Tungsten Control Rods AP1000: Tungsten control rod assemblies have low reactivity values and mechanically provide relatively small reactivity adjustments to the reactor, thereby it reduces the need for chemical and volumetric control systems to adjust reactor coolant boron concentration. As a result, routine treatment of reactor wastewater can be significantly reduced and the design and operation of chemical and volumetric control systems can be simplified. During base load and load following, the gray control rod assembly automatically repositions to maintain programmed average coolant temperatures. Lifting or inserting the ash control rod assembly can also effectively compensate for the effects of fuel and combustible poison burnup, as well as changes in power and temperature, and in core xenon conditions on reactivity.