[en] Experiments which attempt to measure the lifetimes of protons and bound neutrons require detectors which have 10³³ or more fiducial nucleons. If an experiment is to be sensitive to all, or most of the kinematically allowed decay modes, it must also have many samples of information from the nucleon decay products, that is, it must be fine-grained. This report describes a design study for such an experiment carried out at the University of Pennsylvania by E.W. Beier, L.S. Durkin, S.M. Heagy, A.K. Mann and H.H. Williams. The performance of a 160 ton prototype of the nucleon decay detector is also discussed. The detector is based on large, segmented planes of liquid scintillator calorimeter (8m x 8m) viewed by photomultiplier tubes, interleaved with track chambers operated in the proportional mode