The in-core thermionic nuclear reactor is a leading candidate for low power space systems requirements. The thermionic converters are static devices which convert heat directly to electricity in the form of high current, low voltage output power. The nuclear fuel cladding is used as the emitting electrode and is surrounded with close spacing by the collector electrode. The stability and lifetime of the system depend on the maintenance of the interelectrode gap established by the resulting coaxial geometry.Emitter distortion, therefore, can be a life-limiting factor. This is exacerbated by high emitter temperature and high fuel power density present in typical applications. In the particular case of low nuclear power level systems, the added complexity of a fast driver core section is necessary to ensure sufficient excess reactivity for power control.A modification of the present TFE provides design flexibility which allows trade-offs to be made among emitter distortion, emitter temperature and lifetime while increasing fuel-volume ratio and eliminating the driver in the low power level core design. The converter concept PRID (Partial Radiation Insulated Diode), by eliminating a portion of the interelectrode thermal radiation, allows for increased pin size and reduced fuel power density, and can lead to a more efficient and desirable longer TFE cell.