A method and apparatus for online condition monitoring of a spent nuclear fuel dry cask storage system. The method comprises monitoring physical parameters of air flowing through inlet vents and outlet vents of a system and observing successive measurements of the parameter for deviations from the baseline to determine if the condition of the system has changed. The parameters may include temperature, pressure, density, mass and volumetric flow rate, nuclear radiation, impurities, humidity, salt content, acidity, chemistry, and fission product gases. The information may then be used directly or to develop an accumulation profile. The data may also be used in modeling or other simulations and to establish condition change signatures. The apparatus includes sensors placed over inlet and outlet vents, sensor interfacing hardware connected to the sensors, and a computer connected to the hardware to acquire, display, and analyze the sensor data and to display the status of the system.