Techno-economic optimisation methodology for HTGR balance of plant systems

Abstract

The nuclear industry lacks a well documented, systematic procedure defining the requirements for power plant cycle selection and optimisation. A generic technoeconomic optimisation methodology is therefore proposed that can serve in the selection of balance-of-plant configurations and design conditions for High Temperature Gas-cooled Reactor (HTGR) power plants. The example of a cogeneration steam plant coupled to a pebble bed reactor, with or without an intermediate buffer circuit, was used in search of a suitable methodology. The following analyses were performed: - First order thermal hydraulic analysis - Second order thermal hydraulic analysis including cost estimation - Third order steady state analysis to evaluate part-load operation - Third order transient analysis to test operability and controllability The assumptions, level of detail required, modelling methodology and the type of decisions that can be made after each stage are discussed. The cycles under consideration are evaluated and compared based on cycle efficiency, capital cost, unit energy cost and operability. The outcome of this study shows that it is worthwhile spending the effort of developing a second order costing model and a third order model capable of analysing off-design conditions. First order modelling could be omitted from the methodology. The advantage of a second order model is that the cycle configuration can be optimised from a unit energy cost perspective, which incorporates the effects of both capital cost and cycle efficiency. The optimum cycle configuration differs from that predicted by first order modelling, which illustrates that first order modelling alone is insufficient. Third order part-load operation analysis showed operability issues that were not apparent after first or second order modelling. However, transient analysis does not appear justified in the very early design stages. To conclude, the proposed methodology is summarised as follows: - Evaluate the user requirements and design constraints. - Apply design principles from the Second Law of thermodynamics in selecting cycle configurations and base case operating conditions. - Optimise the operating conditions by performing second order thermal hydraulic modelling which includes component design and cost estimation. - Evaluate part-load operation with third order analysis. - Select the cycle with the lowest Levelised Unit Energy Cost (LUEC) and simplest operating strategy.