FissionGasViscoplasticityStressUpdateBase

Base class that calculates the volumetric strain due to gaseous fission product buildup, fission gas inventories, and porosity interconnectivity using viscoplasticity methods.

Description

FissionGasViscoplasticityStressUpdateBase is a base class used to compute an inelastic volumetric strain to account for gaseous swelling in nuclear fuel. This base class provides some common calculations, fission gas inventory models, and porosity interconnection models used to capture the inelastic strain from fission gas bubbles. The actual inelastic strain and gas absorption rates are calculated by inherited classes.

FissionGasViscoplasticityStressUpdateBase utilizes the viscoplasticity methods inherited by ADViscoplasticityStressUpdateBase, similar to ADViscoplasticityStressUpdate. This is different than the historical approach for fission gas swelling that utilized the eigenstrain system. By using the viscoplasticity methods, this model can be coupled to other inelastic strains such as creep, which are accumulated by ADComputeMultipleInelasticStress. The porosity rate of change is then computed from the trace of the inelastic strain rate by ADPorosityFromStrain: In this way, the effects of multiple sources of inelastic strain can be accumulated into a single porosity value.

Several assumptions are utilized to simplify the model, the details of which will be expanded upon in the sections below:

  • Only one, non-constant bubble size is assumed at any given quadrature point;

  • The concentration of bubbles is assumed to remain constant in time, but may vary in space,

  • Bubble growth is monotonic until full interconnection is reached, e.g. bubbles are not allowed to shrink until interconnectivity reaches one;

  • Interconnection to the plenum is defined by local conditions, e.g. percolation is ignored,

  • All fission gas behaves like xenon (e.g. atom size and van der Waal constants).

While the inelastic strain imparted by fission gas bubbles on the fuel is calculated classes that inherit from FissionGasViscoplasticityStressUpdateBase, several common models that can be used to describe the fission gas inventory and porosity interconnection are contained within FissionGasViscoplasticityStressUpdateBase. The details of these models are described below.

Porosity and Interconnection

In general, the swelling due to fission gas bubble growth will terminate once the bubbles become interconnected with an outside surface, and their contents vent to the fuel plenum. Any further gas that reaches the now empty bubbles is assumed to instantly vent to the plenum. In addition, the size of the bubble is assumed to retain its shape once it becomes interconnected. In reality, some hot-pressing of the pores may occur at higher burnup, a phenomenon that is not captured in this model. This is simulated however, by allowing for gaseous swelling to reoccur after full interconnection is reached, which competes against the strain in UPuZrHotPressingStressUpdate. The interconnection fraction is assumed here to be related to the porosity via a clamped smoother step function, (1) (2) where is the interconnection initiating porosity, is the interconnection terminating porosity, and is the current porosity value. The default values of and were determined from phase-field simulations of bubble growth and interconnection (Aagesen et al., 2020).

Due to the highly damaged state of metallic nuclear fuel visible in post irradiation examination micrographs, all quadrature points within the mesh are assumed to be in direct communication with the plenum. As such, a porosity percolation model is not required, and any interconnection of pores will result in instantaneous fission gas release, the details of which will be included in the Gas inventory evolution section.

Gas inventory evolution

A fundamental part of any bubble swelling model is the gas inventory calculation method in order to keep track of the location of gas atoms as they are created during fission. The primary phenomena that must be captured to accurately calculate fission gas population are:

  1. Fission gas creation in the fuel;

  2. Diffusion of the individual gas atoms;

  3. Absorption of the gas atoms by the bubbles;

  4. Bulk volumetric response as porosity increases;

  5. Porosity interconnection;

  6. Release of fission gas to the plenum.

This base class includes calculations for items 1, 5, and 6 above, with the remaining items provided by inherited classes.

All fission gas that is produced (), will exist either as dissolved atoms in the bulk with density (), gas in bubbles (), or as gas in the plenum (), also known as released fission gas. Here, an upper-case signifies the total molar amount per unit volume of fuel, while a lower-case corresponds to the total molar amount per bubble such that the total inventory of fission gas in bubbles can be related to the fission gas present in each individual bubble by, (3) where is the bubble concentration. Since interconnected bubbles vent their contents to the plenum, Eq. (3) includes a to account for loss of fission gas due to interconnection.

The total fission gas conservation can be described as, (4) where is the volume of the fuel.

During irradiation, fission gas is created proportional to the fission rate with a yield . Several elements contribute to the total fission gas production rate, but fission gas primarily consists of the noble gases xenon and krypton, with possible concentrations of iodine and cesium depending on the temperature. All fission gas will be assumed to behave as xenon gas here. The production of fission gas assumes a linear trend in fission rate between any two given timesteps, (5) where is the fission rate and is the time-step size between times and .

These newly created gas atoms contribute to the population of dissolved fission gas atoms in the fuel . Due to an extremely high solution energy of the gas atoms, they will immediately cluster with each other and on other defects, nucleating bubbles within the solid. As the bubbles grow and eventually become interconnected, they vent their contents to the plenum. The resulting fission gas release can be calculated due to diffusional losses (i.e. fission gas release that occurs from gas diffusing to open porosity), and interconnection losses (i.e. or fission gas release that occurs due to bubbles becoming interconnected). The resulting total fission gas release can be calculated as, (6)

Inherited classes must specify the absorption rate of fission gas, thus filling out the terms contained in Eq. (4).

References

  1. Larry K Aagesen, Andrea M Jokisaari, and Jia-Hong Ke. Lower length scale informed improvements to BISON U-Pu-Zr fuel swelling model. Technical Report INL/EXT-20-59990 Rev. 0, Idaho National Laboratory, 2020.[BibTeX]