The pebble bed type High Temperature Gas-cooled Reactor (HTGR) is among the interesting nuclear reactor designs in terms of safety and flexibility for cogeneration applications. In addition, the strong inherent safety characteristics of the pebble bed reactor (PBR) which is based on natural mechanisms improve the simplicity of the PBR design, in particular for the Once-Through-Then-Out (OTTO) cycle PBR design. One of the important challenges of the OTTO cycle PBR design, and nuclear reactor design in general, is improving the nuclear fuel utilization which is shown by attaining a higher burnup value. This study performed a preliminary neutronic design study of a 200 MWt OTTO cycle PBR with high burnup while fulfilling the safety criteria of the PBR design.The safety criteria of the design was represented by the per-fuel-pebble maximum power generation of 4.5 kW/pebble. The maximum burnup value was also limited by the tested maximum burnup value which maintained the integrity of the pebble fuel. Parametric surveys were performed to obtain the optimized parameters used in this study, which are the fuel enrichment, per-pebble heavy metal (HM) loading, and the average axial speed of the fuel. An optimum design with burnup value of 131.1 MWd/Kg-HM was achieved in this study which is much higher compare to the burnup of the reference design HTR-MODUL and a previously proposed OTTO-cycle PBR design. This optimum design uses 17% U-235 enrichment with 4 g HM-loading per fuel pebble.
Dalam high temperature reactor, koefisien reaktivitas temperatur yang didesain negatif menjamin reaksi fisi dalam teras tetap berada di bawah kendali dan panas peluruhan tidak akan pernah melelehkan bahan bakar yang menyebabkan terlepasnya zat radioaktif ke lingkungan. Namun masuknya air (water ingress) ke dalam teras reaktor akibat pecahnya tabung penukar panas generator uap, yang dikenal sebagai salah satu kecelakaan dasar desain, dapat mengintroduksi reaktivitas positif dengan potensi bahaya lainnya seperti korosi grafit dan kerusakan material struktur reflektor. Makalah ini akan menganalisis efek kecelakaan water ingress terhadap reaktivitas Doppler teras RGTT200K. Kapabilitas koefisien reaktivitas Doppler untuk mengkompensasi reaktivitas positif yang timbul selama kecelakaan water ingress akan diuji melalui serangkaian perhitungan dengan program MCNPX dan pustaka ENDF/B-VII untuk perubahan temperatur bahan bakar dari 800K hingga 1800K. Tiga opsi kernel bahan bakar UO2, ThO2/UO2 dan PuO2 dengan tiga model kisi bahan bakar pebble di teras reaktor diterapkan untuk kondisi water ingress dengan densitas air dari 0 hingga 1.000 kg/m3. Hasil perhitungan memperlihatkan koefisien reaktivitas Doppler tetap negatif untuk seluruh opsi bahan bakar yang dipertimbangkan bahkan untuk posibilitas water ingress yang besar. Efek water ingress lebih kuat pada model kisi dengan fraksi packing lebih rendah karena lebih banyak volume yang tersedia untuk air yang memasuki teras reaktor. Efek water ingress juga lebih kuat di teras uranium dibandingkan teras thorium dan plutonium sebagai konsekuensi dari fenomena Doppler dimana absorpsi neutron di daerah resonansi 238U lebih besar daripada 232Th dan 240Pu. Secara keseluruhan dapat disimpulkan bahwa, koefisien Doppler teras RGTT200K mampu mengkompensasi insersi reaktivitas yang diintroduksi oleh kecelakaan water ingress. Teras RGTT200K dengan bahan bakar UO2, ThO2/UO2 dan PuO2 dapat mempertahankan fitur keselamatan melekat dengan cara pasif. Kata kunci: Water ingress, reaktivitas Doppler, RGTT200K In high temperature reactor, the negative temperature reactivity coefficient guarantees fission reaction in the core remain under the control and decay heat will not melt the fuel which cause the release of radioactive substances into the environment. But the entry of water (water ingress) into the reactor core due to rupture of a steam generator tube heat exchanger, which is known as one of the design basis accidents, can introduce positive reactivity with other potential hazards such as graphite corrosion and damage of the reflector structure material. This paper will investigate the effect of water ingress accident on Doppler reactivity coefficient of RGTT200K core. The capability of the Doppler reactivity coefficient to compensate positive reactivity incurred during water ingress accident will be examined through a series of calculations with MCNPX code and ENDF/B-VII library for fuel temperature changes from 800K to 1800K. Three options of UO2, ThO2/UO2 and PuO2 fuel kernels with three lattice models of fuel pebble in the reactor core was applied for condition of water ingress with water density from 0 to 1000 kg/m3. The results of the calculations show that Doppler reactivity coefficient is negative for the entire fuel options being considered even for a large possibility of water ingress. The effects of water ingress becomes stronger in lattice model with lower packing fraction because more volume available for water entering the reactor core. The effect of water ingress is also stronger in the uranium core compared to thorium and plutonium cores as a consequence of the Doppler phenomenon where the neutron absorption in resonance region of 238U is greater than 232Th and 240Pu. It can be concluded overall that Doppler coefficient of RGTT200K core has capability to compensate the reactivity insertion introduced by water ingress accident. RGTT200K core with UO2, ThO2/UO2 and PuO2 fuels can maintain the inherently safety features in a passive way. Keywords: Water ingress, Doppler reactivity, RGTT200K
ABSTRAK ANALISIS LAJU DOSIS NEUTRON TERAS RGTT200K DENGAN MCNP5. Disain koseptual teras reaktor RGTT200K (Reaktor berpendingin Gas Temperatur Tinggi) berdaya 200MWth yang mampu untuk kogenerasi. Teras reaktor berbentuk silinder non anular yang mengadopsi teknologi HTGR (High Temperature Gas-cooled Reactor) berbahan bakar kernel partikel berlapis TRISO dalam bentuk pebble dan berpendingin gas helium. Bahan bakar RGTT200K berbentuk pebble (bola) yang berisikan kernel partikel berlapis TRISO yang berupa uranium oksida (UO2) diperkaya 10 %. Lapisan TRISO terdiri 4 lapisan yaitu lapisan-lapisan: karbon berpori, karbon pirolitik dalam (IPyC, Inner Pyrolitic Carbon), Silikon Karbida (SiC) dan karbon pirolitik luar (OPyC, Outer Pyrolitic Carbon). Perhitungan laju dosis neutron pada teras RGTT200K dilakukan menggunakan program Monte Carlo MCNP5v1.2 yang memanfaatkan file data nuklir ENDF/B-VII, JENDL-4 dan JEFF-3.1 pada temperatur operasi normal Tkernel=1200K dan kondisi kecelakaan Tkernel=1800K. Dengan memanfaatkan program EGS99304, jumlah struktur kelompok energi yaitu 640 (SAND-II group structure) digunakan dalam perhitungan spektrum dan fluks neutron reaktor RGTT200K. Teras reaktor RGTT200K dibagi dalam 25 zona (5 zona arah radial dan 5 arah aksial). Perisai biologis reaktor RGTT200K menggunakan spesifikasi material beton dari LANL-USA. Perhitungan laju dosis neutron yang dipancarkan oleh sumber neutron dengan kartu tally F4 yang tersedia dalam program Monte Carlo yang dinormalisasi terhadap kuat sumber neutron reaktor RGTT200K. Distribusi laju dosis neutron ditentukan menggunakan faktor konversi flux-to-dose dari International Commission on Radiological Protection (ICRP). Hasil perhitungan laju dosis neutron dengan faktor konversi ICRP-74 untuk pekerja radiasi pada arah radial di bagian ujung luar perisai biologis pada temperatur operasi masing-masing adalah: 7,99; 14,30 dan 5,66 µSv/jam, untuk ENDF/B-VII, JENDL-4 dan JEFF-3.1, sedangkan untuk kondisi kecelakaan laju dosis neutron masing-masing diperoleh: 8,77; 5,71 dan 10,70 µSv/jam. Dari hasil analisis tersebut tampak bahwa perisai biologis telah memenuhi standar keselamatan radiasi yang disyaratkan oleh Perka BAPETEN No. 4 tahun 2013. Khususnya untuk perhitungan laju dosis neutron dengan file ENDF/B-VII kedua kondisi operasi reaktor RGTT200K di bawah nilai standar persyaratan yaitu 10 µSv/jam (20 mSv/thn). Pemenuhan persyaratan keselamatan radiasi dengan ketebalan perisai biologis 100 cm menggunakan material beton untuk RGTT200K telah dicapai dengan baik menggunakan file ENDF/B-VII. is the non-annular cylindrical reactor core with TRISO kernel coated fuel particles in the form of balls called pebble and cooled by helium gas. The RGTT200K reactor core design adopts high temperature gas cooled reactor (HTGR) technology with inherent passive safety.. The RGTT200K spherical fuel called pebble fuel containing thousand of TRISO-coated fuel particles of uranium oxide (UO2) 10% enriched. TRISO coating comprises four layers, namely: porous carbon buffer layer, inner pyrol...
Thorium abundance in the Earth's crust is estimated to be three to four times higher than uranium. This is one potential advantage of Thorium as a provider of attractive fuel to produce nuclear energy. Fewer transuranics produced by Thorium during the fuel burn up in the reactor may also be another advantage for reducing the long-term burden of high-level long-lived waste. The scope of this paper is to study the implication of Thorium fraction on neutronic parameters of pebble bed reactor. The reactor model of HTR-10 was selected, and the (Th, 235U)O2 fuel was used in this study. The MCNP6 code was applied to solve a series of neutron transport calculations with various Thorium fractions in (Th,235U)O2 fuel based on the ENDF/BVII library. The calculation results show that the total temperature coefficient of reactivity of Thorium-added pebble bed reactors is generally more negative than those of LEU-fuelled one, except for 10% Thorium fraction. The kinetic parameters, especially prompt neutron lifetime and neutron generation time of pebble bed reactors, are higher, which means the addition of Thorium in the fuel makes the reactor more easily controlled. However, the burn-up calculations show that the introduction of Thorium in the same fuel kernel as LEU within the pebble bed reactor is unable to lengthen the fuel residence time, except for a minimum of 40% Thorium fraction.
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