Reactor Physics Design of Supercritical CO2-Cooled Fast Reactors

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Report Date: September 2004
Appendices: No

Abstract

Gas-Cooled Fast Reactors (GFRs) are among the GEN-IV designs proposed for future deployment. Driven by anticipated plant cost reduction, the use of supercritical CO2(S-CO2) as a Brayton cycle working fluid in a direct cycle is evaluated. By using S-CO2 at turbine inlet conditions of 20 MPa and 550–700°C efficiencies between 45% and 50% can be achieved with extremely compact components.

Neutronic evaluation of candidate core materials was performed for potential use in block-type matrix fueled GFRs with particular concentration on lowering coolant void reactivity to less than $1. SiC cercer fuel was found to have relatively low coolant void worth (+22¢ upon complete depressurization of S-CO2 coolant) and tolerable reactivity limited burnup at matrix volume fractions of 60% or less in a 600 MWth core having H/D of 0.85 and titanium reflectors. Pin-type cores were also evaluated and demonstrated higher keff versus burnup, and higher coolant void reactivity than the SiC cercer cores (+$2.00 in ODS MA956-clad case having H/D of 1). It was shown, however, that S-CO2 coolant void reactivity could be lowered significantly—to less than $1—in pin cores by increasing neutron leakage (e.g. lowering the core H/D ratio to 0.625 in a pin core with ODS MA956 cladding), an effect not observed in cores using helium coolant at 8 MPa and 500ºC. An innovative "block"-geometry tube-in-duct fuel consisting of canisters of vibrationally compacted (VIPAC) oxide fuel was introduced and some preliminary calculations were performed. A reference tube-in-duct core was shown to exhibit favorable neutron economy with a conversion ratio (CR) at beginning of life (BOL) of 1.37, but had a coolant void reactivity of +$1.4. The high CR should allow designers to lower coolant void worth by increasing leakage while preserving the ability of the core to reach high burnup. Titanium, vanadium and scandium were found to be excellent reflector materials from the standpoint of keff and coolant void reactivity due to their unique elastic scattering cross-section profiles: for example, the SiC cercer core having void reactivity of +$0.22 with titanium was shown to have +$0.57 with Zr3Si2. Overall, present work confirmed that the S-CO2-cooled GFR concept has promising characteristics and a sufficiently broad opion space such that a safe and competitive design could be developed in future work with considerably less than $1 void reactivity and a controllable Dk due to burnup. 

Program:     ANP : Advanced Nuclear Power Program

Type:     TR 

RPT. No.:  104