# pflotran / Depreciated / Documentation / QuickGuide / Chemistry / RadioactiveDecayReaction

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Specifies parameters for radioactive decay reaction. This reaction differs from the GENERAL_REACTION in that only one reactant species may be specified with a unit stoichiometry (i.e. the rate is always first order) and the reactant species is decayed in both the aqueous and sorbed phases.

### Required Cards:

REACTION <string>
Reaction equation. Only one reactant species may be listed on the left side of the equation (i.e. or on the right side with a negative stoichiometry). The reactant's stoichiometry is fixed at 1.0. The forward rate is applied to that one species as a first order rate constant [1/sec]. Multiple species are supported as daughter products on the right hand side and stoichiometries can be specified.

RATE_CONSTANT or HALF_LIFE (but not both)

### Optional Cards:

RATE_CONSTANT <float>
rate constant for 1st-order decay reaction [1/sec]. The rate constant may be calculated from -ln(0.5) / half-life
HALF_LIFE <float>
half life of species [sec].

Note that rate constant or half life units other than sec or 1/sec may be specified.

### Examples:

RADIOACTIVE_DECAY_REACTION
REACTION Tracer <-> Tracer2
RATE_CONSTANT 1.7584d-7 ! half life at 0.125 y
/

CHEMISTRY
PRIMARY_SPECIES
A(aq)
B(aq)
C(aq)
/
...
REACTION A(aq) <-> B(aq)
! Calculating forward rate from half-life
! rate = -ln(0.5) / half-life [1/sec]
RATE_CONSTANT 1.75836d-9  ! 1/s  half life = 12.5 yrs
/