Issues of Scale and the Functionality of Polygranular Graphite
Components
The United Kingdom’s nuclear reactors
were built in accordance with design codes for single components
and as such contain elements of redundancy. Currently, the
majority of advanced gas-cooled reactors (AGR’s) are reaching the
second half of their operating lives, which when designed excluded
the possibility of cracked bricks within the core. The
prediction of these bricks cracking within the core is based on the
measurement of small samples of material, even though the actual
size of the bricks is much greater. However, small specimens
taken from the core and tested will not have the same material
properties as that of a whole brick.
AGR core before fuel
insertion
Volume and the
perspective of strength
Weibull theory indicates that the
larger a specimen volume is, the more likely it is to contain flaws
and defects, and therefore the probability of finding a critical
flaw (that would cause failure) increases with volume, i.e. the
figure on the left shows two cubes of greatly differing
volumes. Weibull theory states that the smaller cube would
have a greater strength due to the probability of it having less
critical flaws. However, it is unknown if Weibull methods
work for small sample volumes and it is untested at large sample
volumes. As there are no current valid theories for the
relationship between the strength and volume of graphite, one must
be developed, and can be through the analysis, testing and
modelling of graphite samples.
An abstraction technique is to be used to model the
microstructure of graphite, providing a simple but effective way of
studying the potential flaws and defects within a sample.
This can be performed within a finite element analysis (FEA)
program, and can potentially allow the modelling of graphite
specimen and the production of a strength value without extensive
mechanical testing. However, in order to validate computer
generated results, physical testing must also take place. To
study how the size of a graphite sample affects its various
mechanical properties (e.g. flexural and compressive strength), a
comprehensive testing programme has been developed which will allow
the relationship between volume and strength to be
investigated. Using equipment designed and manufactured
within the University, it is possible to perform standard 3- and
4-pt flexural tests, as well as compression and tension, on a wide
range of samples to gather the appropriate data.
Flexural test equipment
design
3-D representation of a
small section of an AGR core
The other key aspect of the investigation is
that of functionality and in particular, how a graphite-moderated
core will function under various circumstances. Typically,
the graphite core in an AGR reactor is designed to have a radial
keying system (see left), and the ability to predict how a reactor
core will function upon the failure of a number of components is
important to its operation and will serve in determining the
overall lifespan of a nuclear reactor. If bricks fail due to
weakening by radiolytic oxidation or by dimensional changes and
raised internal stress induced by neutron irradiation, it does not
necessarily mean that the reactor is any less functional as
"emergent behaviour" can occur. The phenomenon of emergent
behaviour occurs when a number of components fail but there is no
loss in performance through the system, i.e. the whole is greater
than the sum of the parts. An investigation into the effect
of cracked and broken core bricks on the system as a whole can lead
to a prediction on whether there are a critical number of bricks
that can fail before the system becomes inefficient.
Previous studies have already shown that the stresses and
displacements of a core brick can be modelled using finite element
methods, and it is intended to follow this avenue of investigation
in order to model specific reactions within the core, e.g.
interaction between bricks and keyways, stresses caused at
particular points (effect of notches) and the deformation or
displacement of components. Apart from finite element
modelling, there is access available to a 3-d solid modelling
program which enables the creation of components that can be
manipulated within a 3-dimensional environment. The program
allows the generation of individual parts that can then be
assembled in to a moving component on-screen, which upon completion
can be used to test whether the device functions correctly before
it is manufactured. Obviously the applications for this
software are diverse, but with reference to this project it will
enable the construction and functionality testing of any reactor
core components required. Therefore it is intended to use
this program to develop a model of a full reactor core and test its
functionality.
FE analysis of a beam in
a 4-pt flexural test
Micrographs of the
fracture surface of IM1-24 graphite (138 x magnification, 100 x
magnification respectively)
In summary, the objectives of this research
project are:
- To investigate how specimen volume influences strength
- To study the prediction of flaws and defects within a
specimen
- To determine a relationship between the strength and size
(dependant on scale) of a specimen and model the relationship using
abstraction
- To examine quantitatively how
the failure of a reactor core brick affects the surrounding bricks
and their movement
- To consider whether the emergent behaviour of the system is
dependent on a particular number of failed components.
If you have any queries or questions regarding this research,
please feel free to contact me on the below email address:
m.j.holt@eng.hull.ac.uk