PROGRESS AND CHALLENGES OF HANDLING FUSION RADIOACTIVE MATERIALS
L. El-Guebalya* and M. Zucchettib a
University of Wisconsin, 1500 Engineering Dr., Madison, WI 53706, USA,
[email protected] b Politecnico di Torino, Corso Duca degli Abruzzi 24 - 10129 Torino, Italy,
[email protected]
The importance of handling the sizable mildly radioactive materials that fusion generates received more attention in recent years. Disposing such sizable radwaste in geologic repositories is not a viable option. We suggest changing what is now a costly waste disposal concern for fusion energy into a valued commodity through the further development of the recycling and clearance approaches. This paper reports the outcome of two recent activities that identified the challenges of handling the radioactive materials of ARIES-ACT-2 power plant along with the required design changes and R&D programs that make the recycling/clearance approach a reality, and the development of a new detritiation code that predicts the efficiency of tritium recovery from metallic materials – an essential process before recycling. I. INTRODUCTION There is a strong worldwide interest in building fusion power plants in the second half of the 21st century. Fusion has long been envisioned as possessing an inherent advantage for benign environmental impact, mainly due to the absence of high-level waste (HLW) generation. In recent years, fusion designers have become increasingly aware of the large amount of mildly radioactive materials that fusion tends to generate. The search for a solution has stimulated numerous activation studies at several fusion institutions to identify the origin and nature of fusion radioactive materials. Such a potential problem for fusion has been overlooked in early fusion studies and/or relegated to the back-end as only a disposal issue in lowlevel waste (LLW) repositories, adopting the preferred radioactive waste (radwaste) management approach of the 1960s for nuclear facilities. In fact, disposing the sizable mildly radioactive fusion materials in near-surface geologic repositories is not a viable option. During the past decade and continuing to the present, more attention has been paid to the waste management issue associated with the large volume of radioactive materials discharged from fusion power plants. This has been accomplished by efforts in the US1-3 and throughout the world, particularly in EU, RF, and Japan.4-6 Essential steps included reshaping
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http://dx.doi.org/10.13182/FST14-952
the fusion waste management approach and maximizing the reuse of materials through recycling and clearance in order to reclaim valuable resources in the form of metal alloys and concrete rubble, minimize the environmental impact and radwaste burden for future generations, free ample space in repositories, and, in the long run, save fusion millions of dollars for the high disposal cost. Overall, the main goal is to change the costly waste disposal option for fusion energy into a valued commodity via clever designs, smart choice of materials, and the further development of the recycling approach1-6 (reuse of activated materials within the nuclear industry) and clearance approach7-8 (release of slightly activated materials to the commercial market after a specific cooling period). Numerous fusion studies have indicated the recycling and clearance approaches are technically feasible and identified several critical issues that must be addressed with more in-depth analyses and dedicated R&D programs.1-8 This paper addresses a few critical issues and reports recent results related to: • ARIES-ACT-2 power plant requirements and design changes to support the recycling/clearance approach. We performed rigorous neutronics and activation analyses to identify the alloying elements and/or impurities that impact the recycling process of invessel components and deter the clearance of ex-vessel components. • The development of a new detritiation code that predicts the efficiency of the tritium (T) recovery process (needed before maintenance, recycling, or disposal) and recommends the most efficient detritiation technique to achieve the required T concentration. II. ACTIVATION AND ENVIRONMENTAL IMPACT OF ARIES-ACT-2 The ARIES-ACT-2 power plant9 utilizes the dual cooled LiPb (DCLL) blanket with F82H ferritic steel structure cooled with helium. Having a large major radius of 9.75 m, minor radius of 2.44 m, and fusion power of 2637.5 MW, ARIES-ACT-2 delivers net electric power of
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1000 MW. The design, shown in Fig. 1, generates large amount of fusion radioactive materials.10 To put matters into perspective, we compared in Fig. 2 the power core volumes of the ITER experimental device,11 ARIESACT-1&2,9 and the European Power Plant Conceptual Study (PPCS)12 to ESBWR (Economic Simplified Boiling Water Reactor) – a Gen-III+ advanced fission reactor.13 As noted, fusion power cores generate sizable volumes of mildly activated materials compared to fission. Figure 3 illustrates the volumes of components comprising the fusion power core (FPC) of ARIES-ACT-2. The magnet and its supporting structure generate the largest volume of radioactive materials, followed by the vacuum vessel (VV) and shield, and then the plasma facing components (first wall, blanket, and divertor).
Fig. 3. Volumes of ARIES-ACT-2 FPC components.
Fig. 1. Isometric view of ARIES-ACT-2 showing the 2 m thick bioshield surrounding the FPC.
Fig. 2. Comparison of radwaste from power core of fusion and fission designs (actual volumes of components; not compacted; no replacement; no plasma chamber).
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Surrounding the ARIES-ACT-2 fusion power core is the bioshield – a 2 m thick, steel-reinforced concrete building constructed to withstand natural phenomena and essentially protects the public and workers against radiation. Being away from the plasma, the bioshield is subject to low-level of radiation and contains very low radioactivity. However, its volume (not included in Fig. 3) dominates the waste stream, as shown in Fig. 4. Since burying such a huge volume of slightly activated materials in geologic repositories is impractical, regulatory agencies around the world suggested the clearance concept where such components could temporarily be stored for the radioactivity to decay, then released to the commercial market for reuse as shielding blocks, concrete rubble base for roads, deep concrete foundations, non-water supply dams for flood control, etc.
Fig. 4. Volumes of all ARIES-ACT-2 components: fusion power core, cryostat, and bioshield.
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II.A. Geologic Disposal Issues and Needs Even though this option is the least preferable, we evaluated the waste disposal rating (WDR) for fully compacted components to classify the ARIES-ACT-2 waste according to Nuclear Regulatory Commission (NRC) 10CFR61 and Fetter waste disposal limits.10 The NRC waste classification is based largely on radionuclides that are produced in fission reactors, hospitals, research laboratories, and food irradiation facilities. In the early 1990s, Fetter and others performed analyses to determine the Class C specific activity limits for all long-lived radionuclides of interest to fusion using a methodology similar to that of 10CFR61. Although Fetter’s calculations carry no regulatory acceptance, they are useful because they include fusion-specific isotopes. All ARIES components should meet both NRC and Fetter's limits until the NRC develops official guidelines for fusion waste. Also, the WDR is evaluated at 100 y after shutdown, allowing the short-lived radionuclides to decay. A WDR < 1 means LLW and WDR > 1 means HLW. Reference 10 provides the activation model, codes (PARTISN and ALARA), and alloying elements and impurities for F82H FS. The nominal 18-impurity list (that includes 3.3 wppm Nb and 21 wppm Mo) was measured at JAERI in Japan. For new steels, such as 3Cr3WV, the F82H impurities and density are used since doing physical property measurements on new steels may be too far into the future. In an effort to reduce the longterm radioactivity, Klueh et al.14 provided a list of the lowest 17 impurities (having 0.5 wppm Nb and 5 wppm Mo) that have ever been achieved in large-scale melting and fabrication practices of various steels. In other words, these are the lowest concentrations that have ever been achieved in large-scale melting and fabrication practices. They are not specific to any particular steel composition and should be achievable at present with a relatively modest effort and cost. The F82H FS of the blanket, structural ring (SR), vacuum vessel (VV), and shield was first examined with “nominal” impurities and then reexamined with “present” impurities. Our results showed that the blanket and SR with F82H FS and “nominal” impurities generates HLW, which is unacceptable. On the other hand, the controlled “present” impurity levels allow the blanket and SR to achieve the desired Class C LLW classification. This stresses again the need for strict control of the undesirable impurities (particularly Nb and Mo) that generate HLW. The remaining components qualify as Class C LLW even with “nominal” impurities. The shield and magnets are less radioactive than the in-vessel components and qualify as Class A LLW – the least hazardous type of waste. Excluding the clearable components (cryostat and bioshield), ~40% of the ARIES-ACT-2 waste (blanket, SR, VV, and divertor) qualifies as Class C LLW. The
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dominant radioisotope at 100 y after shutdown is 94Nb (from 0.5 wppm Nb impurity in FS). The remaining ~60% (shield and magnet) would fall under the Class A LLW category. Several critical issues for the disposal option can be identified based on the assessment of disposal situation in the US. These are listed below along with specific needs for the fusion community: Disposal issues: • High disposal cost (for preparation, characterization, packaging, interim storage, transportation, licensing, and disposal) • No commercial HLW repositories exist in US • Limited capacity of existing LLW repositories • Political difficulty of building new repositories • Prediction of repository’s conditions for long time into future • Radwaste burden for future generations. Disposal needs: • Revised activity limits for fusion radioisotopes issued by NRC • Repositories designed for T-containing materials. II.B. Recycling Issues and Needs In this study, the technical feasibility of recycling is based on the dose rate to the remote handling (RH) equipment. Essentially, the dose determines the RH needs (hands-on, conventional, or advanced tools to handle the radioactive components) and the interim storage period necessary to meet the dose limit. Advanced RH equipment has been used in the nuclear industry, in hot cells and reprocessing plants, and in spent fuel facilities.3 While the fission processes may have no direct relevance to fusion, their success gives confidence that advanced RH techniques could be developed to handle high doses (> 10,000 Sv/h) for the recycling of fusion materials. Beside the recycling dose, other important criteria include the decay heat level during reprocessing, recycling of Tcontaining materials, physical properties of recycled products, and economics of fabricating complex shapes remotely. Figure 5 displays the recycling dose for ARIES-ACT-2 outboard (OB) components. The variation with time of the recycling dose shows a strong location dependence. The highly irradiated components can potentially be recycled in less than a year after shutdown with advanced RH equipment that can handle 10,000 Sv/h or more. The FW that surrounds the plasma is an integral part of the blanket. It is shown in Fig. 5 as a separate component to provide the highest possible dose to RH equipment. The average FW/blanket dose is an order of magnitude lower. 54 Mn (from Fe) is the main contributor to the dose of FSbased components (FW, blanket, SR and VV) at early cooling periods (
