Dry storage of Low-Water Reactor (LWR) fuel has been a major issue in the control of waste storage and recycling plants concerned with nuclear projects. In the US and Nuclear Power production countries, there is a need to extend the dry inert storage of the spent fuel beyond its originally anticipated 20-year duration. There are indeed many methodologies developed to support the initial licensing and storage for up to 20years in order to enable the longer storage periods being envisioned. The fact that many plants would rather abandon the reprocessing option and delay in opening a permanent repository where they can dispose the spent nuclear fuel thus changing strategy to utilities that involve reracking spent fuel pools and operating independent spent fuel storage installations. A Nuclear Regulatory Commission approved previously storage cask systems for about 35GWd/MTU but it has come clear that the discharge burn-ups has steadily increased and this is a trend that could probably continue for the better part of a futuristic forecast.
McKinnon and Cunningham suggest that the increased burn-up generally result in increased levels of oxidation and hydriding of the cladding; higher fuel rod internal pressures due to higher fission gas being released from the fuel pellets and consequently, higher hoop stresses in the cladding. Increase in oxidation decreases the effective load-bearing metal thickness of the cladding hence the contribution off high stresses which can be sufficiently high hoop stress and high temperatures to lead to deformations and rupture of the cladding. McKinnon and Cunningham assert that the mechanical properties of specific interest include creep, ductility under impact loading conditions and fracture toughness (2003). Therefore as discharge burnups levels continue to increase, additional studies need to be done to determine how dry storage of the LWR can be achieved to ensure that the storage in lasting about 100years can the effects of the mechanical properties of interest can resolve issues associated with the transportation, handling and disposal operations.
Extending Dry Storage of the Spent LWR fuel
Einzinger, McKinnon and Machiels confirm that the first 20 years of storage of the LWR fuel, properties of the components change due to the elevated temperatures, presence of moisture, effects or radiation and these all imply environmental risks and health hazards. They assert that during the normal storage in an inert atmosphere, there is the potential of the cladding mechanical properties changing as a result of annealing or the interaction of the cask materials. These emissions could also change due to storage conditions and with air leakages into the cask, additional degradation occurs through oxidation in the breached rods that lead to fission gas release (1999). It is postulated that accident scenarios would be similar for the period of storage whether between 20-100years, due to the fact that most storages are governed by operational events. The overall work being critical is the behaviour of the fuel, and as noted by Einzinger, McKinnon and Machiels that spent fuel at the burn-ups below approximately 45 GWd/MTU can be dry stored for 100years. This kind of storage that is long term will require the determination of the temperature limits based on the evaluation of the stress-driven degradation mechanisms of the cladding (1999).
Shenoy and Richards, states that spent fuel elements discharge from a reactor plant are very much ideal waste forms for a permanent disposal in a geologic repository. It is suggested that the graphite fuel elements and the ceramic coating on the fuel particles are as manufactured engineered barriers which will always provide an excellent near field containment of the radio-nuclides and this will minimize reliance on the waste package and surrounding geologic media for a very long period of containment. There are therefore conditions of safe fuel storage bearing in mind critical conditions, conditions of decay heat removal as well as radiation doses. Disposal feasibility, according to Shenoy and Richards, considers a number of categories such as: Proliferation risks and safeguards requirements; Radiological risks to the general public for very long time following a permanent closure of the repository; Suitability and licensability of the final waste form for the permanent disposal; Cost for the disposal inclusive of the waste package costs, tunneling costs, land-area requirements and disposal operation.
Storage and Disposal
It is generally recognized that in the future, the main source of spent fuel would be from the light water reactor (LWR) and that the LWR fuel is very different from magnox that is a major development progamme on the PWR was started. This included the experimentation to validate the theoretical analysis considering decay heat output from the discharged fuel 6mnths out of the reactor up to 100 years. The technical criteria drawn by the British Energy Sector Society, entails the weighting of important criteria with ranking given that includes some most heavily depended on items:
- Temperature of the fuel in storage
- Avoidance of criticality for fuel in the unirradiated condition
- Assurance of heat removal
- The dose uptake to the operations and public
- Environmental protection
- Volume of waste produced
- Physical protection
- Safeguards assurance
- Storage should not prejudice final disposal route.
A modular vault dry storage system has been developed according to the British Nuclear Energy Sector and has the following characteristics to make it suitable for all types of heat producing nuclear materials including the LWR fuel. This design is very specific for storage of research or production fuels with various burn-ups, enrichment and decay heat characteristics and it contains three major systems: The Reception Bay that is where spent fuel is received from the pool on-site and dispatched at the end of life; Secondly, the storage modules where spent fuel is stored and lastly, the fuel handling machine that raises and transfers irradiated fuel assemblies or containers from the cask in the reception bay to the storage position in the storage modules.
The maximum temperature in the cask occurs after the loading and dry-out hence these are limited by various considerations. The fluctuations due to the external temperatures and that of the cask decreases due to the drop in decay heat.
During the 100 year period, the fuel will experience two temperature regimes according to Mckinnon and Machiels. The first occurs during the first 10years when the fuel temperature drops from a maximum of about 380°C, depending on the fuel to approximately 100°C. According to McKinnon and Machiels, the second regime is for the remaining storage period when the temperatures are decreasing slightly but can generally be given a constant upper bound of approximately 100°C. In the cask of the dry storage, the spent fuel is subjected to gamma fields of about 105R/H and a neutron flux that is greater than 1MeV of approximately 104 to 106 n/cm2-s. It is suggested that during the 100year dry storage period the total dosage ranges from 4gamma to 7neutron ordered of the magnitude less than for two to four year residence in the reactor.
McKinnon and Machiels explain that the gamma field decreases with the age of the fuel and increases proportionately to the burn-up and at the maximum expected burn-up for the LWR fuel that should be approximately 60 GWd/MTU, the dose rates would no more than double those calculated for a current cask. An example of the dry storage criteria has been used in several countries like in Canada and Germany. In Canada CANDU fuel has been dry stored in over 200 concrete canisters using an air atmosphere and no adverse or unexpected performance reported since 1985 (McKinnon & Machiels, 1999).
In conclusion, the overall results indicate that based on the fuel behaviour, spent fuel usually at the burn-ups that is approximately below 45 GWd/MTU can be dry stored for 100years and this long duration of storage will not adversely affect the normal transfer and transport activities during or at the end of the storage period. Long storage periods of the higher burn-up fuels or fuels with a newer cladding will then require the determination of the temperature limit based on the evaluation of the stress-driven degradation mechanisms of the cladding process. In this case the best option is the use of high hydrogen content to reduce the allowable strain before the cladding breach and consequently reduce the ability of the cladding to survive an accident impact. This is so far the best technique that has been established and currently in use to dry store LWR spent fuel for longer periods of time.