Chamber Subgroup Question #5 Comments

Mohamed Abdou Wed 30 June 1999 11:37 Subject: Common Grounds Dear David and Mahmoud, I would like to provide you with some quick observations on this good and healthy exchange of views you are having. Please regard my opinions as those of a friendly scientist (not as a convener). My purpose is to help make the discussions converge. If my notes do not help, the just ignore them. Mahmoud gave his views based on certain generic calculations. Dave's thoughtful response is for the specific comparison of ARIES at 4 MW/m2 to the specific value of 10 MW/m2 used in APEX. The specific comparison is more compelling and easier to communicate to people. In the meantime, generic comparisons are useful in the diverse field of fusion with Tokamaks, FRC, RFP, etc. as well as many variants of blankets. Are There Common Grounds? I believe there are ample common grounds between both of your memos as follows: 1) Higher Power Density reduces the volume of the shield and the magnets. This volume reduction is roughly inversely proportional to the wall load ratio. So, for the specific case of 10 MW/m2 versus 4 MW/m2, the volume of the shield plus the magnet is lower by a factor of about 2. (Notes: the 1.15 for the shield thickness that Mahmoud uses becomes about 1.06 for this specific case of 10 versus 4.) 2) For Liquid Walls, the volume of the structure in the high flux region of the first wall/blanket is lower by a factor of 50 to 100 comparing the specific cases of ARIES and APEX conditions (last statement in Dave's message). I believe these common grounds are fine and sufficient for Snowmass. In Snowmass (and actually in most of our fusion research), we just need to know in which direction things improve and why. The actual numbers depend on many details: the actual system, the detailed design, and many other factors that are often uncertain. What we need to do is to indicate to the designers and the R&D community the directions where benefits may be found. Exact numbers will not be available for years to come.

Mohamed Abdou Wed 30 June 1999 11:37 Subject: Other Observations Other Observations I hope that Common Grounds such as those outlined above can be found. I add below some other observations that are not necessary for reaching common grounds but are more of technical curiosity and which are better left for future analysis: 1) I often use a factor of 10 increase in power density in a different context. For example, advocates of FRC are pushing for wall loads of up to 30 MW/m2. Some people in the community believe we need to push that high. Whether we need to or we will be able to are different questions. 2) Some people like to put the current limits on tokamaks as that of ITER and its Technology (e.g. 1 MW/m2). Whether this is fair or not is another debate. 3) In the second paragraph under "(3) Frequency of FW/blanket replacement", Dave raises a question about the two orders of magnitude. I believe that Mahmoud is comparing the flux at the back wall that follows the liquid wall to that at the first wall of a traditional first wall/blanket. If this is what Mahmoud intends, then the two orders of magnitude seem correct (but I let Mahmoud answer this if I misunderstood it.) 4) I agree with Dave that the first wall/blanket does not get replaced every time there is a failure. However, the number of replacements depends on the actual value of MTBF. There are some predictions of such short MTBF that lead to "cumulative" replacements of the module corresponding to one replacement of all modules during the lifetime of the first wall. There are other less pessimistic predictions. Again, it will be nearly impossible to resolve this soon (and it is unlikely that there will be actual blanket testing in a fusion device in the next decade). 5) The more detailed EU blanket designs have much more structure than we see in reactor studies. This is one of the reasons also that their TBR predictions are lower. 6) If traditional blankets have solid breeders (e.g. Li2Zr03, Li2Al02), do we need to account for this volume in the volume of waste?

Mohamed Abdou Wed 30 June 1999 11:37 Subject: Common Grounds Final Notes Thanks for your notes and for giving me the opportunity to provide technical remarks. I believe "Common Grounds" based on the specific case that Dave provided is useful and easier to communicate. One can add to it the remark that numbers will vary based on the actual confinement scheme, the details of the design, the results of future R&D, etc. Best Regards, Mohamed

E.T. Cheng Wed 30 June 1999 11:37 Subject: Comment on Waste I believe the waste quantity difference is about 50 to 100 as analyzed by Mohamed. It is significant, however, as pointed out by Mahmoud. This is like high level waste, a factor of 100 to 1000 does not make a drastic difference in changing the conclusion. I believe the liquid blanket concept has a significant merit if it works. But we also need to keep in mind that reuse of the blanket materials is an option. The liquid blanket is definitely a concept of reuse of the blanket material. The solid blanket material has to be recycled and refablicated. Both options can significantly reduce the waste quantity. When we factor the recycling and reuse of blanket materials into the consideration, the difference in waste quantity between the liquid and solid blanket concepts will be getting small, perhaps about factors of 5 to 10. Ed

Mahmoud Youssef Tue 6 July 1999 9:47 Subject: Response to Comments on Waste Dear Dave and Mohamed Thanks for your response to my memo. I was not able to reply for the last two days (medical reasons) but here is my comments (1) I'm very glad that our opinion on estimating the saving in the radewate volume upon using liquid FW/B concepts is converging. While the estimate could vary based on the underlying assumptions and design details, but at least the advantages of liquid walls over solid wall concepts is clear in this radioactivity waste management area that should be pointed out to our community. Of course there are other stringent requirements pertaining to the success of developing liquid wall concepts, particularly those related to hydrodynamically establishing the thick liquid wall itself. (2) The anticipated much lower failure rate in the liquid wall concepts must be taken as a factor in quantifying the reduction in the waste volume. I agree with both of you that the assumption of replacing the whole FW/B system after each failure may not be so realistic. The actual situation is hard to quantify exactly and requires in-depth analysis that takes into account the actual design details and correct information on MTBF and MTTR which is not fully available at present. But at least one can easily envision the need to replace defective parts (or the whole) FW/B system after frequent failure and this in turn adds to the volume of the waste stream of solid radioactive materials that needs to be disposed. Is a factor of 2-3 reduction a reasonable assumption? This needs to be debated. (3) The assumption of replacing the FW/B system after reaching a dpa level of 200 dpa needs to be re-evaluated. We need from the material community to provide a more precise guidance on this which takes into consideration the type of various candidate materials. For example, the FW/B in ARIES-RS which uses vanadium alloys as the structure is scheduled for replacement every 2.5 years for an average wall load of 4 MW/m2. This translates into ~20 dpa/FPY for a wall load of 1 MW/m2 (a factor 2 higher that ferritic steel). Is it fair then to use the 200 dpa limit as the criterion for FW/B replacement with FS and V-alloys structure? (4) The issue related to the option of recycling the radewaste (raised by Ed) should really be factored in. This is an important point that needs further evaluation. We can discuss this point at the Snowmass meeting. (5) I have no difficulty accepting the final conclusion of the 50-100 reduction in the volume of the radewaste of the "FW/Blanket" with the liquid wall concept over conventional blanket concept. This in itself a significant improvement over conventional blankets.

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