Launch and Reentry of Space Nuclear Systems AC 450.45-1 37 A.2.2 The purpose of the SNS device and mission description is for applicants to communicate to the FAA what the SNS mission is and how the SNS will be operated.
This information enables the FAA to determine the completeness of subsequent nuclear safety analyses. If the applicant considers other information relevant to the overall SNS design or mission, that information may also be included in the SNS device and mission description. There is no need to include nuclear safety analyses or public safety calculations in the SNS design and mission description, as this information is included in subsequent nuclear safety analysis reports.
A.3 RADIOLOGICAL MATERIAL REPORT.
A.3.1 All applications for SNS should include a RMR, as the type and quantity of radioactive material onboard is the foundation for determining the public safety risk of the SNS and the Tier level of the SNS. For every radionuclide on a launch or reentry vehicle, the RMR:
• Identifies the specific isotope, quantity, total activity and A2 value;8
• Specifies the moment in time at which the activity was measured for each source;
• Describes any NRC or agreement state approval for pre-flight ground testing involving that isotope;
8 Applicants may choose to use MM to describe the activity of the isotope.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 38 Table 5 – Sample Radiological Material Report Source Description Element Symbol EZA Half- Life (Years) (IAEA 2021) Activity per Source (Ci) as Measured on (date) Number of Sources Total Activity (Ci) IAEA A2 (Ci) (IAEA 2018) Mission Multiple MM Material Supplier General purpose heat source fuel clads (Contained in 3 Spacecraft Radioisotope Thermoelectric Generators) Pu94238 as PuO294238
87.7 1,908 As measured on 8-20-2019 216 412,128 0.027 1.53x107 DOE Radioisotope Heater Unit Fuel Clads (Onboard Payload) Pu94238 as Pu94238 O2
87.7
33.6 As measured on 12-16-2020 129 4,334 0.027 1.61x105 DOE Instrument Calibration Source (Encapsulated) Co2760 5.27 6x10-6 As measured on 2-15-2021 2 1.2x10-5 10.8 1.1x10-6 Commercial Source Total 416,462 1.53x107 A.3.2 NSPM-20 specifies that the Tier level determination considers both launch and “subsequent stages when accidents may result in radiological effects to the public or environment” (NSPM-20 2019), which drives the potential need for RMR addendums to determine the Tier level of the SNS. For example, a fission reactor may meet Tier II criteria at launch, but after operation the resulting risk of the SNS and mission may meet the Tier III criteria, making it a Tier III system. Robust computer codes exist to model the burnup of nuclear fuel and aid in developing the RMR addendums (Argonne National Laboratory 2016), (Nuclear Regulatory Commission 2018). Table 5 of this AC shows a sample RMR for a mission using multiple radioactive devices, applicants may develop their own format that displays the essential information in a clear manner.
• Includes a RMR addendum addressing the safe terrestrial handling, use, and storage of all the isotopes in the SNS;
• Includes a RMR addendum showing the isotopic makeup of the SNS at the following times:
a. Launch;
b. During all planned flybys;
c. During planned reentry, as applicable for the mission.
• If the SNS can over time develop an isotopic composition more harmful than that at launch, the RMR also includes an addendum for the worst time in life that the SNS will have potential to return to Earth.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 39 A.4 CRITICALITY HAZARD ANALYSIS.
A.4.1 From the RMR, applicants will be able to quickly identify whether their SNS contains fertile material, fissionable material, or fissile material, as defined by the NRC or the IAEA (IAEA 2002), (Nuclear Regulatory Commission 2020). If these isotopes are present, or will become present as the SNS operates, then a criticality hazard analysis is needed to determine if the SNS has potential for criticality during normal SNS operations or off-nominal events in all mission phases (NSPM-20 2019). The criticality hazard analysis step is included even for an SNS with small amounts of fertile, fissionable, or fissile material because critical mass limits have not been established for space environments, since fission can dramatically change the public safety profile of the SNS, including nuclear proliferation concerns and ionizing radiation exposure potential (Connell and Trost 1994).
A.4.2 If a criticality hazard analysis is required, the analysis is conducted by first identifying all reasonably foreseeable hazards which may contribute to fission occurring in the SNS, using valid scientific principles and statistical methods. Examples of relevant information for completing a criticality hazard analysis include:
• The mass of material relative to the theoretical minimum mass limits for criticality emergencies on Earth, as discussed in various standards including American National Standards Institute (ANSI)/ANS-8.1-2014;
• Moderators and reflectors that are inherent to the SNS or spacecraft;
• Deliberate design features of the SNS to control form, shape, or collocation;
• Spacecraft operation, including launch, staging, and release;
• System, subsystem, and component failures or faults;
• Severe physical deformation of the SNS and spacecraft;
• Software operations or malfunctions;
• External environmental changes for all anticipated operational or malfunction environments;
• Interactions with space radiation;
• Proximity to other sources of radioactivity;
• Procedure deficiencies;
• Design inadequacies;
• Human factors;
• Functional and physical interfaces between subsystems; and
• Interactions of any of the above.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 40 A.4.3 There are two possible outcomes for the criticality hazard analysis:
1. Criticality is not possible for the SNS under any reasonably foreseeable criticality hazard scenarios, or;
2. Criticality is possible within reasonably foreseeable criticality hazard scenarios, including deliberately designed capabilities for fission reactors.
A.4.4 When the criticality hazard analysis shows that criticality is possible, the analysis catalogs the scenarios and conditions necessary for criticality. The analysis also identifies and describes the criticality risk mitigation measures used to preclude unintended criticality. Much like a flight hazard analysis, criticality hazard analyses are continually updated throughout the lifecycle of the SNS to ensure the potential for the SNS to achieve criticality is known at all times under all foreseeable operating conditions.
A.5 CRITICALITY SAFETY ANALYSIS.
A.5.1 If the criticality hazard analysis finds that criticality is possible within foreseeable criticality hazard scenarios in any mission phase, a criticality safety analysis is performed to calculate the likelihood and effects of fission in the SNS under each of the foreseeable criticality hazard scenarios that can result in fission. The criticality safety analysis considers scenarios where the SNS is intact, as well as the scenarios that result in uncontrolled radioactive release to the environment. The criticality safety analysis is performed with sufficient fidelity to:
• Determine the conditions and associated probability factors impacting the prompt neutron population and the ability of the SNS to become prompt critical;
• Determine the conditions and associated probability factors impacting the delayed neutron population and the ability of the SNS to become delayed critical;
• Determine the extent to which the SNS can remain subcritical in all reasonably foreseeable environments, including malfunction environments, i.e., the shutdown margin of the device;
• Quantify the performance of the system and radiation emissions for the criticality accidents that could occur;
• Quantify the performance of the system and radiation emissions for various power excursions, including short duration dramatic power excursions and long duration marginal power excursions;
• Quantify the risk of ionizing radiation exposure to the public if criticality were to occur within the Earth’s biosphere;
• Quantify the probability of criticality occurring;
• Quantify the expected rate of energy release and duration of criticality;
• Quantify the maximum possible activity in the SNS;
Launch and Reentry of Space Nuclear Systems AC 450.45-1 41
• Quantify the length of time for the fission products to decay to a level of radioactivity comparable to that of U-235 by the time it reenters the Earth’s atmosphere (per Sec. 3(a)(iii) of SPD-6).
• Quantify the radiation types and exposure pathways if the intact SNS were in proximity to a member of the public with the maximum SNS activity.
• Quantify the expected radiation dose rate at various points and distances around the intact SNS, from on-contact to the distance at which the dose rate is 25 mrem per hour TED for various scenarios, including:
a. During fission;
b. Immediately after shutdown;
c. At the time of maximum SNS activity;
d. Hourly for the first 24 hours after shutdown;
e. Daily for the first 7 days after shutdown;
f. Weekly for the first month after shutdown;
g. Monthly for the first year after shutdown;
h. If deliberate reentry is planned:
i. Hourly for the first 24 hours after reentry;
ii. Daily for the first 7 days after reentry;
iii. Weekly for the first month after reentry;
iv. Monthly for the first year after reentry;
• Quantify how the temperature of the SNS would behave over time if the SNS returned to Earth after fission, considering fission product decay heat; and
• Quantify how containing a critical chain reaction in the space operating environment will alter the material properties of the physical barriers to radioactive release.
A.5.2 Criticality safety analysis may require applicants to apply a combination of computer models from the NRC or DOE national labs and analytical techniques described by various standards setting organizations, as applicable to the specific technologies and isotopes present in the SNS. As commercial SNS use increases and the technology matures, the FAA anticipates that standards will be developed that are uniquely applicable to commercial SNS, however those do not exist at the time of this AC. Until then, applicants should take care to use analysis methods with sound underlying scientific principles and valid statistical methods.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 42 A.6 OPTIONAL COMPARISON TO EXISTING NUCLEAR ANALYSES.
A.6.1 Quantifying the ionizing radiation dose from various isotopes has been an ongoing field of research for decades, and practices continue to evolve and adapt. Some analyses have shown long-standing acceptance and may be suitable for SNS applicants to leverage to shorten their own nuclear safety analysis efforts. Some of these analyses have reached final conclusions on the mass or activity of an isotope required for certain dose thresholds to be met, a particularly useful conclusion for assessing the radiation exposure risk of an SNS. For a small SNS, it may be possible to compare the mass or activity of the isotopes to these pre-established values to establish the public safety risk of the SNS. Applicants have the option to compare their SNS and mission to these existing analyses, or to perform the more comprehensive analysis, which still may be required even if the comparison method is attempted.
A.6.2 If this route is utilized, care should be taken to ensure that rigorous analysis without biases that may prove to be non-conservative are used, as the release, exposure, or inhalation fractions used in the existing analyses may not be appropriate for the accident environments an SNS might experience. To meet or exceed the maximum quantity of radioactive material allowed in the existing analyses, evidence is needed that the release and exposure fractions for the SNS in any hazardous scenario will be equivalent to or less than the release and exposure fractions used in the existing analyses. If evidence is not provided, then FAA conservatively assumes the release and exposure fractions for an SNS accident are 1.0, this effectively reduces the threshold quantities of radioactive material to achieve a given maximum dose listed in the established analyses.
A.6.3 References that may be useful for SNS applicants include but are not limited to:
• DOE publications. The DOE may cancel, revise, or publish a new document at any time, when this AC was written the following series were germane to an SNS safety analysis:
a. DOE-HDBK-3010(series), Airborne Release Fractions/Rates and Respirable Fractions for Nonreactor Nuclear Facilities.
b. DOE-STD-1196(series), Derived Concentration Technical Standard.
c. DOE-STD-1027(series), Hazard Categorization of DOE Nuclear Facilities.
• EPA publications. The EPA may cancel, revise, or publish a new publication at any time. When this AC was written, the following publications were germane to SNS safety analysis:
a. FGR No. 11: Limiting Values of Radionuclide Intake and Air Concentration and Dose Conversion Factors for Inhalation, Submersion, and Ingestion, September 1988.
b. FGR No. 15, External Exposure to Radionuclides in Air, Water and Soil, revised August 2019.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 43
• IAEA Publications. Because NSPM-20 is linked to a specific IAEA document, these references will remain valid unless national policy is updated:
a. IAEA Specific Safety Requirements No. SSR-6 (Rev.1), Regulations for the Safe Transport of Radioactive Material, (2018 edition).
b. IAEA Specific Safety Guide No. SSG-26, Advisory Material for the IAEA Regulations for the Safe Transport of Radioactive Material (2012 Edition).
• International Commission on Radiological Protection (ICRP) publications. The ICRP may cancel, revise, or publish a new publication at any time. When this AC was written, the following publications were germane to an SNS safety analysis:
a. ICRP Publication 60, 1990 Recommendations of the International Commission on Radiological Protection, 1990.
b. ICRP Publication 103, The 2007 Recommendations of the International Commission on Radiological Protection, 2007.
c. ICRP Publication 107, Nuclear Decay Data for Dosimetric Calculations, 2008.
d. ICRP Publication 119, Compendium of Dose Coefficients based on ICRP Publication 60, 2012.
e. ICRP Publication 133, The ICRP Computational Framework for Internal Dose Assessment for Reference Adults: Specific Absorbed Fractions, 2016.
• NRC Regulatory Guides (RG) and Technical Reports (NUREG), including:
a. NUREG-1140, January 1988, A Regulatory Analysis on Emergency Preparedness for Fuel Cycle and Other Radioactive Material Licensees, reprinted August 1991.
b. NRC Regulatory Guide 7.9, Revision 2, Standard Format and Content of Part 71 Applications for Approval of Packages for Radioactive Material.
• United Nations Scientific Committee on the Effects of Atomic Radiation, annual report to the General Assembly Sources and Effects of Ionizing Radiation series.
At the time of this AC, the following reports were of particular interest for SNS:
a. 2016 Report Annex A – Methodology for estimating public exposures due to radioactive discharges.
b. 2008 Report Volume II Annex C – Radiation exposure in accidents.
c. 2000 Report Volume I Annex A – Dose assessment methodologies.
A.6.4 Based on the RMR and comparison to the existing analyses mentioned above, it may be possible for applicants to quickly establish that their SNS is Tier I or Tier II without performing an elaborate nuclear safety analysis. The FAA expects that trace amounts of nuclear materials such as that which is typically used in instrumentation may not require any additional analyses.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 44 A.7 RADIOLOGICAL HAZARD ANALYSIS.
A.7.1 When the comparison of existing analysis is unable to establish the public safety risk of the SNS, when fission is possible in the SNS, or for applicants who opt not to compare to existing nuclear safety analyses, a radiological hazard analysis (RHA) is needed. The purpose of the RHA is to identify all reasonably foreseeable hazards to public safety resulting from the launch, operation, or reentry of an SNS in all mission phases. Like a flight hazard analysis, the RHA is updated continually throughout the lifecycle of the SNS, as events may occur that change the performance of the SNS. Functionally, the RHA provides applicants an opportunity to identify hazards and develop hazard control strategies to minimize the public safety risk of the SNS. The FAA does not mandate the specific type of analysis used to perform the RHA so long as the method:
• Considers all mission phases;
• Identifies all reasonably foreseeable hazards, and the corresponding failure mode for each hazard associated with the launch or reentry of the SNS relevant to public safety, including those relevant from:
a. Ionizing radiation;
b. SNS operation;
c. Scenarios where the boundaries to radioactive release remain intact;
d. Scenarios where the boundaries to radioactive release fail;
e. System, subsystem, and component failures or faults; including launch and reentry vehicle failures, flight abort system failure or activation, etc.;
f. Software operations;
g. Environmental conditions;
h. Human factors;
i. Design inadequacies;
j. Procedure deficiencies;
k. Functional and physical interfaces between subsystems, including SNS interaction with the launch or reentry vehicle and other payloads, if applicable;
and
l. Interactions of any of the above.
• Includes known failure modes, such as those addressed in Advisory Circular (AC)
450.101-1 High Consequence Event Protection; and,
• Has sound underlying scientific principles and statistical validity.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 45 A.8 HIGH FIDELITY RADIOLOGICAL SAFETY ANALYSIS.
A.8.1 The high-fidelity radiological safety analysis (HFRSA) process involves applying probabilistic risk assessment methods and calculating the risk to the public for all reasonably foreseeable events and failures of safety-critical systems during normal and malfunction operations in all mission phases. The HFRSA should be complete enough for the FAA to validate the accuracy and findings of the HFRSA to make a licensing decision. For SNS, it is the consequence analysis side that introduces the greatest challenges and highest uncertainty in the HFRSA because of the complexities of radiation transport, exposure, biokinetics, and dosimetry. The end conclusions of the HFRSA are the PER for the most exposed individual for each event and failure.
A.8.2 The guidance provided in ACs 450.115-1 and -1A, High Fidelity Flight Safety Analysis provide applicants a good starting point for performing the HFRSA, but lacks the specific information needed to calculate the ionizing radiation exposure risks. To compute this additional information, applicants are encouraged to utilize computer codes developed by the NRC, the DOE national labs, or the computational methods underpinning the existing nuclear safety analyses. If none of these options are viable, applicants could make bounding assumptions without biases that may prove to be conservative using the worst-case radioactivity characteristics of the SNS, as identified in the RMR addendums. For all HFRSA, applicants should clearly demonstrate the accuracy of the fundamental scientific principles and statistical validity underpinning the analysis.
A.8.3 The purpose of HFRSA is to calculate the PER for all reasonably foreseeable events the SNS could experience, so the SNS risks can be accurately assessed. Doing this will necessitate determining the probability, location, accident type, radioactive material composition, dispersion, exposure pathways, and exposure magnitudes for ionizing radiation exposure to members of the public. This includes scenarios where the SNS remains intact, and those that result in uncontrolled radioactive release to the environment at some point. The HFRSA is a significant undertaking and will be more involved than the flight safety analysis for a non-SNS mission.
A.8.4 Specific accident environments that may need to be considered when performing the HFRSA include but are not limited to those shown in Table 6 of this AC.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 46 Table 6 – Accident Environments for HFRSA Consideration Accident Environment Physical Mechanisms Dynamic Acceleration, Shock, Vibration Explosive Overpressure Kinetic ΔP, dP/dt Thermal T, ΔT, ∇T, dT/dt Nuclear Radiation Fluence Chemical Chemical Reactivity A.8.5 Where P is Linear Momentum, T is the Temperature, t is the Time, Δ is the Difference Operator, ∇ is the Gradient Operator, d/dt is the Differential Operator.
A.8.6 Accident environments include fireball temperatures, explosive debris fragment velocities, and speeds for ground impact. Other accident environments include pressure gradient, atmospheric lapse rate, wind, and precipitation, which can affect the transport of released radioisotopes on the ground and in the atmosphere. Break-up state vectors – position (location and altitude), velocity, and acceleration are important initial conditions for the transport equations of motion.
A.8.7 These accident scenarios may, in and of themselves, pose hazards to people and property. They are addressed in non-nuclear portions of launch and reentry licensing.
From a nuclear safety perspective, they also impose hazards to the physical barriers to radioactive release that can result in radiological release and dispersal resulting in a PER that must be determined by the HFRSA to satisfy acceptable levels of risk in Chapter 7 of this AC. When complete, the HFRSA provides the information shown in Table 7 of this AC. An iterative process may be beneficial to develop this information by considering the results of safety analyses required for non-nuclear and nuclear missions.
In some cases, the results of the HFRSA may inform decisions regarding the performance of safety mitigations employed by the launch or reentry vehicle. Like all nuclear safety analyses, the HFRSA should be based on sound underlying scientific principles and valid statistical methods.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 47 Table 7 – Information Produced by a HFRSA Information Produced by a HFRSA Rationale Normal operational occurrences analysis for each stage of the mission, including launch, ascent, on orbit, departure from orbit, reentry, and end of mission (EOM).
This information quantifies the baseline risk of the SNS and is used to develop the environmental review.
The PER to the most exposed member of the public during normal operations.
This information quantifies the baseline risk of the SNS and is used to develop the environmental review.
Histograms showing the number of people expected to receive various TEDs above background radiation levels up to the highest dose for normal operations.
This information quantifies the baseline risk of the SNS and is used to develop the environmental review.
Normal operations analysis for return to orbit, on orbit, and EOM disposal, if applicable.
This information is used to assess whether a commercial SNS might impact national assets with radiation detection capabilities.
It is also used during the safety and payload reviews, and potentially to develop the environmental review.
A radiation emission heatmap showing the maximum expected radiation from the SNS out to 100 km from the vehicle when operating in space.
This information is used to assess whether a commercial SNS might impact national assets with radiation detection capabilities.
It is also used during the safety and payload reviews, and potentially to develop the environmental review.
Accident analysis for each stage of the mission, including launch, ascent, on orbit, departure from orbit, reentry, uncontrolled reentry, and EOM disposal.
This information is used to validate the calculated SNS Tier level, to develop the environmental review, and to calculate the maximum probable loss (MPL).
The PER to the most exposed member of the public for each failure chain with a probability greater than 1x10-7.
This information is used to validate the calculated SNS Tier level, to develop the environmental review, and to calculate the MPL.
Launch and Reentry of Space Nuclear Systems AC 450.45-1 48 Information Produced by a HFRSA Rationale Histograms showing the number of people expected to receive various TEDs above background up to the highest dose for each scenario with a probability greater than 1x10-7.
This information is used during the safety and payload reviews, and potentially to develop the environmental review.
Severe accident analysis for each stage of the mission, including launch, ascent, on orbit, departure from orbit, reentry, uncontrolled reentry, and EOM disposal.
This information is used to determine the risk to the public and to assist in the development of the environmental review.
The PER to members of the public for the failure chain that produces the highest PER to a member of the public, and the probability of that failure.
This information is used to determine the risk to the public and to assist in the development of the environmental review.
A histogram showing the number of people expected to receive various TEDs for the failure chain with the highest PER to a member of the public, for various TEDs from background up to the highest dose.
This information is used to determine the risk to the public and to assist in the development of the environmental review..
A.8.8 Additional information on performing probabilistic risk assessment and public exposure calculations for nuclear systems is also available from various nuclear agencies and organizations. Some of these documents are listed in chapter 3 of this AC, but more specific probabilistic risk assessment guidance and standards are available elsewhere.
This information may be very useful to applicants where the SNS merits an RHA or HFRSA. The following is a brief list of some, but not all, of the references available at the time of this AC which may aid applicants in performing their nuclear safety analyses:
A.8.8.1 American Nuclear Society (ANS).
• ANS RA-S-1.4-2021, Probabilistic Risk Assessment for Advanced Non-Light Water Reactor Nuclear Power Plants.
• ANS 6.1.1-2020, Photon and Neutron Fluence-to-Dose Conversion Coefficients.
• ANS 8.23-2019 Nuclear Criticality Accident Emergency Planning and Response.
• ASME/ANS RA-S-1.3-202x, “Standard for Radiological Accident Offsite Consequence Analysis (Level 3 PRA) to Support Nuclear Installation Application” (revision of trial-use standard ASME/ANS RA-S-1.3-2017)