Radiation damage occurs in reactor pressure vessel (RPV) steel, causing microstructural changes such as point defect clusters, changes in dislocation density, and precipitates. These radiation-induced microstructural changes cause material embrittlement. Radiation damage is a crucial concern in the nuclear industry because many nuclear plants throughout the United States are entering the first period of life extension, and older plants are currently undergoing assessment of technical basis to operate beyond 60 years. The result of extended operation is that the RPV and other components will be exposed to higher levels of neutron radiation than they were originally designed to withstand. There is currently no nondestructive evaluation technique with which to unambiguously assess the amount of radiation damage in RPV steels. The development of such a technique would enable the assessment of the integrity of a vessel, allowing operators to determine whether reactors can continue to operate safely, and would directly support the nuclear industry Long Term Operation and U.S. Department of Energy Light Water Reactor Sustainability initiatives. Nonlinear ultrasound (NLU) is a nondestructive evaluation technique that is sensitive to microstructural features such as dislocations, precipitates, and their interactions in metallic materials. The physical effect monitored via NLU is the generation of higher harmonic frequencies in an initially monochromatic ultrasonic wave, arising from the interaction of the ultrasonic wave with microstructural features. Recent research has demonstrated that NLU is sensitive to radiation-induced microstructural changes in RPV steel. NLU measurements were made on various Charpy specimen sets of typical RPV material to investigate the applicability of NLU in characterizing radiation damage over a range of fluence levels, irradiation temperatures, and material compositions. These previous experimental results are interpreted with a newly developed analytical model that combines irradiation-induced precipitate and vacancy contributions to the nonlinearity parameter.