| Organization | ARIES-Pathways Project |
| Boeing | Waganer, Weaver, Seidler (Bill), McEwen (Dave) |
| DOE | Opdenaker |
| FIRE | Meade |
| General Atomics | Turnbull |
| Georgia Tech | Abdel-Khalik, Yoda |
| INL | Cadwallader |
| KIT | Malang |
| LLNL | |
| PPPL | Goldston, Kessel |
| RPI | |
| UCSD | Carlson, Najmabadi, Tillack, Wang |
| U of T, Knoxville | Sheffield |
| UW-Mad | El-Guebaly |
Ref: Agenda and Presentation Links: Meeting Agenda
Administrative
Welcome/Agenda -Al Opdenaker welcomed the team and provided instructions for local internet connections and the emergency exits and safety procedures. He also described the hotel facilities and organized the plans for dinner. Les Waganer summarized the agenda for the next day and half.
Next meeting and call - The next meeting will be held in the October 2010 timeframe in the Washington DC area, preferably in the same hotel. Al Opdenaker will make the hotel arrangements. Les Waganer will send out a Doodle questionnaire on the possible dates.
Plans and General Scope
DOE Expectations for the ARIES Team - Al Opdenaker mentioned that he appreciated having the meeting in a location he can attend. Provided the locations are nearby, he hopes to attend more meetings and become more involved in the project. He said that OFES recognizes and appreciates the ARIES contributions and values its objective results. Al affirmed that DOE will continue to support ARIES efforts and may ask for specific help in the future, however, flat budgets are currently foreseen.
Fusion Nuclear Science - Pathway Assessment - Chuck Kessel is leading the Fusion Nuclear Science Pathways "Program" for OFES with a mission to:
Magnetic Fusion: Issues, Metrics, Gaps and a Road Map to Energy - Dale Meade explained that the ARIES studies have identified the desirable characteristics of attractive (magnetically confined) fusion power plants, noting that significant advances remain to be planned, funded, and executed. He emphasized that we, as the fusion community, must establish guiding goals and requirements, define the present state of technology and then lay out a credible R&D technology roadmap. The 2007 FESAC Greenwald Panel provided an excellent outline of issues and needs for fusion with three high level themes. Identification of current gaps between the existing technology database and the desired end goal must be accomplished. The role of ITER to help achieve these goals was discussed. A heightened US fusion initiative is needed to achieve the goal of commercial fusion in the "near" future.
(The) Use of (a) Simple Analytic Expression in Tokamak Design Studies - John Sheffield highlighted the need to have computer codes to help define and quantify viable conceptual designs for fusion power plants, ultimately leading to a more fully engineered design based on validated technologies. John highlighted a few of the key plant parameters to be analyzed and optimized. Following detailed discussion of these parameters, he concluded it was possible to have simple formulae to understand how a pilot plant might be a staged element on the road to the demonstration and commercial power plants. John also mentioned a few of the main engineering considerations necessary, such as the ability to replace a superconducting TF coil, neutronic shielding for the superconducting coils and the effective shaping of the plasma.
Planned and Possible New ARIES Efforts - Farrokh Najmabadi affirmed the ARIES project will continue to investigate the edge plasma physics and plasma materials interaction (PMI) and plasma facing components. He said ARIES will also continue to support the DOE Fusion Nuclear Science (FNS) program. This is primarily accomplished by defining and analyzing the parametric operating space of conservative to aggressive plasma and technology conditions. We are also pushing technology boundaries by examining high heat flux solutions, pushing design codes beyond ASME codes and analyzing transient and non-uniform loading conditions. We are also trying to strengthen and broaden our system code capabilities.
ARIES-Pathways Task Results
ARIES Systems Analysis: Four Corners Identification - Chuck Kessel (and Lane Carlson) noted that extensive scans by the systems code have produced a large database of thousands of possible operating points with most interest in the four corners of:
The basic parameters and results of these cases were shown in a table that Chuck presented. The aggressive physics and technology had the lowest COE, but carried the highest physics and technology risk. The dependence on betaN was rather weak. Chuck suggested going to a larger major radius to help reduce the risk. He also increased the number of points examined in the corner regions to help validate and smooth the results.
The conservative physics/aggressive technology corner showed that higher magnetic fields can compensate for lower betaN values, while COE values are higher. This corner can be improved by tweaking some physics values.
The aggressive physics/conservative technology option requires higher fusion power to achieve the same net electric power so the device is much larger (with higher COE values). The COE still weakly relates to betaN. Certain physics values may be too aggressive.
The conservative physics/conservative technology is a poor performer as both the physics and technology underperform so the fusion power and power core get larger or bigger, hence the COE is much higher. It also needs higher fields to compensate for higher betaN values.
Chuck noted that all aggressive physics points have a plasma elongation (kappa) of 2.2; however, an elongation of 1.8 for the conservative physics case would move the vertical stabilizing shell from the middle of the blanket to behind the blanket. Chuck described a complete set of modified physics and technology parameters to refine the database and thicken the database. Chuck suggested a new set of starting parameters.
Exploring the Parameter Space with the Visual ARIES Systems Scanning Tool - Lane Carlson graphically displayed the general parameters for the four-corner operating point scan. Lane noted the preliminary filtering parameters to obtain the data point sets. Lane described the VASST GUI as a visualization tool that allows real-time display of the selected parameter sets. As an example, he showed a set of points for the aggressive physics/aggressive technology parameter set displayed as the Greenwald density function vs. major radius with various constraints. He also reiterated Chuck's points, trends and observations.
Lane used a graphic that indicated the SC magnet representation may be too optimistic. Lane also showed a graphic of COE for a range of size plants using an advanced SiC/LiPb blanket.
No ELM, Small ELM and Large ELM Strawman Scenarios - Alan Turnbull suggested that both ELM-free and small ELM plasma regimes may be available for tokamak power plants. Some ELM-free scenarios have been observed on C-Mod, DIII-D and NSTX. Small ELM scenarios include shaping with large squareness, type II (grassy) ELMs and type III ELMS. Both of these conditions may be restricted to a fairly narrow operating regime. However, it was also pointed out that in recent years some of the initially restricted operating regimes have been considerably expanded as a result of concerted research programs to do so. Alan pointed out that an MHD instability driven by steep edge gradients is the driver for the onset of ELMs. With this, ELM onset can be predicted by using the EPED1.6 model. Alan proposed to study the possibility of analyzing ELM-free and small ELM scenarios in the ARIES systems code, generally based on the H- mode cases from the ARIES 2000 study. Further data can be obtained from DIII-D and NSTX experimental data. He is concluded by pointing out that the key question is how to scale the discharges to power plant conditions given the current limited understanding of the physics behind the limitations in operational conditions in each of the scenarios.
Overview of (ARIES) Engineering Activities - Mark Tillack explained that the Engineering Group had established several themes to push the limit of high heat flux components, understand (via analysis) the transients and off-normal events, explore design improvements and define programmatic interfaces.
Update on the FW Armor Design and Inelastic Analysis - Xueren Wang described his analysis of a change in the thickness of the RAFS front plate from 1 mm to 2 mm to reduce the primary stress and plastic deformation and ratcheting. The cooling channels were reduced to 27 mm similar to the FW. Xueren observed that the temperature loading profiles for the fabrication steps and transient operations are similar. The RAFS material temperature limits are met during transient operations. No ratcheting is observed during transient operations with either warm or cold shutdowns.
Xueren re-evaluated the ARIES-CS DCLL FW design to determine how much heat flux could be tolerated. The temperature of the F82H is above the temperature limit at 1 MW/m2; however the structural behaviors stay within the plastic limit at heat fluxes up to 2 MW/m2. Creep modeling has begun, but no results have been obtained yet.
Extrapolating Model Divertor Studies to Prototypical Conditions - Minami Yoda explained their objective is to experimentally evaluate the thermal performance of an ARIES gas-cooled divertor design. Specifically, they are evaluating the use of fins to enhance the thermal performance for the plate-type and HEMP/HEMJ designs to levels exceeding 10 MW/m2. Using experimental test modules that mimic the design, GT has conducted experiments spanning the non-dimensional parameters expected of the proposed divertor design. Then they compared the experimental data with predicted results. The baseline designs rely on jet impingement, but can the heat transfer coefficient be enhanced with an array of cylindrical pin fins? What is the expected pressure drop?
Minami described the baseline plate-type divertor along with its expected operating parameters. She then explained the test set up with the pin-fin arrays. She also described the effective and actual heat transfer coefficients (HTC) and how these were calculated from the experiments. The effective HTC of the pin-covered surfaces are up to 180% greater that the HTC of the bare surface models. The actual HTC for the pin-covered surfaces are actually lower than the bare surface, but the cooled area is increased by 276%. There are some differences between the test media of air and that of helium. The maximum heat flux that can be increased up to 18-19 MW/m2 or, alternatively, the divertor can be operated at the lower design heat flux with lower pressure drops.
The helium-cooled modular divertor with pin array (HEMP) has been developed by KIT (formerly FZK) to accommodate heat fluxes up to 10 MW/m2. It uses pins in a circular array with inwardly flowing helium. This arrangement was experimentally modeled and tested with inward and outward flows. The pins do help the effective HTC. Flow outward was also tested, but it was not as effective as the inward flow of the original design. Numerical simulations show qualitative agreement.
Concept Development of Pin-Fin Cooled Divertors - - Mark Tillack summarized that UCSD has designed and analyzed jet-cooled divertor concepts, supplemented with Georgia Tech experimental results of pin-fin cooling enhancements. The goal is to integrate these two activities to yield an enhanced divertor reference concept.
Several fabrication approaches have been proposed including EDM, slitting with a saw, brazing pins, laser machining and electrochemical machining. Each of these approaches was explained.
Pins offer a higher level of thermal performance at the cost of added fabrication challenges, cost and reliability issues. So the question is whether the complexity of the pins is worth the extra challenges?
Update on High Performance Divertor Design and Analysis - Xueren Wang described the three high performance plate-type divertor concepts that have been investigated by the ARIES team (plate type, combination of plate and finger and finger modules over entire divertor surface). The ARIES design team devised transition joints constructed with tantalum. Two dimensional thermo-mechanical analyses including both elasticity and plasticity were performed on the joints to confirm acceptable stresses. They also analyzed the ODS- and W-rib thicknesses on the plastic deformations. Full 3-D non-linear analyses are needed to confirm an actual design.
Xueren showed a tapered tube divertor design window of the allowable tungsten tube temperature versus the plasma surface heat flux (8-15 MW/m2). The thickness of the W-armor has minimal effect on the temperature. Of course, the heat flux has a significant effect on the temperature. The limiting factor on the heat flux is the allowable pumping power limit, presently set at 0.21 MPa. The heat transfer coefficient can be increased by reducing the width of the slot jet. Both the tapered and non-tapered T-tube designs have stresses well below the 2Sm stress limit, thus temperature is the limiting constraint at present.
The plate-type divertor covered by small fingers will be re-investigated for plastic deformation to assess the acceptable design limits.
3-D Assessment of Neutron Streaming Through Assembly Gaps: Outboard Options - Laila El-Guebaly reviewed the background and concerns on neutron streaming via straight, stepped, and multiple steps during operations. She noted that a 2-cm gap is a reasonable gap during operation although some gaps may close due to thermal swelling. The goal is to redesign gaps with shielding blocks to reduce to a no-gap level, i.e., below acceptable radiation limits.
Laila went on to summarize the April 2009 inboard gap analysis results showing the inboard model with radial manifolds and 5-cm stepped shielding in the gaps to reduce neutron streaming. These stepped shield blocks are passively cooled. The associated analysis indicated stepped WC shielding blocks are effective in protecting inboard components. The overall arrangement is compatible with the sector maintenance scheme of ARIES tokamaks.
The current analysis on the outboard used the current CAD definition of the outboard radial build with a peak NWL of 5 MW/m2. The University of Wisconsin - Madison created a CAD model for the 3-D streaming analysis using the CAD-MCNP approach. Cases were analyzed with straight 2-cm gaps and with a shielding block. The skeleton ring, VV and magnets can be lifetime components in both cases, but the VV could not be rewelded with the straight gap. On the other hand, the shield block would allow the VV to be rewelded at a 20-cm lateral distance. Some redesign with the shield block is necessary to allow maintenance of the outer sectors. A slim wedge shield block was analyzed, but it provided inadequate shielding. A stepped wedge and an external local shield are other design options to be considered in the future. There seem to be reasonable shielding solutions for the outboard gaps.
Electrical Resistivity Changes with Neutron Irradiation and Implications for W-Stabilizing Shells - Laila El-Guebaly summarized the rationale for using tungsten as the material for the stabilizing shells in the outboard blanket: reasonable resistivity, high temperature allowables, no active cooling required and acceptable influence on tritium breeding if placed between blanket segments. However, the resistivity of tungsten increases with temperature and neutron irradiation, which may dictate thicker shells that will impact TBR, temperature gradients, thermal stresses and cooling. Laila showed the resistivity of W with temperature (up to 1200ºC). As tungsten is irradiated, its composition changes with the addition of Re, Ta, Os and other transmutation radioisotopes, which contribute to the dominant resistivity changes with temperature. She then deduced the shell thickness as a function of temperature and irradiation.
A potential option would be to consider the LiPb as the stabilizing shell. She outlined several options for this approach.
PMI Issues and Concerns
PMI-Driven Mission Risks, Status, and Data Sources for the Next DT Device - Rob Goldston summarized the key risks and status of the technologies for the next DT device including:
Rob mentioned the operational time periods of the future experimental facilities that might answer some of these important questions. He provided a simplified set of PMI parameters for these facilities as well as other proposed DT facilities. He proposed a series of time-phased experiments through 2028 to answer each of these PMI issues and concerns. He concluded there are significant PMI-driven mission risks for the next (large) DT device. Mission risks, status and data source analyses should be expanded to other PMI risk areas. The timing of the R&D, design and construction of the next large DT facility should employ risk analyses to quantify the acceptable mission risk.
ARIES Town Meeting Discussion
Highlights of the ARIES Town Meeting regarding "Edge Plasma Physics and Material Interactions in the Fusion Power Plant Regime" - The intent of the town meeting was to determine the status of the current understanding and predictive capabilities for edge plasmas and PMI, what R&D is needed to design the power plants and what new facilities are needed to advance our knowledge.
Mark Tillack noted that there were 42 participants from 17 international organizations that participated in the ARIES town meeting. The presentations are archived on the ARIES web site, Edge Plasma Physics and Matl Interactions Town Meeting, highlighted on the DOE OFES VLT and a journal article is being prepared.
The meeting produced recommendations for the ARIES project in seven topic areas:
Mark highlighted the discussion and the supporting data for each topic area, followed by specific recommendations for ARIES. For the added detail, see Mark's slides via the agenda link, ARIES July 2010 Project Meeting Agenda..