The Monte Carlo codes MCNP and MCU for RBMK criticality calculations

The core design of a Small Modular Pressurised Water Reactor for commercial marine propulsion

“…These discrepancies are much higher than those found when modelling conventional PWRs (discrepancies < 0.5%) (Alexeev et al (1998)) and should preferably be below 0.2%. In order to determine the likely reason for the discrepancies between SIMULATE-3 and MONK-9A, 2D lattice calculations (using the lattice geometry for the marine reactor) were performed in MONK and CASMO in order to benchmark the two codes; noting that any discrepancy 15 at the lattice level (CASMO) will directly impact the accuracy at the nodal level (SIMULATE).…”

“…CASMO-4 is a multi-group two-dimensional neutron transport theory code for modelling fuel assemblies, whereas SIMULATE-3 is a 3D nodal code that utilises diffusion theory. SIMULATE employs the output from CASMO-4 to model the entire reactor core and is used for determining the spatial and time dependence of the neutron flux throughout core life, where the slow variation of the global flux permits the use of the diffusion equation (Stammler and Abbate (1983)).…”

“…To ensure an inherently stable core design, the power coefficient of reactivity must be kept negative for all conceivable operating conditions. The two dominant effects in most reactors are the Doppler coefficient and Moderator Temperature Coefficient (MTC) (Duderstadt and Hamilton (1976)). When licensing a core design the whole gamut of reactivity coefficients must be studied, but to simplify analysis only the MTC and Doppler coefficient were considered here.…”

“…Xenon clearly has minimal impact on core reactivity throughout life -for comparison conventional PWRs can experience negative reactivity insertions of up to approximately 1000 pcm. The reason behind the core's relative insensitivity to xenon transients (which is advantageous for a reactor expected to routinely undergo significant power changes) is due to the low thermal flux in the core, since the behaviour of xenon is heavily dependent on the steady-state flux level (Duderstadt and Hamilton (1976)). The average thermal flux across the core within this reactor at end of life, calculated by SIMULATE, was found to be around 6×10 12 neutrons s −1 cm −2 .…”

“…Besides overall core reactivity changes, the dependence of 135 Xe on flux also creates the possibility for localised variations in 135 Xe concentration within the core, which can lead to so-called xenon oscillations. Xenon oscillations occur due to perturbations in the flux within a reactor (Duderstadt and Hamilton (1976)). Given the limited effect of xenon within the core (due to the low thermal flux) the marine core was found to be robust against axial and radial xenon oscillations.…”