Nuclear power and hydrogen production systems have distinct dynamic characteristics and their interaction can produce results hard to predict intuitively. Therefore, it is necessary to use mathematical models describing the transient behavior of these systems. Dynamic models have assisted engineers for several decades in the assessment of safety of nuclear power plants [4, 10]. On the other hand, hydrogen production alternatives utilizing heat and/or electricity from nuclear power plants have been mostly studied within the steady-state regime to make design decisions. Some efforts have been made to understand the dynamics of this system [6, 11]. The hydrogen production plant adds extra complexity to the system because it includes chemical reactions with nonlinear dynamics. The simulation environment used should be able to capture these physical and chemical phenomena. The long term goal of this research is to develop a suite of mathematical models that enable dynamic simulation, sensitivity analysis and optimization studies of the interaction of a nuclear reactor and a hydrogen generation plant.
This paper focuses on dynamic simulation of the heat transfer loop. This loop removes the heat from the nuclear reactor coolant and uses it to generate high temperature steam to feed the hydrogen plant. High-temperature electrolysis is considered as the electrochemical process to produce hydrogen. The main components of the heat transfer loop are two heat exchangers and a compressor. The heat exchanger removing the heat from the nuclear reactor coolant is a compact heat exchanger and the heat exchanger heating the steam is of the tube-in-shell kind [1]. The compressor regulates the gas flowrate and it is responsible for smooth operational changes. Helium is considered as the heat transfer fluid. The use of helium in the intermediate loop simplifies the heat exchanger design, and avoids chemical consequences of leakage of different coolants into the reactor loop.
Previous attempts to simulate similar systems involving a nuclear reactor and PCUs have used numerical methods that introduce errors and produce unphysical numerical artifacts. The most common error is to solve the partial differential equations for gas dynamics using the method of lines in an implicit integrator without limiting the time step according to the CFL condition. A simulation of gas dynamics performed in this way cannot capture the fast dynamics of a gas flowing in a pipe. The second error is to use numerical methods without high-resolution corrections; the consequence of this is artificial oscillations in the simulations. Here, this issue is addressed by developing accurate methods for the simulation of the helium gas dynamics in a pipe. The gas dynamics simulator is based on CLAWPACK [3], which is a transient simulator for hyperbolic equations based on an explicit integrator with high-resolution methods. The CLAWPACK boundary conditions have to be reformulated to be able to model closed loop systems involving compressors. Simulation results for the nuclear reactor coupled to the heat transfer interface during start-up and shut-down will be presented.
The dynamic profiles of the different variables involved are fundamental in understanding possible issues related to safety and operability of the plant. The thermal and pressure profiles dictate the necessary characteristics of the materials to be used. The analysis of the power profiles helps to determine and size the auxiliary heat sources/sinks, and the best operation of the plant. Also these simulations help to identify the most important process parameters that can be modified to improve the operation of the plant.
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