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TitleLight Water Reactor Sustainability Program
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For baseline calculation purpose, we choose FRAPCON [16] code for our fuel
performance steady state calculations. Power history and axial power profile from the output of
core design codes such as PHISICS are required as FRAPCON inputs. The linear heat generation
rate (kW/m) at each time step of FRAPCON simulation should also be provided.

Table 1. Common Data from Fuel Rod Design for Different Physics in LOCA Analysis.
Fuel Rod Data Fuel Performance Core


Rod geometry information such as cladding outer
diameter, cladding thickness, fabricated gap, active
fuel length, and plenum length

Spring dimensions such as outer diameter of plenum
spring, diameter of the plenum spring wire, and
number of turns in the plenum spring

Pellet shape such as height (length) of each pellet,
height (depth) of pellet dish, pellet end-dish shoulder
width, Chamfer height and width

Pellet isotopics such as fuel pellet U-235 enrichment,
oxygen-to-metal atomic ratio, weight fraction of
gadolinia in urania-gadolinia fuel pellets, Boron-10
enrichment in ZrB2, parts per million by weight of
moisture in the as-fabricated pellets, and parts per
million by weight of nitrogen in the as-fabricated
Pellet fabrication such as as-fabricated apparent fuel
density, open porosity fraction for pellets, the fuel
pellet surface arithmetic mean roughness, etc.
Cladding fabrication such as cladding type, the
cladding surface arithmetic mean roughness, as-
fabricated hydrogen in cladding, etc.
Rod fill conditions such as initial fill gas pressure,
Initial fill gas type and their mole fractions
Fuel assembly geometry such as pitch

System code RELAP5-3D already has some simple fuel performance models such as the
rupture model and ballooning model, but it could not provide detailed analysis of fuel rods’
behaviors such as the fission gas released, rod internal pressure, and fuel-cladding mechanical
interaction, etc., which requires the simulation from fuel performance codes.

The most important issue when coupling the system code and fuel performance code is to
make sure that the stored energy in the fuel pin for the RELAP5 steady state result equals to the
stored energy calculated by the fuel performance code. The stored energy in the fuel rod is
calculated by summing the energy of each pellet ring calculated at the ring temperature. The
expression for stored energy is


where is mass of ring segment , is temperature of ring segment , is specific
heat evaluated at temperature , is total mass of the axial node, is the number of annular
rings. The stored energy is calculated for each axial node.

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