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Sandia monitors nuclear safety of mission to Mars

NASA’s Mars Science Laboratory spacecraft, sealed inside its payload fairing atop a United Launch Alliance Atlas V rocket, clears the tower at Space Launch Complex 41 at Cape Canaveral Air Force Station in Florida. (Photo courtesy of United Launch Alliance)

The Mars Science Laboratory is just beginning its eight-month journey to Earth’s neighbor after a successful launch Friday, Nov. 26, but for the past five years, a team of Sandia engineers has been working behind the scenes to ensure its smooth launch.

NASA’s $2.5 billion MSL rover, the largest and most sophisticated vehicle to visit the Red Planet, is powered by a multi-mission radioisotope thermoelectric generator, or MMRTG. The generator turns heat from the decay of 10.6 pounds of plutonium dioxide into 110 watts of electricity to move the rover and run a suite of 10 instruments, which can do things like find water 32 feet below the surface and analyze chemical composition of rocks three car-lengths away.

While the MMRTG significantly increases the rover’s range and lifetime from previous rovers, which relied on solar panels, launching nuclear material requires diligent attention to safety, and Sandia has been tasked with the tremendous responsibility of conducting the safety analysis report. Since 2006, Sandia engineers have analyzed millions of combinations of potential scenarios to ensure risks to people, animals, and the environment were minimal as the Atlas V rocketed out of Earth’s atmosphere.

The first time the US launched a nuclear battery was on a satellite in 1961, less than eight weeks after Alan Shepard became the first American in space. Every launch of nuclear material since requires DOE to perform and write up a rigorous safety analysis, which is then sent to the Office of the President for final launch approval. Sandia was selected by DOE in 2006 to conduct the safety analysis for MSL and all future nuclear missions.

“We look at the probabilities of all the different accidents that could happen. Because each event can happen at a particular time and a different way, we simulate the trajectory of a launch. There are parameters that represent those times and ways, and we randomly select each of these every time we run the code. We run the code more than a million times, so we build up a large statistical database,” says Ron Lipinski (6223), team leader for Sandia’s safety analysis report. Ultimately, Sandia provides probabilities of risk to decision makers. Sandia does not make the decision to launch.

Built with safety in mind

The MMRTG is built with safety in mind. The marshmallow-sized plutonium pellets are encased in four layers of iridium and graphite, designed to withstand heat from a fire or reentry. Plutonium-238 dioxide was a deliberate choice; the alpha particles it gives off can be stopped by a sheet of paper. In fact, about the only way it could pose a health risk is if it’s ground into fine particles and inhaled. It is manufactured in ceramic form, and if something goes wrong, it is designed to break into large chunks, which would drastically minimize environmental hazards.

While the risk of a launch failure is small, and the chance of any plutonium being released is even smaller, accidents do happen, so the model simulates anomalies in every part of the launch sequence, including rocket trajectory, accident times, explosions and fires, debris impact, and orbital reentry. The team uses the Launch Accident Scenario Evaluation Program (LASEP) to analyze how the plutonium-powered generator would respond to a given incident. Sandia’s experts in blast and impacts, fire and thermal, reentry dynamics, health physics, atmospheric transport, and contamination work together to develop a robust picture of any potential risks.

In the event of an explosion, the blast and subsequent impacts could damage the fuel and its casings, so one team was focused on running hundreds of scenarios over the course of several years to understand how the fuel and fuel containment structures would react.

“We track how much plutonium-238 would be released and what form it would be released in, should there be a blast or impact. We then provide that data to LASEP, which combines many different scenarios to perform its calculations,” says team leader John Bignell (6223), who came to Sandia from NASA’s Jet Propulsion Laboratory, where he did structural analysis for the rover suspension and chassis. “It’s pretty amazing to think that something you had direct contact with ends up on another planet, and I was excited to get the opportunity to continue working on this project when I came to Sandia.”

A long history of testing

Launching something as large as the MSL required tons of rocket fuel, so fire is another hazard, particularly in the first 50 seconds after launch, when the rocket is still relatively close to Earth. Tim Bartel (6223) led a team to analyze the risks of high temperatures on nuclear cargo, either from an explosion or accidental reentry of the rocket.

“Liquid propellant makes a big explosion, but it’s over quickly and doesn’t really do much damage. Solid propellant is different. It’s reliable and provides the extra thrust needed, but it can cause a really hot-burning environment, and the temperature can reach 3,000 Kelvin, which is hot enough to vaporize plutonium and present health hazards,” Tim says. “Sandia has a long history of testing and characterizing burns, and we do thousands of calculations over the course of several years to better understand and mitigate those risks.”

All of those calculations are used to determine the source term analysis, which is how much plutonium could be released to the atmosphere and the particle size distribution. “We are trying to analyze things that rarely happen, and we dig into the details of those rare occurrences. Then we have multiple layers of people looking at those results because of the high level of scrutiny and the possible impacts to the public. So we have to do a good job and show that we do a good job, too,” says Daniel Clayton (6223), team lead for the source term analysis.

Nathan Bixler (6223), who also conducts safety analysis for commercial nuclear reactors, led the team to understand consequences, including how the release could be transported by wind and how it would affect the public. He used Daniel’s analysis and combined that with historical meteorological data, which is considered to be a good indication of future weather patterns.

“The chance of an accident is low; even lower is the chance that plutonium would be released, and the chance that human health could be affected by an accident is fractions of a percent,” Nathan says. “If an accident were to happen, we calculate that a little over a square kilometer would be contaminated which is likely to be confined to Kennedy Space Center and the Cape Canaveral Air Force Station, and could be cleaned up without any impact to the surrounding area.”

A few members of the Sandia team went to Florida for the launch; Ron was part of the Radiological Control Center at Mission Control, which included representatives from NASA, DOE, Lawrence Livermore National Laboratory, and state and local agencies in Florida, to do rapid response in case of an accident.

Sandia will monitor the safety of future missions, and the team is thrilled to be a part of this effort. Greg Lucas (6223) was hired after an internship with the group, and this is his third year with the program. “It’s a great project to be a part of so early in my career,” Greg says. “It’s nice to know that your analysis does help this launch proceed, and you feel that you have a part in making the MSL mission happen.”

 “This is an opportunity to be part of history, and to be a part of this mission is wonderful,” says Ron. “We’ve gotten some tremendous insights into other planets, as well as our own. It’s very exciting.”