VOL. 11, NO. 6, MARCH 2016 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences © 2006-2016 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com 4070 SCOPING STUDY ON THE OPTIMUM FUEL COMPOSITION AND FUELING SCHEME OF A PEBBLE-BED HTGR Tagor Malem Sembiring 1 and Peng Hong Liem 2 1 Center for Nuclear Reactor Technology and Safety - National Nuclear Energy Agency of Indonesia, Kawasan Puspiptek Serpong, Tangerang Selatan, Indonesia 2 Nippon Advanced Information Service (NAIS Co. Inc.), Muramatsu, Tokaimura, Ibaraki, Japan E-Mail: tagorms@batan.go.id ABSTRACT An optimum fuel composition is a very important parameter in the operation of a pebble bed high temperature gas-cooled reactor (HTGR). In the present scoping study, the optimum ranges of heavy metal (HM) loading per pebble and the uranium enrichment are investigated. The HM loading range covers 4 to 10 g per pebble, while the uranium enrichment covers 5 to 20 w/o. Two fuel loading schemes typical to pebble-bed HTGRs are also investigated, i.e. the OTTO and multi- pass schemes. All calculations are carried out using BATAN-MPASS, a general in-core fuel management code dedicated for pebble-bed type HTGRs. The reference reactor design case is adopted from the German 200 MWth HTR-Module but with core height of half of the original design. Other design parameters follow the original HTR-Module design. The results of the scoping study show that, for both once-through-then-out (OTTO) and multi-pass fueling schemes, the optimal HM loading per pebble is around 7 g HM/ball. Increasing the uranium enrichment minimizes the fissile loading however higher enrichment than 15 w/o is not effective anymore. The multi-pass fueling scheme shows lower fissile loading requirement and a significantly lower axial power peaking than the OTTO scheme. It can be concluded that the optimum range of HM loading and uranium enrichment are found to be around 7 g per pebble and 15 w/o. In addition the multi-pass fueling scheme shows superior burnup and safety characteristics than the OTTO fueling scheme. Keywords: optimum fuel composition, fuel loading scheme, pebble-bed HTGR, HTR-Module, Batan-MPASS. INTRODUCTION An optimum fuel composition and fuel loading scheme in the operation of a pebble-bed high temperature gas-cooled reactor (HTGR) are very important design parameters since they will directly affect the fuel cost, new and spent fuel storage capacity as well as other back-end environmental burden. A scoping study on the fuel composition parameters, namely heavy metal (HM) loading per pebble and uranium enrichment, and on the fuel loading scheme, i.e. once-through-then-out (OTTO) and multi-pass, is conducted. The main goal of this study is to obtain optimum range of HM loading per pebble and uranium enrichment for the both of OTTO and multi-pass schemes. The HM loading per pebble strongly affects the neutron moderation while the uranium enrichment is correlated directly with the achievable discharge burnup. The fuel loading schemes, on the other hand, will influence the overall burnup performance as well as the axial power profile which is an important safety aspect. In the past, Liem (1996) has proposed a two-step design procedure for small-sized pebble bed HTGRs [1]. The German 200 MWth HTR-Module [2] was taken as the reference case for designing smaller-sized pebble-bed HTGRs. In the first step of the design procedure, keeping the core diameter (D) constant, the core height (H) is reduced until the burnup performance starts to deteriorate considerably. It has been found that the HTR-Module original core height can be reduced to its half value with negligible penalty on the burnup performance, that is, from around 9 m to 4.8 m. This core dimension is taken as the reference case for the present scoping study while keeping other design parameters identical with the ones of the original HTR-Module. The main reason to choose the above-mentioned height is the use of OTTO fueling scheme (HTR-Module considered only multi-pass fueling scheme) where the bottom half of the 9 m core produces almost no power. We aim at a more compact core design where the core pressure drop is expected to be much lower. In addition, from the point of view of xenon stability, the shorter core height is expected to improve the stability against xenon. The neutron migration length (M) is estimated to be approximately 25 cm, so that the H/M ratio decreased from 36 to 18, i.e. to more stable regime which allow higher level of thermal neutron flux in the core. However, reducing the active core volume resulted in a higher average power density. It should be noted also that the fuel compositions used in the two-step design procedure in Ref [1] were identical with the HTR-Module (7 g/pebble, 8 w/o enrichment), and no effort has been conducted to check whether the fuel composition is optimum. The vendor of HTR-Module may have conducted a similar scoping study however the results are not available in open publications. Hence, this work is expected to contribute in providing engineering arguments of optimum fuel composition not only for the HTR-Module but also for smaller-sized pebble-bed HTGRs derived from the design. METHODOLOGY The reactor core main parameters used in the present scoping study are shown in the Table-1. For other main design parameters, readers should consult Ref. [2]. For the multi-pass fuel loading scheme, the number of passes is 8 times.