Launch and Reentry of Space Nuclear Systems AC 450.45-1 69 Guideline Rationale Planned movements to lower orbits after operating fission systems at power are identified and analyzed during the nuclear safety analysis.
Intentionally moving an SNS closer to Earth shortens the natural orbital decay period; in the event of a failure this would minimize the time for the SNS to return to Earth, resulting in more fission products onboard when an uncontrolled reentry occurs. These risks are quantified and analyzed to ensure that an accurate assessment of the overall risk to the public is understood at the time the mission is licensed.
SNS are analyzed for the effects of corrosion.
Corrosion is a major concern for SNS, and materials science expertise is applied to determine the correct materials for each component to avoid premature or unexpected material failure. For example, in the case of a lithium heat-pipe fast reactor SNS at elevated operating temperatures, any contaminants within the lithium heat pipe can accelerate corrosion and heat pipe failure (Obal 2011).
SNS are analyzed for the effects of material fatigue.
In addition to possible low-cycle and high-cycle metal fatigue, in the case of a fission power reactor used in long transport missions, neutron embrittlement will raise the DBTT over time and may also contribute to metal fatigue (Finckenor and de Groh 2020), (Obal 2011).
SNS are analyzed for the effects of outgassing.
Outgassing can change the material properties or have undesirable effects on structural components.
For example, when choosing refractory materials in an SNS, designers are aware of vacuum welding and contaminant outgassing (Obal 2011).
SNS are analyzed for radiation effects on materials and electronics.
In addition to DBTT, in the case of polymers, particulate radiation exposure can lead to chain scission or cross-linking, causing polymer embrittlement (Finckenor and de Groh 2020). In the case of electronics, radiation-induced single event upsets, latch-ups, and bit errors can occur (Finckenor and de Groh 2020). Once again, material science is applied to the design of the SNS to choose suitable materials for each component.
SNS are analyzed for the effects of shock.
The explosive force of a launch mishap can range from thousands of kilograms TNT-equivalent up to one thousand tons TNT-equivalent (Camp, et al.
2019).
Launch and Reentry of Space Nuclear Systems AC 450.45-1 70 Guideline Rationale SNS are analyzed for the effects of thermal cycles.
The cyclic stress from thermal cycles can contribute to material failures. Depending upon the mission orbit, as many as 16 spacecraft thermal cycles a day could take place (Finckenor and de Groh 2020).
Material failure increases the risk to the public, as it can create a path for radioactive material to reach the environment.
SNS are analyzed for the effects of vibration.
Vibration can harm structural and internal components of SNS and may be caused by the spacecraft or the SNS. For instance, in the case of an SNS using fuel particles, the vibration analysis accounts for the possibility of damage to or shifting of the fuel particles, resulting in reactor plugging, or local coolant flow blockage (Idaho National Laboratory 2015).
B.2.9 Guidelines Applicable to SNS Validation, Verification and Testing.
Validation, verification, and testing of the SNS are a key step to ensure the as-built device performs like the as-designed device. In this context, testing an actual nuclear system provides the ability to generate actual test data, but naturally presents greater risk to the surrounding people and property than a computer simulation or mathematical model, since the nuclear system is actually operating. During this process, applicants may discover that the actual performance differs significantly enough to impact the safety analysis conclusions. To aid applicants in developing a robust validation, verification, and testing program, the guidelines in Table 14 of this AC are provided.
Table 13 - Guidelines for SNS Validation, Verification, and Testing Guideline Rationale Validate, verify, and test SNS to ensure the as-constructed system and its host spacecraft are within design specifications.
Validating the constructed SNS meets the design requirements minimizes the probability of failure leading to the release of radioactive material to the environment under normal operating conditions and minimizes the probability of the physical barriers to radioactive release failing during abnormal or accident conditions, thus minimizing the risk to the public and environment (IAEA 1999).
Launch and Reentry of Space Nuclear Systems AC 450.45-1 71 Guideline Rationale SNS have well understood operating characteristics for anticipated operational occurrences.
Significant modeling, simulation, validation, verification and testing will be conducted prior to launch approval to ensure that SNS can be operated safely within expected operational parameters (NSPM-20 2019), (IAEA 1999).
The verification and testing of SNS should demonstrate structural integrity for anticipated operational occurrences and design-basis accidents.
Design-basis accidents ensure the SNS can withstand the most severe failures expected. For systems to be operated in Earth orbit, the designbasis accidents include uncontrolled reentry. The risk to the public can be minimized by proper design, informed by modeling, and testing of SNS.
SNS to be operated in orbits that naturally decay to Earth before the activity of the system will decay to about the activity of uranium-235 have overall system reliabilities meeting or exceeding those of highly reliable flight safety systems,
0.999 at 95% confidence (SPD- 6 2020), (Title 14 CFR Part 450 2022).
SNS are designed and manufactured with the reliability standards of highly reliable flight safety systems to minimize the risk to the public if an SNS is not able to remain on orbit. Because SNS have significant public safety risks, they are designed to be as reliable as the most reliable systems on the spacecraft.
To the maximum extent practical, minimize critical operations and testing for a fission based SNS before arriving at a safe orbital altitude (SPD-6 2020).
Starting up a nuclear fission reactor increases the quantity and concentration of shorter lived, higher energy emitting radioactive isotopes and increases the potential source term and radiological risk to the public (Fullwood 1992). In general, fission products pose more exposure risk to the public than nuclear fuels, therefore the amount of fission products in the system is minimized to the extent practical prior to launch (IAEA 2021). Minimizing fission product generation will lessen radiation levels after shutdown and thus the risk to the public.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 72 B.2.10 Guidelines Applicable to Mission Planning for SNS Launch.
B.2.10.1 Launch is a time of particularly high risk to the SNS and the public. Applicants are encouraged to consider the full spectrum of design and operations concepts to identify the one that best minimizes the risk to the public. Guidance for planning SNS launch is provided in Table 15 of this AC. The areas overflown during launch determine the potential locations for the debris field impact during a launch mishap. The populations and land use of the overflown areas, as well as the nuclear proliferation concerns of the overflown areas drive the risk to the public from the SNS crashing during launch.
Because NSPM-20 directs the nuclear safety analyses to consider any subsequent stages of SNS operation, which “may result in radiological effects on the public or the environment,” the overflight areas are relevant to determining the overall risk of the SNS (NSPM-20 2019).
B.2.10.2 The vehicles used for launch, operation, and reentry of an SNS directly affect the risk to the public, as a vehicle failure significantly increases the risk of a nuclear system failure. Additionally, overall vehicle reliability, vehicle control and contingency management capabilities can greatly influence the safety of a launch attempt, including if the launch is unsuccessful. For example, the controllability of a liquid-fueled vehicle might minimize the stress on an SNS payload, or a vehicle with a thrust termination system may mitigate safety risk more effectively than a vehicle with a destruct system.
Table 14 – Guidelines for SNS Launch Planning Guideline Rationale Risk to the public is minimized when the SNS launch vehicle is highly reliable.
Highly reliable launch vehicles are more likely to have launch success, and less likely to expose the public to ionizing radiation.
Maximize overflight of oceans during launch.
Crashing into the ocean versus onto land minimizes the risk to individuals and the cleanup costs of the event, due to the radiation shielding and contamination dilution effects of large bodies of water (National Institute of Standards and Technology 2019).
For a given SNS and launch vehicle, the risk to the public can be changed through the selection of different launch sites and azimuths. Proper risk mitigation for SNS includes a comprehensive review to identify the right combination to best accomplish the mission objectives while minimizing risk.
It is prudent for launch applicants to consider a variety of launch sites and azimuths for launches with SNS to make an informed, riskbased decision when planning missions.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 73 Guideline Rationale When considering alternative launch sites, it is best to include comparisons for nominal, off nominal, and flight abort system triggering flight profiles. Each profile will produce a unique probability of exposure to the public and contamination of land, which can aid applicants in determining the most appropriate launch site.
It is possible that the risk to the public for a site might be lower for one specific flight profile, but the composite risk from all flight profiles is higher. For example, it would be inappropriate to select a launch site with the lowest risk for a nominal flight profile if that site also had the highest composite risk when all possible flight profiles are considered.
Radiological risk is just one part of the overall risk of the mission, consideration is also given to:
• Any hazard from a launch vehicle, vehicle component, payload or radioactive material that can reach any protected area at any time during flight.
• A failure of the launch vehicle that would have a high consequence to the public.
• Launch vehicle failures that could place severe stresses on an SNS, increasing the probability of a nuclear system failure.
Evacuations are a method to reduce the radiation exposure risk to the public, but the practicality varies. If an applicant intends to evacuate an Emergency Planning Zone (EPZ) as determined during the NRC licensing or support for launch authorization to reduce risk during launch, the following information is relevant:
• The amount of risk reduction gained by the evacuation;
• The duration of the evacuation;
• The impacted population;
• The land use in the EPZ;
• The local jurisdictional boundaries within the EPZ;
• The willingness and ability of local authorities to enact an evacuation; and The economic impact of the evacuation.
Evacuations are an effective way to reduce the radiological risk to individuals but can be costly and cumbersome to enact. Sheltering is often preferable to evacuations, and evacuating people also does nothing to reduce the hazard of ground contamination, and thus could have only a slight impact to the MPL. For evacuation to be a viable risk reduction strategy, evidence is needed that the evacuation tangibly reduces the risk to the public, and that the evacuation can be carried out at a cost commensurate to the risk reduction.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 74 B.2.11 Guidelines Applicable to Mission Planning for SNS Operations in Space.
Mission planning plays an outsized role in determining the risk of unintentional or uncontrolled reentry of an SNS. Reentry mechanisms vary, but the consequences of unsuccessful reentry can be severe, and are best avoided by mindful planning (Gummer, et al. 1980), (NSPM-20 2019). Guidelines for planning SNS destinations are found in Table 16 of this AC.
Table 15 – Guidelines for Planning SNS Operations in Space Guideline Rationale The safest place for an SNS to operate is a location with no potential to return to Earth.
If the system cannot return to Earth, the risk of radiation exposure to the public on the ground or ground contamination is eliminated.
Plan missions with SNS to minimize the probability of nuclear material having an uncontrolled reentry.
Mission planning helps determine the best operational orbits to minimize the possibility of the SNS returning to Earth in an uncontrolled reentry due to loss of operational control, collision with other space systems or orbital debris, or atmospheric drag.
SNS missions are planned to operate above the highest altitude where the spatial density of objects exceeds 1x10-8 objects per km3 at launch or is expected to exceed 1x10-8 objects per km3 during the mission lifetime.
In orbits where orbital density is high, the risk of collision and subsequent uncontrolled reentry is also high. In lower, more densely occupied altitudes, orbits also decay faster, providing less time for radioactive decay to reduce the radiation exposure hazard of the SNS. Consistent with national policy, these risks are minimized by operating SNS at sufficiently high altitudes in accordance with the principles in SPD-6 Section 3 to avoid space traffic and extend the uncontrolled reentry timeline (SPD-6 2020).
Missions using SNS with a scheduled Earth flyby are planned to not approach Earth closer than the highest orbit where the spatial density exceeds 1x10-8 objects per km3.
Earth flybys in general present unique challenges in that they are not routinely tracked with an orbital Element Set the way orbital spacecraft are tracked as they continuously orbit the Earth (18th Space Control Squadron 2020).
Spacecraft performing a flyby of Earth have a hyperbolic trajectory and depend primarily on the owner operator’s assessment of position as it enters the Earth’s gravitational pull. The possibility of collision with another orbiting object is always greater in congested regions. This can cause damage to the SNS worse yet, an unintended reentry into the Earth’s atmosphere. The public safety risk during flybys is minimized when they are planned to avoid orbital altitudes with the heaviest concentrated satellites and debris.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 75 B.2.12 Guidelines Applicable to Mission Planning for SNS EOM.
Applicants should plan SNS missions with a cradle to grave philosophy that includes the long-term disposal plan for the nuclear system at the end of its useful life (IAEA 1999). This EOM plan informs the design of the overall system design and operation, and guidelines for developing the EOM plan are provided in Table 17 of this AC.
Table 16 – Guidelines for Planning SNS EOM Guideline Rationale Intentional reentry for disposal on Earth is riskier than disposal in space.
The chance of radiation exposure to the public is minimized when the disposal location has no possibility of natural decay while the activity of the fission products is greater than the activity of uranium-235 (SPD-6 2020).
SNS that utilize fission are not intentionally reentered after operation at power.
Hundreds to thousands of years may be required for fission product decay to reduce the activity of a fission reactor to about the activity of uranium-235, as needed for reentry (SPD-6 2020). Planning on deorbiting this far in the future is unrealistic, impractical and neglects the long-term in-space storage that will need to occur.
Preferably plan SNS disposal locations with no possibility of return to Earth to minimize the risk to the public.
Disposal in a decaying Earth orbit is riskier than disposal in a location that has no likelihood of future return to Earth such as L4 and L5 (Cornish 2020).
Maximize overflight of oceans during reentry and target an oceanic landing point.
Crashing into the ocean versus onto land minimizes the risk to individuals and the cleanup costs of the event, due to the radiation shielding and contamination dilution effects of large bodies of water (National Institute of Standards and Technology 2019).
Evaluate planned de-orbits to minimize the risk that restricted or embargoed nations, as defined in 10 CFR §110.28, could acquire the nuclear material during or after de-orbit.
Nuclear proliferation is a significant threat to global security and stability (National Nuclear Security Administration 2021). Careful measures are taken to minimize the probability of commercial space activity providing nuclear material to embargoed or restricted nations, and during the safety review the FAA reviews these measures to ensure the launch satisfies the legal requirements to protect the national security and foreign policy interests of the United States (Title 14 CFR Part 450 2022).
Launch and Reentry of Space Nuclear Systems AC 450.45-1 76 Guideline Rationale SNS disposal for Tier I devices in Earth orbit may involve controlled reentries or raising maneuvers into disposal orbits.
Mission planners account for disposal in spacecraft design and strive to place aging SNS in orbits of sufficient altitude to minimize the chance of orbital decay. Although less desirable, mission planners may also consider controlled reentries of the SNS to decommission and dispose of the system. Orbital decays present a risk to all resident space objects in the decay path.
For planned de-orbits select reentry sites that minimize the risk of exposing the public to radiation if there is a reentry failure, consistent with ALARA (as low as reasonably achievable).
Predetermined reentry sites (over water or land) are planned with a significant buffer on either side of the reentry location to allow for unintended reentry failures to protect the environment and population. If, in the event the de-orbit is conducted over a land mass for payload retrieval, considerations for radioactive environmental cleanup and remediation may become paramount if any radioactive material is released during the reentry or landing, and the MPL may be significant.
Evaluate planned de-orbit sites to minimize the risk that restricted or embargoed nations, as defined in 10 CFR, could acquire the nuclear material during or after de-orbit.
The U.S. Government updates and publishes a list of restricted or embargoed nations as necessary due to unfavorable economic or political circumstances between the two countries (Title 10 CFR Chapter I 2021). Hostile nations or groups gaining control of an SNS during or after reentry could harm the national security or foreign policy interests of the U.S.
Minimize the risk by selecting sites well clear of restricted or embargoed nations.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 77 B.2.13 Guidelines Applicable to SNS Launch Operations.
Launch operations with SNS merit additional attention and steps to minimize the risk of the public being exposed to ionizing radiation, or the launch facility being contaminated with radioactive material. To aid applicants in identifying these additional steps, the guidelines of Table 18 of this AC are provided.
Table 17 – Guidelines for SNS Launch Operations Guideline Rationale SNS are installed as late in the launch countdown as possible.
The largest amount of propellant is present at spacecraft launch, presenting an explosion hazard. By delaying system installation until absolutely essential in the launch countdown, the time window for the SNS to be exposed to this explosion risk is minimized.
Also, nuclear safety requirements for storage are proven effective for maintaining custody of nuclear material. Minimizing the time with modified security requirements and physical barriers gives the greatest assurance of the nuclear material remaining in custody of the licensed owner.
Launch collision risks are fully evaluated and every effort is taken to minimize the risk of collision during launch.
Collision during or shortly after launch could lead to damaged reentry of the SNS and subsequent public radiation exposure or radioactive contamination. Therefore, it is recommended that the most thorough launch collision risk calculation process possible is used for SNS launches. At the time of this AC, the most thorough approach utilizes multiple screening methods for launch time planning. Specifically, probability of collision screening should be done for all human spaceflight objects as well as active payloads. Debris objects should be screened for both probability of collision and standoff distance violations.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 78 Guideline Rationale Meteorological conditions from the time of launch through departure from Earth’s atmosphere play a significant role in determining the spread of the debris field and radioactive debris, and ultimately in the radiological risk associated with the launch. Meteorological conditions vary with the launch site, and daily at the launch site, not all meteorological conditions at a given launch site may be appropriate for SNS launch.
Considering the impact of meteorological conditions to nuclear risks may result in a different determination of acceptable meteorological conditions for a launch with a nuclear payload than an otherwise identical launch without a nuclear payload.
The nuclear safety analyses are sufficiently detailed for operators to quantify how wind and weather impact the risk to the public, providing justification for the weather commit criteria for launch of the SNS.
B.2.14 Guidelines Applicable to SNS Operations in Space.
SNS operations, and the operations of their host spacecraft mission can increase the likelihood of an uncontrolled reentry, particularly for maneuvers to lower orbits or returns to Earth’s orbit from interplanetary orbits. Guidelines for planning in-flight maneuvers to minimize risks to the public are provided in Table 19 of this AC.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 79 Table 18 – Guidelines for SNS Operations in Space Guideline Rationale SNS that will linger in Earth orbit are on spacecraft with sufficient maneuverability that the space operator can keep the risk of collision less than 1x10-6 while in orbit.
Earth orbits are becoming increasingly congested, necessitating in-flight maneuvers to avoid collision on an increasingly frequent basis. Consequently, SNS are designed with the capability to evade imminent collisions to minimize the risk of collision and radioactive material release.
Earth gravity assist fly-bys are performed at sufficient distance that the risk of collision with a tracked space object remains below 1x10-6.
Earth flybys in general present unique challenges in that they are not routinely tracked with an orbital Element Set the way orbital spacecraft are tracked as they continuously orbit the Earth (18th Space Control Squadron 2020). Spacecraft performing a flyby of Earth have a hyperbolic trajectory and depend primarily on the owner operator’s assessment of position as it enters the Earth’s gravitational pull.
The possibility of collision with another orbiting object is always greater in congested regions. This can cause damage to the SNS worse yet, an unintended reentry into the Earth’s atmosphere. The public safety risk during flybys is minimized by staying far enough from Earth to minimize the risk of collision.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 80 B.2.15 Guidelines Applicable to SNS Operations at EOM.
SNS disposal is non-trivial, and in the case of terrestrial disposal can significantly add to the public safety risk of the SNS. Specific guidelines for safely completing EOM disposal are provided in Table 19 of this AC.
Table 19 – Guidelines for EOM Disposal Guideline Rationale Ensure SNS that are disposed in Earth orbit are passivated at EOM.
Passivation minimizes the risk of on-orbit explosions and prevents any possible reactivation by malicious actors (The United States Government 2019).
Perform planned de-orbits only if it can be proven that the nuclear fuel and physical barriers to radioactive release are intact.
Having intact fuel and physical barriers to release of radioactive material before de-orbit are crucial to ensure the remaining radioactive material is not released to the environment and populated areas during reentry or on impact.
Prior to reentry, perform analysis to ensure adequate heat removal while remaining safe to touch (~140°F) after landing.
Reentering a system with a temperature too high to handle and move to a safe location is a significant health hazard and nuclear material security challenge, even if the system releases no radioactive material to the environment. Inadequate SNS heat removal can result in temperatures that are thousands of degrees (Nuclear Regulatory Commission 1979). Heat removal methods and rates will be significantly different between the space and Earth environments. Also, the space environment subjects any spacecraft to extreme temperature variations depending on whether the spacecraft is in direct sunlight or eclipse and is different on Earth.
Proper design will give adequate heat removal capabilities regardless of the EOM disposal plan.
Radiative cooling via high thermal flux, which may be effective in space, is generally not a viable heat removal option on Earth.
Prior to reentry, verify heat removal systems that are at least single-fault tolerant if they rely on means other than convective and radiative losses to ambient to protect personnel and SNS integrity.
A fault-tolerant design enables a system to continue its intended operation, rather than failing completely, when some part of the system fails, even if at a reduced level. SNS design includes the ability to remove heat even if a prescribed means of heat removal fails to prevent overheating the SNS and causing a radioactive release to the environment.