Post Irradiation Examination (PIE) is the study of used
nuclear materials
Nuclear material refers to the metals uranium, plutonium, and thorium, in any form, according to the IAEA. This is differentiated further into "source material", consisting of natural and depleted uranium, and "special fissionable material", co ...
such as
nuclear fuel. It has several purposes. It is known that by examination of used fuel that the failure modes which occur during normal use (and the manner in which the fuel will behave during an accident) can be studied. In addition information is gained which enables the users of fuel to assure themselves of its quality and it also assists in the development of new fuels. After major accidents the core (or what is left of it) is normally subject to PIE in order to find out what happened. One site where PIE is done is the
ITU which is the
EU centre for the study of highly
radioactive
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...
materials.
Materials in a high radiation environment (such as a reactor) can undergo unique behaviors such as swelling and non-thermal creep. If there are nuclear reactions within the material (such as what happens in the fuel), the stoichiometry will also change slowly over time. These behaviors can lead to new material properties, cracking, and fission gas release:
Fission gas release
As the fuel is degraded or heated the more volatile fission products which are trapped within the
uranium dioxide
Uranium dioxide or uranium(IV) oxide (), also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear re ...
may become free.
[J.Y. Colle, J.P. Hiernaut, D. Papaioannou, C. Ronchi, A. Sasahara, ''Journal of Nuclear Materials'', 2006, 348, 229.]
Fuel cracking
As the fuel expands on heating, the core of the pellet expands more than the rim which may lead to cracking. Because of the thermal stress thus formed the fuel cracks, the cracks tend to go from the centre to the edge in a star shaped pattern.
In order to better understand and control these changes in materials, these behaviors are studie
https://web.archive.org/web/20070710110024/http://www.ornl.gov/~webworks/cppr/y2001/pres/114819.pdf
Due to the intensely radioactive nature of the used fuel this is done in a hot cell
Shielded nuclear radiation containment chambers are commonly referred to as hot cells. The word "hot" refers to radioactivity.
Hot cells are used in both the nuclear-energy and the nuclear-medicines industries.
They are required to protect in ...
. A combination of nondestructive and destructive methods of PIE are common.
In addition to the effects of radiation and the fission products on materials, scientists also need to consider the temperature of materials in a reactor, and in particular, the fuel. Too high fuel temperatures can compromise the fuel, and therefore it is important to control the temperature in order to control the fission chain reaction.
The temperature of the fuel varies as a function of the distance from the centre to the rim. At distance x from the centre the temperature (Tx) is described by the equation where ρ is the power density (W m−3) and Kf is the thermal conductivity
The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa.
Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
.
:Tx = TRim + ρ (rpellet2 - x2) (4 Kf)−1
To explain this for a series of fuel pellets being used with a rim temperature of 200 °C (typical for a BWR) with different diameters
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid fo ...
and power densities of 250 Wm−3 have been modeled using the above equation. Note that these fuel pellets are rather large; it is normal to use oxide pellets which are about 10 mm in diameter.
Further reading
Radiochemistry and Nuclear Chemistry, G. Choppin, J-O Liljenzin and J. Rydberg, 3rd Ed, 2002, Butterworth-Heinemann,
References
External links
IAEA (International Atomic Energy Agency) - Post Irradiation Examination Facilities Database
{{Nuclear Technology
Nuclear materials
Nuclear reprocessing
Nuclear technology
Nuclear chemistry
Radioactive waste
Actinides