performance of concrete structures in containing liquid and solid … · 2019-04-27 · underwater...
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Performance of concrete structures in containing liquid and solid radioactive wastes at Nuclear sites in England and Wales
John Kernaghan
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Aims
• To review the condition of concrete radioactive waste containment structures
• To identify areas where there is a need for improved monitoring and remediation of concrete
• To explore technologies that could potentially be used to monitor concrete
• To explore methods of repairing defects that exist in the concrete
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Scope • Concerns all concrete fuel ponds and silos in England
and Wales, but focused mainly on legacy facilities at Sellafield
• Interested in technologies that have been demonstrated in other applications
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Background
• Spent fuel from reactors is stored on-site in spent fuel ponds before being transferred to Sellafield
• It remains here awaiting final disposal in a Geological Disposal Facility
Diagram – Neil Hyatt, University of Sheffield
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Background
• Some of the storage facilities are over 50 years old and have gone beyond their design life
• There have been leakages involving cracks in concrete containment structures
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Background
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Is it really a disaster waiting to happen?
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Condition of the Concrete Structures
• Quality of construction was good! – Strong, densely packed concrete – Rebar cover generally 40-50mm
• Chemical Degradation is not a huge threat – Carbonation depth around 20mm – Very little chloride ingress – Little sulphate content
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Condition of the Concrete Structures
• The major problem is cracking – Mostly due to early shrinkage and settlement. – Generally around construction joints. – Little evidence of new cracks forming, but those
that exist can open and close.
• Damage to water bars and seals is also a concern
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Condition of the Concrete Structures • These defects can cause leaks of radioactive material
into the environment • Can also allow the liquid to reach the rebar,
accelerating corrosion
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Condition of the Concrete Structures • The lifetime of the structures may have to be
lengthened • Defects may be made worse by proposed sludge
retrieval operations • A better understanding of the condition of the
concrete is therefore required
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Monitoring of the Concrete Structures
• Areas where better monitoring is required: – Inside the containment structures – Beneath the ground – Continuous remote monitoring
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Monitoring of the Concrete Structures
• Three methods of concrete monitoring have been identified and evaluated: – Fibre Optic Sensors – Underwater Visual Inspection using ROVs – Gamma Imaging
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Fibre Optic Sensors • Have been used to monitor the condition
of many large concrete structures, e.g. dams, tunnels and bridges.
• Could be used in concrete fuel ponds and silos in three different applications – Strain monitoring – Crack monitoring – Leak detection
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Strain Monitoring • Can be attached to the surface of the concrete
structures • Small strains in the sensor are detected as
changes in the properties of the propagating light.
• Could be particularly useful during sludge retrieval
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Crack Monitoring • Can monitor existing cracks working similarly
to a strain sensor • Can also detect cracking by monitoring
changes in light intensity caused by bending in the fibre
Bending of the fibre
FOS
Crack formation
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Leak Detection • There is a limited knowledge of the condition
of the structures beneath the ground • Temperature sensitive FOSs have been used in
dams to detect seepage • Could be applied to fuel ponds, but would
require the water temperature to be changed
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Advantages • A proven technology • Provide continuous monitoring • Processing equipment can be located away from the
structure • Can operate underwater, and in corrosive and
radioactive environments
Challenges • Fitting the fibres will be difficult, especially inside
and underneath the structures.
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ROVs in Concrete Inspection
• ROVs have been used extensively in the nuclear industry • Some have been designed specifically for use in fuel ponds
and silos
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ROVs in Concrete Inspection
• Video cameras could be used to monitor the concrete inside
the containment structures • However, even with powerful lighting it is unclear and it is
difficult to assess the depth of defects
Image taken inside FGMSP
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ROVs in Concrete Inspection
• 3D imaging techniques can be
used to improve underwater visual inspection
• They use either sonar or laser to produce clearer images
• Laser scanners can also quantify defects
• Scanners have been used to inspect concrete dams, and can be mounted on ROVs
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Gamma Imaging
• Technology designed specifically for the nuclear industry • Used to locate and monitor gamma radiation, mainly in
decommissioning projects
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Gamma Imaging
• Could be used in the same way as radiographic testing, using
the radioactive liquor in the fuel ponds as the source. • Fission products, such as Cs-137, emit gamma radiation that
can be detected on the outside surface of the structures • Cracks and defects on the concrete would show a higher
activity, and periodic monitoring could determine whether they are growing.
• Although the technique could not be used to quantify the sizes of defects, it does directly measure the performance of the concrete in terms of its shielding ability.
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Concrete Repair
• Most concrete repairs on the fuel ponds and silos are the
same as in any other concrete structure. • Problems arise, however, when defects occur inside the
structures where the liquid present and the high activity make access difficult.
• The ability to repair cracks and seals inside the structures would help prevent future leaks into the environment
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Underwater Grouting with ROV
• ROVs have been used to repair underwater concrete in marine
structures that are too deep for divers • Has been demonstrated by ASI group on the Ponce Deep Sea
Outfall, Puerto Rico • Defects are washed with a high pressure hose, before being
sealed with a self-sealing gelatinous grout • An ROV with a mechanical arm was used to apply the sealing
material • Inspection confirmed a 100% seal of the joint
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Underwater Grouting with ROV
• Could be integrated into currently used ROVs
• The gelatinous grout material has not be proven to be resistant to radiation damage, but there are many other materials available
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Conclusions
• The structures are stable, but leakages into the environment
are possible and must be prevented • FOSs, Underwater 3D imaging, and Gamma Imaging have
promising potential applications in inspecting the concrete structures
• Repair of cracks inside the containment buildings is possible
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Thank You For Your Attention
Any Questions?