Optimization of Radioisotope Production at Rsg-Gas Reactor Using Deterministic Method

Teknologi Indonesia 35 (2) Edisi Khusus 2012: 37 45 Teknologi Indonesia LIPI Press 2012 Optimization of Radioisotope Production at Rsg-Gas Reactor Using Deterministic Method S. Pinem, J. Susilo, Tukiran, and T.M. Sembiring Pusat Teknologi Reaktor dan Keselamatan Nuklir BATAN E-mail: pinem@batan.go.id ABSTRACT RSG-GAS is a research reactor operated for radioisotope production, material testing, research and development and also industry and university needs. The radioisotope productions are Mo-99/Tc-99m,, Iridium,,, Mo and others which are needed for hospital and industry. To meet all demand, optimization of irradiation has been done for increasing the target irradiation in the core so that the utilization of RSG-GAS core is also increasing. This is important to be done because the target insertion to the core will change the core characteristic and can disturb the reactor operation safety. This paper will analyz the influence of each target insertion and also combination of the targets insertion to the core while considering the reactor operation safety factor. The calculation of the optimization was done by a deterministic method using Standard Reactor Analysis Code (SRAC) computer code. Meanwhile the determination of optimum target was done by analyzing the influence of changes in the reactivity and radial power peak factor due to the insertion of the target in the core. Based on the calculation the number of maximum target insertion for fission product molybdenum-low enrichment uranium (FPM-LEU) to get Mo-99/Tc-99m in all irradiation positions has been achieved. The E-4 and G-7 are the best core grid positions for the FPM-LEU target with the maximum mass of 21 g. Beside that, several data for calculation of, Iridium,,, Mo, targets with different numbers and also combination of all targets have been achieved as well. Research works is ready to be applied in the management of irradiation target for optimum radioisotope production in the RSG-GAS reactor. Keywords: Optimal, radioisotope, deterministic method, RSG-GAS reactor ABSTRAK Reaktor RSG-GAS adalah reaktor riset yang dioperasikan untuk keperluan produksi radioisotop, penelitian, uji material, permintaan industri, lembaga litbang, serta universitas. Produksi radioisotop adalah Mo-99/Tc-99m,, Iridium,,, Mo, dan beberapa jenis produk yang lain yang digunakan untuk kesehatan dan kebutuhan industri. Untuk memenuhi seluruh keperluan iradiasi telah dilakukan optimalisasi untuk peningkatan iradiasi target dalam teras reaktor untuk meningkatkan pendayagunaan reaktor RSG-GAS. Hal ini penting dilakukan karena pemasukan target ke dalam teras akan mengubah karakteristik teras sehingga dapat mengganggu keselamatan operasi reaktor. Dalam makalah ini telah dilakukan analisis pengaruh masing-masing target yang diiradiasi dalam teras dan kombinasi antar target dengan mempertimbangkan faktor keselamatan operasi reaktor. Perhitungan optimasi ini dilakukan dengan metode deterministik menggunakan program Standard Reactor Analysis Code (SRAC). Penentuan target yang optimum dilakukan dengan menganalisis pengaruh perubahan reaktivitas dan faktor puncak daya radial akibat masuknya target ke dalam teras reaktor. Berdasarkan hasil perhitungan maka telah diperoleh jumlah target maksimum fi ssion product molybdenum-low enrichment uranium (FPM LEU) untuk menghasilkan Mo-99/Tc-99m pada seluruh fasilitas iradiasi. Fasilitas iradiasi E-4 dan G-7 adalah posisi teras yang terbaik untuk target FPM-LEU dengan massa maksimum 21 g. Selain itu telah diperoleh data beberapa analisis perhitungan target, Iridium,,, Mo, dengan jumlah yang berbeda dan kombinasi antara semua target. Hasil penelitian ini siap diaplikasikan dalam manajemen target iradiasi untuk menghasilkan produksi radioisotop yang optimum di reaktor RSG-GAS. Kata kunci: Optimasi, radioisotop, metode deterministik, reaktor RSG-GAS Off print request to: S. Pinem, J. Susilo, Tukiran, and T.M. Sembiring 37

Jurnal Teknologi Indonesia 35 (2) Edisi Khusus 2012 INTRODUCTION RSG-GAS reactor uses silicide fuel (U 3 Si 2 -Al) with 19.75% enrichment of U-235 and 2.96 gu/ cc uranium density or equivalent to 250 g of 235 U in a standard fuel element. RSG-GAS reactor is operated for research of science and nuclear technology and also radioisotope production. To fulfill the demand of isotope production, the RSG-GAS reactor is utilized for producing many radioisotopes (RI) for medical or industry, such as Mo-99/Tc-99 using the low enrichment uranium target, I-131 using target, Sm-153 using target and Gd-153 using target. There are 8 core grid positions, ie 4 positions in Central Irradiation Position (CIP) and 4 positions in Irradiation Position (IP) are available for the RI targets. Meanwhile the demand for radioisotope (RI) progressively increases from domestic and abroad, especially that of Mo-99, since some research reactors have limitation in RI production and others have ageing problem. Currently, 2 core grid positions are dedicated for the gemstone colorization and the remaining positions are able to be used for RI targets. To fulfill the safety requirement, the neutronic analysis is necessary to be carried out to achieve an optimum RI target irradiation for RSG-GAS reactor. Although the mass of RI target is relatively small, but it can change some core parameters, especially that of the low enrichment of uranium target for Mo-99 RI (FPM-LEU), such as reactivity, neutron flux distribution and power peaking factor distribution. The changes depend on the neutronic characteristics of the target, absorption and fission macroscopic cross section, which give an influence on neutronic interaction between targets and core. If the mass increases, the changes of neutronic parameters become significant. Therefore the optimization of the mass of target or the number of the target is limited to the safety criteria of each parameter. In this paper, the combinations of the RI targets are analyzed, too [1,2]. The objective of this research is to obtain the maximum capacity of RSG-GAS reactor in production RI targets based on number, mass and material of targets as well as the core grid positions. Based on this research, the core configuration can be easily set up by the management of reactor operation to fulfill the safety criteria and quality of the RI production. Therefore, the optimum RI target irradiation can be well managed, so the effective and efficient of RSG-GAS reactor operation will be done if this research is applied. The neutronic analysis is carried out for the silicide equilibrium core. Two main core parameters, reactivity and maximum radial power peaking factors, have been calculated using a deterministic method of SRAC 2006 code system, with the neutron transport method (PIJ) and the neutron diffusion method (CITATION) codes [3,4]. The Monte Carlo method code (MCNP) [5], was used as a reference to obtain an accurate model of RI targets. METHODOLOGY Reactor Core Model A standard fuel element of RSG-GAS reactor consists of 21 plates of U 3 Si 2 -Al fuel with U-235 enrichment of 19.75 wt% and density of uranium of 2.96 gu/cc. Nominally mass uranium every fuel element is 250 g. Between each plate of fuel slot at fuel rack at both of its side, there is light water (H 2 O) as a moderator and also as a coolant. A control fuel element is basically same with a standard fuel element, but both sides of 3 outside plates of fuel were removed for the absorber blade, so the element has 15 fuel plates. Meanwhile absorber materials made for control element are Ag-In-Cd with composition of 80%, 15%, 5%, respectively. When control rods are fully withdrawn, the absorber materials position is replaced with light water (H 2 O). The length of absorber materials and active height of fuel element 60 cm. As seen in Fig. 1, the equilibrium of RSG- GAS core consists of 40 fuel elements, 8 control elements, 4 Irradiation Positions (IP), 4 Central Irradiation Positions (CIP), beryllium reflector elements and 4 positions for rabbit systems. As mentioned previously, the core grid positions of B-6, D-9, E-4 and G-7 (IP) and D-6, D-7, E-6 and E-7 (CIP) are the position for the optimizing the RI targets. In the real operation, a stringer with 3 holes is used for one core grid position. 38

S. Pinem, dkk. Optimization of Radioisotope Production... The first step of this research is to generate the macroscopic cross section of the RI target using PIJ code. The cross sections were generated for 8 neutron energy group. The RI targets were modeled based on the following conditions: 1) single capsule of target at single and multiple core grid positions with various mass for all positions, 2) multiple capsules of target at single and multiple core grid positions with various mass for all positions, and 3) multiple capsules and materials of targets at single and multiple core grid positions with various mass for all positions. The second step is core calculation using CITATION code. In this research, the RSG-GAS core is modeled in a 3-D geometry model of X-Y-Z, therefore the RI target irradiation is homogenized into rectangular shape as seen in the next sub-section. The output calculation of reactivity change (Δρ, %Δk/k) and the maximum radial power peaking factor (PPF r ) is the focus in the optimizing the RI target. The parameter of the core without target is reference value in determination effect of the RI target. In optimizing the RI targets at RSG-GAS core, the following constraints should not be violated: 1) the maximum reactivity change for single core grid position is 0.5 %Δk/k, 2) the maximum reactivity change for multiple core grid positions is 2.0 %Δk/k. These value is derived from the minimum shutdown margin at one stuck rod condition, 3) PPF r maximum is 1.4 Model of Radioisotope Target Since the Mo-99 isotope is the most requested RI in world market, the main RI target in this research is FPM-LEU (fission product Mo-99 Figure 1. Typical working silicide core of RSG-GAS reactor [6] 39

Jurnal Teknologi Indonesia 35 (2) Edisi Khusus 2012 using low enrichment of uranium, LEU) target, then followed by W,,, and Ir targets. Size and dimension of the targets are presented in Figure 2, Table 1 and Table 2. For the FPM-LEU target, the height of the active zone is 60 cm, so the thickness of target is corrected by mass and volume of the target. For other RI targets, the height of active zone is shorter than 60 cm, so the axially active area is divided into some layers that accommodate the height of every target. The maximum number of target capsules in one pile per hole is 3. Table 1. Size and Dimension of the FPM LEU Target Name Material Inner diameter (mm) Outer diameter (mm) LEU UO 2 (19.75%) 26.52 26.60 Tube Al 26.60 28.37 Stringer Al 33.00 38.00 RESULTS AND DISCUSSIONS Validation of the Model of RI Target Fig. 3 showed that the model of RI targets by deterministic method (PIJ and CITATION codes) is accurate since the excess reactivity change, as function of mass of 235 U, calculated by CITATION code is very closed to the MCNP results by 8%. The calculations were carried out by various mass of 235U from 2,5 g to 15 g per capsule. Although not discussed in this work, the MCNP result is used as a reference because the core and the RI targets are modeled in detail. Therefore, the model can be used for the core calculations to determine the optimum RI target. Table 2. Size and Dimension of the W / / / /Ir Targets Name Target Target Target Material Inner diameter (mm) 17.7 18.1 W 20.27 20.32 S Ir 15.5 10.6 Outer diameter (mm) 21.4 idem Idem Idem Idem Idem Capsule Al 21.4 24.4 Tube Al 25.4 30.0 Stringer Al 33.0 38.0 Figure 3. The calculated reactivity change using SRAC and MCNP codes Since the FPM-LEU target gives positive reactivity when inserting in the core, the first step to do is to determine the optimum mass of 235 U at RSG-GAS core. To find the most sensitive core grid position in reactivity change for the irradiation of FPM-LEU target, we selected 4 positions of IP (B-6, D-9, E-4 and G-7) and 1 position of CIP (D-7). As we assumed that the 4 positions FPM/LEU 8.10 cm G d 2 T eo 2 F e 2 7.71 cm 60 cm Figure 2. Model of the target in radial and axial directions 40

S. Pinem, dkk. Optimization of Radioisotope Production... of CIP have same effect in reactivity change, the D-7 core grid position is selected. In this single position, the mass of 235 U was varied from 2.5 g to 27.5 g per capsule per core grid position. As seen in Fig. 4, the E-4 and G-7 core grid positions have the less sensitivity in reactivity changes. Meanwhile the D-9 core grid position has the most sensitive. By using the limit of 0,5%Δk/k, the E-7 and G-7 core grid positions can be used for irradiating the FPM-LEU target up to 20 g of 235 U. For other core grid positions, the optimum mass of 235 U at D-7, B-6 and D-9 are 14 g, 8g and 7.5 g, respectively. These calculations indicated that E-4 and G-7 core grid positions could give the most optimum of 235 U mass of FPM-LEU target by 20 g per capsule. For single core grid position, it is clear that the irradiation of 7.5 g FPM-LEU target can be carried in all IP and CIP positions. Figure 4. Reactivity change vs. mass of uranium. Reactivity Change Table 3 shows the reactivity change for single capsule of FPM-LEU target at single and multiple core grid positions with a mass of 3 g 235 U per capsule. It shows that if the CIP and IP positions are inserted with FPM-LEU target with total of 24 g the will give total reactivity change of 1.84 %Δk/k. Higher number of used core grid positions makes higher mass of 235 U, then the reactivity change increases, but it is not linear. It is because the neutronic interaction strongly depend on the core grid position. Since the total reactivity change of 1.84 %Δk/k is lower than 2.0 %Δk/k, the 8 core grid positions of CIP and IP can be inserted of the FPM-LEU target with a total mass of 24 g 235 U. For the multiple capsules of FPM-LEU targets, a core grid position has 3 capsules (Fig. 2) with a total mass of 9 g 235 U because the mass of 235 U per capsule is 3 g. As seen in Table 4, the reactivity change of multiple capsules is higher than that of single capsule for the same number of core grid position, so while 5 core grid positions are inserted, the reactivity change is larger than the safety limit of 2 %Δk/k. It is clear from Table 4 that 12 FPM-LEU targets with total mass of 36 g of 235U at 4 CIP position can be irradiated safely because the reactivity change is less than 2 %Δk/k. Table 3. Reactivity Change for Single Capsule of the FPM-LEU. Irradia on Posi on Total mass (g) No target - 8.32 - D-6 3 8.52 0.20 D-6 +E-7 6 8.74 0.42 D-6+E-7 + E-6 9 8.97 0.65 D-6 +E-7+E-6 +D7 12 9.23 0.91 D-6 +E-7+E-6 +D7 + B-6 15 9.49 1.17 D-6 +E-7+E-6 +D7 +B-6 + D-9 18 9.71 1.39 D-6 +E-7+E-6 +D7 + B-6 + D-9 + E-4 21 9.96 1.64 D-6 +E-7+E-6 +D7 + B-6 + D-9 + E-4 +G-7 24 10.16 1.84 41

Jurnal Teknologi Indonesia 35 (2) Edisi Khusus 2012 Table 4. Reactivity Change for Multiple Capsules of the FPM LEU Target (3 capsules) Irradia on posi on Total mass (g) No Target - 8.32 - D-6 9 8.71 0.39 D-6 +E-7 18 9.16 0.84 D-6+E-7 + E-6 27 9.64 1.32 D-6 +E-7+E-6 +D7 36 10.17 1.85 D-6 +E-7+E-6 +D7 + B-6 45 10.61 2.29 D-6 +E-7+E-6 +D7 +D-9 54 10.99 2.67 D-6 +E-7+E-6 +D7 + E-4 63 11.38 3.06 D-6 +E-7+E-6 +D7 + E-4 + G-7 72 11.72 3.40 Note: the 235 U mass per capsule is 3 g The next calculation is conducted with the com bination of multiple materials and masses of target at single core grid position of D-6. The material targets are: 1) The mass of target is varied from 0.04 g to 0.20 g. But the masses of FPM-LEU, and Mo are set to 3g, 125 g and 0.50 g, respectively. The calculation results are shown in Table 5. 2) The mass of target is varied from 125 g to 675 g. But the masses of FPM-LEU, and Mo are set to 3g, 0.04 g and 0.50 g, respectively. The calculation results are shown in Table 6. 3) The mass of target varied from 0.05 g to 0.25 g. But the masses of FPM-LEU, and are set to 3g, 125 g and 0.04 g, respectively. The calculation results are shown in Table 7. Tables 5-7 show that contribution of the FPM-LEU target in the reactivity change is higher compared to other targets. Even mass of one material target increases, so all reactivity changes are positive. It means, the neutron fission reaction rates is higher than neutron absorption reaction rates in the neutronic interaction among the targets and the core. Compared to the single capsule of FPM-LEU target at D-6 core grid position (Table 3), the addition of non-fission material, such as and, gives higher reactivity change by 0.05 % Δk/k. As seen in Tables 5-7, the increase of mass of the and targets that have high neutron absorption cross section did not decrease the reactivity change. This fact indicates the CIP of the RSG- GAS core has over moderation, since the addition of capsules of target making loss of H 2 O so that the effective multiplication factor decreases. The Table 5. Reactivity Changes as Function of the Mass of Target at D-6 Core Grid Position Mass of target Excess Reac vity FPM-LEU 3 g 125 g Mo 0.50 g 0.04 g 8.57 0.25 0.08 g 8.57 0.25 0.12 g 8.57 0.25 0.16 g 8.57 0.25 0.20 g 8.57 0.25 42

S. Pinem, dkk. Optimization of Radioisotope Production... Table 6. Reactivity Changes as Function of the Mass of Target at D-6 Core Grid Position FPM-LEU 3 g Mass of target 0.04 g Mo 0.50 g Excess Reac vity 125 g 8.57 0.25 300 g 8.56 0.24 425 g 8.54 0.22 550 g 8.53 0.21 675 g 8.52 0.20 Table 7. Reactivity Changes as Function of the Mass of Target at D-6 Core Grid Position FPM-LEU 3 g Mass of target 125 g 0.04 g Excess Reac vity 0.05 g 8.57 0.25 0.10 g 8.57 0.25 0.15 g 8.57 0.25 0.20 g 8.57 0.25 0.25 g 8.57 0.25 contribution of the over-moderation of the CIP is higher than the neutron absorption reaction rate, so that there is no significant effect to reactivity change. However, for the target, the higher mass gives a decrease in the reactivity change, because the volume of the target is higher compared to FPM-LEU, and. Therefore the neutron absorption reaction rate in the area of target has an important role in decreasing the excess reactivity of the core. The calculation results for the combination of multiple materials and masses of target at multiple core grid positions are shown in Table 8-10. Table 8, 9 and 10 show the result for the mass variation of Ir-191, Mo and Fe 2, respectively. In these combinations, the FPM- LEU targets are inserted in D-6 and E-7 core grid positions with fixed mass of 6 g 235 U (3 g per capsule). Since the Ir-191 has high neutron absorption cross section, the higher mass of the target will give higher negative reactivity as seen in Table 8. For the Mo, the higher mass of the target did not change the reactivity change, means the effect of the over-moderation at E-7 core grid position dominates neutronic interaction so that the total of reactivity changes are constant. Same as the Ir-191, the higher mass of Fe 2 target gives higher negative reactivity, so that the reactivity change goes to more negative. Since the Fe has high absorption, it has a great influence to reduce the excess reactivity. Table 8. Reactivity Changes as Function of the Mass of Ir-191 Target at D-6 Core Grid Position Irradia on posi on and mass of target D-6 E-7 D-6 FPM-LEU 3 g FPM LEU 3 g Ir-191 8.4 g 8.783-0.04 g Ir-191 16.8 g 8.781-0.002 125 g 0.05 g Ir-191 25.2 g 8.779-0.004 Ir-191 33.6 g 8.778-0.005 Ir-191 42.0 g 8.776-0.007 43

Jurnal Teknologi Indonesia 35 (2) Edisi Khusus 2012 Table 9. Reactivity Changes as Function of the Mass of Mo Target at E-7 Core Grid Position Irradia on posi on and mass of target D-6 E-7 E-7 FPM-LEU 3 g Mo - 0.50 g 8.76 0.44-125 g MoO - 0.04 g 3-1.00 g 8.76 0.44 Mo - 1.50 g 8.76 0.44 Mo - 2.00 g 8.76 0.44 Mo - 2.50 g 8.76 0.44 FPM-LEU -3 g Ir-191-8.4 g Table 10. Reactivity Changes as Function of the Mass of Fe 2 Target at E-7 Core Grid Position Irradia on posi on and mass of target D-6 E-7 E-7 FPM-LEU - 3 g Fe 2 200 g 8.78 - - 125 g Fe - 0.04 g 2 400 g 8.76-0.02 Fe 2 600 g 8.75-0.03 Fe 2 800 g 8.74-0.04 Fe 2 1000 g 8.72-0.06 FPM-LEU - 3 g Ir-191-8.4 g Maximum Power Peaking Factor As discussed above, the insertion of FPM-LEU target can change the maximum radial power peaking factor, PPF r, at the RSG-GAS core since some power is generated in the active zone of the target. Table 11 shows the maximum PPF r at two standard fuel elements positions, E-5 and E-8, and at one control fuel element, C-8, for the insertion of single target at single position of the D-6 core grid positions. It is clear the PPF r at the core does not change even there is an increase mass of uranium. Table 11. Maximum PPF r while single FPM-LEU target is inserted at IP position Mass of UO 2 (g) Core grid posi on C-8 E-5 E-8 without target 1.16 1.10 1.10 5 1.16 1.11 1.10 10 1.16 1.11 1.10 15 1.16 1.11 1.10 If the single FPM-LEU target is inserted at CIP positions (Table 12), the maximum PPF r is increased with the difference of 0.01, 0.02, 0.01 and 0.04 for the mass of 3 g, 6 g, 9 g and 12 g 235 U, respectively. It is noted that the mass of 3 g, 6 g, 9 g and 12 g 235 U are corresponded to 1, 2, 3, and 4 core grid positions at CIP. Based on the maximum PPF r, the FPM-LEU target can be irradiated up to 12 g because it is less than the limit value of 1.4. Compared to the IP positions, the CIP position shows more sensitivity in the change of the power distribution. It means it is safer to irradiate the FPM-LEU target at IP position. Tabel 12. Maximum PPF r While Single FPM-LEU Target is Inserted at CIP Position. Mass of UO 2 (g) Core grid posi on C-8 E-5 E-8 3 1.17 1.10 1.11 6 1.18 1.11 1.11 9 1.17 1.13 1.13 12 1.20 1.15 1.16 As disccused previously, for combination of single capsule (Table 3) and multiple capsules (Table 4) of FPM-LEU targets at multiple core grid position, the maximum mass of targets are 24 g and 36 g, respectively. Table 13 shows that if those combinations are carried out, it will give an increase to maximum PPF r by 0.03 and 0.04, it is even less than the limit value of 1.4. 44

S. Pinem, dkk. Optimization of Radioisotope Production... Table 13. Maximum PPF r while multiple FPM-LEU targets is inserted at CIP/IP positions. Mass of UO 2 (g) Core grid posi on C-8 E-5 E-8 without target 1.16 1.10 1.11 24 1.19 1.12 1.14 36 1.2 1.15 1.16 CONCLUSION The optimization of radioisotope (RI) target at RSG-GAS care has been carried out by using deterministic code of CITATION. This research showed that the optimum target strongly depend on mass and irradiation position of the target. Based on the reactivity change and maximum radial power peaking factor, the FPM-LEU target is the most sensitive compare to other RI targets. By combination of irradiation position and number of capsule, the optimum mass of FPM-LEU target can be irradiated safely up to 36 g. By this research the optimum mass of,,, Mo, Ir-191, Fe 2 targets is also determined. This result of research is ready to be applied in the core and RI target management to get an higher utilization and a safer operation of the RSG-GAS reactor. ACKNOWLEDGMENT The authors express their gratitude to the Research and Technology Ministry of Republic of Indonesia for funding this research under the Incentive Program of PKPP Degree of Research and Technology Ministry Number: 053/M/Kp/ II/2010 (09 February 2010). REFERENCES [1] M. Q. Huda. M.S. Islam. (2009). Studies on the overall safety aspects during irradiation of in the central thimble of the TRIGA research reactor. Annals of Nuclear Energy. 36: pp.199 212. [2] A. Mushtaq et. al. (2008). Neutronic and thermal hydraulic analysis for production of fission molybdenum-99 at Pakistan Research Reactor. Annals of Nuclear Energy. Elsevier. 35 (2); pp. 345 352. [3] Okumura. K. et al.. (2002). SRAC: The Comperhensive neutronics calculation system. Tokaimura: Japan Atomic Energy Research Institute. [4] Surian Pinem. Jati Susilo. (2006). Validation of the SRAC code on the First Core of RSG-GAS Reactor. FNCA 2006 Workshop on the Utilization of Research Reactor; pp. 226 242. [5] P. H. Liem. (1998). Monte Carlo Calculations on the First Criticality the multipurpose reactor G.A. Siwabessy. Atom Indonesia. 24 ( 2); pp. 51 73. [6] BATAN. (2002). Safety Analysis Report of RSG-GAS. Rev.8. Serpong: BATAN. Received: 25 May 2012 Revised: 9 July 2012 Accepted: 12 July 2012 45