Low temperature thermal conductivity, κ, of optimally-doped Bi2212 was studied before and after the introduction of point defects by electron irradiation. The amplitude of the linear component of κ remains unchanged, confirming the universal nature of heat transport by zero-energy quasiparticles. The induced decrease in the absolute value of κ at finite temperatures allows us to resolve a nonuniversal term in κ due to conduction by finite-energy quasiparticles. The magnitude of this term provides an estimate of the quasiparticle lifetime at subkelvin temperatures.Fifteen years after their discovery, high-T c superconductors continue to attract significant attention and provoke intense debate [1]. One central issue is the extent of validity of the Fermi-liquid picture for describing the electronic excitations in these systems. The properties of the metallic state (even at optimal doping) appear to remain beyond such a picture. But, are there well-defined quasiparticles (qp) deep in the superconducting state as suggested by ARPES measurements [2]? And if yes, at which energy scale do they break down? To answer these questions, low-energy excitations in the superconducting state are under intense scrutiny [3,4].Low temperature thermal conductivity, κ, has proven to be an instructive probe of such excitations. A non-vanishing linear term in thermal conductivity of optimally-doped Y123 for T → 0 was the first solid evidence for a finite density states of nodal quasiparticles at zero energy [5]. Moreover, as expected for the case of a d-wave gap [6,7], the amplitude of this term was found to be universal; i.e. independent of impurity concentration [5]. Recently, Durst and Lee [3] showed that, regardless of Fermi-liquid corrections, this amplitude is intimately related to the fine structure of the superconducting gap in the vicinity of the nodes. Subsequently, Chiao et al. [4] found a quantitative agreement between this gap structure as deduced from thermal conductivity and the one directly observed by ARPES studies [8] in optimally-doped Bi2212. On the other hand, data on electronic specific heat [9][10][11] and penetration depth [12,13] are limited to T >2K and probe excitations within a higher energy interval where the density of states is a linear function of energy. The temperature dependence of electronic specific heat in Y123 is in agreement with theoretical expectations for the gap structure near the nodes [4,11].