Fragmentation, in other words the breaking of particulate material into smaller pieces can be observed in nature and in modern life on a wide range of length scales in all kinds of technical applications. Most studies on dynamic failure focus on the behaviour of bulk systems in one, two and tree dimensions under impact and explosive loading, showing universal power law behaviour of fragment size distribution, however, hardly any studies have been devoted to fragmentation of shells. We present a detailed theoretical and experimental study on the fragmentation of closed thin shells made of disordered brittle material, due to an excess load inside the system. Based on large-scale discrete element simulations with spherical shell systems under different extreme loading situations, we prove a power law for the fragment mass distribution and give evidence that it arises due to an underlying phase transition. Satisfactory agreement is found between the numerical predictions of the exponent of the power law for the fragment mass distribution and the experimental findings.