Impact of ballistic effectson the gel layer properties of simplified nuclear glasses: a Monte Carlo simulation approach
Beginning: October 2016
Duration: 3 years
Supervisor for PNNL:S. Kerisit, Geochemistry Scientist, Pacific Northwest National Laboratory, PO Box 999, MSIN: K8-96, Richland, WA 99352, USA, email:
Supervisors for CEA:J.-M. Delaye (DTCD/SECM/LMPA, CEA Marcoule), O. Bouty (DTCD/SECM/LMPA, CEA Marcoule), email: ;
PhD director: S. Gin (DTCD/SECM, CEA Marcoule), email:
General context
This PhD topic falls within the framework of a collaboration between PNNL and CEA Marcoule and the candidate will be prompted to share his time between these two institutions.
Scientific context
As a confinement matrix for the long-lived radioactive wastes, high-level waste glasses in general and R7T7 glass in particular (the most studied nuclear glass to date)are subjected to high internal irradiation (nuclear and electronic effects). In addition, after some thousands of years in a deep geological repository, the glass will be subjected to water alteration.
The study of alteration mechanisms is therefore a crucial point in the demonstration of thesafety of the geological disposal. To be complete, the demonstration of glass performance has to consider the synergy between alteration by ground water and internal radiation effects. It is in fact possible that the structural changes induced by irradiation modify the glass durability and hence the release of the radionuclides in the geosphere.
When a nuclear glass of R7T7-type is in contact with water, the alteration rate decreases by several orders of magnitude until theresidual rate regime is reached. An alteration layer is formed at the glass surface within which three regions can be distinguished: a hydrated layer in contact with the pristine glass,a gel layer between the hydrated layer and the aqueous solution, and crystalline phases on top of the gel. Roughly speaking, the hydrated layer corresponds to the glassy structure whose soluble elements (B and Na especially) have been removed. The gel layer is formed by thehydrolysis/condensation of the sparingly soluble species (Si, Al, and Ca especially) and can incorporate exogenous elements provided by the aqueous medium. Depending on the glass composition and alteration conditions, the crystalline phases that formduring alteration can be clay-type minerals, calcium silicate hydrates, or zeolites.
As the alteration progresses, the alteration rate is largely slowed down because of the formation of a protective layer (Figure 1) [1]. Recent experimental analysis by TOF-SIMS [2] allowed locating this protective layer inside the hydrated zone. Indeed, Figure 2 shows that the concentration profiles of the soluble species fall steeply in the hydrated layer.
Other recent studies conducted on glasses pre-irradiated by heavy ions and placed in pure water or in a Si-enriched solution showed an increase of the alteration rate compared to the non-irradiated standard sample. Similar results had been also obtained for a glass doped with 244Cm. These experiments suggest that the structural changes of the glass skeleton induced by the heavy ion ballistic effects modify the glass alterability and the residual rate.
Figure 1: Phenomenological scheme of the interface between a nuclear waste glass and the solution
Figure 2: Concentration profiles in the hydrated zone of a SON68 glass: lixiviation at 90°C and 100 bars in a weakly renewed granitic water solution [2]. The pristine glass is on the left and the solution on the right.
PhD topic
Objective
On the basis of these recent results, the thesis will aim to understand the impact of the structural changes induced by ballistic effects on the properties of protective layer formed during nuclear glass alteration.
It is proposed to employ the Monte Carlo algorithm developed first in collaboration between Ecole Polytechnique and CEA Marcoule [3], and then at PNNL (see below), to simulate the alteration layer growth when a simplified nuclear glass is put in contact with water.
It will be necessary to improve this method, currently dedicated to the simulation of the gel layer formation, to allow us to represent also the hydrated layer.
Reminders about the Monte Carlo method
In the current Monte Carlo method [3], a glass is represented by three-dimensional cubic lattice (referred to asthe underlying network hereafter), on the nodes of which the glassy network formers (Si, Al, B) and the insoluble elements (Zr) are located. By removingsome of the bonds ofthe underlying network, it is possible to represent elements with coordination numbers equal to 3, 4, or 6 as well as non-bridging oxygens.
The chemical disorder is taken into account by locating randomly the atoms on the nodes. In this sense, the underlying network is topologically ordered but chemically disordered and can be considered as a model of the glass.
Then, a set of water molecules simulating the solution is introduced to be in contact with the glass surface (each water molecule is also located on a node of the underlying network)and the alteration is simulated by applying a series of elementary mechanisms whose frequencies are governed by probabilities fitted todifferent experimental and numerical data. The elementary mechanisms can be, for example,B release, chemical bond hydrolysis, or the condensation of dissolved Si or Al atoms at the glass surface.
This simplified model of the glass alteration enables in particular to reproduce the different stages of the gel layer formation at the glass surface (Figure 3).
Figure 3: Examples of gel layers simulated by the Monte Carlo method at the surface of a SiO2-B2O3-ZrO2-Na2O-CaO glass. The gel layer morphology is greatly impacted by the Zr concentration [4].
The different stages of the PhD
The objective of this research will be met by organizingthe work in four tasks.
Task1: Quantification of the ballistic disorder induced by the ballistic effects
In this task, the classical molecular dynamics tools used for several years to simulate the ballistic effects in simplified nuclear glasses [5] will be used to quantify the disordering of the glassy network when it is irradiated by an heavy projectile representing a recoil nucleus.
In parallel, the behavior of preexisting pores under irradiation will be also analyzed. Indeed, it is important to understand how local free volumes evolve when they are subjected to displacement cascades.
Task2: Modification of the Monte Carlo codeto introduce the hydrated layer formation
The Monte Carlo method has been recently taken up by the simulation team atPNNL [6] to introduce a finer description of the diffusion mechanisms inside the alteration layer. Thanks to this modification, it is nowpossible to consider the impact of the Si chemical concentration gradienton the gel layer formation, which improves the realism of the simulation. This technique will be used here.
It will be also necessary to simulate the water penetration upstream of the hydrolysis front and the leaching of B and Na to the solution in order to model the hydrated layer growth.
Task3: Implementation of the ballistic effects in the Monte Carlo code
In this task, the disorder induced by the ballistic effects on the underlying network will be implemented. The objective will be to prepare different glassy structures characterized by the same chemical composition but with different topologies, one corresponding to the pristine glass, and another modified by displacement cascades.
Task4: Simulation of the glass alteration by the Monte Carlo method
Simulations of glass alteration will be carried out for irradiated and non-irradiated structures to compare and contrast their behavior. The impact of the initial disorder on the alteration processes, more specificallyon the growth and on the transport properties of aqueous species through the alteration layer,will be estimated. It will be also possible to apply the algorithm to a series of glasses with different degrees of polymerization to determine the relationship between the extent of irradiation and the alteration behavior.
The results will be compared to experimental data obtained in parallel. Experiments will be designed to validate the modeling approach and to gain insight into glass alteration mechanisms.
Profile of the applicant
The applicant must have an extensive background in computer programming (C++) and materials science. For job applications, please send a CV and a motivation letter to the supervisors listed above. Application is open until April 15th. The selection ends on April 29th. A decision will bemade beginning ofJune.
References
[1] Gin S., Jollivet P., Fournier M., Angeli F., Frugier P., Charpentier T., Nature Communications 6 (2015) 6360.
[2] Gin S., Ryan J.V., Schreiber D.K., Neeway J., Cabié M., Chemical Geology, 349-350 (2013) 99.
[3] Devreux F., Ledieu A., Barboux P., Minet Y., Journal of Non-CrystallineSolids 343 (2004) 13.
[4] Cailleteau C., Devreux F., Spalla O., Angeli F., Gin S., The Journal of Physical Chemistry C 115 (2011) 5846.
[5] Delaye J.-M., Peuget S., Bureau G., Calas G., Journal of Non-Crystalline Solids 357 (2011) 2763.
[6] Kerisit S., Pierce E.M., Ryan J.V., Journal of Non-Crystalline Solids 408 (2015) 142.