Explosives are a popular choice among terrorists for causing disruption, casualties and destruction. Although chemical, biological, radiological and nuclear (CBRN) weapons may cause much more damage, explosives can still be the first choice because they are relatively easy to make, transport and use. The Department of Homeland Security (DHS) Science and Technology Directorate (S&T) wants to make sure that state and local leaders have choices, too, by arming them with technology to plan for worst-case scenarios and mitigate the fallout of terrorist attacks.
“Explosives work and are effective weapons,” said Dr. David Reed, a chemist at S&T’s Chemical Security Analysis Center (CSAC). “The trade-off between difficulty to build and use versus the ability to cause harm is a decision that a terrorist probably considers. On the flip side, a decision-maker like a mayor might ask, ‘What are the most damaging scenarios that can occur and how can I protect against them?’ An analysis that considers the different buildings, explosive types, injuries and medical response can provide the information that begins to answer that question.”
With the very real possibility of explosive attacks in public spaces—like stadiums, special event routes, airports, places of worship, theaters, and others—the U.S. needs to be prepared by having security measures in place to stop them and effective medical response for attacks that do occur. With limited resources, knowing which security measures to use and which medical capabilities to emphasize are critical questions to answer before an attack happens.
S&T’s CSAC is developing a modeling tool called Homeland Explosive Consequence and Threat (HExCAT) that estimates the hazard and related human health consequences from thousands of plausible scenarios. HExCAT is currently focused on single event assessments of special events such as parades. After validation and further development, it will be integrated into national- and regional-level risk analysis.
“HExCAT is a holistic risk assessment that informs decision-makers like governors and mayors how to invest in security, plan for operations and mitigation, and make important decisions for securing public spaces,” said Reed, who works with CSAC’s Chemical Threat Characterization Team. “If a terrorist were to detonate a bomb in a building vs. a bomb during a marathon vs. a car bomb near a stadium, what physical security and medical countermeasures will be most effective?”
HExCAT is part of a suite of hazard and response models
HExCAT is based on the Homeland Security Presidential Directive-19, which establishes a national policy and requires the development of a national strategy and plan for combatting terrorist use of explosives in the U.S.
This tool is standalone, but it is planned to be part of a suite of models called the Countermeasure Assessment and Planning Tool (CAPT WEB). These models, covering CBRN hazards, help federal agencies analyze threats, vulnerabilities and consequences of potential attacks to prioritize resources for the most effective defense and response. S&T collaborated with the Department of Defense, the Federal Emergency Management Agency, and the Federal Bureau of Investigation, among others, on the development of HExCAT.
When considering a terrorist attack, thinking of only CBRN leaves out a frequently-used tactic by terrorists.
“The obvious missing piece was explosives—we needed to create something that could easily integrate with the other models and fill in the missing piece,” said Reed. “With the suite of CAPT WEB models, we can compare different types of weapons of mass destruction attacks against each other to prioritize planning decisions. Then with HExCAT specifically, we can assess specific scenarios in terms of consequence, chokepoints in the response, or particular vulnerabilities.”
HExCAT helps cities calculate hazards and medical response
HExCAT maintains a library of 25 different types of military and homemade explosives including combinations of fuels and oxidizers, which increase the effect of the explosion. The tool can also model different scenarios based on a variety of indoor and outdoor public spaces. For general areas, such as city centers or parade routes, HExCAT can simulate city layouts and buildings to model potential damage to representative buildings and determine medical response according to local capabilities.
HExCAT also estimates the human health impact from the simulated explosions and determines the predicted medical response based on local capabilities. A key feature is that its medical mitigation model is fully integrated with the blast effects and injury model. The medical mitigation model can estimate the effectiveness of the medical response using EMT and hospital data (typical equipment, coordinates, trauma center level, number of beds, etc.) for both immediate onsite care and hospital-based care, and the logistical weak points connecting the two. While tracking the delivery of medical care in critical time, the model will look for alternative treatments if the preferred treatments are unavailable or exhausted. By capturing the severity of injuries and tracking the personnel, countermeasures and facility resources throughout the simulation, potential chokepoints in the system could be identified and improvements can be made. So, the current ability to respond, both onsite and in the hospital, is captured for every scenario.
“Scenarios can be assessed proactively before major events. The specific locality, staging and location of the event could indicate some explosives and explosive configurations could be more damaging than others,” Reed explained. “We can model the blast, overpressure, thermal and fragmentation effects, as well as building failure and collapse,”
Reed pointed out that the tool could identify particularly impactful scenarios to guide mitigation and security strategies, such as barrier placement, package and person screening, and the medical resources needed if an explosion occurred. All these measures are designed to limit the severity of an attack.
Integration with the medical mitigation model allows a clear assessment of the critical onsite and hospital-based medical response. In a real emergency, if victims are seriously wounded, they have very little time. So, the availability of equipment, location of medical response and location of trauma centers is critical. For special events, HExCAT can assist with an assessment of the required resources needed to respond to an attack and provide lifesaving operations in time.
It only takes milliseconds to run a single simulation of a venue, and a full range of variables that characterize a venue can be run in a few minutes.
“A very important difference between HExCAT and more traditional, single scenario, models lies in probabilistic treatment of key variables that define a scenario, including building and device configuration,” said CSAC Research Scientist Alex Dolan.
Dolan explained that CSAC is not modeling a single explosive or the path of shrapnel but thousands of different explosion scenarios to predict consequences from an event, including variations in building construction, device placement, device enhancements (such as nails or ball bearings), population density, barriers, etc. This way, HExCAT can identify the most damaging sets of conditions that lead to maximum casualties. Focusing on reducing or eliminating these conditions can improve public safety.
It is important to note that HExCAT does not calculate risk. DHS considers a terrorist risk assessment as including a terrorist decision model—the what, how and where they would strike—as well as potential for failure or interdiction by law enforcement at any step in the process. In the future, HExCAT could be integrated with S&T’s Probabilistic Analysis for National Threats Hazards and Risk program to generate a full risk assessment for explosives.
HExCAT will work with decision makers
Ultimately, HExCAT can help decision makers evaluate the potential severity of an event and the impact of various response and mitigation strategies.
“We can’t know exactly what a terrorist will do, so we need to plan for all of the reasonable possibilities,” said Dolan. “By analyzing the impact from the thousands of possibilities, we can work with officials to identify opportunities to eliminate or minimize the results of an explosive attack.” And this can help save more lives.
CSAC is very interested in working with local planners to validate the model and evaluate its capabilities for modeling specific localities and/or events.
“We are looking for actual explosive cases to see how the model compares to reality and adjust the model as needed to better reflect reality,” said Reed. “Once validated, decision makers like mayors or police chiefs could ask CSAC to run HExCAT and adjust it for their specific cities’ first response and medical response capabilities depending on various aspects like medical infrastructure and transportation systems. They could also ask to modify the parameters of scenarios to simulate the effects of additional hazards.”
After the validation, CSAC will finalize the medical mitigation component and integrate HExCAT into the CAPT platform. Future plans include adding advanced analytics to allow tracking medical resource use, time resolution for scenarios, and refinement of device and material characteristics. HExCAT is expected to be available for use mid-2021.