Small modular reactors (SMRs) offer a promising avenue for revolutionizing the traditional role of nuclear plants, transforming them from serving as baseload to flexible con- tributors in both power generation and ancillary services. This paper develops a steady-state model for SMRs, with a focus on incorporating constraints related to â\euro˜xenon poisoningâ\euro™. These constraints are essential to prevent issues during nuclear plant ramp-up following a ramp-down event. These â\euro˜xenon poisoningâ\euro™ constraints have been integrated into a multi-timescale power system operation framework, which also encompasses the formu- lation of inter-temporal coupling constraints. A comprehensive investigation is undertaken to evaluate the impact of integrating SMRs into a power grid with a high penetration of renewable en- ergy, specifically the NREL-118 bus system. A capacity expansion planning analysis is first conducted over multiple years to identify the optimal locations and sizes for deploying SMRs across the network. Additionally, weâ\euro™ve developed various reserve rules that adapt to the ramping status of the SMRs and include different â\euro˜hold-timeâ\euro™ for â\euro˜xenon poisoningâ\euro™ mitigation. Results obtained from a day-long simulation illustrate that the implementation of minimal â\euro˜xenon poisoningâ\euro™ hold-time, coupled with a steady- state guided reserve provision rule, yields the highest revenue â\euro“ approximately 4.14% more than the base case.Â