DOE OSTI2015
Small modular reactors (SMRs) are being considered by the commercial nuclear power industry as an option for more distributed generation and for replacement of older fossil fuel generating facilities. SMRs are more compact than operating pressurized water reactors (PWRs), producing about 200 MWe as compared to 1000 MWe or higher for their full-sized cousins, and are offered as “expandable” units; that is, their modular design allows the utility to add more units progressively at the same site. In Summer 2014, CASL reported on a progression of demonstrations illustrating VERA’s ability to simulate a typical SMR of the iPWR type, while developing a simple core cycle design for cycle economic studies. The work included development of fuel assembly lattices, 3D single assembly and full core (quarter symmetry) simulations of the proposed core cycle design using the CASL Virtual Environment for Reactor Applications (VERA). The report concluded that it would be difficult for the SMR to operate for the target 1400 EFPD cycle length and that the fuel cost would be on the order of 50% higher as compared with a PWR. Unfortunately, the Reference 4 core cycle design did not include control component management. The iPWR is intended to be operated using the control rods for reactivity control, as opposed to using soluble boron (like most currently operating PWRs), and the introduction of the control rods is expected to affect the length of the cycle. Therefore, this work continues the previous CASL demonstration of VERA capability to simulate a typical iPWR core, including the development of a control component management strategy using the Reference 4 core design and utilizing the soon-to-be released VERA3.3 tools. Note that this study is not intended as a validation study; rather it is merely an illustration of VERA’s capabilities to simulate a typical iPWR. Simulations included two variations on control rod banks – a two-bank scheme and a ten-bank scheme. Both schemes were modelled using the VERA Common Input, and more than six iterations on each control rod scheme were simulated to arrive at a combination of rod banks and insertion depths that provided a core reactivity near the target k eff of 1.000 ± 0.001 (more than 50 cases run) over two calendar months. In particular, VERA was used to determine the hot excess reactivity for the core design with all rods out (ARO), and then iterative simulations with the rod banks inserted to determine the critical control rod bank positions were completed. VERA options to simplify the process were exercised as possible, particularly restart functionality, coupled thermal-hydraulics using the internal T-H solver, xenon, and depletion capability. The fuel shuffling functionality was not exercised, since the iPWR SMR features a once-through cycle. The VERA method of characteristics (MOC) neutronics subcomponent was used to predict power and reactivity with control rod maneuvering, both with and without thermal-hydraulic feedback. The problem was scaled from 35 CPU cores to 3815, effectively demonstrating the solution time with respect to computing capacity. Computer clusters at Oak Ridge National Laboratory, Idaho National Laboratory and Tennessee Valley Authority were used. These studies provide an additional illustration of the application of VERA to a typical iPWR SMR. Reference 4 previously demonstrated that the codes could handle the differences in geometry for the iPWR, and this study expanded the illustration to include control rod management. Further, the work illustrates the need for in-depth simulations to support effective control rod maneuvering for an iPWR and highlights the need for control rod reactivity search capability within VERA. Additionally, the development of a control rod bank reactivity search capability will be extremely useful for not only this application, but also for BWRs as CASL develops BWR capability in Phase 2.
22 GENERAL STUDIES OF NUCLEAR REACTORS↗