A transient model of the Sulfur-Iodine (SI) cycle is coupled to a THERMIX thermal hydraulic model of the Pebble Bed Modular Reactor 268 (PBMR 268) and a point kinetics model. A transient is initiated via reactivity insertion in point kinetics model. In a coupled nuclear hydrogen production system, a transient is initiated on either the nuclear reactor side or the chemical side of the plant. There are many potential transients that occur in a nuclear reactor system. Some examples are, startup, shutdown, reactivity insertion, or removal, off-normal operation, and design basis accident. In the chemical plant, examples of transient driving forces are reaction chamber temperature change, small pipe break or leak, vapor explosion, and chemical plant fire or other event. In terms of nuclear reactor response, each of these chemical plant events is a loss-of-heat-sink accident. An investigation of the coupling of the chemical plant and nuclear reactor consists of evaluating the response of the chemical plant and feedback to the nuclear reactor in a reactivity insertion or removal. As reactivity is removed, nuclear reactor power and coolant temperature will drop. The heat sink for the nuclear reactor is the chemical plant. Section 2 has a very high threshold temperature, around 800°C. Below this threshold temperature, the reaction will not proceed. The heat transferred through the IHX is split into the vaporization, and the heat required for the endothermic chemical reaction. Relatively small amount of the power increase is actually sent to the chemical plant, since much of the additional heat is used to raise the temperature of the fuel, moderator, and containment of the reactor. After the response from the reactivity insertion, a new steady state is attained within several hundred seconds. A relatively small amount of the power increase is actually sent to the chemical plant, since much of the additional heat is used to raise the temperature of the fuel, moderator, and containment of the reactor. After the response from the reactivity insertion, a new steady state is attained within several hundred seconds. The maximum fuel, average fuel temperature, and average core temperature were obtained. Integrating the average core temperature, over the first 20 seconds and comparing the power deposited in the core to the approximate power required to raise the temperature of the reactor core by 41 °C it is determined that the predicted amount of heat deposited in the core is reasonable. The radial and axial fuel and coolant temperatures at the initial steady state, the peak temperature, and the final steady state, were obtained. A case of large reactivity insertion such as control rod ejection was also studied. Results of these transient are presented and discussed.