The Very High Temperature Reactor (VHTR) such as pebble bed modular reactor (PBMR) is one of the most likely Next Generation Nuclear Plants (NGNP) for co-generation of process heat and electricity and for the production of hydrogen from water for industrial applications in the chemical and petrochemical sectors. Due to its very high operating temperature (1000°C) various thermochemical processes such as coal gasification, biogas and methane reformation, and hydrogen from water splitting are possible in addition to electrical energy production.. The understanding of interface and coupled integral system is important in the safety and operational analysis. The fact that during a planned transient or an accident, the reactor and the chemical plant system act like a single unit coupled via the intermediate heat exchanger (IHX) which poses unique challenges for routine transient, safety and accident isolation. The IHX transfers energy from nuclear plant side to chemical plant side and acts as an isolation device between the two plants. If a nuclear reactor is coupled via a heat exchanger to a chemical plant, the entire coupled system becomes subject to transients in response to changes in either the reactor loop or the chemical plant. When unifying two systems, which are dynamic and provide feedback to each other, the nature of the response is dictated by the relative time constants of the plants. A VHTR is a thermal reactor, and thus delayed neutrons play an important role in reactor transients. A thermal reactor has a time constant of about 55 seconds. In the chemical plant, different reactor sections have different response times. The limiting reaction rate in the chemical plant is that of the component that has the largest time constant, which provides at least a first order estimate of the overall plant response. A transient event, which is driven by the chemical plant, might consist of a small leak or break in piping. Such an event can be characterized as a partial-loss-of-heat-sink accident. A more catastrophic chemical plant driven accident, such as an explosion, would cause a complete loss of heat sink for the nuclear reactor. In this work, extensive modeling methodology is developed for a VHTR, nominally a PBMR, coupled to a hydrogen generation plant, nominally based on the Sulfur Iodine (SI) cycle. A transient chemical model of the Sulfur-Iodine (SI) cycle is coupled to a thermal hydraulic model of the Pebble Bed Modular Reactor 268 (PBMR 268) and a point kinetics model through an IHX.