ASTM D8091-21

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Designation: D8091 − 16 D8091 − 21
Standard Guide for
Impregnation of Graphite with Molten Salt
This standard is issued under the fixed designation D8091; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope Scope*
1.1 This guide covers procedures for the impregnation of graphite with molten salt under a consistent pressure and temperature.
Such procedures are necessary if the user wishes to prepare graphite specimens for testing that have represent material that has
been exposed to a molten salt environment that may not necessarily represent material exposed to an operating reactor
environment.in a molten salt nuclear reactor. The user will need to ensure that impregnation temperature and pressure conditions
reflect those pertaining to the molten salt environment, noting that the properties of the material will change once it becomes
irradiated.
NOTE 1—The term impregnation is used throughout this guide as this is the correct term for the described process. Other terms such as infiltration and
intrusion may be encountered by the user in other texts and the term intrusion is commonly used to describe penetration of open porosity in graphite in
a molten salt reactor environment.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this guide.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
B923 Test Method for Metal Powder Skeletal Density by Helium or Nitrogen Pycnometry
C559 Test Method for Bulk Density by Physical Measurements of Manufactured Carbon and Graphite Articles
D7775 Guide for Measurements on Small Graphite Specimens
3. Terminology
3.1 Definitions:
3.1.1 impregnation pressure (P ), n—the differential pressure of between the cover gas used in the impregnation.pressure and the
I
pore pressure of the graphite specimen.
This guide is under the jurisdiction of ASTM Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcommittee
D02.F0 on Manufactured Carbon and Graphite Products.
Current edition approved Dec. 1, 2016July 1, 2021. Published January 2017July 2021. Originally approved in 2016. Last previous edition approved in 2016 as D8091 – 16.
DOI: 10.1520/D8091-16.10.1520/D8091-21.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D8091 − 21
3.1.1.1 Discussion—
If the impregnation starts at a pore pressure of atmospheric pressure, the impregnation pressure is the gauge pressure of the cover
gas; if the impregnation starts at a pore pressure of “0” (vacuum), the impregnation pressure is the gauge pressure plus atmospheric
pressure. For a pore pressure between 0 and atmospheric pressure, the impregnation pressure is (gauge pressure + atmospheric
pressure – pore pressure).
3.1.2 impregnation temperature (T ), n—the system temperature before the graphite samplespecimen has been immersed in the
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molten salt.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 Parameter D—D , n—provides a measure of the extent of penetration of the graphite porosity by the molten salt. If there
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was no damage to the microstructure of the graphite during impregnation, then parameter D based upon open pore volume would
be unity at saturation. At high impregnation pressures, closed porosity may be broken into by the molten salt and parameter salt
expressed D based upon open pore volume could have values greater than unity. It is therefore useful to express parameter Din
terms of boththe open pore volume and total (open and closed) pore volume.of the specimen.
3.2.1.1 Discussion—
Parameter If there was no damage to the microstructure of the graphite during impregnation, then parameter D is based upon open
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pore volume (typically applicable at low impregnation pressures) would be unity at saturation. This parameter is applicable when
damage to the graphite microstructure during impregnation is absent or low. Mercury porosimetry studies indicate that the
3,4
threshold pressure for microstructural damage is 13.8 MPa to 20.0 MPa (2000 psi to 3000 psi). This threshold should be used
as a guide by users when evaluating D .and At high impregnation pressures, closed porosity may be broken into by the molten salt
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and parameter D based upon open pore volume could have values greater than unity. At such pressures, the user may wish to
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express parameter D in terms of total (open and closed) pore volume, termed D .is calculated as Evaluation of this parameter is
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provided in 9.2follows:.
W 2 W
2 1
D 5 (1)
o
ρV
o
where:
W = the weight of the graphite sample before impregnation,
W = the weight of the graphite sample after impregnation,
ρ = the density of molten salt, and
V = the open pore volume in the sample.
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3.2.1.2 Discussion—
Parameter D based upon total pore volume (typically at high impregnation pressures) is calculated as follows:
W 2 W
2 1
D 5 (2)
t
ρV
t
where:
V = the total pore volume in the sample.
t
4. Summary of Guide
4.1 This guide provides guidance on the impregnation of graphite with molten salt. The guide gives the various factors which need
to be considered to perform the impregnation procedure. These include pretreatment of graphite samples,specimens, immersion of
graphite in the molten salt, safe handling of the molten salt, and selection and control of the impregnation pressures and
temperatures.
5. Significance and Use
5.1 The molten salt reactor is a nuclear reactor which uses graphite as reflector and structural material and fluoride molten salt
as coolant. The graphite components will be submerged in the molten salt during the lifetime of the reactor. The porous structure
Dickinson, J. M., Shore, J. W., “Observations Concerning the Determination of Porosities in Graphites,” Carbon, Vol 6, 1968, pp. 937–941.
Baker, D. J., Morris, J. B., “Structural Damage in Graphite Occurring during Pore Size Measurements by High Pressure Mercury,” Carbon, Vol 9, 1971, pp. 687–690.
D8091 − 21
of graphite may lead to molten salt permeation, which can affect the thermal and mechanical properties of graphite. Consequently,
it is important to assess the effect of impregnation of molten salt on the properties of the as-manufactured graphite material.
5.2 The purpose of this guide is to report considerations that should be included in the preparation of graphite samplesspecimens
representative of that after exposure to a molten salt environment. The degree to which the molten salt will infiltrate the graphite
will depend upon a number of factors, including the type of graphite and the type and extent of porosity, the properties of the
molten salt, the impregnation pressure and temperature, and the duration of the exposure of the graphite to the molten salt.
5.3 The user of this guide will need to select impregnation parameters sufficiently representative of those in a molten salt reactor
based on parameters provided by the designer. Alternatively, the user may select a standard set of impregnation conditions to allow
comparisons across a range of graphites.
5.4 This guide is not intended to be prescriptive. A typical apparatus and associated procedure isare described. Some indication
of the sensitivity of the procedure to graphite type and impregnation conditions is given in He, et al.
5.5 There are four major practical issues that must be addressed during the impregnation process:
5.5.1 The density of molten salt is greater than that of graphite. A specially designed tool is required to submerge graphite samples
in the molten salt during the impregnation process.
5.5.2 Some molten salts (for example, FLiBe) are poisonous and it is therefore necessary to provide containment by performing
procedures within a glove box.
5.5.3 The graphite must be kept away from air to avoid oxidation at high temperature. This can be achieved by performing the
impregnation process within a glove box with a controlled atmosphere.
5.5.4 Pressure control of the molten salt can be difficult to achieve. A specially designed autoclave is needed to hold the
samplespecimen and molten salt.
5.6 In order to assess the quantity of molten salt in the graphite, parameter D is used as a variable in measuring the mechanical
and thermal material properties. For a low impregnation pressure, the parameter Parameter D is the ratio of salt volume to open
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pore volume. For a high impregnation pressure, the parameter Parameter D is the ratio of salt volume to total pore volume. The
t
saturated value of D can be greater than 1 when the molten salt impregnation takes place at high pressure. It is postulated that
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the internal microstructure of graphite has been damaged by the high impregnation pressure and some closed pores have been
opened. In this case, the parameter D is more appropriate to represent the impregnation process.
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6. Apparatus
6.1 Autoclave:
6.1.1 Since fluoride molten salts are toxic and a small amount of water can significantly affect the wetting behavior of graphite,
the conditions for the procedure should be strictly controlled during molten salt impregnation. The schematic diagram of an
impregnation setup is shown in Fig. 1.
6.1.2 An autoclave is used as a sealed container and should meet a pressure leakage rate <0.25 % ⁄h. ⁄h at the maximum operation
pressure. To avoid corrosion by molten salt, the autoclave should be made of a high temperature and corrosion-resistant alloy.
6.1.3 A graphite crucible is placed inside the autoclave to hold the molten salt. There should be enough clearance between the wall
of autoclave and graphite crucible to prevent damage of the graphite crucible due to differences in thermal expansion.
6.1.4 The graphite samplesspecimens must be secured in a samplespecimen holder. For example, the samplespecimen holder
might comprise two graphite plates with the samplesspe
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