530 Nuclear Nonproliferation Policy: General

A Comparison of Activation Products in Different Types of Urban Nuclear Melt Glass

Joshua J. Molgaard1, John D. Auxier II2,3, Howard L. Hall2,3,4

1United States Military Academy, Science Building 753, West Point, NY, 10996, [email protected] 2Department of Nuclear Engineering, University of Tennessee, Knoxville, TN, 37996, [email protected] and [email protected] 3Radiochemistry Center of Excellence (RCOE), University of Tennessee, Knoxville, TN, 37996 4Institute for Nuclear Security, 1640 Cumberland Avenue, Knoxville, TN, 37996, [email protected]

metropolitan area. The details of the emplacement INTRODUCTION scenario are not specified. In each case the device is assumed to produce a yield equivalent to 1 kiloton of Countering nuclear proliferation is one of the primary TNT explosives. Both 235U and 239Pu are considered as challenges facing our nation and the world today. fuels. Other device components (e.g. tamper, initiator, Scientists and engineers working in the nuclear industry wiring) are not included. All constituents are assumed to have a vested interest in this issue, as do politicians, mix equally within the fireball and be distributed government officials, and members of the armed forces. uniformly within the debris. This may seem to be an over- The academic community can also contribute to nuclear simplification, however, if the same assumptions are used non-proliferation efforts by conducting research and for each modeling prediction the comparisons should training experts in the fields of radiochemistry, nuclear remain valid. forensics, and nuclear security. The Radiochemistry The fuel quantities are calculated in accordance with Center of Excellence (RCOE), established at the the method developed by Giminaro et al [2]. For uranium University of Tennessee (UT) and funded by the National fueled devices the mass of fuel in a 1-gram melt glass Nuclear Security Administration (NNSA), is training sample is estimated at 66.7 micrograms. For plutonium students and developing new analysis techniques to the fuel mass is set at 21.3 micrograms. These improve the timeliness of nuclear forensics results. To calculations are based on significant quantities reported support these efforts UT has developed and patented by the International Atomic Energy Agency [4]. It is methods for creating surrogate nuclear melt glass for assumed that the fuel mass is a fixed quantity and the forensic analysis [1]. Most recently, researchers at UT efficiency of the weapon determines the yield. It is also have designed an analytical method for developing urban assumed that the mass of melt glass produced is directly matrix formulations to be used in the synthesis of urban proportional to the yield of the device [5]. nuclear melt glass surrogates [2]. The work presented here focuses on modeling efforts designed to predict and Irradiation compare activation products found in urban nuclear melt glass produced by notional events in two different For each sample FAT is used to initiate a very short metropolitan areas within the United States. reactor run in SCALE 6.1 and the output is analyzed using the f71 file analyzer in FAT. For a 1-gram sample a MODELING scaled yield of 2.67x10-9 kilotons is desired [2]. This is obtained by running the reactor for 1 microsecond at For this study the methods developed by Giminaro et. 1.24x104 megawatts. For each model a 27 group neutron al. [2] were employed to predict the elemental spectrum (27GrpSCALE6) is used and the usable energy composition of nuclear debris produced by events in New per fission is set at 180 MeV. York, NY (NYC) and Houston, TX. For each city, the Two cities (NYC and Houston) are modeled. For composition of a notional 1-gram sample was entered into each city both fuels are considered, and for each fuel two the Fallout Analysis Tool (FAT) which is run in concert ORIGEN libraries (fast and thermal) are used. This results with SCALE 6.1 [3] to generate fission and activation in a total of eight distinct models. The thermal reactor products in the sample. Only the activation products are runs are included for comparison to synthetic nuclear melt analyzed in this study. Differences in the compositions of glass samples which will be irradiated at the High Flux debris matrices developed for NYC and Houston are Isotope Reactor (HFIR) using the pneumatic tube system. expected to lead to different radioactive signatures. The fast library is clearly more appropriate for simulating a nuclear weapon. Scenarios

For both cities the nuclear detonation is modeled as a surface burst with ground zero located at the center of the

Transactions of the American Nuclear Society, Vol. 112, San Antonio, Texas, June 7–11, 2015 Nuclear Nonproliferation Policy: General 531

RESULTS Table II. Top Five Activation Products (AP) by Activity at t=24 hours (after detonation) The elemental composition of each urban matrix is NYC Houston shown in Table I. The Houston matrix has a higher silicon Fuel Library AP Act. (bq/g) AP Act. (bq/g) and calcium content while the NYC matrix is enriched in 235 24 10 37 10 sodium, iron, and magnesium by comparison. The U Fast Na 1.76x10 Ar 8.54x10 37 24 elemental differences observed here will give rise to Ar 1.64x1010 Na 7.06x1010 unique radioactive signatures for each scenario. 42 42 K 1.88x109 K 5.50x109

32 8 32 9 Table I. Urban Matrix Compositions [2] P 6.11x10 P 2.88x10 47 47 Element NYC Houston Sc 3.52x108 Sc 1.56x109 24 24 Si 6.05E-01 6.38E-01 Therm. Na 8.19x109 Na 2.04x109 42 42 K 3.55E-02 2.20E-02 K 1.13x109 K 6.98x108 37 37 Al 1.51E-01 1.79E-01 Ar 1.36x108 Ar 1.60x108 56 31 Ca 6.35E-02 7.48E-02 Mn 9.26x107 Si 8.71x107 31 56 Na 3.23E-02 7.51E-03 Si 8.36x107 Mn 4.54x107 239 24 37 Fe 7.63E-02 6.23E-02 Pu Fast Na 3.81x1010 Ar 4.17x1010 37 24 Mg 2.65E-02 8.69E-03 Ar 3.54x1010 Na 3.45x1010 42 42 S 6.02E-04 6.58E-04 K 4.06x109 K 2.68x109 32 32 Ba 5.61E-04 8.60E-04 P 1.32x109 P 1.40x109 47 47 Mn 1.16E-03 5.61E-04 Sc 7.62x108 Sc 7.57x108 24 24 P 9.66E-04 2.85E-04 Therm. Na 1.33x1010 Na 3.32x109 42 42 Ti 6.08E-03 6.03E-03 K 1.84x109 K 1.13x109 235 37Ar 8 37Ar 8 U 6.67E-05 6.67E-05 2.22x10 2.60x10 56 31 239 Mn 8 Si 8 or Pu 2.13E-05 2.13E-05 1.52x10 1.42x10 31Si 8 56Mn 7 Total 1.00E+00 1.00E+00 1.36x10 7.38x10

24 For each of the eight distinct models the activities of Activation Product Na

the top five activation products are reported at t=24 hours 24 after the detonation. The results are shown in Table II. The Na found in each notional sample may be Seven different activation products are seen here, produced in a variety of ways. The primary production route for thermal neutron irradiations is the (n, γ) reaction however, the top five and the order of precedence vary 23 24 depending on the details of each model. Table III lists the which converts stable Na to Na with a half-life of 15 likely production routes for the seven activation products hours. The comparison between NYC and Houston considered in this study. samples is thus straightforward for thermal neutron 37 irradiations (the NYC matrix contains more sodium and The Ar which appears in each model is produced by 24 neutron bombardment of 40Ca via an (n, α) reaction. The more Na as expected). This holds true for both uranium 37 40 and plutonium fueled models. However, for samples quantity of Ar produced depends on the quantity of Ca 37 produced using uranium fuel and fast neutron irradiations in the matrix, and the Houston samples have higher Ar the results may seem surprising at first glance. The cross activities then the corresponding NYC samples, as section for the (n, γ) reaction is very small for fast neutron expected. However, noble gases, such as argon, produced energies, and the higher activity of 24Na in the Houston during a surface burst are likely to be dispersed and 37 samples irradiated with fast neutrons can be explained by carried away from ground zero by winds. Thus, the Ar considering another production route for 24Na. Neutron content of nuclear melt glass is expected to be much 27 24 lower than what is reported here. Only a small fraction bombardment of Al can produce Na via an (n, α) may be trapped within vesicles formed in the melt glass. reaction and the cross section for this reaction is much For this reason 37Ar is not given further consideration in larger for fast neutron energies (see Table III). Because the Houston matrix contains more aluminum, the higher this study. The remaining activation products are 24 235 discussed individually in the following sections. Na activity in the U/Fast/Houston samples is reasonable if this second production route is assumed to

Transactions of the American Nuclear Society, Vol. 112, San Antonio, Texas, June 7–11, 2015 Transactions of the American Nuclear Society, Vol. 112, San Antonio, Texas, June 7–11, 2015 532 Nuclear Nonproliferation Policy: General dominate. For plutonium fueled models a similar Once again, for models using plutonium fuel and the fast argument may explain why the 24Na activities in the NYC neutron library, the activation product quantities deviate and Houston samples are closer than expected (based on from the general trend. This is attributed to the significant elemental comparison alone). Differences in the neutron differences in the shapes of the prompt fission neutron spectra produced by the two fuels will give rise to other spectra of 235U and 239Pu for fast incident neutron energies variations between the samples. These subtleties will be [6]. analyzed in a future study. The prevalence of calcium and potassium in the environment leads to the inevitability of 42K production. Activation Product 42K However, the 12.4 hour half-life of 42K will make it important primarily for analysis of early sample The production of 42K arises primarily from one of collections (within a few days after the detonation). the two reactions shown in Table III. The (n, γ) reaction dominates at thermal neutron energies while the (n, p) Activation Product 32P reaction dominates for fast neutrons. For thermal irradiations, using either uranium of plutonium fuel, the 32P only makes the top five list in samples produced NYC samples contain more 42K than the Houston via fast neutron irradiation, and the 32P activity is samples. This follows directly from the higher potassium consistently higher in the Houston samples. It appears that content of the NYC matrix. While the natural abundance the primary production route at fast neutron energies is of 41K is only 6.8 percent, the relatively large cross neutron capture by 32S and subsequent emission of a section for the (n, γ) reaction, where 41K is the target, proton to form 32P. The Houston matrix contains more provides sufficient compensation. The natural abundance sulfur and it follows that more 32P will be found in the of 42Ca is also small (less than 1 percent) yet at fast Houston samples. With a 14.3 day half-life, 32P may be an neutron energies there is a small contribution from both important radioisotope in samples analyzed up to several production routes which together produce sufficient weeks after the detonation. quantities of 42K. It is also possible that other production routes, which are not considered here, contribute to the Activation Product 56Mn total quantity of 42K (and other radioisotopes). For models using uranium fuel and the fast neutron 56Mn is produced primarily by neutron bombardment library the dominance of the (n, p) reaction, where 42Ca is of 55Mn via an (n, γ) reaction with a substantial cross the target, leads to a higher 42K content in the Houston section for thermal neutrons (see Table III). This explains samples. This follows from the higher calcium content in why 56Mn only appears in the top five for samples the Houston matrix. The natural abundance of 42Ca is also produced via thermal neutron irradiation, and why the small (less than 1 percent) yet at fast neutron energies activity is highest in the NYC samples (the NYC matrix there is a small contribution from both production routes contains more manganese than the Houston matrix). It which together produce sufficient quantities of 42K. should be noted that the forensic utility of 56Mn may be limited due to its short half-life. Table III. Production Routes for Activation Products with Cross Sections for Thermal and Fast Neutron Energies Activation Product 31Si AP t1/2 Production Cross Section (barns) 31 -2 7 Reactions 2.53x10 eV 1.40x10 eV The discussion of Si is straightforward. This 24 23 24 -1 -4 radioisotope is produced primarily by neutron Na 15.0 h Na(n, γ) Na 5.28x10 2.30x10 bombardment of 30Si via the (n, γ) reaction shown in 27Al(n, α)24Na 0.00 1.23x10-1 Table III. 30Si is one of three stable isotopes of silicon and 37Ar 35.1 d 40Ca(n, α)37Ar 2.40x10-3 1.53x10-1 has a natural abundance of approximately 3.1 percent. It follows that the 31Si activity is slightly higher in the 42K 12.4 h 41K(n, γ)42K 5.16x10-1 1.03x10-5 Houston samples. The short half-life will limit the 42 42 -1 Ca(n, p) K 0.00 2.04x10 importance of 31Si for samples analyzed more than 24 32P 14.3 d 31P(n, γ)32P 1.70x10-1 3.00x10-4 hours after the detonation. 32 32 -1 S(n, p) P 0.00 2.54x10 47 47 47 47 -1 Activation Product Sc Sc 3.35 d Ti(n, p) Sc 0.00 1.46x10 56Mn 2.58 h 55Mn(n, γ)56Mn 1.35x101 6.15x10-4 The production of 47Sc is likely due to fast neutron 47 47 56Fe(n, p)56Mn 0.00 1.17x10-1 capture by Ti. The natural abundance of Ti is 31 30 31 -1 -4 approximately 7.4 percent and the titanium mass fraction Si 2.62 h Si(n, γ) Si 1.10x10 6.00x10 is similar in both matrices studied here. It is not surprising 31P(n, p)31Si 0.00 9.00x10-2 that 47Sc only appears in the top five for the samples

Transactions of the American Nuclear Society, Vol. 112, San Antonio, Texas, June 7–11, 2015 Nuclear Nonproliferation Policy: General 533

produced using fast neutron irradiation. Furthermore, the MORILLON, F. J. HAMBSCH, J. C. SUBLET, “235U, quantities of 47Sc found in samples containing platinum 233U, and 239Pu Prompt Fission Neutron Spectra,” Journal fuel are comparable, as expected based on the elemental of the Korean Physical Society, 59, 2 (August 2011). composition of the NYC and Houston matrices. However, for the samples containing uranium fuel, it is not clear why the 47Sc activity is significantly higher in the Houston sample. Additional analysis of this activation product may be worthwhile as its 3.35 day half-life will allow it to linger for up to a few weeks after the detonation.

CONCLUSIONS

The modeling and analysis presented in this study reveal that different urban environments will give rise to nuclear melt glass samples with distinct activation product signatures. It is also clear that the neutron energy spectrum is a key parameter. In order to produce realistic radioactive signatures in synthetic nuclear melt glass samples the irradiation settings must be carefully controlled to compensate for the inherent differences between nuclear reactors and nuclear weapons. As the surrogate melt glass production process is further developed and refined, the irradiation parameters and specific fuel characteristics will be important considerations. Future studies will focus on modeling of both fission and activation products in order to motivate potential variations in sample chemistry and adjustments to the activation procedure.

REFERENCES

1. J. MOLGAARD, J. D. AUXIER II, A. GIMINARO, C. J. OLDHAM, M. COOK, S. YOUNG, H. HALL, “Development of Synthetic Nuclear Melt Glass for Forensic Analysis,” J. Radioanal. Nucl. Chem., DOI 10.1007/s10967-015-3941-8 (20 January 2015). 2. A. GIMINARO, A. STRATZ, J. GILL, J. P. AUXIER, C. J. OLDHAM, M. COOK, J. D. AUXIER II, J. MOLGAARD, H. HALL, “A Method for Development of Synthetic Urban Nuclear Melt Glass for Rapid Forensic Analysis,” J. Radioanal. Nucl. Chem., DOI 10.1007/s10967-015-4061-1 (21 March 2015). 3. I. GAULD, G. RADULESCU, G. ILAS, B. MURPHY, M. WILLIAMS, D. WIARDA, “Isotopic Depletion and Decay Methods and Analysis Capabilities in SCALE,” Nuclear Technology, Volume 174, pgs 169-195 (May 2011) 4. INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA), "IAEA Safeguards Glossary", 1st Ed., Vienna, Austria (2001). 5. S. GLASSTONE, P. DOLAN, Effects of Nuclear Weapons, 3rd ed. United States DOD/DOE (1977). 6. V. M. MASLOV, N. A. TETEREVA, V. G. PRONYAEV, A. B. KAGALENKO, K. I. ZOLOTAREV, R. CAPOTE, T. GRANIER, B.

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