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WP6: Release and Capture Studies for Radioisotopes

Work package number: 6
Start date or starting event: M1
Work package title: Release and Capture Studies for Radioisotopes
Activity Type: RTD
Participant number: 11 - 1 - 6
Participant short name: PSI - SCK•CEN - UGent
Person-months per participant 64.8 - 38 - 32.4

Objectives
The aim of this work package is to determine the key parameters that influence the volatilization of radionuclides from liquid LBE and their capture on various materials, both in order to facilitate reliable predictions of radioactivity transfer to the gas phase during MYRRHA operation and in accident scenarios as well as for the development of efficient filters for the removal of volatile radionuclides from the gas phase. These tasks will be tackled using a combination of experimental and theoretical methods:
Experiments studying the evaporation of volatiles from liquid LBE as well as their gas phase transport and their capture on suitable getter materials will be performed under variation of various experimental parameters with the following goals:

  • identify those parameters that influence the evaporation, gas phase transport and deposition processes
  • characterize the chemical species involved in these processes
  • determine physicochemical parameters such as thermodynamic activity coefficients, saturation vapour pressure, Henry constants, deposition temperatures, adsorption enthalpies and entropies, which are necessary for the prediction of vapour phase concentrations of radionuclides in an ADS and the development of gas phase filtering systems for their removal.

In a complementary approach, quantum mechanical calculations will be used to calculate crucial properties of the evaporation and deposition processes theoretically:

  • Temperature-dependent solubility and heat of solution of volatiles in solid and liquid metals, in particular LBE and potential filter materials
  • Heat of adsorption of volatiles on filter material surfaces
  • Heat of formation of chemical compounds of the volatiles that may precipitate from the liquid metal
  • Formation energies of simple molecular compounds of the volatile elements

The results of the theoretical calculations will contribute essentially both to the interpretation and confirmation of experimental results as well as to the prediction of materials that have favourable properties for the capture of volatiles and chemical processes that can enhance or reduce evaporation. Thus, the interplay of experimental and theoretical studies will result in more reliable data, enhanced understanding of the underlying phenomena and streamlining of efforts. Because of their central importance, the focus of the proposed studies will be on polonium and mercury and their compounds and to a lesser extent their lighter chemical homologues.

Description of work (PSI, SCK•CEN, UGent)
Work package 6 is subdivided in 3 Tasks.

T6.1: Release, gas phase transport and deposition studies for volatile radionuclides (PSI)
The objective of this task is to determine the key parameters that influence the volatilization of radionuclides and their deposition on various materials. In particular, the existence of volatile chemical species and their chemical nature will be studied to obtain evidence on how and under which conditions they can be formed and captured. Thermodynamic and kinetic parameters concerning the evaporation of volatiles from LBE will be determined. The results will provide crucial input to assess the relevance of volatile chemical species of radionuclides for ADS systems and fundamental data for calculation of their gas phase concentrations and evaporation rates.
T6.1.1: Thermochromatography
To achieve these objectives, the transport of Po, Hg and their lighter chemical homologues will be studied in a temperature gradient in a carrier gas under variation of gas atmospheres (purified/moist gases, reductive, oxidizing and containing radicals), gas flows, temperature gradients, experiment duration, column material and pre-treatment of the column material. The results will allow proving the existence of different gas phase species of the studied elements under various conditions, the identification and characterisation of their chemical nature as well as determination of their thermodynamic properties, e.g. enthalpies and entropies of adsorption.
This task will provide important input and complementary information for several other tasks within this work package: Proving the existence of different gas phase species under certain conditions will allow to systematically select relevant conditions for evaporation studies from liquid LBE (Tasks 6.1.2 and 6.2.2 and facilitate the choice of suitable filter materials through their chemical nature and the data obtained on adsorption enthalpies and entropies (Task 6.2.1). The results obtained can also provide input on which gas phase species are important to study using quantum theory (Task 6.3.4), and vice-versa the results on stability of gas phase molecules (Task 6.3.4) and adsorption enthalpies on metal surfaces (task 6.3.2) obtained by first principles calculations will be extremely valuable for the interpretation and understanding of the thermochromatography results. The comparison of experimental and theoretical results will also facilitate a validation of the reliability of first principles methods for the prediction of these properties.

T6.1.2: Evaporation studies using the transpiration method
Dynamic evaporation experiments studying the release of radionuclides, especially Po and Hg and their chemical homologues, from liquid LBE will be carried out under variation of gas phase composition (purified/moist gases, reductive, oxidizing and containing radicals), specific activity/concentration of the respective radionuclide in LBE, temperature, gas flow (saturation and non-saturated conditions) and sample geometry. Goal of this task is the determination of qualitative and quantitative effects of experimental conditions on vapour phase concentration and release rates of volatile radionuclides from LBE solution. The results will allow determining fundamental thermodynamic and kinetic parameters for the evaporation process, e.g. thermodynamic activities and Henry constants for the case of gas phase saturation as well as evaporation rates under various concentrations and gas flow conditions for unsaturated gas conditions. The latter can be compared with evaporation rate calculations of different degrees of sophistication. The thermodynamic data will be compared to those obtained by first principles calculations (Task 6.3.3-6) and independent investigations performed in Task 6.2.2 to interpret the results and to achieve a consistent data set for ADS safety assessments.

T6.2: Studies of polonium and mercury evaporation and capture on filters (SCK•CEN)
In this task, the objective is to identify and evaluate adsorbents suitable for capturing gaseous Po and Hg and to study the evaporation of Po and Hg from LBE at low (second independent data set for licensing) and elevated concentration. The two subtasks are defined as:
T6.2.1: Adsorption experiments
Po and Hg adsorption experiments under variation of adsorbent type (porous carrier material, impregnant), velocity and composition of carrier gas, adsorbate concentration and concentration of other contaminants, adsorbent bed properties (depth, temperature) will be performed. For this purpose, breakthrough and pulse chromatography experiments will be carried out using an isothermal packed bed of adsorbent particles. Such experiments will allow determining the effective (dynamic) adsorption capacity and long-term stability of an adsorber system under conditions relevant to ADSs. To gain insight into the fundamental processes that occur in the adsorber, kinetic (mass transfer resistances) and equilibrium adsorption parameters (Henry constants, enthalpies) will be derived by matching the experimental data with appropriate mathematical models. The selection of suitable adsorbent materials will be based both on experiences reported in the literature and on experimental results produced in Tasks 6.1.1, 6.3.2 and 6.3.4 of this work package. In addition, the thermodynamic parameters determined in these Tasks will be used as input for modelling the experimental adsorption data.
From the results obtained here, optimized filtering systems for the retention of Hg and Po in ADS can be developed.
T6.2.2: Evaporation experiments
Evaporation experiments of volatile elements dissolved in LBE will be carried out using the transpiration method. Experiments at low concentrations (10-6 appm) will be performed to obtain additional, independently measured data sets for the most hazardous elements Po and Hg to support licensing of MYRRHA. Furthermore, a gradual increase up to concentrations expected in MYRRHA (0.1 appm) is envisaged. Here, the goal is to identify and characterize anomalous Po evaporation processes that may occur at elevated concentrations. Progress with the experiments at increasing Po concentration will depend on the licensing of glove box/hot cell test setup.
The results of this task will be compared with those of Task 6.1.2 and Tasks 6.3.3-5 to obtain a consistent data set concerning the extremely important volatilization data.
In addition, the data from tasks 6.1 and 6.3 will be used to assess the release of Po into the environment in various accident scenarios involving damage to the confinement structure.

T6.3: Theoretical thermochemistry of Po interacting with LBE and filter materials (UGent)
The objective of this task is the determination of fundamental properties describing the chemical interaction of polonium with components of the MYRRHA ADS, i.e. the target material LBE and impurities contained therein, promising materials for the development of filters to remove Po from the gas phase and volatile molecular Po-compounds. The task is divided in the following five subtasks:
T6.3.1: Po solubility in solid LBE
The temperature-dependent solubility of Po in solid LBE will be predicted following a computational procedure that relies on density functional theory for 0 K ground state energies and on a Debye model and/or ab initio calculated phonons for the behaviour above 0 K. The goal is to determine the solubility of Po in solid LBE to establish a lower limit on its solubility in liquid LBE.
T6.3.2: Search for most promising filter materials
The heat of solution of Po in noble metals, and the heat of adsorption of Po on noble metal surfaces will be predicted in the framework of density functional theory. Several low-index surfaces will be considered ((100), (110), (111), ...). This will allow assessing in an accurate, quantitative way which noble metal, and which specific surface type, is most effective to capture Po. This information will allow interpreting the results of thermochromatography experiments (Task 6.1.1) on a theoretical basis and performing the difficult adsorption experiments (Task 6.2.1) in a much more focused way.
T6.3.3: Predict the existence of Po-alloys
Precipitation of Po-containing alloys within liquid LBE affects the Po evaporation rate. Therefore, the stability (heat of formation) of a number of Po-X and Po-X-Y compounds will be calculated by density functional theory. Earlier simplified assessments based on the Miedema model suggest which alloys are worth to be considered. The crystal structure will be taken into account, either by examining crystal structures that are known to exist in chemically similar compounds, or by making use of unbiased crystal prediction tools. The results facilitate the interpretation of evaporation experiments (Tasks 6.1.2 and 6.2.2) where the formation of Po-compounds may play a role.
T6.3.4: Po molecules in the gas phase
The formation energies of several Po-containing molecules will be determined by density functional theory and/or quantum chemical methods: monomeric Po, Po2, PoH, PoH2, PoOH, Po(OH)2, PoBi, PoPb, PoHg,.. The results obtained by this task are important for all considerations concerning evaporation, gas phase transport and deposition processes. Thus, they will provide important input for all the experimental sub-tasks of work package 6. This information will allow eliminating molecules that are unstable at the relevant temperatures from these considerations and to assess probable volatisation paths. Several proposed/likely reaction paths will be examined by the same methodology, in order to point out the most favourable of those paths. The stability of different gas phase molecules also affects their specific adsorption properties, thus influencing the choice of filter materials (Task 6.2.1).
T6.3.5: Po solubility in liquid LBE
The solubility of Po in liquid LBE will be calculated from first principles. It has been shown in the recent literature how crucial quantities as the Henry constant can be written in terms of thermodynamic expressions that depend on a few quantities which all can be obtained from static density functional theory and/or molecular dynamics. We will apply these computational schemes to assess Po solubility in liquid LBE as a function of concentration and temperature. This will complement and expand the information that is available from experiments. In particular, the enthalpy of solution of Po in liquid LBE is one of the crucial parameters needed for interpreting the results of evaporation experiments in Tasks 6.1.2 and 6.2.2.

Deliverables
D6.1 Report on results of screening experiments (PSI) (M15)
D6.2 Report on tasks 6.3.1, 6.3.2 and 6.3.3 (UGent) (M22)
D6.3 Final report on volatilization and deposition studies (PSI) (M36)
D6.4 Final report on the evaporation, release and capture of Po (SCK•CEN) (M36)
D6.5 Final report on the evaporation and capture of Hg (SCK•CEN) (M36)
D6.6 Report on tasks 6.3.4 and 6.3.5 (UGent) (M36)