Thermochemical Study of Hardening Cement Mixtures Which Contained Simulated Radioactive Waste by Co. SEP

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Thermochemical Study of Hardening Cement Mixtures Which Contained Simulated Radioactive Waste P.V. Kozlov1, O.M. Slyunchev1, K.V. Kirʹyanov2, I. V. Myalkin2, A.V. Knyazev*2 Federal State Unitary Enterprise Mayak Production Association, str. Lenina, 31, 456780, Ozersk, Russia

N.I. Lobachevsky State university of Nizhni Novgorod , Gagarin Prospekt 23/2,603950, Nizhni Novgorod, Russia

knyazevav@gmail.com (A. Knyazev)

Abstract Heat generation of cement mixtures of various compositions was measured with the calorimetric method during hardening of solutions, which simulated liquid medium‐level wastes (MLW) from the radiochemical plant and distillation residues obtained from evaporation of Nuclear Power Plant (NPP) liquid radioactive wastes. Thermal capacity was determined and heat generation was calculated for the processes of hydration and solidification of cement mixtures. Impact of dry mixture and solution compositions on heat generation was determined, as well as that of the solution‐cement ratio. A scaled‐up experiment was carried out to solidify a 4 m3 block of compound using the developed cement mixture with reduced heat generation. It was found that at hardening, the temperature in the specimen didn’t exceed 57ºС. This fact confirmed the efficiency of the measures undertaken to decrease heating up of the compound in the course of hardening. Applicability of the cementation technology was considered in relation to NPP distillation residues with subsequent storage of the obtained compound in pour‐ type compartments. It was demonstrated that the developed dry mixture compositions ensured compliance of the cement compound with the respective process and regulation requirements during NPP distillation residues hardening. Results of the study will help verify and, if necessary, adjust the mathematical model used in preliminary calculations to describe the process of storage compartment heating. It will allow developing a safe mode of filling compartments of the storage facility with the cement compound containing solidified MLW. Keywords Medium‐level Waste; Cementation; Specific Heat Generation; Heating; Calorimetric Studies

Introduction Cementation is the primary way for hardening distillation residues resulted from evaporation of NPP liquid medium‐level and low‐level wastes. It can be explained by a relative simplicity of practical realization of cementation and by a satisfactory quality of the produced cement compound [1]. Mayak PA intends to use the cementation technology to solidify liquid medium‐level waste (MLW) from the radiochemical plant. A storage facility of pour ‐ type is planned to be used at the cementation complex now under construction at the Mayak PA to locate the cement compound with solidified radioactive wastes. It will be a surface construction consisting of 100 reinforced concrete compartments to be filled with cement grout. A compartment (5m L x 5m W x 11m H) will contain about 260 m3 of solidified waste. Annually, it is planned to fill 4 ‐ 6 such compartments with a total volume of up to 1600 m3. Filling compartments with the cement mixture will be performed successively so that to ensure optimum temperature conditions in the storage facility. When the operating life (20‐50 years) of the MLW cementation complex comes to an end, all equipment and easily detachable building structures located above the storage facility will be dismantled, while remaining auxiliary rooms, openings, corridors and other voids will be filled with cement. The storage facility will be isolated and then transferred to a category of repository. Despite obvious advantages of the pour ‐ type storage facilities, there are numerous specific requirements imposed upon the given technology:

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

reduced hydration heat of the cement mixture due to hindered heat removal from the block; temperature of the compound on the boundary with a reinforced concrete wall of a storage facility compartment should not exceed 363 К;



mobility of the cement grout should ensure uniform filling of storage facility compartments;



the compound shouldn’t break down in the course of concrete setting, water gain on its surface shouldn’t exceed 1 % by volume.

Complying with these requirements is to be carried out in parallel with meeting the regulatory requirements to the cement compound. Consequently, there is a need to thoroughly select parameters of hardening. It should be noticed that, due to a large volume of the block, solidified waste located into the pour ‐ type storage facility could not be removed, transferred or put into another container. Therefore, design of the storage facility and characteristics of the solidified compound are to comply in full with the most up‐to‐date and strict requirements to a similar type of objects. Nomenclature m: amount of the dry mixture taken for the experiment; V: volume of NaNO3 solution with a concentration of 3.53 mole/l, which was used for the mixture solidification; SCR: the solution‐cement ratio; ‐∆hardН0 : change in a standard enthalpy (exothermal heat effect) in the course of hardening of the mixtures under study; Wmax.1 : maximum value of heat generation (heating capacity) for a given mixture composition. Experimental This work demonstrates results of the studies aimed at the development of cement compound compositions ensuring reduced heat generation in the course of the cement compound hardening. Reduction in heat generation was achieved by replacing a larger part of Portland cement in the compound with low‐calcium ashes from Thermal Power Plants (the Argayash TPP and the Reftinsk State‐owned Power Plant, which are located near the Mayak Production Association), with clay additives, and also by an increase in salts concentration in wastes. Portland cement in a dry mixture accounted for 20 ‐ 30 mass %. Necessity to comply with the regulatory requirements concerning the durability of the hardened compound (not less than 50 kg/cm2) determined the minimal content of Portland cement (20%) in the dry mixture of the cement compound. Salts concentration in the hardening solution (primarily, that of sodium nitrate) ranged from 300 tо 600 g/l. This high solution ‐ cement ratio coupled with the use of plasticizer С‐3 in the amount of 0.3 mass % ensured the appropriate mobility of the cement mixture (20‐23 cm in terms of the slump cone spread). At that, water gain accompanying the cement mixture setting did not exceed 1 % by volume. Due to the use of bentonite and cloniptilolite in the dry mixture composition (in the amount of 10 % mass), and due to the preliminary solution treatment followed by the calcium carbonate precipitation in an alkaline region of pH (10‐12), radionuclides were firmly fixed in the compound [2]. Leaching rate for 137Cs and 90Sr, determined according to the GOST R 52126‐2003 standard, was (2‐6)∙10‐4 g/cm2∙day, and that for 241Am was 2∙10‐5 g/сm2∙day. Leaching degree for 137Cs and 90Sr did not exceed 10 %, and that for 241Am did not exceed 0.5 %. The cement compound obtained with the use of these additives and component ratios satisfied the regulatory requirements of water resistance, low temperature resistance and resistance to radiation damage [3]. While considering the process of NPP waste solidification, note should be taken that chemical composition of NPP distillation residues is quite specific. They include sodium borate, sodium and potassium hydroxides in significant concentrations (tens and hundreds grams per liter), oxalates, complexones (for ex., EDTA) and surface active substances (SAS, for ex., ОP‐10) [4, 5]. The above ‐ mentioned fact significantly complicates the processes of hydration and solidification of the cement material and may negatively affect the cement compound properties. It is known that tetraborates considerably slow down the cement hydration [4]. Therefore, for hardening in this work, a solution simulating distillation residues resulted from NPP LRW evaporation was used with a composition selected on the basis of data from works [4‐6]. The composition is provided in Table 1 (composition 7).

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TABLE 1 THE DRY MIXTURES UNDER STUDY AND BASIC CONDITIONS OF THEIR BLENDING WITH HARDENING SOLUTIONS

Comp Mixtu osition re No. PC‐ No. 400

Component content in the mixture, % ChBFS

Clinoptilolite

Bentonite

Aerosil

Ash R.

Ash А.

Composition of the hardening solution, g/l

SCR, ml/g

99.7

NaNO3‐300

0.35

29.7

NaNO3‐300

0.60

49.7

NaNO3‐300

0.65

NaNO3‐300, CaCO3 ‐5, IER NaNO3‐600, CaCO3 ‐5, IER NaNO3‐300, CaCO3 ‐5 H3BO3‐ 110, NaNO3 ‐180, Na2SO4‐8, NaCl‐1, NaOH‐ 80, KOH‐20, Na2C2O4‐1, EDTA‐1, (ОP‐10) ‐1, Ca(OH)2‐5

0.65

49.7

NaNO3‐600

0.65

1.5

48.2

NaNO3‐300

0.60

69.7

NaNO3‐300

0.55

79.7

99.7

NaNO3‐300 NaOH‐100, CaCO3 ‐5, IER NaNO3‐600 NaOH‐100, CaCO3 ‐5, IER NaNO3‐300 NaOH‐100, CaCO3 ‐5 NaNO3‐300, NaOH‐100

0.65 0.65

0.55

0.55 0.55 0.55 0.35

Table 1 presents compositions of all the dry mixture samples under study and main conditions of their blending with hardening solutions. The following abbreviations were introduced: 

PC‐400 –Portland cement of grade 400;



ChBFS –crushed Chelyabinsk blast furnace slag;



ash R. and ash А. – flue ashes from the Reftinsk State‐owned Power Plant and the Argayash TPP, correspondingly;



IER – a mixture of neutralized swollen ion‐exchange resins КU‐2‐8 and АV‐17‐8;



ОP‐10 –a surface–active substance, component of washing solutions with the composition C18H17‐C6H4O‐ (CH2‐CH2O)10‐CH2CH2OH;



SCR – the solution ‐ cement ratio (a volume of solution taken for preparing cement grout with 1 gram of dry mixture).

For comparison reasons, a solution, which simulated MLW of the radiochemical plant and contained sodium nitrate in the concentration of 300 g/l, was used in the experiments. In order to provide additional fixation of radionuclides before hardening, calcium nitrate and sodium carbonate were additionally introduced into solutions in a dry form to precipitate carbonate, or calcium hydroxide (in a solution simulating distillation residues). At that, formation of 5 g/l of precipitate was expected. After precipitation, with the help of sodium hydroxide, a pH value in the MLW simulating solution was adjusted to 11. In all cases, the dry mixture contained super‐plasticizer C‐3 in the amount of 0.3 mass %. The С‐3 super‐plasticizer is an organic synthetic substance based on the product of condensation of naphthalene sulfonic acid and formaldehyde, which contains admixtures of sodium sulfate and resinous substances. Hardening solutions with IER were prepared in the form of sludge. To prepare the sludge, a mixture of neutralized resins KU‐2‐8 and АV‐17‐ 8 was taken in the 4:5 mass ratio. After thorough blending, water was added to the mixture. In a day, water was

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decanted. Remaining water was removed by a pipette. Finally, jelly‐like water‐swollen IER was obtained. Before preparing hardening solutions with IER sludge using Table 1 data, precipitation of CaCO3 was carried out in NaNO3 solution by introducing Ca(NO3)2 and Na2CO3 (at that precipitation of CaCO3 in the amount of 5 g/l was expected). Then just prepared solutions were adjusted to a рН value of 11 by introducing NaOH solution (for compositions of mixtures 4‐6), or by introducing dry NaOH (for compositions of mixtures 11‐13) to achieve NaOH concentration in the solution of about 100 g/l. At hardening a mixture of composition 7, the precipitation of Ca(OH)2 in NaNO3 solution was performed at the final stage of the solution making by introducing Ca(NO3)2 in the required amount. Then a mixture with IER was prepared in accordance with the below stated component ratios. In case of solidification of compositions 4 and 11, NaNO3 solution was mixed with IER sludge in the volume ratio 7.2:1 (resin was added by mass, its volume being recalculated considering its density of 1.07 g/cm3). While working with compositions 5 and 12, NaNO3 solution was mixed with IER sludge in the volume ratio 3.6:1. All salt solutions were prepared by introducing “chemically pure” or “analytically pure” dry components (salts) into distilled water. The sludges prepared by the way described simulated wastes of the Radiochemical Plant, which were intended for cementation. Before blending with a hardening solution, the dry mixture was thoroughly stirred. Then a sample of the dry mixture was put into a glass vial with a thin bottom. The vial with the substance was sealed with paraffin and secured in a thin‐walled Teflon holder of a special design [7]. Then the vial, secured as described above, was placed into an upper part of the reaction cell of the calorimeter thermostat. The reaction cell was filled beforehand with a required amount of simulating solution, which complied with the selected composition and set solution‐cement ratio (Table 1). Once thermal equilibrium was established between the calorimetric block and the cell with the substances under study, reacting agents were mixed in one of the reaction cells by breaking the thin lower part of the glass vial over the bottom of the reaction cell. This was done by pushing the ceramic rod going beyond the calorimeter, which in the lower end was connected via the Teflon holder with the glass vial. After the vial breaking, the spring‐actuated ceramic rod with remnants of the upper part of the vial returned to the original position. The same actions resulted in stirring of the reaction mixture. At that, remnants of the glass vial acted as a stirrer. Another reaction cell was used as a comparison cell. Stirring was performed till the mixture was hardened. All experiments were carried out at a temperature of 298.15 К [8‐10]. Results and Discussion Table 2 and Fig. 1‐5 present data obtained from experiments, in which only solutions with different NaNO3 concentrations were used to harden the mixtures under study. TABLE 2 RESULTS OF THE EXPERIMENTS AIMED AT DETERMINATION OF HARDENING ENTHALPY FOR THE MIXTURES UNDER STUDY, 298.15 К

‐hardН0, J/ (g of dry mixture)

Wmax1, mW/ (g of dry mixture)

0.35

133

11.8

0.68

200

13.2

0.35

140

12.0

0.60

95.7

14.1

0.601

0.60

96.9

14.5

0.6850

0.445

0.65

82.6

20.1

1.0011

0.651

0.65

83.9

20.4

0.7025

0.456**

0.65

75.1

15.5

m(of dry mixture), g

V(NaNO3, 300 g/l), ml

SCR, ml/(g of dry mixture)

Experiment No.

Dry mixture No.*

0.8501

0.297

0.5276

0.357

0.9997

0.350

0.8649

0.513

1.0008

0.9949

0.647**

0.65

67.0

16.2

0.4874

0.314**

0.65

66.5

17.8

0.6266

0.374

0.60

61.8

22.4

0.9886

0.593

0.60

62.0

22.2

0.9763

0.537

0.55

59.5

13.1

1.0063

0.352***

0.35

78.9

2.3

1.4090

0.493***

0.35

78.5

2.3

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Designations used in Table 2 are: 

m (of dry mixture) –amount of the dry mixture taken for the experiment;



V (NaNO3, 300 g/l) – volume of NaNO3 solution with a concentration of 3.53 mole/l, which was used for the mixture solidification;



SCR – the solution‐cement ratio;



‐∆hardН0 – change in a standard enthalpy (exothermal heat effect) in the course of hardening of the mixtures under study;



Wmax.1 – maximum value of heat generation (heating capacity) for a given mixture composition.

At blending the studied mixture samples with NaNO3 solution, two heat generation regions were observed, which were characterized by maximums of thermal capacity Wmax1 and Wmax2. At that, for all mixtures in the course of their hardening, after the first region a slowly growing small heat generation was recorded. Afterwards, it increased relatively quickly, reached Wmax2 and over some time it had constant heat release, which was equal to Wmax2 within the limits of measurement errors. Then there was a slow decrease in heat generation. Prolonged small heat generation before the second region can be associated with the presence of some “induction period”, which preceded the second region of heat generation. Therefore, the process of hardening of the mixtures under study could be split into three stages. Duration of each stage and Wmax values depended on a mixture composition and on a concentration of NaNO3 solution taken for hardening. Intensive heat generation with a clearly expressed maximum Wmax1 was observed at the first stage of hardening during the first 15 minutes (Fig. 1, Table 2). At that, a heat generation curve represented a ballistic curve typical for rapid processes with a duration of about 40 minutes, at the end of which heat generation decreased to values less than 0.5 mW/(g of dry mixture). The largest value of heat generation of 22.4 mW/(g of dry mixture) during the first hour of hardening was observed for mixture 4, containing highly dispersed microsilica (aerosil) (Fig. 1, experiment 11, Table 2) in addition to ashes from the Argayash Thermal Power Plant. The least thermal capacity of hardening of not more than 2.3 mW/(g of dry mixture) was recorded for slag‐alkaline cement (mixture 7, experiment 15, Fig. 1; experiments 14 and 15, Table 2).

FIG 1 CHANGE IN HEAT GENERATION (THERMAL CAPACITY W) OVER TIME (T) AT MIXTURE HARDENING (TABLE 1) IN THE FIRST 60 MINUTES CALCULATED PER GRAM OF DRY MIXTURE. NUMBERS OF COMPOSITIONS AND EXPERIMENTS FROM TABLE 2 ARE INDICATED ON CURVES

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It should be noted that inappropriate blending of the mixture and the hardening solution resulted in a considerable increase in the duration of the second stage and in a significant shift of Wmax2 in time (mixture 3, experiment 8, Fig. 2‐6).

FIG 2 CHANGE IN HEAT GENERATION (W) OVER TIME (T) AT MIXTURE HARDENING (TABLE 1) DURING 50 H CALCULATED PER GRAM OF DRY MIXTURE. NUMBERS OF MIXTURES AND EXPERIMENTS FROM TABLE 2 ARE INDICATED ON CURVES

FIG 3 CHANGE IN HEAT GENERATION (W) OVER TIME (T) AT MIXTURE HARDENING (TABLE 1) DURING A WEEK CALCULATED PER GRAM OF DRY MIXTURE. NUMBERS OF MIXTURES AND EXPERIMENTS FROM TABLE 2 ARE INDICATED ON CURVES

The second stage of hardening for PC was accompanied by slow heat generation with W values of about 0.23 at SCR = 0.35 (mixture 1, experiment 1) and of about 0.02 W/(g of dry mixture) at SCR = 0.68 ml/(g of dry mixture) (mixture 1, experiment 2, Fig. 2,3). Following 4 hours of hardening, there was an increase in heat generation, and during the third stage of hardening, the highest values of Wmax2 of about 1.3 W/(g of dry mixture) were recorded. Introduction of flue ashes and other highly dispersed mineral additives did not change the behavior of W curves, but resulted in an increased heat generation during the first and second process stages and a decreased W during

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its third stage.

FIG 4 CHANGE IN ENTHALPY (ΔН0) OVER TIME (T) AT MIXTURE HARDENING (TABLE 1) DURING 15 HOURS CALCULATED PER GRAM OF DRY MIXTURE. NUMBERS OF MIXTURES AND EXPERIMENTS FROM TABLE 2 ARE INDICATED ON THE CURVES

FIG 5 VARIATION IN ENTHALPY (ΔН0) OVER TIME (T) AT MIXTURE HARDENING (TABLE 1) DURING 50 HOURS CALCULATED PER GRAM OF DRY MIXTURE. NUMBERS OF MIXTURES AND EXPERIMENTS FROM TABLE 2 ARE INDICATED ON THE CURVES

When clinoptilolite and bentonite in the amount of 10 mass .% of each were added to the dry mixture simultaneously with the introduction of 29.7 mass.% of Reftinsk State‐owned power plant ash (mixture 2), at SCR values of about 0.6 ml/(g of dry mixture), there was an increase in heat generation and a growth of Wmax1 from 13.2 (mixture 1, experiment 2) tо 14.1 mW/(g of dry mixture) (experiment 4, Table 2, Fig. 1). Increasing Reftinsk SPP ash content in the cement compound up to 49.7 mass.% (mixture 3, composition 3) or its replacing with Argayash TPP ash in the amount of 48.2 mass.% together with adding 1.5 mass % of aerosil (mixture 4, composition 9, Table 1), under close mixing conditions, resulted in a sharp increase in heat generation and a growth of Wmax1 tо 20.1 (mixture 3, experiment 6) or 22.4 mW/(g of dry mixture) (mixture 4, experiment 11, Table 2, Fig. 1). Therefore, both clinoptilolite and Refrinsk SPP ashes together with PC participate in hydration processes at mixture hardening,

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which proves active participation of these additives in the formation of the compound structure already at the initial stage of the process.

FIG 6 VARIATION IN ENTHALPY (ΔН0) OVER TIME (T) AT MIXTURE HARDENING (TABLE 1) DURING ONE WEEK CALCULATED PER GRAM OF DRY MIXTURE. NUMBERS OF MIXTURES AND EXPERIMENTS FROM TABLE 2 ARE INDICATED ON THE CURVES

At SCR of about 0.65 ml/(g of dry mixture), a value of Wmax2 at the third stage of the cement compound hardening decreased from 1.3 for PC (mixture 1, experiment 1,2) to values of 0.79 (mixture 2, experiment 4) and 0.54 mW/(g of dry mixture) (mixture 3, experiment 6, Fig. 2, 3). It should be noted that for mixture 4 containing aerosil (experiment 11, Fig. 2‐5), the second stage of hardening was characterized by a short duration and relatively high values of heat generation. For this mixture exothermal processes of hardening which produced a typical ballistic heat generation curve in the first region with the largest maximum of intensity Wmax1 = 22.4 mW/(g of dry mixture) observed in 3 minutes, while, afterwards, constant heat generation was being observed for about 3 hours at a level of 0.7 mW/(g of dry mixture) (second stage) and for 10 hours at a level of 0.55 mW/(g of dry mixture) at the third stage of hardening. Then, a smooth decrease in heat generation occurred. Data obtained from Table 2 and Fig. 2‐5 prove that, under other equal conditions, enthalpy of the cement compound hardening depended directly on PC content in the dry mixture. Its replacement with Argayash TPP flue ashes in the amount of 69.7%, with a simultaneous introduction of 10 mass.% of bentonite, at SCR of about 0.6 ml/(g of dry mixture), led to a decrease in an absolute enthalpy value of the mixture hardening from 200 (mixture 1) tо 59.5 J/(g of dry mixture) (mixture 5) or from 119 tо 38 J/(g of cement grout). The same decrease of exoeffect was observed at an introduction of 48.2 mass.% of Argayash TPP ashes into PC with a simultaneous addition of clinoptilolite and bentonite in the amount of 10 mass.% each, and a small amount of aerosil (mixture 4). Therefore, cliniptilolite (one of the structural types of natural zeolite) also reduces exoeffect of the cement compound hardening. The same holds true for Argayash TPP ashes. This fact could be explained by their ability to absorb some water and by less complete flowing of cement hydration processes. The solution‐cement ratio has a considerable impact on exoeffect of hardening. A SCR increase at PC hardening (mixture 1, Table 2) from 0.35 tо 0.68 ml/(g of dry mixture) resulted in its growth from 135 tо 200 J/(g of dry mixture) or from 98.5 tо 119 J/(g of cement grout). For PC, relatively high values of Wmax2 in the second region of heat generation were observed in 15 and 20 hours, correspondingly, in experiment 1 at SCR = 0.35 ml/(g of dry mixture), and in experiment 2 at SCR = 0.68 ml/(g of dry mixture) (Fig. 2,3). Thus, without interfering into the mechanism of hardening, SCR slows it down and extends it in time. Increase of NaNO3 concentration in the hardening solution from 300 to 600 g/l (mixture 3, table 2) resulted in

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reduction of Wmax1 by 4 mW/(g of dry mixture) and decrease of hardening exoeffect from ‐83 to a mean value of ‐ (70±4) J/(g of dry mixture) (experiments 6‐10, table 2, figure 6). This fact can be explained by decreasing availability of water molecules needed for hydration in the presence of large amount of salts around which hydrate envelopes are generated. This fact is confirmed by the results obtained at determination of heating of cement blocks of considerable volume at hardening of solutions of various composition [9]. The process of slag‐alkaline cement hardening (mixture 7, experiment 15, fig. 2‐5) was accompanied by moderate heat generation, both at the first and the second stages of the hardening process. The result obtained in the experiments is that Wmax1 = 2.3 mW/(g of dry mixture). However, heat generation at the third stage of the process is comparable with that for the Portland cement compounds and amounts to Wmax2 = 0.6 mW/(g of dry mixture). And the shapes of the heat generation curves are analogous to those for the Portland cement mixtures suggesting similarity in hardening mechanisms of these types of cementing materials. Enthalpy variation during hardening of the crushed blast‐furnace slag based compound was equal to ∆hardН0 = ‐ (78.7± 0.4) J/(g of dry mixture) or ‐(58.3±0.4) J/(g of cement grout) (mixture 7, experiments 14,15), which agrees with data [1]. This quantity is significantly lower than ∆отвН0 = – (103±2) J/(g of mixture) obtained for the Portland cement (mixture 1, experiments 1, 3, table 2). Thus, in terms of heat generation, slag‐alkaline cements theoretically can be used along with Portland cement mixtures containing various additives (Table 1). Drawback of slag cements is connected with the need for arrangement of such processes as crushing, classification and monitoring of the slag composition. Figures 7‐12 and table 3 provide the results of the experiments that used the solutions simulating medium‐level wastes from the radiochemical plant for hardening of the studied mixtures. TABLE 3 THE RESULTS OF THE EXPERIMENTS AIMED AT DETERMINATION OF HARDENING ENTHALPY OF THE STUDIED MIXTURES AT 298.15 К

m (dry mixture), g

SCR, ml/g

‐hardН0, J/ (g mixture)

Wmax1., mW/ (g mixture)

0.3405

0.65

74.3

14.9

NaNO3‐300, CaCO3 ‐5, IER

0.584

0.65

75.0

15.0

0.6121

NaNO3‐600, CaCO3 ‐5, IER

0.398

0.65

28.8

11.2

0.9754

NaNO3‐600, CaCO3 ‐5, IER

0.634

0.65

28.9

11.5

0.4806

NaNO3‐300, CaCO3 ‐5

0.312

0.65

76.8

15.8

0.9879

NaNO3‐300, CaCO3 ‐5

0.642

0.65

76.6

15.9

0.494

0.55

16.2

12.2

0.309

0.55

16.6

12.0

0.551

0.55

16.9

12.5

0.548

0.55

48.9

10.1

0.3755

0.55

49.0

10.0

0.549

0.55

48.7

12.0

0.397

0.55

48.6

12.1

No of experime nt

No of mixture

0.5238

NaNO3‐300, CaCO3 ‐5, IER

0.8985

Solution composition, g/l

V (solution), ml

H3BO3‐ 110, NaNO3 ‐180, NaOH‐80, KOH‐20 ** H3BO3‐ 110, NaNO3 ‐180, NaOH‐80, KOH‐20 ** H3BO3‐ 110, NaNO3 ‐180, NaOH‐80, KOH‐20 ** NaNO3‐300 NaOH‐100, CaCO3 ‐5, IER NaNO3‐300 NaOH‐100, CaCO3 ‐5, IER NaNO3‐600 NaOH‐100, CaCO3 ‐5, IER NaNO3‐600 NaOH‐100, CaCO3 ‐5, IER

0.8989

0.5623

1.0021

0.9956

0.6828

0.9987

0.7216

0.5510

NaNO3‐300 NaOH‐100, CaCO3 ‐5

0.303

0.55

75.7

17.8

0.9989

NaNO3‐300 NaOH‐100, CaCO3 ‐5

0.549

0.55

75.6

17.7

Blending of the studied mixture samples with the above mentioned solutions was also associated with heat generation ranges that corresponded to the three stages of mixtures hardening. For all the mixtures in the course of their hardening, the first range of heat generation had a well‐defined heat generation peak Wmax1 (table 3, figure 7).

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Adding of CaCO3 and a mixture of neutralized swollen ion‐exchange resins KU‐2‐8 and AV‐17‐8 in the form of sludge to the hardening solution results in the reduction of heat generation at all stages of the hardening process. Thus, Wmax1 for mixture 3 drops down from 20.1 (experiment 6, table 2, figure 1) to 15.8 (experiment 5, table 3, figure 7), and when IER are also added – to 14.9 mW/(g of dry mixture) (experiment 1, table 3, figure 7).

FIG 7 VARIATION IN HEAT GENERATION (W, THERMAL CAPACITY) OVER TIME (T) IN THE COURSE OF MIXTURE HARDENING IN THE FIRST 60 MINUTES CALCULATED PER GRAM OF DRY MIXTURE. NUMBERS OF MIXTURES AND EXPERIMENTS FROM TABLE 3 ARE SPECIFIED ON THE CURVES

FIG 8 VARIATION IN HEAT GENERATION (W) OVER TIME (T) IN THE COURSE OF MIXTURE HARDENING DURING 40 HOURS CALCULATED PER GRAM OF DRY MIXTURE. NUMBERS OF MIXTURES AND EXPERIMENTS FROM TABLE 3 ARE INDICATED ON THE CURVES

Heat generation peak Wmax2 for mixture 3 drops down from 0.54 (observed in 12 hours, experiment 6, figures 2, 3) to 0.39 (observed in 25 hours, experiment 5, figure 8, 9) after adding CaCO3 to the hardening solution, and to 0.47 mW/(g of dry mixture) in the presence of IER (observed in 16 hours, experiment 1, figures 8, 9). These additives in the hardening solution slow down and decrease hardening exoeffect recorded during a week from ‐82.6 (mixture 3, experiment 6) to ‐76.8 (mixture 3, experiment 5), and to ‐74.3 J/(g of dry mixture) (mixture 3, experiment 1, table 3, figure 9) in the presence of IER.

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FIG 9 VARIATION IN HEAT GENERATION (W) OVER TIME (T) IN THE COURSE OF MIXTURE HARDENING DURING 7 DAYS CALCULATED PER GRAM OF DRY MIXTURE. NUMBERS OF MIXTURES AND EXPERIMENTS FROM TABLE 3 ARE INDICATED ON THE CURVES

FIG 10 VARIATION IN ENTHALPY (ΔН0) OVER TIME (T) IN THE COURSE OF MIXTURE HARDENING DURING THE FIRST 60 MINUTES CALCULATED PER GRAM OF DRY MIXTURE. NUMBERS OF MIXTURES AND EXPERIMENTS FROM TABLE 3 ARE INDICATED ON THE CURVES

For ChBFS introduction of clinoptilolite and bentonite additives in amounts of 10 mass.% each was also accompanied by three stages of heat generation. And the heat generation at all stages is characterized by relatively high values of W that are comparable with those observed at hardening of PC compounds. The values of Wmax2 for ChBFS‐based mixtures are twice as large as those for the PC compounds. Besides, Wmax2 for ChBFS was observed in 3 hours (mixture 6, experiments 11, 13 and 14, figures 8 and 9), while for PC mixtures containing also solutions simulating medium‐level wastes from the radiochemical plant Wmax2 was observed no less than in 16 hours (mixture 3, experiments 1, 3 and 5). Therefore, the second stage of the hardening process of ChBFS‐based mixtures was significantly shorter than that observed for the PC‐based mixtures (figures 8, 9). For this reason, about 50 % of the total hardening energy has been released already in 10 hours of hardening of ChBFS‐based mixtures (figures 11, 12). However, hardening enthalpy of ChBFS‐based mixtures by the solutions containing calcium carbonate or its mixture with IER is lower in absolute value than that for PC compounds (table 3).

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FIG 11 VARIATION IN ENTHALPY (ΔН0) OVER TIME (T) IN THE COURSE OF MIXTURE HARDENING DURING 40 HOURS CALCULATED PER GRAM OF DRY MIXTURE. MIXTURE AND EXPERIMENT NUMBERS FROM TABLE 3 ARE INDICATED ON THE CURVES

FIG 12 VARIATION IN ENTHALPY (ΔН0) OVER TIME (T) IN THE COURSE OF MIXTURE HARDENING DURING 7 DAYS CALCULATED PER GRAM OF DRY MIXTURE. MIXTURE AND EXPERIMENT NUMBERS FROM TABLE 3 ARE INDICATED ON THE CURVES

At hardening of crushed ChBFS in the hardening solution with NaNO3 concentration of 3.53 mole/l (300 g/l) in the presence of CaCO3, the process enthalpy was found to be ∆hardН0 = ‐75.7 J/(g of dry mixture) (mixture 6, experiment 14,), and at adding IER ΔН0 = ‐49.0 J/(g of dry mixture) (mixture 6, experiment 11, table 3, fig. 10‐12). At hardening of ChBFS in the hardening solution with CaCO3 and IER, the value of ∆hardН0 is the same within the measurement errors at variation of NaNO3 concentration in the solution from 3.53 to 7.06 mole/l (600 g/l). Consequently, increasing concentration of sodium nitrate solution from 300 to 600 g/l and adding IER and CaCO3 to the hardening solution result in sharp decrease of the process enthalpy in terms of absolute value in case of PC compound hardening (mixture 3, experiments 1‐4) and identical (within the limits of measurement errors) exoeffect for mixtures with crushed ChBFS (mixture 6, experiments 10‐13, table 3). Adding H3BO3, Na2SO4, NaCl, NaOH, KOH, Na2C2O4, EDTA‐1, OP‐10, Ca(OH)2 into the hardening solution results in considerable reduction of heat generation (mixture 3, experiments 7‐9, table 3, figures 10‐12) and, correspondingly, of the cement compound temperature in the course of its hardening.

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It follows from the obtained data that three stages can be singled out in the hydration and hardening processes. This fact is in complete agreement with classical conception of the above‐mentioned processes [1]. The first stage (about 0.5 h) is characterized by a sharp increase of heat generation (3‐5 minutes) followed by its decrease. In the course of this process, the hydrolysis of tricalcium silicate (one of the main minerals in the cement) is accompanied by generation of calcium hydroxide forming oversaturated solution from which the first new formations begin to settle during several minutes. This process is followed by the second stage of hydration, the so‐called “induction” period that is characterized by generation of very fine calcium hydrosilicates. Decreased heat generation is typical of this stage which lasts for 4‐5 hours. The above‐mentioned stages are practically identical for all compositions with slightly lower intensity for compositions with NPP distillation residues. Then, in the cement compound, there is an increase in heat generation, during which the thermal capacity for mixtures with nitrate‐containing medium‐ level wastes grows from 0.15 to 0.4 mW/(g of dry mixture) (figures 8, 9). The above‐stated increase is related to the third stage of the cement hydration during which crystallization of calcium hydroxide from the cement grout begins. For the mixture with the NPP distillation residue, the third stage of hydration is not so much pronounced in terms of thermal effects – heat generation capacity decreases practically monotonically achieving zero in 60 hours after blending the mixture. The above‐mentioned differences in thermal parameters of hardening of the compounds containing the employed simulating solutions are indicative of considerable influence of their components on structuring of the matrix material. Boric acid salts that are present in the NPP distillation residues retard significantly the cement hydration and hardening processes, to a considerably greater extent than sodium nitrate at that. This fact is confirmed by the increased by two‐three times (relatively MLW solution composition) hardening time of boron‐containing compound, that is about 3 days. At the same time, strength test results for the samples suggest that in 28 days these processes level down, which results in achieving equal durability by the compounds. Further storage of the samples under humid air conditions up to 160 days results in achieving strength at a level of 145‐150 kg/cm2, and the given results have been obtained already on the 70‐th day of hardening [8]. Lower heat generation of boron‐containing (as opposed to nitrate‐containing) compound during hardening allows forecasting substantially less heating in the storage facility compartments, all other conditions being equal. This fact in its turn makes it possible to increase fraction of Portland cement in the dry mixture, if further improvement of the compound quality is needed. In order to determine kinetics of the cement compound heating in the enlarged model of the compartment in industrial conditions, a compound block of 4 m3 was hardened. The dry mixture used in the experiment contained mass.%: PC‐400 ‐ 30; ATPP ash – 59.7; bentonite – 10; S‐3 – 0.3. The solution subject to hardening contained sodium nitrate with concentration of about 300 g/l. The averaged SCR during the entire experiment made 0.485 ml/g. The sample size was 2x2x0.9 m3. While the compound was being hardened, measurements of temperature in the sample were carried out. The maximum temperature in the sample central part was determined to be about 57 °С and was achieved in 3 days of hardening. The air temperature in the room was 20‐21 °С. Compared the obtained data with the results of hardening of pure Portland cement with a volume of 150 l (130 °С in 15 hours of hardening) [9], it should be noted that the maximum temperature in the modified compound turned out to be lower by more than two times, while the sample volume was greater by a factor of 25 [10]. This fact confirms the efficiency of the employed methods aiming at reducing compound heating in the course of hardening. Conclusions The experiments were carried out related to cementing the solutions that simulate medium‐level wastes from the radiochemical plant and distillation residues resulting from NPP LRW evaporation, with the use of binding agents on the basis of Portland cement and metallurgical slag. The obtained results made it possible to evaluate thermal effects associated with hardening of cement mixtures, and to assess the effect of various factors on heat generation, such as composition of the dry mixture and the solution, SCR. The work confirmed the efficiency of the employed methods aiming at reducing heat generation associated with hardening of the cement compound (introducing mineral additives, for example, low‐calcium TPP ashes; increasing concentration of the solution to be hardened). It is determined that slag‐alkaline cements are

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characterized by the thermal effect values at hardening comparable with those inherent in the developed Portland cement mixtures. Hardening of the solution simulating the NPP distillation residues by the studied dry mixtures was accompanied by extremely low heat generation which is related to inhibiting effect of the boric acid that is present in these wastes. This fact makes it possible to increase the fraction of active components in the dry mixture, if needed. The scaled‐up experiment on hardening of the compound block with a volume of about 4 m3 (based on the developed Portland cement blend and the simulating solution of the radiochemical plant MLW) demonstrated that the temperature in the sample in the process of hardening did not exceed 57 ºС. This fact confirms the efficiency of the measures taken to reduce the compound heating during hardening. The obtained calorimetric test data associated with heat generation of the perspective cement mixtures and the results of the experiment with the aim of determination of the compound sample heating in the scaled‐up compartment model allow further verifying the mathematical model of the storage compartment heating employed for preliminary calculations and introducing changes into this model when needed. This, in its turn, will help develop safe mode of filling the compartments of the storage facility of cement compound with hardened MLW, ensuring the absence of overheating in the building constructional elements and in the compound itself. REFERENCES

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N.G. Chernorukov, A.V. Knyazev, A.A. Sazonov. Isomorphism in the(NH4)2Th(NO3)6 ‐ K2Th(NO3)6 ‐ Rb2Th(NO3)6 System. Russian Journal of Inorganic Chemistry. 2009. Vol.54. №7. P.1002‐1008.

[10] N.G. Chernorukov, A.V. Knyazev, S.A. Gavrilova. Chemical thermodynamics of Nickel, Copper and Zink Uranyl Sulfates. Russian Radiochemistry. 2003. Vol.45. №.5. P. 484‐487. [11] P.V. Kozlov, О.М. Slyunchev, К.V. Kiryanov, I.V. Myalkin. ”Cementation of the NPP distillation residues with subsequent compound disposing in the pour‐type storage facilities.” Atomic Energy 111 (2011) Issue 3: 148‐154. [12] P.V. Kozlov, О.М. Slyunchev, P.A. Bobrov, V.I. Karpov, G.M. Medvedev, S.I. Rovny. ”Thermal properties of the cement compound as a factor of LRW cementing technology.” Radiation Safety Problems (2008) Issue 1: 37‐47. [13] P.V. Kozlov, О.М. Slyunchev, S.I. Rovny, К.V. Kiryanov. ”Determination of the cement compound heat generation in the process of hardening.” Radiation Safety Problems (2009) Issue 3: 14‐21.

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