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QUABOX-CUBBOX provides a detailed analysis of the reactor core behavior based on 3D neutronics models which solve the two-energy group neutron diffusion equations including reactivity feedback effects caused by changes of coolant flow conditions and changes of fuel rod temperatures. The efficient solution of spatial and time-dependent neutronics equations is based on coarse mesh methods or nodal expansion methods achieving a high accuracy even for radial nodes corresponding to the fuel assembly size.

The methods deployed in QUABOX-CUBBOX may be characterized by the solution of two-group diffusion equations with six groups of delayed neutrons. The solution method is based on a flux expansion method by local polynomials, enabling to calculate 1D-, 2D- and 3D-core configurations. The code has been developed at GRS for more than 30 years; today it is one of the first nodal diffusion codes with capabilities for assembly-wise and pin-by-pin full core calculations.

QUABOX-CUBBOX has been successfully validated at international benchmarks, and is constantly subject to advanced development and validation. For about 15 years, QUABOX-CUBBOX has been coupled to ATHLET. This coupling allows modeling of the thermal-hydraulic feedbacks. The coupling aims at providing realistic calculations, replacing the conservative model calculations by best estimate calculations; acceptance criteria based on core local parameters can be evaluated more precisely. Application fields of the coupled code systems are e.g. cool-down transients with strongly negative moderator temperature reactivity coefficient in PWR, particularly for high burnup fuel or extended use of MOX fuel; the local boron dilution accident in PWR; ATWS analyses with 3D neutronics models, which reduce the high uncertainties of inherent feedback determining power production and consequently pressure increase.

Today, the coupled code system is the backbone for performing full core production calculations for transient and accident analyses of nuclear power plants with PWR, BWR and RBMK reactors.