ASSESSMENT OF THE IMPACT OF NEUTRONIC THERMAL-HYDRAULIC COUPLING ON THE DESIGN AND PERFORMANCE OF NUCLEAR-REACTORS FOR SPACE PROPULSION

A series of studies has been performed to investigate the potential im pact of the coupling between neutronics and thermal hydraulics on the design and performance assessment of solid core reactors for nuclear t hermal space propulsion, using the particle bed reactor (PBR) concept as an example system. For a given temperature distribution in the reac tor, the k(eff) and steady-state core power distribution are obtained from three-dimensional, continuous energy Monte Carlo simulations usin g the MCNP code. For a given core power distribution, determination of the temperature distribution in the core and hydrogen-filled annulus between the reflector and pressure vessel is based on a nonthermal equ ilibrium analysis. The results show that a realistic estimation of fue l, core size, and control requirements for PBRs using hydrogenous mode rators, as well as optimization of the overall engine design, may requ ire coupled neutronic/thermal-hydraulic studies. However, it may be po ssible to estimate the thermal safety margins and propellant exit temp eratures based on power distributions obtained from neutronic calculat ions at room temperature. The results also show that, while variation of the hydrogen flow rate in the annulus has been proposed as a partia l control mechanism for PBRs, such a control mechanism may not be feas ible for PBRs with high moderator-to-fuel ratios and hence soft core n eutron spectra.