L-3 MAPPS' Core Model based on Equivalence Theory (COMET™) is an advanced model for the simulation of reactor neutron kinetics based on a rigorous application of first principle physics and advanced numerical techniques. COMET™ has been installed worldwide on numerous simulators currently certified for training.

COMET™ is a true three-dimensional model. A minimum of one radial node per fuel assembly for PWRs and one radial node per fuel channel for BWRs is used. The neutron diffusion equations are solved at each node at each time step without relying on approximate methods based on a space-time factorization. The finite difference form of the neutron diffusion equations are solved using a homogenization method (equivalence theory) similar to those used by certain advanced lattice codes.

Each node takes into account:

	Xenon, Iodine, Samarium and Promethium concentrations
	Six delayed neutron groups
	Decay heat calculations are carried out using a set of eleven decay heat precursors
	Neutron source strength
	Fuel burn-up, Plutonium build up and depletion of burnable poisons

Control rods are treated as individual entities and shadowing effects are fully simulated. Variable enrichment fuel and other modern fuel designs can be accounted for.

The time-dependent neutron diffusion equations are spatially discretized over the nodes representing the core and reflectors by using a mesh-centered finite difference technique. In the process of discretizing the diffusion equations, equivalence theory is used to calculate the homogenized nuclear cross sections over each node. The homogenization method preserves integral quantities while eliminating truncation errors due to finite differences. The solution of the resulting equations reproduces the reaction rates, leakage rates and eigenvalues of the off-line, fine mesh heterogeneous models used to provide the reference states.

One of the ultimate goals to be achieved in simulating the core neutronics consists of calculations of the local flux at the in-core detector locations. COMET™ accomplishes this by reconstructing the fine mesh flux, as a function of the fine mesh nuclear properties, in the nodes where detectors are present. As a result, very localized effects, such as the effect of movements of neighboring rods on detector responses, are fully simulated.

Cycle-specific parameters required by COMET™ are calculated by Chorus™. This powerful off-line tool features a user-friendly Windows NT graphical interface in order to maximize ease of use. For example, adding a detector, changing the real-time model nodalization or performing the off-line calculations can easily be done graphically. Chorus™ also provides features allowing the user to rapidly validate the input lattice code neutronic data as well as the output real-time homogenized nuclear parameters. Inputs to Chorus™ mendation 6 of INPO's SOER 96-02 to provide cycle-specificsimulator training.

Complementing L-3 MAPPS' Total Simulator Solutions, COMET™ and Chorus™ are advanced simulation software packages that demonstrate L-3 MAPPS' long-term commitment to excellence and innovation.