Joint International Workshop: Nuclear Technology and Society – Needs for Next Generation Berkeley, California, January 6-8, 2008, Berkeley Faculty Club, UC Berkeley Campus NEUTRONIC DESIGN OF THE PB-AHTR Massimiliano Fratoni, Ehud Greenspan and Per F. Peterson Nuclear Engineering Department University of California at Berkeley Berkeley CA 94720-1730 maxfratoni@nuc.berkeley.edu ABSTRACT This study (1) investigates the neutronic characteristics of the Pebble Bed Advanced High Temperature Reactor (PB-AHTR); (2) compares the PB-AHTR neutronic performance against those of the alternative design options of He-cooled high temperature reactors using either pebble (PBMR) or prismatic (VHTR) fuel and against a liquid-salt cooled design version of the latter (LS-VHTR); (3) studies the possibility of incinerating TRU in the PB-AHTR feeding TRU from LWRs spent fuel. It is found that the optimal features a graphite-to-heavy metal ratio of ~360 and its reactivity coefficients are all negative. A comparison with the helium-cooled pebble-bed reactor and with a prismatic-fuel reactor that is cooled with either flibe or helium is also presented. It is found that the PB-AHTR offers similar discharge burnup as the other three designs. As compared to the gas-cooled pebble bed, the PB-AHTR uranium loading and energy generated per pebble are ~2.5 times higher. When loading TRU in the pebbles it is found that they can reach a burnup as high as 685 GWd/tHM and the core average reactivity coefficients are all negative. About 70% of the initial load of HM is incinerated in a single pass. Key Words: pebble, AHTR, VHTR, TRU 1. INTRODUCTION The PB-AHTR is a Pebble Bed Advanced High Temperature Reactor that is cooled with the liquid salt flibe (LiF-BeF 2 ) rather than helium 1,2,3,4 . Recent experiments have demonstrated that pebble beds can be formed with a liquid salt coolant and pebbles can be recirculated 5 . This study presents a preliminary neutronic and depletion analysis for the PB-AHTR. Section II describes the computational model and the methodology developed to determine attainable burnup. Section III summarizes the parametric study conducted to search for the optimal uranium loading per pebble, for a given power density and total core power, that maximizes the attainable burnup. Reactivity coefficients are evaluated, too. Comparisons with the helium-cooled Pebble Bed Modular Reactor (PBMR) and with a prismatic-fuel reactor that is cooled with either flibe (Liquid Salt-cooled Very High Temperature Reactor – LS-VHTR) or helium (Very High Temperature Reactor – VHTR) are summarized in Section IV. A preliminary study for incineration of TRU in the PB-AHTR is presented in Section V. 2. MODEL AND METHODOLOGY This section presents the model applied in the analysis and the methodology developed to determine the attainable burnup.