Bertrand, Johan

Betelu, Stéphanie

Daoudi, Jordan

Ignatiadis, Ioannis

Tzedakis, Théo

English

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HAL Id: hal-02160857
https://hal-brgm.archives-ouvertes.fr/hal-02160857
Submitted on 20 Jun 2019
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Investigation of dental amalgam electrode behaviour for
the long term monitoring of nuclear waste disposals
Jordan Daoudi, Stéphanie Betelu, Johan Bertrand, Théo Tzedakis, Ioannis
Ignatiadis
To cite this version:
Jordan Daoudi, Stéphanie Betelu, Johan Bertrand, Théo Tzedakis, Ioannis Ignatiadis. Investigation
of dental amalgam electrode behaviour for the long term monitoring of nuclear waste disposals. Clay
Conference 2017 - the 7th International Conference on Clays in Natural and Engineered Barriers for
Radioactive Waste Confinement, Sep 2017, Davos, Switzerland. 2017. ￿hal-02160857￿
Poster presentation; Topic 10: Monitoring (from initial state to post-closure) 
 
Investigation of dental amalgam electrode behaviour for the long term monitoring of 
nuclear waste disposals. 
Jordan Daoudi
1,2,3
, Stéphanie Betelu
1
, Johan Bertrand
2
,
 
Théo Tzedakis
3
, Ioannis Ignatiadis
1 
 
1 
BRGM (French Geological Survey) – Orléans, France j.daoudi@brgm.fr; s.betelu@brgm.fr; 
i.ignatiadis@brgm.fr 
2 
ANDRA (French national radioactive waste management agency) – France 
Johan.Bertrand@andra.fr  
3 
LGC (Chemical Engineering Laboratory) – Toulouse, France . tzedakis@chimie.ups-tlse.fr 
 
Nuclear waste disposals are being installed in deep excavated rock formations in some places 
in Europe to isolate and store radioactive waste. Near-neutral pH and low redox potentials 
(Eh) are considered favourable conditions for immobilization of most of radionuclides in deep 
disposal systems in clay geological formations. These are the reason why deep excavated rock 
formations such as Callovian-Oxfordian formation (COx) in France, or Opalinus Clay in 
Switzerland (OPA) are potential candidate for nuclear waste disposal.  
Based on this overall strategy, Andra has analysed the technical requirements that must be 
met by adapted monitoring equipment. First, these must be able to provide information on key 
THMCR (Thermal-Hydraulic-Mechanical-Chemical and Radiological) processes, to provide a 
three-dimensional image of its behaviour and thus to understand the underground installation 
functioning, in particular the cell interactions with the near-field. 
The main difficulty lies on the fact that no maintenance of the devices will be feasible once 
installed on site. 
Dental amalgam (DA) has been used by dentists for more than a century and is the most 
thoroughly researched and tested restorative material among all those in use (Castaño et al., 
2008; Colon, 2003; Brett et al. 2002; Acciari et al., 2001; Toumelin-Chemla et al., 1998; 
Marek, 1993). It is made by combining elemental mercury (approximately 50%), silver, tin, 
copper and possibly other metallic elements. During the trituration process, the surface layer 
of a silver-tin alloy dissolves in the liquid mercury and the reaction causes new phases to be 
formed. For low-copper amalgams, these new solid phases cause solidification such as: 
                                                     
           ϒ-phase                              ϒ1-phase     ϒ2-phase               ϒ-phase 
All the works described in the bibliography show that the dental amalgam corrosion depends 
essentially on the constitutive phases of the material. Briefly, ϒ1-phase is the most noble 
(Acciari et al., 2001), ϒ-phase is weakly altered due to its electrochemical stability in the 
buccal environment (Toumelin-Chemla et al., 1998) and ϒ2-phase is described as the most 
reactive (Acciari et al. (2001)), probably by the formation of a Sn(IV) oxide. Dental amalgam 
is nevertheless described as stable alloy, i.e. highly resistant to wear. Dental amalgam is 
therefore of great interest to be tested in conditions close to those which can be encountered in 
the context of geological storage. 
Performances, reliability and robustness of a homemade dental electrodes, containing 
respectively 8, 15, 34 and 43% of Cu, Sn, Ag and Hg, were examined by potentiometric 
measurements at 25°C. Data were compared to those obtained by three individual electrodes: 
coper, silver and tin electrode. Investigations were limited in pH, ranging from 5.5 to 12, 
close to those encountered in the environment of the nuclear waste disposals. Experiments 
were conducted into two buffer solutions based on NaH2PO4/NaHPO4, NaCl (0.1 mol.L
-1
) or 
NaHCO3/Na2CO3, NaCl (0.1 mol.L
-1
). When necessary, NaOH was used to extend the curve 
to the region of high pH value. In addition to measurements done under atmospheric oxygen 
saturation (with PO2 about 0.2 atm) while the influence of the O2/H2O redox couple was being 
studied, several measurements were also done in a glove box in an oxygen-free atmosphere 
(100 % nitrogen).  
Under the investigated conditions, the open circuit potential taken by the homemade dental 
amalgam electrode (EOCDA) is more analogous to those taken by silver or copper electrodes. 
Under atmospheric conditions and for pH values closed to neutral pH (5.5-9), the EOCDA 
appeared to be fixed by Cu-Ag/AgCl NaCl 0.1M at 222 mV/ENH. This result is explained 
by a double phenomenon. The presence of chloride ions in an oxygenated solution promotes 
the oxidation of Ag (Ag + Cl- AgCl↓+ e-) in the form of an AgCl coating on the surface of 
the Ag electrode. This phenomenon results in a decrease of the oxidizing power of Ag. Over 
the investigated pH range, this phenomenon gives rise to an anodic protection of copper by 
silver. That is why the expression of EOCDA is influenced by Ag/AgCl 0.1M NaCl. Between 
pH values 9 and 11, in carbonate medium, the EOCDA is rather be fixed by a mixed Ag-
Cu(II)/Cu(I) such as EOCDA (mV/ENH)=-24pH+422. For higher pH values, the EOCDA tends to 
decrease strongly such as the dental amalgam electrode is fixed by a mixed potential, 
including Sn, Cu and Ag redox couples.  
Under anaerobic conditions, the dental amalgam is rather fixed by a Ag-Cu2O/Cu redox 
potential such as EOCDA(mV/ENH) = -62 pH + 514. 
Under the investigated conditions, the home made dental amalgam electrode showed different 
behaviours. The EOCDA values tend to demonstrate the influence of the residual Ag-Cu phase. 
Under the atmospheric conditions, the home made dental amalgam electrode can be used as a 
reference electrode in the range 5.5-9. For higher pH values, work is in progress to precisely 
calibrate the electrode. Under anoxic conditions, the dental amalgam was established to 
exhibit a near-Nernstian behaviour in the pH domain ranging from 5.5 to 12 at 25°C. 
Electrode response was slightly affected by the direction of the pH change. Electrode 
reliability was clearly demonstrated for pH monitoring. The investigation of the electrode 
robustness is in progress in a synthetic solution whose composition in major elements and pH 
was representative of COx pore water. 
References 
Acciari H.A., Antonio C., Guastaldi A.C., Brett C.M.A. (2001) - Corrosion of dental 
amalgams: electrochemical study of Ag–Hg, Ag–Sn and Sn–Hg phases. Electrochimica Acta ; 
46: 3887-3893. 
Brett C.M. A., Acciari H.A. Guastaldi A.C. (2002) - Corrosion of Dental Amalgams. Studies 
of Individual Phases Key Engineering Materials; 230-232: 463-466 
Castaño P., Echavarria A., Gómez G.J. (2008) - Evaluation of the galvanic corrosion of high 
copper dental amalgams by means of electrochemical techniques. Rev. Fac. Ing. Univ. 
Antioquia ; 45: 77-86. 
Colon P., Pradelle-Plasse N., Galland J. (2003) - Evaluation of the long term corrosion 
behaviour of dental amalgams: influence of palladium addition and particle morphology. 
Dent. Mat.; 19: 232-9. 
Marek M. (1983) - The corrosion of dental materials, Treatise on materials science and  
technology. Academic Press, London ed. J. C. Scully. 
Toumelin-Chemla T., Toumelin J.P., Degrange M. (1998) - L’amalgame dentaire et ses 
alternatives - Evaluation et gestion des risques, Edition Lavoisier tec & doc, chapitre 3 : 23-
25. 
 


Investigation of dental amalgam electrode behaviour for the long term monitoring of nuclear waste disposals

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Investigation of dental amalgam electrode behaviour for the long term monitoring of nuclear waste disposals

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