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Nuclear radiation effects on semiconductor devices and circuits are modelled using flowgraph techniques. The usefulness and applicability of a model adequate for both transient and permanent radiation effects in devices and circuits is based on several recent developments in modelling techniques: ?? the lumped-parameter semiconductor diffusion-controlled model. ?? the systematic formulation of equivalent circuits with independent and controlled sources. ?? the development of the closed system approach to assess variation in terms of sensitivity for systems with large number of variables. ?? the utilization of a computer routine which provides circuit response to given excitations or due to given parameter changes. The lumped semiconductor device model proposed by Linvill is modified to include effects of both neutron and pulse radiation sources. The network-like nature of the lumped device model provides insight into the physical processes involved as well as into circuit changes produced by irradiation. Examples of circuit applications illustrate the use of the lumped model with radiation induced generators of carriers. To account for variations for circuit response due to parameter changes, a technique of evaluating sensitivities for models of any complexity are developed. Criteria for optimization of circuits under parameter changes are established. Emphasis is not on experimental data but on methods and criteria for establishing and evaluating models of circuits in a radiation environment.