NEUTRONICS CHARACTERIZATIONS OF THE DUPLEX TRISO THORIUM FUEL ASSEMBLY BLOCKS USING MCNPX CODE

Mohamad Hairie B. Rabir; Aznan Fazli Ismail; Mohd Syukri Yahya

Mohamad Hairie B. Rabir Malaysian Nuclear Agency (Nuclear Malaysia)
Aznan Fazli Ismail Universiti Kebangsaan Malaysia (UKM)
Mohd Syukri Yahya, Dr Universiti Tenaga Nasional (UNITEN)

Keywords: Thorium Reactor, High Temperature Reactor, Micro Modular Reactor, TRISO-Duplex, MCNPX

Abstract

Abstract

The UO₂ seed and ThO₂ blanket in the duplex fuel pellet were originally designed to increase the breeding rate of fissile materials in light water breeder reactors. In this configuration, most of the fission reactions occur in the seed portion, resulting in high temperature of the seed region. As such, duplex feel pellet could possibly be advantageous for an application in a high-temperature reactor (HTR). In light of this observation, the paper aims to investigate the potential of duplex fuel pellet with TRISO fuel at the fuel block level. Specifically, the block-level neutronics performance of the typical TRISO fuel was compared to that of the TRISO duplex fuel using the MCNPX simulations. For the TRISO duplet pellet, a variety of packing fractions and seed's ²³⁵U enrichment were also simulated. In addition, the duplex TRISO fuel block was also modeled and compared to the standard UO₂ block, homogeneous (Th,U)O₂ fuel block, and seed and blanket unit (SBU) fuel block. It was noted that the duplex TRISO fuel compact has a noticeable longer operational cycle length than the UO₂ TRISO fuel compact – these TRISO duplex fuels’ cycle length and burnup were found to be significantly affected by variations in its packing fraction and seed enrichment. In addition, the duplex-rodded fuel block and the SBU model also have comparable neutronics properties, as well as the highest power peaking. The combined method, in which the blanket ThO₂ rods in SBU were replaced with duplex rods, can be used to manage these high power peaking issue by up to 23% reduction but with a shorter cycle length than the all duplex rods and SBU model. Nonetheless, the combined duplex + S&B configuration still has a longer cycle length than the standard UO₂ block and homogeneous (Th,U)O₂ fuel block models. These findings demonstrate the advantages of the duplex TRISO fuel design as a major optimization strategy for the future development of a thorium-loaded HTR.

Abstrak

Bijian UO₂ dan selimut ThO₂ dalam pelet bahan api dupleks pada asalnya direka untuk meningkatkan kadar pembiakan bahan mudah pecah dalam reaktor pembiakan air ringan. Dalam konfigurasi ini, kebanyakan tindak balas pembelahan berlaku dalam bahagian benih, mengakibatkan suhu tinggi kawasan benih. Oleh itu, pelet rasa dupleks mungkin berfaedah untuk aplikasi dalam reaktor suhu tinggi (HTR). Berdasarkan pemerhatian ini, kertas kerja ini bertujuan untuk menyiasat potensi pelet bahan api dupleks dengan bahan api TRISO pada tahap blok bahan api. Secara khususnya, prestasi neutronik tahap blok bahan api TRISO biasa dibandingkan dengan bahan api dupleks TRISO menggunakan simulasi MCNPX. Untuk pelet duplet TRISO, pelbagai pecahan pembungkusan dan pengayaan ²³⁵U benih juga telah disimulasikan. Selain itu, blok bahan api TRISO dupleks juga telah dimodelkan dan dibandingkan dengan blok UO₂ standard, blok bahan api homogen (Th,U)O₂ dan blok bahan api unit benih dan selimut (SBU). Telah diperhatikan bahawa padat bahan api TRISO dupleks mempunyai panjang kitaran operasi yang lebih ketara berbanding padat bahan api UO₂ TRISO - panjang kitaran bahan api dupleks TRISO dan pembakaran didapati terjejas dengan ketara oleh variasi dalam pecahan pembungkusan dan pengayaan benih. Selain itu, blok bahan api berrod dupleks dan model SBU juga mempunyai sifat neutronik yang setanding, serta memuncak kuasa tertinggi. Kaedah gabungan, di mana rod ThO₂ selimut dalam SBU digantikan dengan rod dupleks, boleh digunakan untuk menguruskan isu memuncak kuasa tinggi ini dengan pengurangan sehingga 23% tetapi dengan panjang kitaran yang lebih pendek daripada semua rod dupleks dan model SBU. Namun begitu, konfigurasi dupleks + S&B gabungan masih mempunyai panjang kitaran yang lebih panjang daripada blok UO₂ standard dan model blok bahan api homogen (Th,U)O₂. Penemuan ini menunjukkan kelebihan reka bentuk bahan api TRISO dupleks sebagai strategi pengoptimuman utama untuk pembangunan masa depan HTR yang dimuatkan torium.