An International Conference on Storage of Spent Fuel from Power Reactors was held in Vienna from 2-6 June 2003. The Conference was organised by the International Atomic Energy Agency in co-operation with the OECD Nuclear Energy Agency. One hundred twenty five participants from 35 countries attended the Conference reflecting the worldwide interest in spent fuel storage from power reactors. The Conference consisted of four major oral sessions and an invited panel session. In addition, the programme included opening overviews by the IAEA and the OECD/NEA, two poster sessions, and a concluding session featuring related IAEA presentations and summaries by session chairs.
2. Opening Overviews
Storage is not a final activity, but an interim, albeit important, step in the nuclear fuel cycle. While the next step can be reprocessing or disposal, all spent fuel (SF) or high-level waste (HLW) from reprocessing must sooner or later be disposed off. That means that the SF must be capable of being handled in a safe way after storage.
There is technical consensus that the present technologies for SF storage provide adequate protection to both people and the environment. Wet and dry storage are proven technologies, with, to date, longer experience for wet storage compared to dry storage. However, there is continued pressure for further improvements and efficiencies as anticipated SF volumes and storage duration increase and reactor fuels evolve (e.g., higher burnup, higher initial enrichment, MOX fuels, modified cladding). It is therefore essential to investigate not only fuel behaviour during long term storage, but also materials, equipment, installations, procedures and sites used for safe spent fuel storage.
Long term storage of SF requires social stability to maintain institutional control. Storage is a necessary phase of SF management, in which the issue of public trust must be addressed. Since perpetual storage is not feasible for periods extending over the hazardous lifetime of the radioactive isotopes, there is no alternative to disposal over the long term. In addition, it is important to communicate to stakeholders that there is no credible endpoint other than disposal. When elaborating on strategies for storage, issues of retrievability and transport need to be also considered.
3. National Programmes - Session 1.
Presentations during session one demonstrated significant national developments in the area of SF management. These can be summarized as follows:
The attention paid by the relevant national authorities for the safe storage of spent fuel from power reactors is evident for both the short and long term (interim) storage and for the repository. The schedule for the construction of repositories is mainly linked to the decommissioning of the NPPs and strongly depends on the national economic, political and social environment. In national strategies, the role of governments was underlined as a major influence on safe SF interim storage for long durations. In this context, governments should ensure that:
Competence, necessary for the safe and adequate treatment and disposal of radioactive materials, will be available;
Knowledge (both human resources and documentation) will be preserved.
Several technological and scientific projects related to spent fuel storage are under development at national and international levels, by governmental and/or private organizations. The papers, in general, included estimations of the milestones considered in national programmes for siting, construction and operation of facilities, including repositories. In general, the data presented showed that the expected time horizon for repository site definition would be between 2005 and 2030. Operation of repositories is generally planned in the period of 2020-2050.
The current options, under analysis by the relevant Governmental Authorities, take into consideration deep geological disposal, centralised storage, either above or below ground and surface storage at nuclear reactor sites. In some cases, the strategy for the back-end of the fuel cycle was not yet defined. The policy presented for some countries was not to reprocess the spent fuel and to consider spent fuel as radioactive waste.
Three key policy objectives were identified: establishment of (1) a separate waste owners fund, (2) a reporting relationship with the Waste Management National Authority and (3) a national review and approval process. New legislative initiatives, which have been set-up during the last few years, have included a number of requirements, which establish a process for addressing social impacts, ethical considerations, equity aspects and societal principles. Also, there is high national interest in identifying concerns and interests, in mitigating these concerns, and responding to public interests. The need to increase the public confidence is considered in medium and long term management plans at the national level. Some countries have adopted the “wait and see” approach for SF management. In this context, they focus on interim SF storage while monitoring related developments.
Presentations on national strategies highlighted the following important aspects:
4. Technologies on Spent Fuel Storage - Session 2.
Steady progress (since the last symposium in 1998) in accumulating practical experience either at reactor or away from reactor sites around the world was reported. This was driven by the continuing need for interim SF storage in many countries encountering delays with reprocessing, disposal or wait-and-see policies. This progress is also due to the safe and secure deployment of SF storage technologies around the world.
Presented information included progress in development and use of dual purpose casks made of metal and metal-concrete, modular vault storage and horizontal silos for various types of fuel. A wet storage facility with natural convection cooling was presented as a significant innovation since the last symposium. Japanese representatives introduced extensive dry cask demonstration test programmes. One programme dealt with the long term integrity of dual-purpose metal casks, particularly the evolution of the sealing function with time. Another programme dealt with thermal and seismic performance of concrete casks.
Various storage technologies are now available with significant experience. Storage technologies have matured to meet primary requirements, but the challenge for improvement is still ongoing. It was pointed out that in order to meet anticipated trends and changes in requirements, it would be important to share information and to collaborate internationally to leverage human and financial resources.
5. Experience & Licensing - Session 3.
Several countries reported on licensing criteria as well as the analyses and data that were used to develop those criteria. In particular, one paper (USA) presented licensing criteria for the storage and transportation of high burnup and damaged fuel. The Czech Republic and the Russian Federation presented information on the safety requirements for their dry storage technologies, while Lithuania presented a comparison of the performance of the CASTOR and CONSTOR casks.
Several countries presented descriptions of the history and process used to license various dry storage technologies. Sweden reported on the underground wet storage facilities of CLAB 1 and 2 and Finland discussed the wet pool storage facility for spent fuel as well as the facilities used to store other types of waste. Czech Republic, Canada and Germany shared their experience in operating SF storage facilities and the procedures used to load spent fuel and transfer dry casks to the interim storage facilities. The United States presented information on the licensing experience and technical issues facing the at-reactor SF pools in the US. France presented an overview of the centralized storage facility in Switzerland and Germany shared its analysis of the economics of wet versus dry storage.
Most countries use common licensing processes and follow the IAEA guidance. Common elements of those processes include:
Participants noted that assistance from the IAEA and the information exchanged at this and previous IAEA meetings on dry storage have helped some countries advance the development of their own licensing processes and implement dry storage technologies.
Both wet and dry storage technologies have evolved significantly over the last 20 or so years as evidenced by the wide range of papers presented at this conference. Furthermore, there is a remarkable safety record with the use of both wet and dry storage through out the world. To support the continued safety of wet and dry SF storage, Participants expressed an interest in hearing more about operational difficulties that are encountered with the various technologies and the solutions that were developed to overcome those obstacles in future IAEA meetings.
6. Research & Development - Session 4.
Most of the research topics were conducted in support of dry spent fuel storage. R&D projects and results were not only presented in this session but also in the papers presented in other sessions and poster presentations. Broadly speaking the topics covered could be grouped in four main categories:
Regarding the fuel itself, a good number of isotopic data and properties of different fuel types were presented in the session, with an emphasis on high burnup fuel data. Several aspects are still under investigation, which may be relevant to long term dry storage, such as helium and fission gas release from the matrix. A free access international database compiling isotopic compositions of LWR fuel, named SFCOMPO has been set up. Administered by the OECD/NEA, the intention is to broaden the spectrum of fuels covered if new data is donated.
There was a general consensus on the important mechanisms that affect fuel cladding behaviour/degradation in dry storage technologies: creep, hydrogen pick-up/mobility and redistribution of hydrides. Experimental data from different countries (France, Germany, Japan, Russian Federation, USA) showed that generally accepted safety criteria would not be compromised for the fuel and storage systems studied. The most relevant conclusion was the establishment of conditions (i.e., maximum cladding hoop stresses and storage/transient temperatures) under which re-orientation of the hydrides and creep behaviour of the cladding does not lead to its failure. Newly developed cladding materials with greater corrosion resistance were shown to lead to greater margins, which could be counterbalanced by the tendency to higher fuel burnup. A continuation of the IAEA’s Spent Fuel Performance and Research Programme (SPAR), which shares and exchanges information in the above areas, ongoing fuel performance and technology developments would help to ensure that best practice is continued to be shared with Member States.
Fuel behaviour modelling has been undertaken for WWER fuels mainly through modifications or validation of broadly accepted computational codes. The Republic of Korea is in the early stages of evaluating the use of molten salt technology for fuel conditioning prior to long term storage and disposal. France is carrying out studies into extremely long term (300 years) behaviour of the fuel under the conditions of dry storage in the PRECCI project. Preliminary results were presented on fuel matrix and cladding evolution both in an intact configuration and after cladding failure, which will be used to support long term storage and disposal. One paper reported on the work sponsored by the US industry on cladding creep deformation and creep rupture criteria that were part of the discussions with the US regulators, which eventually resulted in regulatory guidance on “Cladding Considerations for the Transportation and Storage of Spent Fuel”.
Burnup credit, was shown to be a widespread practice in at-reactor spent fuel wet storage applications, but is not so widely applied in dry storage, perhaps due to the absence of internationally accepted methodologies for its application. A continuation of the initiatives undertaken by the IAEA and the NEA, which provide a forum for the exchange of information on burnup credit between the industry and the regulatory bodies from different countries, could potentially help to develop a unified approach in this area.
7. Technical & Regulatory Challenges Raised by Long Term Storage - Panel Session
Challenges for long term storage in the context of the entire fuel cycle.
It is essential to discuss the technical and regulatory challenges to long term storage in the context of the entire fuel cycle. Given the options (once-through, reprocessing, recycle, or wait-and-see), the specific function to be achieved for the storage phase will eventually dictate its typical duration as well as the desirable attributes for the stored radioactive material. Estimated storage durations have been trending upward during the past few years. An upper limit of 300 years is generally discussed; more typical durations are expected to be 50 to 100 years. While requirements for nuclear safety (criticality, confinement and shielding) and operational flexibility (retrievability) must be maintained, specific acceptance criteria for spent fuel cladding integrity must be discussed in the context of the storage technology (wet versus dry) and expected role of cladding in subsequent fuel cycle operations.
Future conferences should consider storage challenges in the broader context of its role in the entire fuel cycle. Topics such as long term decay heat rates, transportation challenges, expectations for cladding integrity in the disposal phase should be included as these topics are intimately related to storage system performance requirements.
Risk assessments of storage systems performance.
Storage can be regarded as a well-understood and rapidly maturing technology. The documented safety performance of storage systems has been excellent. In the US, probabilistic safety analyses of dry storage systems have been initiated by the US NRC. Preliminary results show extremely low levels of risks. If these results are confirmed and shown to be representative of a typical independent spent fuel storage installation (ISFSI), the potential for simplifying or streamlining the regulatory process would exist.
Technical, regulatory, legislative issues.
Technical issues of interest at this time include the condition of a dual-purpose cask and its contents prior to shipping and its response to accident conditions. This interest is heightened by the trend toward increasing discharge fuel burnup. In addition, specific countries deal with specific issues: e.g.: (1) the U.S. and the process for extending the storage license beyond 20 years; (2) the Russian Federation and the legislative and organizational issues for accepting non-domestic spent fuel; (3) Japan and the optimum approach for monitoring containment and spent-fuel condition; (4) Bulgaria and the finalization of a licensing framework for dry storage system.
Optimizing public safety during transportation.
On the issue of criticality safety, especially in the context of transportation of spent fuel, do the acceptance criteria for the criticality analyses support the overall objective of minimum risks to the public? Of primary concern is limited acceptance of burnup credit application. Wider adoption of burnup credit would lead to a reduction in the number of shipments, and a corresponding reduction in transportation risks. The relative contributions of the radiological and non-radiological (more common) risks need to be explicitly considered when defining the protocols for performing criticality safety analyses (fresh fuel assumption versus burnup credit methodology).
8. Poster Sessions
The majority of the posters were complementary to the oral presentations, but some addressed scenario analyses of spent fuel arisings from possible nuclear power programmes in developing Member States.
Pakistan presented a study on a container for handling and storing the KANUPP reactor fuel. Indonesia gave an analysis of spent fuel arisings from 3 scenarios based on 600 and 900 MW(e) nuclear power plants and combinations thereof. The Czech Republic looked at the results of simulations of thermal effects on the CASTOR 440/80 cask from the operation involved in the evacuation process from the water pool, while Slovakia presented results of subcriticality analyses of the spent fuel storage baskets being used at Bohunice NPP for compact storage of spent fuel in the AFR pool. Romania reported the recent construction of the dry storage facility at the Cernavoda NPP site. Coincidentally, the first spent fuel bundles were loaded into MACSOR type storage modules on the first day of the conference.
Slovakia showed progress on the implementation of the AFR pool capacity expansion at the Bohunice NPP site by use of a new compact basket. Slovenia reported on the reracking work and future plans for further expansion of the pool capacity to accommodate lifetime operation of the Krsko NPP. Turkey evaluated the spent fuel arisings and management scenarios in a possible nuclear power programme in Turkey. Kazakhstan reported on the status of spent fuel storage at the BN-350 reactor, which has been shut down. Russia presented results of tests carried out in the development of the metal-concrete cask. The tests included measurement of shielding by gamma ray test and demonstration of a monitoring technique for cladding integrity of RBMK fuel for long term storage. Ukraine described the research project being conducted in co-operation with RIAR in Russia, using actual spent fuel rods from a WWER-1000 in tests at dry storage conditions.
9. Salient Overall Observations and Conclusions
Currently about 10,000 tonnes heavy metal (HM) spent fuel are unloaded every year from nuclear power reactors worldwide, of which 8,500 t HM need to be stored (after accounting for reprocessed fuel). This is the largest continuous source of civil radioactive materials generated, and needs to be managed appropriately. This annual discharge amount is estimated to increase to some 11,500 tonnes HM by 2010.
From a global perspective, the storage capacity currently available and under construction would accommodate spent fuel arisings through 2015. While there is sufficient fuel storage capacity on a global basis, certain national situations differ and may require urgent attention. All national programmes anticipate long storage durations. Accordingly, long term storage of spent fuel is becoming a progressive reality. Several experts said that long term storage of spent fuel requires social stability to maintain institutional control. All current operations in storage programmes rely on industrially mature technologies. The experts demonstrated a high level of confidence in the wet and dry storage technologies applied, the performances of the facilities and the good safety records.
Several presentations referred to a 100 years storage period (and even beyond) and as storage periods grow longer new challenges are arising:
With a growth in storage duration anticipated, storage is increasingly facing the very same problems and challenges faced by disposal: involvement and stability of society, monitoring and ability to predict performance over longer time spans. Participants noted that storage cannot be looked into in isolation, but has to be seen within the entire back-end of the fuel cycle. For instance one will have to assure that spent fuel can be handled after storage. Related measures include data management and material identification systems. Also, risks associated with storage of power reactor fuel have been analysed and shown to be very low and far lower than conventional industrial risks.
Possible Agency initiatives include: