ML20212R522
| ML20212R522 | |
| Person / Time | |
|---|---|
| Issue date: | 09/25/1986 |
| From: | Krauskopf K Advisory Committee on Reactor Safeguards, STANFORD UNIV., STANFORD, CA |
| To: | Merrill O Advisory Committee on Reactor Safeguards |
| Shared Package | |
| ML20212R512 | List: |
| References | |
| ACRS-CT-1861, NUDOCS 8702020694 | |
| Download: ML20212R522 (3) | |
Text
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STANFORD UNIVERSITY STANFORD, CALIFORNIA 94305 /'M TELEPHONE:(415) 723 2537 DEPARTMENT OF GEOLOGY -
School of Eanh Sciences -
- 25 September 1986 Mr. Owen Merrill, Staff Engineer Advisory Committee on Reactor Safeguards Nuclear Regulatory Commission .
Washington, DC 20555 '
D2ar Owen:
This is a long-delayed answer to your letter of 29 August, in which you ask I'm sorry for com=ents on Generic Technical Positions on solubility and sorption.
about the delay, but I have been out of the country for much of the past month and have only now found time to go over the volume you sent with the care it deserves.
I found the volume very interesting, especially the transcript of the 22 July maeting of the ACRS subcommittee -- it must have been a lively session!
Let me lead off with a couple of general comments. First, I like the emphasis on experimental work rather than theory. Of course it would be nide to know the intricate details of the dissolving of complex waste forms, and the mechanisms of the many processes that are lumped as " sorption,"I but to get with disagree this the kindconclusion of information would require a century of concentrated research.
of the NRC/ORNL workshop (which you also included) that complete understanding of corption processes is essential; we should work toward such understanding, sure, but in the time available for waste disposal we must depend more on empirical data.
Second, I agree wholeheartedly with the subcommittee's judgment that the two GTP's are too prescriptive. Parts of them are also too much like elementary text-books. I gather that the documents are supposed to serve as guides to chemists torking for DOE, telling them what sort of information DOE will be expected If I were to produce a DOE chemist, to enable NRC to make rational judgments about licensing.
it would be an insult to my intelligence to read that "the redox potential and acidity of groundwater can strongly influence the mechanisms of dissolution," or " sorptionIf Exparf-ents can be subdivided into two types, closed systems and Seemsopento systems."
me the approach I didn't know that sort of thing, I shouldn't have the job.
' of the GTP's ought to be to spell out in detail what NRC will consider to be impor-tant, and to list the kinds of back-up evidence that DOE will be asked for in support <
of its conclusions and reconmndations -- rather than to give an elementary course t
in physical chemistry and then describe what experiments "should be done.
l' mo, Then for some more specific comments. (1) Seems to me not enough attention is
'O paid to details of the dicsolution process. In spent fuel, the actinides and Tc h jS will be minor contamin,ents in the UO crystal structure, and will dissolve only as y Reversible equilibrium here has no meaning;
.c fas)hasjis, structure breaks down. 2 crystals wouldn't sure y gdontaminated UOp precipitate};j ifk o so, i y o u - from an oversaturated solution. Elements whose dioxides are more soluble than UO N would go into solution as fast as the uranium does, and would be unsaturated with g\
y respect to dioxide precipitation; elemnts less soluble would In this sense notangoequilibrium into solution, E tsor- but would form their own dioxides as precipitates.
ma.o might be set up, but it would be an equilibrium between solution and pure Ac0 ,at n would yg the waste form. Then a difficulty might arise because the Ac02 ( r Ac(OH)4) 2 e (This 3 i
be in very tiny particles, hence would be more soluble than largerBom crystals.
ailicate glass M is a form of metastability that should be specifically flagged.) $
f o e _
i STANFORD UNIVERSITY STANFORD, CALIFORNIA 94305 TELEPHONE:(415) 723-1537 DEPARTMENT OF GEOLOGY , '
School of Eanh Sciences ~ .
would presumably _have the radionuclides randomly distributed thru the glass structure, cnd here again reversibility of dissolution is absurd; the contyninants would be releasedastheglassdisintegrates,andeachwouldbecontro1gedbythesolubility of its dioxide (or whatever most insoluble compound it forms in an aqueous environment),
(2) Another detail of the dissolution process that isn't mentioned is the prob-obility that radiolysis of the groundwater will produce Thisan oxidizing would environinent certainly acceleratein the immediate vicinity of the waste-form surface.As these nuclides move away from the waste dissolution of U, Tc, Np, and probably pu.
they would encounter the normal reducing conditions of the general repository environ-cent and would prsumably precipitate; but
- he solubility equilibrium Probably would beof the existence operating such at a distance rather than at the waste-fo..n surface.
a redox front is not of great importance, but it is something that DOE should be raminded of, to keep in mind.
(3) Nothing is said about the composition of gas in contact with the groundwater.
Immediately after a repository is closed the gas will be ordinary air; its compo-sition will gradually change by reaction with water and rock and waste; presumably eventually the composition will be largely detennined by radiolysis, and close to the waste may be a nixture of Hp , 02, and various nitrogen gases.
(4) Fage 5. third sentence of 4.1: I think " total range . . . anywhere and at ony time ... " should be toned down a bit, because it could be so easily used by intervenors.
(5) Fage 6, just before 4.2: I would say " radionuclides whose compounds have It makes no sense to high solubilities under thplanned experimental conditions."
cpeak of a "radionuclide having high solubility" without qualification.
(6) Fage 7, top: It is so easy to talk about measuring Eh and pH at high tem-I don't think it's right to toss out such perature, but so very difficult to do it!
a glib recommendation without noting how nearly impossible it is to fojlow it.
(7) Fages 7, 8, 9: surely DOE wouldn't hire anybody who doesn't know all this?
(8) Page 12, section 4.4.2: This is all true enough, but what is DOE supposed to do about it? Can't NRC give some sort of advice about what meaning should be or will be attached to Eh measurements?
(9) Page 13, end of first paragraph: As noted above, this has meaning only if the solid in question is specifically defined. It does not apply to waste forms.
(10) Seems.to e not enough is said about competition for sorption sitesToamong me, different ions and about the limited sorption capacity of most surfaces.
' sorption is a dynamic process which eventually establishes a stesdy state between ions landing on a surface and ions escaping from it. This means that surfaces near the waste rapidly become saturated and can have only a temporary retardation effect.
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1 i STANFORD UNIVERSITY
' STANFORD, CALIFORNIA 94305 TELEPHONE: (415) 723-2537 DEPARTMENT OF CEOLOGY School of Eanh Sciences -
(11) (sorption GTP) Page 5, bottom: I guess this suggestion is theoretically OK, but for ions that will be present at such very low concentrations I wonder if o change in Kd with concentration has much significance?
(12) Page 6, top: In-situ tests are a great idea, and they certainly should be r com.m ended, but some cautions should be mentioned if such tests are to have any meaning.
If grounwater flow is chiefly thru fissures, the tgsts should be run at more than ene spot; results from a single place would be capricious, because fissures very so much in size, number, and orientation. Care must be taken that addition of the tracer doesn't change groundwater chenistry. Air must either be excluded or an ion used and CO . And if an anal g element is to be that is not sensitive to dissolved 0 2 2 used in place of a radionuclide, the element must be chosen with great care.
(13) Tage 7, middle: Why not mention specific elenents? For example, under cost repository conditions solubility controls the concentration of Pu, As, and Cm over a wider range of Eh and pH than the concentration of Np and Tc; hence good data on sorption are more critical for these latter two.
(14) Nitpicking:
Vhitfield's article is dated 1972 on p.12 and 1969 on p.16 (solubility GTP).
After all this destructive criticism, I should say that the authors of the two GTF's have done a good job in assembling and organizing a lot of useful information, My They have covered the field well, and most of it is up-to-date and accurate.
criticisms are mostly.on minor matters. The principal change I'd like to see is just a change in emphasis: from prescriptions about what experiments should be done to guidance for DOE with regard to the kind and depth of information it will need to furnish NRC as a basis for licensing decisions.
I hope this will be of some use, Owen -- and again my apologies for getting it to you so late.
"' Woth best regards, j tot Konrad B. Krauskopf
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