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

Background

Is it really a disaster waiting to happen?

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

Thank You For Your Attention

Any Questions?