Implicit Collision Multiplicity Adjustment for Efficient Monte Carlo Transport Simulation of Reactivity Excursion

We present an implicit collision method with on-the-fly multiplicity adjustment based on the forward weight window methodology for efficient Dynamic Monte Carlo (MC) simulation of reactivity excursion transport problems. Test problems based on the Dragon experiment of 1945 by Otto Frisch are devised to verify and assess the efficiency of the method. The test problems exhibit the key features of the Dragon experiment, namely nine orders of magnitude neutron flux bursts followed by significant post-burst delayed neutron effects. Such an extreme reactivity excursion is particularly challenging and has never been solved with Dynamic MC. The proposed implicit collision multiplicity adjustment, in conjunction with a simple forced delayed neutron precursor decay technique, profitably trades simulation precision for reduced runtime, leading to an improved figure of merit, enabling efficient Dynamic MC simulation of extreme reactivity excursions.