OC ~l CXU ^J'JI. T KJ

STATENS KÄRNBRÄNSLE NÄMND

NATIONAL BOABD FOR SPENT NUCLEAR FUEL SKN REPORT 57 .

Stability of Metallic in the Near Surface Environment

MARCH 1992 Where and how will we dispose of spent nuclear fuel?

There is political consensus to dispose or spent nuclear fuel from Swedish nuclear power plants in Sweden. No decision has yet been reached on a site for the final repository in Sweden and neither has a method for disposal been determined. The disposal site and method must be selected with regard to safety and the environment as well as with regard to our responsibility to prevent the proliferation of materials which can be used to produce nuclear weapons. In 1983, a disposal method called KBS-3 was presented by the nuclear power utilities, through the Swedish Nuclear Fuel and Waste Management Company (SKB). In its 1984 resolution on permission to load fuel into the Forsmark 3 and Oskarshamn 3 reactors, the government stated that the KBS-3 method - which had been thoroughly reviewed by Swedish and foreign experts - "was, in its entirety and in all essentials, found to be acceptable in terms of safety and radiological protection." In the same resolution, the government also pointed out that a final position on a choice of method would require further research and development work.

Who is responsible for the safe management of spent nuclear fuel?

The nuclear power utilities have the direct responsibility for the safe handling and disposal of spent nuclear fuel. This decision is based on the following, general argument: those who conduct an activity arc responsible for seeing that the activity is conducted in a safe manner. This responsibility also includes managing any waste generated by the activity. This argument is reflected in the wording of major legislation in the field of nuclear power, such as the Act on Nuclear Activities (1984) and the Act on the Financing of Future Expenses for Spent Nuclear Fuel etc. (1981). The Act on Nuclear Activities and the Act on the Financing of Future Expenses for Spent Nuclear Fuel etc. stipulate that the nuclear power utilities arc responsible for conducting the research which is necessary for the safe management of spent nuclear fuel. This legislation stipulates that the utilities are also responsible for the costs incurred in connection with the handling and disposal of the waste. There arc four nuclear power utilities in Sweden: Vattenfall AB, Forsmarks Kraft- grupp AB, Sydsvenska Värmckraft AB and OKG AB. Together, these four utilities own the Swedish Nuclear Fuel and Waste Management Company (SKB). SKB's tasks include the practical execution of the work which the utilities arc responsible for carrying out. The govemr -m has the overall responsibility for safely in connection with waste handling and c* »sal. Three authorities - the National Board for Spent Nuclear Fuel (SKN), the Swedish Nuclear Power Inspectorate (SKI), and the National lastitutc of Radi ation Protection (SSI) - arc responsible for di ffcrcnt aspects of government supe"/ ision of the utilities' waste activities. The government has also appointed a special agency, the National Council for Nuclear Waste -KASAM, to deal with these matters. (The task of this commission was completed by June 1990.)

Continued en the bock inside cover. STATENS KÄRNBRÄNSLE NÄMND SÖM REPORT 57 NATIONAL BOARD POR S»ENT NUCLEAR FUEL MARCH 1992

DNR 60/90

Stability of Metallic Copper in the Near Surface Environment

ÖRJAN AMCOFF AND KATALIN HOLÉNYI UPPSALA UNIVERSITY DECEMBER 1991

THIS WORK HA» BEEN FINANCED BY THE NATIONAL BOARD FOR SPENT NUCLEAR FUEL. THE AUTHORS ARE RESPONSIBLE IOR OPINIONS EXPRESSED IN THIS REPORT. Knnimi-niiis lorliie 7572IM»5 Tryck: (irnpliic Systems AH. (inlcborg IW2 TABLE OF CONTENTS

1. INTRODUCTION

2. CHARACTERIZATION OF THE PROBLEM 1

3. NATURAL ANALOGUES 4 3.1. Metallic copper in nature 4 3.2 Solubility of copper at low temperature in nature 7

4. DISCUSSION 8 4.1. Stability of copper phases 8 4.2. Solubility of copper in the presence of complexing agents. 15 4.3. Supply of reduced sulphur 1 8 4.3.1. Sulphate reduction 1 8 4.3.2. Influence from sulphide in the enclosing rocks 20 4.4. Eh-pH conditions 23 4.5. Kinetics of sulphidation and metastable behaviour 26

5. CONCLUSIONS AND RECOMMENDATIONS 29

6. REFERENCES 35

7. APPENDIX 41

Thermodynamic calculations.

8. FIGURE LEGENDS 53

Reports published by the National Board for Spent Nuclear Fuel 67 INTRODUCTION.

The present study was initiated by the National Board for Spent Nuclear Fuel (SKN). It may be regarded as a review of the state of the art of copper stability - copper mobility in a low temperature - near surface environment. In the discussion, we have emphasized geological - geochemical milieus that have a direct bearing on the problem of final storage of spent nuclear fuel in copper canisters. The literature review has concentrated on copper in connection with: (a) low-temperature environments, and (b) stability-mobility, with particular emphasis on a chloride- rich, sulphur-rich milieu. The possible influence on the present processes of radiolysis and engineered barriers besides copper is not discussrd in this report.

In order to faciliate the discussion, a number of examples on copper stabilities and copper solubility etc. are given below, based on thermodynamic calculations. These calculations are simplified to a certain degree and the discussion is based on differences in orders of magnitude rather than on exact figures. The thermodynamic foundation for the calculations is given in an appendix. Conclusions and recommendations are outlined in general terms in a separate report.

The geological circumstances are explained at some length because we assume that the readers will be mainly non- geologists. All conclusions drawn are our own.

CHARACTERIZATION OF THE PROBLEM.

The purpose of the natural as well as what is known as the engineered barriers, of which the copper canisters are only one. are to prevent radioactive leakage for a long time, probably more than 100 000 years. During this time the temperature will drop from perhaps 80°C to 5-10°C (SKB 83-24). Studies on the behaviour of copper in the geological - geochemical literature have been mainly concerned with formation and ore alteration.

The present problem may be stated as follows;

(i) What physico-chemical milieu can we expect at the repository site?

(ii) Which solid phase(s) will be thermodynamically stable? This is a function of the total system, including interactions between the metallic copper - fluid phase - engineered barriers - neighbouring rocks.

(iii) What is the total solubility of copper in the fluid phase? This depends on the activities of suitable ligands for copper complexing in the fluid phase.

(iiii) Kinetioal considerations including; a) reaction rates of copper mineral formation, b) reaction rates of copper mineral dissolution, c) possible rate inhibiting and rate-enhancing factors (The expected rate-inhibiting capacity of technical barriers besides metallic copper is not discussed in this study), and d) metastable mineral formation.

All factors are strongly dependent on temperature.

The question of long-term metallic copper stability in a future repository site milieu can, at present, not be answered in a strict sense. This is due to the fact that almost all research has been concerned with equilibrium relations in "simple" systems while data on kinetics mainly consist of studies on corrosion rates of various archaeiogical bronzes at a temperature of 5-10°C (e.g. KBS 83-05), and of rate studies based on the availability of "reactive species" at the repository site (KBS 83-24, SKB 90-31). As a rule of thumb, for many reactions where a solid phase is included, the reaction rate is about doubled for each 10°C temperature increase. This means that the copper corrosion rate might increase by a factor of 100-1000 at 100°C compared with rates at room temperature.

Further complications arise from the fact that "alternative metastable behaviours" (Putnis and McConnell, 1980) may be expected in the present low temperature environment. This is especially important for the solid state reactions that may take place (Cu-Fe-S-CI-0 mineral formation). Equilibrium thermodynamics is concerned with the states of a system but not with the reaction path. In the real world, however, intermediate products are often formed because of a lower activation energy compared with formation of the stable end product and a further gain in free energy may be so small that the reaction is arrested. This is a rule rather than an exception in comparatively sluggish solid state systems and may affect kinetics.

As regards mineral changes, for example, reactions involving metallic copper at low temperatures in a complex water system, predictions about reaction rates and product phases are thus plagued by a significant uncertainty. Rate studies of possible reactions are considerably rare. Moreover, geologists working with metamorphic rocks (= rocks that have been subjected to changes in pressure, temperature, and composition) are becoming increasingly aware of the fact that apparently simple solid state reactions can be catalyzed by aqueous species which may increase reaction rates in a way that is difficult to predict. An example is given in Appendix (i).

NATURAL ANALOGUES.

Storage of copper canisters at a depth of 500 m may be regarded as an artificial formation of a native copper mineralization. Two geological environments have a direct bearing on the question of long-term storage; (1) Occurrences of native copper in nature, and (2) Natural environments where a significant mobility of copper at low temperature is evident.

Metallic copper in nature.

(1) Copper is a comparatively noble element, although its natural occurrences depeM on its sulphophilic character. Cu-Fe sulphides, of which (CuFeS2) is the most common, are most important, while simple Cu- sulphides are found in oxidized sulphide : for example in ores formed at low temperature near the surface of the earth (Stanton, 1972; Guilbert and Park, 1986). Native copper (= metallic copper in nature), which occurs as an exception compared with other copper minerals, can be found in a few different environments:

(a) In very small quantities in meteorites, in "Moon-rocks", and in ultramafic magmatic rocks (= extremely Si02-poor, Mg.Fe-rich rocks formed at great depths on Earth). This environment suggests strongly reducing conditions. In fact, Fe-Ni meteorites, reflect the "primitive" conditions when the solar system, including the earth, was formed. Thus, the primitive atmosphere was strongly reducing because it equilibrated with respect to metallic iron (e.g. Henderson, 1982)

(b) In oxidized copper-rich sulphide ores native copper is sometimes found (e.g. Evans, 1987). Its presence in this milieu can be explained as follows; During weathering and oxidation of the upper portion of a copper-rich sulphide ore body, FeS and FeS2 are oxidized to Fe2C>3 (or FeO(OH)) and sulphuric acid. According to e.g. Guilbert and Park (1986)

+ 2 2FeS2 + 15/2O2 + 4H2O -> Fe2O3 + 8H + 4SO4 ' (D and

2 2FeS + 9/2O2 + 2H2O -> Fe2O3 + 4H+ + 2SO4 " (2)

In an analogous way, the primary mineral chalcopyrite (CuFeS2) may be oxidized according to

2+ 2 + 2CuFeS2 + 17/2O2 + 2H2O -> Fe2O3 + 2Cu + 4SO4 - + 4H (3)

Under these strongly acid, oxidizing conditions copper; Cu2+(aq), is leached downwards. A gradual increase in pH and decrease in Eh, because of reactions with the country rocks, results in

precipitation of CuO (), followed by Cu2O () (as well as more complex copper minerals, e.g. Sikka et al, 1991). At a certain level the oxygen activity (aQ?) is too low for copper oxide formation and the sulphur activity (&$„) is still too low for sulphide formation. (Note that a§9 is equivalent to the ideal pressure of S2(gas) in a reaction and should not be confused with a§, which is equivalent to the activity of pure sulphur in a reaction and which is unity for pure liquid or solid sulphur. In thermochemical calculations 85- is used as a measure of sulphur activity rather than a§. The concepts of activity and activity coefficients are further discussed in Appendix, ii). This may result in precipitation of native copper. Deeper still CU2S and CuS are formed until, finally, the unaffected ore is encountered (Fig. 1). It is important to note that in this environment native copper is formed at higher Eh conditions compared with environment (a). This is further discussed on page 12 and the two environments are shown in Fig. 5.

(c) Native copper is sometimes found in mafic (SiC^-poor) lavas and associated sedimentary rocks formed in the near surface environment (e.g. type, Ramdohr, 1980). The genesis of this type(s) is not known in detail but the associated hydrothermal activity and the sharp changes in the Eh-pH conditions near the surface of the earth suggest similar conditions when compared with environment (b) or occasionally even (a). Reactions between water and SiC^-poor volcanic rocks may result in strongly reducing conditions.

Here, it may be noted that much less noble than copper are sometimes precipitated in their elemental states in nature. As an extreme example "natural smelters" producing native iron may result when Fe-rich molten rocks approaching the surface of the earth encounter a coal seam (Ovifak in West Greenland is famous, Ramdohr, 1980).

Solubility of copper at low temperatures in nature.

(2) Copper-rich sulphide deposits are known to be formed in sedimentary rocks at "room temperature conditions" (e.g. Gilbert and Park. 1986). This suggests significant solubility of copper in the aqueous phase. Most prominent in this connection are the "red bed copper deposits", which are believed to be formed in what is known as sabkha environment. A sabkha is a tidal marsh that is formed along the coast of desert areas. (The word is Arabic and sabkhas are well-known from Saudi Arabia). Decay of algal mats and periodic influx of seawater result in bacterological sulphate reduction. Strong evaporation results in a hydraulic updraft of continental waters through the reduced beds (e.g. Renfro, 1974; Haynes and Bloom, 1987). These waters are highly saline because of the presence of salt beds in neighbouring rocks. Under relatively oxidizing conditions heavy metals are leached from the porous country rocks (copper in the form of Cu1+-CI complexes) and precipitated as sulphides when the reducing sabkha beds are encountered. A prominent zoning pattern of precipitation can be observed with: copper - lead - zinc - iron, in a seaward direction (Haynes and Bloom, 1987, Fig. 2). This reflects the order in which the solubility products of the various sulphides are exceeded (in relation to the stability of the complexes), and is equivalent with the zoning pattern in other types of sulphide deposits (Stanton, 1972). The mobility of metals, leading to the formation of ores at low temperatures, has resulted in much research on the stability of metal complexes. Barnes (1979), and Crerar et al (1985) discussed this in general terms and the latter 8 suggested that stabilities of metal complexes in aqueous solutions partly depends on the ionic potential (Z/r) versus the elecironegativity relation of the metal.

DISCUSSION.

Stability of copper phases.

In this report, the activity concept is being used consequently both for gases, dissolved species, and solid phases. Activity is assumed to equal concentration in the calculations, i.e. the activity coefficient is taken to be unity. This is discussed in Appendix (ii). The Eh-pH diagram is also employed. The thermodynamic foundation is outlined in Appendix (iii).

The natural analogue examples show that native copper is thermodynamically stable in nature only under special circumstances. In a water system lacking species suitable for copper mineral formation (e.g. reduced sulphur species) metallic copper is stable up to fairly high Eh-conditions, depending on the pH. With an increase in the Eh, copper is oxidized to Ci^O, which in turn is oxidized to CuO. The equilibrium relations;

2Cu + 1/2 O2 = Cu2O (4) copper cuprite and

Cu2O + 2CuO (5) cuprite tenorite establish BLQ at approximately 10"^1 and 10"38, respectively at

25°C (see Appendix, iiii). Oxygen activity can be related to the Eh-pH relations. Thus

H2O = 1/2O2 + 2H+ + 2e" for which

Eh = 1.229 - 0.059pH + 0.059/4 log QQ2 (7)

(Handbook of Chemistry and Physics).

51 Inserting SQ2 = 10" (=co-existing Cu(s) + CU2O) yields

Eh = 0.477 - 0.059 pH (8)

In an analogous way aQp = 10*38 (= co-existing CU2O + CuO) yields

Eh = 0.699 - 0.059 pH (9)

Eh-pH calculations of groundwater that may be present near the repository site (KBS 83-59, SKB 86-03, SKB 87-07) suggest pH ~ 8 and Eh < 0. pH = 8, Eh - 0, are equivalent with aQ ,J0"51 at room temperature (from equ. 7, Fig. 3). Thus, in an environment where sulphur is lacking, metallic copper rather than copper 10 oxides is the stable solid phase. However, only a small increase in Eh or pH will stabilize C112O. The effect of complex cupric (Cu2+) species like malachite; Cu2(OH)2CC>3, azurite; Cu3(OH)2(CO3)2, brochantite; Cu^OHJgSO^ and atacomite; Cu2(OH)3CI, will not affect this result since they are formed under more oxidizing conditions (Hägg, 1966; Klockmanns, 1978; Ramdohr, 1980; and Guilbert and Park, 1986). However, formation of solid species, besides CU2O, in their metastable states can not be excluded. This is discussed below (p 26-28).

When the thermodynamic stability of sulphophilic elements like copper is considered, the concentration of sulphur species is a critical factor. The stability of simple metal sulphides can be calculated in terms of as? at room temperature, using AGf° data (see also Appendix, iiii).

For example

2Cu + 1/2 S2(g) = Cu2S, (10) copper chalcocite

-44 yields as2~10

Under increasing agp conditions a number of intermediary copper sulphides form until finally

8CU-) 75S + 3S2(g) = 14CuS (11) anilite covellite 11

1 which yields as?~ 10"^ using AGf° data from Woods et al (1987).

Thus, sulphidation of metallic copper starts already at about _ io"4^, at room temperature, when the most sulphur-poor mineral starts to form (CU2S). With an increase in a§ sulphur- rich copper minerals are stabilized, and the most sulphur-rich

21 variety (CuS) is formed at as?~10' . Elemental sulphur

14 condenses at as? ~10" at room temperature (Barnes and Kullerud, 1961).

In the present case, we are interested in the stability of metallic copper in relation to copper sulphides in an aqueous system. These phase relations can be illustrated in terms of Eh-pH conditions (see Appendix, iiiii).

The boundaries between the fields of dominance of aqueous sulphur species do not depend on the total concentration of anc sulphur. However, as2 * hence the stability limits of copper sulphides in an aqueous system do depend on the total concentration of dissolved sulphur species.

Using AGf° data, activities of S2 can be contoured in the Eh-pH diagram if the total sulphur content is known by considering reactions where S2 is included (Vaughan and Craig, 1978). This is exemplified in Appendix (iiiiii) and shown in Fig. 4. 12

Thus, if sulphur activities of critical sulphidation reactions are known, the stability limits of sulphide minerals can be shown in the Eh-pH diagram (for. ex. equ. 10 and 11).

Fig. 5 shows the stability limits of copper sulphides at a total sulphur concentration of 10"^ and 10'1 molar, respectively. The total stability field of the sulphide minerals decreases when total sulphur concentration decreases. It may be noted that the conditions at a future repository site are expected to correspond to 10~5 rather than to 10"1 molar reduced sulphur (SKB 87-07;

SKB 91-142). Note that 1 mg/l S - 10'4-5 m .

As seen in Fig. 5, metallic copper in nature can be stable under two quite different physico-chemical environments. Thus,

starting from strongly reducing conditions, at constant pH, as? increases as the Eh increases, reaches a maximum and then decreases rapidly with a further increase in the Eh, because of

^" formation.

Low a$p conditions under (a) strongly reducing conditions (Fig. 5) are equivalent to the "primitive solar system environment" mentioned above (page 4). On Earth, these conditions are found in rocks which are formed at very great depths and have little bearing on the expected repository site milieu.

As seen in Fig. 5 metallic copper may also remain stable under more oxidizing conditions (intermediate Eh). This is equivalent to environment (b) (see page 5), i.e. with the lower part of the oxidation zone of sulphide ores. The Eh-pH conditions suggest that 13 this environment may correspond to the repository site milieau.

As the total amount of dissolved sulphur increases, the upper portion of the metallic copper area decreases in the Eh-pH system (Fig. 5) and is stable in a narrow Eh range only, depending on the pH (Woods et al, 1987; Guilbert and Park, 1986).

The conditions at the repository site may thus favour metallic copper ox copper oxide QI copper sulphide depending on small changes in the Eh only. (Here we disregard the fact that the canisters may remain "intact" for a very long period of time because of the "kinetic buffering capacity", especially of the bentonite, as well as because of an expected sluggishness of the SO42" reduction etc.).

The phase telations in the Eh-pH diagrams shown above (Fig. 5) are to a certain degree oversimplified regarding the expected repository site milieu. Besides kinetic factors, which are discussed later in this report, this is due to the fact that aqueous iron (Fe^+) is present in the natural system , as well as additional components (Mn, Mg etc., SKB 87-07) which may be present in sulphide phases to some degree. These latter components are disregarded in the discussion, since we believe them to be of minor importance compared with iron.

With the exception of the simplest sulphides, there are no AGf° data for phases in the Cu-Fe-S system. The system includes some 20-25 species (Craig and Scott, 1973), depending on which minerals are regarded as being thermodynamically stable at room temperature. However, some data do exist, for stability relations in the system, based on experimental results extrapolated to 14 room temperature as well as on studies of naturally occurring Cu-Fe- sulphides (Craig and Scott, 1973; Vaughan and Craig 1978; Sugaki et al, 1975).

If the phase relations in the system are known it is possible to relate the sulphidation reaction to as? in a qualitative way, i.e. there are no equilibrium data but we can relate different phase assemblages to one another as a function of increasing or decreasing ay_. Fig. 6 shows that, for example bornite Cu5FeS4 may be formed at lower as? conditions compared with simple copper sulphide in a natural system containing iron. This is further discussed in Appendix

Thus, even if e.g. CU2S is precipitated initially, for kinetical reasons, it may be transformed into bomite (Cu5FeS4) and/or chalcopyrite (CuFeS2). The relative stabilities of Cu-Fe- sulphides in an aqueous system, were also defined by Sikka et al

(1991) as a function of ape2+ and acu2+.

In this context, we would like to stress that formation of various metastable compounds is more likely in a low-temperature Cl- rich water environment than the thermodynamically stable minerals. This is discussed below.

The fact that metallic copper may not be stable thermodynamically at the repository site has been considered by SKB and calculations have been carried out on the rate of copper corrosion based on limitations of the supply of reduced sulphur (e.g. Neretnieks in KBS 83-24). 15

Solubility of copper in the presence of complexing agents.

Dissolution of metallic copper in water takes place via redox processes. The relative importance of the "pure" dissolved species Cu^+ and Cu2+ is given by the equations;

Cu = Cu1+ +e" , E° =0.522 V (12) and

E° = 0.158 V (13)

(Handbook of Chemistry and Physics). The relative areas of dominance of totally dissolved cuprous and cupric species in the Eh-pH diagram are offset if the solution contains ligand-forming species.

In acid solutions cupric species in typical "freshwater" are 2+ 1 + strongly dominated by Cu ( 90%), with subordinate Cu(HCO3) (10%) and CUSO40 (aq) (<1%). In alkaline "freshwater" solutions Cu(OH)2° (aq) is dominant. In an acid seawater environment Cu2+ is still dominant ( 70%), while Cu(HCO3)1+, CUSO40 (aq), and CuCI1+ make up about 10% each. In an alkaline seawater environment Cu(OH)2° (aq) is still very dominant (Long and Angino, 1977). According to Rose (1976, 1986) the solubility of Cu2+ or complexes of common natural anions with Cu2+ is very low at pH > 6.

Cupric (Cu2+) species are probably of limited importance in the 16 repository site context. However, the situation is potentially hazardous as regards Cu1+ since chloride complexes are strongly stabilized (Rose, 1976, 1986; Rose et al, 1986; Merino et al, 1986, and Wallin in KBS 83-24), while HS~ complexing is probably of less importance due to expected low concentrations of reduced sulphur species.

According to Rose (1976), even at room temperature, the reactions:

1+ 54 (14) Cu + 2Cr = CuCI2" K = 10 and

1+ 2 5 8 Cu + 3d" = CuCI3 - K = 10 (15) are considerably shifted to the right. These complexes are important at low temperatures and their stabilization offsets the limits of dominance between cuprous and cupric species from 0.158 V for pure Cu1+ and Cu2+ to 0.43V in a solution with 0.5 molar Cl", at room temperature. Since aQU1+ increases tenfold for each increase in Eh of 0.06 V, if metallic copper is present, so do aQUQ|o1- and aQuQ|_2-. In the expected environment" (Eh

A calculation based on equ. 14 and 15 shows that normal seawater (Cl~ = 19350 mg/kg, Drever 1982 in SKBF 83-59) would yield a total amount of dissolved copper (CuCl2~ + CUCI32") of 16 mg/l at Eh = 0, if it is assumed that metallic copper is present (= conditions where aQu1+ is independent of pH). Under identical conditions the most CT-rich groundwater analysed in SKB 87-07 (CT = 5200 mg/l, brackish water) would yield a total amount of dissolved copper of 0.66 mg/l, and the most Cl'-rich groundwater at Äspö; 12600 mg/l Cr (SKB 91-142), would yield 5.4 mg/l dissolved copper. This shows that the Cl" concentration is an important factor. Expected solubilities are further discussed under "Eh-pH conditions" below.

An increase in the reduced sulphur species concentration leading to stabilization of Cu-S or Cu-Fe-S compounds, rather than metallic copper, results in a decrease in total solubility because Cu1+ is "used up" in sulphide formation. Thus, the special situation regarding the copper canisters seems to be that the total solubility of copper may be significant under conditions where metallic copper (or CU2O) is the stable solid phase while a stabilization of e.g. CU2S results in a sharp decrease in solubility but may lead to corrosion because of sulphide formation or formation of metastable compounds (Fig. 7).

Similar results regarding the solubility of copper in a chloride - rich environment was obtained by Wallin, and Wallin and Lewis in KBS 83-24. Moreover, their data indicate that the total solubility increases by a factor of around 10 when the temperature is 18 increased from 25 to 100°C. Unfortunately no further comment on their results were made in the report. Also the results of Crerar et al (1978), Barnes (1979), and Seaward (1981, 1984) indicate a stabilization of metal chloride complexes as the temperature is increased (see also "Eh-pH conditions" below).

Supply of reduced sulphur.

Sulphate reduction.

In sulphide ores formed in the near surface environment at low temperatures there is a strong connection between sulphur and carbon concentrations. Typically, iron sulphides, which are the most important quantitatively, are more common in sediments of marine origin when compared with fresh water sediments. This is due to the much higher sulphate content in marine waters. Reduced sulphur is supplied through the agency of SO^" reducing bacteria under anoxic conditions (Berner, 1984), while metals may be derived both locally at the precipitation site and transported from a larger distance. Elements like copper, zinc, and lead can be supplied to the precipitation site in the form of metal complexes, diffusing through the water-saturated sediments or carried passively by groundwater movements (e.g. Stanton, 1972). In the "copper canister stability" context it is interesting to note that both in ores precipitated at "room temperature" as well as in ores formed at higher temperatures, in a reduced marine environment, there is a zoning pattern showing that copper sulphides are precipitated very rapidly and efficiently compared with sulphides of zinc, lead, and iron (e.g. Stanton, 1972). In accordance with thermodynamic calculations this shows that the stability of copper (Cu-CI) complexes 19

decreases sharply when asp increases (presence of reduced sulphur species).

Analyses of various types of ground water by SKB suggest that SC>42~ concentrations can be high, depending on the origin of the water, but that the content of reduced sulphur species is low (< 1 ppm). Hallberg and Grenthe, in KBS 83-24, discussed the possibility of inorganic SO^- reduction and referred to various sources. They concluded that such processes take place at insignificant rates below 100°C. This result was supported by Grauer in SKB 91-16, who gave several examples showing the improbability of inorganic SO^" reduction. Hallberg and Grenthe in SKB 83-24 also gave an example of a native copper deposit (Quincey Mine, Lake Superior district) where prolonged exposure of the metal to SO42", Fe2+ rich waters did not result in extensive copper sulphide formation. In relation to this it seems appropriate to add that even the formation of native copper in the oxidation zone of sulphide ores takes place from solutions rich in SO42", i.e. precipitation and stability of the metal is a question of suitable Eh-pH conditions (Fig 5).

The low cocentrations of reduced sulphur species in the analysed groundwater of interest may owe their origin to organic reduction of SO42" (Berner, 1984). Thus, the water contains some organic carbon (KBS 83-24, Appendices 1-2). According to KBS 83-24, Appendix 5, a possible copper sulphide formation would not result in serious corrosion during the time period considered. 20

Influence from sulphide minerals in the enclosing rocks.

In the present case, there is a possibility that reduced sulphur can be contributed to the copper canisters from other sources except organic sulphate reduction. The enclosing rocks can be expected to contain a certain amount of reduced sulphur in the form of sulphide minerals. Among these pyrite (FeS2) and pyrrhotite (Fe^.xS) are normally dominant.

The Earth has a mean concentration of sulphur of about 2 wt.%, most of which is concentrated in the core and the mantle (Henderson, 1982). According to Bailey (1979) the average sulphur content in igneous rocks (= rocks originating from a melt) is 450 ppm. The average granite (= SiO2-rich) contains 200 ppm S and the average basalt (= gabbroic composition, SiC^-poor) 500 ppm S. Individual rocks direr strongly as regards their sulphur content. It is therefore impossible to give a realistic figure for the rocks at a future repository site.

However, for the sake of argument we assume a certain concentration of pyrite + pyrrhotite in the vicinity of the buried canisters (high concentrations are possible if a mineralized zone is encountered). The sulphides may be present as scattered grains throughout the rock mass or concentrated in small veins.

Metallic copper is not thermodynamically stable in the presence of pyrite or pyrrhotite (Only stoichiometric FeS; troilite, co- exists with metallic copper. Troilite is not expected in the present environment, it is mostly found in ultramafic rocks; = rocks with a very low total SiC>2 content, in iron meteorites etc.). However, the copper canisters will be separated from the 21 enclosing rock mass by engineered barriers. Thus, no direct physical contact is expected to take place between copper and iron sulphides. This means that it is only meaningful tr discuss disequilibrium between copper and iron sulphides on the scale of the whole system; including copper canisters, engineered barriers, and the surrounding rocks.

Since no direct contact will exist between the reactants, a possible reaction will take place through the agency of aqueous species diffusing through the groundwater. Besides diffusion rates through the engineered barriers, which are not discussed here, a crucial question as regards reaction rates will be the dissolution rates of sulphide minerals and the concentrations of aqueous sulphide species in the neighbouring fluid phase.

Primary sulphide minerals have been present near the surface of the Earth, in the Precambrian crystalline bedrocks of Sweden, for many millions of years with no apparent corrosion of the grains, except where contact with oxygenated waters has faciliated redox processes. A sulphide grain may be in chemical equilibrium with the surrounding grain boundary fluid, and, as long as the grain boundary fluid is isolated from the groundwater, it may persist intact for very long periods of time even in a geological perspective, i.e. diffusion of aqueous sulphide species from the sulphide grain is practically nil in this case, at least at room temperature conditions.

Alternatively, sulphide minerals may equilibrate with respect to the bulk of the groundwater and still persist almost indefinitely as long as the Eh-pH conditions are within the stability range of the minerals. This may be the case if the water is more or less 22

stagnant under reducing conditions.

We can envisage a worst case situation where aqueous sulphide species diffuse through stagnant groundwater from sulphide grains in the surrounding rocks to the reaction site at the copper canister surface, it is not possible to give a time perspective for such a complex process. However, we do have some indirect indications which allow us to make a qualitative prediction. Thus, microscopic observations of sulphide minerals from sulphide ores and from sulphide grains, scattered in a crystalline bedrock, almost invariably show that chemically incompatible phases occur in close prcximity with one another (mm-scale) in the rock (e.g. Ramdohr, 1980). This suggests that groundwaters or grain boundary fluids have not acted as efficient reaction agents at room temperature conditions, even in a long time perspective. In other words, the very existence of chemically incompatible solid sulphide phases in close proximity in nature suggests that a contribution of aqueous sulphide species, from dissolving sulphide minerals in the crystalline rock mass to the copper canisters, vould result in only insignificant corrosion even in a 100 000 year perspective.

However, sulphide minerals at shallow depths in crystalline bedrock, which are affected by oxygenated groundwater, are strongly altered even in the cold climate of Sweden. This is seen in massive sulphide ores, for example in Bergslagen, where tectonic zones are characterized by intense alteration of primary sulphide minerals, leading to the formation of soluble copper and iron sulphates etc. (e.g. Magnusson, 1973). Oxidation zones (see page 5) in sulphide ores in Sweden are thus controlled by the vicinity to tectonic zones rather than by the actual depth. 23

Primary sulphide minerals can persist for many millions of years quite near the surface of the Earth as long as they are protected from the hydrosphere, the atmosphere, and the biosphere by the surrounding rock. Alternatively, they can be altered within minutes if they are directly exposed to the atmosphere.

The conditions discussed could change because of elevated temperatures near the copper canisters. This is, however, difficult to quantify. Also, if an oxidation zone is created during construction work the stability of sulphide minerals in the country rocks will be strongly affected. Such an event could have an adverse effect on the copper canisters.

EH -pH conditions.

Both the dissolution of native copper (see page 14) and a possible formation of sulphide compounds depend on the chemical environment in terms of Eh and pH. The analyzed groundwater suggest a pH which is slightly on the alkaline side of neutrality. This is expected since almost all rock - water reactions tend towards increasing pH. In the case of oxidation of sulphides leading to released sulphuric acid, the pH will be rapidly buffered because of reactions with neighbouring rock-forming minerals. Moreover, seawater is close to a buffer minimum at around pH=8. This is caused by a combination of (1) HCO3" supply from limestone drainage areas, and (2) plankton formation and biogenic calcite formation in the ocean. Atmospheric carbon dioxide concentration and the solubility product of calcite constitute critical factors. Chemical equilibria are discussed in Krauskopf (1979) . The engineered barriers are also expected to take part in H+ consuming reactions. 24

Even if pH conditions turn out to be neutral to alkaline, at the repository site, the exact conditions will be of utmost importance as regards the total solubility of copper. Fig. 8 shows conditions at 100°C, CI" = 1 molar, and XS = 10~4 m, based on Merino et al (1986). Under the conditions where metallic copper co-exists with CU2S; pH = 8 is equivalent to a copper solubility of around 20 ppm, pH = 7 of around 400 ppm, and pH = 6 of around 10000 ppm. Even if we assume that the water is brackish and divide the solubility figures by a factor on the order of 30-100, they are still significant.

In terms of a change in Eh, at a constant pH, the change in total copper solubility is drastic since each increase in Eh of 0.06 V will be equivalent to an increase in total solubility of a factor of 10 at room temperature, assuming conditions where metallic copper is the stable solid phase (Fig. 7). This may be the case at the repository site. Thus, it is essential to have a good knowledge of the expected redox conditions, which are given by the bulk system including the country rock. An influx of decaying organic materials for example can lead to a rapid decrease in Eh, while the long-term redox buffering capacity of the groundwater may be given by the rock composition in terms of reactive minerals with red-ox couples like Fe^+/Fe^+ and Mn^+/Mn^+/Mn^+, especially at elevated temperatures. Thus, we would expect a mafic rock like a gabbro to have a larger buffering capacity because of a much higher content of minerals rich in e.g. iron

As reflected in the composition of oxide minerals, rocks create their own red-ox conditions during heating. Thus, acid magmatic rocks contain oxide assemblages suggesting relatively high 25 conditions, while ultramafic rocks (=Fe-Mg rich, SiC>2 poor) contain oxide phases which indicate strongly reduced conditions. Moreover, in sulphide ores, magnetite ^304) is the dominant iron oxide mineral, reflecting the reducing conditions, and oxidation to hematite (Fe2C>3) takes place in parts of the ore subjected to surface oxidation only.

A drastic example of very local ao? control in an iron ore, subjected to a temperature of around 600°C, was reported by

Miyashiro (1973). Thus, the composition of an ao?-indicator mineral showed considerably higher aQ2 conditions in a rich rock layer when compared with a neighbouring unit. The rock layers were only a few centimetres apart. This indicates that aQ is strictly a function of the local bulk composition at such temperatures. It should be stressed that the mobility of e.g. water under these conditions is limited, and is mainly due to grain boundary diffusion.

It is difficult to extrapolate such results to the 0-100°C environment since variations in reaction rates, surface reactions etc. may be expected to play an increasingly important role at low temperatures. As a rule of thumbs it can be stated that the reaction rates at "near room temperature conditions" are many orders of magnitude lower compared with the rates at 600°C (the example discussed). This means that if chemical disequilibrium among solid phases is evident on the cm-scale at 600°C it will be already evident on the microscale at low temperatures (see also the discussion under "Influence from sulphide minerals in the enclosing rocks" above). A complication arises from the fact that mineral reactivities differ enormously. According to Barton 26

(1970) some sulphides react with about the same rate at room temperature (Ag- and Cu- sulphides) as others do at 600° C (FeS2)- This implies a difference in reactivity that may amount to a factor of 1010. Even larger differences are apparent if we compare reactive sulphides with the most sluggish silicate minerals. Thus we can expect groundwater to react with surfaces of reactive minerals, e.g. clay minerals and reactive sulphides, while sluggish minerals may remain completely unaffected. In addition, Eh conditions may be affected by radiolysis. This complication is outside the scope of this report.

In conclusion, the question of expected Eh conditions at the repository site is a difficult one, especially when measurements at 5-10°C are translated to some 100°C. We can hope for more information in the future.

Kinetics of sulphidation and metastable behaviour.

All questions discussed here are concerned with kinetics. Unfortunately it is the least understood subject. There is also a potential "worst case" risk that metastable reactions may result in rapid mineral formation and disintegration of the copper canisters. This risk should be evaluated.

The mechanism of sulphidation of meta'lic copper might be considered unimportant in the present context, since calculations by SKB (e.g. Neretnieks in KBS 83-24) suggest a very limited supply of reduced sulphur. However, the mechanism is important to study since copper sulphide forrria..:on might catalyze or initiate metastable oxi^e-chloride mineral formation in a natural 27 complex environment (McNeal and Little, 1990).

Since the Eh-pH conditions can be interpreted in terms of as?, we can envisage the sulphidation in terms of a simple redox process.

Compositional changes in sulphide minerals are essentially a question of metal atom diffusion in a more or less static sulphur substructure (Birchenall, 1974: Condit et al, 1974). Sulphidation of copper can be regarded as a rapid electron transfer process followed by an outward diffusion of copper ions through the sulphide layer which is formed and a reaction on the surface. Such a process can lead to the formation of pores in the metal, where the "fresh" metal surface is exposed to the sulphur atmosphere. This is equivalent to rapid kinetics.

A similar mechanism in a sulphide mineral was indicated by one of us (Amcoff, 1987, 1988), performing experiments on phase changes in the "dry" Cu-Fe-S system. Moreover, a simple sintering experiment by Kucsynski and Stablein (1961) using metallic copper and nickel in close contact showed a mechanism where the original pieces of nickel grew in size, because of Ni-Cu alloying, when heated, while the copper pieces gradually disintegrated and showed a typical development of pores. This anisothropic behaviour is due to the fact that copper diffuses much more rapidly in nickel than vice versa. A simple experiment on the kinetics of copper sulphidation was performed by Vorobyev (1982) who heated metallic copper + sulphur in evacuated silica ampoules. He calculated the energy of activation of the process and claimed that the sulphidation follows first-order kinetics, i.e. a logarithmic time dependance was observed, where diffusion of copper through CuS constituted the rate-determining factor. In 28 this respect it is important to note that the rate of sulphidation at high 85- conditions is extremely high. In fact, metallic copper is practically instantly "blackened" when put in contact with sulphur, even at room temperature.

It is quite conceivable that the rate of mineral formation will increase in a water system compared with the "dry" system because of the presence of species that can catalyze the reaction(s), as well as of the possibility of metastable mineral formation. It may be noted that chlorine can substitute for sulphur in sulphides and "protonization" processes, i.e. formation of HS-ligands in the mineral structure can be expected to play a part in the complex system (Craig and Scott, 1973).

Under certain circumstances very high corrosion rates of copper are observed. According to McNeal and Little (1990) copper can be subjected to sudden disintegration in soils or porous rocks, rich in chloride ions and undersaturated with respect to water. They claim that anaeorobic bacteria create a reducing milieu on the surface of the metal, in an otherwise oxidizing environment, leading to the formation of nantokite (CuCI), under dry conditions. Subsequent hydration reactions result in the formation of concentrated hydrochloric acid and sudden copper dissolution. The formation of other copper chloride - oxide minerals may lead to similar results.

The reactions outlined by McNeal and Little (1990) are presumably very different from those expected in a crystalline bedrock environment. However, they demonstrate that the question of organic matter at the repository site is important, not only because of possible SO42" reduction but also because of 29 possible metastable mineral formation.

CONCLUSIONS AND RECOMMENDATIONS.

The reports presented by SKB on the problem of copper corrosion show that an impressive amount of work has already been accumulated on the subject. The main conclusion of these reports is that under "expected" circumstances, where e.g. the Eh-pH conditions, as reflected in the groundwater analyses, will remain unchanged, and where the bentonite buffer will remain intact etc., corrosion will be negligible even in a 100 000 year perspective. The analogue studies of archaelogical bronzes are valuable in predicting what to expect as regards corrosion rates, but we would like to stress that a "80°C environment" may prove to be rather different in this respect as compared with the milieus to which these objects have been subjected (£-10°C). Moreover, as regards pitting corrosion studies on archaelogical bronzes such studies aie inherently biased because, using the very words of Birgit Arrhenius in SKBF 83-05, "Special problems have been encountered in this measurement of pitting corrosion, since pitting corrosion can only be studied on relatively well-preserved bronzes, whereas in most cases general corrosion has completely ruined the original surface. The selected material is also skewed due to the fact that the studied material has already undergone one selection, so to speak, in that it has exhibited a corrosion resistance that has enabled it to survive at all. We can deduce from the many tombs that have been found empty of artifacts or where the only metal artifacts are of gold that only a very small portion of the original archaelogical metal material has been preserved through the millenia" (underlined by the present authors). 30

In the present report, various physico-chemical factors, which may affect the long-term stability of metallic copper in a crystalline environment, have been discussed. In qualitative terms we have tried to evaluate the relative importance of these factors, which include (a) expected chemical environment in terms of Eh-pH relations, presence of certain complex-forming species etc., (b) elevated temperature near the canisters, and (c) kinetics-metastability. Gaps in our knowledge of complex geochemical processes become apparent when we are confronted with practical, well-defined problems like the present one.

We think, the most serious lack of knowledge is concerned with metastability and kinetics. Several issues have been addressed where this lack of knowledge is apparent;

(i) It has been argued, based mainly on studies of low- temperature sedimentary sulphide mineral formation processes, that SO4.2" reduction, at low temperatures, only occurs with significant rates in the presence of certain bacteria, whose existence depends on organic materials (Hallberg and Grenthe in SKB 83-24; Berner, 1984). Can we be confident that this is really the case as long as no sulphidation experiments have been undertaken at T = 10-100°C, in an environment that resembles the expected, chemically very complex milieu? Can we be certain that e.g. clay mineral surfaces (bentonite) will not catalyze inorganic reduction processes as well as other "unwanted" reactions? 31

The paper by McNeal and Little (1990) suggests that microbiologically induced copper corrosion can be very rapid under certain circumstances, e.g. formation of nantokite (CuCI) and CU2S on the copper surface in a Cl'-rich, alternating wet-dry environment. Even if conditions at a future repository site probably will be rather different, the results of McNeal and Little (1990) illustrate that much remains to be learned concerning reactions on the copper surface in a complex environment.

(iii) An uncertainty also applies to pitting corrosion studies on archaelogical materials (Bresle et al. in KBS 83-05). If the selection of samples for study is inherently biased (see above), of what value are the presented figures? (iiii) The change in reaction rate with temperature depends on the activation energy, which varies strongly for different processes. As stated above; "the reaction rate is about doubled for each 10°C temperature increase for many reactions where a solid phase is involved". This rule of thumb is too crude to be used in the present context. As far as we know no real attempt has been made to translate the expected chemical conditions at a future repository site from say 10°C to 80°C. This is an important issue both as regards sulphidation reactions, copper chloride complex formation, possible formation of metastable phases, etc.

Our lack of knowledge, especially regarding conditions at 80°C, makes it difficult to predict whether there is a risk of significant corrosion. The most impressive calculations, based on large thermodynamic data, are to little avail if the geochemical processes are governed by kinetics and metastable behaviour rather than by equilibrium thermodynamics. 32

Our knowledge of kinetics is mainly based on empirical studies, and there is no possibility of making precise statements about reaction rates in the present complex system based on theoretical arguments and chemical equilibrium calculations. The very complexity of the system makes it virtually impossible to predict all probable and less probable reactions that can take place.

We believe that such uncertainties can be substantially reduced if a series of experiments is undertaken, where metallic copper is placed in a physico-chemical milieu that corresponds to an expected future repository site milieu. This means a milieu where components and species that may participate in copper corrosion processes should be included simultaneously. To place an upper limit on the corrosion rate and to increase reaction rates (1) temperature, and (2) concentration of species that participate in reactions with copper, like Cl" and dissolved sulphide species, might have to be exaggerated. In our opinion, this study should include both actual dissolution rates of copper and processes taking place on the metal surface, i.e. stable and metastable mineral formation and reaction mechanisms. Besides, we think both a biotic and an abiotic environment should be considered.

It might be argued that these experiments would be superfluous since stabilities of Cu-CI complexes, Cu- sulphides etc., have already been studied. The point is, however, that the total complex system can be expected to evolve differently as compared with the sum of the participating simpler binary systems.

It would be advantageous to include "dry" sulphidation 33 experiments of copper at different &§ conditions in the proposed study. These reference runs, which are simple to perform, should be compared with results from experiments in an aqueous environment, performed under identical as conditions. An observed difference would show the influence of additional aqueous species, like Cr and Fe2+, as well as a possible catalyzing effect of e.g. bentonite, on the "simple" sulphidation process, especially as regards phases that form on the copper surface.

In conclusion, if a long-term durability for copper in a crystalline bedrock environment is to be considered, the following interdependent points should be considered:

(i) dissolution of copper in the presence of Cl~ and other complex-forming species.

(ii) sulphide mineral formation in the presence of reduced sulphur species.

(iii) the effect of elevated temperature near the canisters.

(iiii) Eh-pH relations as a function of chemistry and temperature.

(iiiii) kinetics, in a long-term perspective.

(iiiiii) possible metastable reactions on the canister surface.

The engineered barriers are intended to function as multiple shields against radioactive leakage. If one should prove defective, 34 for some reason, leakage should be limited by the remaining barriers. For this reason, points (i)-(iiiiii) should be addressed regardless of the long-term fate of the bentonite buffer.

The present discussion does not cover all aspects of the question of copper corrosion, e.g. possible effects of radiolysis have been excluded. We may have overemphasized some facts and underestimated others. In our opinion this uncertainty reflects the state of the art of the subject. 35

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Appendix

THERMODYNAMIC CALCULATIONS.

(i) As an example of a catalyzed solid-state process we can regard the apparently simple mineral transformation

AI2S1O5 (kyanite) = AI2Si05 (sillimanite) (A1) which is shifted towards the right with an increase in temperature and towards the left with an increase in pressure. These minerals are common in Al-rich metamorphic rocks. However, "very rarely does sillimanite ever seem to have been formed directly from kyanite, it is usually formed as a result of other reactions within the rock" (Ehlers and Blatt, 1982, p 595). Carmichel (1969) considered the reactions

+ 3AI2Si05 (kyanite) + 3SiO2(quartz) + 2K + 3H2O =

2KAI3Si3O10(OH)2 (muscovite) + 2H+ (A2) and

(muscovite) + 2H+ =

3AI2Si05 (sillimanite) + 3SiO2 + 2K+ + 3H2O (A3)

A2 + A3 yield reaction A1.

Even if this reaction takes place at a very high temperature and 42 pressure (T>500°C, P>375 MPa; Ehlers and Blatt, 1982) it illustrates that aqueous species may catalyze apparently straightforward reactions.

(ii) Activity and fugacity are thermodynamic entities, exact definitions are found in standard textbooks, for example, in Klotz and Rosenberg (1977). They may be regarded as a measure of the "effective concentration" of constituents participating in a reaction, where fugacity (f) is commonly used for gases as a measure of "effective gas pressure", while activity (a) is employed both for solids, liquids, gases, and dissolved species. It has been found convenient to relate the effective concentration to some reference state and f=1 for an ideal gas at P=1 atm. and a=i for a pure liquid or solid are most often used. To avoid confusion we have consequently used activity in the present report, both for solids, gases, and dissolved species.

"Real" substances often deviate from the ideal. Fugacity and activity are then related to pressure and concentration, respectively, by means of the fugacity or activity coefficient ( ,).

Activity coefficients of aqueous species may be derived by successive measurements at different ionic strenghts, or, in reasonable dilute solutions, by using the Debye-Huckel or Davies equations (Vaughan and Craig, 1978; Klotz and Rosenberg, 1977).

In the simple calculations presented in this report the concentration of a species is assumed to equal its activity. This can be justified since the changes discussed, resulting from large differences in the chosen concentration levels, exceed the error 43 introduced by the simplification. The present discussion is an attempt to illustrate differences and trends in a qualitative way rather than to present exact figures.

(iii) The present discussion is much concerned with stability relations, in terms of Eh and pH. The well-known Eh-pH diagrams, which have become a standard method of illustrating equilibrium relationships of dissolved and solid species in water are based on the Nernst equation which, in turn, is a special form of the free energy equation (see for example Klotz and Rosenberg, 1977)

AG = AG° + RTInK (A4)

where AG is the free energy difference in a reaction, AG° is the free energy difference under standard conditions (T = 298,15 Kelvin, P = 1 atm. etc), R is the gas constant, T is the absolute temperature and K is the equilibrium constant.

The Nernst equation can be obtained from equ. (A4) by dividing both sides by nF, where n is the number of moles of electrons in the redox reaction and F is the Faraday constant (23 061 cal mole"1 eV"1, Craig and Vaughan, 1978).

Thus

AG/nF = AG°/nF + RT/nF InK (A5) where AG/nF = Eh and AG°/nF = E° 44 inserting yields

Eh = E° + 0.059/n logK (Nernst equ.) (A6) at T = 298.15 K, P « 1 atm. ( e.g. Klotz and Rosenberg, 1977). From a thermodynamic point of view the Eh-pH diagram may be regarded as a special case of an activity - activity diagram, where Eh is a measure of the electron activity and pH is a measure of the proton activity.

(iiii) To calculate agp from the copper oxidation reactions the free energy equation can be used

AG = AG° + RTInK (A4) at equilibrium AG = 0, which yields

AG° = -RTInK (A7) or log K = - AG°/RT2.303 = - AG°/1364.4 (A8) at T = 298.15 K (Vaughan and Craig, 1978).

AG° can be calculated if the standard free energy of formation

(AGf°) for the participating constituents is known, since 45

AG° (process) = lAGf0 (product) - lAGf0 (reactants) (A9) AGf° values can be obtained from standard reference books, for example "Handbook of chemistry and physics".

As an example we can consider the reaction

2Cu + 1/2 O2 = Cu2O (A10)

From "Handbook of chemistry and physics" we obtain

AGfC 34980 cal Cu2O (s) = - < AGf°Cu(s)> AGf°Op(g) s ° (standard states).

This yields log K = -AG°/1364.4 = 34980/1364.4 = 25.64 but

/a a 1/2 from ec u log K = log [acu2O Cu( O2^ l ^ * -

and a DOth e( ual 1 ure but aQu o Cu ' (P solids) this yields

1/2 log K - - log (aoj = -1/2log a0 = 25.64 46

51 thus, aoo = 10' at 298.15 K.

Performing analogous calculations for

CU2O + I/2O2 =2CuO (A11)

38 yield ao2 = 10" at 298.15 K

(iiiii) The fields of dominance of stable aqueous sulphur species can be constructed by means of AGf° data.

For example

2 H2S (aq) + 2O2 = SO4 "(aq) + 2H+ (A12)

1 0 where AGf°So42-(aq) - -177 340 (cal mole" ), AGf H2s(aq)

6660, AGf°o2> AGf°H+ = 0 (standard states).

Thus

AG° = -177 340 -(-6660) = -170 680 (cal mole'1) at T = 298.15 Kelvin, P • 1 atm.

2 2 log K = log [aSo42- (aH+) / a^s ao2 ] = -AGO/1364.4 = 126.77

2 a wnicn at the boundary line; aso4 " • HoS' yields

s log a|_j+ - log ao2 63.39 (A13) 47 but

Eh = 1.229 -0.059 pH + 0.059/4 logao2 (7)

Substituting equ. (A13) into equ. (7) yields

Eh = 0.294 - 0.074 pH (A14)

This linear relation constitutes the boundary line between the fields of dominance of H2S (aq) and ^

(iiiiii) Activities of S2(g) can be calculated by considering reactions between S2(g) and the aqueous species.

For example in the H2S(aq) field of dominance

2H2S (aq) = S2(g) + 2H2(g) (A15)

AG° = 19100 -(-2x6660) = 32420 since AGf°H = 0 (standard state).

1 Note that AGf°s2 - 19100 (cal mole" ) is calculated from the reaction

2S(s) = S2(g) (A16)

14 as2 =10' at T = 298.15 K (Vaughan and Craig, 1978). thus 48

2 2 + 2 lo a 2 lo a log K = log [aS2(aH2) /(aH2s) ] - '09 as2 9 H2 - 9 H2S .4 = -23.76

Thus, if the total amount of dissolved sulphur species (IS) is known and if we simplify the calculation by equalizing

14 concentration and activity we can contour e.g. as? = 10~ ( = activity where sulphur condenses at room temperature) in the Eh- pH diagram.

For example, in the H2S(aq) field of dominance at £S = 0.1, i.e. aH2S - IS = 0.1 thus

log as2 + 2 log aH2 - 2 log aH2s » -14 + 2 log aH2 -2(-1) which yields

log aH2 =-5.88 (A17) but

H2 = 2H+ + 2e" (standard electrode)

Eh = -0.059 pH - 0.059/2 log aH2 (A18)

Inserting equ. (A17) into equ. (A18) yields 49

Eh = 0.173 - 0.059 pH (A20)

Thus this linear relation shows the boundary line at which elemental sulphur appears in the H2S field of dominance at £S 0.1 molar, T = 298.15 K.

Inserting LS = 10*5 molar in an identical calculation yields

Eh = 0.291 - 0.059 pH (A21) which shows that the stability field of elemental sulphur decreases as IS decreases.

In a similar way sulphur activities in e.g. the SO^'faq) field of dominance can be calculated based on

2 S2(g) + 2H2O + 3O2(g) = 2SO4 -(aq) + 4H+ (A22) and so on. Note that these calculations are based on Vaughan and Craig (1978).

(iiiiiii) In the presence of dissolved iron we may expect formation of copper iron sulphides rather than simple copper

sulphides on the canister surface, depending on e.g. ape2+ and ag^. There are few thermochemical data for copper iron sulphides at room temperature, because of sluggish kinetics, and Eh-pH relations that are shown in the literature should be regarded with caution. 50

In Fig. 6 simplified phase relations in the "dry" Cu-Fe-S system are shown. The tie-lines connect minerals that are believed to co-exist stably in a thermodynamic sense at room temperature. For example, metallic copper + pyrrhotite + cubanite constitute a

stable mineral assemblage that establishes a$? at a certain value, depending on temperature, the assemblage; metallic copper + cubanite + bornite establishes a$p at another value etc.

Thus, when a tie-line is crossed a$p changes abruptly, while it remains fixed within three-phase fields bordered by tie-lines.

According to the thermodynamic phase rule, as? increases when a tie-line is crossed in a direction towards the sulphur corner in the diagram (see the arrows in Fig. 6). The crossing point constitutes a sulphidation reaction.

Copper is supplied to the reaction site on the canister surface by means of diffusion through forming mineral phases and onto the surface (Vorobyev, 1982; Amcoff, 1987, 1988). The supply of iron is limited by the rate with which it diffuses through the engineered barriers to the reaction site. In Fig. 6 two cases are shown where arrow(1) indicates a limited amount of iron compared with copper at the reaction site, while arrow(2) indicates iron-richer conditions. In this respect, the expected repository site environment is probably more in line with arrow(1).

Thus, in Fig. 6, arrow(1) shows phase changes taking place when as« is increased at a fixed Cu/Fe ratio (10:1 in the figure), i.e. the arrow points straight towards the sulphur corner. For 51 example, if we start from the base in fig. 6, where Cu(s) + Fe(s) co-exist, and follow the stippled line, a reaction takes place when we cross over into the Cu(s) + Fe(s) + FeS(s) three-phase field

Fe + 1/2S2(g) = FeS (A23)

AGf° data yield a$ = 10"48 for this reaction.

When the Cu - FeS tie-line is crossed all metallic iron is consumed because of the reaction

Cu(s) + 2Fe(s) CuFe2S3 (cubanite) (A24)

and we cross over into the Cu + FeS + three-phase field.

At the Cu - tie-line the next sulphidation reaction takes place

5Cu + FeS +3/2S2(g) = Cu5FeS4 (bornite) (A25)

in which all FeS is consumed, etc.

In this way, a number of sulphidation reactions can be

constructed that take place at higher and higher as«. Even if AGf° data are missing for most phases it would be possible to show the phase assemblages in relation to one another in an Eh-pH diagram. 52

The conditions indicated in Fig. 6, starting from Cu(s) + Fe(s) base-line, are comparable with the strongly reducing "primitive solar system environment" (environment (a) on page 4). However, as regards the expected future repository site environment (environment (b) on page 5) we must take into consideration the assumption that the Eh is considerably higher (Fig. 5). Under these conditions metallic iron and cubanite and possibly also pyrrhotite are unstable. Still, we can deduce from Fig. 6 that at least bornite may be formed at lower as« conditions when compared with simple copper sulphide. 53

FIGURE LEGENDS.

Fig. 1: Vertical section through the oxidized upper portion of a hypothetical copper ore. Note formation of native copper, intermediate between copper oxide and copper sulphide. Modified after Guilbert and Park (1986).

Fig 2: Sedimentary copper ore formation in a sabkha environment . After Haynes and Bloom (1987).

Fig. 3: Stability of copper phases in the Cu-O-H system as a function of Eh and pH at room temperature. Based on equations (8) and (9). Note that in "the real case" no solid phases are stable under acid-oxidizing conditions. ao2 = 0-2 (atmosperic pressure) and a|-|? = 1, correspond to the upper and lower boundary for geological processes near the surface of the earth. Oxygen activities have been included.

Fig. 4: Activity of S2 as a function of Eh and pH at room temperature . (i) IS = 10"^ molar, (ii) IS = 10"1 molar. Based on AGf° data and equilibrium relations where S2(g) is included. (For example equation A15 in Appendix 1, iiiiii).

Fig. 5: Stability of copper sulphides as a function of Eh and pH at room temperature, (i) IS = 10*5 molar, (ii) IS = 10"1 molar. Based on sulphidation reactions, for example, equations (10) and (11), and on AGf° data by e.g. Woods et al (1986). (see also Fig. 4). Note that the total copper sulphide area increases and the native copper area decreases as the IS increases. Note also that (a) and 54

(b) in the figure correspond to environments (a) and (b) on pages 4-5. Abbr: dj; djurleite (Cu-j 965S), an; anilite (Cu-| 75S).

Fig. 6: Simplified phase relations in the "dry" Cu-Fe-S system at room temperature. Tie-lines connect minerals that are believed to be thermodynamically stable with one another. Arrow(1) shows mineral changes taking place during progressive sulphidation at a bulk Cu/Fe ratio of 10:1, and arrow(2) shows sulphidation at a bulk ratio of 3:1. This is further explained in Appendix 1 (iiiiiii). Note that native iron, cubanite, and possibly pyrrhotite, are not stable in the expected repository site milieu. Abbr. Cu; native copper. Fe; native iron, S; condensed sulphur, cc; chalcocite (CU2S), dg; digenite (Cu-j 3S), cv; covellite (CuS), bn; bornite (CusFeS^, cp; chalcopyrite (CuFeS2), cb; cubanite

(CuFe2S3), po; pyrrhotite (Fe-|.xS), py; pyrite (FeS2). According to Craig and Scott (1973), and Vaughan and Craig (1978).

Fig. 7: Solubility of copper (in mg/l) at room temperature as a function of Eh and pH, with a total sulphur concentration of 10~4 molar, (i) Solubility of copper in the "pure" Cu-O-H-S system, (ii)

Solubility of copper in the Cu-O-H-S-CI system with a Cl" concentration of 0.5 molar. (Note that normal seawater corresponds to -0.55 m Cl", the most Cl'-rich groundwater analysed in SKB 87-07; 5200 mg/l Cl"; brackish water, corresponds to -0.15 m CI", and the most Cl'-rich groundwater from Äspö; 12600 mg/l Cl", corresponds to -0.36 m CI"). Based on 55

Rose (1976).

Fig. 8: Solubility of Cu (in mg/l) as a function of Eh and pH at T =

100°C, CI' = 1 molar, IS = 10"4 molar. Total content dissolved Cu essentially contained in CuCI0, CuCl21', and CuC^2'. Note that

1 molar C\~ is about twice the chloride concentration of ordinary seawater (The solubility figures for seawater should be about 5 times lower and those of brackish water should be of the order of 30-100 times lower, compared to the figure). Abbr.: cc; chalcocite (CU2S), bn; bornite (CusFeS^, Cu; native copper. (Modified from Merino et al. (1986). 56

Eh Q) O "O OJ N Fig. 1

X O

primary ore

evaporation t

sabkha flat sea .)/.../; groundwater

depositional seawater zoning metal-Cl of sulphides complexes

Fig. 2 57

O

Fig. 3 58

Fig. 4(1) 59

0.5"

Fig. 60

Fig. 5(i) 61

-1.0

Fig. 5(ii) 62

ev po

dg cc

Fig. 6 63

-0.3

Fig. 7(1) 64

Fig. 7(ii) Fig. 8 67 Reports published by the National Board for Spent Nuclear Fuel, SKN

NAK RAPPORT 1 SAMMANSTÄLLNING AV NUKLEÄRA KÄLLDATA FOB ANVÄNT KÄRNBRÄNSLE (Compilation of Nuclear Source Data for Spent Nuclear Fuel) Göran Olsson Studsvik Energiteknik AB, april 1983 (19 pages of tables and diagrams, Swedish text)

NAK RAPPORT 2 MATERIAL OCH PROCESSUTVECKLING FOR DIREKT SLUTFÖRVARING AV ANVÄNT KÄRNBRÄNSLE (Material and Process Development of Ceramic Encapsulation of Spent Nuclear Fuel for Disposal) Caj Airola Studsvik Energiteknik AB, november 1983 (119 pages mainly on engineering aspects of ceramic encapsulation, in Swedish)

NAK REPORT 3 ENCAPSULATION OF SPENT NUCLEAR FUEL IN CERAMIC MATERIALS. AN INTRODUCTORY STUDY S Forsberg, T Westermark Department of Nuclear Chemistry, The Royal Institute of Technology, Stockholm, Sweden, March 1983 (37 pages mainly on material properties, in English)

NAK RAPPORT 4 INTERNATIONELL UTVECKLING FÖR KOMMERSIELL UPPARBETNING AV KÄRNBRÄNSLE, BEHANDLING AV UPPARBETNINGSAVFALL OCH ÅTERANVÄNDNING AV PLUTONIUM (International Development of Commercial Reprocessing of Spent Nuclear Fuel, Treatment of Reprocessing Wastes and Recycling of Plutonium) Göran Carleson, Åke Hultgren Studsvik Energiteknik AB, mars 1983 (44 p?<>es in Swedish) 68

NAK RAPPORT 5 SAMMANSTÄLLNING OCH VÄRDERING AV UTLÄNDSKA SÄKERHETSSTUDIER ÖVER SLUTFÖRVARING AV KÄRNBRÄNSLEAVPALL (Review af Foreign Safety Studies on Disposal cf Spent Nuckor Fuel Waste) Ragnar Gelin Studsvik Energiteknik AB, april 1983 (109 pages in Swedish)

NAK RAPPORT 6 NÅGRA HUVUDINRIKTNINGAR INOM MODERN RISKTEORI (Some Main Trends in Recent Development of Risk Theory) Nils-Eric Sahlin AB Episteme, Lund, juli 1983 (23 pages in Swedish)

NAK RAPPORT 7 RISK PÅ LÅNG SIKT OCH DISKONTERING AV RISK (Risk in the Long Term and Discounting of Risk) Bengt Hansson AB Episteme, juli 1983 (17 pages in Swedish)

NAK RAPPORT 8 RISK PÅ LÅNG SIKT OCH DISKONTERING AV RISK I SAMBAND MED FÖRVARING AV RADIOAKTIVT MATERIAL (Long Term Risk and Discounting of Risk in Relation to Disposal of Radioactive Materials) Ola Svenson, Gunnar Karlsson Stockholms universitet, september 1983 (43 pages in Swedish)

NAK RAPPORT 9 UPPFÖLJNING AV UTLÄNDSKA SÄKERHETSSTUDIER ÖVER SLUTFÖRVARING AV KÄRNBRÄNSLEAVFALL (Review of Foreign Safety Studies on Disposal of Spent Nuclear Fuel Waste) Ragnar Gelin Studsvik Energiteknik AB, juni 1984 (40 pages in Swedish) 69

NAK RAPPORT 10 INTERNATIONELL UTVECKLING AV MELLANLAGRING OCH UPPARBETNING AV ANVÄNT KÄRNBRÄNSLE SAMT PLUTONIUMÅTERFÖRING (International Development of Intermediate Storage and Reprocessing of Spent Nuclear Fuel and on Recycling of Plutonium) G Carleson, Å Hultgrcn Studsvik Energiteknik AB, augusti 1984 (56 pages in Swedish)

NAK REPORT 11 THE DISPOSAL OF HIGH-LEVEL RADIOACTIVE WASTE 1984, Vol. I and D Frank L Parker, Robert E Broshears, Janos Pasztor The Beijer Institute of the Royal Swedish Academy of Sciences October 1984 (Vol 1116 pages. Vol II209 pages, in English)

NAK RAPPORT 12 KÄRNBRÄNSLENÄMNDENS FORSKNINGSINVENTERING 1983-1984 DELI SLUTRAPPORT DEL II FORSKARUTLÅTANDEN (An inventory of research needs Vol I Summary and Conclusions, 30 pages Vol II Research proposals, 196 pages) The report identifies knowledge gaps and presents proposals on items which ought to be addressed in the on-going Swedish research programme on disposal of spent nuclear fuel (in Swedish) Gas-Otto Wene et al, december 1984

NAK RAPPORT 13 RISKFORSKNING OCH BESLUT OM RISKER (Risk Research and Decisions about Risks) Sven Ove Hansson, februari 198S (14 pages in Swedish)

NAK RAPPORT 14 UPPFÖLJNING AV UTLÄNDSKA SÄKERHETSSTUDIER ÖVER SLUTFÖRVARING AV KÄRNBRÄNSLEAVFALL (Review cf Foreign Safety Studies on Disposal of Spent Nuclear Fuel Waste) Ragnar Gelin Studsvik Energiteknik AB, april 1985 (37 pages in Swedish) 70

NAK RAPPORT 15 UPPFÖLJNING AV DEN INTERNATIONELLA UTVECKLINGEN AV MELLANLAGRING OCH UPPARBETNING AV ANVÄNT KÄRNBRÄNSLE (International Development of Intermediate Storage and Reprocessing of Spent Nuclear Fuel and on Recycling of Plutonium) Göran Carieson Studsvik Energiteknik AB, juni 1985 (39 pages in Swedish)

SKN REPORT 16 NAK WP-CAVE PROJECT • REPORT ON THE RESEARCH AND DEVELOPMENT STAGE, MAY 1984 TO OCTOBER 1985 Boliden WP-Contech AB, November 1985 (129 pages in English)

SKN REPORT 17 TECHNICAL AND SOCIO-POLITICAL ISSUES IN RADIOACTIVE WASTE DISPOSAL 1986 Vol I Safety, Siting and Interim Storage (184 pages) Vol IA Appendices (197 pages) Vol II Subseabed Disposal (95 pages) Frank L Parker, Roger E Kasperson, Tor Leif Andersson, Stephan A Paiker The Beijer Institute of the Royal Swedish Academy of Sciences, November 1987

SKN RAPPORT 18 INTERNATIONELL UTVECKLING INOM KÄRNBRÄNSLE- CYKELN MED TYNGDPUNKT PÅ TRANSPORTER OCH SAFEGUARDS {International Development within the Spent Nuclear Fuel Cycle with Emphasis on Transports and Safeguards) Karin Brodén, Göran Carieson, Åke Hultgren Studsvik Energiteknik AB, november 1987 (52 pages in Swedish)

SKN REPORT 19 RISK DECISIONS AND NUCLEAR WASTE Sven Ove Hansson The National Board for Spent Nuclear Fuel, November 1987 (38 pages in English)

SKN REPORT 20 TIME ORIENTATION, PLANNING HORIZONS AND RESPONSIBILITY INTO THE FUTURE Ola Svenson, Göran Nilsson University of Stockholm, January 1988 (40 pages in English) 71

SKN REPORT 21 PSYCHOLOGICAL ASPECTS OF NUCLEAR WASTE DISPOSAL: Long time perception and the question of discounting of risks Gunnar Karlsson, Ola Svenson University of Stockholm, January 1988 (40 pages in English)

SKN RAPPORT 22 KÄRNKRAFT OCH RADIOAKTIVT AVFALL: Riskperception och attityder hos gymnasieelever i Stockholm (Nuclear Power and Radioactive Waste: Risk Perceptions and Attitudes of High-school Students in Stockholm, Sweden) Britt-Marie Drottz, Lennart Sjöberg Psykologisk Metod AB, Stockholm, januari 1988 (ISO pages in Swedish)

SKN RAPPORT 23 ATTITYDER TILL RADIOAKTIVT AVFALL (Attitudes to Radioactive Waste) Lennart Sjöberg, Britt-Marie Drottz Psykologisk Metod AB, Stockholm, januari 1988 (160 pages in Swedish)

SKN RAPPORT 24 OPINIONSBILDNING I KÄRNKRAFTENS AVFALLSFRÅGA: Mediestudierna - teoretiska utgångspunkter och empiriska undersökningar (Formation of Public Opinion on the Question of Nuclear Waste: Media Studies - Theoretical Premisses and Empirical Investigations) Kent Asp, Per Hedberg Göteborgs Universitet, juni 1988 (12 pages in Swedish)

SKN RAPPORT 25 SAMHÄLLSDEBATT I ENERGI- OCH KÄRNKRAFTSFRÅGOR: Utbudet av skrifter och tidskriftsartiklar om kärnavfall, kärnkraft och energiförsörjning under åren 1971-1987 (Public Debate on Questions of Energy and Nuclear Power: Books and Articles on Nuclear Waste, Nuclear Power and Power Supply during the Years 1971-1987) Monica Djerf Göteborgsuniversitetjuni 1988 (46 pages in Swedish) 72

SKN RAPPORT 26 BIBLIOGRAFI: Utbudet av skrifter och tidskriftsartiklar om kärnavfall, kärnkraft och energiförsörjning under åren 1971-1987 (BIBLIOGRAPHY: Books and Artides on Nuclear Waste, Nuclear Power and Power Supply during the Years 1971-1987) Monika Djerf, Per Hedbets Göteborgs universitet, juni 1988 (44 pages in Swedish)

SKN RAPPORT 27 INTERNATIONELL UTVECKLING INOM KÄRNBRÄNSLE- CYKELN (International Development within the Spent Nuclear Fuel Cycle) Karin Brodén, Åke Hultgrcn Studsvik Nuclear, mars 1988 (80 pages in Swedish)

SKN RAPPORT 28 ETIK OCH KÄRNAVFALL: Rapport frän ett seminarium om ETISKT HANDLANDE UNDER OSÄKERHET i Stockholm den 8-9 september 1987 (Ethics and Nuclear Waste: A report from a seminar on Ethical Action in the Face of Uncertainty in Stockholm, Sweden, September 8-9,1987) Mars 1988 (210 pages in Swedish)

SKN REPORT 29 ETHICAL ASPECTS ON NUCLEAR WASTE: Some points discussed at a seminar on Ethical Action in the Face of Uncertainty in Stockholm, Sweden, September 8-9,1987 April 1988 (14 pages in English)

SKN RAPPORT 30 SVENSKA FOLKETS ÅSIKTER OM KÄRNKRAFT OCH SLUTFÖRVARING EFTER TJERNOBYL (Opinions of the Swedish People on Nuclear Power and Final Disposal of Nuclear Waste after Chernobyl) Sören Holmberg Göteborgs universitet, oktober 1988 (41 pages in Swedish) 73

SKN RAPPORT 31 INTERNATIONELL UTVECKLING INOM KÄRNBRÄNSLE- CYKELN (International Development within the Nuclear Fuel Cycle) Karin Brodén, Ragnar Gelin Studsvik Nuclear, februari 1989 (60 pages in Swedish)

SKN RAPPORT 32 TEKNIK OCH POLITIK KRING FÖRVARING AV RADIO- AKTIVT AVFALL. EN INTERNATIONELL JÄMFÖRELSE (Short verson in Swedish of SKN Report 17) Tor Leif Andersson Mars 1989 (24 pages in Swedish)

SKN RAPPORT 33 INTERNATIONELL UTVECKLING INOM KÄRNBRÄNSLE- CYKELN (International Development within the Nuclear Fuel Cycle) Ingrid Aggeryd, Karin Brodén, Ragnar Gelin Studsvik Nuclear, januari 1990 (63 pages in Swedish)

SKN RAPPORT 34 FINNS DET SÄKRA SVAR? Rapport frän ett seminarium om DEN NATURVETENSKAPLIGA KUNSKAPSBASEN FÖR SLUTFÖRVARINGEN AV DET ANVÄNDA KÄRNBRÄNSLET i Saltsjöbaden den 5 - 7 september 1989 (Are there Safe Answers? A Report from a Seminar on the Scientific Basis for the Disposal of Spent Nuclear Fuel in Sweden, Saltsjöbaden 5 - 7 September 1989.) Tor Leif Andersson (ed.) Tellus Energi AB, mars 1990 (296 pages in Swedish)

SKN REPORT 35 VIEWS ON THE CALCULATION OF FLOW AND DISPERSION PROCESSES IN FRACTURED ROCK Lennart Jönsson University of Lund, March 1990 (57 pages in English)

SKN REPORT 36 WATER FLOW CHARACTERISTICS OF ROCK FRACTURES Lennart Jönsson Univeisity of Lund, March 1990 (54 pages in Swedish) 74

SKN REPORT 37 MODELLING OF FLOW AND CONTAMINANT MIGRATION IN SINGLE ROCK FRACTURES Peter Dahlblom, Lennart Jönsson University of Lund, March 1990 (54 pages in Swedish)

SKN REPORT 38 REPROCESSING IN SWEDEN: HISTORY AND PERSPECTIVE Å Hultgren, C-G Osterlundh Studsvik Nuclear, August 1990 (32 pages in English)

SKN RAPPORT 39 SEDIMENTÄR BERGGRUND SOM HYDRAULISK BARRIÄR, FÖRSTUDIE (Sedimentary Rocks as Hydraulic Barrier) Kaj Ahlbom, Jan-Erik Andersson, Leif Carlsson, SvenTirén, Anders Winberg Sveriges Geologiska AB, december 1990 (110 pages in Swedish)

SKN RAPPORT 40 EN KVALITATIV STUDIE I LEKMÄNS UPPLEVELSE AV RISK I SAMBAND MED OMHÄNDERTAGANDE AV KÄRNAVFALL (A Qualitative Study ofLaymens' Experiences of Risk in Connection With Storage of Nuclear Waste) Gunnar Karlsson, Anneli Ljungberg Stockholms universitet, december 1990 (36 pages in Swedish)

SKN REPORT 41 AN OVERVIEW OF DECISION THEORY Sven Ove Hansson Uppsala University, January 1991 (60 pages in English)

SKN RAPPORT 42 SKNS GRANSKNING AV FÖRUNDERSÖKNINGARNA INFÖR BYGGANDET AV ÄSPÖLABORATORIET (SKN's review of the preinvestigations for the Äspö Hard Rock Laboratory) Kai Palmqvist, BERGAB Tommy Olsson, GoldcrGeosystcm AB Januari 1991 (108 pages in Swedish) 75

SKN RAPPORT 43 GEOGASTRANSPORTI BERG, FÖRSTUDIE (Geogas - a carrier or a tracer?) Hans-Peter Hermansson, Jan Chyssler, Studsvik AB Gustav Åkerblom, Anders Linden, Sveriges Geologiska AB Januari 1991 (43 pages in Swedish)

SKN RAPPORT 44 INTERNATIONELL UTVECKLING INOM KÄRNBRÄNSLE- CYKELN (International Development within the Nuclear Fuel Cycle) Ingrid Aggeryd, Åke Hultgren, Karin Brodén Studsvik Nuclear, feb"iar 1990 (44 pages in Swedish)

SKN RAPPORT 45 OSÄKERHET OCH BESLUT, RAPPORT FRÅN ETT SEMINARIUM OM BESLUT UNDER OSÄKERHET I ANKNYTNING TILL KÄRNAVFALLSFRÅGAN PÅ HÄSSELBY SLOTT, DEN 4-6 APRIL 1990 (Decisions in the Face of Uncertainty. A report from a seminar on decisions in the face of uncertainty in connection with nuclear waste in Stockholm, Sweden, April 4-6,1990) Redaktörer: Christian Munthe och Folke Tersman Statens kämbränslenamnd, februari 1991 (100 pages in Swedish)

SKN RAPPORT 46 RISKUPPLEVELSER I SAMBAND MED LOKALISERING AV ETT SLUTFÖRVAR FÖR ANVÄNT KÄRNBRÄNSLE (Experiences of Risk in Connection with Site Selection for a Repository for Spent Nuclear Fuel) Anders Biel och Ulf Dahlstrand Göteborgs universitet, mars 1991 (57 pages in Swedish)

SKN RAPPORT 47 KÄRNAVFALLSFRÅGAN I SVENSKA MASSMEDIER (The Nuclear Waste Issue in Swedish Mass Media) Per Hedberg Göteborgs universitet, april 1991 (74 pages in Swedish) 76

SKN REPORT 48 THE IMPACT OF PARTY ON NUCLEAR POWER ATTITUDES IN SWEDEN Sören Holmberg Göteborg University, April 1991 (38 pages in English)

SKN RAPPORT 49 KÄRNAVFALL OCH SAMHÄLLSFORSKNING (Nuclear Waste and Social Science Research) Christian Munthe och Folke Tersman Statens kämbränslenämnd, april 1991 (70 pages in Swedish)

SKN RAPPORT 50 KVINNOR, MÄN OCH KÄRNKRAFT (Women, Men and Nuclear Power) Maria Oskarson Göteborgs universitet, augusti 1991 (60 pages in Swedish)

SKN REPORT 51 GEOGAS - A CARRIER OR A TRACER? Hans-Peter Hermansson, Jan Chyssler, Studsvik AB Gustav Åkerblom, Anders Linden, Sveriges Geologiska AB October 1991 (66 pages in English)

SKN REPORT 52 GEOGAS IN CRYSTALLINE BEDROCK Hans-Peter Hermansson, Studsvik AB Rolf Sjöblom, Statens kämbranslenämnd Gustav Åkerblom, Sveriges Geologiska AB, nu pä SSI October 1991 (32 pages in English)

SKN REPORT 53 IS THERE A DEFINITIVE ANSWER? The Scientific Base for the Final Disposal of Spent Nuclear Fuel. A condensed summary based on a report (SKN 34) of a seminar held at Saltsjöbaden, Sweden on September 5-7th, 1989. J. A. T. Smellie Conterra AB, October 1991 (43 pages in English) 77

SKN REPORT 54 SPECIALIST MEETING ON ACCELERATOR-DRIVEN TRANSMUTATION TECHNOLOGY FOR RADWASTE AND OTHER APPLICATIONS 24-28 June 1991, Saltsjöbaden, Stockholm Sweden Compiled by: R. A. Jameson Los Alamos National Laboratory, USA, November 1991 (798 pages in English)

SKN REPORT 55 NUCLEAR WASTE MANAGEMENT REVIEW WORK - Part of the Decision Making Process Proceedings from a symposium in connection with the 10 year anniversary of the Swedish National Board for Spent Nuclear Fuel Stockholm, Sweden, September 25-26,1991. Statens kärnbränslenämnd, February 1992 (145 pages in English)

SKN RAPPORT 56 INTERNATIONELL UTVECKLING INOM KÄRNBRÄNSLECYKELN (International Development within the Nuclear Fuel Cycle) Ingrid Aggeryd och Karin Brodén Studsvik Nuclear, december 1991 (71 pages in Swedish)

SKN REPORT 57 STABILITY OF METALLIC COPPER IN THE NEAR SURFACE ENVIRONMENT Örjan Amcoff and Katalin Holényi Uppsala University, December 1991 (77 pages in English) The Swedish nuclear power programme

After the 1980 referendum on nuclear power, the Riksdag decided that nuclear power in Sweden would be phased out no later than the year 2010 and that the number of reactors would be limited to twelve. Since 198S, these reactors have all been in operation at the nuclear power plants in Barsebäck, Forsmark, Oskarshamn and Ringhals.

Different kinds of radioactive waste

Different kinds of rodioocnve woste ore generoted during the operation of a nuclear power plant - low-level waste, intern»ediote level waste and high-level waste.

LOW- AND INTERMEDIATE-LEVEL WASTE Low- and intermediate-level waste arising from the continuous operation of a nuclear power plant arc known by the common name of reactor waste. Reactor waste consists of scrap material and metal, protective matting, clothing and suchlike which are used within the controlled areas of the nuclear power plants. This waste also consists of Filter material which is used to trap radioactive substances in the reactor coolant. The radiation level of low-level waste is so low that it can be handled without any particular safety measures and so it is packed in plastic bags or sheet metal drums. However, certain protective measures arc required when handling intermediate-level waste. This waste is cast in concrete or asphalt. In the spring of 1988, SFR (the final repository for reactor waste) was taken into service. SFR is located under the seabed near to Forsmark nuclear power plant. The utilities plan to deposit all reactor waste as well as low- and intermediate-level waste from decommissioning in SFR.

HIGH-LEVEL WASTE High-level waste mainly consists of spent nuclear fuel, i .c. fuel elements in which so many of the fissile atoms arc spent that the elements can no longer be used. However, the spent fuel still generates heat on account of its radioactivity and must be cooled. The fuel is, therefore, stored in special pools Tilled with water in the reactor building for at least one year. The fuel is then transported by a specially built ship, called Sigyn, to CLAB (the central interim storage facility for spent nuclear fuel), located close to Oskarshamn nuclear power plant. CLAB was taken into service 1986. Since the radiation level is very high, the fuel is transported in specially built containers. The walls of the containers arc made of thick steel so as to shield the personnel and surroundings from harmful radiation and to protect the fuel from damage. The fuel is then placed in storage pools in an underground room at CLAB where it will be stored for at least forty years. During this time, the radioactivity and the heat generated by the fuel will decline thereby facilitating handling and disposal of the fuel. THE NATIONAL BOARD FOR SPENT NUCLEAR FUEL

One of the main tasks of the National Board for Spent Nuclear Fuel (SKN) is to review the utilities' research and development programme for the management of spent nuclear fuel and for the decommissioning of the nuclear power plants. The Board also supervises the way in which the utilities carry out the programme, in order to accomplish this task, The Board keeps abreast with international research and de- velopment work within the area and initiates such research that is important to its own supervisory functions. The research conducted by the Board is both scientific/ technical and sociological in nature. The results from this research are published in the SKN Reports series. A list of published reports is available at the end of each publication. Another of the Board's main tasks is to handle issues concerning the financing of costs within the area of nuclear waste. Each year, the Board estimates the size of the fee to be paid by the utilities to cover the current and future costs of waste manage- ment. The proposal on fees for the coming year is reported in SKN PLAN, which is submitted to the government before the end of October. The Board is also responsible for seeing that the public is granted insight into the work on the safe disposal of spent nuclear fuel. The Board will continually issue short publications on this matter in the series, DISPOSAL OF SPENT NUCLEAR FUEL. The following publications have so far been issued:

1. Comments on the research programme for 1986. (In Swedish) 2. (Now replaced by number 5) 3. How do we choose a suitable site for a final repository? (In Swedish) 4. Radioactive waste: technology and politics in six countries. (In Swedish) 5. This is how nuclear waste management is financed. (In Swedish and English) 6. Evaluation of SKB's research programme 89. (In Swedish) 7. 100 questions and answers concerning spent nuclear fuel. (In Swedish)

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NATIONAL BOARD FOR SPENT fJUCLF AR FUEL

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