Physical Effects of Radiation-II

         Figure: Increase in NDT Temperatures of Steels from Irradiation below 232ºC

The commonly accepted explanation of irradiation-induced swelling is totally based on the features of interstitial loops and voids or vacancy loops.   If the temperature is high sufficient to allow vacancies and interstitials, but not high sufficient to permit recombination, an associatively large (supersaturated) concentration of defects could be maintained under irradiation.Under this situation, the interstitials tend to agglomerate, or cluster, to form roughly circular 2- dimensional disks, or platelets, generally known as interstitial loops.  A dislocation loop is established while the collapse (or readjustment) of adjacent atomic planes takes place.  Alternatively, vacancies  could  agglomerate  to  form  2-dimensional   vacancy  loops,  that  collapse  into dislocation  loops,  or  3-dimensional  clusters  known  voids.  This difference in behavior among interstitials and vacancies has a significant effect on determining the swelling which several metals suffer as a result of exposure to fast neutrons and other particle radiation over a contain temperature range. While irradiation-induced swelling occurs, it is commonly significant just in the temperature range of roughly 0.3 Tm to 0.5 Tm that Tm is the melting point of the metal in Kelvin degrees.

Swelling could also result from gases generates in materials, such as helium formed through (n, α) reactions and another gaseous impurities present in the metals.   Those traces of gas increase the concentration of voids established upon exposure to radiation.For instance, the (n, α) and (n, 2n) reactions among fast neutrons and beryllium form helium and tritium gases which built swelling.Below certain conditions, embrittlement could be enhanced through the presence of the helium bubbles (helium embrittlement).The accepted view is in which this embrittlement is the result of stress- induced growth of helium gas bubbles at the grain boundaries.  A Bubble eventually link up and cause intergranular failure.

Fissionable metals suffer from radiation damage in a manner same to in which encountered in structural alloys. Further problems are introduced through the high energy fission fragments and the heavy gases xenon and krypton that appear between the fission products.  Two fragments which share 167 MeV of kinetic energy, within inverse proportion to their atomic masses, are generates from every fission.  Every fragment will have a range of various hundred angstroms as it generates a displacement spike. A core of vacancies is surrounded through a shell of interstitials,generating growth and distortion. Below Figure displays the growth in a uranium rod upon irradiation.

The gas formation generates eventual swelling of the fuel and might place the cladding under considerable pressure as well. One of the main challenges in alloying metallic uranium is the attainment of better stability under irradiation.Small additions of zirconium have displays marked improvement in decreasing distortion and growth.

                      Figure: (a) Growth of Uranium Rod; (b) Uranium Rod Size Dummy

The physical effects of ionizing radiation in metals are a uniform heating of the metal.  Ions are generates  through  the  passage  of  gamma  rays  or  charged  particles  by  the  metal,  causing enough electrical interaction to remove an external (or orbital) electron from the atom.  Metals along with shared electrons, that are associatively free to wander by the crystal lattice, are affected extremely little through ionization.