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Chemical Threat
Analysis
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| WPE Independent Design Verification of HABIT 1.1
EXTRAN/CHEM |
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Regulatory Guide
1.78, Revision 1, provides a methodology to evaluate
potential hazardous chemical releases in the vicinity of
a nuclear power plant for impact on the safety of its
Control Room operators. It provides screening criteria
for stationary and mobile chemical sources to assess
Control Room habitability dependant upon the quantity of
chemical released, the distance from the plant,
prevailing meteorological conditions, the inleakage
characteristics of the Control Room Envelope, and the
specific toxicity limits. Where a particular chemical
constituent can not be eliminated from consideration by
this screening process, the Regulatory Guide endorses
the use the HABIT 1.1 EXTRAN/CHEM computer codes for
more in-depth analysis.
The HABIT 1.1 EXTRAN/CHEM
computer code was developed for the NRC after reviewing
procedures and regulatory guidance used for estimating
atmospheric transport and diffusion for Control Room
Habitability assessments. The EXTRAN code combines
procedures for estimating the amount of airborne
material, a Gaussian puff model, and the most recent of
the building-wake diffusion coefficient algorithms.
Given a hazardous material in a tank, an analyst can
postulate a release scenario, the environmental
conditions, and the physical and chemical
characteristics of the material. EXTRAN uses this
information to compute a concentration history from the
time the substance first arrives at the air intake until
actions can be taken to protect Control Room occupants.
The CHEM module uses the
mean toxic chemical concentration at the Control Room
intake(s) produced by EXTRAN for each time step. The
code also has the capability of introducing bottled
fresh air into the Control Room volume if applicable.
CHEM calculates a concentration in the Control Room
volume by balancing the infiltration of toxic chemicals
with outflow through the envelope. It assumes that toxic
chemicals are not effectively removed by filters,
settling, or chemical reaction.
WPE has performed
Hazardous Chemical Analysis for various nuclear
facilities using the EXTRAN and CHEM computer codes in
response to Generic Letter 2003-01, Control Room
Habitability. In addition, WPE has developed a
stand-alone spreadsheet calculation to verify the
FORTRAN 77 algorithms used by EXTRAN and CHEM. The
alternate analysis confirms conformance with NUREG/CR-6210
and PNL-10496 technical standards, the manipulation of
the formulas, output and consistency in use of
engineering units. This alternate calculation is
available to nuclear facilities in support of their
software verification programs when using the EXTRAN and
CHEM computer software to perform 10CFR50 Appendix B
calculations and analyses.
Use of this modeling
software is not limited to nuclear power. It is a
powerful tool which can be used in assessment of the
risk to any facility from accidental or deliberate
hazardous chemical spills. Following a methodology
similar to that prescribed in Regulatory Guide 1.78, WPE
can survey potential sources of toxic gas and quickly
evaluate potential concentrations for any commercial or
government facility. |
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