Answer to Question #2218 Submitted to "Ask the Experts"

Q

I am looking for a model with which to predict the fallout protection factor of existing structures. Is such a thing available either in book form or as software for a PC?

A

By "fallout protection factor" I assume that you wish to determine the attenuation of external gamma radiation levels indoors compared to outdoors for various types of buildings and for various distributions of fallout radionuclides on the outside of the building and in the immediate surroundings.

This is a fairly complex shielding problem, and the answer will be a function of building construction, fallout radionuclide gamma energies, and building and source geometries (is it on the roof? on the ground outside?).

You are probably going to want a general-purpose shielding code for the calculation. The selection will depend on your familiarity with computers, your familiarity with shielding concepts, and your budget.

Many available codes trace their lineage back to the point-kernel integration techniques implemented in the old ISOSHLD (Isotope Shielding) computer code (Engel, Greenborg, and Hendrickson 1966). Many currently available codes are descendants of ISOSHLD. This code was used to determine dose rates from relatively simple arrangements of sources (point, line, plane, simple geometric volumes) and shields (slabs and volumes). The user can manually build up dose estimates for complicated exposure geometries by summing the appropriate simple cases. This code was a real workhorse in the 1960s and 1970s.

If you are reasonably proficient with DOS, an inexpensive option is the ISO-PC code, which is a PC version of ISOSHLD. It is available from the Radiation Shielding Information Computational Center (RSICC). Look for Code Package CCC-636.

A more user-friendly version of ISOSHLD, available through Grove Engineering (now a subsidiary of Framatome), is called MicroShield. This has all of the ISOSHLD geometries, plus some additional improvements, with a very useful user interface and graphical visualization tools to help set up the problems. However, it will set you back about $1,325. More information is available at Framatome.

Another approach you might try is one of the RESRAD code family from Argonne National Laboratory. The RESRAD-BUILD code is intended for use in determining the dose rates inside of modestly complicated structures. The primary intent is in decommissioning, where the contamination is in or on the building, but I suspect that the problem inputs could be structured to accommodate outdoor fallout. The RESRAD-BUILD code was used to determine the stochastic nature of the external gamma shielding factor for the main RESRAD code by applying it to a series of different structure designs (Yu et al. 2000, Attachment C, Section 7.10). The RESRAD external gamma shielding factor is the ratio of the external gamma radiation level indoors to that outdoors—which is what I assume you want to determine. These codes are freely available for download on the RESRAD home page.

Bruce Napier, CHP
Staff Scientist
Pacific Northwest National Laboratory

References

Engel RL, Greenborg J, Hendrickson MM. ISOSHLD—A computer code for general purpose isotope shielding analysis. Richland, Washington: Pacific Northwest Laboratory; BNWL-236; 1966.

Yu C, LePoire D, Gnanapragasam E, Arnish J, Kamboj S, Biwer BM, Cheng J-J, Zielen A, Chen SY. Development of probabilistic RESRAD 6.0 and RESRAD-BUILD 3.0 computer codes. Washington, DC: U.S. Nuclear Regulatory Commission; NUREG/CR-6697; 2000.