Pebble Bed High Temperature Reactor for Electric and Cogeneration Application

Specific energy demand of Indonesia with its distributed energy demand required a special energy supply solution. The energy supply solution should be able to fulfil the electric demand of huge and growing population which spread in many different islands also to fulfil the energy demand to process abundances yet distributed natural sources. This paper proposes a pebble bed high temperature reactor as an energy supply in the form electric and/or cogeneration application. A parametric survey of the core geometry of a 150MWt Pebble Bed Reactor called PeLUIt is performed, follow by an equilibrium analysis of the optimized design. An accident analysis of depressurized loss of forced cooling (DLOFC) is performed to show the strong passive safety features of the design. Finally the heat-mass balance of the plant is also presented.


Introduction (copas dr Equilibrium Core Design of RDE)
Specific energy demand of Indonesia with its distributed energy demand required a special energy supply solution. This solution should be able to supply the electric demand of huge and growing population which spread in many different islands and energy demand to process natural resources. A small modular reactor (SMR) concept with the pebble bed high temperature gas-cooled reactor (HTGR) design is considered to be one of the best solutions for this specific energy demand. As an initial step, a 10MWt experimental reactor of this type called Reactor Daya Eksperimental (RDE) was proposed and designed [1]. Site license of RDE in the PUPIPTEK Area at Serpong was already achieved, and currently it is in progress to achive the design permit from BAPETEN as the national regulatory body. The RDE design development is mainly following the HTR-10 design [2] and also to help a fast progress, without  [3] was also adopted.,As the preparation of the follow-up step of RDE, a 150MWt pebble bed reactor, called 'Pembangkit Listrik dan Uap-panas Industri' or simply PeLUIt is being designed. This power level is considered to be the appropriate size for the Indonesian electricity and energy demand and appropriate to supply the small unit electricity demand part of Indonesia, as can be seen in Figure 1.
The purpose of this paper is to perform an initial conceptual design of a 150MWt pebble bed reactor. A parametric survey of the core geometry of a 150MWt Pebble Bed Reactor called PeLUIt is performed, follow by an equilibrium analysis of the optimized design. An accident analysis of depressurized loss of forced cooling (DLOFC) is performed to show the strong passive safety features of the design. Heat-mass balance of the plant is also performed.   Figure 2. Pebble Bed type HTGR composition from active core to TRISO coated fuel particle [1]

Reactor Model
The configuration of Pebble Bed type HTGR from the general in-vessel component including the active core down to its TRISO coated fuel particle can be seen in Figure 2. A ceramic based material including its fuel guarantee the capability to withstand a high temperature condition. In addition, the high heat conductivity and high heat capacity of the graphite ceramic make a good thermal hydraulic performance of the core. The diameter of the pebble bed type HTGR generally limited to 300cm also to assure the capability of the core in releasing the heat outside passively. The PeLUIt design development is based on the HTR-PM design including its pebble fuel design of 7g/pebble and 8.5% enrichment. The HTR-PM design then downscale to have an optimum design of PeLUIt 150MWt.

Calculation Model
The equilibrium core analysis of the pebble bed type HTGR in this study was perfomed using PEBBED code [4,5]. It covers the neutronic analysis for eigen-value and depletion analysis of the moving core. In general, computational method to analyse equilibrium core is divided into two methods. Equilibrium analysis is important as it represent the general performance of the core over the entire range of its operation. Physically, the moving core reactor will have an initial loading, running-in phase, and finally reach the equilibrium condition [6]. However, PEBBED code directly calculate the equilibrium core as the BATAN-MPASS code [7]. On the other way, VSOP [8] code, adopt a non-direct approach by following the physical change of the moving core up to its equilibrium. VSOP code use diffusion method for the neutron transport simulation. A Monte Carlo based analysis with non-direct method can be done by MCPBR [9] software. Basically, to simulate a moving core of pebble bed reactor, the governing equation need to be solved [10]  = yield of isotope k due to fission of isotope i = probability that neutron absorption of isotope s produces isotope k = probability that decay of isotope j produce isotope k In solving the above equation, PEBBED converges directly to the equilibrium (or asymptotic) core solution and thus assumes that the first term on the left in Eq. 1 vanishes. By solving that new equation and the diffusion equation iteratively until burnup convergence is achieved which represent the static nuclide distribution in the core. The thermal-fluid and spectrum equations are also solved in in this loop to yield a fully consistent core solution [1,5]. Flowchart of the PEBBED code is shown in Figure 3.

Results and Discussions
Parametric survey considering the possible core geometry of the PeLUIt based as a downscale of HTR-PM are given in Table 2.

Table 2. Relation between Core Radius and Height
Results of the parametric survey are shown in Figure 4. As can be seen from Figure 4, discharge burnup is increasing for bigger core radius, while the maximum power generated per fuel is decreasing. The optimum condition is the design with power density 3.2 W/cm3, active core radius 1.5m, and core height 6.63m.  Results of the pass recirculation parametric survey is shown in Figure 5. From the axial power density profile, it can be seen that increasing the pass will decrease the power density peak up to certain number of pass. The results in Figure 5 shows that the 15-pass is the optimum pass for the design.  Variation of U-235 enrichment for the optimum discharge burnup is shown in Figure 7. The maximum discharge burnup include in this study is 150 MWd/Kg, based on this limit, the optimum discharge burnup of 147.81 MWd/Kg can be achieved by enrichment 13.5%. However, this discharge burnup value might be too high, several option with lower discharged burnup is possible as given in Figure 7. The enrichment value of 8.5% as in the HTR-PM is used in further optimized equilibrium design. Result of the conceptual design based on the PEBBED calculation are as following. The parameter of the optimized equilibrium core is given in Table 3. Among the acceptance criteria of the design is that the maximum fuel temperature after hypotetical DLOFC accident is below 1620°C. The fuel temperature transient after DLOFC as calculated by PEBBED is shown in Figure 7 which shows that current design have an acceptable peak fuel temperature even after DLOFC accident. Steam Turbine for current PBR 150MWt is selected based on the target power rate of 150MWt or about 65MWe. For the cogenreation application, a live steam from steam generator is targeted to have a temperature of 540°C and pressure140bar. This steam parameter used as the design parameter of the steam turbine which currently a Siemens SST400 steam turbine was chosen. Mass balance analysis of the coupled primary and secondary system of current PBR 150MWt was performed using ChemCAD software. Input to the ChemCAD is the livesteam parameter for the steam turbine (540°C and 140bar) also the helium parameter in the primary system. Data for the power conversion system is given in Table 4 and schematic results of the general mass balance analysis is given in Figure 10.