First of all, lead has an incredibly high boiling point, far beyond the range of operating temperatures that are envisaged. This means that it will never boil, not even in accidental conditions, ensuring that the core, always cooled, will not melt down.
Then, it has a high density. Though this has been perceived in the past as a limitation, it permits instead to favor natural circulation of the coolant, adding in reliability by guaranteeing the effective removal of heat from the core even in case of pumps failure.
From a chemical point of view, lead reacts with almost everything, but gently: with no significant.
This means, first of all, that the reactor will never have to deal with such additional energy, making any abnormal condition easier to face, and the system’s response much more reliable and effective. But even more: the gentle chemical interaction of lead, notably with the radioactive fission products contained in the fuel implies that, even in the postulated case of failure in some fuel element, the radioactive inventory is maintained within the lead melt and not released outside, thereby protecting people and the environment.
Finally, from the neutronics standpoint, the chain reaction efficiency is not tigthly associated to narrow bundles of fuel pins: this permits to space the pins apart having very low pressure drops through the core, hence a very effective heat removal by natural circulation, when required.