INTRODUCTION AND BACKGROUND

For approximately a decade, the United States Department of Energy has been running advanced fuel cycle research in the current Fuel Cycle R&D Program and its predecessors. A key objective of this research has been improved waste management by lessening both the environmental burden of nuclear energy and the proliferation risk of accumulating used nuclear fuel.

Until recently, this programme was technically focused on achieving an optimized symbiosis between fuel cycle options, on the one hand, and the US geological repository on the other. In previous years, a relatively short term deployment focus was being pursued. On the basis of detailed technical analyses, this focus led to the selection of a limited set of technologies that were expected to meet specific geology related criteria, and which would be based on limited extrapolations of existing technologies.

Recent developments in the United States of America indicate that alternative repository sites will be considered and that Yucca Mountain may not

FINCK and HILL

150

be the choice for final disposal. With delayed geological disposal, advanced fuel cycles could be postponed until mid-century, with increased reliance on temporary storage of used nuclear fuel in the interim.

2. FUEL CYCLE R&D OBJECTIVES AND APPROACH

Consequently, the Fuel Cycle R&D Program is being redirected towards a longer term science based research approach. The work will be conducted with a goal oriented focus, driven by the following three considerations:

(1) The programme is currently examining a broad set of options, including different geological media and transmutation technologies in order to understand their relationships and provide information for later decisions.

(2) The R&D component of the programme is focused on acquiring the basic understanding of key phenomena, defining the relevant challenges and acquiring the basic tools necessary to resolve them.

(3) The timeline of the programme allows for deployment of the successfully demonstrated technology in the 2040–2050 time frame; allowing consider-ation of technologies that are not yet mature but that might provide significant improvements in performance.

The science based research approach will integrate theory, experiment and high performance modelling and simulation to promote development of the needed technologies. The focus for a science based approach shifts to smaller scale experiments of phenomenological and separate or coupled effects. This approach provides a fundamental understanding of targeted phenomena and data for model development. New and innovative experimental design and novel measurement techniques are anticipated to support improved fidelity and small scale detail.

Theory development is an essential element of the science based approach.

This requires first a deep understanding and a database of existing knowledge and theories to identify and explain the key phenomena. In the long term, theory must span quantum mechanics to continuum mechanics in order to explain the behaviour of physical systems. A well-integrated balance between experiments and theory development is required.

The knowledge and data gained under the experimental and theoretical elements of the science based approach will be incorporated into advanced modelling and simulation tools that take advantage of state of the art computing capabilities. Owing to the complex and formal nature of the nuclear licensing

PLENARY SESSION 2

151 process, procedures to demonstrate the validity of the new tools must be clearly addressed.

The technical programme is articulated along the following elements:

(a) A systems integration task that analyses the relationships between technologies and defines requirements to achieve overall system objectives.

(b) A separation research programme that is aimed at understanding the funda-mentals of actinide chemistry in order to develop processes that achieve specific separation goals with very low losses.

(c) A better understanding of used nuclear fuel geological repository options and development of better safeguards techniques.

(d) A fuels research programme that is also aimed at a better understanding of the fundamentals of fuel behaviour in order to design minor actinide containing fuels with high burnup capabilities.

(e) A fast reactor research programme aimed at reducing the cost of fast reactors, with increased safety performance. This work is conducted as part of the Generation IV programme in close collaboration with the Fuel Cycle R&D Program.

3. FAST REACTOR R&D ACCOMPLISHMENTS

The role of the reactor in a closed fuel cycle is to utilize materials recovered from spent fuel for both electricity production and fuel cycle management.

Recycling of key elements is required to satisfy both waste management and resource extension objectives. A fast spectrum reactor with an associated closed fuel cycle is required to close completely the fuel cycle because practical limitations to extended recycle have been identified for thermal systems.

A variety of fast reactors can be considered for transmutation. Three options being considered in the Generation IV advanced reactor programme are:

(i) the sodium cooled fast reactor (SFR), (ii) the lead alloy cooled fast reactor (LFR) and (iii) the gas cooled fast reactor (GFR). Similar transmutation performance for these systems has been demonstrated. The SFR technology was favoured in predecessor programmes because of its maturity for near term appli-cation, while the alternative LFR and GFR technologies offer some advantages for high temperature applications.

Current fast reactor R&D is focused on the primary issues that have inhibited fast reactor introduction in the past:

(a) A perception of higher capital costs as compared to conventional LWR technology;

FINCK and HILL

152

(b) Unique concerns related to alternative coolants (e.g. sodium reactions with air/water, corrosion by lead alloys, component access under liquid metal, decay heat removal with gas).

Thus, the objective of fast reactor R&D is to research and develop advanced technologies that significantly improve both economic and safety performances of fast recycle systems. This outcome requires concurrent efforts on science based R&D for innovative technologies, integration of new features into reactor systems and development of fast reactor recycle fuels.

3.1. R&D on innovative technologies

Because capital investment in reactors is the dominant cost of any nuclear fuel cycle, this work is critical to assure an economically viable closed fuel cycle.

To reduce the cost of future fast reactors, a variety of innovative solutions are being researched:

(a) Advanced modelling and simulation. Reactor simulation requires modelling diverse, coupled physics, including neutronics, thermal fluid dynamics and structural phenomena. New techniques will exploit modern computational hardware and visualization software. The improved modelling will make reactor design tools more predictive, reducing the reliance on calibration and conservative margins. Improved accuracy and better integration of methods will also promote design optimization. Improved nuclear data is also important for both system optimization and safety assurance. Prioritized high accuracy experiments are conducted for key actinides and materials used to predict key reactor parameters such as criticality, transmutation rates and reactivity feedback coefficients.

(b) Advanced materials. Advanced structural materials could improve reactor costs by enabling compact configurations, higher operating temperatures, higher reliability and longer lifetimes. Modern material science techniques are being used to optimize variants of existing alloys for fast reactor appli-cations. Qualification of these materials requires resolution of code and licensing issues and irradiation testing, and initial testing of candidate alloys is being conducted.

(c) Advanced energy conversion systems. Refined energy production systems such as a supercritical CO₂ Brayton cycle offer the promise of improved thermal efficiency. Research needs for advanced heat exchangers (e.g.

small tube configurations) and compact components are also being pursued with objectives of both reduced cost and high reliability.

PLENARY SESSION 2

153 (d) Safety research. Inherent safety is a key approach for licensing assurance

and cost reduction. A wide variety of design features for prevention and mitigation of severe accidents have been proposed for advanced design concepts. The benefit and performance of features such as core restraint, seismic isolation and ‘core catchers’ are being assessed. In addition, the validation of safety analysis methods and advanced techniques with existing data is being aggressively pursued.

3.2. System integration and concept development

Another important aspect of this work is the analysis of diverse fast reactor technology options (e.g. refined coolants, fuels) and system configuration options (e.g. pool, loop, hybrid, elimination of intermediate loop). The assessment of performance impacts can only be compared in a systematic manner by meticulous application of system constraints and performance criteria.

This work guides the other research activities by providing a fundamental understanding of the technical utilization and feasibility of advanced technology options in an integrated reactor system. Favourable applications for innovative features are developed and the cost reduction benefits are evaluated.

3.3. Fast reactor fuels

The objective of fast reactor fuels R&D is to develop transmutation fuels for use in fast reactors with associated closed fuel cycles. This requires the trans-mutation fuel to cover a wide range of compositions to account for variability of recycle material feeds and flexible fast reactor fuel cycle modes (e.g. burner or converter). To this end, irradiation testing has been conducted on metal, oxide and nitride fuel forms to assess the impact of including minor actinides and other recycle impurities.

To improve the economic performance of fast reactor recycle, research is also conducted to extend fuel burnup and improve fabrication costs. The realization of high burnup requires the development of radiation tolerant fuel forms and core structural materials. As described above, extensive theory and modelling efforts of fuel behaviour and performance are being pursued to understand and to optimize performance of diverse options. With regard to fuel fabrication, advanced technologies are being developed to allow ‘remotized’

operation, minimize losses and waste, and streamline operations.

.

155