Abstract
The fission product releases from spherical fuel elements used in modern high
temperature gas cooled reactors are one of the first source terms used in
describing the safety of planned nuclear plants during normal and accident
conditions. The verification and validation of the model and code used to predict the
gaseous fission product behaviour and release from spherical fuel elements for the
PBMR were documented in this dissertation. The PBMR is the latest design in high
temperature gas cooled reactor technology utilizing spherical fuel elements based
on the LEU TRISO-coated particle design. Fission products can be divided into
relatively short-lived noble gas and halogens, and relatively long-lived metallic
fission and activation products. Each group is described by its own models and sets
of transport parameters. The noble gases and halogen fission product releases
from the fuel elements are direct indications of fuel performance and are modelled
by the Booth equation. The fission product release legacy code NOBLEG for noble
gases and halogens was developed previously to calculate this diffusion model for
high temperature reactors. The model and code are verified and validated for use in
PBMR design and analyses under normal operating conditions.
The history of irradiation experiments conducted on coated fuel particles and
spherical fuel elements was investigated, and the most suitable irradiation tests
with their post irradiation investigations were identified for the purpose of validation
of the model and code. The model used to determine gaseous fission product
behaviour and release from spherical fuel elements is described in detail. The
application of this model in the code is verified mathematically with the Booth
model, and by inspection of the source code. The thermohydraulic model used by
NOBLEG to calculate fuel temperatures is verified with code to code comparisons
with the core neutronics code VSOP. The irradiation tests HFR-K5 and -K6 were
selected to validate the gaseous fission product code NOBLEG.
An investigation was done into the development of NOBLEG to calculate gaseous
fission product release under oxidizing conditions caused by water ingress events.
New relationships were derived from water vapour injection tests done during the
irradiation experiment HFR-K6, that allows NOBLEG to estimate the increase in
gaseous fission product release under oxidizing conditions. A new model was
proposed to explain peculiarities observed during the water injection tests.
Prof. P.P. Coetzee