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

Graphite is used as the moderator, fuel barrier material, and core structure in High Temperature Reactors (HTRs). However, despite its good thermal and mechanical properties below the radiation and high temperatures, it cannot avoid corrosion as a consequence of an accident of water/air ingress. Degradation of graphite as a main HTR material and the formation of dangerous CO gas is a serious problem in HTR safety. One of the several steps that can be adopted to avoid or prevent the corrosion of graphite by the water/air ingress is the application of a thin layer of silicon carbide (SiC) on the surface of the fuel element. This study investigates the effect of applying SiC coating on the fuel surfaces of pebble-bed HTR in water ingress accident from the reactivity points of view. A series of reactivity calculations were done with the Monte Carlo transport code MCNPX and continuous energy nuclear data library ENDF/B-VII at temperature of 1 200 K. Three options of UO2, PuO2, and ThO2/UO2 fuel kernel were considered to obtain the inter comparison of the core reactivity of pebble-bed HTR in conditions of water/air ingress accident. The calculation results indicated that the UO2-fueled pebble-bed HTR reactivity was slightly reduced and relatively more decreased when the thickness of the SiC coating increased. The reactivity characteristic of ThO2/UO2-fueled pebble-bed HTR showed a similar trend to that of UO2, but did not show reactivity peak caused by water ingress. In contrast with UO2- and ThO2-fueled pebble-bed HTR, although the reactivity of PuO2-fueled pebble-bed HTR was the lowest, its characteristics showed a very high reactivity peak (0.33 Δk/k) and this introduction of positive reactivity is difficult to control. SiC coating on the surface of the plutonium fuel pebble has no significant impact. From the comparison between reactivity characteristics of uranium, thorium and plutonium cores with 0.10 cm thick SiC coating, it can be concluded that the effect of SiC coating on core reactivity in water ingress accident is more dominant for the pebble-bed HTR fuelled with thorium than those with uranium and plutonium fuels.

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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...