PERANCANGAN FUEL ASSEMBLY SMALL-PWR BERBAHAN BAKAR URANIUM MONONITRIDA BERBASIS FULLY CERAMIC MICROENCAPSULATED MENGGUNAKAN SCALE DAN SERPENT

ARIEF RAHMAN HAKIM, Dr. Alexander Agung, S.T., M.Sc. ; Dr. Ir. Andang Widi Harto, M.T.

2017 | Skripsi | S1 TEKNIK NUKLIR

Setelah kecelakaan pada unit PLTN Fukushima Daichii, konsep sistem bahan bakar Accident Tolerant Fuel (ATF) diajukan. Sebagai salah satu bahan bakar yang mengadopsi konsep ATF, fully ceramic micro-encapsulated fuel (FCMF) dengan menggunakan uranium mononitrida (UN) memiliki keunggulan dari aspek keselamatan meskipun memiliki performa neutronik yang tidak lebih baik seperti berkurangnya muatan bahan bakar dikarenakan penggunaan matriks dan kerugian ekonomi neutron dikarenakan N14. Untuk menerapkan teknologi UN-FCM pada Small-PWR, penelitian pada tingkat fuel assembly perlu dilakukan untuk memastikan perancangan fuel assembly yang dilakukan memiliki fitur keselamatan yang melekat. Penelitian ini akan mengevaluasi dampak variasi pitch batang bahan bakar, packing fraction, pengayaan N15, dan pengayaan U235 terhadap koefisien reaktivitas suhu bahan bakar, koefisien reaktivitas void, burnup dan panjang siklus untuk mendapatkan zona perancangan fuel assembly dengan fitur keselamatan yang melekat. Kemudian, performa UN-FCM dan UO2-FCM akan disimulasikan pada desain fuel assembly yang diajukan untuk mengetahui perbandingan performa neutronik yang meliputi koefisien reaktivitas suhu bahan bakar, koefisien reaktivitas void, dan panjang siklus. Perangkat bahan bakar yang menggunakan bahan bakar UN-FCM dimodelkan menggunakan kode SERPENT and SCALE. Pada kode SERPENT, partikel FCM yang terdispersi pada batang bahan bakar dimodelkan menggunakan repeated regular cuboid lattice. Pada kode SCALE, partikel FCM yag terdispersi pada batang bahan bakar akan dihomogenisasi menggunakan doublehet-cell menghasilkan cell-weighted. Penelitian menunjukkan bahwa desain perangkat bahan bakar yang diajukan memiliki fitur keselamatan yang melekat terkait koefisien reaktivitas suhu bahan bakar dan koefisien reaktivitas void pada awal siklus. Hasil perhitungan koefisien reaktivitas suhu bahan bakar menggunakan SCALE & SERPENT pada suhu rendah: -4,403 ± 0,1304 pcm/K & -4,251 ± 0,03385 pcm/K, suhu operasi: -1,959 ± 0,02516 pcm/K & -2,323 ± 0,09892 pcm/K, dan suhu transien: -2,083 ± 0,3196 pcm/K & -1,592 ± 0,01780 pcm/K. Hasil perhitungan koefisien reaktivitas void menggunakan SCALE & SERPENT pada kondisi low-void: -0,01757 ± 0,0066 /% void & -0,02544 ± 0,006 /% void dan high-void: -0,12660 ± 0,0144 /% void & -0,13490 ± 0,0192 /% void. Meskipun demikian, saat operasi perangkat bahan bakar akan kehilangan fitur keselamatan melekat karena meningkatnya nilai moderator-to-fuel ratio ketika beroperasi. Hasil simulasi burnup menunjukkan bahwa untuk meningkatkan performa bahan bakar UN-FCM, penambahan pengayaan N15 diperlukan. Bahan bakar UN-FCM dengan tingkat pengayaan N15 terbukti memiliki burnup yang lebih tinggi: 25,99 GWd/MTU jika dibandingkan dengan bahan bakar UO2-FCM: 24,62 GWd/MTU.

After Fukushima Daiichi nuclear power plant accident, Accident Tolerant Fuel (ATF) was proposed. As one of the fuels categorized as ATF, fully ceramic micro-encapsulated fuel (FCMF) using uranium mononitride (UN) has better safety aspects than UO2 pellet fuel although it might not have better neutronic performance such as decreased fuel loading because the use of matrix and neutron economy disadvantage because of N14. To implement UN-FCM technology in Small-PWR, further research must be done to make sure the proposed design of fuel assembly has inherent safety feature. This paper focuses to evaluate the impact of varying fuel rod pitch, packing fraction, N15 enrichment, and U235 enrichment to fuel temperature reactivity coefficient, void reactivity coefficient, burnup and cycle length to produce fuel assembly design zone which has inherent safety feature. Furthermore, UN-FCM and UO2-FCM will be simulated in proposed fuel assembly design to compare the fuel performance including fuel temperature reactivity coefficient, void reactivity coefficient, and cycle length. UN-FCM based fuel assembly was modelled in SERPENT and SCALE. In SERPENT, FCM particle was modelled using repeated regular cuboid lattice in fuel rod. In SCALE, UN-FCM was modelled using doublehet-cell which the dispersed particle was homogenized into cell-weighted. This paper shows that the proposed fuel assembly design at beginning of life has an inherent safety feature according to fuel temperature reactivity coefficient and void reactivity coefficient. Fuel temperature reactivity coefficients were calculated by SCALE&SERPENT in low temperature: -4.403 ± 0.1304 pcm/K & -4.251 ± 0.03385 pcm/K, operating temperature: -1.959 ± 0.02516 pcm/K & -2.323 ± 0.09892 pcm/K, and transient temperature: -2.083 ± 0.3196 pcm/K & -1.592 ± 0.01780 pcm/K. Void reactivity coefficients were calculated by SCALE & SERPENT in 2 conditions: low void condition: -0.01757 ± 0.0066 /% void & -0.02544 ± 0.006 /% void and high void condition: -0.12660 ± 0.0144 /% void & -0.13490 ± 0.0192 /% void. However, during the operation the system will loose inherent safety feature due to the increase of moderator-to-fuel ratio. The burnup simulation shown that to improve UN-FCM based fuel, an increase of N15 enrichment is necessary. UN-FCM based fuel with 50% enrichment of 15N has a higher burnup: 25.99 GWd / MTU compared to UO2-FCM based fuel: 24.62 GWd / MTU.

Kata Kunci : Accident Tolerant Fuel, Fully Ceramic Micro-encapsulated, Uranium Mononitrida.