The fuel U-235, used in PWRs in its oxide form UO2, is enriched up to 5% (maximum) and is used as dried pellets of about 10 mm in diameter and 10 mm in height with theoretical density (TD) ~95%. The stoichiometry of UO2 is kept such that the ratio of O:U is never allowed to go beyond 2:1. The surface temperature of the fuel can reach ~1400°C with the centre temperature still higher. The oxide pellets are enclosed in a Zircaloy clad tube and the tube is capped on both sides to make a fuel rod. The fuel-clad gap is filled with high thermal conductivity helium gas and the conductiv­ity degrades slowly with the fission gases diluting helium. Many such fuel rods (17 x 17) are bundled to form a fuel assembly. Many such fuel assem­blies (~200) are immersed in a pool of light water, flowing at a pressure of ~16 MPa, which is the heat transfer fluid in the primary loop of a PWR. During start up, the pellet-clad gap gets reduced due to thermal expansion of the fuel but soon increases as the fuel densifies under irradiation. At high burnups, the gap slowly reduces and eventually an intimate contact is estab­lished with the fuel swelling and the clad collapsing under creep due to the coolant pressure (clad creep-down). All cladding tubes have some ovality which increases with creep-down and the direct impact of creep-down is the increased gap (between rods) and increased water volume. This increased water volume increases the moderation effect and gives rise to power peaks in the neighbouring fuel pellets.68 The swelling of the fuel applies a severe hoop stress on the clad which slowly gets embrittled by irradiation and by absorption of hydrogen. The thin oxide layer on the clad breaks and bonding between fuel and clad becomes established, leading to a condition called pellet clad interaction (PCI). In extreme cases, these factors lead to cracking of the clad and to release of the fission gases into the coolant. Such situa­tions may demand reduction of reactor power or the shutdown of the reac­tor if safe discharge limits are crossed. The strain concentration produced in the cladding by the sharp corners of radially cracked pellet during a power increase is increased by (i) the increasing coolant pressure, (ii) the pellet/ cladding friction coefficient, (iii) the pellet radius and (iv) the circumferen­tial temperature gradient. This severity can be marginally reduced by using a stronger clad material and by reducing the number of radial pellet cracks. For burnups greater than 70 MWd/kgU, a high burnup structure (HBS) with bubble size much larger than the grain size forms and traps the fission gases. A rapid rise in temperature can lead to shattering of the HBS and can put severe stress on the cladding.69