Recently, radiation-hardened embedded systems are demanded for robots that must function in high-radiation environments such as those prevailing at the Fukushima Daiichi and Chernobyl nuclear power plants. Recent news reports have described the discovery of a 650 Sv/h radiation region near the containment vessels of the Fukushima Daiichi nuclear power plant. Even under such a dauntingly intense radiation environment, workable robots are required. This paper therefore present a high-soft-error-tolerant 16-bit adder unit based on penta-modular redundancy (SMR) in addition to triple modular redundancy (TMR). The soft-error tolerances of the TMR and SMR were analyzed theoretically. Such TMR and SMR circuits were implemented onto a field programmable gate array (FPGA). Then error-injection experiments for the TMR and SMR were done on the FPGA. Analytical results have revealed that SMR 16-bit adder circuit is useful for heavy radiation environments.