Gaseous swelling and fission gas release are described below. For conciseness, the term ‘fuel’ is used to refer to fuel pellets (oxide fuel) or fuel bars (metallic fuel).

Fission gas — predominantly xenon and krypton — atoms are generated uniformly within each fuel grain, each with a significant kinetic energy. Since the fission fragment range in the fuel is appreciable — of the order of 6 pm in UO2 (Noggle and Stiegler, 1960) — a significant fraction of atoms in grains close to the fuel surface are ejected into the pin free volume — this is known as recoil release, and is an athermal release mechanism. The remaining fission gas atoms come to rest in the fuel matrix, where they are effectively in solution. They are then subject to diffusion, and tend to either become trapped in intra-granular fission gas bubbles (which are nucleated in the wake of energetic fission fragments), or reach the grain boundary. The amount of fission gas that diffuses to the grain boundaries is dependent upon both the fuel temperatures and grain size, especially the former. In the case of external grains (those at the fuel surface), the fission gas atoms are released to the pin free volume. In contrast, for internal grains, the atoms that reach the grain boundary are rapidly accommodated into grain face bubbles via grain boundary diffusion. It is also possible that diffusing fission gas atoms near to the fuel surface are knocked out of the fuel by energetic fission fragments in an athermal process known as knockout release, but experimental measurements of radioactive fission gas release have shown that release due to this mechanism is negligible (Lewis, 1987). Any grain growth alters the amount of fission gas that reaches grain boundaries via two effects: the increase in the average distance over which fission gas atoms have to diffuse to reach a boundary; and the accumulation of intra-granular gas at moving grain boundaries (a process known as grain boundary sweeping). The first effect tends to decrease the rate of fission gas release to the pin free volume, while the second effect tends to increase it.

The intra-granular and grain face bubbles (which are actually quasi-crystallites, and not bubbles in the traditional sense) grow as fission gas atoms and vacancies diffuse to them and as intra-granular gas is swept up by grain boundary migration. However, both gas atoms and vacancies (which together comprise the quasi­crystallite material) are also ejected from the bubbles by energetic fission fragments in a process known as irradiation-induced re-solution. Thus, the size of the bubbles is governed by the net result of these two competing processes. Because of the large difference in the volume occupied by a gas atom in the fuel matrix and in a bubble, the presence of the intra-granular and grain face bubbles leads to gaseous swelling of the fuel.

In steady-state conditions, the radius of the (spherical) intra-granular bubbles tends towards an equilibrium value, while the grain face bubbles tend to grow inexorably. The inter-granular bubbles are initially lenticular, but as they grow and coalesce they can, if the imposed stress is small enough, become first elongated and then vermicular (‘worm-like’) (Barker et at., 2009). The grain face bubbles that intersect a grain edge are subsumed into grain edge bubbles. Once the areal density of fission gas on the grain face reaches a critical value, the isolated grain edge bubbles become long enough that they interlink to form tunnels to the fuel surface. The gas in these interlinked tunnels is then rapidly vented to the pin free volume. Once vented, the tunnels may close due to sintering, but they can re-open with time as more fission gas arrives at the grain boundaries. The dependence of bubble morphology and size on temperature, fission rate, burnup, etc., means that gaseous swelling strains can vary by a large amount depending on the precise irradiation conditions.

To summarise the above: gaseous swelling occurs due to nucleation and growth of both intra — and inter-granular fission gas bubbles; and fission gas release (release of fission gas from the fuel pellets or bars to the pin free volume) occurs via (i) recoil (athermal), (ii) diffusion to free surfaces (both athermal and thermal) and (iii) diffusion plus interlinkage (both thermal and athermal) mechanisms. A fourth (thermal) release mechanism — bubble migration — occurs at high temperature (above ~ 1800°C (Turnbull, 1976) for UO2 fuel). Finally, there is the possibility of enhanced fission gas release from thermal reactor oxide fuel at high burnup due to saturation of the fuel matrix with fission gas, either locally at the pellet rim (see the discussion in 14.2.14 on formation of high burnup structure), or throughout the fuel pellets (Sontheimer and Landskron, 2000).

The fission gas released to the pin free volume has a significantly lower thermal conductivity than the helium fill gas. The gap conductance is therefore noticeably reduced, leading to an increase in fuel temperatures. The released fission gas also increases the pin internal pressure. If the fission gas release becomes overly large, the high pin internal pressure can cause creepout of the cladding. If the rate of creepout is greater than the rate of fuel swelling, there is an opening of the gap between the pellets/bar and cladding. This increases fuel temperatures, which in turn can lead to further fission gas release. This positive feedback mechanism has the potential to cause rupture of the cladding.

The fission gas fractions releasedby each of the four mechanisms, and therefore their relative importance, are dependent upon fuel type. All mechanisms are important for fast reactor, HTR and CANDU fuel, where fuel temperatures are high. In contrast, only the first mechanism is important in Magnox fuel, where fuel temperatures are low. Finally, only the first three mechanisms are important for LWR and AGR fuel (diffusion plus interlinkage is dominant once interlinkage occurs), where fuel temperatures are intermediate. The magnitude of fission gas release is dependent upon fuel type, fuel pin design, the irradiation history (in particular the fuel burnup), and the fuel grain size. Typical end-of-life values for LWR (Johnson et al. , 2004), AGR (Barrable et al., 1997) and CANDU (Floyd et al., 1992) UO, fuel are in the range 0.1 to 10%, whereas typical end-of-life values for metallic and oxide fast reactor fuel are 60% (IAEA, 2003b) and 80% (Maeda et al., 2005), respectively. Both gaseous swelling and fission gas release are significantly enhanced during any transient increases in pin power.