HERACLES-ACSEPT Integration

Growing interest in the field of nuclear chemistry has promoted extensive studies of advanced reactor materials during the last few decades. Although new fuels are benifitial from the economical point of view (fabrication, improved neutronic characteristics, reactor operation period etc.), their actual application is still under the discussion. That is due to the special conditions that are required for handling of certain fuel materials (e.g. carbide, nitride, IMF etc.).

One of key issues is reprocessing of the irradiated fuel material. Methods of reprocessing, which are compatible with already existing technology, will be preferable because only limited changes in the sequence of reprocessing steps will be needed. Another key question is the possibility of fissile material recycling in order to be able to get more energy from the fuel being once used. Recycling of the irradiated material requires efficient separation of U/Th from Transuranic elements (TRU) and fission products (FPs).

For these reasons, the speciation of TRU and FPs in UO₂ or ThO₂ matrix is the key information in reprocessing technology, as well as for the risk assessment in the final storage repositories. Moreover, with the increased fuel burnup, materials become more difficult to handle because of their high dose rates. From this point of view theoretical modeling of chemical speciation in nuclear fuel is a potentially important tool in development of new reprocessing stages and improvement of the whole nuclear fuel cycle. For that purpose a number of computer codes were developed that predict chemical speciation of irradiated fuel components. The Gibbs Energy Minimization (GEM) approach is one of the most frequently used method to evaluate the chemical state of the elements in the fuels. More advanced codes, that are briefly described below, combine thermodynamic modeling with simulation of mechanical evolution of the matrix under relevant conditions.

In 2008 the internal PSI project HERACLES (Head-End Reprocessing studies by thermAl and thermoChemicaL treatment of fuElS) has been started, that is encouraged to support experimental work on advanced nuclear fuel reprocessing techniques with theoretical calculations of fuel thermodynamics. For the purposes of the project, thermochemical data for U, Th, TRU and FPs compounds were collected from various sources, compiled into into a project-specific database HERACLES-TDB and introduced into the computer code GEM-Selektor v. 3 package (see below). After the first compilation was accomplished, the database has been used for simulation of the FP release from irradiated fuels during thermal annealing induced by the inductive heating. The theroretical calculations were meant to support the experimental work on FP behavior in irradiated fuel under elevated temperature conditions, performed in the framework of FP7 research program ACSEPT (Actinide reCycling by SEParation and Transmutation). The first results were published in the internal project report were evaluated to be successful.

The outcomes of our activity within the ACSEPT project demonstrate, that GEMS code in conjunction with the HERACLES-TDB can be used for prediction of the complex chemistry of multicomponent fuel systems and possible phase transitions. The on the basis of chemical properties of individual components (U, Th, TRU and FPs) and possible interactions between them, code simulates chemical speciation of nuclear materials at ambient pressure under normal up to high temperature conditions (2500K).

Thermochemical data were collected from a number of publications, as listed in the HERACLES-TDB v. 0.2 database reference list.

The auxiliary data were taken from CODATA Key values for Thermodynamics.

This code, developed at PSI (http://gems.web.psi.ch), computes equilibrium chemical speciation and activities/fugacities of components in all co-existing phases at given pressure P, temperature T, and bulk composition of the system. Standard molar Gibbs energy functions ΔG⁰(Tr) must be available for all components in all phases included in the system. Correction to T (and P) occurs automatically in GEM-Selektor code using the ΔG⁰₂₉₈, molar absolute entropy S⁰₂₉₈ and heat capacity function coefficients Cp⁰ = f (T) from the bulit-in database. At this stage, any kinetic/metastability effects are not considered in GEM calculations, although, in principle, such effects can be simulated in GEMS package.

Up to now, several computer codes are internationally developed, aimed at various simulations such as:

• Modeling of the single fuel rod performance (e.g. Single Fuel Rod Performance (SFPR), ORIGEN, Svecha/Quench)
  codes for modelling of single rod behavior under various regimes of LWR operation (normal and off-normal, including severe accidents)

• Modeling of the fission gas release and swelling (e.g. Gaseous Release And sWelling-A, FASTGRASS, VICTORIA)
  simulation of the microscopic characteristics of the fuel: volume distribution of the vacancies and interstitials un the uranium and oxygen sub-lattice in the thermodyamic equilibrium and as a functions of time

• Modeling of the fission products migration and release from the irradiated oxide fuel (e.g. Module for Fission Product Release)
  mechanistic code describing the evolution of various deffects and their interaction with gas atoms and bubbles migrating from the grain boundaries; these simulations are performed for transient (annealing) conditions, where the approximation of equilibrium bubbles is no longer valid. Various chemical interactions of the active FPs are taken into consideration (thermochemical database contains thermodynamic data for U and other 15 FPs)

• Thermodynamic modeling on U, TRU and FP speciation (e.g. Gibbs Energy Minimization Selector with HERACLES-TDB; MFPR)
  modeling of the U, TRU and FP speciation in the wide temperature intervals (300 - 3000K)

The first two groups of methods allow prediction of the fuel and fission gas behavior in the entire rod under normal reactor operation conditions as well as under severe accident conditions. In contrast, codes MFPR and GEMS are able to compute the U, MA and PF speciation in the fuel matrix and can be utilized for modeling the local distribution of the particular FP or fuel stoichiometry. Both utilize the Gibbs energy minimization approach.

The MFPR and HERACLES-TDB contain thermochemical data for the most of relevant FPs, under the assumption that their diffusion and re-solution processes take place on the grain boundaries. In many cases, this is sufficient for prediction of the FPs speciation and release.

The MFPR code database also contains the information on FP diffusion coefficients, which makes it capable to model the kinetics of FPs migration in the bulk fuel matrix. Although the various diffusion and interfacial processes are comprehensively assumed in the MFPR, its application is limited to the uranium oxide fuel. Even the behavior of MOX fuel or materials simulating irradiated oxide fuel (SIMFUEL) cannot be described by MFPR. This problem was can be potentially solved in future development of the HERACLES-TDB, which, if used with the GEMS code, can predict speciation of FPs in any type of fuel.

• In the "Setup of aqueous and gas phases..." check the last option "Do not include aqueous electrolyte phase into the system definition",

• Proceed further with the creation of the first Single System Equilibria (SysEq) record key, for example:

Thermodynamic potential to minimize - =G= Name of chemical system definition - =Test_1= Variant number of bulk composition - =0= Volume of the system (L) or 0 (no volume constraint) - =0= Pressure in bar - =1=. ********************************* At present, thermochemical data available in database correspond to the ambient pressure. Pressure from 1 to 50 bar is assumed to have negligible impact on the goT values of condensed compounds, therefore no additional data for pressure corrections are provided. Gaseous species are assumed to be ideal ********************************* Temperature (deg. C) - =25= Variant number for CSD constraints - =0=

• Give the name for CSD - e.g. U-FPs-O and any comment, if needed. Use default setting in for CSD and Step 2 and finish with OK,

• Give the composition for you system by picking the species from Dependent conmponent or Independent component lists. In the right part enter the quantity and the units. An examples of the FP inventories corresponding to the different burnups of irradiated UO₂ fuel can be found here.
  Proceed further as its writen in GEMS Sample Project tutorial.

• ORIGEN - The ORNL Isotope Generation and Depletion Code Bell M.J.
  OFW-4628 (May 1973).
• Development of SVECHA/QUENCH Code for Modeling of Fuel Cladding Degradation in QUENCH tests Berdyshev A.V., Boldzrev A.V., Paladin A.V., Shestak V.E. and Veschunov M.S.
  Transactions SMiRT 16, Paper 2028 (2001).
• CODATA Key Values for Thermodynamics Cox J.D., Wagman D.D., Medvedev V.A. Eds.: Cox J.D., Wagman D.D., Medvedev V.A.
  Hemisphere Publishing Corporatoin (1989).
• VICTORIA: a Mechanistic Model of Radionuclide Behavior in the Reactor Coolant System Under Severe Accident Conditions Heames T.J., Williams D.A., Bixler N.E., Grimley A.J., Wheatley C.J., Johns N.A., Domagala P., Dickson L.W., Alexander C.A., Osborn-Lee I., Zawadzki S., Rest J., Mason A., Lee R.Y.
  JNUREG/CR 5545, SAND90-0756 (1992).
• Minimization of Gibbs Free Energy in geochemical systems by convex programming Karpov I.K., Chudnenko K.V., Kulik D.A., Avchenko O.V., Bychinski V.A.
  Geochemistry International 39, 1108 (2001).
• A Dynamic Model for Fission Gas Release and Gaseous Swelling Integrated into the FALCON Fuel Analysis and Licensing Code Khvostov G.
  Proceedings of Top Fuel 2009 Paper 2085 (2009).
• FASTGRASS, A Mechanistic Model for the Prediction of Xe, I, Cs, Te, Ba and Sr release from Nuclear Fuel under Normal and Severe-Accident Conditions Rest J., Zavadski S.A.
  NUREG/CR 5840, TI92 040783 (1994).
• Development of the mechanistic code MFPR for modelling fission-product release fro irradiated UO₂ fuel Veschunov M.S., Ozrin V.D., Shestak V.E., Tarasov V.I., Dubourg R., Nicaise G.
  Nuclear Energy and Design 236, 179 (2006).
  DOI: 10.1016/j.nucengdes.2005.08.006
• SFPR: an Advanced Mechanistic Code for Modeling of Single Fuel Rod Performance under Various Regimes of LWR Reactor Operation Veschunov M.S., Boldyrev A.V., Ozrin V.D., Shestak V.E., Tarasov V.I.
  Proceedings of Top Fuel 2009 Paper 2011 (2009).