Physics study of block/prismatic-type HTGR design option for the Indonesian Experimental Power Reactor (RDE)

•The block-type HTGR is proposed as an alternative option for Indonesian RDE.•Lattice physics calculations have been conducted to find the optimum fuel composition.•3 different cores with single-batch, 2-batch, and 4-batch refuelling scenarios have been analysed.•The proposed core design satisfied the RDE user design requirement. In our previous works, we have investigated the pebble-bed type high-temperature gas-cooled reactor (HTGR) as a candidate design for the 10 MWth Indonesian Experimental Power Reactor (“Reaktor Daya Experimental,” RDE). The once-through-then-out (OTTO) and multipass fuelling schemes with both uranium and thorium fuel cycles were considered. The heavy metal loading per ball and fissile enrichment as free design parameters were surveyed to obtain the optimum fuel compositions (the smallest fissile requirement per unit energy generated) for an average burnup target of 80 MWd/tU. Steady-state thermal-hydraulic analyses were also conducted, and no critical challenge was found since the reactor power density is low. Geographically the RDE site and the country, in general, are located in the so-called Ring of Fire (Circum-Pacific Belt) which is vulnerable against frequent seismic events. The nature of a block- or prismatic-type HTGR does not allow large and random movement of its fuel elements during a large seismic event which makes this type of HTGR relatively safer for these regions. The design also allows control and safety rods (absorbers) to be located inside the core by providing holes in graphite blocks which ensure the sub-criticality during an abnormal event. In the present work, under the same RDE design requirements and constraints, the block/prismatic-type 10 MWth HTGR is studied based on the Japanese 30 MWth High-Temperature Engineering Test Reactor (HTTR). Although the design study emphasized on the optimal fuel composition in the reactor physics lattice phase, a preliminary result of the full core reactor physics analytical work is given.