3.7 m MgCl2.6H2O, 7.43 for brine 1, and 8.23 for Brine 2.  The temperature was (24  2 C).  After irradiation, a UV Vis absorption spectrum was taken.          WM'02 Conference, February 24-28, 2002, Tucson, AZ   3  Table I: Chemical Composition of Brine 1 and Brine 2   Brine 1 Brine 2 Reagent Mass Fraction Mass Fraction  g in 100 mL water g in 100 mL water Na2SO4 2.07 2.37 NaBr 0.22 0.11 Na2B4O7.10H2O 1.23 0.60 NaCl 14.72 26.16 NaHCO3  0.13 KCl 2.85 0.72 MgCl2•6H2O 16.97 0.38 LiCl 0.01  CaCl2•2H2O 0.16 0.17   RESULTS AND DISCUSSION  Irradiation caused some changes in the UV Vis absorption spectra of the brines (Figs. 1 and 2).  These changes are relatively small for 5 M NaCl solution, a little larger for 3.7 M MgCl2 solution and much larger for brines 1 and 2.  The biggest change was observed for brine #1.  Changes increased while increasing the radiation dose.  Qualitative explanation of these changes in spectra will be discussed below.  Because geological salt formations are considered possible sites for radioactive waste disposal, numerous studies on the radiolysis effects of salt brines are reported in the literature [7].  Chloride ions in aqueous solution are inert towards aqe  and hydrogen but undergo rapid electron-transfer-type reactions with OH. to give 2Cl  which incorporate a second chloride ion and are stochiometrically equivalent to ClCl :  OHClClOH Cl 2.        (Eq. 1)  Dimerization of the species formed by the above reaction produces the corresponding halogen molecule ( 2Cl ) or an ion derived from it (3Cl ),  )(22 322 ClClorClClCl        (Eq. 2)  Bjergbakke et al. [8] surveyed the compounds that may form in NaCl brines exposed to an assumed dose rate of 1Gy/s for 100s. They predicted by extrapolation the yields of the production of different chloride species.  According to their observations, the concentration of  WM'02 Conference, February 24-28, 2002, Tucson, AZ   4hypochlorous acid (HOCl) remained in a steady-state at micromolar-level, and the concentration of chlorine gas was below the detection limit even for NaCl concentration of 5.5 M. 00.10.20.30.40.50.60.70.8220 240 260 280 300 320Wavelength (nm)abcdefa - 5 m NaCl, Irradiation dose 120 kGyb - 5 m NaCl, Irradiation dose 160 kGyc - 5 m NaCl, Irradiation dose 216 kGyd - 3.7 m MgCl 2, Irradiation dose 120 kGye - 3.7 m MgCl 2, Irradiation dose 160 kGyf - 3.7 m MgCl 2, Irradiation dose 216 kGy  Fig. 1: Selected UV Vis absorption spectra of two pure salt solutions 5 m NaCl and 3.7 m MgCl2.6H2O irradiated at three different doses: 120 kGy, 160 kGy, 216 kGy by a 13/9 MeV Linear Accelerator of Electrons (LAE).  WM'02 Conference, February 24-28, 2002, Tucson, AZ   500.511.522.5240 280 320 360 400Wavelength (nm)g - Brine 1, Irradiation dose 120 kGyh - Brine 1, Irradiation dose 160 kGyi - Brine 1, Irradiation dose 216 kGyj - Brine 2, Irradiation dose 120 kGyk - Brine 2, Irradiation dose 160 kGym - Brine 2, Irradiation dose 216 kGygihjkm Fig. 2: Selected UV Vis absorption spectra of two synthetic brines irradiated at three different doses: 120 kGy, 160 kGy, 216 kGy by a 13/9 MeV Linear Accelerator of Electrons (LAE).   Alpha-particle irradiation of 5 M NaCl results in the formation of transitory equilibrium system of  ClClOHClOClCl ,,,, 23  [9].  According to Büppelmann K., Kim J.I., and Lierse Ch., at pH<7, the formation of chlorine gas is favored.  At 4<pH<7, the formation of hypochlorous acid (HClO) is observed while at pH>7, the formation of hypochlorite (ClO-) is preferential [10].  This change may be easily explained by H+ + ClO- - HClO equilibrium (pKa  =7.23 in pure water).  Several chloride species have been identified in the literature [11-16].  The UV Vis absorption spectra of the individual species HClO2, Cl2, KClO3, ClO-, NaClO2 and ClO2 are plotted in Fig. 3.  The generation of Cl2 may be well distinguished by a relatively broad absorption band at 332.5 nm.  ClO2 formed by disproportionation radiolysis is also easy to detect with a broad absorption band at 360 nm.  Hypochlorite ion ClO- exhibits a peak at 292 nm and chlorite ion ClO2- - at 260 nm. WM'02 Conference, February 24-28, 2002, Tucson, AZ   6050100150200250300350400200 250 300 350 400 450Wavelength (nm)3.10-2 M HClO2 , pH 1.98 (Buser  et al. 1952)Cl2 (Gillam et al. 1929)0.1 m KClO3 (Buser et al. 1952)10 -2 M ClO- ( this paper)10 -2 M NaClO2 (this paper)4.10-3 MClO2 (Buser et al. 1952) Fig. 3: UV Vis absorption spectra of chloride species in water  The spectroscopic characteristics of important radiolysis products known in chloride solutions are summarized in Table II.  The comparison of the data in Table II to the spectra of irradiated brines reveals that in our experiments Cl2, Cl3- and ClO2 were not the main products of radiolysis.  Traces of Cl2, Cl3- and ClO2 may be suspected only for spectra “i” and “m” (“i”- brine 1 at 216 kGy, “m”- brine 2 at 216 kGy).  Taking into account the molar absorption coefficients and very small rise in absorbance in region 330 – 400 nm, we conclude that non-ionic products of radiolysis are only a very small fraction of the radiolysis by-products.  The biggest changes in the spectra of irradiated brines are observed in the 220 – 300 nm region.  This is the region where ClO-, ClO2- and their protonated counterparts strongly absorb.  We suspect that these ions (molecules) are the main contribution in the total overall radiolysis by-products.  The presence of higher oxidation steps of chlorine in irradiated brines cannot be confirmed by direct UV Vis spectrum.  Chlorate and perchlorate have much too weak absorption of UV light to be determined by direct UV Vis spectroscopy. WM'02 Conference, February 24-28, 2002, Tucson, AZ   7The presence of oxochlorides seriously influences the stability of Pu waste-forms.  In the absence of reductants, hypochlorite causes the high redox potential of the brine, which (1) may accelerate the dissolution of waste-forms, and (2) oxidizes actinides to their highest oxidation state, which is usually the most soluble.  The complexing properties of ClO- towards Pu(VI), as an example, can further increase its solubility.  Besides its oxidizing properties, hypochlorite has the ability to form stable complexes with Pu(VI) [17, 18].  These complexes control the solubility of Pu in nearly neutral saline solutions.  Table II. Spectroscopic characteristics of important radiolysis products in water and chloride solutions  Species Absorption band (nm)  (L mol-1 cm-1) Reference 2Cl  332.5 65 [11] 3Cl  325 190 [9] HClO 235 100 [9] ClO  in water, pH 7 290 163 This work ClO in water, pH 13 292 345 This work ClO in 5 m NaCl, pH 6.87 291 324 This work ClO in 3.7 m MgCl2.6H2O, pH 4.42 284 197 This work ClO in Brine 1, pH 7.43 370 60 This work ClO in Brine 2, pH 8.23 330 49 This work 2ClO  260 98 This work 2ClO  in 5 m NaCl, pH 6.87 259 108 This work 2ClO  in 3.7 m MgCl2.6H2O, pH 4.42 256 101 This work 2ClO  in water 360 1000 [11]  We have determined the UV Vis absorption spectra of hypochlorite at different concentrations (10-1 M to 10-4 M) in water, pH 7 and in four different salt solutions, 5 m NaCl, 3.7 m MgCl2.6H2O, brine 1 and brine 2 (Fig. 4).    WM'02 Conference, February 24-28, 2002, Tucson, AZ   8050100150200250300350400240 280 320 360 400 440 480Wavelength (nm)10-2 M ClO- in water10-2 M ClO- in 5 m NaCl10-2 M ClO- in 3.7 m MgCl2.6H2O10-2 M ClO- in synthetic brine 110-2 M ClO- in synthetic brine 2 Fig. 4: UV Vis absorption spectra of hypochlorite in water and in four different salt solutions  The formation of the ClO- ion appears at pH 7, in water with a characteristic absorption spectrum at 291 nm, and a molar extinction coefficient  of 163  1.91.  Depending on the medium, the absorption band is shifted towards lower wavelength at 284 nm for a 3.7 m MgCl2.6H2O solution with a molar extinction coefficient  of 197  1.54, or towards higher wavelengths for brine 1 and brine 2, with an absorption band at 370 nm and 330 nm respectively and a molar extinction coefficient  of 60.34  1.65 for brine 1 and 49.21  1.36 for brine 2.  The absorption spectrum of ClO- ion is depending strongly on the medium, as well as on pH.  CONCLUSION   From the preliminary experimental results, it appears evident that some radiolytically induced chloride species are produced by irradiating brines with different dose rate from a 13/9 MeV LAE.  These oxidizing chlorine species, which are produced by the radiolysis processes of brines, may be responsible of the oxidation of actinides to their highest oxidation state, which is usually the most soluble.  Much insight into the radiation process of brines and radiation induced radical reactions will be gained by means of the pulse radiolysis method, based on irradiation of brines with a short pulse of ionising radiation. We are planning to investigate the WM'02 Conference, February 24-28, 2002, Tucson, AZ   9transient behavior of the radiolytically generated short-lived species by optical absorption spectroscopy. The time resolution will be in the nanosecond range.  ACKNOWLEDGEMENTS  The presented collaboration between laboratories named as authors affiliations was initiated during the NATO and US DOE sponsored Advanced Research Workshop: “The Environmental Challenges of Nuclear Disarmament” held in Cracow, Poland in November 1998, later was supported by NATO Expert Grant No EV 977200 and currently is supported by NATO, under the Collaborative Linkage Grant No SST.CLG.997936.  REFERENCES  1. Kelm M., Bohnert E., Radiolytic compounds formed by dissolution of irradiated NaCl and MgCl2 . 6H2O in water. Radiochimica Acta, 1996. 74: p. 155. 2. Kelm M., Bohnert E., Radiation chemical effects in the near field of a final disposal site I: Radiolytic products formed in concentrated NaCl solutions. Nuclear Technol., 2000. 129: p. 119. 3. Kelm M., Bohnert E., Products formed from alpha radiolysis of chloride brines. 2001: Karlsruhe, Germany, Personal Communication. 4. Kelm M., Bohnert E., Strahlenchemische Effekte im Endlagernahbereich: Gamma radiolyse in konzentrierter NaCl - Loesung. 2001, Personal Communication 5. Kelm M., Pashalidis I., Kim J.I., Spectroscopic investigation on the formation of hypochlorite by alpha radiolysis in concentrated NaCl solutions. Applied Radiation and Isotopes, 1999. 51: p. 637. 6. Zagorski Z. P., Dependence of depth-dose curves on the energy spectrum of 5 to 13 MeV electron beams. Radiat. Phys. Chem., 1983. 22: p. 409. 7. Tandon L., Radiolysis of Salts and Long-Term Storage Issues for Both Pure and Impure PuO2 Materials in Plutonium Storage Containers. 2000, Report LA-13725-MS, Los Alamos National Laboratory. 8. Bjergbakke E., Dragnic Z., Sehested K., Dragamic I., Radiolytic products in waters, Part II Computer simulation of some radiolytic processes in nature. Radiochimica Acta, 1989. 48: p. 73. 9. Büppelmann K., Kim J.I., Lierse Ch., The redox behavior of Pu in saline solutions under radiolysis effects. Radiochimica Acta, 1988. 44/45: p. 65. 10. Pourbaix M., in Atlas of electrochemical equilibria in aqueous solution. 1966, Pergamon Press: London. p. 590. 11. Buser W., Hanisch .H., Spektrophotometrische Untersuchung saurer Chloritloesungen, in Fasciculus VII. 1952. p. 2547. 12. Chen T., Spectrophotometric determination of microquantities of chlorate, chlorite, hypochlorite and chloride in perchlorate. Analytical Chemistry, 1967. 39: p.804. 13. Adam L., Fabian I., Suzuki K., Gordon G., Hypochlorous acid decomposition in the pH 5-8 region. Inorg. Chem., 1992. 31: p. 3534. 14. Adam L., Gordon G., Direct and sequential potentiometric determination of hypochlorite, chlorite and chlorate ions when hypochlorite ion is present in large excess. Anal. Chem., 1995. 67: p. 535. WM'02 Conference, February 24-28, 2002, Tucson, AZ   1015. Denis M., Masschelein W., Determination des ions chlorite, chlorate et bromate en melange en solution aqueuse. Analusis, 1983. 11: p. 79. 16. Gady G., Naughton J., Dexter T., Chlorine compounds. Inorg. Synth., 1957. 5: p. 156. 17. Pashalidis I., Kim J.I., Lierse Ch., Sullivan J., The chemistry of Pu in concentrated aqueous NaCl solution: Effects of alpha self-radiolysis and the interaction between hypochlorite and dioxoplutonium (VI). Radiochimica Acta, 1993. 60: p. 99. 18. Pashalidis I., Kim J.I., Ashida T., Grenthe I., Spectroscopic study of the hydrolysis of PuO22+ in aqueous solutions. Radiochimica Acta, 1995. 68: p. 99. 

Spectroscopic Investigation of the Formation of Radiolysis By-Products By 13/9 MeV Linear Accelerator of Electrons (LAE) in Salt Solutions

https://digital.library.unt.edu/ark:/67531/metadc784661/m2/1/high_res_d/833788.pdf

Spectroscopic Investigation of the Formation of Radiolysis By-Products By 13/9 MeV Linear Accelerator of Electrons (LAE) in Salt Solutions

Abstract

Similar works

Full text

Available Versions

UNT Digital Library