ACT2 and REACT not in agreement

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From: Andrew L. Scott

Subject: ACT2 and REACT not in agreement

I have been trying to model the Yucca Mountain J-13 well water in preparation for performing a sensitivity analysis for U speciation. Attached you will find the Statement of the Problem, some REACT output, the thermo dB, scripts for ACT2 and REACT and some calculation macros. I updated the thermo.com.v8.r6+ dB with the NEA 2003 numbers for U species only at 25 deg C. I hope someone is able to understand and help with my problem. I have tried to be as complete as possible with the materials I have provided, but if there is anything that I have not included or you do not understand something I have done, please feel free to ask. [A note from the admin: The files are not available

From: Mark Logsdon

Subject: Re: ACT2 and REACT not in agreement

You certainly have been complete in your information. I regret that I don't have time to go throuygh all your calculations, but I have a quick question on the two bases you use. Your REACT basis gives Na at 2 mmolar and Cl at 0.2 mmolar. But the ACT2 basis has log a Na at - 3.5 and log a Cl at -2.8. Are you sure you are calculating the acivities properly? If you just take log C (as a quick consistency check) I get -2.7 and -3.7, respectively, so the log a values look funny to me - and in fact, their order doesn't look right either. I haven't tried to do this for other components or to check your spreadsheet.

From: Tom Meuzelaar

Subject: Re: ACT2 and REACT not in agreement

I think I see the problem. If you allow NO3- to speciate over your activity diagram (in Act2, Basis tab, click the right-most arrow for the NO3- entry, and choose "Speciate over X-Y"), you'll get a very similar diagram to your Act2 plot (the "correct" plot) without NO3- (if you additionally suppress Coffinite, the diagrams look even more similar). In your original "incorrect" activity diagram, there is an implicit assumption that NO3- is stable over your entire Eh-pH range- React predicts, however, that most of your NO3- component will occur as the N2(aq) species- you can confirm this by looking at your React output file. By choosing the "speciate" option in Act2, you're allowing the program to predict which nitrogen species is stable- the outcome greatly effects the final diagram.

From: Andrew L. Scott

Subject: RE: ACT2 and REACT not in agreement

Thanks for the feedback, particularly Tom Meuzlaar and Mark Logsdon. Based on their comments, I recalculated the basis and allowed speciation over the X-Y axis in ACT2, and now have perfect agreement (and a greater understanding of process) between ACT2 and REACT.

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If the silicon species is not allowed to speciate, quartz will not precipitate, and allows the precipitation of Haiweeite, a uranyl silicate mineral, at equilibrium. This calls into question the kinetics of the two processes (quartz and Haiweeite precipitation/dissolution) with regard to the mobility of the U in the Yucca Mountain J-13 water (represented by the top-most green dot).

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