Calculation of particulate dispersion in a design-basis tornadic storm from the Atomics International Nuclear Material Development Facility, Santa Susana, California Page: 4 of 28

                                
ABSTRACT

A three-dimensional numerical model is used to calculate
ground-level air concentration and deposition (due to precipita-
tion scavenging) after a hypothetical tornado strike at the
Atomics International Nuclear Material Development Facility at
Santa Susana, California. Plutonium particles less than 20 Vm
in diameter are assumed to be lifted into the tornadic storm cell
by the vortex. The rotational characteristics of the tornadic
storm are embedded within the larger mesoscale flow of the storm
system. The design-basis translational wind values are based
on probabilities associated with existing records of tornado
strikes in the vicinity of the plant site. Turbulence exchange
coefficients are based on empirical values deduced from experi-
mental data in severe storms and from theoretical assumptions
obtained from the literature. The method of moments is used to
incorporate subgrid-scale resolution of the concentration within
a grid cell volume. This method is a quasi-Lagrangian scheme
which minimizes numerical error associated with advection.
In all case studies, the effects oe updrafts and downdrafts,
coupled with scavenging of the particulates by precipitation,
account for most of the material being deposited within 50 km
downwind of the plant site. Ground-level isopleths in the x-y
plane show that most of the material is deposited behind and
slightly to the left of the centerline trajectory of the storm.
Approximately 5% of the material is dispersed into the stratosphere
and anvil section of the storm.