SERPENT Guide
Materials & Compositions
Defining nuclear materials for neutron transport calculations
The Material Card
Material definitions tell Serpent what atoms are present in each region of your model and in what proportions. Every material gets a unique name that you reference in cell definitions. The material card specifies the bulk density, an optional temperature for cross-section selection, and a list of nuclide identifiers paired with their fractional contributions to the composition.
Basic Syntax
mat MATERIAL_NAME DENSITY [OPTIONS]
NUCLIDE1 FRACTION1
NUCLIDE2 FRACTION2
NUCLIDE3 FRACTION3The material name is an arbitrary string that you choose. The density is specified as a signed number: negative values indicate mass density in g/cm3, while positive values indicate atomic density in atoms per barn-cm. Optional keywords after the density control temperature (tmp), burnup tracking (burn), thermal scattering (moder), and other properties.
Density and Fraction Conventions
Serpent uses the sign of both the density and the individual nuclide fractions to distinguish between mass-based and atom-based specifications.
When using mass fractions (negative fraction values), the fractions represent weight percentages and do not need to sum to exactly 1.0 — Serpent normalizes them automatically. When using atom fractions (positive values), the fractions represent atomic ratios. For UO2, specifying U with a fraction of 1.0 and O with a fraction of 2.0 correctly captures the stoichiometric 1:2 ratio regardless of the absolute values.
Mass Fractions vs. Atom Fractions
% Mass density with atom fractions (most common for fuel)
mat fuel -10.4
92235.09c 0.031 % 3.1% enrichment (atom ratio)
92238.09c 0.969 % Remaining U-238 (atom ratio)
8016.09c 2.0 % Stoichiometric oxygen
% Mass density with mass fractions
mat clad -6.56
40000.09c -0.9816 % 98.16 wt% zirconium
50000.09c -0.0184 % 1.84 wt% tin
% Atom ratios for water (Serpent normalizes automatically)
mat water -0.7
1001.09c 2 % 2 hydrogen atoms per molecule
8016.09c 1 % 1 oxygen atom per moleculeTemperature and Thermal Scattering
Temperature significantly affects neutron cross sections through Doppler broadening of resonances in heavy nuclei and through changes in thermal scattering behavior in moderator materials. Serpent handles temperature effects through the tmp keyword, which selects the appropriate temperature-dependent cross sections from the nuclear data library.
For light nuclei in bound molecular systems — hydrogen in water, deuterium in heavy water, carbon in graphite — the free-gas scattering model is inadequate at thermal energies. The moder keyword links a specific nuclide in the material to a thermal scattering library (S(alpha,beta) data) that accounts for molecular binding effects. The corresponding therm card identifies the actual data file.
Temperature and Thermal Scattering Example
% Fuel at reactor operating temperature
mat fuel -10.4 tmp 900
92235.09c 0.045
92238.09c 0.955
8016.09c 2.0
% Cladding at intermediate temperature
mat clad -6.56 tmp 600
40000.09c -0.9816
50000.09c -0.0184
% Water with thermal scattering treatment
mat water -0.7 tmp 574 moder lwtr 1001
1001.09c 2
8016.09c 1
therm lwtr lwj3.11t
% Graphite moderator with thermal scattering
mat graphite -1.7 tmp 900 moder grph 6000
6000.09c 1
therm grph grj3.11tThe moder lwtr 1001 on the water material means "apply thermal scattering library lwtr to nuclide 1001 (H-1)." The therm lwtr lwj3.11tcard tells Serpent which data file provides the lwtr scattering kernel. Common thermal scattering libraries include lwj3.11t for hydrogen in light water, hwj3.11t for deuterium in heavy water, and grj3.11tfor carbon in graphite.
Reactor Material Library
Reference compositions and densities for common LWR materials:
Fuel Materials
% Low enriched uranium (3.1 at%)
mat fuel_3p1 -10.4 tmp 900 burn 1
92235.09c 0.031
92238.09c 0.969
8016.09c 2.0
% Medium enriched uranium (4.5 at%)
mat fuel_4p5 -10.4 tmp 900 burn 1
92235.09c 0.045
92238.09c 0.955
8016.09c 2.0
% Mixed oxide fuel (MOX, ~7 at% Pu)
mat mox -10.8 tmp 900 burn 1
92238.09c 0.93
94239.09c 0.05
94240.09c 0.015
94241.09c 0.005
8016.09c 2.0The burn 1 flag enables depletion tracking for the material in burnup calculations. Without this flag, the material composition remains fixed throughout the simulation. The integer argument is essentially a boolean — any positive value enables depletion. Each material with a distinct name is depleted as its own independent zone. For spatial subdivision within a single material (e.g., radial zones in a fuel pellet), use the div card.
Structural and Cladding Materials
% Zircaloy-4 (typical PWR cladding)
mat zirc4 -6.56 tmp 600
40000.09c -0.9816
50000.09c -0.0150
26000.09c -0.0021
24000.09c -0.0010
8016.09c -0.0012
% 304 Stainless Steel
mat ss304 -8.0 tmp 600
26000.09c -0.695
24000.09c -0.190
28000.09c -0.095
25055.09c -0.020
% 316 Stainless Steel
mat ss316 -8.0 tmp 600
26000.09c -0.650
24000.09c -0.170
28000.09c -0.120
42000.09c -0.025
25055.09c -0.020
14000.09c -0.010
6000.09c -0.003Coolant and Moderator Materials
% Light water at room temperature
mat water_cold -1.0 tmp 293 moder lwtr 1001
1001.09c 2
8016.09c 1
% Light water at PWR conditions
mat water_pwr -0.7 tmp 574 moder lwtr 1001
1001.09c 2
8016.09c 1
% Borated water (1000 ppm natural boron)
mat bwater -0.7 tmp 574 moder lwtr 1001
1001.09c 2.0
8016.09c 1.0
5010.09c 0.000332
5011.09c 0.001335
therm lwtr lwj3.11tThe density of water changes significantly with temperature — from 1.0 g/cm3 at room temperature to approximately 0.7 g/cm3 at typical PWR operating conditions. Always use the density appropriate for the thermal-hydraulic conditions in your model. Boron concentrations in PWR coolant typically range from 0 to 2000 ppm and must include both B-10 and B-11 in their natural isotopic ratio.
Control Materials
% Ag-In-Cd control rod
mat aic -10.17 tmp 600
47000.09c -0.80
49000.09c -0.15
48000.09c -0.05
% Boron carbide (B4C)
mat b4c -2.52 tmp 600
5010.09c 3.184
5011.09c 12.816
6000.09c 4.0
% Hafnium control rod
mat hafnium -13.3 tmp 600
72000.09c -1.0Advanced Material Features
Material volumes can be specified explicitly with the vol keyword when Serpent cannot calculate them automatically (e.g., in universe-based geometries). Explicit volumes are necessary for correct normalization of reaction rates and isotopic inventories in burnup calculations.
Burnup and Volume Specification
% Fuel with burnup tracking and explicit volume
mat fuel -10.4 tmp 900 vol 125.6 burn 1
92235.09c 0.031
92238.09c 0.969
8016.09c 2.0
% Multiple independent burnup zones (separate names = separate zones)
mat fuel_inner -10.4 tmp 900 burn 1
92235.09c 0.031
92238.09c 0.969
8016.09c 2.0
mat fuel_outer -10.4 tmp 900 burn 1
92235.09c 0.031
92238.09c 0.969
8016.09c 2.0Common Mistakes
The most frequent error in material definitions is confusing mass fractions with atom fractions. If you intend to specify weight percentages, the fraction values must be negative. Using positive values when you meant weight fractions will produce an entirely different composition, often with no error message since both conventions are valid.
Forgetting thermal scattering data for hydrogen in water is another common oversight. Without the moder and therm cards, Serpent treats hydrogen as a free gas, which produces an incorrect thermal neutron energy spectrum. The free-gas kernel ignores molecular binding effects in water, leading to k-effective errors of several hundred pcm — enough to give qualitatively wrong answers.
Always use Kelvin for temperature specifications. Serpent's tmp keyword expects absolute temperature. Specifying 300 when you meant 300 degrees Celsius would give you a temperature of 300 K (27 degrees Celsius), not the 573 K (300 degrees Celsius) you intended. Verify that your material densities are consistent with the temperatures you specify — hot materials have lower densities than cold ones due to thermal expansion.