Implications for Venting Scenarios in UG Nuclear Tests

Activity concentrations are calculated assuming prompt venting of fresh fission products. Activity concentrations at surface ground zero are estimated based on fallout curves from [4]. For a 1-kT underground explosion, venting fractions greater than 10-7 are detectable at 4 weeks post-event with spectral anomaly detection. Underground nuclear tests conducted in the United States have shown venting fractions from 10-10 to 10-1 , from [5].

1.E-­‐11  

1.E-­‐10  

1.E-­‐09  

1.E-­‐08  

1.E-­‐07  

1.E-­‐06  

1.E-­‐05  

1.E-­‐04  

1.E-­‐03  

1.E-­‐02  

1.E-­‐01  

1.E+00  

10/1/1960   10/1/1963   10/1/1966   10/1/1969   10/1/1972   10/1/1975   10/1/1978  

Frac%o

nal  release  

Date  

Frac%onal  release  vs.  date  of  shot  (US  Underground  Tests,  DOE-­‐NV-­‐317)  

Exact  yields  reported  

References

[1] Hendricks, T, and S Riedhauser. 1994. An Aerial Radiological Survey of the Nevada Test Site. Report No. DOE/NV/11718-324W, Bechtel Nevada, Las Vegas, Nevada.

[2] Proctor, AE. 1997. Aerial Radiological Surveys. Report No. DOE/NV/11718-127, Bechtel Nevada, Las Vegas, Nevada.

[3] Pfund, DM, et al. 2007. "Examination of Count-Starved Gamma Spectra Using the Method of Spectral Comparison Ratios." Nuclear Science, IEEE Transactions on 54(4):1232-38.

[4] Glasstone, S, and PJ Dolan. 1977. The Effects of Nuclear Weapons. 3rd ed., United States Department of Defense/ERDA.

[5] Schoengold, CR, ME DeMarre, and EM Kirkwood. 1996. Radiological Effluents Released from US Continental Tests, 1961 through 1992. Revision 1. Report No. DOE/NV-317, U.S. Department of Energy Nevada Test Site

Conclusions

▶  It is possible to distinguish fresh fission products from legacy nuclear-testing debris using spectroscopic analysis of airborne gamma-ray measurements.

▶  Specialized anomaly detection methods (like NSCRAD) can achieve 100 times better MDCs than gross-count methods alone.

▶  Aerial systems are capable of achieving MDCs as low as 1.5 nCi/m2 (56 Bq/ m2) under favorable conditions, and average performance ranges from 10-100 nCi/m2 (0.37 – 3.7 kBq/ m2) in high backgrounds.

▶  The calculated MDCs imply a minimum required venting fraction from a 1kT underground nuclear test of 10-7 to 10-5 in order to detect the debris from an aerial system.

▶  Venting fractions of this magnitude have been observed in a number of US nuclear tests.

Sensitivity Analysis Of Aerial Radiological Over Flights And Implications For Nuclear Testing Scenarios CE Seifert, R Detwiler, D Pfund, M Myjak, J Fast

Data Processing Methods Gross Counts Total measured count rate observed by the radiation detection system is used to determine the presence of radioactive material in an environment with background radiation. An alarm is typically triggered by a measureable elevation of counts above the background, defined in terms of the standard deviation in the background.

Man-Made Excess Counts The man-made algorithm uses a two-window extraction from the gamma-ray spectrum to predict the number of events at low energies based on the observed number of events at high energies [1][2]. The low -energy window is less than 1394 keV (just below the background peak from naturally-occurring 40K) and the high-energy window is greater than 1394 keV. The ratio of low-energy to high-energy counts is generally constant for natural background, but the low-energy window contains additional counts when anthropogenic radiation sources, such as industrial, medical, or nuclear materials, are present.

CMM = man-made excess counts  

E = energy in keV

C(E) = counts in energy bin E

Cbkg(E) = counts in background in energy bin E

Spectral Anomaly Detection Anomaly detection uses a limited number of regions of interest to compare observed spectra to the known or expected radiation background [3]. Such methods can be implemented in a spectrally blinded way, because no isotope identification is performed. These methods have potentially the best sensitivity, but require proper selection of the energy windows to filter out non-background radionuclides irrelevant to the Treaty.

Method Minimum Detectable Concentration (µCi/m2)

Gross Counts 3.0 (110 kBq/m2 )

Man made 1.5 (56 kBq/m2 )

NSCRAD 0.03 (1.1 kBq/m2 )

Only the anomaly detection method detects the presence of 11 kBq/m2 contamination on top of the legacy 137Cs contamination. Calculated MDCs for each method are shown in the table.

   

A   B  

C  

 

A   B  

C   D  

 

A   B  

C   D  

 

A   B  

C   D  

Gross  Counts   Man  Made   Anomaly  DetecFon  

Aerial Radiological Overflights Aerial radiological surveys provide a rapid method to characterize radioactive dispersal over a large area. Aerial surveys can detect gamma emissions from expected fission products from a nuclear explosion, depending on the activity concentration and (in the case of underground testing) venting of the released material. Aerial detection systems: ▶  Traverse a large area efficiently ▶  Are not reliant on passable roads and favorable terrain conditions ▶  Serve as a tool to identify regions within the large inspection area to concentrate further inspection activities ▶  Can also be re-deployed in ground-based assets to carry out more focused inspection surveys in the identified regions

Minimum Detectable Concentrations

Isotope Half-life (days)

Gamma-Ray

Energy (keV)

Gamma-Ray Yield (g/decay)

% of total Fission Product Activity at

4 weeks

Ba-140 12.8 537.3 0.244 18.01

I-131 8.02 364.5 0.816 23.95

637 0.071

La-140 1.68 487 0.455 20.96

1596.2 0.954

Ru-103 39.3 497.1 0.909 37.08

Simulated OSI Source (11 kBq/m2)

Response to short-lived fission products above was simulated and injected at the indicated location on top of measured aerial backgrounds from legacy contamination due to U.S. nuclear testing. Contour maps of detection metrics determined across the survey area from gross counts, man-made excess counts, and anomaly detection are shown below.

Contact: Carolyn Seifert (509) 375-1987 Carolyn.Seifert@pnnl.gov

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