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physico chemical characteristics: cement and fiber characterization
CEMENT and CONCRETE RESEARCH. Vol. 22, pp. 351-358, 1992. Printed in the USA. 0008-8846/92 $5.00 + .00. Copyright © 1992 Pergamon Press Ltd.
Fiber concrete overpacks/physico chemical characteristics: cement and fiber characterization Richard PECH President, Chief Executive Officer, SOGEFIBRE, 1, rue des Herons Montigny-le-Bretonneux, 78182 Saint-Quentin-en-Yvelines Cedex, France
Sogefibre, a subsidiary of SGN and Compagnie G6nerale des Eaux, is in charge of producing fiber concrete containers for nuclear waste producers and of developing and promoting this new material: glassy-metal fiber reinforced concrete•
Abstract Radioactive waste conditioning is a major step in the processes implemented in nuclear installations. The objective is to contain the radioactive materials in nuclear waste as satisfactory as possible for man and the environment; containment integrity has to be guaranteed over very long periods. Medium-level (ML) and even very low-level (LL) waste is no exception to this rule. Cogema conducted research for many years and developed a new process to condition nuclear waste in containers reinforced with metal fibers, called fiber concrete containers. This process is welcomed by the French Safety Authorities and Andra, the French radioactive waste management agency, as the most satisfactory up to now.
1. WHY FIBER CONCRETE? Extensive research has been conducted on the incorporation of various fibers in materials such as mortar, cement or concrete, in order to enhance their mechanical properties. Such materials, and more particularly fiber reinforced concrete, are being used with increasing success in Germany, the USA, Japan, Great Britain and France. In view of the advantages of fiber concrete, a research and development program was initiated in early 1985 to define: • the optimum fiber concrete formulation (proportions of water, sand, cement, aggregates, additives and fibers), • the type of fiber, . the waste encapsulation method, the shape of the fiber concrete containers (cubical, cylindrical). The incorporation of fibers in concrete has the following major advantages: • the matrix is reinforced, thus providing better tensile, bending, impact fatigue and abrasion strength, • rupture strength is improved; a less fragile and more ductile material is thus obtained, • concrete microcracking is reduced, which is the key to long term containment of nuclear waste. 351
352
R. Pech
Vol. 22, Nos. 2/3
A characterization program of the selected material has been undertaken and has demonstrated that Andra requirements for packages holding immobilized heterogeneous waste delivered in durable containers and intended for disposal in a near-surface facility were met. These requirements are expressed in the Andra specification STE 119.581.S. So fiber reinforced concrete package got Andra agreement for nuclear waste surface disposal. During the characterization program, mechanical, physical, containment and irradiation characteristics have been evaluated. Main results are presented in the tables 1 to 3 where they are compared with Andra requirements. When available, data issued from industrial manufacturing of fiber reinforced concrete are presented. Most advantage of Sogefibre fiber reinforced concrete rest in the specific type of fiber selected called Fibraflex. The corroding, conventional carbon steel, used for armoured concrete is replaced by a non corroding amorphous metal fiber, Fibraflex is produced by quenching a liquid metal jet on a cooled wheel rotating at high speed. The main characteristics of Fibraflex fiber are given in the table 4. The very high resistance to corrosion of Fibraflex fiber, due to the presence of chromium in the alloy, has been confirmed by test. Results of these tests performed on both carbon steel rebars and Fibraflex fibers are presented in the table 5. The advantage procured by Flbraflex fiber is very noticeable. Based on all these characteristics, computer simulation of the radionuclides migration taking account of the container surfaces degradation have been carried out to check the integrity of Sogefibre fiber-reinforced concrete containers for a long period of time. Although some assessments are still to be completed, it is clear today that the lifetime is consistant with the Institutional Control Period for a low-level and medium level waste French disposal facilities, i.e. 300 years. The figure 1 shows the typical fibers distribution inside the concrete.
Figure ].
Cylindrical
container,
local
section.
Vol. 22, Nos. 2/3
OVERPACKS, FIBER CONCRETE, CHARACTERIZATION
2. PRODUCT
353
LINE
The reference of the French patents for Sogefibre fiber-reinforced concrete are 88.16337 of 12 December 1988 and 89.08050 of 16June 1989. Fiber concrete containers can be tailored to the specific needs of each customer, as regards: - geometry (cylindrical, cubical, etc.), - dimensions, - wall thickness, - closure system, - gripping system, - surface finish of outer walls. Since fiber concrete containers are produced by moulding, customization is relatively easy. Three typical containers are currently available: - 3 cylindrical
containers:
• C B F - C I : interior diameter: 690 mm, height: 1,200 mm; useful volume : 330 I minimum wall thickness : 74 mm . CBF-C2: interior diameter: 850 mm, height: 1,500 mm; useful volume : 7001 minimum wall thickness : 74 mm - 1 cubical
• CBF-K:
container:
outer dimensions 1,700 x 1,700 x 1,700 mm useful volume : 30001 mininu~ wall thickness : 100 mm
A cross section of a cylindrical container is given in the figure 2. The figure 3 shows the cover of a cylindrical container• The purpose of the elaborated shape is to avoid the floating of the waste drum when filling the container.
Figure 2. Cylindrical container after cutting
354
R. Pooh
Vol. 22, Nos. 2/3
Figure 3. Fiber concrete container CBF-C1 Anti-float cover Series fabrication started on these bases in July 1990. Our product line will next include two additional types: nestable containers fitting within another to increase the biological shield thickness and diminish external irradiation, - containers adapted to dismantling works: smooth outer walls, several handling means available and wall thickness adapted to the different kinds of waste to be conditioned... The cubical container recently introduced into service is presented in figure 4. -
Figure 4. Cubical container. 1.7 x 1.7 x 1.7 m
Vol. 22, Nos. 2/3
OVERPACKS,FIBERCONCRETE,CHARACI'ERIZATION
355
3. VALOGNES FACTORY (FRANCE) The number of containers required at the Cogema La Hague reprocessing plant, and especially the need to control their quality to satisfy Andra specifications, involved very stringent control by the Cogema Group and led to the construction of a factory in Valognes, France, in the vicinity of La Hague.
840 dia 690 dia
O Q
Figure 5.Fiber concrete container CBF-C1 Dimensions of Cogema specified container
356
R. Pech
Vol. 22, Nos. 2/3
Figure 6. Removal of cylindrical containers from moulds This factory entered service in July 1990. The three typical containers presented in the paragraph 2 may be manufactured in this factory. In the figure 5 is presented a sketch of one of the manufactured containers• An important manufacturing step is presented in the figure 6. It has a rated capacity of 12,000 containers per year that can be increased to satisfy the potential needs of other clients in the nuclear or nonnuclear fields, since this new material, developed for applications in the La Hague reprocessing plant using high quality control and high quality assurance levels, can surely find applications in other sectors of the industry (prefabrication, thin concrete walls, environmental protection, etc.). The quality assurance procedures implemented in the Valognes factory (French standard "AQ2") are focused on the following major points: • very stringent control for incoming raw materials (cement, sand, aggregates, fibers, etc.), • control of process parameters (proportioning of concrete ingredients, mixing time, etc.), • systematic measurement of concrete shrinkage and mechanical strength on test samples, • dimensional and visual inspection of products, • product traceability.
Vol. 22, Nos. 2/3
OVERPACKS, FIBER CONCRETE, CHARACTERI7~TION
357
Table I Fiber-Reinforced Concrete Material: Mechanical and physical properties (after 28 days) INDUSTRIAL RESULTS (after one year operation)
ANDRA CRITERIA
- Specific gravity
2.4
- C o m p r e s s i v e strength
60 to 70 M Pa
> 50 M Pa
- Tensile strength
4.5 to 5.5 M P a
> 4.5 M P a
- Shrinkage
about 290/~m/m
< 300
- Weight loss
0.7%
pm/m
Table II Fiber-Reinforced Concrete Material: Confinement properties (after 28 days)
TESTS
CONDITIONS
- Effective diffusion factor = 1 year . Tritiated w a t e r 1 cm pellet • Caesium 2 cm pellet - W a t e r porosity - Nitrogen permeability
-
TESTS RESULTS
ANDRA CRITERIA
< 7.1 10-5 cm2/d < 2.4 10-5 cm2/d
< 1.5 10-3 cm2/d < 10-3 cm2/d
< 1.5 10-20 m2 3 10-20 m2
< 5.10-18 m2
B
358
R. Pech
Vol. 22, Nos. 2/3
T a b l e III F R C Material: Irradiation s t r e n g t h 106 Gray 1.2 to 1.4 Gy h-1 5 kg
- Dose integrated - Dose rate - Sample weight RESULTS: - V o l u m e of H2 generated - Radiolytic return ratio G (H2) - V o l u m e of 0 2 c o n s u m e d - Radiolytic return ratio G (02) - No cracks
0.220 I 0.017 1.141 - 0.09
T a b l e IV F I B R A F L E X fiber c h a r a c t e r i s t i c s - Size • length ............................................................................. --- 30 mm • width .............................................................................. 1 to 2 mm • thickness ................................................................................ 30 p - Specific surface ............................................................... 10 kg/m2 - Tensile strength ........................................................ = 2 000 M P a - Modules of elasticity ............................................. - 140 000 M P a
Table V R e s i s t a n c e to c o r r o s i o n of fiber ( F I B R A F L E X ) Length of test
Corrosion speed mm/year
Current of corrosion 10-6 A/cm2
Potential corrosion mV/ecs
Carbon steel Rebars
240 h
413 _+ 15
36
- 386
Fibraflex
720 h
13_+6
1.1
- 40
(solution of pH = 11.5 + chlorides and sulfates; temperature 50°C)
Fiber concrete overpacks/physico chemical characteristics: cement and fiber characterization Richard PECH President, Chief Executive Officer, SOGEFIBRE, 1, rue des Herons Montigny-le-Bretonneux, 78182 Saint-Quentin-en-Yvelines Cedex, France
Sogefibre, a subsidiary of SGN and Compagnie G6nerale des Eaux, is in charge of producing fiber concrete containers for nuclear waste producers and of developing and promoting this new material: glassy-metal fiber reinforced concrete•
Abstract Radioactive waste conditioning is a major step in the processes implemented in nuclear installations. The objective is to contain the radioactive materials in nuclear waste as satisfactory as possible for man and the environment; containment integrity has to be guaranteed over very long periods. Medium-level (ML) and even very low-level (LL) waste is no exception to this rule. Cogema conducted research for many years and developed a new process to condition nuclear waste in containers reinforced with metal fibers, called fiber concrete containers. This process is welcomed by the French Safety Authorities and Andra, the French radioactive waste management agency, as the most satisfactory up to now.
1. WHY FIBER CONCRETE? Extensive research has been conducted on the incorporation of various fibers in materials such as mortar, cement or concrete, in order to enhance their mechanical properties. Such materials, and more particularly fiber reinforced concrete, are being used with increasing success in Germany, the USA, Japan, Great Britain and France. In view of the advantages of fiber concrete, a research and development program was initiated in early 1985 to define: • the optimum fiber concrete formulation (proportions of water, sand, cement, aggregates, additives and fibers), • the type of fiber, . the waste encapsulation method, the shape of the fiber concrete containers (cubical, cylindrical). The incorporation of fibers in concrete has the following major advantages: • the matrix is reinforced, thus providing better tensile, bending, impact fatigue and abrasion strength, • rupture strength is improved; a less fragile and more ductile material is thus obtained, • concrete microcracking is reduced, which is the key to long term containment of nuclear waste. 351
352
R. Pech
Vol. 22, Nos. 2/3
A characterization program of the selected material has been undertaken and has demonstrated that Andra requirements for packages holding immobilized heterogeneous waste delivered in durable containers and intended for disposal in a near-surface facility were met. These requirements are expressed in the Andra specification STE 119.581.S. So fiber reinforced concrete package got Andra agreement for nuclear waste surface disposal. During the characterization program, mechanical, physical, containment and irradiation characteristics have been evaluated. Main results are presented in the tables 1 to 3 where they are compared with Andra requirements. When available, data issued from industrial manufacturing of fiber reinforced concrete are presented. Most advantage of Sogefibre fiber reinforced concrete rest in the specific type of fiber selected called Fibraflex. The corroding, conventional carbon steel, used for armoured concrete is replaced by a non corroding amorphous metal fiber, Fibraflex is produced by quenching a liquid metal jet on a cooled wheel rotating at high speed. The main characteristics of Fibraflex fiber are given in the table 4. The very high resistance to corrosion of Fibraflex fiber, due to the presence of chromium in the alloy, has been confirmed by test. Results of these tests performed on both carbon steel rebars and Fibraflex fibers are presented in the table 5. The advantage procured by Flbraflex fiber is very noticeable. Based on all these characteristics, computer simulation of the radionuclides migration taking account of the container surfaces degradation have been carried out to check the integrity of Sogefibre fiber-reinforced concrete containers for a long period of time. Although some assessments are still to be completed, it is clear today that the lifetime is consistant with the Institutional Control Period for a low-level and medium level waste French disposal facilities, i.e. 300 years. The figure 1 shows the typical fibers distribution inside the concrete.
Figure ].
Cylindrical
container,
local
section.
Vol. 22, Nos. 2/3
OVERPACKS, FIBER CONCRETE, CHARACTERIZATION
2. PRODUCT
353
LINE
The reference of the French patents for Sogefibre fiber-reinforced concrete are 88.16337 of 12 December 1988 and 89.08050 of 16June 1989. Fiber concrete containers can be tailored to the specific needs of each customer, as regards: - geometry (cylindrical, cubical, etc.), - dimensions, - wall thickness, - closure system, - gripping system, - surface finish of outer walls. Since fiber concrete containers are produced by moulding, customization is relatively easy. Three typical containers are currently available: - 3 cylindrical
containers:
• C B F - C I : interior diameter: 690 mm, height: 1,200 mm; useful volume : 330 I minimum wall thickness : 74 mm . CBF-C2: interior diameter: 850 mm, height: 1,500 mm; useful volume : 7001 minimum wall thickness : 74 mm - 1 cubical
• CBF-K:
container:
outer dimensions 1,700 x 1,700 x 1,700 mm useful volume : 30001 mininu~ wall thickness : 100 mm
A cross section of a cylindrical container is given in the figure 2. The figure 3 shows the cover of a cylindrical container• The purpose of the elaborated shape is to avoid the floating of the waste drum when filling the container.
Figure 2. Cylindrical container after cutting
354
R. Pooh
Vol. 22, Nos. 2/3
Figure 3. Fiber concrete container CBF-C1 Anti-float cover Series fabrication started on these bases in July 1990. Our product line will next include two additional types: nestable containers fitting within another to increase the biological shield thickness and diminish external irradiation, - containers adapted to dismantling works: smooth outer walls, several handling means available and wall thickness adapted to the different kinds of waste to be conditioned... The cubical container recently introduced into service is presented in figure 4. -
Figure 4. Cubical container. 1.7 x 1.7 x 1.7 m
Vol. 22, Nos. 2/3
OVERPACKS,FIBERCONCRETE,CHARACI'ERIZATION
355
3. VALOGNES FACTORY (FRANCE) The number of containers required at the Cogema La Hague reprocessing plant, and especially the need to control their quality to satisfy Andra specifications, involved very stringent control by the Cogema Group and led to the construction of a factory in Valognes, France, in the vicinity of La Hague.
840 dia 690 dia
O Q
Figure 5.Fiber concrete container CBF-C1 Dimensions of Cogema specified container
356
R. Pech
Vol. 22, Nos. 2/3
Figure 6. Removal of cylindrical containers from moulds This factory entered service in July 1990. The three typical containers presented in the paragraph 2 may be manufactured in this factory. In the figure 5 is presented a sketch of one of the manufactured containers• An important manufacturing step is presented in the figure 6. It has a rated capacity of 12,000 containers per year that can be increased to satisfy the potential needs of other clients in the nuclear or nonnuclear fields, since this new material, developed for applications in the La Hague reprocessing plant using high quality control and high quality assurance levels, can surely find applications in other sectors of the industry (prefabrication, thin concrete walls, environmental protection, etc.). The quality assurance procedures implemented in the Valognes factory (French standard "AQ2") are focused on the following major points: • very stringent control for incoming raw materials (cement, sand, aggregates, fibers, etc.), • control of process parameters (proportioning of concrete ingredients, mixing time, etc.), • systematic measurement of concrete shrinkage and mechanical strength on test samples, • dimensional and visual inspection of products, • product traceability.
Vol. 22, Nos. 2/3
OVERPACKS, FIBER CONCRETE, CHARACTERI7~TION
357
Table I Fiber-Reinforced Concrete Material: Mechanical and physical properties (after 28 days) INDUSTRIAL RESULTS (after one year operation)
ANDRA CRITERIA
- Specific gravity
2.4
- C o m p r e s s i v e strength
60 to 70 M Pa
> 50 M Pa
- Tensile strength
4.5 to 5.5 M P a
> 4.5 M P a
- Shrinkage
about 290/~m/m
< 300
- Weight loss
0.7%
pm/m
Table II Fiber-Reinforced Concrete Material: Confinement properties (after 28 days)
TESTS
CONDITIONS
- Effective diffusion factor = 1 year . Tritiated w a t e r 1 cm pellet • Caesium 2 cm pellet - W a t e r porosity - Nitrogen permeability
-
TESTS RESULTS
ANDRA CRITERIA
< 7.1 10-5 cm2/d < 2.4 10-5 cm2/d
< 1.5 10-3 cm2/d < 10-3 cm2/d
< 1.5 10-20 m2 3 10-20 m2
< 5.10-18 m2
B
358
R. Pech
Vol. 22, Nos. 2/3
T a b l e III F R C Material: Irradiation s t r e n g t h 106 Gray 1.2 to 1.4 Gy h-1 5 kg
- Dose integrated - Dose rate - Sample weight RESULTS: - V o l u m e of H2 generated - Radiolytic return ratio G (H2) - V o l u m e of 0 2 c o n s u m e d - Radiolytic return ratio G (02) - No cracks
0.220 I 0.017 1.141 - 0.09
T a b l e IV F I B R A F L E X fiber c h a r a c t e r i s t i c s - Size • length ............................................................................. --- 30 mm • width .............................................................................. 1 to 2 mm • thickness ................................................................................ 30 p - Specific surface ............................................................... 10 kg/m2 - Tensile strength ........................................................ = 2 000 M P a - Modules of elasticity ............................................. - 140 000 M P a
Table V R e s i s t a n c e to c o r r o s i o n of fiber ( F I B R A F L E X ) Length of test
Corrosion speed mm/year
Current of corrosion 10-6 A/cm2
Potential corrosion mV/ecs
Carbon steel Rebars
240 h
413 _+ 15
36
- 386
Fibraflex
720 h
13_+6
1.1
- 40
(solution of pH = 11.5 + chlorides and sulfates; temperature 50°C)