IAEA project on research reactor spent fuel management options

March 1, 2020, 10:35PMRadwaste SolutionsFrances M. Marshall

International Atomic Energy Agency member states operating or having previously operated a research reactor are responsible for the safe and sustainable management of associated radioactive waste, including research reactor spent nuclear fuel (RRSNF). Management includes storage and ultimate disposal of RRSNF, or the corresponding equivalent waste generated and returned following reprocessing of the spent fuel. Currently, there are 259 research reactors operating, planned, or under construction around the world [1]. An additional 147 research reactors are in extended or permanent shutdown, or under decommissioning.

One key challenge to developing general recommendations for RRSNF management options lies in the diversity of spent fuel types, locations, and national or regional circumstances, rather than mass or volume alone, particularly since typical RRSNF inventories are relatively small. Currently, many countries lack an effective long-term strategy for managing RRSNF. Many research reactor organizations know they have responsibility for the spent fuel, however, they do not know how to decide among multiple options for its management. A methodical review and compilation of technology options for RRSNF management is needed.

The core of the TRIGA Mark-III research reactor at the National Institute for Nuclear Research in Mexico City. Photo: Conleth Brady/IAEA

To help member states address the challenges mentioned above, the IAEA organized a coordinated research project on “Options and Technologies for Managing the Back End of the Research Reactor Nuclear Fuel Cycle” to address the issues facing research reactor owners relative to management and disposition of their RRSNF. This project emphasized concerns specific to countries that do not possess large commercial nuclear energy infrastructures. The project was conducted from 2015 to 2018 with the participation of 16 representative organizations from 15 member states: Argentina, Australia, Brazil, Chile, France, Ghana, Greece, Jamaica, Malaysia, Norway, Portugal, Russian Federation, Slovak Republic, South Africa, and the United States.

The goals of the project were to identify a comprehensive set of currently available technical options for RRSNF management, and to develop tools to enable member states to compare multiple RRSNF management strategies, with a focus on both economic and noneconomic aspects. The options analysis is intended to inform decision-makers about the advantages and disadvantages of each option, assisting new and existing research reactor organizations in making decisions on future spent fuel disposition strategies. A publication is currently being developed by IAEA with the project team that includes technical details of the RRSNF management options and the decision support tools.

Technologies for RRSNF management

To develop comprehensive RRSNF management strategies, it is first necessary to identify and understand the different elements of the spent fuel cycle. The primary steps are discussed here: storage (wet and dry), transportation, conditioning, processing, and geologic disposal.

Storage

Storage of RRSNF is a challenge for operators as it could be expected to last over 50 years until a final disposal decision is made. Spent fuel storage is an interim step in the back end of the nuclear fuel cycle. The general trend however has been ever-increasing storage periods, and durations of more than 100 years are now being envisaged.

Wet storage of RRSNF is used as the first cooling stage after irradiation of the fuel in the reactor core, allowing the decay heat to dissipate before further management and handling of the fuel. Depending on the reactor and facility design, the storage pools or ponds are located either within the reactor building or in another on-site building. Most research reactor fuel assemblies are not designed for unlimited duration of wet storage.

Currently, many research reactor operators are considering moving their RRSNF from wet storage into dry storage, either at the reactor site or at a centralized location that may contain fuel from several reactors. The main attractions of dry storage are its passive cooling capability, reduced concern for cladding corrosion, and reduced up-front costs through the ability to add incremental capacity. Dry storage facilities can store fuel for periods of more than 50 years with an appropriate level of monitoring, surveillance, and ageing management planning.

Conditioning

For spent fuel management, conditioning is defined as operations that produce a waste or spent fuel package suitable for handling, transport, storage, and/or disposal. Conditioning may include the conversion of the waste to a solid waste form, enclosure of the waste in containers, and if necessary, provision of an overpack [2].

Several conditioning options are currently under development [3], although there is no standard process available to address the diversity of RRSNF materials and designs. Possible conditioning methods for RRSNF include:

Removal of inactive material, fuel meat melting, or compaction in order to reduce the volume, followed by specific conditioning for meeting the expected waste acceptance criteria;

Fuel meat encapsulation in a matrix like glass or ceramics, with a long-term behaviour that is well-known;

Chemical treatment (without fissile material removal).

Storage options for conditioning products include metal and concrete casks, concrete vaults, and concrete silo systems. In principle, all are suitable for the materials under consideration.

Reprocessing and waste conditioning

RRSNF reprocessing allows separation of the valuable materials from the waste for recycling any remaining fissile material into fresh fuel, and optimizes the final waste form (both amount and composition) for disposal. RRSNF reprocessing is, or has been, used by several countries such as Australia, Belgium, France, Germany, Italy, Japan, Russia, United States, United Kingdom, Poland, and Romania. The final waste form is specifically designed to ensure safe interim storage during extended periods, and it may be released from IAEA safeguards. For more information about RRSNF reprocessing, see Ref. 4.

Geological disposal

Disposal is the end point of every RRSNF management strategy. The final radioactive waste form (conditioned RRSNF or post-reprocessing waste) ultimately needs to be placed in a location with no intention of retrieval. Disposal facilities use a combination of natural and engineered barriers to contain radionuclides within the engineered barriers, as well as to delay and retard their release to the environment until their activity has decayed to acceptable levels.

Several countries have developed or are in the process of designing geological repositories for radioactive wastes. The costs associated with developing, siting and licensing, constructing, operating, and eventually closing these facilities are high and range in the billions to several tens of billions of euros. These high costs are justified in the national context only when there is a large expected waste volume.

It is not practical to build one of these facilities for the relatively small waste inventories from research reactor fuel. Thus, there is a significant incentive to look for alternative disposal concepts. One potential approach would be to use a suitable, existing underground excavation (e.g., from an old mine), however, a full safety review and licensing is needed for nuclear fuel use. A variant to this would be creating access to an underground formation from the side of a hill or mountain. Another proposed disposal option for small waste inventories is borehole disposal. None of the geologic disposal options have been finalized for research reactor fuel.

Transportation

All spent fuel management strategies will include at least one transportation step, and most will include more than one. From the reactor pool, fuel could be moved to dry storage near the reactor facility, centralized storage off the reactor site, a reprocessing facility, or final disposal. RRSNF can be transported via road, rail, air, or ship, and a variety of approved containers are available for virtually all types of RRSNF shipments. Some containers are designed to serve as both transportation and storage containers, such that the fuel will be loaded into the container and not be removed until it reaches its final disposal location. For general information about RRSNF transportation, see Ref. 4.

Fuel return programs

Commencing in the 1990s, in an effort to reduce the amount of high-enriched uranium (HEU) at civilian facilities, the United States and the Russian Federation initiated programs to allow the return of U.S.-origin and Russian-origin HEU fuel to the country of origin [5]. From a member state’s perspective, this option may be attractive because it transfers the final disposition responsibility to the fuel-receiving country. However, while there are still some bilateral agreements that will allow the return of HEU and some LEU to the country of origin, member states cannot rely on the indefinite availability of these return programs as a long-term RRSNF management strategy.

Some fuel vendors have mentioned the possibility that the fuel supply contract contains the option for the buyer to return the RRSNF to the country where the fuel was enriched and manufactured, subject to negotiated intergovernmental agreement. This is not standard protocol, but there is no restriction against it.

Spent fuel management options

From the existing RRSNF management technologies, a comprehensive RRSNF management strategy can be developed. All RRSNF management starts in the reactor facility, typically in wet storage, and a complete RRSNF management strategy will result in final disposal. The steps in between can include storage, transportation, processing, or conditioning, and can include multiple steps of similar types. Project participants identified 11 options (plus Option 0, which is to do nothing) for managing their spent fuels. The options include:

Option 0: On-reactor-site storage. This is essentially the “do nothing” option and is not suitable for the long term.

Option 1: Direct disposal. This involves moving the fuel directly from the reactor pool storage to the final disposal facility.

Option 2: Storage, direct disposal. Fuel is moved from the reactor pool and placed in storage away from the reactor (either wet or dry), then moved to final disposal.

Option 3: Conditioning, disposal. Fuel is conditioned (i.e., structural parts cut to minimize waste volume and the fuel assembly is encased in a stabilizing container or matrix) prior to placement in the final disposal site.

Option 4: Conditioning, storage, disposal. Fuel is sent from the reactor pool to be conditioned, placed in storage away from the reactor, then sent for final disposal.

Option 5: Storage, conditioning, disposal. Fuel is moved away from the reactor facility, stored again, then moved to a conditioning facility prior to moving the fuel to the final disposal site.

Option 6: Storage, conditioning, storage, disposal. This is the same as Option 5, however there is an additional storage step between the conditioning and the final disposal site.

Option 7: Reprocessing, disposal. The fuel is moved directly from the reactor facility to a reprocessing facility, then the waste product from the reprocessing is moved directly to the final disposal site.

Option 8: Reprocessing, storage, disposal. This is the same as Option 7, except there is an additional storage between the reprocessing facility and transport to the final disposal site.

Option 9: Storage, reprocessing, disposal. This is the same as Option 7, except the fuel is removed from the reactor facility and stored prior to transport to the reprocessing facility.

Option 10: Storage, reprocessing, storage, disposal. This is the same as Option 7, with the addition of two storage steps in between the reactor facility and the reprocessing facility, then another storage step between the reprocessing facility prior to transport to the final disposal site.

Option 11: Fuel return. Fuel is returned to the country of origin (i.e., the country where the nuclear material was enriched), which takes responsibility for the final disposal.

A single flow chart of the spent fuel management life cycle is shown in Fig. 1.

Fig. 1. RRSNF management options.

Decision support tools

To assist member states in making decisions among multiple RRSNF management options, two electronic, Excel-based, decision-support tools were developed: Backend Research Reactor Integrated Decision Making Evaluation (BRIDE), which enables qualitative comparison of the identified strategies, and Fuel Cycle Cost Estimation for Research Reactors in Excel (FERREX), which estimates detailed costs for the chosen strategy. Within BRIDE is a subroutine, Backend Analytical Scenario Cost Estimation Tool (BASCET), to provide a comparative cost comparison of multiple strategies.

Noneconomic factors that could influence the viability, and probability of success, of implementing any given RRSNF management scenario need to be considered. The following is a list of general factors considered in the course of this project:

Legal and regulatory situation;

Industrial and technical capabilities and development opportunities;

Political support;

Public acceptance;

Human resource availability;

Environmental impact;

Regional and international partnerships.

BRIDE is a multi-attribute utility methodology and can be used by a member state to compare options for RRSNF management, combining the noneconomic factors with a comparative cost estimate to determine the optimum strategy. BRIDE is structured such that the strengths and potential weaknesses of each scenario, along with measures that can remediate weaknesses, are identified during the evaluation.

Once a preferred option is identified by using BRIDE, FERREX is used to develop a detailed cost estimate of the preferred strategy. These tools are expected to be used by decision-makers within the research reactor organization, funding organizations, or in the government organizations responsible for the RRSNF management.

Both FERREX and BRIDE were tested and simulated by the project participants, and adjustments were made as needed. Fig. 2 shows a simplified flowchart of the relationship between BRIDE and FERREX.

Fig. 2. Decision workflow for BRIDE and FERREX.

The key to obtaining a useful output from the BRIDE/BASCET tools is the use of several experts from multiple organizations outside the immediate research reactor facility and a facilitator to ensure that the rationale behind the scoring of the scenarios is discussed among the decision-makers and recorded in the BRIDE report. Fig. 3 shows an example of the BRIDE scoring summary output.

Fig. 3. BRIDE output scoring summary.

Conclusion

In response to a lack of understanding of the available RRSNF management options, IAEA completed a project to help identify the available RRSNF management options and created a set of decision-support tools to assist member states in deciding among several options. The emphasis is on ensuring that member states understand their RRSNF management responsibilities, and for the information generated by the project to enable the member states to more effectively determine the appropriate RRSNF management option for their unique situation.

A publication summarizing the results of the project is expected to be published in 2020, and the decision-support tools will be available online when the publication is released. Future work will include facilitation of BRIDE and FERREX workshops in member states with their evaluation and decision teams.


Frances M. Marshall is a research reactor fuel cycle expert with the International Atomic Energy Agency.

References

1. IAEA, IAEA Research Reactor Database, http://nucleus.iaea.org/RRDB/RR/ReactorSearch.aspx?filter=0.

2. IAEA, IAEA Safety Glossary, Vienna (2016).

3. IAEA, “Cost Aspects of the Research Reactor Fuel Cycle,” Nuclear Energy Series NG-T-4.3, Vienna (2010).

4. IAEA, “Available Reprocessing and Recycling Services for Research Reactor Spent Nuclear Fuel,” Nuclear Energy Series NW-T-1.11, Vienna (2017).

5. IAEA, “Experience of Shipping Russian-origin Research Reactor Spent Fuel to the Russian Federation,” IAEA-TECDOC-1632, Vienna (2009).


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