ARIES-Pathways Project Meeting Minutes
25-26 October 2010
Documented by L. Waganer
Ref: Agenda and Presentation Links: Meeting Agenda
Welcome/Agenda -Al Opdenaker welcomed the team and provided instructions for local internet connections and the hotel emergency exits and safety procedures. Les Waganer summarized the agenda for the next day and half meeting.
Next meeting and call - The next meeting will be held in the late January 2011 timeframe. Les Waganer will send out a Doodle questionnaire on the possible dates.
Plans and General Scope
DOE Expectations for the ARIES Team - Al Opdenaker said that the 2012 budget is currently being defined and discussed. He anticipated no drastic cuts in the ARIES design study funding. The team is viewed by DOE as a valuable resource that provides unbiased assessments.
Status and Update of the ARIES Study - Farrokh Najmabadi is pleased with the progress of the study that is now underway in 2011. The primary thrusts are investigating the plasma edge physics and the plasma interaction with the plasma facing materials. The main issues involve high heat flux on the core components and the anticipated off-normal plasma events with high energy discharges and ELMs. To help understand the complex technical implications, the systems code is being updated to provide detailed power core information about the extremes of conservative to aggressive physics and technology areas (i.e., the four corners parameter chart). We are also pushing the design and analysis envelope of divertors and first walls with advanced materials (tungsten) and plastic stress/strain analyses.Gene Nardella mentioned that he was appreciative of the ARIES efforts to advance the HHF knowledge involving current technologies and design code extensions beyond ASME code. He applauded the addition of Arnie Lumsdaine of ORNL to help the team understand the material issues. Such investigations might involve the use of tungsten, other advanced materials and innovative processes. It would be especially useful to identify new required (material) properties, technology and physics knowledge gaps and analytical predictions for better operating regimes.
Comments about the Recent ARIES PMI Town Meeting - Don Steiner mentioned this town meeting was a forum for many innovative divertor concepts and perhaps ARIES should investigate and assess some of the more promising approaches. This meeting also discussed new materials and new uses for existing materials.
Pilot Plant: Volt-Seconds Needed for Flexible Operation - As a matter of background into the pilot plant technical development, John Sheffield discussed on the volt-seconds (V-S) needed for the operation of a pilot plant, undertaken by PPPL as a study (neither affiliated with nor funded by the ARIES program). The ARIES Starlite study in the 1995 time frame is the basis for the Demo goal to which the Pilot Plant will be contributing. John mentioned that the Starlite study preferred a steady-state power core, but pulsed operation would be acceptable if a massive heat storage system were provided. If the technically simpler, pulsed plasma operation were considered, what would be its design characteristics?
John described the present Pilot Plant (PP) design features he used in his V-S analysis involving the required plasma inductance and resistance. John showed a table of key power core parameters highlighting those from ITER, ARIES-AT, Pilot Plant 1 and Pilot Plant 2 (two cases for consideration). Key questions involved determining the maximum solenoid current density and field, peak electron temperatures, Greenwald fraction, pulse length and plasma density and temperature. John concluded that the considered case of Pilot Plant 2 is larger, has a lower (solenoid?) current density, has a longer flat-top time (audience questioned this statement) and is more flexible with respect to plasma conditions. Dale Meade posed the question of how many cycles a large machine could sustain over its anticipated life.
ARIES-Pathways Task Results
Choosing Preliminary Strawmen, Utilizing VASST and Documenting (the) Systems Code - Lane Carlson told the team that he had completed two strawmen, namely the aggressive SiC blanket paired with the aggressive physics ßN ~ 0.045 and conservative physics, ßN ~ 0.03. The results are shown in a table (defined by a few million points). Lane described the filtering process to arrive at these tentative points. A few of the code parameters have been fixed as noted in the table on presentation page 5. The COE for these two points are about 69 and 91 mill/kWh in 2009 dollars. Other parameters are given in the tables on pages 6 and 7. On page 8, the power core cross sections of the two cases are shown. The next few pages illustrate the visualization capabilities of the systems code. The code displays many computed trial points for each "corner" that results in distinct clustered regions for each set of graphed parameters. A question was asked about the H98 vs. COE chart, "Why are some points at better COE values than the strawmen?"
Lane described his method of documenting the changes and evolution of the systems code as it improves and becomes more flexible. He continues to work with Laila to verify the output results with ARIES AT and other baseline data. He is planning on giving a paper at the TOFE conference on the systems code with some preliminary results.
Improvements to Power Flow Modeling in the ARIES System Code - Mark Tillack noted that in past meetings, several errors were noted regarding the modeling of the thermal conversion and power flow modeling in the code. The current models were not well documented or adaptable to new conditions. Specific topics addressed by Mark were:
There are currently two different ARIES graphical interpolation methods to determine the thermal conversion efficiency for the DCLL blankets and the SiC blanket that have been derived from specific blanket and divertor designs. However, bulk outlet blanket and divertor coolant temperatures really determine the thermal conversion efficiency. Advanced Brayton cycle efficiencies can be based on data provided by General Atomics, R. Schleicher, et. al. The power density establishes the limits for the bulk temperature due to structural temperature limits and heat transfer delta T's
The pressure drop in the blanket and divertors is related to the heat transfer capability as well as the heat flux limits. Mark discussed two charts displaying the current pumping power models for the DCLL and the SiC blankets. He also stressed that we need new pumping power models, especially for a helium-cooled divertor.
Mark mentioned that it is not clear how the power density limits are defined in the systems code. He cited the current divertor and FW peak heat fluxes and peak neutron wall load for the two blanket systems.
Mark also noted the pump efficiencies are defined by look-up table for the DCLL blanket while the SiC blanket pump efficiencies are obtained from a data file. He recommended using a 90% efficiency for a helium compressor, but the efficiency of the LiPb pump is dependent on the type of pump used. It was noted that most of the pump work is recoverable as it increases the temperature of the working fluid via friction losses.
Presently, the code partitions the fusion power and core radiation power by using the FW and divertor fractional area. This area method over-estimates the power flow to the divertor - a better estimate would involve scaling using neutronic and radiation models.
Comments on the ARIES-ACT 10/20/2010 Pre-Strawman - Laila said that the revised code now contains the new costing algorithms provided by Les Waganer. The results are shown in 2010$. No LSA factors are used in the estimates. Two cases were analyzed, one for the aggressive physics and one for the conservative physics, both with the SiC/LiPb blanket (aggressive technology). Laila showed the power core cross-sections along with the key parameters of the two cases compared to the published ARIES-AT design. She also showed the respective radial builds and the volumes of the major systems. She then compared the cost of those systems, noting the significant differences and possible causes. The recirculating power had some serious differences. Laila concluded with the areas remaining to be checked for accuracy.
Modeling Boundary Plasma/Neutral Characteristics for ARIES - Tom Rognlien discussed his plans for edge modeling of the four point-designs for ARIES, which involved baseline analyses, sensitivity studies, transient heat and particle flux analyses, and ash and DT fuel modeling. He acknowledged with the advent of more powerful plasmas, the issue of heat flux is becoming more important. Tom will utilize the UEDGE 2D plasma/neutral fluid transport code. He explained its features and capabilities and described its modular elements. UEDGE has been used on ARIES in the past - in 2001, Tom and others used ARIES-AT and FIRE plasmas to illustrate options to reduce divertor heat flux (e.g., from flat to tilted divertor plates in ARIES-AT). In FIRE, neon was seeded in the plasma to increase radiation to the first wall and reduce divertor heat flux. In 2006, UEDGE simulations used orthogonal plates and a simpler impurity model. Neutrals also can be important in the modeling of the divertor region, perhaps using an efficient fluid model or a Monte Carlo model. He would also consider analyzing the classical ExB and grad_B drifts for argon transport into the core plasma, e.g., DIII_D. He referenced the OFES FY10 Joint Research Target on scaling of the heat-flux width to help model the relationship of plasma current to the divertor heat flux.
Tom then provided more details on his plans for ARIES plasma modeling of the four point-designs relating to his basic areas:
Parametric Design Curves for Divertor Thermal Performance at Prototypical Conditions - Minami Yoda outlined her objectives to determine if the fins enhance thermal performance of finger-type modular divertor designs and to provide design guidance for future divertor design development. GT personnel constructed test module experiments with and without fins and then tested those experimental set-ups over a representative set of operating conditions and fluids. Both the Helium-cooled Modular divertor with Pin array (HEMP) was modeled, test fixtures fabricated and tested. An action item was generated to consider modeling and testing hexagonal pin arrangements. Minami explained the differences between effective and actual heat transfer coefficients (HTC) as well as the correlation between the test fluid (air) and working fluid (helium). Minami showed how to calculate the maximum heat flux, q", as well as the test values of q" for HEMJ, HEMP and forward flow (radially inward) with fins. They also tested the reverse flow (outward) without fins, which had poorer performance across most of the test range. She also explained the loss coefficients (pressure drop) and some results for the various test configurations - forward (radially inward) had the highest loss and reverse flow without fins the lowest loss.
Minami advocated the development and use of divertor thermal and hydraulic parametric performance and design curves. She illustrated such curves for the HEMJ and HEMP divertor coolant concepts. She suggested the next steps for their group might involve testing with helium in place of air and validating the experimental results with numerical simulations.
Updates on High Performance He-Cooled Tungsten Divertor Concepts - Xueren Wang reported he is striving for high heat flux performance on the ARIES divertor concept by analyzing the jet configuration, heat flux and stress-strain conditions using elasto-plastic structural analyses. He showed his combined finger-plate divertor design that is tailored to the anticipated heat flux. Tungsten is the current best candidate plasma-facing armor or tile. These tiles can operate from 1000°C up to 2000°C with the vacuum metalized tungsten (VM-W). They can be fabricated with Hot Isostatic Pressing or injection molding. Previous analyses indicated the finger design could handle a surface heat flux of 10 MW/m2. With some design optimization and plastic analysis, the ARIES finger divertor can handle a heat flux up to 15 MW/m2 without exceeding temperature or pumping power limits. After stress relaxation by plastic deformation, it also meets ASME code (3 Sm).
Xueren described the 2D non-linear structural analyses of the ODS-Ta-W joints for both fabrication and transient operations. The joints are subject to thermal cycling during diffusion welding from 1050°C down to RT, back up to 700°C and back to RT. The brazing operation also includes a thermal cool down from 1050°C down to RT. Operational cycles are from RT up to 700°C many times. Xueren showed the design criteria for the transition joint materials. His analysis indicated that the joints could survive during fabrication and warm shutdown, but ratcheting will occur with each cycle of cold shutdown, reaching a maximum strain value after 100 cycles. The 3D nonlinear structural analyses are underway. The preliminary 3D strain results are 7 times higher than those calculated in the 2D case, but future design improvements may reduce the 3D results.
Xueren Wang is continuing to improve his T-tube divertor design with tapered manifold tubes and reduced slot size, which results in a maximum heat flux of 11 MW/m2. Higher heat flux performance can be obtained with increased pumping power. Xueren is also working on the design and analysis of transition joints, similar to the plate and finger designs.
Methodology for Scaling Plant Availability to First-of-a-kind and One-of-a-kind Plants - Les Waganer noted the described work was co-sponsored by ARIES and PPPL Pilot Plant study. He explained that plant availability is a very important parameter for the competitiveness of power-producing plants. The expected availability for all types of new power plants is currently around 90%, however there is little or no guidance on what level of availability would be necessary for the developmental or demonstration plants leading to the first power plant. These plants before the first power plant must look like and act like the first power plant in order to adequately verify the predicted availability with acceptable program risk. In the distant past, fusion power plant studies assumed an availability goal with no valid basis. In the last three ARIES studies, Les has analyzed, with increasing detail, the necessary actions and hardware necessary to achieve competitive availability levels.
Les explained the definition of plant availability as a function of operational days (full power years) (FPY) divided by the total time including maintenance periods (both scheduled and unscheduled). He organized this maintenance time into five categories to simplify the analysis.
The ARIES-AT availability analysis yielded maintenance days/FPY goals for each of these classes of maintenance activities that established a plant availability goal of 87.6%. Les outlined the logic for the scaling of availability for the plants leading to the mature power plant (10th of a kind). This plant would have refined subsystems, validated reliability and lifetime databases, proven failure and end-of-life prediction methods and sophisticated autonomous maintenance systems. The first of a kind plant would have first generation subsystems and procedures with limited validation experience with an availability goal around 60-70%. The one of a kind plants (e.g., Demo) would have prototype subsystems and test articles to gain and establish reliability and maintainability data, with an availability goal in the range of 40-50%.Moving backward in time from the "suggested" maintenance durations of ARIES-AT, Les recommended multiplying factors for each maintenance category that applied to the first of a kind plant and the one of a kind plant, ranging from twice as long to ten times a long, depending on the maintenance category. These factors were intended to establish a yardstick to help guide baseline parameters for each facility. These factors were then applied to the ARIES-AT mature power plant maintenance durations to yield representative maintenance durations for the prior developmental and demonstration plants. These maintenance durations were converted to systems and plant availability, which yielded values very close to the goals suggested earlier. The exact values are not important, but they do set up reasonable maintenance and availability goals for these preparatory facilities.
Tritium Technology and Regulation Issues for (fusion) Power Plants - Lee Cadwallader explained that ITER is the best reference machine for handling, storing, delivering and re-purifying tritium for fusion demo and power plants. This experimental tritium system is designed based on a rich history of tritium systems in many large fusion experimental facilities. Lee thought ITER would be representative of a Technology Readiness Level (TRL) of 7 when operational. However the tritium breeding and extraction would still remain at the TRL 3-4 level as ITER will only have a few breeding modules. The ITER tritium system is quite complex as shown in a graphic of the planned subsystem arrangement.
The regulation of tritium in the U.S. is mainly administrated by DOE for its fusion experiments. ITER is limited to an occupational dose of 1 rem/y (10 mSv/y) as well as several other key safety parameters. Lee continued by explaining the differences between DOE, NRC and the ITER regulatory agencies (governed by the host country). Basic tritium accounting is accomplished by using a mass balance - shipment of tritium in (or bred) minus losses in effluent streams (including hardware). ITER believes they can determine tritium inventories to within 3% during a shift and within 1% over longer periods of time. NRC has not made any new regulations on tritium and they are not likely to do so until they assume regulation of larger fusion plants in the U.S. In summary, near term tritium handling systems are fairly mature except for tritium production and extraction.