A Literature Review of Radiolytic Gas Generation as a Result of the Decomposition of Sodium Nitrate Wastes

8/12/2019 A Literature Review of Radiolytic Gas Generation as a Result of the Decomposition of Sodium Nitrate Wastes

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ORNL I M;.11632

Chemical Tcclmo ogy Division

ALlTERATURE REVIEW OFR.ADIQLYTICGAS GENERATION ~ R E S I J L T OFTIlE DECOMPOSmONOFSODII JM NlTRA1E WASTE S

J. L Kasten

Date Published: JanuaIJ: 1991

Prepared y theOAK RIDGE NATIONAL LABORATORY

Oak Ridge, Tennessee 37831managed yMARTIN MARIETTA ENERGY SYSTEMS, INC. r th

u S DEPARTMENT OF ENERGYunder Contract No. DE-AC05-840R21400

3 4456 325244 1


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ONTENTS

EXEClJTI\ E SUMMARY 0 O O 0 0 0 VABS1 R.ACI . 0 1

IN1RODUCfION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 BACKGROUND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1 RADIATION ENERGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 ALPHA RAYS 42.3 BETA RAYS 42.4 GAMMA RAYS 52.5 ENERGY PENETRATION CHARACTERISTICS . . . . . . . . . . . . . . 52.6 ENERGY ABSORPTION CHARACTERISTICS 5

3 RADIOLYTIC DECOMPOSmON OF..50DIUM NITRATE WASTES . . . . . 63.1 RADIOLYTICDECOM POSITIONOFWATER 63.2 RADIOLYTIC DECOMPosmONOF NITRATES . . 9

4 RESUL1 S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125. SUMMARY 36 RECOMMENDATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 REFERENCES 19

iii


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LIST O TABLES

G values for the generation of hydrogen, G Hz, due to theradiolytic decomposition of water 142 G value for the generation of oxygen, G Oz , due to theradiolytic decomposition of sodium nitrate 16

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EXECUTIVE SUMM RY

Wastes containing radioactivity are capable of generating gases due to radiolyticdecomposition of the waste constituents. Wastes destined for final disposal in repositories, such asthe proposed Department of Energy DOE Waste Isolation Pilot Plant WIPP , must complywith regulations-including restrictions on gas generation. The radioactive sodium nitrate wastesstored in the Melton Valley Storage Tanks MVST at Oak Ridge National Laboratory ORNLare an example of such wastes that may eventually be destined for disposal at the WIPP. Aliterature survey was conducted to determine the chemical reactions and the expected gasgeneration as a result of the radiolytic decomposition of such wastes.

There are many factors which affect radiolytic decomposition of wastes including the typeof radiation and its penetration and absorption characteristics as well as waste properties such aspH, temperature, and chemical species present. The chemical reactions and associated gasgeneration for a sodium nitrate waste are determined y evaluating experimental studies pertainingto the radioIytic decomposition of water and sodium nitrates.

Basic chemical reactions which are capable of producing a gaseous product are identifiedfrom the literature study. The radiolytic decomposition of water s represented y the initialreactions

andH2


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This reaction suggests that oxygen is the gaseous product generated y radiolytic decomposition ofnitrates. h mechanisms of radiolytic decomposition are complex. Besides these initial reactionsback reactions and recombination reactions of the products of decomposition occur.

h gaseous products generated are measured y their G value which is the expressionfor radiation chemical yields as molecules of gas formed p r 100 eV of absorbed energy. h Gvalues identified in this literature study were measured during experimental studies involving theradiolytic decomposition of water and sodium nitrate. These values can only indicate a range forexpected products to be generated as th various types of sodium nitrate wastes have specificcharacteristics such as radiation type pH temperature chemical species present etc. that willaffect gas generation due to radiolytic decomposition. Experimental studies on a laboratory scales well as a full scale demonstration are recommended to determine G values by th actual

measurement th gas generation due to radiolytic decomposition of a specific sodium nitratewaste.

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A LlTERAnJRE REVIEWOF.R.AJ:)IOLYTICGAS GENERATIONASA RESULT OF1HE DECOMPOSmONOFSODIUM NITRA1E W AS FESKasten

ABSTRACI

The objective of this literature review is to determine expected chemical reactionsand the gas generation associated with radiolytic decomposition of radioactivesodium nitrate wastes such as the wastes stored in the Melton Valley StorageTanks (MVST) at Oak Ridge National Laboratory (ORNL). The literaturesurvey summarizes expected chemical reactions and identifies the gases expectedto be generated as a result of the radiolytic decomposition. The literature surveyalso identifies G values, which are the expression for radiation chemical yields asmolecules of gas formed per 100 eV of absorbed energy, obtained fromexperimental studies of the radiolytic decomposition of water and sodium nitrate.

1 INTRODUCTION

The Department of Energy (DOE) Waste Isolation Pilot Plant (WIPP), located insoutheastern New Mexico, is the proposed repository for the permanent disposal of transuranic1RU radioactive wastes. The WIPP requirements include compliance with the EnvironmentalProtection Agency (EPA) standards for TRU waste disposal as presented in Standards for theManagement and Disposal of Spent Nuclear Fuel, High Level and Transuranic RadioactiveWastes contained in the ode ederal Regulations 40 CPR Part 191 (U.S. EPA, 1985).Although at this time EPA standards do not specify a gas generation limit, knowledge of gasgeneration rates from wastes destined for disposal at WIPP necessary. A limit of 0 1 to 0 3 molof gas generated per drum per year has been suggested y AI Lappin of Sandia NationalLaboratory as an acceptable gas generation rate for the proposed WIPP repository.l

The Department of Energy (DOE) has signed an agreement with the state of New Mexicothat mandates shipment of 1RU wastes in Nuclear Regulatory Commission (NRC) certified


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packages. Thus, the NR requirements on combustible mixtures of gases in shipping containers

must b e m et y OE in shipping TRU wastes to the WIPP. The requirements are given in theNRC s Inspection Enforcement Information Notice No 84-72: larification onditions for WasteShipments Subject to Hydrogen Gas Generation September 1984). The gas generation incontainers of wastes approved for transport may be demonstrated y actual measurement of thegas or y predictions from acceptable calculations.

The radioactive sodium nitrate wastes stored in the Melton Valley Storage Tanks MVST)at Oak Ridge National Laboratory ORNL) may eventually be disposed of at the WIPP. Thus, itis necessary to determine the type and amount of gases t ha t may b e p ro du ce d as a r es ult of theradiolytic decomposition of the wastes.

Sodium nitrate wastes containing radioactivity are capable of generating gases due to theradiolytic decomposition of compounds present in the wastes, including water and sodium nitrate.This mechanism for gas generation, called radiolysis, is the chemical dissociation of compoundsdu e to energy from radiation. The nature of the radiolytic decomposition of these compounds isdependent on the waste characteristics i.e., temperature, pH, chemical species, etc.) as well as thetype of radiation. The expression for radiation chemical yields is the G value, which is the numberof any species ion, radical, molecules, etc.,) decomposed, formed, or rea cte d p er 100 e V ofabsorbed energy. Fo r example, G X) refers to the number of molecules of product X formedupon irradiation per 100 eV of energy absorbed.

This report presents a literature review of expected chemical reactions and the associatedG values recorded in studies pertaining to the radiolytic decomposition of water and sodiumnitrates. These G values are necessary in order to perform calculations for gas generation rates spresented in the ORNL report, Prediction Hydrogen and Oxygen Generation Rates due to


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Radiolysis in Solidified Melton Valley Storage Tank MVST Waste, ORNLffM-11717, byH WGodbee, T. C Wright, and J. L Kasten in preparation .

CKGROUND

A knowledge of the basic principles regarding energy from radiation will assist in thedetermination of gas generation due to radiolytic decomposition of waste. The following presentsa brief summary of the properties associated with radioactive particles.2.1 R DI TION ENERGY

Chemical decomposition is induced by the absorption of ionizing radiation produced byradioactive nuclei including alpha, beta, and gamma rays n alpha ray is a stream of alphaparticles; a beta ray a stream of beta particles; and a gamma ray a photon. Photons, whetheremitted from the nucleus gamma rays or the shell X rays of an atom, have the same effect.Each particle or photon can ionize or excite a large number of molecules that are distributedalong its path. The energy lost by a radioactive particle in passing through matter producesconsiderable numbers of ions and excited molecules in its track. The particles are not selectiveand may react with any molecule lying in their path. The rate of energy loss is generally expressedin terms of the linear energy transfer LET , which is defined as the linear rate of loss of energylocally absorbed by an ionizing particle traversing a material medium.2 The following chargedparticles are listed in order of increasing LET values: gamma rays, beta rays, and alpha rays

The interaction of ionizing radiation can be described as:primary interactions that involve electronic transitions of the molecules prodUcing ions,excited species, radicals, etc. and, in certain circumstances, atom displacements; and


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2 secondary interactions that usually involve the reactions the ions, excited species,radicals, etc., to give the final products. In most cases, only the final stable products areactually observed and the nature of the intermediate or primary species, which are theprecursors the final prodUCts is highly speculative.3

ALPHA RAYSAlpha rays are heavy charged particles that arc easily absorbed. Alpha particles arc the

nuclei helium atoms, that is, helium atoms that have lost both electrons and, hence, have adouble positive charge. On passing through matter, alpha particles lose energy principally byinelastic collisions with electrons lying in their path, which leads to excitation and ionization ofthe atoms and molecules to which these electrons belong. The great difference in mass betweenthe alpha particle and the electron means that the alpha particle loses only a small fraction of itsenergy and is virtually undeflected by collision. As a consequence, alpha particles are slowed downgradually because a large number small energy losses and travel in a nearly straight path.Since each the alpha particles from a radioactive element have the same energy, they will each

have about the same range; the random nature the collisions gives rise to small variations inthe range individual particles. The extent of the ionization caused by an alpha particle dependson the number molecules it hits along its path and on the way in which it hits them.

BETA RAYS

Beta particles are negatively charged electrons emitted by radioactive nuclei. In contrastto alpha particles, beta particles from a particular radioactive element are not all emitted with thesame energy but with energies ranging from zero up to a maximum value that is characteristic ofthe element. Beta particles are much more penetrating than alpha particles. The beta particle hasa much smaller mass and greater speed than the alpha particle. A beta particle may lose a largefraction its energy in a single collision with an atomic electron. Beta particles are scatteredmuch more easily by nuclei than are alpha particles, so their paths are usually not straight 2 4


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GAMMA RAYSGamma radiation is electrically neutral. The gamma rays emitted by radioactive isotopes

are either monoenergetic or have a small number of discrete energies. Unlike alpha and betaparticles, which usually lose their energies gradually through a number of small energy transfers,gamma rays tend to lose the greater part of their energy through a single interaction.2 Themechanism of the absorption of gamma rays by matter is different from that of charged particlessuch as alpha or beta particles. The difference is apparent in the much greater penetrating powerof gamma rays, and the characteristic exponential absorption in matter. Gamma rays do not havea definite range as is found for charged particles.4S ENERGY PENETRATION CHARACTER1STICS

Positively charged short-range alpha radiation, negatively charged intermediate-range betaradiation, and electrically neutral penetrating gamma radiation are three types of radiationresulting from the decay of radioactive substances. Alpha emission is observed n all nuclides ofatomic number greater than 83 that are stable to other modes of decay and in a few lighternuclides, principally among the rare earth metals. The alpha particles interact very strongly withmatter as they penetrate only a few centimeters of air and are stopped by paper thin films of solidsubstances. The usual beta radiation can penetrate a millimeter or more of solid matter or abouta meter of air, although some of the very low energy beta radiation is no more penetrating thanalpha particles. Gamma rays are very penetrating 4 S6 ENERGY ABSORPTION CHARACTER1STICS

The amount of energy absorbed by a waste as a result of the radioactive decay process is afunction of curie loading, waste properties, and container geometry. t is reasonable to assumethat the alpha and beta emission energies are absorbed in the waste. This is because of therelatively short travel length, or range, as compared with gamma rays required for alpha and beta


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particles to give up their energy in the waste material, and the extremely low probability of suchparticles escaping the container. The gamma energy absorbed by the waste depends on thestrength of the gamma emission, the amount of gamma ray energy absorbed by interactioncollision with a waste particle, and the number of particles with which the gamma ray interacts.Because an interaction will either absorb or, more likely, attentuate an emission, the fraction ofgamma energy absorbed depends on the number of interactions possible. Therefore, gammaenergy absorption increases with increasing numbers of waste particles. For this reason, gammaabsorption is a function of the contained waste density and geometry.6

3. RADIOLYTIC DECOMPOSmON OF SODIUM NTIR E W SfES

Chemical effects produced by ionizing radiations are known to depend on the energy andmass of the ionizing particles. Some radioactive materials have complex decay schemes. Thedecay spectra may include alpha energies, beta energies as well as gamma energies. Complete

knowledge of the decay schemes are important for evaluation of the energy generated in theradioactive sources.s From the information presented in the literature, the chemical speciespresent in sodium nitrate wastes that are capable of producing gas by radiolytic decompositioninclude water generating hydrogen and nitrates generating oxygen .3.1 RADIOLYTIC DECOMPOSmON OF W ER

The radiolysis of water has been studied more thoroughly than any other compound.7- 7The two initial reactions include the initial formation of product molecules, which is known as theforward reaction F , and the initial formation of radicals-the radical reaction R ,

F H20 - 2H2 t + 2 zR H 20 - H+ + OH


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Reaction F represents the molecular yield of hydrogen Hz) and hydrogen peroxide HzOz), andreaction R represents the radical yield of hydrogen and hydroxide ions H+ and OH- .S-13

During the early work in radiation chemistry, it was found that pure water sealed in a tubeand irradiated with X rays or gamma rays appeared to be quite stable, showing no decompostion.This apparent lack of decomposition of water is now understood to result from the reaction offree radicals, H+ and OH , with the molecular products, Hz and H z which leads them torecombine to water. With alpha rays, the number of molecular decomposition products formed istoo great to be converted back to water by the relatively small number of free radicals produced.Although oxygen gas is found among the water decomposition products, it seems to be formed notdirectly from the water but as a result of the action of the radicals on H ZOZ4

The different yields for water decomposition by different kinds of irradiation may be dueto the existence of recombination and back reactions of the products of the decomposition. Therecombination of radicals refers to the union of H and OH- radicals to yield water. Thecombination of radicals refers to reactions that do not yield water e.g., H H 2 z). Backreaction refers to any reaction that causes ultimate conversion of products to watere.g., OH Hz H20 H .IS.10

In the absence of any solute, the free radicals formed by the decomposition of water willdisappear by reaction with one another. For example, if an OH- radical reacts with an H+ atom,water molecules result again; but if an H reacts with another H+ or an OH- with another OH-,the new molecules Hz and HzOz will be produced. These products, hydrogen and hydrogenperoxide, insofar as they stay dissolved in the water, will be able to react with the free radicals, Hand OH-, formed by the decomposition of further water molecules. The result will be destructionof the products and subsequent formation of water. The most probable equations are:

OR Hz - H20 H


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andH H20 2 H20 OH. 6

When pure water is irradiated the products H2 and H20 2 are expected to build up to a steadystate concentration at which the rate of back reaction of these products to reform water s equalto the rate of their production from waterY

A basic obseIVation in the irradiation of dilute aqueous solutions with light-particleradiation s that the major chemical change occurs in the dissolved material while the waterundergoes little or no decomposition. 4 Since water as the major component must have originallyabsorbed practically all the energy of the radiation some way must exist for this energy to befunneled into the molecules of dissolved materials. In an early study by Hugo Fricke 14 thisphenomena was explained as the conversion of the water by the radiation to a form calledactivated water which was chemically reactive but stable enough to diffuse through the solutionand react with solute molecules. The general opinion today s that this activated water consists ofthe molecular fragments or free radicals H and O resulting from the break up of watermolecules that react with dissolved materials present.14 6

Light-particle radiations yield predominately free radicals whereas h e v y ~ p r t i l eradiations yield principally hydrogen and hydrogen peroxide. Heavy-particle radiations lead toextensive decomposition of water while the effect on dissolved solutes s considerably less for agiven energy input than with the light-particle types. These molecular decomposition products canalso be found to exist with light-particle radiations although their number is much smaller thanthe number of free radicals that are produced simultaneously.14 18

Although the independent formation of radical and molecular products is sufficient toexplain the chemical reactions in irradiated solutions the difference between the heavy- and lightparticle radiations may result from the differing geometrical arrangements in which radicals are


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produced. As radicals diffuse randomly mostoflhemwill escape into the solution and react asfree radicals with whatever solutes they may/find; but a proportion will e n o u n t e r o n ~ n o t h e rbefore diffusing from the vicinity. unlike tadicals thus meet,they will recombine toform water,so that their existence will not be evident,bul those that meet in like pairs 9Iillcombine tcr formthe moleculesH2 and Hz0 which are th e observed molecular products. The.larger the number ofradicaisinagroup the greater the proportionwhiclrwill combine instead o f ~ c p i n g i n t o t h esolution. Hence, as the LET of the radiationincreases., the proportion of theradicalspro(1ucedthat combine to the molecular products increases, while th e proportion thatdiff uses intothesolution and manifests itself as free radicals decreases}4

Larger yields of molecular products are obtained in the radiolysis with radiation of highLET (e.g., aUpha particles). These concepts are shown in the comparison of alpha radiolysis withgamma or beta radiolysis. High molecular yields, such as hydrogen, are observed from the alpharadiolysis of pure water giving a G HJ 1.2 molecules/loo eV, while results from gammaradiolysis show a H = 0.45 molecules/tOO eV. The lower yield from low LET radiation hasbeen ascribed to radiation-induced recombination reactions.193.2 RADIOLYTIC DECOMPOSmONOiEtNl l RATES

The ease of decomposition of various nitrate crystals is influenced by a number of factorsrelated to the chemical nature and crystal structure of the material. The actual mechanism isprobably very complex-possibly involving reactions of several ionic. free radical, and molecularintermediates (which might include 3, Ob and 0J.1J

Large differences have been reported between the rates of radiation-induceddecomposition of solid nitrates.l 9-22 Although there have been a number of studies on theirradiation of inorganic nitrates, the decomposition mechanism is still open to question. Thechemical effect of irradiation can, however, be summarized by a few general observations.

1. The net chemical reaction is


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N03- NOz- 1 2. Ozwith a significant radiation induced back reaction

NOz- 1 2. Oz N03-as well as the reaction

1 2. Oz N03- NOz- Oz2. There s very little, if any, decomposition of the nitrite ion. 19 Thus, assuming the

dissociation of the nitrate ions into NOz- and 02 the Oz atom might: recombine with the NOzion produced simultaneously; react with an adjacent N03- ion to form NOz- and 02; or diffusethrough the lattice until it reacts with anothertom, N02-, or 0/.0 21

Covalent bonds present in solids may be broken upon irradiation and bring aboutchemical changes. Nitrates decompose giving oxygen 0 2 and nitrite NOz-) , and it has beendemonstrated that the final products are formed in the crystal.z

Conflicting results are found in the literature pertaining to the G value for oxygengenerated as a result of the radiolytic decomposition of nitrates. Most studies have shown that ifthe irradiated nitrate is dissolved in water, both of the products can be measured and the ratio ofnitrite to oxygen is found to be 2:1, as expected, for

N03- NOz- If}. 022 23Initial G NOz-) values of solid nitrate salts are believed to be dependent on the cation of

the salt and the radiolysis temperature. Oxygen production s generally colinear with NOzproduction in the correct stoichiometric ratio.2 3 The detectable amount of oxygen will declinewith respect to NOz- when reaction of oxygen with metal atoms s favorable.19

One study, however, concluded that for irradiated nitrates, oxygen and nitrite were formedin equivalent amounts. Following radiation exposure, sodium nitrate salts evolved a gas that wasshown to be mainly oxygen gas trapped in small pockets in the crystal during irradiation. When


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irradiated crystals were heated below the melting point, the gas pockets grew and coalesced-thisbeing accompanied by a decrease in crystal density 22

Largedifferences h a v e b e e I 1 r e p ( ) r t e c l l . ' > e t w e ~ . n l h e rates of radiation-induceddecomposition of various solid nitrates. While there is general agreement that the primary processisunimolecular dissociation of an O J ' s p e ( ~ e s p r o d u c i n g N02- and 02 the varying sensitivitieshave been attributed to: 1) differentpolarizillg power.of the cations, 2 competition l e l l 1 ~ y p a r t i a U yaccount for the higher yield in the potassiumsalt.

Although the concept of free space, which is related to the closeness ofpackingoflhecrystalstructures, has been used t o e ~ l a i n t h e d i f f e r e n c e in G values for the decomposition ofvarious inorganic nitrates, there arediscrcpallciesinlheconcept. N o t a b l y , c e s i u m n i l l ' a t ~ ( C ' s N 0 3 )hasless free space than KN03, but the initialyield is appreciably higher 2 S

The observed chemical reactions in the radiolysis of aqueous NaN03 solutions indicatedthat the oxygencame from two sources: lJthejointparticipation of HzO and N03


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performed at p to determine the origin of the O2 produced by radiolysis. The results of these

measurements indicate that most O2 originates from water in dilute solutions, with an increasingfraction originating from nitrate at higher nitrate concentrations. The radiolysis of the nitrate ionin aqueous solutions is consistent with the reaction presented for the radiolysis of solid nitrates

N03- => N0 2- + 1 2 6 28

4. R SULTS

According to the literature, the gases generated as a result of the radiolytic decompositionof water and sodium nitrate will most likely be hydrogen from water and oxygen from sodiumnitrate . The basic initial chemical reactions due to the radiolytic decomposition of water include

H 20 => 1 2 H 2 + 1 2 H 20 2and

H 20 H OHwith the following probable back reactions:

OH H 2 H 20 Hand

H + H 20 2 H 20 + OH .The reaction for the decomposition of nitrate is given by

N03- = N02- + 1 2 O2with a significant radiation induced back reaction given by

N02- + 1 2 N03as well as the combination reaction1 2 02 + N03- => N0 2- + 02


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From these equations, as well as other probable equations representing recombination andback reactions of the products of decompositions, i l is apparent that the gases expected to begeneratedasaresult of the radiolytic decomposition sodium nitrate wastes include hydrogenand oxygen. The literature review gives insighltochemical reactions expected in th e radiolyticdecomposition of water generating hydrogen and nitrates generating oxygen . Tables 1 and 2present experimentally obtained Gvaluesgiven in th e literature associated withthe radiolyticdecomposition of water and sodium nitrates,.respectively.

5 SUMMARY

Basic principles that apply to the decomposition of water have been presented, althoughthe actual mechanisms of radiolytic decompOSition are complicated and not easily ascertained.Studies indicate that four main factors affect th e decomposition of water:

The decomposition of water by radiation is dependent on the impurities present. Thepresence of other elements affects the combination of th e free radicals and molecular products.

2. The amount of decomposition is a function of free volume above th e water. Theirradiation of water enclosed with a large free volume in a vacuum system resulted in large yieldsof hydrogen as opposed to small yields of hydrogen from the irradiation of water enclosed with asmall free volume in a pressurized system.

3. A reduction in LET results in a shift in the equilibrium toward a greaterrecombination of products in th e gas phase. Conversely, th e yield of molecular products increasesrelative to the free radical yield at higher LET Values for the radical and molecular productsfrom water in alpha radiolysis differ by factors ranging from to 5 from those in gamma radiolysis.


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SourceJ. W. T. Spinks, R. J. Woods,Introduction to RadiationChemistry 1964J. W. T. Spinks, R. J. Woods,Introduction to Radiation Chemistry1964J. W. T . Spinks, R. J. Woods,Introduction to Radiation h mistry1964

W. T. Spinks, R. J. Woods,Introduction to Radiation OIemistry1964

F. S. Dainton, W.S. Watt,Nature 195, 1294 1962A. O. Allen, J. Phys ColloidChem 52, 479 1948

Table 1 continued

Original authorE Collinson, F. S. Dainton, J. Kroh,Nature 187, 475 1960

J. Ralani, G. Stein, hem Phys 37, 1865 1962

J. H. Baxendale, G. K. Thomas,T. Woodward, reported by M. S.Matheson, Ann Rev Phys Chem13, 77 1962A. O. Anen, H. A. Schwarz,Proc 2nd Intern ont PeacefulUses Atomic Energy UN, Geneva,29, 30 1958Same as source

Same as source

Molecules of H generated per 100 eV of radiationenergy absorbedlpna eta Gamma

0.6OC

0.5


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Table 2. G values for the generation of oxygen, G(O;', due to the radiolytic decomposition of sodium nitrate

Molecules of O generated per eV of radiationenergy absorbedSource (original author)C. J. Hochandel, T. W Davis,J. hern Phys. 27 333 1957e J. Hochandel, Radiat Res16, 286 (1962)G. Hennig, R Lees, M. S. Matheson,J. 1em Phys. 21, 664 1953H. A. Mahlman, J. Phys. hern67, 1466 (1963)

M. Hyder, J. Phys. hern 69, 1858(1965)

SystemSolid sodium nitrate crystals

Solid sodium nitrate crystals

Solid sodium nitrate crystals

Aqueous sodium nitrate solutions1.0MNaN32.0 M NaN033 M NaN034.0MNaN35.0 M NaN036 M NaN03

Aqueous sodium nitrate solutions0.12 M NaN030.50 M NaN031.0 M NaN032 5 M NaN034.0 M NaN03

Alpha Gamma0.12 0.021O 13S I CO SI e

0.09,10.2h0.18, 0.31 h0.25, 0.42h0.30, 0 5h0.36,1: 0.56h0.43, 0.63h

0.06,1: 2.0 h0.21,1: 2.3Sh0.38,1: 2.4er0.70, 2.3Sh0.90,1: 2 3e>h

IThe G(NO;' value is given in the source; the G(O;' value is determined from the reaction N0 3- - N02- 1/2 O2 where the ratio ofnitrite to oxygen is 2: 1:Temperature of 25C.cTemperature of 30C.dTemperature of 120C.eTemperature of 150C.The oxygen gas was measuredin the sodium nitrate crystals.I:The value for G(O;' is believed to be due to the radiolytic decomposition of the nitrate ions.hThe value for G OJ is believed to be due to the radiolytic decomposition of the combination of the water and the nitrate ions.


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4. Water decomposition is a function of pH. It has been observed that the radical yieldsH and OH- from the decomposition of water due to ionizing radiations on aqueous solutionsare markedly influenced by changes in pH. For the molecular yield of Hb however, the G H2value remains constant despite changes in the pH.14,18-19.29-31

Most experimental evidence for the radiolysis of solid nitrates as well as aqueous nitratesolutions indicates that the G value, G N0 2- for the nitrate ion is equal to twice that for G 02)substantiating the stoichiometry indicated by the reaction

N03- NOz- + 02t has been concluded that 02 is produced by a direct effect on the radiolysis of NO - in alpharadiolysis as in gamma radiolysis for solid nitrates only.29

Although the radiolysis of solid nitrates is complex, possible mechanisms of decompositionof nitrate crystals have been observed.

1. The nitrite and oxygen are produced n the solid and most of the oxygen is retained bythe crystals at room temperature. Sodium nitrate must be heated to near melting in order torelease all its oxygen. Both the initial yields and the s t e y ~ s t t e concentrations increase withincreased temperature for decomposition by gamma rays. A 50 increase in the G valuesresulting from the decomposition of NaNO was observed when the temperature was increasedfrom 25 to QOc32 The influences of increased temperature may be due to the loosening of thelattice structure or supplying activation energy for various processes. For irradiation by alphapanicles, however, the yield of NaN03 actually decreased slightly with increased temperature.

2. Yields generally decrease with increased dose of radioactivity, suggesting an approachto steady state; however, the decrease in yield may merely result from loss of energy to productmolecules.


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3 There an effect of LET both on the initial yield and also apparently on steady states.The inital yield for NaNG3 has been observed to be about five times as great for alpha irradiationas for gamma irradiation at room temperature.20

6. RE OMMEND TIONS

Radiological, physical, and chemical conditions each playa role in radiolytic gasgeneration from wastes. The characterization of the waste composition must be complete todetermine defensible gas generation rates. The radionuclides plus the daughters, if present, mustbe considered, and knowledge to ascertain the stage of decay of the waste is necessary. Unless itcan be conclusively proven that the daughters of a radionuclide are not present either by analysesor knowledge of the complete history of the waste the daughters should be included so that gasgeneration results are conservative i.e., the gas generation at a maximum . The capabilities ofthe wastes to absorb radiation energy influences the decomposition of the waste constituents.Chemical reactions such as recombination and back reactions affect the amount of gas generateddue to radiolytic decomposition.

The G values associated with radiolytic decomposition indicating molecules of gasgenerated by waste constituents per 1 eV absorbed by the waste constituents is complex. The Gvalues obtained from literature can only indicate a range for expected G values due to radiolyticdecomposition of a sodium nitrate waste since there are many waste-specific factors that influencegas generation rates. Experimental studies are recommended where the G values are obtained bythe actual measurement of the gas generation.


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7 REFERENCES

NucL Waste News 9 (47),441, November 30,1989.2 J. W T. Spinks and R. J.Woods, nIntroduction to Radiation Chemistry John.Wiley Sons, Inc., 1964.

E. R. Johnson, The Radiationlnducet1 Decomposition of Inorganic Molecular Ions Gordanand Breach Science Publishers, 1970.4. Irving Kaplan, Nuclear Physics Addison-Wesley Publishing Company, Inc., 1955.5. Sigfred Peterson and R. G.Wyrner,Chemistryin Nuclear Technology Addisan.WesleyPublishing Company, Inc , 1963.6. J. E.Flaherty, Akira Fujita,C P I)eltetcLandiG. J. Quinn, A Calculational.Technique toPredict Combustible as Gi nerationdnSealedRadioactive Waste COnt4mersGEND-041, May 1986.7. NF.Barr and A O. Allen, H} drogen AtolllSin the Radiolysis.atWater, 1 Phys. Chem.63, June 1959.8 H. Samuel and J. L Magee, Theory of Radiation Chemistry. II Track Effects inRadiolysis of Water, Chem. Phys. 21, (6), June 1953.9. J. K. Thomas, Elementary Processes and Reactions in the Radiolysis of Water, inAdvances in Radiation Chemistry eds. M Button and J. Magee, Wiley-Interscience, 1969.1 A O. Allen, The Yields of Free Hand OH in the Irradiation of Water; Radiat. Res.1954.11 H. A Dewhurst, A H. Samuel,.andJ.I.. Magee, A Theoretical Survey ofthe.RadiationChemistry of Water and AqueQ1lS Solutions, Radiat. Res. 1954.12 E. Hayon, Role of Oxygen in the Primary Formation of Hand H20- in the RadiationChemistry of Water, Nature 96 November 1962.13. C. J.Hochanadel, EffectsofC()baltAlpha-Radiation on Water and Aqueous Solutions,liPkys Chem. ~ y 1952.14 A O Allen, The Radiation Chemistry ofiWaterandAqueous So[utions D. VanNostrand

Company, Inc., 1961.15. E.Hayon, Nature of the PI ecursors Molecular Products in the Radiation .Chemistry ofWater, Nature 194, May1962.


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A O. Allen, Radiation Chemistry of Aqueous Solutions, 1. Phys. Coll. Chern. 52 (3),March 1948.A O. Allen, C. J. Hochanadel, J. A Ghormley, and T. W. Davis, Decomposition ofWater and Aqueous Solutions Under Mixed Fast Neutron and Gamma Radiation, 1. PhysChern. 56, May 1952.Edwin J. Hart Warren J. Ramler, and Sol R. Rocklin, Chemical Yields of IonizingParticles in Aqueous Solutions: Effect of Energy of Protons and Deuterons, Radiat. Res.4,1956.G. L. Tingey and W. D. Felix, Radiofytic Gas Generation From Calcined Nuclear WasteBNHL-2381, August 1977.C. J. Hochanadel, Evidence for 'Thermal Spikes' in the Alpha-Particle Radiolysis ofNitrate Crystals, Radiat. Res. 16, 1962.C. J. Hochanadel and T. W. Davis, Radiolysis of Solid Nitrates , 1. hern Phys. 27, 1951.G. Hennig, R. Lees, and M. S. Matheson, The Decomposition of Nitrate Crystals byIonizing Radiations, 1. hem Phys. 21 (4) , April 1953.Arnold Frances and Everett R. Johnson, Mechanism of the Radiation-InducedDecomposition of Sodium Nitrate. Part 1 1. Phys. Chern. 79 (1), 1975.J. Cunningham, Radiation Chemistry of Ionic Solids. I Diffusion-Controlled Mechanismfor Radiolysis of Ionic Nitrates, 1. Phys. Chern. 65, April, 1961.Tung-Ho Chen and E. R. Johnson, Mechanism of the Decomposition of InorganicNitrates, 1. Phys. Chern. 66, November, 1962.M. L. Hyder, The Radiolysis of Aqueous Nitrate Solutions, 1. Phys. Chern. 69 (6), 1965.H. Mahlman, The Direct Effect in the Radiolysis of Aqueous Sodium NitrateSolutions, 1. Phys. Chern. 67, 1963.Malcolm Daniels andn Eric E. Wigg, Radiation Chemistryof the Aqueous NitrateSystem. I. Gamma Radiolysis of Dilute Solutions, 1. Phys. Chern. 71 (4), March 1967.N. E. Bibler, Curiurn 244 lpha Radiolysis ofNitric Acid. Oxygen Production From DirectRadiolysis ofNitrate Ions DP-MS-72-68.F. S. Dainton and W. S. Watt, Radiation Chemistry The Effect of pH o n the RadicalYields in the Gamma-Radiolysis of Aqueous Systems, Nature 195, September 1962.J. T. Allan and G. Scholes, Effects of p H and the Nature of the Primary Species in theRadiolysis of Aqueous Solutions, Nature 187, July, 1960.


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32. veretlR Johnson Effect of Intensity.on the Radiation Induced Decomposition ofInorganic Nitrates hys Chem 66 April 1962.


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A Literature Review of Radiolytic Gas Generation as a Result of the Decomposition of Sodium Nitrate Wastes