PHYSOR 2004 -The Physics of Fuel Cycles and Advanced Nuclear Systems: Global Developments Chicago, Illinois, April 25-29, 2004, on CD-ROM, American Nuclear Society, Lagrange Park, IL. (2004) Modelling of HTRs with Monte Carlo: Sensitivity due to Different Isotopic Fuel Composition Rita Plukiene1, Danas Ridikas2*, Arturas Plukis1 and Vidmantas Remeikis1 1Physics Institute, Savanoriu pr. 231, LT-2053 Vilnius, Lithuania 2DSM/DAPNIA/SPhN, CEA Saclay, F-91191 Gif-sur-Yvette, France The gas turbine modular helium-cooled reactor (GT-MHR) is a potential candidate for the maximum plutonium destruction in once-through cycle. A particular feature of GT-MHR is that its refractory coated fuel (TRISO particles) is supposed to provide an impermeable barrier to the release of fission products and, at the same time, to resist very deep burn-up rates (more than 90% for 239Pu). In this work we performed detailed Monte Carlo simulations of the GT-MHR operation by loading with different plutonium fuel vectors: plutonium from military applications, plutonium from LWR and RBMK spent nuclear fuel. The comparison of the main GT-MHR performance parameters: keff eigenvalues, the length of the fuel cycle, neutron characteristics and the evolution of fuel composition in particular were obtained. We show that the performance of GT-MHR may be considerably influenced by the plutonium isotopic composition vector used as initial fuel material. This is the first time when incineration possibility of the RBMK-1500 based plutonium isotopic composition was tested using high temperature reactor. Finally, the propagation of statistical errors for the fuel burn-up was examined in detail showing no uncertainty accumulation. KEYWORDS: High Temperature Reactor, Monte Carlo, Pu Incineration 1. Introduction In principle, GT-MHR can be used to burn all types of fuel and offers significant advantages in accomplishing the transmutation of plutonium isotopes and high destruction of 239Pu in particular [1]. The GT-MHR features are: the helium coolant, which remains in a single phase under all conditions; the graphite core, which provides high heat capacity, slow thermal response, and structural stability at high temperatures; the refractory coated particle fuel, which allows high burn-up, retains fission products during operation in high temperatures and retains their integrity in a repository environment for hundreds of thousands years [2]. It utilizes natural erbium as a burn-able poison with the capture cross section having a resonance at a neutron energy such that ensures a strong negative temperature coefficient of reactivity. The lack of interaction of neutrons with coolant (helium gas) makes sure that temperature feedback of fuel and graphite is the only significant contributor to the power coefficient. These features are combined with a closed Brayton energy conversion cycle giving a net efficiency in the range of 45-47 % [2]. In this work we performed detailed simulations of GT-MHR operation by loading it with different Pu fuel vector: plutonium isotopes separated from LWR and RBMK-1500 spent nuclear fuel and plutonium from military applications. The main goal of our study is to compare the main GT-MHR reactor performance parameters as keff - values, the length of the fuel cycle, neutron characteristics, temperature coefficients, and the evolution of fuel composition as a function of different initial fuel isotopic composition. The modelling of GT-MHR and plutonium based fuel cycles within a Monte * Corresponding author, Tel. +33 1 69087847, FAX +33 1 69087584, E-mail: [email protected] Carlo approach was made using double-heterogeneous reactor core geometry including micro-particles [3]. The propagation of Monte Carlo errors was addressed in detail in the last section of this paper. This is the first time when transmutation possibility of the plutonium, originating from the RBMK-1500 spent nuclear fuel (Ignalina nuclear power plant in Lithuania) was tested using GT-MHR. 2. Modeling details The investigation of transmutation feasibility of different isotopic plutonium composition in GT-MHR reactor is based on 3D double-heterogeneous reactor core geometry with micro-particles as described in our recent studies [3, 4]. In brief, the main feature of a double-heterogeneous geometry is an exact active core structure, where fuel and erbium compacts include ceramic - coated fuel and erbium particles mixed uniformly in a graphite matrix. Each coated fuel particle consists of spherical kernel of PuO2-x (200 µm diameter) surrounded by triple protective coatings from pyrocarbon and SiC (so called TRISO coating). The same structure is valid for particles containing burn-able poison - natural erbium in the form of Er2O3. In this work we used the MCNP code and associated data libraries to obtain keff, neutron fluxes and temperature coefficients. As soon as keff is smaller than 1 (within statistical errors) the length of the fuel cycle is determined. In the more realistic description of GT-MHR, control rods would need to be modeled. Here the control rods were not designed since their effect would be very similar in the case of a comparative analysis (relative comparison) if modeled-omitted exactly in the same way. For burn-up calculations we employed the Monteburns code (MCNP+ORIGEN) [5]. ENDF/B-VI data library (as the most often employed with MCNP) was used for the fuel and structure materials, JENDL-3.2 data files (having the biggest number of fission products) were employed for fission products. Neutron spectrum in GT-MHR changes considerably during the fuel burn-up [3]. In this work the neutron fluxes and cross sections were recalculated in every fifty days for the follow-up ORIGEN burn-up calculations. As it was mentioned above, our major interest in this study was to look at the GT-MHR performance parameters by investigating different plutonium fuel isotopic configuration, i.e. plutonium originating from: a) the spent nuclear fuel of LWR (light water reactor), b) the spent nuclear fuel of RBMK - 1500 (graphite - moderated water - cooled reactor), and c) plutonium found in military applications (see Table 1). Table 1. Different GT-MHR fuel and erbium load at the beginning of the fuel cycle.*Note that in RBMK Pu1 case GT-MHR performance parameters where investigated two times – with burnable poison erbium (in Figure 1), and without erbium (in Figure 2). Fuel Military LWR RBMK RBMK RBMK RBMK Pu Pu Pu1* Pu2 Pu3 Pu4 Pu composition, kg 7.0 9.5 238Pu -- 32.4 2.5 6.0 685.5 652.2 239Pu 659.0 661.0 802.1 673.3 429.6 439.0 240Pu 7.8 277.0 361.8 443.1 13.0 14.1 241Pu 4.2 142.0 8.4 12.4 64.9 85.2 242Pu -- 87.6 25.2 65.2 Total plutonium, kg : 701.0 1200.0 1200.0 1200.0 1200.0 1200.0 Er composition, kg : 166Er 132.0 16.8 18.0 -- -- -- 167Er 94.2 11.4 12.2 -- -- -- 170Er 61.4 7.5 8.0 -- -- -- Total erbium, kg : 293.6 35.7 38.2 -- -- -- Since RBMK-1500 SNF (Spent Nuclear Fuel) isotopic composition strongly depends on the initial 235U enrichment and its final burn-up, for the RBMK plutonium several cases were tested: “RBMK Pu1” – 2.0 % 235U initial enrichment, 14 MWd/kg burn-up, “RBMK Pu2” – 2.0 % 235U initial enrichment, 20 MWd/kg burn-up, “RBMK Pu3” – 2.4 % 235U initial enrichment, 22 MWd/kg burn-up, “RBMK Pu4” – 2.6 % 235U initial enrichment, 26 MWd/kg burn-up. RBMK-1500 reactor spent fuel 1.20 composition was calculated using SCALE 4.4a code package (with 1.15 ENDF/B-V nuclear data library) [6]. The corresponding GT-MHR 1.10 plutonium fuel masses for each composition are presented in Table 1. 1.05 eff 1200 kg for civil plutonium cases and k RBMK Pu1 (k =1.156) 700 kg for military plutonium were the 1.00 eff RBMK Pu2 (k =1.087) optimal fuel loads for GT-MHR. The eff RBMK Pu3 (k =1.103) burnable poison (erbium) in natural 0.95 eff RBMK Pu4 (k =1.088) isotopic composition was added to eff 0.90 reactor core to achieve the same keff 0 200 400 600 800 1000 eigenvalue at the beginning of the fuel Time, d cycle for LWR Pu, RBMK Pu1 and Military Pu. A high load of 240Pu in Figure 1. A behaviour of keff for different RBMK other three cases Pu2, Pu3 and Pu4 plutonium cases without burnable poison in GT-MHR. limited the possibility to add burnable poison (see next section). 3. Burn-up results We perform our calculations by simulating the once-through fuel cycle at the constant 600 MWth power. Our primary goal was to check the RBMK plutonium as an initial GT-MHR fuel load. For this purpose GT-MHR performance parameters were analyzed for four RBMK fuel cases without burnable poison. The keff ± 1σ time dependence 1.20 (consequently, its dependence on Military Pu (k =1.132) eff burn-up) for RBMK Pu1, Pu2, Pu3 and LWR Pu (k =1.138) 1.15 eff Pu4 compositions are presented in RBMK Pu1 (k =1.132) eff Figure 1. The most attractive RBMK 1.10 plutonium composition is that of 14 MWd/kg burn-up (up to now there is 1.05 eff about 15% of all SNF), but the use of k other burn-up RBMK SNF plutonium 1.00 in GT-MHR is also possible. In RBMK Pu3 case more than 90% of 239Pu 0.95 transmutation was obtained (see the 0.90 discussion below). 0 200 400 600 800 1000 1200 The GT-MHR with military Time, d plutonium composition was tested in our previous studies [3, 4]. In order to obtain Figure 2. A behaviour of keff for Military, LWR and relative fuel cycle length for different 14 MWd/kg burn-up RBMK plutonium with burnable GT-MHR fuel cases the appropriate poison in GT-MHR.