GUARDYAN is a GPU-based dynamic Monte Carlo code developed at the Budapest University
of Technology and Economics, Hungary. Dynamic Monte Carlo computes the neutron population
evolution by calculating the direct time dependence of the neutron histories in multiplying
systems. Some well-established Monte Carlo codes have DMC versions with coupling to
Thermal-Hydraulic solvers. GUARDYAN has the computational advantage of applying GPUs,
thus calculation burden can be carried by commonly available hardware, and is capable
of handling power plant size systems for kinetics problems. GUARDYAN has been recently
coupled to the subchannel thermal-hydraulics code SUBCHANFLOW in order to carry out
dynamic calculations with TH feedback. This paper describes some convergence studies
regarding reaching the initial equilibrium state. A literature-suggested set of TH
input settings and high sample numbers resulted in very low statistical errors of
the power estimates and stable global measures (L-2) of power release, fuel, and coolant
temperatures for both static and dynamic convergence. Dynamic mode low-sample simulations
provided surprisingly stable global L-2 measures, correct fuel temperatures, and power
release, while coolant temperatures were off, without any indication of the incorrectness
of the result. Static convergence showed an alternating, fluctuating L-2 behavior
that did not affect the final stable state.
