SNOWMASS HOT TOPIC

CHAMBER SCIENCE AND TECHNOLOGY

KEY QUESTION NUMBER 5

RE-EVALUATION OF THE USE OF LOW ACTIVATION MATERIALS IN WASTE MANAGEMENT STRATEGIES FOR FUSION

Background

Materials choice has long been recognized as a key factor in realizing the full safety and environmental potential of fusion power. Because the materials are de-coupled from the fusion energy source (the plasma), the long-term induced activation of components can be tailored by proper selection of materials to avoid generation of waste that would require deep geological disposal. Thus, the idea of “low activation” materials was conceived for the fusion program with the hope that such material could be disposed of as low level waste (e.g., shallow land burial) and would not pose a burden to future generations.

The environmental impact of waste material is determined not only by the level of activation which is a measure of radiological hazard, but also the total volume of activated waste. A tokamak power plant is large, and there is a potential to generate a correspondingly large volume of activated material. The adoption of low activation materials, whilst important to reduce the radiotoxicity of the most active components, should be done as part of a strategy that also minimizes the volume of waste material that might be categorized as radioactive, even if low level.

Preliminary review of current designs indicates that the use of low activation materials (e.g., Li/V) in a design may exacerbate the waste volume problem because such materials are not necessarily good moderators of neutrons. Thus, the neutrons are spread all over the tokamak causing many components to be activated to some degree. Although most of this material would qualify as low level waste, the volume is quite large. Current estimates are that the total waste from a tokamak power plant will be about 5 to 8 times greater than that from a fission plant of equal power.

Further, we support the work that different researchers have tried to characterize the radiological hazard of fusion waste compared to fission waste and the radiological content of coal ash, [1] These comparisons are favorable for fusion, but like all metrics, are not perfect and may not mean anything to the general public. By contrast, the public does understand the concept of volume.

Some activated materials in the tokamak may be candidates for recycling, and others may be cleared from regulatory control by meeting prescribed criteria that have yet to be agreed upon internationally. Recently these concepts of recycling or clearance have been recognized as options for reducing the volume of radioactive waste from a fusion power plant. Determining if a material can be recycled or cleared from regulatory control depends largely on our ability to limit the induced activation of the component. Thus, there is a need to re-optimize the blanket and shield of conceptual fusion designs to achieve an optimal balance of the parameters which influence neutronic behavior and thereby substantially reduce the activation of the large ex-vessel components which contribute significantly to the overall volume of activated material. The impact of these parameters on other aspects of plant performance must also be considered.

In this hot topic, we review scoping studies with neutronics and activation models to examine these issues, and identify the trends which allow improved in-vessel shielding to result in reduced ex-vessel activation. The performance of typical fusion power plant designs with respect to recycling and clearance criteria are also assessed, to show the potential for improvement in waste volume reduction by careful selection of materials combinations. The implications of the results on the development path for fusion power are discussed.

Subtopics

Key subtopic questions to be addressed are:

What are the absorption and scattering characteristics of different fusion materials (e.g., Li, Pb, water, B, LAFS, V, SiC, W) at both fast and thermal energies?
What criteria should be considered to allow recycling of fusion materials? Contact dose for handling both hands on and remote operation? Very low activation for reuse or clearance?
What would be the impact of changing the current constraints on fusion ex-vessel components from insulator dose on the magnet and re-weldability of the vacuum vessel to constraints that limit the ex-vessel activation to very low values? Very low activation might allow these large components to meet recycling/reuse criteria and thus would not have to be counted as waste. Would this new constraint require increasing the radial build of the machine significantly or would other materials need to be considered? If other material need to be considered, can we meet such new constraints with low activation materials or do we have to consider higher activation materials for shielding?
Are current designs at an optimum in regard to the overall tradeoff between waste hazard and waste volume? How would a fusion tokamak reactor design be different if we changed the optimization and focused more on volume and a little less on hazard?
What is the impact on the overall COE of reducing waste volume but perhaps increasing waste hazard? Is there a simple scaling relation between radial build and cost of energy that could be compared with current estimates of waste disposal cost so that we could examine the overall tradeoff?
What would such a shift in design optimization mean for fusion technology development, the low activation materials program and our new advanced power extraction initiative?

Speakers/Core Group of planners:

D. Petti – INEEL (Co-lead)

Ed Cheng – TSI (Co-lead)

Don Steiner – RPI

Mohamed Sawan/Hesham Khater - UW

Mahmoud Youssef - UCLA

[1] Holdren did work in the 80s with the concept of biological hazard potential. Our UK colleagues have tried to develop a radiological metric and compare fusion with fission and coal ash