Exploding bridgewire detonators (EBWs) containing pentaerythritol tetranitrate (PETN) exposed to high temperatures may not function following discharge of the design electrical firing signal from a charged capacitor. Knowing functionality of these arbitrarily facing EBWs is crucial when making safety assessments of detonators in accidental fires. Orientation effects are only significant when the PETN is partially melted. The melting temperature can be measured with a differential scanning calorimeter. Nonmelting EBWs will be fully functional provided the detonator never exceeds 406 K (133 °C) for at least 1 h. Conversely, EBWs will not be functional once the average input pellet temperature exceeds 414 K (141 °C) for a least 1 min which is long enough to cause the PETN input pellet to completely melt. Functionality of the EBWs at temperatures between 406 and 414 K will depend on orientation and can be predicted using a stratification model for downward facing detonators but is more complex for arbitrary orientations. A conservative rule of thumb would be to assume that the EBWs are fully functional unless the PETN input pellet has completely melted.
Exploding bridgewire detonators (EBWs) containing pentaerythritol tetranitrate (PETN) exposed to high temperatures may not function following discharge of the design electrical firing signal from a charged capacitor. Knowing functionality of these arbitrarily facing EBWs is crucial when making safety assessments of detonators in accidental fires. Orientation effects are only significant when the PETN is partially melted. Here, the melting temperature can be measured with a differential scanning calorimeter. Nonmelting EBWs will be fully functional provided the detonator never exceeds 406 K (133 °C) for at least 1 h. Conversely, EBWs will not be functional once the average input pellet temperature exceeds 414 K (141 °C) for a least 1 min which is long enough to cause the PETN input pellet to completely melt. Functionality of the EBWs at temperatures between 406 and 414 K will depend on orientation and can be predicted using a stratification model for downward facing detonators but is more complex for arbitrary orientations. A conservative rule of thumb would be to assume that the EBWs are fully functional unless the PETN input pellet has completely melted.
Large rocket motors may violently explode when exposed to accidental fires. Even hot metal fragments from a nearby accident may penetrate the propellant and ultimately cause thermal ignition. A mechanistic understanding of heated propellants leading to thermal runaway is a major unsolved problem. Here we show that thermal ignition in propellants can be predicted using a universal cookoff model coupled to a micromechanics pressurization model. Our model predicts the time to thermal ignition in cookoff experiments with variable headspace volumes. We found that experiments with headspace volumes are more prone to deformation which distorts pores and causes increased permeability when the propellant expands into this headspace. Delayed ignition with larger headspace volume correlates with lower headspace pressures during decomposition. We found that our predictions matched experimental measurements best when the initial propellant was impermeable to gas flow rather than being permeable. Similar behavior is expected with other energetic materials with rubbery binders. Our model is validated using data from a separate laboratory. We also present an uncertainty analysis using Latin Hypercube Sampling (LHS) of thermal ignition caused by a steel fragment embedded in the propellant.
Understanding titanium particle combustion processes is critical not only for characterizing existing pyrotechnic systems but also for creating new igniter designs. In order to characterize titanium particle combustion processes, morphologies, and temperatures, simultaneous spatially-resolved electric field holography and imaging pyrometry techniques were used to capture post-ignition data at up to 7 kHz. Due to the phase and thermal distortions present in the combustion cloud, traditional digital in-line holography techniques fail to capture accurate data. In this work, electric field holography techniques are used in order to cancel distortions and capture the three-dimensional spatial locations and diameters of the particles. In order to estimate the projected surface temperatures of the titanium particles, an imaging pyrometry method that ratios emission at 750 and 850 nm is utilized. Using these diagnostics, joint statistics are collected for particle size, morphology, velocity, and temperature. Results show that, early in the combustion process, the titanium particles are primarily oxidized by potassium perchlorate inside the igniter cup, resulting in projected surface temperatures near 3000 K. Later in the process, the particles interact with ambient air, resulting in lower surface temperatures around 2400 K and the formation of flame zones. These results are consistent with adiabatic flame temperature predictions as well as particle morphology observations of a titanium core with a TiO2 surface. Late stage particle expansion, star fragmentation, and molten droplet breakup events are also observed using the time-resolved morphology and temperature diagnostics. These results illustrate the different stages of titanium particle combustion in pyrotechnic environments, which can be used to inform improvements in next-generation igniters.
Hobbs, Michael L.; Britt, Phillip F.; Hobbs, David T.; Kaneshige, Michael J.; Minette, Michael; Mintz, Jessica; Pennebaker, Frank M.; Parker, Gary R.; Rosenberg, David; Schwantes, Jon; Williams, Audrey; Pierce, Robert
In previous work, commercially available downward facing exploding bridgewire detonators (EBWs) were exposed to elevated temperatures. These detonators were then initiated using a firing set which discharged a high amplitude short duration electrical pulse into a thin gold bridgewire. Responses of the detonators were measured using photonic doppler velocimetry (PDV) and high-speed photography. A time delay of 2 μs between EBW initiation and first movement of an output flyer separated operable detonators from inoperable detonators or duds. In the current work, we propose a simple method to determine detonator operability from the calculated state of the detonator at the time the firing set is initiated. The failure criterion is based on the gap distance between the exploding bridgewire (EBW) and the adjacent initiating explosive within the detonator which is low-density pentaerythritol tetranitrate (PETN) that melts between 413–415 K (140–142 °C). The gap forms as PETN melts and flows to the bottom of the input pellet. Melting of PETN is modeled thermodynamically as an energy sink using a normal distribution spread over a temperature range between the onset temperature of 413 K and the ending temperature of 415 K. The extent of the melt is determined from the average temperature of the PETN. The PETN liquid is assumed to occupy the interstitial gas volume in the lower part of the input pellet. The vacated volume from the relocated liquid forms the gap between the EBW and the PETN. The remaining sandwiched layer consists of solid PETN particles and gas filling interstitial volume. We predict that a threshold gap between 17–27 μm separates properly functioning detonators from duds.
Operability thresholds that differentiate between functional RP-87 exploding bridge wire (EBW) detonators and nonfunctional RP-87 EBW detonators (duds) were determined by measuring the time delay between initiation and early wall movement (function time). The detonators were inserted into an externally heated hollow cylinder of aluminum and fired with current flow from a charged capacitor using an exploding bridge wire (EBW initiated). Functioning detonators responded like unheated pristine detonators when the function time was 4 μs or less. The operability thresholds of the detonators were characterized with a simple decomposition cookoff model calibrated using a modified version of the Sandia Instrumented Thermal Ignition (SITI) experiment. These thresholds are based on the calculated state of the PETN when the detonators fire. The operability threshold is proportional to the positive temperature difference (ΔT) between the maximum temperature within the PETN and the onset of decomposition (∼406 K). The temperature difference alone was not sufficient to define the operability threshold. The operability threshold was also proportional to the time that the PETN had been at elevated temperatures. That is, failure was proportional to both temperature and reaction rate. The reacted gas fraction is used in the current work for the reaction correlation. Melting of PETN also had a significant effect on the operability threshold. Detonator failure occurred when the maximum temperature exceeded the nominal melting point of PETN (414 K) for 45±5 s or more.
This is the Sandia report from a joint NSRD project between Sandia National Labs and Savannah River National Labs. The project involved development of simulation tools and data intended to be useful for tritium operations safety assessment. Tritium is a synthetic isotope of hydrogen that has a limited lifetime, and it is found at many tritium facilities in the form of elemental gas (T2). The most serious risk of reasonable probability in an accident scenario is when the tritium is released and reacts with oxygen to form a water molecule, which is subsequently absorbed into the human body. This tritium oxide is more readily absorbed by the body and therefore represents a limiting factor for safety analysis. The abnormal condition of a fire may result in conversion of the safer T2 inventory to the more hazardous oxidized form. It is this risk that tends to govern the safety protocols. Tritium fire datasets do not exist, so prescriptive safety guidance is largely conservative and reliant on means other than testing to formulate guidelines. This can have a consequence in terms of expensive and/or unnecessary mitigation design, handling protocols, and operational activities. This issue can be addressed through added studies on the behavior of tritium under representative conditions. Due to the hazards associated with the tests, this is being approached mainly from a modeling and simulation standpoint and surrogate testing. This study largely establishes the capability to generate simulation predictions with sufficiently credible characteristics to be accepted for safety guidelines as a surrogate for actual data through a variety of testing and modeling activities.
Transforming polymorphs, melting, and boiling are physical processes that can accelerate decomposition rates during cookoff of PETN and make measurements difficult. For example, splashing liquids from large bubbles filled with decomposition products clog pressure tubing in sealed experiments. Boil over can also extinguish thermal excursions in vented experiments making ignition difficult. For better measurements, we have modified the Sandia Instrumented Thermal Ignition (SITI) experiment to obtain better sealed and vented cookoff data for PETN by reducing the sample size and including additional gas space to prevent clogged tubing and boil over. Ignition times were not affected by 1) increasing the gas space by a factor of 3 in sealed SITI experiments or by 2) venting the decomposition gasses. That is, thermal ignition of PETN is not pressure dependent and the rate-limiting step during PETN decomposition likely occurs in the condensed phase. A simple decomposition model was calibrated using these observations and includes rate acceleration caused by melting and boiling. The model is used to predict internal temperatures, pressurization, and thermal ignition in a wide variety of experiments. The model is also used with SITI data to estimate the previously unreported latent enthalpy (5 J/g) associated with the α (PETN-I) to β (PETN-II) polymorphic phase transformation of PETN.
Cookoff experiments of powdered and pressed TATB-based plastic bonded explosives (PBXs) have been modeled using a pressure-dependent universal cookoff model (UCM) in combination with a micromechanics pressurization (MMP) model described in a companion paper. The MMP model is based on the accumulation of decomposition gases at nucleation sites that load the surrounding TATB crystals and binder. This is the first cookoff model to use an analytical mechanics solution for compressibility and thermal expansion to describe internal pressurization caused by both temperature and decomposition occurring within closed-pore explosives. This approach produces more accurate predictions of ignition time and pressurization within high-density explosives than simple equation-of-state models. The current paper gives details of the reaction chemistry, model parameters, predicted uncertainty, and validation using experiments from multiple laboratories with errors less than 6 %. The UCM/MMP model framework gives more accurate thermal ignition predictions for high density explosives that are initially impermeable to decomposition gases.
We have completed a series of both vented and sealed cookoff experiments of black powder and smokeless powder in our Sandia Instrumented Thermal Ignition (SITI) apparatus at bulk densities of 1078 and 729 kg/m3, respectively. The confining aluminum cylinder was ramped from room temperature to a set point temperature and then held at the setpoint temperature until ignition. The setpoint temperatures varied between 495 to 523 K for the black powder and 401 to 412 K for the more sensitive smokeless powder. The vented experiments show a significant delay in thermal ignition, indicating that the ignition is dependent on pressure. Post experimental debris shows greater violence for our smokeless powder experiments than our black powder experiments. A simplified universal cookoff model (UCM) was calibrated using the black powder and smokeless powder SITI data and used to predict pressurization and thermal ignition. The current work presents the first calibration of the UCM with a double base propellant. This work also presents the first pressure-dependent cookoff model for black powder and smokeless powder.
We have completed a series of vented and sealed cookoff experiments of the ε-polymorph of CL-20 in our Sandia Instrumented Thermal Ignition (SITI) apparatus using both powder and pressed pellets at nominal densities of 313 ± 8 kg/m3 and 1030 ± 4 kg/m3, respectively. The boundary temperature of our aluminum confinement cylinder was ramped in 10 minutes from room temperature to a prescribed set-point temperature ranging between 448 nd 468 K and held at the set-point temperature until ignition. A universal cookoff model (UCM) has been calibrated using the ε-CL-20 SITI data to predict pressurization and thermal ignition of ε-CL-20. The ignition model was validated by using one-dimensional time-to-explosion (ODTX) ignition data from a different laboratory. We found that a thirtyfold increase in the reaction rates due to liquefaction at 520 K could explain the high temperature ODTX cookoff data. The model gives a plausible explanation of why melting is important in fast cookoff events involving CL-20. Our model also gives support to 520 K as the liquefaction point of CL-20, which has different values in the literature.
The solubility of RDX (hexahydro-1,3,5-tri-nitro-1,3,5-triazine) in TNT (2,4,6-trinitrotoluene) at elevated temperatures is required to accurately predict the response of Comp-B3 (60:40 RDX:TNT) during accidents involving fire. As the temperature increases, the TNT component melts, the RDX partially dissolves in the liquid TNT, and the remaining RDX melts (203 ∘C) as the Comp-B thermally ignites. In the current work, we used a differential scanning calorimeter (DSC) to estimate the solubility of RDX in TNT at the melting point of RDX. Most DSC measurements of Comp-B3 do not show an RDX melt endotherm. The absence of an endotherm associated with the RDX melt has been interpreted as RDX being completely dissolved in TNT before reaching the melting point. We have observed that the endotherm is not absent, but is masked by exothermic reactions occurring at these elevated temperatures. We have inhibited the exothermic reactions by venting our DSC samples and measuring the RDX melt endotherm in our Comp-B3 samples at about 203 ∘C. Using the measured heat flow associated with the RDX melt and the latent melting enthalpy of RDX, we have approximated the solubility of RDX in TNT to be roughly 50–100 g RDX per 100 g TNT. The broad range is based on corrections for exothermic reactions occurring as the RDX melts.
Thermal conductivity has been determined for a variety of energetic materials (EMs) using finite element analysis (FEA) and cookoff data from the Sandia Instrumented Thermal Ignition (SITI) experiment. Materials studied include melt-cast, pressed, and low-density explosives. The low-density explosives were either prills or powders with some experiments run at pour density (not pressed). We have compared several of our thermal conductivities with those in the literature as well as investigated contact resistance between the confining aluminum and explosive, multidimensional heat transfer effects, and uncertainty in the thermocouple bead positions. We have determined that contact resistance is minimal in the SITI experiment, the heat transfer along the midplane is one-dimensional, and that uncertainty in the thermocouple location is greatest near the heated boundary. Our values of thermal conductivity can be used with kinetic mechanisms to accurately predict thermal profiles and energy dissipation during the cookoff of explosives.
Validated models of melt cast explosives exposed to accidental fires are essential for safety analysis. In the current work, we provide several experiments that can be used to develop and validate cookoff models of melt cast explosives such as Comp-B3 composed of 60:40 wt% RDX:TNT. We present several vented and sealed experiments from 2.5 mg to 4.2 kg of Comp-B3 in several configurations. We measured pressure, spatial temperature, and ignition time. Some experiments included borescope images obtained during both vented and sealed decomposition. We observed the TNT melt, the suspension of RDX particles in the melt, bubble formation caused by RDX decomposition, and bubble-induced mixing of the suspension. The RDX suspension did not completely dissolve, even as temperatures approached ignition. Our results contrast with published measurements of RDX solubility in hot TNT that suggest RDX would be completely dissolved at these high temperatures. These different observations are attributed to sample purity. We did not observe significant movement of the two-phase mixture until decomposition gases formed bubbles. Bubble generation was inhibited in our sealed experiments and suppressed mixing.