LDR Lite Benchmark: Coupled 3D Neutronics and Thermal-Hydraulics Analysis of a Control Rod Drop Transient

The innovative designs of small modular reactors (SMR) pose new challenges for demonstrating their safety. We introduce a new benchmark for the LDR lite design, which addresses the issue of there being little public data and no international SMR benchmarks. The benchmark specifications are based on a public version of the LDR-50 design developed at VTT. It is a small light water reactor for low temperature applications with special characteristics, such as natural circulation, passive decay heat removal through containment, self-pressurization, boron-free operation, and a small core size. The full LDR lite benchmark specifications i nclude a d escription o f t he r eactor g eometry, which allows building a system code model, and a detailed description of the core composition for Monte Carlo and nodal method neutronics codes. The specifications c an b e f ound f rom t he benchmark website (https://www.ldr-reactor.fi/en/ldr-lite-benchmark/). T hey w ill b e u pdated i n t he f uture and more calculation tasks added. A control rod drop transient is introduced as one of the benchmark tasks. Dropping of a single control rod leads into an asymmetric radial power distribution and the reactor finds a new equilibrium at a lower power level, which makes this an interesting test case for coupled 3D neutronics and system thermal-hydraulics. A reference solution is calculated for this task using VTT's nodal neutronics solver Ants and process simulation software Apros.