Transient modelling of sulphur-iodine cycle thermochemical hydrogen generation coupled to pebble bed modular reactor

A transient control volume model of the sulphur iodine (S-I) and Westinghouse hybrid sulphur (HyS) cycles is presented. These cycles are some of the leading candidates for hydrogen generation using a high temperature heat source. The control volume models presented here are based on a heat and mass balance in each reaction chamber coupled to the relevant reaction kinetics. The chemical kinetics expressions are extracted from a relevant literature review. Two assumptions regarding reaction chamber pressure are identified, namely a constant pressure condition and a differential form of ideal gas law. The HyS model is based on an application of the Nernst equation. This application of the Nernst equation suggests that in the HyS cycle the hydrogen generation rate is directly proportional to the SO2 production rate. The observed chemical kinetic response time of the sulphuric acid decomposition section is on the order of 30 seconds, whereas the response time of the hydrogen iodide decomposition section is on the order of 500 seconds. It is concluded that the decomposition of hydrogen iodide (HI) is the rate limiting step of the entire S-I cycle. High temperature nuclear reactors are ideal candidates for use as a driving heat source for both the S-I and HyS cycle. The pebble bed modular reactor is a type of very high temperature reactor (VHTR ) suitable for nuclear hydrogen generation. A methodology for coupling of the S-I or HyS cycle to a pebble bed modular reactor (PBMR) via an intermediate heat exchanger (IHX) is developed. A 2-D THERMIX heat transfer model of a PBMR-268 is presented, and this model is coupled to a point kinetics model. The point kinetics model was developed to meet the same specifications as the RELAP5 point kinetics module. A steady-state integration of the S-I and HyS cycle models to the PBMR 268 heat transfer model is performed. The integration assumes that 100% of the heat energy from the PBMR-268 is deposited into the chemical plant via the IHX. The steady-state energy balance suggests that, for the S-I cycle, 74% of the heat energy from the PBMR-268 is used for the decomposition of HI, with the remaining 26% used for the decomposition of sulphuric acid (H2SO4) . The S-I and HyS hydrogen generation models are coupled to the PBMR-268 heat transfer and point kinetics models. The coupling of the PBMR-268 models to the hydrogen generation models is a fully transient coupling through the intermediate heat exchanger. Two step insertions of USD +0.25 and USD -0.25 are initiated on the nuclear reactor side of the coupled plant and the response is observed on the chemical plant side. (authors)