Utilizing integrated gate-commutated thyristor (IGCT) devices within resonant converters facilitates the realization of megawatt-scale direct current transformers (DCTs). The clamp circuitry, typically used to restrict the current slew rate at the turn-ON of the IGCT is not strictly needed, thanks to the presence of the resonant tank impedance. Yet, its operation and performance inside the resonant converters have never been investigated and quantified. The well-known hard-switching IGCT clamp working principles and design metrics are not directly applicable for the resonant converters, since the mentioned circuit exhibits distinctly different operating conditions and stresses. This paper investigates the clamp circuit operation in the bidirectional CLLC resonant neutral-point clamp (NPC) DCT topology by providing a mathematical model of the clamp circuit operation and deriving its design rules. Additionally, the impacts of the IGCT clamp circuits on the resonant network parameters in the operation are analysed and equivalent models are presented. The analysis, design and operation are validated experimentally on a megawatt-scale DCT prototype.