Restoration of the historical repositories

The main advantage offered by RAW storage is the guaranteed security afforded by the integrity of each separate element of the multi-barrier complex. If any one of these multiple shielding barriers is damaged, the potential risk of migration of radionuclides into the environment increases. Any number of factors can disrupt the integrity of the construction material of the repository and/or of the massif of solidified RAW, principally: (1) the deformation of structural elements as a result of significant temperature variations in the environment (e. g., seasonal freezing and thawing) or as a result of shrinkage; (2) the decompression of seams; and (3) any construc­tion defects including where the different elements meet, such as joints between sides of walls and partitions and the upper overlap. In addition, cracks and microcracks can appear on the surface of the construction (the outer duct), which may be connected to each other, leading to the formation of a system of interconnected channels in the walls of the repositories.

An estimation of the conditions of the man-made barriers, carried out in the early 2000s [6, 7] , included comprehensive geological, hydrogeological, geophysical and radiometric studies of the condition of the RAW massif, the soils forming the edge zone and the massif of the surrounding rocks. In the selected zones boreholes 8 m in depth were drilled, and various meas­urements were taken: the layer-by-layer permeability of the medium was measured; gamma — and thermo-logging were carried out; and the core ma­terial (RAW cement compound and soils, as shown in Fig. 10.2) and water was sampled for further radiochemical, chemical, physical, mechanical, micro-structural and microbiological analysis.

image148

image149"These comprehensive studies, carried out with the help of boreholes drilled in the near-contour zone, determined that the high sorption property loams of the near-contour zone are indeed a reliable obstacle that prohibits the radionuclides from entering the adjacent massif [8]. During the inves­tigation of the cement compound samples containing RAW (core material), it was established that a cement matrix that has been age-hardened for more than 40 years generally preserves its basic immobilization properties. The majority of samples tested (Table 10.3) have a compressive strength higher than the normative value of 5 MPa [9] and the rate of radionuclide 137Cs leaching is lower than the required level of 1 x 10-3 g/(m2-day). However, sufficient humidity and porosity together with structure friability and low compressive strength attested in several samples from different depths were

Sample description

Specific activity E/J on 137Cs (Bq/kg)

Porosity (%)

Humidity (%)

Compressive strength (MPa)

Leaching rate 137Cs (g/(m2-day))

Dense cement block with cellulose

3.46 x 107

19.5

5.6-8.9

7-18

5.3 x 104

inclusion, without visible damage

(1960, encapsulation in 2004)

Scattering or friable cement

0.1-1.0

(1.2-6.5) x 107

20.1

25.9-38.1

5.7-23.9

(1.61-2.4) x 10-3

block with wood and rubber

1.0-2.5

44.2

3.9-13.0

(1.98-3.2) x 10-3

inclusions and cracks,

2.5-3.8

36.4

2.3-10.2

(0.66-7.4) x 104

crumbles (1961), at depth,

3.8-6.5

28.0

1.3-3.5

(0.58-7.9) x 10-5

m

6.5-8.0

11

0.3-1.4

(1.06-5.2) x 10-2

Without visible structure damage

(0.16-7.85) x 106

32.8

15.9-32.5

14-20

(0.28-9.4) x 10~4

(1965)

Without visible structure damage

1.59 x 106

17.7-32.3

2.30-30.0

5.3-5.6

1.6 x 106

(1987)

 

Подпись: © Woodhead Publishing Limited, 2013

also observed, showing that destructive processes were taking place in the cement matrix [10].

Microbiological investigations found that the cement compounds, the pore water in the cement compound and the penetrating waters contained microorganisms of different physiological groups, such as anaerobic fer­menting, denitrifying, nitrifying or sulphate-reducing bacteria, as well as fungi, which were capable of destroying silicate materials (Table 10.4). Studies were carried out to determine the form and number of bacteria and produced metabolites as well as the influence of these metabolites on the microstructure and immobilization properties of the cement matrix [9,11].

RAW intended for cementation contains components that may provide nutrition for microorganisms, which can then grow under the favourable conditions offered by near-surface repositories, such as insufficient exchange of air, humidity, temperatures of 6-28°C, and a pH of between 5 and 9. The main nutrients for microorganisms are provided by LRAW in the form of aqueous solutions of nitrates, sulfates and chlorides at concentrations of up to 300 g/l, mineral oils and organic liquids (extractants, scintillators), and also wood, plastic and cloth remnants. Gas-liquid chromatography was used to determine that the denitrifying and fermenting bacteria are capable of secreting (8.6-10.6) x 10-2ml of N2 within 24 hours from 1 cm2 of the RAW cement compound surface.

The products of the metabolism of microorganisms can cause changes in the way in which cement hydrated minerals crystallize, as well as distur­bances in the cement matrix microstructure. Consequently, the microstruc­ture of samples of cement compound cores taken from more than 4 m below the surface was examined. At this depth, the action of aggressive factors such as freezing and the penetration of surface and groundwater are not factors. The cores were examined using petrography and scanning electron microscopy (Fig. 10.3). Numerous local sections of the cement matrix dis­played internal cracks (Fig. 10.3a), and 20-200 pm micropores in which thin long fibres of new mineral formations were nucleated (Fig. 10.3c). The resultant body of hydrated cement minerals was not the uniform dense structure expected for this significant period of hydration, but a friable ‘cor­roded’ structure (Fig. 10.3b, d). The products of the hydration of calcium silicates, which determine the strength of the cement matrix, are heteroge­neous in their composition (refractive index N = 1,573-1,590). Calcium aluminate hydrates are heterogeneous in size (2-50 pm), represented by heterogeneous cubic and hexagonal crystals (Fig. 10.3e), surrounded by single ferrite gel. This is the characteristic crystallization nucleation of nee­dle-shaped and rhombic crystals of ettringite 3CaSOAl2O3-3CaSO4-32H2O and calcite CaCO3 (Fig. 10.3d, f), which fill the cement pores and bridge the crack that has been generated [12]. Destruction of the type observed here

Table 10.4 Microbiological characteristics of the samples of cement compounds with RAW from near-surface repositories (conservation in 1960-1961, 1965, 1987)

Physiological groups of microorganisms and their characteristics

Number of bacteria, isolated at

Samples of cement compound, kl/g

Samples of soil, kl/g

Samples of ground water, kl/ml

Total amount of microorganisms (colony

9.1 x 102 … 1.9 x 104

4.9 x 106 CFU/g

1.4 x 105 CFU/ml

forming units/g or ml )

CFU/g

Nitrifying bacteria

0

1.5 x 102

0

Denitrifying bacteria 1 phase

0

1.2 x 105

1.3 x 104

2 phase

3.6 x 104

6.0 x 104

Sulphate-reducing bacteria

2

1

0

Fermenting bacteria

60 … 3.0 x 104

3.6 x 105

2.5 x 102

Iron-reducing bacteria

0

2

2

Iron-oxidizing bacteria

33

0

0

Thione bacteria

0

0

0

Micromycetes

45 … 1.5 x 104

2.6 x 106

8.0 x 104 CFU/ml

Generic assignment

Bacillus Pseudomonas

Bacillus

Arthrobacter Rhodococcus

Rhodococcus, Alcaligenes

Pseudomonas

Alcaligenes

Micrococcus

Rhodococcus

Rhodococcus Bacillus

Mycobacterium

Flavobacterium

Arthrobacter

Properties of Microscopic picture

Rod-shaped bacteria, in

Rods, cocci

strains

pairs or singles,

containing spores

Grams-stain

Gram +

Gram +

Gram -, Gram +

The reaction with catalase

+

+

+

 

Подпись: © Woodhead Publishing Limited, 2013

image224

(e)

 

(f)

 

image150"image152"

image144

10.3 I nternal chippings of cement compound core from near-surface repositories of 1960-1961 (microphotography by scanning electron microscope): (a) internal cracks; (b) friable structure, (c) pores, filled with the fibrous new formations, (d) needles of ettringite,

(e) hexagonal plates of hydro-aluminate, (f) the mutual germination of the needle-shaped and rhombic crystals of ettringite and calcite.

may be the result of neutralization and carbonization caused by the bio­genic acid products of the alkaline minerals in the cement matrix, mainly portlandite Ca(OH)2 and the hydrosilicates of calcium, and of recrystalliza­tion and an anomalous growth of ettringite crystals in the space of the microcracks of biogenic origin [13].

A number of biocidal additives are being investigated as possible means of preventing microbiological destruction in cement compounds. The minimum concentration of these additives required to suppress the growth of the bacteria characteristic of near-surface repositories is 0.0015-0.003 mg/ ml, and for complete disinfection, 0.003-0.006 mg/ml. Effective biocides of the poly-hexamethyleneguanidine class are chemically compatible with the components of the cement solution [14, 15] , allowing the development of the highly penetrating cement compositions known as ‘Bison — BPl’ and ‘SPCK’, which were successfully tested in 2003-2007 during the hermetic sealing of RAW near-surface repositories from the 1960s. In the RAW technological cementation processes, these compositions [16, 17] were first used in 2004 to prevent undesirable microbiological processes in repositor­ies for solidified RAW.

To correct the disturbances formed in the man-made barriers of the his­torical near-surface repositories, a method of repeated grouting was devel­oped for cemented RAW massifs. The method relies on boring into the RAW cement massif forcing a mixture of biocidal and highly-penetrating cement compositions such as Bison — BPl and SPCK into the cement mortar. This repeated grouting allows the restoration of the impermeability of the repository and prevents the migration of radionuclides out of the construction. An upper layer of protective coating was also developed [6] with the aim of counteracting the influence of sediments and seasonal tem­perature differences on the construction of near-surface type repositories and their contents [ 6]. This coating is a multilayer screen 2.7-3 m thick, made mainly from natural materials, that cover the entire surface of the construction. The stages in the creation of this sealing coating are presented in Fig. 10.4.

The creation of protective coating for historical RAW repositories involves the following steps: [25]

(a)

 

(b)

image155

 

image156

(f)

 

image153"image154"image157"

10.4 ( a-f) Complex of works of preservation coating for ‘historical’ near-surface RAW repositories.

• creation of drainage systems for rainfall outlet;

• creation of a system of boreholes for observation and radio-ecological monitoring of the repository;

• creation of drainage and protective layers of a preservation coating from sand, clay, gravel and topsoil.

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