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Radionuclide observables of underwater nuclear explosive tests
Journal of Environmental Radioactivity 192 (2018) 160–165
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Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Radionuclide observables of underwater nuclear explosive tests
T
∗
Jonathan L. Burnett , Brian D. Milbrath Pacific Northwest National Laboratory, PO Box 999, Richland, WA, USA
A B S T R A C T
There remain technical challenges for an On-site Inspection (OSI) in the high seas environment, which gathers evidence of a violation of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). For terrestrial nuclear explosions, the radionuclide observables are well defined and States Parties have chosen 17 particulate radionuclides that allow discrimination from other nuclear events. However, an underwater nuclear explosion generates induced radionuclides from the neutron activation of seawater, which has the potential to interfere with the measurement of the radionuclide observables using gamma-spectrometry techniques. To understand these effects the inventory of OSI relevant (6.0 × 1016 Bq) and activation (1.6 × 1019 Bq) radionuclides has been calculated for a 1 kT underwater nuclear explosion. The activation products consist predominantly of 38Cl and 24Na, which decay to 5.56% and 0.0007% of their initial activity within 1 and 14 days. Monte Carlo techniques have been used to assess spectral interferences within this timeframe. It is demonstrated that during this period they do not interfere with the measurement of the existing radionuclide observables. Additionally, 24Na has been identified as useful for inspection purposes.
1. Introduction
141
Ce, 144Ce, 144Pr and 147Nd) and four gaseous radionuclides (131mXe, Xe, 133mXe, 37Ar). These radionuclides are predominantly fission products, with the exception of 134Cs and 37Ar produced by neutron capture of 133Cs and 40Ca. Neutron activation becomes more important in the underwater environment, as seawater contains dissolved amounts of the majority of the 92 naturally occurring elements (Wright and Colling, 1995). This creates radioactive species that have potential to interfere with the measurement of the radionuclide observables. 133
1.1. On-site inspection An On-site Inspection (OSI) is a key aspect of the verification regime of the Comprehensive Nuclear-Test-Ban Treaty (CTBT), which bans all nuclear explosions, whether for military, civil or any other purposes (UN, 1996). As conditions warrant, an OSI may be conducted in underground, underwater, sub-seafloor and atmospheric inspection environments (CTBTO, 2013), with the object of verifying whether a nuclear explosion has occurred in violation of the Treaty and to gather facts which might assist in identifying any possible violator. While the Treaty has not yet entered into force, and remains in provisional operations, primary emphasis has been on developing the OSI capability for an underground nuclear explosion (UNE) scenario (Burnett and Davies, 2015; Burnett and Milbrath, 2016; Burnett et al., 2016, 2017; Zucca, 2014). There remain technical challenges to undertaking an OSI in the underwater, or high seas environment. Among these, and the focus of this research, are the radionuclide observables suitable for OSI measurement that provide discrimination of an underwater nuclear explosion (often referred to as an UNDEX or underwater explosion) from other radiation sources, such as reactor incidents or releases, nuclear waste and medical isotopes. These have been defined for the UNE terrestrial scenario, as 17 particulate radionuclides (95Zr, 95Nb, 99 Mo, 99mTc, 103Ru, 106Rh, 132Te, 131I, 132I, 134Cs, 137Cs, 140Ba, 140La, ∗
Corresponding author. E-mail address: [email protected] (J.L. Burnett).
https://doi.org/10.1016/j.jenvrad.2018.06.007 Received 30 March 2018; Received in revised form 23 May 2018; Accepted 7 June 2018 0265-931X/ © 2018 Published by Elsevier Ltd.
1.2. Underwater nuclear explosions Of the 2057 nuclear explosive tests undertaken during 1945–2017, only 8 were conducted underwater during 1946–1962 (Table 1). This included 5 by the United States in the Pacific Ocean and 3 by the former Soviet Union in the Arctic Ocean (close to Novaya Zemlya). There are characteristic phenomena associated with an underwater nuclear explosion that depend on yield, device detonation and water depth, and the area of the water body. When a nuclear weapon explodes underwater, energy is transferred to the surrounding water mass by radiation. This causes superheating of the seawater, resulting in expansion and the formation of a shockwave. The shockwave propagates outward transferring approximately half of the explosion energy and leaving behind a bubble of steam at high pressure and temperature (Pritchett, 1971b). The bubble rises due to buoyancy, and expands until its pressure is below the ambient pressure (over expansion), and then contracts,
Journal of Environmental Radioactivity 192 (2018) 160–165
J.L. Burnett, B.D. Milbrath
timeframe from one week to two years after an alleged nuclear explosion, and be measurable in field conditions with no common or otherwise expected interferences that could allow misidentification. It is the latter requirement that is problematic in the underwater environment, where significant quantities of induced radionuclides may also be produced by neutron activation of the seawater. Whilst many of these elements also occur in terrestrial soils and rocks, their concentration (salinity) is typically an order of magnitude less than seawater, such that fewer activation products are generated. The initial inventory of OSI relevant radionuclides can be calculated using fission product yield data (England and Rider, 1994) in the manner described by the authors' previous research (Burnett et al., 2018). This depends on the yield, which for a fission-based explosion, is the amount of explosive energy (in trinitrotoluene equivalents) discharged from the detonation of fissionable nuclear material (e.g. 235U, 239 Pu). As one kT is equivalent to 1.45 × 1023 fissions, and if the fission probability of a particular fission product is f, and its decay constant is λ s−1, the activity following a 1 kT nuclear explosion is 1.45 × 1023 fλ Bq (Chamberlain, 2004). As the relevant radionuclides are produced directly by fission and indirectly by the decay of other fission products, a computer program called RadICalc (Radiation Intensity Calculator) was used for inventory calculations (Robinson et al., 2015). This computes initial radionuclide activities for all fission products (using independent fission yield) and undertakes complex decay and in-growth calculations using the Bateman's Laplace transform-based solution to the differential equations for radioactive decay (Bateman, 1910). RadICalc utilises nuclear data (version B-VII.1 for 239Pu pooled fast neutron fission decay) from the evaluated nuclear data file (ENDF) hosted by Brookhaven National Laboratory (NNDC, 2018). The exception to this approach, was the activation produced 134Cs, which can be derived by direct linear scaling of reported nuclear explosive test values (Battelle, 1972; IAEA, 2008). Also consistent with previous research (Burnett and Davies, 2015; Burnett and Milbrath, 2016; Burnett et al., 2016, 2017), a239Pu fission-based nuclear explosion with explosive yield of 1 kT was modelled.
Table 1 Historic underwater nuclear explosions. Test name
Country
Date
Bomb depth (m)
Yield (kT)
Baker Wigwam 22 (Joe 17) 48 Wahoo Umbrella 122 (Korall-1) Swordfish
USA USA USSR USSR USA USA USSR USA
25-Jul-46 14-May-55 21-Sep-55 10-Oct-57 16-May-58 8-Jun-58 23-Oct-61 11-May-62
27 610 10 30 150 50 20 198
20 30 3.5 10 9 9 4.8 < 20
thereby increasing its pressure and causing condensation. This process continues at a decreasing rate until the bubble erupts from the surface, releasing mostly gaseous fission products (of which iodine is the most abundant), and hurtling large masses of water aloft as the plume (Pritchett, 1971a). When the plume water falls back to the surface a radioactive cloud called the base surge is produced (Dolan, 1972). This travels outwards and is typically highly radioactive (Weary et al., 1981). This is followed by late-time explosion effects, such as residual upwelling along the explosion axis and turbulent diffusion of the radioactive surface pool. The explosive process typically results in three sources of radioactive contamination: airborne activity, residual fallout and water contamination. Notably, during the Wigwam nuclear explosion only 1% of the radioactivity was present in the airborne fraction with the potential to become fallout (Weary et al., 1981). This value is in agreement with observations for other underwater nuclear explosions (Lee, 1979).
2. Methodology 2.1. OSI relevant radionuclides The OSI relevant radionuclides are produced from a nuclear explosion either as fission or activation products (Table 2). They can provide discrimination from different nuclear sources, especially when used in combination, and in the instance of 137Cs/134Cs for reactor accidents. This is because 134Cs is produced predominantly by neutron capture of 133Cs (133Cs(n, γ)134Cs) due to a low fission yield (0.0000279% per fission). Thus nuclear explosion debris samples would be expected to have a low 137Cs/134Cs ratio. Other ratios are also useful (e.g. 95Zr/95Nb, 99Mo/99mTc) and can provide timing information relating to the explosion (Harms et al., 2009; Nir-El, 2006; Yamba et al., 2016). The list of radionuclides was chosen so as to encompass an OSI
2.2. Seawater activation Prompt neutrons and gamma-rays produced by an underwater nuclear explosion, are rapidly attenuated by the surrounding seawater and typically travel < 1 m (Weary et al., 1981). Water molecules (H2O) do not activate, as neutron capture of hydrogen (1H) and oxygen (16O) produces stable isotopes of deuterium (2H) and 17O. On the short-time scales of an underwater detonation, there is relatively little double capture to form radioactive tritium (3H) commonly associated with
Table 2 The OSI relevant radionuclides. Each radionuclide is categorized as volatile (V), refractory (R) or intermediate (V-R). The fission product yields are given per 100 fissions for239Pu pooled fast neutron fission decay. Nuclide
Half-life
E (keV)
I (%)
Ind. Yield
Cum. Yield
V or R
Activity
95
64.03 d 34.99 d 2.75 d 6.0 h 39.26 d 373.61 d 8.02 d 3.20 d 2.30 h 2.07 y 30.08 y 12.75 d 1.68 d 32.51 d 284.95 d 17.28 m 10.98 d
756.7 765.8 739.5 140.5 497.1 621.9 364.5 228.2 667.7 604.7 661.7 537.3 1596.2 145.4 133.5 696.5 91.1
54.4 99.8 12.1 89.1 91.0 9.9 81.7 88.0 98.7 97.6 85.1 24.4 95.4 48.3 11.1 1.3 27.9
9.30 × 10−2 1.92 × 10−4 1.31 × 10−2 1.49 × 10−6 1.29 × 10−3 3.85 × 10−1 2.01 × 10−2 3.00 1.74 × 10−1 Activation 1.00 9.17 × 10−1 9.87 × 10−3 2.46 × 10−4 1.64 × 10−1 5.10 × 10−5 1.72 × 10−3
4.67 4.67 5.98 5.26 6.83 4.36 3.88 5.15 5.33 Activation 6.58 5.32 5.33 5.15 3.67 3.67 1.99
R R R R V V V V V V V V-R V-R V-R R R R
1.8 × 1013 6.4 × 1010 5.6 × 1013 6.9 × 1010 3.8 × 1011 3.7 × 1016 2.9 × 1013 1.1 × 1016 1.1 × 1016 6.1 × 1011 1.1 × 1012 1.1 × 1015 6.9 × 1013 8.8 × 1010 7.3 × 1012 4.9 × 1013 1.8 × 1012
Zr Nb 99 Mo 99m Tc 103 Ru 106 Rh 131 I 132 Te 132 I 134 Cs 137 Cs 140 Ba 140 La 141 Ce 144 Ce 144 Pr 147 Nd 95
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Table 3 Estimated activity of induced radionuclides for a 1 kT underwater nuclear explosion. Note that35S and32P have no gamma-emissions. The relative proportion of each radionuclide is given by the ratio. Isotope
Half-life
Ratio
Activity (Bq)
Isotope
Half-life
Ratio
Activity
38
37.230 m 14.997 h 12.355 h 35.282 h 87.37 d 2.5789 h 82.93 m 12.701 h 18.642 d 14.268 d 26.24 h 4.536 d 64.849 d 14.10 h 1.6786 d
8.3 × 10−1 1.7 × 10−1 9.2 × 10−4 8.3 × 10−4 7.0 × 10−6 1.7 × 10−6 3.3 × 10−7 9.9 × 10−8 5.8 × 10−8 5.8 × 10−8 5.0 × 10−8 4.1 × 10−8 8.3 × 10−9 6.6 × 10−9 5.0 × 10−9
1.3 × 1019 2.7 × 1018 1.5 × 1016 1.3 × 1016 9.8 × 1013 2.7 × 1013 5.4 × 1012 1.6 × 1012 9.3 × 1011 1.1 × 1012 8.0 × 1011 6.7 × 1011 1.3 × 1011 1.1 × 1011 8.0 × 1010
165
2.334 h 56.4 m 65.976 h 24.000 h 243.93 d 2.0652 y 60.20 d 1925.28 d 44.56 d 44.495 d 119.78 d 27.701 d 83.79 d 46.594 d 249.83 d
3.3 × 10−9 3.3 × 10−9 1.7 × 10−9 1.7 × 10−9 8.3 × 10−10 8.3 × 10−11 8.3 × 10−11 3.3 × 10−11 3.3 × 10−11 1.7 × 10−11 9.1 × 10−12 8.3 × 10−12 8.3 × 10−12 4.1 × 10−12 1.7 × 10−13
5.3 × 1010 5.3 × 1010 2.7 × 1010 2.7 × 1010 1.3 × 1010 1.2 × 109 1.3 × 109 5.3 × 108 5.3 × 108 2.7 × 108 1.5 × 108 1.3 × 108 1.3 × 108 6.7 × 107 2.7 × 106
Cl Na 42 K 82 Br 35 S 56 Mn 139 Ba 64 Cu 86 Rb 32 P 76 As 47 Ca 85 Sr 72 Ga 140 La 24
Dy Zn 99 Mo 187 W 65 Zn 134 Cs 124 Sb 60 Co 115m Cd 59 Fe 75 Se 51 Cr 46 Sc 203 Hg 110m Ag 69m
nuclear reactors (18O is also stable). The probability of triple capture to form 19O or 16O undergoing high-energy neutron-proton reaction to form 16N is also low. It is the activation of ambient materials in the water that is substantial, and the activation products are produced largely within the volume of seawater vaporized by the explosion. This contribution can be estimated using linear scaling of reported UNE test values (as for the OSI relevant radionuclide 134Cs) with corrections applied to account for the different elemental composition of seawater. Additional activation products can also be extrapolated from neutron irradiation experiments of seawater and the relative proportions of induced radionuclides measured (Robertson and Carpenter, 1974). Using this approach, the activity of the main induced radionuclides in seawater for a 1 kT underwater nuclear explosion has been calculated (Table 3).
products on the measurement of the OSI relevant radionuclides. For these purposes a point source was modelled 50 mm from a Broad Energy Germanium (BEGe) detector (1500 mm3 crystal volume, 30% relative efficiency) with a 0.5 mm beryllium window (for low-energy Xray and gamma-ray transmission). Using this geometry, detector response functions were then modelled for the given radionuclides and activities. The simulated count time was 3600 s with normalisation to a total spectra count of 1 × 108 counts. Simulated spectra were then analysed using the Canberra Gamma Acquisition and Analysis software (version 3.3).
2.3. Mixing of relevant and induced radionuclides
Undertaking an OSI for an underwater nuclear explosion is especially challenging as the relevant radionuclides are quickly dispersed and diluted by the surrounding waters. Following the Wigwam nuclear explosive test, which had the highest yield (30 kt) and deepest depth (610 m) of any underwater test, the surface activity was largely dispersed within 4 days of the explosion (as measured using surveying instrumentation) (Weary et al., 1981). This reduction is a consequence of horizontal and vertical mixing in addition to radioactive decay. The reduction in maximum radiation (Rmax, mR/h) with time (t, days) can be approximated by Rmax = 130t−2 (Isaacs, 1955). Based on the Wigwam test data, this means that the signal is reduced to 4.57% after 1 day, 0.09% after 7 days and 0.02% after 14 days. This has implications for an OSI on the high seas. Terrestrial UNEs are often discussed within a timeframe of 1 week–2 years (Burnett and Davies, 2015; Burnett and Milbrath, 2016; Burnett et al., 2016, 2017). For a nuclear UNDEX, this timeframe is likely to be much more restricted and for the purposes of this research is limited to within 14 days of the detonation. To measure such a transient signal presents logistical difficulties, and would likely require the mounting of a ship-based inspection (CTBTO, 2013).
3. Discussion 3.1. A transient signal
Following nuclear detonation, both refractory (R) and volatile (V) radionuclides should be initially contained within the gaseous bubble. The induced radionuclides would also be contained within this volume, as they are produced within the vaporized volume that gives rise to the gaseous bubble. The high-boiling point refractory elements will condense at 1500–3000 °C whilst the volatile radionuclides will condense as the ambient temperature is approached (∼50 °C). Cooling occurs on short timescales due to the high specific heat capacity of seawater of 3993 J kg−1 K−1 at 20 °C and the bubble moves rapidly towards the surface. For example, for the 1955 Wigwam test the bubble traveled at 200 mph and reached the surface within 10 s (Weary et al., 1981). In addition, the contamination extended to a diameter of 3170 m, with 28% of contamination found above the thermocline and 72% below. The thermocline extended from 60 to 100 m depth (Weary et al., 1981). Hence, fractionation effects would be expected to be minimal within the contaminated waters produced. As such dispersion occurs on relatively rapid timescales and is locally contained, fission (Table 2) and activation (Table 3) product inventories can be assumed to be mixed and diluted equally in the surrounding waters. The exception is for the atmospheric fraction (1% of the activity measured during the Wigwam test) which would consist of predominantly volatile species such as 131I.
3.2. Radioactive decay The detonation of a239Pu device with a 1 kT explosive yield produces 6.0 × 1016 Bq of OSI relevant radionuclides (Table 2) and 1.6 × 1019 Bq of induced radionuclides from seawater activation (Table 3). The composition of the relevant radionuclides changes dramatically during the first 2 weeks (Fig. 1a) due to radioactive decay and in-growth from precursory fission products. This causes the activity to initially increase to 8.5 × 1016 Bq (141.1%) at 1 day after detonation, and then to decrease to 3.3 × 1016 Bq (54.3%) at 1 week and 1.5 × 1016 Bq (24.9%) at 2 weeks. During this time the composition of
2.4. Simulated spectra The calculated activities of OSI relevant radionuclides and seawater activation products were used with Monte Carlo software, Gamma Spectrum Generator Pro++ (Nucleonica, 2014), to provide gammaspectra at 1 day, 1 week and 2 week intervals. The purpose of the modelling was to understand the effects of the seawater activation 162
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Fig. 1. The OSI relevant radionuclides (a) and seawater activation products (b) from a239Pu device with a 1 kT explosive yield. The activation products 46Sc, 51Cr, 59 Fe, 60Co, 65Zn, 75Se, 85Sr, 110mAg, 115mCd, 124Sb, 134Cs and 203Hg, have been omitted for clarity. Their activity is approximately constant from the values given by Table 3 on the timescales shown. 32P and 35S have also been excluded as these are not gamma-emitting radionuclides.
most challenging conditions for performing gamma-spectrometry measurements. It is during this time there are the highest radionuclide activities, which provide difficulties for peak deconvolution and identification purposes.
the radionuclides also changes dramatically as the shorter half-life radionuclides (e.g. 99Mo/99mTc, 132Te/132I, 131I) decay and the longer half-life radionuclides (e.g. 137Cs, 134Cs, 144Ce) persist. For example, at 1 day the most abundant radionuclides are 99Mo (23.5%), 99mTc (20.9%), 132I (17.7%) and 132Te (17.2%) and at 2 weeks this has changed to 140La (17.2%), 140Ba (15.0%), 131I (11.6%) and 103Ru (10.2%). The induced radionuclides decrease at a faster rate (Fig. 1b) from 5.56% (1 day), to 0.01% (1 week) to 0.0007% (2 weeks) with almost complete decay of the shorter-lived radionuclides such as 38Cl, 56 Mn, 165Dy, 139Ba and 69mZn and persistence of the longer-lived induced radionuclides persist, such as 35S, 82Br, 86Rb and 32P. At 1 day the composition is dominated by 24Na (98.6%), 82Br (0.9%), 42K (0.4%) and 35 S (0.01%) and at 2 weeks it is 35S (81.4%), 82Br (16.9%), 32P (0.5%) and 86Rb (0.5%). Notably, the contribution from 38Cl and 24Na which account for 99.8% of the initial induced activity, decays almost completely within 2 weeks of the nuclear detonation. Within this 2 week period, and without considering dispersal and dilution effects, are the
3.3. Spectral interferences Simulated spectra show the effects of the induced radionuclides from seawater activation on the measurement of the OSI relevant radionuclides (Fig. 2). At 1 day the main seawater activation products are 24Na (1368.6 keV), 42K (1524.6 keV), 64Cu (511 keV), 69mZn (438.6 keV) and 82Br (multiple lines including 776.5 keV) with detection of 14 OSI relevant radionuclides including 95Nb, 95Zr, 99Mo, 99mTc, 103 Ru, 106Rh, 131I, 132I, 132Te, 140Ba, 140La, 141Ce, 144Ce and 147Nd. None of the activated radionuclides are direct interferences on the measurement of the OSI relevant radionuclides. The main detrimental effect is from the Compton continuum resulting from the 24Na peak at 163
Journal of Environmental Radioactivity 192 (2018) 160–165
J.L. Burnett, B.D. Milbrath
Fig. 2. Simulated spectra for samples containing OSI relevant radionuclides and seawater activation products after a 1 kT underwater nuclear explosion. Spectra are shown at 1 day (top), 1 week (middle) and 2 weeks (bottom). The energy of the OSI relevant radionuclides is denoted by the dashed vertical lines. The main activation products are also indicated.
1524.6 keV). Whilst the 38Cl has a higher initial activity (1.3 × 1019 Bq), its relatively short half-life (37.2 min) makes it largely unsuitable as an indicator except for the most immediate measurements (within 2–4 h). The other activation products occur at too lower activity (6.7 × 108 Bq to 9.3 × 1011 Bq) to be advantageous in addition to the 17 OSI relevant radionuclides. Beyond this timeframe, further radioactive decay and dispersion within the water column occurs (99.98% signal reduction within 14 days), such that a later stage OSI on the high seas is unlikely to be a viable option. Exceptions include a nuclear UNDEX conducted within relatively shallow waters (e.g. the 20 kT Baker nuclear test at Bikini Atoll in 1946), where significant radionuclide contamination of the underlying sediments might occur or in areas with very limited mixing. Alternatively, the detonation could be from beneath the seafloor, such that the OSI could be searching for radionuclide seepage into the water column from below. Importantly, the capability to detect the initial nuclear UNDEX is far greater than for nuclear explosions in any other medium. This is largely attributable to the efficient transmission of hydroacoustic and seismic signals generated by the shock wave, such that an explosive yield of just a few kilograms is detectable (Dahlman et al., 2011). Satellite observations and other national technical means (NTMs) may also facilitate surveillance and attribution (e.g. tracking ships).
1368.6 keV. This scattered gamma-radiation results in an elevated continuum (note the Compton edge at 1150 keV on Fig. 2a) that acts to degrade the detection sensitivity for lower-energy radionuclides (Burnett and Davies, 2013). This is apparent in the detection of 115 peaks at day 1, compared to 176 peaks and 167 peaks at 1 and 2 weeks respectively. At day 1, many of the peaks identifiable in the later spectra are concealed beneath the Compton continuum resulting from the 24Na. At 1 week the 24Na has undergone decay resulting in a significant reduction in the continuum. At this time, the main activation products are 24Na, 51Cr (320.1 keV), 82Br and 203Hg (81.6 keV) with an additional OSI relevant radionuclide (137Cs) being detectable. At 2 weeks, the 24Na is no longer detectable in the spectra, with the main activation products being 51Cr, 76As (559.1 keV) and 82Br. The OSI relevant radionuclide 106Rh is also not present at this time.
4. Considerations The inventory calculations and simulated spectra show that the detection of the OSI relevant radionuclides is largely unaffected by the induced radionuclides from seawater activation within the timescale of 1 day–2 weeks. This is because the activation products largely decay (e.g. 38Cl and 24Na), or those that remain (e.g. 35S and 32P) do not present spectral interferences to the OSI relevant radionuclides. Within this timeframe, the 24Na could be a useful indicator of a nuclear explosion, as its activity is within a similar order of magnitude (2.7× 1018 Bq to 4.8 × 1011 Bq) to the fission products produced (average 3.6 × 1015 Bq to 8.8 × 1014 Bq). This radionuclide is also well-suited to measurement by gamma-spectrometry, with a gamma-intensity of 100% at 1368.6 keV (and also 99.9% at 2754.0 keV although this exceeds the typical calibration range for a gamma-spectrometer). The 42K (1.5 × 1016 Bq to 9.6 × 107 Bq) would be useful to a lesser extent due to its lower activity and gamma-intensity for measurement (18% at
5. Conclusions The 17 particulate radionuclide observables, as defined for a terrestrial UNE, are suitable indicators of a nuclear UNDEX. This is despite large amounts of induced radionuclides generated by the neutron activation of seawater by the nuclear explosion. A total of 30 activation products have been considered, with 1.6 × 1019 Bq being generated by the detonation of a 1 kT 239Pu device. This compares to 6.0 × 1016 Bq 164
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of OSI relevant radionuclides. The activation products consist predominantly of 38Cl and 24Na, and decay rapidly to the extent they do not interfere with an OSI conducted within 1–14 days of the explosion. At 1 day the activated activity has decreased to 5.56% and at 2 weeks it has decreased to 0.0007%. During this timeframe the activation products do not present significant spectral interferences to the OSI relevant radionuclides. Furthermore, the relatively high activity of 24Na would make it useful as an additional UNDEX indicator. After 2 weeks the dispersion effects of horizontal and vertical mixing with the surrounding waters are likely to impede radionuclide detection for OSI purposes.
Chamberlain, A.C., 2004. Radioactive Aerosols. Cambridge Environmental Chemistry Series, UK. CTBTO, 2013. Model Text for the Draft OSI Operational Manual, Task Leader Paper. Comprehensive Nuclear-Test-Ban Treaty Organization, Austria. Dahlman, O., Mackby, J., Mykkeltveit, S., Haak, H., 2011. Monitoring Nuclear Explosions in the Oceans, Detect and Deter: Can Countries Verify the Nuclear Test Ban? Springer Netherlands, Dordrecht, pp. 111–127. Dolan, P.J., 1972. . Capabilities of Nuclear Weapons. Defense Nuclear Agency, United States. England, T., Rider, B., 1994. Evaluation and Compilation of Fission Product Yields. Los Alamos National Laboratory ENDF-349, LA-UR-94-3106. Harms, A., Johansson, L., MacMahon, D., 2009. Decay correction of 95Nb. Appl. Radiat. Isot. 67, 641–642. IAEA, 2008. Handbook of Nuclear Data for Safeguards: Database Extensions. International Atomic Energy Agency, Austria. Isaacs, J.D., 1955. Mechanism and Extent of the Early Dispersion of Radioactive Products in Water. Scripps Institution of Oceanography, University of California, USA. Lee, H., 1979. Scoping the Impact of Nuclear Bursts at Sea on the Environment. Defense Nuclear Agency, United States. Nir-El, Y., 2006. Dating the age of a recent nuclear event by gamma-spectrometry. J. Radioanal. Nucl. Chem. 267, 567–573. NNDC, 2018. Evaluated Nuclear Data File (ENDF). National Nuclear Data Center (NNDC). Brookhaven National Laboratory, USA. Nucleonica, 2014. Gamma Spectrum Generator Pro++, 3.0.66.0001 Ed. Nucleonica Nuclear Science Portal, Karlsruhe. Pritchett, J.W., 1971a. An Evaluation of Various Theoretical Models for Underwater Explosion Bubble Pulsation. Information Research Associates, USA. Pritchett, J.W., 1971b. Incompressible calculations of underwater explosion phenomena. In: Holt, M. (Ed.), Proceedings of the Second International Conference on Numerical Methods in Fluid Dynamics: September 15–19, 1970 University of California. Berkeley. Springer Berlin Heidelberg, Berlin, Heidelberg, pp. 422–428. Robertson, D.E., Carpenter, R., 1974. Neutron activation techniques for the measurement of trace metals in environmental samples. In: Technical Information Center, Office of Information Services. United States Atomic Energy Commission. Robinson, J.W., Dion, M.P., Eiden, G.C., Farmer, O.T., Liezers, M., 2015. RadICalc: a program for estimating radiation intensity of radionuclide mixtures. J. Radioanal. Nucl. Chem. 303, 1955–1960. UN, 1996. The Comprehensive Nuclear-test-ban Treaty. United Nations, Vienna, Austria. Weary, S.E., Ozeroff, W.J., L, S.J., Collins, B., Lowery, C.W., Obermiller, S.K., 1981. Operation Wigwam. Defense Nuclear Agency, United States. Wright, J., Colling, A., 1995. CHAPTER 6-THE SEAWATER SOLUTION, Seawater: its Composition, Properties and Behaviour, second ed. Pergamon, pp. 85–127. Yamba, K., Sanogo, O., Kalinowski, M.B., Nikkinen, M., Koulidiati, J., 2016. Fast and accurate dating of nuclear events using La-140/Ba-140 isotopic activity ratio. Appl. Radiat. Isot. 112, 141–146. Zucca, J.J., 2014. CTBT on-site inspections. In: AIP Conference Proceedings 1596, pp. 105–114.
Acknowledgements The views expressed here do not necessarily reflect the opinion of the United States Government, the United States Department of Energy, or the Pacific Northwest National Laboratory. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract DE-AC05-76RL01830. References Bateman, H., 1910. Solution of a system of differential equations occurring in the theory of radiactive transformations. Proc. Camb. Phil. Soc. 15, 423–427. Battelle, 1972. Plowshare Geothermal Steam Chemistry, Introductory Studies, AEC Research and Development Report. Battelle Pacific Northwest Laboratories, United States. Burnett, J.L., Davies, A.V., 2013. Compton suppressed gamma-spectrometry for comprehensive nuclear-test-ban treaty samples. J. Radioanal. Nucl. Chem. 295, 497–499. Burnett, J.L., Davies, A.V., 2015. On-site inspection for the radionuclide observables of an underground nuclear explosion. J. Radioanal. Nucl. Chem. 303, 2073–2079. Burnett, J.L., Milbrath, B.D., 2016. Radionuclide observables for the Platte underground nuclear explosive test on 14 April 1962. J. Environ. Radioact. 164, 232–238. Burnett, J.L., Miley, H.S., Bowyer, T.W., Cameron, I.M., 2018. The 2014 integrated field exercise of the comprehensive nuclear-test-ban treaty revisited: the case for data fusion. J. Environ. Radioact. 189, 175–181. Burnett, J.L., Miley, H.S., Milbrath, B.D., 2016. Radionuclide observables during the integrated field exercise of the comprehensive nuclear-test-ban treaty. J. Environ. Radioact. 153, 195–200. Burnett, J.L., Mueller, W.F., Milbrath, B.D., 2017. Evaluation of gamma-spectrometry equipment for on-site inspection. J. Radioanal. Nucl. Chem. 312, 377–383.
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Radionuclide observables of underwater nuclear explosive tests
T
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Jonathan L. Burnett , Brian D. Milbrath Pacific Northwest National Laboratory, PO Box 999, Richland, WA, USA
A B S T R A C T
There remain technical challenges for an On-site Inspection (OSI) in the high seas environment, which gathers evidence of a violation of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). For terrestrial nuclear explosions, the radionuclide observables are well defined and States Parties have chosen 17 particulate radionuclides that allow discrimination from other nuclear events. However, an underwater nuclear explosion generates induced radionuclides from the neutron activation of seawater, which has the potential to interfere with the measurement of the radionuclide observables using gamma-spectrometry techniques. To understand these effects the inventory of OSI relevant (6.0 × 1016 Bq) and activation (1.6 × 1019 Bq) radionuclides has been calculated for a 1 kT underwater nuclear explosion. The activation products consist predominantly of 38Cl and 24Na, which decay to 5.56% and 0.0007% of their initial activity within 1 and 14 days. Monte Carlo techniques have been used to assess spectral interferences within this timeframe. It is demonstrated that during this period they do not interfere with the measurement of the existing radionuclide observables. Additionally, 24Na has been identified as useful for inspection purposes.
1. Introduction
141
Ce, 144Ce, 144Pr and 147Nd) and four gaseous radionuclides (131mXe, Xe, 133mXe, 37Ar). These radionuclides are predominantly fission products, with the exception of 134Cs and 37Ar produced by neutron capture of 133Cs and 40Ca. Neutron activation becomes more important in the underwater environment, as seawater contains dissolved amounts of the majority of the 92 naturally occurring elements (Wright and Colling, 1995). This creates radioactive species that have potential to interfere with the measurement of the radionuclide observables. 133
1.1. On-site inspection An On-site Inspection (OSI) is a key aspect of the verification regime of the Comprehensive Nuclear-Test-Ban Treaty (CTBT), which bans all nuclear explosions, whether for military, civil or any other purposes (UN, 1996). As conditions warrant, an OSI may be conducted in underground, underwater, sub-seafloor and atmospheric inspection environments (CTBTO, 2013), with the object of verifying whether a nuclear explosion has occurred in violation of the Treaty and to gather facts which might assist in identifying any possible violator. While the Treaty has not yet entered into force, and remains in provisional operations, primary emphasis has been on developing the OSI capability for an underground nuclear explosion (UNE) scenario (Burnett and Davies, 2015; Burnett and Milbrath, 2016; Burnett et al., 2016, 2017; Zucca, 2014). There remain technical challenges to undertaking an OSI in the underwater, or high seas environment. Among these, and the focus of this research, are the radionuclide observables suitable for OSI measurement that provide discrimination of an underwater nuclear explosion (often referred to as an UNDEX or underwater explosion) from other radiation sources, such as reactor incidents or releases, nuclear waste and medical isotopes. These have been defined for the UNE terrestrial scenario, as 17 particulate radionuclides (95Zr, 95Nb, 99 Mo, 99mTc, 103Ru, 106Rh, 132Te, 131I, 132I, 134Cs, 137Cs, 140Ba, 140La, ∗
Corresponding author. E-mail address: [email protected] (J.L. Burnett).
https://doi.org/10.1016/j.jenvrad.2018.06.007 Received 30 March 2018; Received in revised form 23 May 2018; Accepted 7 June 2018 0265-931X/ © 2018 Published by Elsevier Ltd.
1.2. Underwater nuclear explosions Of the 2057 nuclear explosive tests undertaken during 1945–2017, only 8 were conducted underwater during 1946–1962 (Table 1). This included 5 by the United States in the Pacific Ocean and 3 by the former Soviet Union in the Arctic Ocean (close to Novaya Zemlya). There are characteristic phenomena associated with an underwater nuclear explosion that depend on yield, device detonation and water depth, and the area of the water body. When a nuclear weapon explodes underwater, energy is transferred to the surrounding water mass by radiation. This causes superheating of the seawater, resulting in expansion and the formation of a shockwave. The shockwave propagates outward transferring approximately half of the explosion energy and leaving behind a bubble of steam at high pressure and temperature (Pritchett, 1971b). The bubble rises due to buoyancy, and expands until its pressure is below the ambient pressure (over expansion), and then contracts,
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timeframe from one week to two years after an alleged nuclear explosion, and be measurable in field conditions with no common or otherwise expected interferences that could allow misidentification. It is the latter requirement that is problematic in the underwater environment, where significant quantities of induced radionuclides may also be produced by neutron activation of the seawater. Whilst many of these elements also occur in terrestrial soils and rocks, their concentration (salinity) is typically an order of magnitude less than seawater, such that fewer activation products are generated. The initial inventory of OSI relevant radionuclides can be calculated using fission product yield data (England and Rider, 1994) in the manner described by the authors' previous research (Burnett et al., 2018). This depends on the yield, which for a fission-based explosion, is the amount of explosive energy (in trinitrotoluene equivalents) discharged from the detonation of fissionable nuclear material (e.g. 235U, 239 Pu). As one kT is equivalent to 1.45 × 1023 fissions, and if the fission probability of a particular fission product is f, and its decay constant is λ s−1, the activity following a 1 kT nuclear explosion is 1.45 × 1023 fλ Bq (Chamberlain, 2004). As the relevant radionuclides are produced directly by fission and indirectly by the decay of other fission products, a computer program called RadICalc (Radiation Intensity Calculator) was used for inventory calculations (Robinson et al., 2015). This computes initial radionuclide activities for all fission products (using independent fission yield) and undertakes complex decay and in-growth calculations using the Bateman's Laplace transform-based solution to the differential equations for radioactive decay (Bateman, 1910). RadICalc utilises nuclear data (version B-VII.1 for 239Pu pooled fast neutron fission decay) from the evaluated nuclear data file (ENDF) hosted by Brookhaven National Laboratory (NNDC, 2018). The exception to this approach, was the activation produced 134Cs, which can be derived by direct linear scaling of reported nuclear explosive test values (Battelle, 1972; IAEA, 2008). Also consistent with previous research (Burnett and Davies, 2015; Burnett and Milbrath, 2016; Burnett et al., 2016, 2017), a239Pu fission-based nuclear explosion with explosive yield of 1 kT was modelled.
Table 1 Historic underwater nuclear explosions. Test name
Country
Date
Bomb depth (m)
Yield (kT)
Baker Wigwam 22 (Joe 17) 48 Wahoo Umbrella 122 (Korall-1) Swordfish
USA USA USSR USSR USA USA USSR USA
25-Jul-46 14-May-55 21-Sep-55 10-Oct-57 16-May-58 8-Jun-58 23-Oct-61 11-May-62
27 610 10 30 150 50 20 198
20 30 3.5 10 9 9 4.8 < 20
thereby increasing its pressure and causing condensation. This process continues at a decreasing rate until the bubble erupts from the surface, releasing mostly gaseous fission products (of which iodine is the most abundant), and hurtling large masses of water aloft as the plume (Pritchett, 1971a). When the plume water falls back to the surface a radioactive cloud called the base surge is produced (Dolan, 1972). This travels outwards and is typically highly radioactive (Weary et al., 1981). This is followed by late-time explosion effects, such as residual upwelling along the explosion axis and turbulent diffusion of the radioactive surface pool. The explosive process typically results in three sources of radioactive contamination: airborne activity, residual fallout and water contamination. Notably, during the Wigwam nuclear explosion only 1% of the radioactivity was present in the airborne fraction with the potential to become fallout (Weary et al., 1981). This value is in agreement with observations for other underwater nuclear explosions (Lee, 1979).
2. Methodology 2.1. OSI relevant radionuclides The OSI relevant radionuclides are produced from a nuclear explosion either as fission or activation products (Table 2). They can provide discrimination from different nuclear sources, especially when used in combination, and in the instance of 137Cs/134Cs for reactor accidents. This is because 134Cs is produced predominantly by neutron capture of 133Cs (133Cs(n, γ)134Cs) due to a low fission yield (0.0000279% per fission). Thus nuclear explosion debris samples would be expected to have a low 137Cs/134Cs ratio. Other ratios are also useful (e.g. 95Zr/95Nb, 99Mo/99mTc) and can provide timing information relating to the explosion (Harms et al., 2009; Nir-El, 2006; Yamba et al., 2016). The list of radionuclides was chosen so as to encompass an OSI
2.2. Seawater activation Prompt neutrons and gamma-rays produced by an underwater nuclear explosion, are rapidly attenuated by the surrounding seawater and typically travel < 1 m (Weary et al., 1981). Water molecules (H2O) do not activate, as neutron capture of hydrogen (1H) and oxygen (16O) produces stable isotopes of deuterium (2H) and 17O. On the short-time scales of an underwater detonation, there is relatively little double capture to form radioactive tritium (3H) commonly associated with
Table 2 The OSI relevant radionuclides. Each radionuclide is categorized as volatile (V), refractory (R) or intermediate (V-R). The fission product yields are given per 100 fissions for239Pu pooled fast neutron fission decay. Nuclide
Half-life
E (keV)
I (%)
Ind. Yield
Cum. Yield
V or R
Activity
95
64.03 d 34.99 d 2.75 d 6.0 h 39.26 d 373.61 d 8.02 d 3.20 d 2.30 h 2.07 y 30.08 y 12.75 d 1.68 d 32.51 d 284.95 d 17.28 m 10.98 d
756.7 765.8 739.5 140.5 497.1 621.9 364.5 228.2 667.7 604.7 661.7 537.3 1596.2 145.4 133.5 696.5 91.1
54.4 99.8 12.1 89.1 91.0 9.9 81.7 88.0 98.7 97.6 85.1 24.4 95.4 48.3 11.1 1.3 27.9
9.30 × 10−2 1.92 × 10−4 1.31 × 10−2 1.49 × 10−6 1.29 × 10−3 3.85 × 10−1 2.01 × 10−2 3.00 1.74 × 10−1 Activation 1.00 9.17 × 10−1 9.87 × 10−3 2.46 × 10−4 1.64 × 10−1 5.10 × 10−5 1.72 × 10−3
4.67 4.67 5.98 5.26 6.83 4.36 3.88 5.15 5.33 Activation 6.58 5.32 5.33 5.15 3.67 3.67 1.99
R R R R V V V V V V V V-R V-R V-R R R R
1.8 × 1013 6.4 × 1010 5.6 × 1013 6.9 × 1010 3.8 × 1011 3.7 × 1016 2.9 × 1013 1.1 × 1016 1.1 × 1016 6.1 × 1011 1.1 × 1012 1.1 × 1015 6.9 × 1013 8.8 × 1010 7.3 × 1012 4.9 × 1013 1.8 × 1012
Zr Nb 99 Mo 99m Tc 103 Ru 106 Rh 131 I 132 Te 132 I 134 Cs 137 Cs 140 Ba 140 La 141 Ce 144 Ce 144 Pr 147 Nd 95
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Table 3 Estimated activity of induced radionuclides for a 1 kT underwater nuclear explosion. Note that35S and32P have no gamma-emissions. The relative proportion of each radionuclide is given by the ratio. Isotope
Half-life
Ratio
Activity (Bq)
Isotope
Half-life
Ratio
Activity
38
37.230 m 14.997 h 12.355 h 35.282 h 87.37 d 2.5789 h 82.93 m 12.701 h 18.642 d 14.268 d 26.24 h 4.536 d 64.849 d 14.10 h 1.6786 d
8.3 × 10−1 1.7 × 10−1 9.2 × 10−4 8.3 × 10−4 7.0 × 10−6 1.7 × 10−6 3.3 × 10−7 9.9 × 10−8 5.8 × 10−8 5.8 × 10−8 5.0 × 10−8 4.1 × 10−8 8.3 × 10−9 6.6 × 10−9 5.0 × 10−9
1.3 × 1019 2.7 × 1018 1.5 × 1016 1.3 × 1016 9.8 × 1013 2.7 × 1013 5.4 × 1012 1.6 × 1012 9.3 × 1011 1.1 × 1012 8.0 × 1011 6.7 × 1011 1.3 × 1011 1.1 × 1011 8.0 × 1010
165
2.334 h 56.4 m 65.976 h 24.000 h 243.93 d 2.0652 y 60.20 d 1925.28 d 44.56 d 44.495 d 119.78 d 27.701 d 83.79 d 46.594 d 249.83 d
3.3 × 10−9 3.3 × 10−9 1.7 × 10−9 1.7 × 10−9 8.3 × 10−10 8.3 × 10−11 8.3 × 10−11 3.3 × 10−11 3.3 × 10−11 1.7 × 10−11 9.1 × 10−12 8.3 × 10−12 8.3 × 10−12 4.1 × 10−12 1.7 × 10−13
5.3 × 1010 5.3 × 1010 2.7 × 1010 2.7 × 1010 1.3 × 1010 1.2 × 109 1.3 × 109 5.3 × 108 5.3 × 108 2.7 × 108 1.5 × 108 1.3 × 108 1.3 × 108 6.7 × 107 2.7 × 106
Cl Na 42 K 82 Br 35 S 56 Mn 139 Ba 64 Cu 86 Rb 32 P 76 As 47 Ca 85 Sr 72 Ga 140 La 24
Dy Zn 99 Mo 187 W 65 Zn 134 Cs 124 Sb 60 Co 115m Cd 59 Fe 75 Se 51 Cr 46 Sc 203 Hg 110m Ag 69m
nuclear reactors (18O is also stable). The probability of triple capture to form 19O or 16O undergoing high-energy neutron-proton reaction to form 16N is also low. It is the activation of ambient materials in the water that is substantial, and the activation products are produced largely within the volume of seawater vaporized by the explosion. This contribution can be estimated using linear scaling of reported UNE test values (as for the OSI relevant radionuclide 134Cs) with corrections applied to account for the different elemental composition of seawater. Additional activation products can also be extrapolated from neutron irradiation experiments of seawater and the relative proportions of induced radionuclides measured (Robertson and Carpenter, 1974). Using this approach, the activity of the main induced radionuclides in seawater for a 1 kT underwater nuclear explosion has been calculated (Table 3).
products on the measurement of the OSI relevant radionuclides. For these purposes a point source was modelled 50 mm from a Broad Energy Germanium (BEGe) detector (1500 mm3 crystal volume, 30% relative efficiency) with a 0.5 mm beryllium window (for low-energy Xray and gamma-ray transmission). Using this geometry, detector response functions were then modelled for the given radionuclides and activities. The simulated count time was 3600 s with normalisation to a total spectra count of 1 × 108 counts. Simulated spectra were then analysed using the Canberra Gamma Acquisition and Analysis software (version 3.3).
2.3. Mixing of relevant and induced radionuclides
Undertaking an OSI for an underwater nuclear explosion is especially challenging as the relevant radionuclides are quickly dispersed and diluted by the surrounding waters. Following the Wigwam nuclear explosive test, which had the highest yield (30 kt) and deepest depth (610 m) of any underwater test, the surface activity was largely dispersed within 4 days of the explosion (as measured using surveying instrumentation) (Weary et al., 1981). This reduction is a consequence of horizontal and vertical mixing in addition to radioactive decay. The reduction in maximum radiation (Rmax, mR/h) with time (t, days) can be approximated by Rmax = 130t−2 (Isaacs, 1955). Based on the Wigwam test data, this means that the signal is reduced to 4.57% after 1 day, 0.09% after 7 days and 0.02% after 14 days. This has implications for an OSI on the high seas. Terrestrial UNEs are often discussed within a timeframe of 1 week–2 years (Burnett and Davies, 2015; Burnett and Milbrath, 2016; Burnett et al., 2016, 2017). For a nuclear UNDEX, this timeframe is likely to be much more restricted and for the purposes of this research is limited to within 14 days of the detonation. To measure such a transient signal presents logistical difficulties, and would likely require the mounting of a ship-based inspection (CTBTO, 2013).
3. Discussion 3.1. A transient signal
Following nuclear detonation, both refractory (R) and volatile (V) radionuclides should be initially contained within the gaseous bubble. The induced radionuclides would also be contained within this volume, as they are produced within the vaporized volume that gives rise to the gaseous bubble. The high-boiling point refractory elements will condense at 1500–3000 °C whilst the volatile radionuclides will condense as the ambient temperature is approached (∼50 °C). Cooling occurs on short timescales due to the high specific heat capacity of seawater of 3993 J kg−1 K−1 at 20 °C and the bubble moves rapidly towards the surface. For example, for the 1955 Wigwam test the bubble traveled at 200 mph and reached the surface within 10 s (Weary et al., 1981). In addition, the contamination extended to a diameter of 3170 m, with 28% of contamination found above the thermocline and 72% below. The thermocline extended from 60 to 100 m depth (Weary et al., 1981). Hence, fractionation effects would be expected to be minimal within the contaminated waters produced. As such dispersion occurs on relatively rapid timescales and is locally contained, fission (Table 2) and activation (Table 3) product inventories can be assumed to be mixed and diluted equally in the surrounding waters. The exception is for the atmospheric fraction (1% of the activity measured during the Wigwam test) which would consist of predominantly volatile species such as 131I.
3.2. Radioactive decay The detonation of a239Pu device with a 1 kT explosive yield produces 6.0 × 1016 Bq of OSI relevant radionuclides (Table 2) and 1.6 × 1019 Bq of induced radionuclides from seawater activation (Table 3). The composition of the relevant radionuclides changes dramatically during the first 2 weeks (Fig. 1a) due to radioactive decay and in-growth from precursory fission products. This causes the activity to initially increase to 8.5 × 1016 Bq (141.1%) at 1 day after detonation, and then to decrease to 3.3 × 1016 Bq (54.3%) at 1 week and 1.5 × 1016 Bq (24.9%) at 2 weeks. During this time the composition of
2.4. Simulated spectra The calculated activities of OSI relevant radionuclides and seawater activation products were used with Monte Carlo software, Gamma Spectrum Generator Pro++ (Nucleonica, 2014), to provide gammaspectra at 1 day, 1 week and 2 week intervals. The purpose of the modelling was to understand the effects of the seawater activation 162
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Fig. 1. The OSI relevant radionuclides (a) and seawater activation products (b) from a239Pu device with a 1 kT explosive yield. The activation products 46Sc, 51Cr, 59 Fe, 60Co, 65Zn, 75Se, 85Sr, 110mAg, 115mCd, 124Sb, 134Cs and 203Hg, have been omitted for clarity. Their activity is approximately constant from the values given by Table 3 on the timescales shown. 32P and 35S have also been excluded as these are not gamma-emitting radionuclides.
most challenging conditions for performing gamma-spectrometry measurements. It is during this time there are the highest radionuclide activities, which provide difficulties for peak deconvolution and identification purposes.
the radionuclides also changes dramatically as the shorter half-life radionuclides (e.g. 99Mo/99mTc, 132Te/132I, 131I) decay and the longer half-life radionuclides (e.g. 137Cs, 134Cs, 144Ce) persist. For example, at 1 day the most abundant radionuclides are 99Mo (23.5%), 99mTc (20.9%), 132I (17.7%) and 132Te (17.2%) and at 2 weeks this has changed to 140La (17.2%), 140Ba (15.0%), 131I (11.6%) and 103Ru (10.2%). The induced radionuclides decrease at a faster rate (Fig. 1b) from 5.56% (1 day), to 0.01% (1 week) to 0.0007% (2 weeks) with almost complete decay of the shorter-lived radionuclides such as 38Cl, 56 Mn, 165Dy, 139Ba and 69mZn and persistence of the longer-lived induced radionuclides persist, such as 35S, 82Br, 86Rb and 32P. At 1 day the composition is dominated by 24Na (98.6%), 82Br (0.9%), 42K (0.4%) and 35 S (0.01%) and at 2 weeks it is 35S (81.4%), 82Br (16.9%), 32P (0.5%) and 86Rb (0.5%). Notably, the contribution from 38Cl and 24Na which account for 99.8% of the initial induced activity, decays almost completely within 2 weeks of the nuclear detonation. Within this 2 week period, and without considering dispersal and dilution effects, are the
3.3. Spectral interferences Simulated spectra show the effects of the induced radionuclides from seawater activation on the measurement of the OSI relevant radionuclides (Fig. 2). At 1 day the main seawater activation products are 24Na (1368.6 keV), 42K (1524.6 keV), 64Cu (511 keV), 69mZn (438.6 keV) and 82Br (multiple lines including 776.5 keV) with detection of 14 OSI relevant radionuclides including 95Nb, 95Zr, 99Mo, 99mTc, 103 Ru, 106Rh, 131I, 132I, 132Te, 140Ba, 140La, 141Ce, 144Ce and 147Nd. None of the activated radionuclides are direct interferences on the measurement of the OSI relevant radionuclides. The main detrimental effect is from the Compton continuum resulting from the 24Na peak at 163
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Fig. 2. Simulated spectra for samples containing OSI relevant radionuclides and seawater activation products after a 1 kT underwater nuclear explosion. Spectra are shown at 1 day (top), 1 week (middle) and 2 weeks (bottom). The energy of the OSI relevant radionuclides is denoted by the dashed vertical lines. The main activation products are also indicated.
1524.6 keV). Whilst the 38Cl has a higher initial activity (1.3 × 1019 Bq), its relatively short half-life (37.2 min) makes it largely unsuitable as an indicator except for the most immediate measurements (within 2–4 h). The other activation products occur at too lower activity (6.7 × 108 Bq to 9.3 × 1011 Bq) to be advantageous in addition to the 17 OSI relevant radionuclides. Beyond this timeframe, further radioactive decay and dispersion within the water column occurs (99.98% signal reduction within 14 days), such that a later stage OSI on the high seas is unlikely to be a viable option. Exceptions include a nuclear UNDEX conducted within relatively shallow waters (e.g. the 20 kT Baker nuclear test at Bikini Atoll in 1946), where significant radionuclide contamination of the underlying sediments might occur or in areas with very limited mixing. Alternatively, the detonation could be from beneath the seafloor, such that the OSI could be searching for radionuclide seepage into the water column from below. Importantly, the capability to detect the initial nuclear UNDEX is far greater than for nuclear explosions in any other medium. This is largely attributable to the efficient transmission of hydroacoustic and seismic signals generated by the shock wave, such that an explosive yield of just a few kilograms is detectable (Dahlman et al., 2011). Satellite observations and other national technical means (NTMs) may also facilitate surveillance and attribution (e.g. tracking ships).
1368.6 keV. This scattered gamma-radiation results in an elevated continuum (note the Compton edge at 1150 keV on Fig. 2a) that acts to degrade the detection sensitivity for lower-energy radionuclides (Burnett and Davies, 2013). This is apparent in the detection of 115 peaks at day 1, compared to 176 peaks and 167 peaks at 1 and 2 weeks respectively. At day 1, many of the peaks identifiable in the later spectra are concealed beneath the Compton continuum resulting from the 24Na. At 1 week the 24Na has undergone decay resulting in a significant reduction in the continuum. At this time, the main activation products are 24Na, 51Cr (320.1 keV), 82Br and 203Hg (81.6 keV) with an additional OSI relevant radionuclide (137Cs) being detectable. At 2 weeks, the 24Na is no longer detectable in the spectra, with the main activation products being 51Cr, 76As (559.1 keV) and 82Br. The OSI relevant radionuclide 106Rh is also not present at this time.
4. Considerations The inventory calculations and simulated spectra show that the detection of the OSI relevant radionuclides is largely unaffected by the induced radionuclides from seawater activation within the timescale of 1 day–2 weeks. This is because the activation products largely decay (e.g. 38Cl and 24Na), or those that remain (e.g. 35S and 32P) do not present spectral interferences to the OSI relevant radionuclides. Within this timeframe, the 24Na could be a useful indicator of a nuclear explosion, as its activity is within a similar order of magnitude (2.7× 1018 Bq to 4.8 × 1011 Bq) to the fission products produced (average 3.6 × 1015 Bq to 8.8 × 1014 Bq). This radionuclide is also well-suited to measurement by gamma-spectrometry, with a gamma-intensity of 100% at 1368.6 keV (and also 99.9% at 2754.0 keV although this exceeds the typical calibration range for a gamma-spectrometer). The 42K (1.5 × 1016 Bq to 9.6 × 107 Bq) would be useful to a lesser extent due to its lower activity and gamma-intensity for measurement (18% at
5. Conclusions The 17 particulate radionuclide observables, as defined for a terrestrial UNE, are suitable indicators of a nuclear UNDEX. This is despite large amounts of induced radionuclides generated by the neutron activation of seawater by the nuclear explosion. A total of 30 activation products have been considered, with 1.6 × 1019 Bq being generated by the detonation of a 1 kT 239Pu device. This compares to 6.0 × 1016 Bq 164
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of OSI relevant radionuclides. The activation products consist predominantly of 38Cl and 24Na, and decay rapidly to the extent they do not interfere with an OSI conducted within 1–14 days of the explosion. At 1 day the activated activity has decreased to 5.56% and at 2 weeks it has decreased to 0.0007%. During this timeframe the activation products do not present significant spectral interferences to the OSI relevant radionuclides. Furthermore, the relatively high activity of 24Na would make it useful as an additional UNDEX indicator. After 2 weeks the dispersion effects of horizontal and vertical mixing with the surrounding waters are likely to impede radionuclide detection for OSI purposes.
Chamberlain, A.C., 2004. Radioactive Aerosols. Cambridge Environmental Chemistry Series, UK. CTBTO, 2013. Model Text for the Draft OSI Operational Manual, Task Leader Paper. Comprehensive Nuclear-Test-Ban Treaty Organization, Austria. Dahlman, O., Mackby, J., Mykkeltveit, S., Haak, H., 2011. Monitoring Nuclear Explosions in the Oceans, Detect and Deter: Can Countries Verify the Nuclear Test Ban? Springer Netherlands, Dordrecht, pp. 111–127. Dolan, P.J., 1972. . Capabilities of Nuclear Weapons. Defense Nuclear Agency, United States. England, T., Rider, B., 1994. Evaluation and Compilation of Fission Product Yields. Los Alamos National Laboratory ENDF-349, LA-UR-94-3106. Harms, A., Johansson, L., MacMahon, D., 2009. Decay correction of 95Nb. Appl. Radiat. Isot. 67, 641–642. IAEA, 2008. Handbook of Nuclear Data for Safeguards: Database Extensions. International Atomic Energy Agency, Austria. Isaacs, J.D., 1955. Mechanism and Extent of the Early Dispersion of Radioactive Products in Water. Scripps Institution of Oceanography, University of California, USA. Lee, H., 1979. Scoping the Impact of Nuclear Bursts at Sea on the Environment. Defense Nuclear Agency, United States. Nir-El, Y., 2006. Dating the age of a recent nuclear event by gamma-spectrometry. J. Radioanal. Nucl. Chem. 267, 567–573. NNDC, 2018. Evaluated Nuclear Data File (ENDF). National Nuclear Data Center (NNDC). Brookhaven National Laboratory, USA. Nucleonica, 2014. Gamma Spectrum Generator Pro++, 3.0.66.0001 Ed. Nucleonica Nuclear Science Portal, Karlsruhe. Pritchett, J.W., 1971a. An Evaluation of Various Theoretical Models for Underwater Explosion Bubble Pulsation. Information Research Associates, USA. Pritchett, J.W., 1971b. Incompressible calculations of underwater explosion phenomena. In: Holt, M. (Ed.), Proceedings of the Second International Conference on Numerical Methods in Fluid Dynamics: September 15–19, 1970 University of California. Berkeley. Springer Berlin Heidelberg, Berlin, Heidelberg, pp. 422–428. Robertson, D.E., Carpenter, R., 1974. Neutron activation techniques for the measurement of trace metals in environmental samples. In: Technical Information Center, Office of Information Services. United States Atomic Energy Commission. Robinson, J.W., Dion, M.P., Eiden, G.C., Farmer, O.T., Liezers, M., 2015. RadICalc: a program for estimating radiation intensity of radionuclide mixtures. J. Radioanal. Nucl. Chem. 303, 1955–1960. UN, 1996. The Comprehensive Nuclear-test-ban Treaty. United Nations, Vienna, Austria. Weary, S.E., Ozeroff, W.J., L, S.J., Collins, B., Lowery, C.W., Obermiller, S.K., 1981. Operation Wigwam. Defense Nuclear Agency, United States. Wright, J., Colling, A., 1995. CHAPTER 6-THE SEAWATER SOLUTION, Seawater: its Composition, Properties and Behaviour, second ed. Pergamon, pp. 85–127. Yamba, K., Sanogo, O., Kalinowski, M.B., Nikkinen, M., Koulidiati, J., 2016. Fast and accurate dating of nuclear events using La-140/Ba-140 isotopic activity ratio. Appl. Radiat. Isot. 112, 141–146. Zucca, J.J., 2014. CTBT on-site inspections. In: AIP Conference Proceedings 1596, pp. 105–114.
Acknowledgements The views expressed here do not necessarily reflect the opinion of the United States Government, the United States Department of Energy, or the Pacific Northwest National Laboratory. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract DE-AC05-76RL01830. References Bateman, H., 1910. Solution of a system of differential equations occurring in the theory of radiactive transformations. Proc. Camb. Phil. Soc. 15, 423–427. Battelle, 1972. Plowshare Geothermal Steam Chemistry, Introductory Studies, AEC Research and Development Report. Battelle Pacific Northwest Laboratories, United States. Burnett, J.L., Davies, A.V., 2013. Compton suppressed gamma-spectrometry for comprehensive nuclear-test-ban treaty samples. J. Radioanal. Nucl. Chem. 295, 497–499. Burnett, J.L., Davies, A.V., 2015. On-site inspection for the radionuclide observables of an underground nuclear explosion. J. Radioanal. Nucl. Chem. 303, 2073–2079. Burnett, J.L., Milbrath, B.D., 2016. Radionuclide observables for the Platte underground nuclear explosive test on 14 April 1962. J. Environ. Radioact. 164, 232–238. Burnett, J.L., Miley, H.S., Bowyer, T.W., Cameron, I.M., 2018. The 2014 integrated field exercise of the comprehensive nuclear-test-ban treaty revisited: the case for data fusion. J. Environ. Radioact. 189, 175–181. Burnett, J.L., Miley, H.S., Milbrath, B.D., 2016. Radionuclide observables during the integrated field exercise of the comprehensive nuclear-test-ban treaty. J. Environ. Radioact. 153, 195–200. Burnett, J.L., Mueller, W.F., Milbrath, B.D., 2017. Evaluation of gamma-spectrometry equipment for on-site inspection. J. Radioanal. Nucl. Chem. 312, 377–383.
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