The number of intragranular hydrogen bubbles or micro-cavities in the metal is important. It is inside of these tiny pockets of hydrogen gas that pressures can reach enormous values. Interestingly, these features increase as hydrogen loading increases. They are also found mostly near the surface of the metal powder (why LENR is more or less a surface phenomenon). And I've also found out why "cracks" and SURFACE cavities seem to be associated with excess heat generation. Hydrogen can be "trapped" by surface cracks and pores. In these areas, there can be greater levels of hydrogen absoprtion into the lattice. Basically, as the crack propagates into the metal via hydrogen embrittlement, intergranular bubbles form nearby. But it is important we make a distinction between surface cavities (in which the trapped hydrogen has a clear path of migration out during desorption) and INTERIOR bubbles/cavities inside the lattice. In these bubbles, there is no path for the hydrogen to escape except back through the lattice. This means when the hydrogen bubbles are re-heated rapidly, the already very high pressures can be boosted to incredible levels. If the rate is fast enough and the pressure peaks, LENR can form. Or, in some cases, the cavities can literally rupture damaging the lattice.
All of this was in the literature when Andrea Rossi launched the JONP. I want to slap myself for not tracking down all this literature sooner.