1. A single quantification method of an external event probabilistic safety assessment (PSA) model containing multi-compartment scenarios using a computer system, comprising: loading an internal event PSA logic model having core damage as a top event from a database of the computer system; using the computer system to constitute a maping table between internal events and external events containing the multi-compartment scnarios based on information of occurring frequencies of external events, initiators induced by external events, and equipment damaged by external events; establishing an external event PSA model, wherein establishing an external event PSA model comprises: replacing the initiators, wherein the computer system replaces the initiators of the internal event PSA lgic model with OR logics of related external events comprising single external events in single compartments and said single external events propagating to an adjacent compartment, by using the mapping table; and adding external events to the basic events, wherein the computer system performs OR logic operations on the basic events of equipment damage caused by external events, among the basic events of the internal event PSA logic model, and wherein the external events make the equipment unusable; calculating a final minimum cut set (MCS) based on the external event PSA model; and calculating a core damage frequency (CDF) value accordinng to the final MCS.