Performance of concrete structures in containing liquid and solid ...
Transcript of Performance of concrete structures in containing liquid and solid ...
Performance of concrete structures in containing liquid and solid radioactive wastes at Nuclear sites in England and Wales
John Kernaghan
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
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
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
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
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
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
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
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
Monitoring of the Concrete Structures
• Areas where better monitoring is required: – Inside the containment structures – Beneath the ground – Continuous remote monitoring
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
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
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
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
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
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.
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
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
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
Gamma Imaging
• Technology designed specifically for the nuclear industry • Used to locate and monitor gamma radiation, mainly in
decommissioning projects
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.
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
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
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
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