Reactivity analysis of a direct nuclear heated gas turbine with nitrogen coolants

Abstract

This paper investigates the effects on core reactivity of using a nitrogen coolant within a prismatic High Temperature Gas Reactors (HTGR). The intent is that the nitrogen coolant will be directly compatible with existing technologically mature gas turbine rotating machinery and therefore simplify the design path to a direct nuclear heated closed cycle gas turbine. The 14N isotope of nitrogen makes up the majority (99.6%) of natural nitrogen and has a reasonably significant absorption cross-section for thermal neutrons. An understanding is therefore required of any implications for reactivity insertions during coolant pressure and temperature changes. The analysis was undertaken using the Serpent continuous-energy Monte Carlo particle transport code modelling an infinite lattice core of fuel blocks containing tristructural-isotropic (TRISO) coated fuel particles. The results of the analysis show that for low pressure ratio turbo machinery (10:1) with atmospheric compressor inlet pressure the reactivity swings are small (<60pcm). Equally the effects of temperature transients were also shown to provide only a small positive reactivity coefficient (∼0.09pcm/K) which inversely diminishes with absolute temperature. The conclusion of this paper is that the effect of nitrogen on core reactivity does not prevent its utilisation as a potential reactor coolant for small prismatic HTGRs, potentially enabling direct coupling of existing air derived turbomachinery.