Mechanisms Governing ⁹⁰Sr Removal and Remobilisation in a VLLW Surface Disposal Concept

Round 1

Reviewer 1 Report

Brief Summary

This paper aims to determine how different barrier materials (i.e. rock, bentonite, steel) may interact with Sr under Finnish near-surface repository conditions, to help inform a safety design case.  Overall, the results determine that Sr retention is largely due to outer-sphere sorption to rock flour and bentonite, with a minor fraction more strongly bound.  During remobilization experiments, more Sr is released during seawater exposure (higher ionic strength) compared to rainwater in bentonite-containing columns, consistent with ion-exchange.  Interestingly, seawater did not increase Sr release in rock flour only systems, indicating that Sr is more strongly bound to the rock flour material. The paper is well written, the data and conclusions are clearly presented, and the authors demonstrate how the results contribute to the current literature and the Finnish near-surface repository safety design case.

General concept comments

I have only minor suggestions for the authors, highlighting a few typos/inconsistencies (detailed below), and addressing some issues with their EXAFS fitting.  Specifically, the second shell (Sr-Si/Al) seems to need further refinement, given that the debye-waller factor is unrealistically small, with a relatively large uncertainty.  However, as the fitted k-range is only up to 9, this may be an unavoidable effect of poor data quality, presumably due to the low Sr concentration.

Specific comments

L228-230: “The fitting procedure was constrained to fixed values for the passive electron reduction factor (S02) and Debye−Waller factor (σ2) to determine Sr coordination number and additional shells.”

This statement implies that the debye-waller factor was fixed, and the coordination number was unrestrained.  However, in the SI (Table S4), the debye-waller factor seems to have been unrestrained (uncertainties are provided) and instead the coordination number appears to have been fixed (no uncertainty provided).  More clarity here and/or in Table S4 would be helpful.

 

L353-355: “Attempts to fit subsequent shells using sensible parameters (i.e., Fe, Al, Si, and / or C )  yielded unsatisfactory fits (unrealistic bond lengths or R-factors > 0.02; SI Table S4)”

The SI Table S4 doesn’t show these unsatisfactory fits, only the plotted ones. These fits could be added to Table S4.

 

L355-359: “In all other samples, the EXAFS spectra were best fit with a first backscattering shell containing 8.5 O backscatterers at 2.55 Å – 2.56 Å, and a second shell containing 0.7 Si/Al backscatterers at 3.33 Å – 3.35 Å , which statistically improved the fits and is consistent with Sr sorption to bentonite (F-test 92 – 99%, SI Table S4)”

Slight typo here, the O backscatterers in Table S4 are at 2.54-2.56 Å, not 2.55-2.56 Å.

Also, the addition of a second shell might statistically improve the fit, but the dybe-waller factor seems incorrect for all fits.  In particular, the “Rock flour + bentonite (6 wt.%) + steel coupons 5.0 cm ‒ 5.5 cm” has a σ² of only 0.0001(3), which is unrealistically small.  In fact, all the second shells have an error larger than the σ² (despite negative σ² not being possible), and have values significantly smaller than the first shell.  This suggests that these fits need further refinement if possible. 

Author Response

In attached word document.

Author Response File: Author Response.pdf

Reviewer 2 Report

This reviewer considers this manuscript an excellent piece of work, very well written and structured. The topic covered by the authors, i.e. radionuclide behaviour under surface disposal conditions, is challenging and the results reported in this work will be of great help for the forthcoming investigations in this area. It’s is important also to stress, as already reported by the authors in their conclusions, that additional data/measurements are needed to verify their findings.

I have just a comment about the possible effect of carbon steel coupons in the sorption behaviour of Sr. According to the plots included in the manuscript the column test were running for almost a year. Is this enough time to induce/promote corrosion in such coupons without any catalytic agent (i.e. microbes, external potential, etc.)? I wonder if authors may add, if possible, information about carbon steel coupons characterization after the column tests. Basically to clearly indicate if iron corrosion products have been formed (or not) in the experiment, as they are a well know sink for elements.

Typos:

I’ve found a missing parenthesis in line 426.

Let me emphasize again the excellent work performed by the authors.

Author Response

In attached word document.

Author Response File: Author Response.pdf

Reviewer 3 Report

Summary

The study investigates the retention properties of rock flour intended as backfill in a low-level nuclear waste repository with regard to Sr-90. With the aid of column tests, it is clearly shown that the rock flour in leads to significant retention and that it takes long test periods until the original concentration of strontium is reached in the effluent. Bentonite or iron additions have only a minor influence on retention; they bind strontium only weakly. Inflow of seawater leads to remobilization and should be considered in the design of the repositories.

General comments

Is the article well-organized?

The article introduces the reader to the topic and describes in a logical, clear sequence the motivation, the approach, the performance of the experiments and their interpretation.

Does the article contain all the components you would expect?

The article contains the essential elements that would be expected for this purely empirical study. A good addition would be a modelling part to support the interpretation of the results (e.g., saturation indices for Sr phases, transport modelling). E.g.: strontium carbonate could be saturated at least at the beginning of the experiment at high pH values, if the partial pressure of CO2 in the atmosphere is used. However, this is only a recommendation and not mandatory.

Are the sections well-developed?

All sections are well developed. The open questions for the disposal of low-level radioactive waste in near-surface landfills are clearly identified. For reading the article, it would be useful to emphasize the specific research question addressed in the present paper (lines 99ff.). In some parts of the results section, experimental details are repeated that would be better placed in the methods section. The introduction could be somewhat shorter. It seems somewhat long in relation to the later parts.

Does the author do a good job of synthesizing the literature?

The introduction mentions relevant aspects of the presence of Sr in radioactive waste and the behaviour of strontium in the geosphere. This compilation appears to be sufficient in the context of the work. The microbial degradation of the organic components of the waste materials could be relevant to the question (pH value, CO2 production). This should be mentioned briefly. The article would benefit from adding an illustration showing the concept of the planned repository so that the reader can get an idea of the situation.

Does the author answer the questions he/she sets out to answer?

The question outlined in the introduction is specifically addressed in the thesis and clearly answered in the conclusion section.

Is the methodology clearly explained? Are the manuscript’s results reproducible based on the details given in the methods section? 

The basic approach to clarify the question is clearly justified and elaborated in the method description. Beside some minor gaps, them experimental setup and the analytical methods are reproducibly outlined.

Especially for column experiments, however, it is important to discuss the expected or assumed geochemical and physical initial and boundary conditions in the respective system that shall be represented or approximated by the experiment, e.g. redox conditions, temperature, chemical processes that haven taken place before, presence/ concentration of CO₂(g)/CO₃² (important for the formation of strontianite):

 - In the introduction, for example, reference is made to the large temperature fluctuations to be expected. What temperatures are to be expected in the repository . Why is it justified to work at  21°C (e.g., for practical reasons, but 21°C is a conservative assumption as retention is lower at this temperature/ difference are considered negligible?).

- The addition of steel coupons is an interesting variation. But it is important to discuss the condition of the waste containers at the time of radionuclide release (they could already be heavily corroded or an early failure is assumed, so that corrosion is only slightly advanced.

- What influence can microbial degradation of organic waste ingredients have (it is perfectly legitimate to exclude this process and its effects in the experiments for practical reasons, but it should not be ignored).

Are the figures/tables/images/schemes appropriate? Do they properly show the data? Are they easy to interpret and understand?

The diagrams explain well the data described in the text. Some additional information in the diagrams may help to better and quicker understand what is shown (see specific comments)

Is the article well-written and easy to understand?

The article is for the most part clearly and comprehensibly described and at a high linguistic level.

Are you convinced by the author’s results? Why or why not?

The results presented are convincingly presented. The conclusions on the retention capacity of backfill from rock flour/bentonite will be convincing as well when the questions on the initial and boundary conditions have been clarified.

Other comments

In some places, name/year is used as a citation in the text instead of a number [11]. Is this intentional? E.g., line 124/125

I would suggest using mg/l throughout the text and not ppm which may have different meanings. Same applies to solids in table S1: µg/g instead of ppm

line 83 (pellets or pallets?)

line 154-159: These lines anticipate some results

line 175: What is the shape/ diameter of the steel coupons used? Is there any further information which carbon steel type has been used (not chemical composition, but material code)?

Table 2: entries in the first column should be move upwards so that it is clear which entries in the other columns belongs to them

Line 232: the sentence is not easy to read

Line 250-255: seem to repeat parts of chapter 2

Line 265: The increase of Sr concentrations was interpreted as a result of saturation of the sorption sites. A different but compatible interpretation would be that the continuous decrease of the pH (10à6) reduced the sorption capacity of the solids. This relationship may be discussed as well

Line 286ff. As it appears, colloids do not occur in the experiments. Therefore, the number of citations on concerning colloids may be reduced significantly [57] – [67]

Line 302-308: seem to repeat parts of chapter 2

Figure 2 (B): This graph is said to show the percentage of 0.5 N HCl extractable Fe(II), but no method has been described to determine extractable Fe(II). The only method (line 200) refers to extractable Fe content. The x-axis title Fe (%) is not readily comprehensible. 

Figure 3: Instead of 'A', 'B', 'C' the three diagrams may be labelled with abbreviated compositions, e.g. 'rock', 'rock + bent', 'rock + bent. + Fe'. Would make it easier to read it

Line 360; where there any XRD measurements of the solids after the experiments? Would be interesting to see whether there were definite iron corrosion phases

Figure 4. EXAFS spectra and Forurier transformations are not easy to interpret, especially if shown without further explanation. Without such they are mere illustrations and proof that such measurements have taken place. But they don't tell much. So, if possible some parts of the transformations may be highlighted to show the key characteristics/differences and the interpretations. 

Lines 381-386: seem to better fit in the introduction section

Lines 386-388: seem to repeat parts of chapter 2

Line 392: What is completely unclear is that the column experiments until day 295 produced Sr concentrations of max. 2.1 mg/l, but the remobilization experiments started with up to 5 mg/l. This concentration jump is not explained

Figure 5: Adding 'rainwater' and 'seawater' to the left and right half of the diagram would allow a quicker understanding of the diagram.

Line 436: speaking of the 'remobilization potential' of rainwater sounds odd when the deposition of strontium was made by pumping actually rainwater through the column. In this specific case, ‘leaching’ may be an alternative way to describe this observation

Line 450: the text says that corroding waste packages do not seem to significantly bind strontium. That is correct for the specific conditions of these experiments. But as fresh coupons were used, the amount (and active surface) of corrosion was probably very limited, whereas in reality, massive corrosion may have taken place before any radionuclide could be mobilized from the waste matrix. So, a general statement on the relevance of waste packages can probably not be done.

Line 466: Data availability statement. This is currently only text from a template. Please insert some appropriate statement. Consider providing the raw data shown in Fig 1-5 and Fig. S.3 in the supplementary information or add link to a report / thesis(?) where such data is or will be published.

Supporting Information

 

Table S1: According to this table, the steel coupons contained only 7000 ppm Fe. Some figures must be missing here

 

 

Author Response

In attached document

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

All of the issues raised in my first review have been addressed in detail and in an appropriate manner. In my view, no further changes are necessary.