Wet fuel storage facilities

One consensus opinion from the TCM is, that material characteristics and properties will vary with time. This may be a result of general corrosion. A second point is, that the demands required for ten-year storage periods are different from those required for 100 years. For example the following parameters will change with time:

(1) Fuel decay heat, (2) Total activity,

(3) Degradation of materials,

(4) The conditions that these materials experience over time (e.g. thermal, environmental, radiation).

Several questions were raised with regard to wet storage of spent fuel. First, considering the role played by the pool water, as coolant and biological shield, does there come a point in time when it is no longer required as the required duty, as thermal conditions and dose have changed dramatically from the starting position? The answer, in principle yes, if doing so does not impact on fuel assembly integrity, the residual radiation and airborne activities etc. can be managed.

Second, can the new duty be performed by another material, for example, a glass or a polymeric cover? Possibly. Although the above questions appear to be radical, there are examples, where time is used to downgrade the storage conditions/requirements. For example the management of Magnox fuel at Wylfa NPP has changed over the years where initial fuel cooling is undertaken in carbon dioxide cooled stores, and subsequent storage is in air.

Storage conditions are defined to ensure that safety margins are not compromised in time.

Secondly, the conditions should also be defined to ensure best practice.

1.4.1.1. Water chemistry

Water chemistry requirements go hand in hand with those required to ensure that fuel assembly integrity is not compromised over the storage period. The need to ensure that aggressive ions, such as chloride, are removed or kept to an absolute minimum is paramount, in order to maximize material safety margins and to ensure premature material failure is avoided. The only potential problem will be, if the requirements to maintain fuel assembly integrity have a detrimental effect on the fuel storage components.

With a move to much longer storage duration, the impact on fuel storage facility materials also needs to be taken into consideration. This may lead to the introduction of corrosion inhibitors, to minimise the rate of general corrosion.

Concerns must be re-examined. For example, are welds a weak point, where pools are lined with stainless steel? Is water ingress between liner and primary structure a problem? It depends upon original weld standard and techniques. There is experience of such failures and repair techniques have been developed. The main issue is the need for spare storage capacity, for clearing some space, to enable the repair to be undertaken. It is believed that water ingress should be avoided, as this is a potential concrete degradation mechanism.

The particular fuel being stored defines the water chemistry required. For example the typical water chemistry for LWR at AFR pools is:

pH 4–6 Conductivity 100–350 µS/m

Chloride <0.1 ppm

Sulphate <0.1 ppm

Fluoride <0.1 ppm

Na ⁺ and Ca ⁺⁺ <0.5 ppm 1.4.1.2. Temperature

Environmental factors have to be considered. A question to countries that experience extremes in temperature is the impact of thermal cycling (repeated freeze-thaw cycles) on the concrete structure (especially the external surfaces). Does this define a requirement to protect against this function? The consensus opinion from global experience is, that it is an issue that needs to be closely monitored, and measures taken to minimise the effect.

Decay heat natural cooling requirements become less onerous with time. Depending upon the type of fuel and storage duration cooling may no longer be required. In fact the reverse is true, the need to heat the water may become applicable.

One of the prime functions of minimising pool temperature is the impact on activity leach rate. Minimising the activity release during storage will impact on the lifetime cost of plant cleanup operations (volume of ion exchange resin used), and the cost of final pool decommissioning.

1.4.1.3. Activity levels

See also comments given above.

The long term management is the issue. It can be concluded that in order to avoid problems associated with activity buildup the fuel should be either canned or contained (to introduce a secondary barrier between pool water and fuel) or have a cleaning system.

The choice would be determined through a lifetime cost/benefit analysis.

1.4.1.4. Corrosion products

Controlling corrosion products can be important to avoid secondary electrochemical reactions between the fuel and the storage rack or container. This relies on good housekeeping policy.

Uncontrolled release of corrosion products will impact on final pool decommissioning cost, and potentially lead to early failure of the fuel, or affect its retrievability.

The general conclusion in moving to longer storage periods invokes an extension to current best practices, but there needs to be some consideration to the effects of fuel decay heat, total activity, material degradation and the conditions that these materials experience over time (e.g. thermal, environmental, radiation). Effective management of water chemistry, temperature, activity levels and corrosion products are required, to ensure that safety is not compromised, the fuel can be retrieved, to minimise lifetime operating costs, and final decommissioning cost/requirements.

1.4.2. Long term behaviour of materials with safety functions 1.4.2.1. Loss of pool water

Although covered here, the loss of pool water does not affect the safety of the fuel within the pool, in reality the criticality safety case is improved. It does, however, present other problems, e.g. loss of containment, radiological, etc.

1.4.2.2. Storage racks, etc.

The robustness of most neutron absorbing materials has been questioned at some point in time. Examples include swelling of Boraflex and Boral and the durability of borated stainless steel under impact conditions.

The life limiting factors include general corrosion, changes in environmental conditions, and radiation damage. These issues can be addressed by research and monitoring programmes.

1.4.2.3. Fuel cladding defects

It is important to minimise the effects of cladding defects:

− For dose reasons, impact on operators integrated over time,

− Impact on maintenance activities,

− Activity buildup in cooling systems,

− Cost of remediation,

− Impact on final decommissioning as activity will leach into cracks, pool coatings etc.,

− Ion exchange usage.

Resolution is affected by restoring the primary containment barrier. Identification that a defect has occurred during prolonged pool storage can be determined through activity release monitoring in conjunction with activity release models.

1.4.3. Research & development and operational needs

R & D activities should be geared to being able to demonstrate safe storage for the projected storage duration and at any point in time.

Investigations to support projected new life may be required.

1.4.3.1. Development of detection systems

Analysis of material samples should be developed.

1.4.3.2. Monitoring techniques

Multiple monitoring techniques involve a mixture of traditional techniques and on-line state of art monitors. NDT monitors linked to the facility control room. Ideally they will alarm, when there is a change in conditions, not when a failure is underway.

1.4.3.3. Operational needs

Some consideration should be given to the issue to plan up front the lifetime cost and to make financial provision (managed fund) as per decommissioning activities. The way the facility is operated will impact on lifetime cost. Examples of factors affected include:

− Minimising activity levels within the storage pool,

− Minimising degradation of materials,

− The operation of auxiliary plant,

− Management of water leakage,

− Temperature, dose levels, physical protection.

1.4.3.4. People

− Continuity of expertise,

− How they respond to an event,

− Operator behaviour as a function of time (behavioural safety),

− Quality assurance programme also implies more reliance on automatic alarms. Regular inspections by regulators unannounced to ensure standards are being maintained.

1.4.3.5. Documentation

− Information for decommissioning,

− Records (how it was build, activity levels, the fuel being stored, etc.),

− The systems used to store the information,

− Plant items, continuity of plant spares.

1.4.3.6. Non-proliferation aspects

This may be easier for wet storage as it is feasible to inspect the fuel in situ at any point in

1.4.3.7. Licence extension

The requirements for licence extension (or periodic safety review) are different from the previous, because there is always change, for example building codes. The impact of these changes needs to be evaluated and assessed against the ability to safely store the fuel. If there are no safety implications then modifications to meet the last standard and should only be carried out if cost effective to do so. Where safety is compromised modifications will be necessary, however, this may lead to facility closure on the grounds that the modifications can not be carried out, or the costs are too restrictive.

1.4.4. Conclusions

There are no foreseen issues presented by increasing the operating life of wet storage facilities, provided that safe operation can be demonstrated through continued monitoring and maintenance of quality assurance systems.

1.4.5. Recommendations

− There is need for spare capacity or contingencies for unforeseen events,

− Need to have a monitoring programme for key plant areas to demonstrate safety,

− Provision for funding to cover plant life needs to be considered,

− Further assessment of the technical issues outlined above should be considered.

1.5. REGULATORY CONCERNS RELATED TO THE LONG TERM STORAGE OF