NASA Procedural Requirements

NPR 8715.26 Effective Date: February 03, 2022 Expiration Date: February 03, 2027

Subject: Nuclear Flight Safety

Responsible Office: Office of Safety and Mission Assurance

Appendix C. Additional Information Regarding NSPM-20 and Nuclear Flight Safety

C.1 The following additional information is provided pertaining to NSPM-20, relative to the requirements contained in Chapter 3. . This is clarifying information and does not contain any additional requirements.

C.1.1 NSPM-20 sets tier boundaries based on material-at-risk, technology, and radiological risk estimates stemming from the nuclear safety analysis, with the ultimate tier depending on a combination of factors. Therefore, final determinations are made after completion of the nuclear safety analysis. However, the characteristics of safety analysis review depend on the tier, and so earlier evaluation of the likely tiering outcome is also needed. Figure 1 provides an illustration of the NSPM-20 tiering criteria, while Table 2 provides a tabular capturing of the same criteria.

Figure 1: Illustration Showing NSPM-20 Tiering Factors

(LWRHU = Light-weight radioisotope heater unit; MMRTG = Multi-mission radioisotope thermoelectric generator; SAR = Safety Analysis Report; SER = Safety Evaluation Report; SNS = Space nuclear system)

Table 2: NSPM-20 Tiering Criteria

C.1.2 A few features of NSPM-20 implementation within this directive warrant elaboration.

C.1.2.1 NSPM-20 clearly states in Section 1 that it "updates the process for launches of spacecraft containing space nuclear systems," while later using the terminology "radioactive sources" in the definitions of Tier I and Tier II. In this directive, NSPM-20 is only applied to SNS (with the expectation that no other payload would have an A2 mission multiple greater than 1,000), while the nuclear safety analysis would need to consider any additional radioactive material in the payload (in addition to the SNS). Other requirements in this directive ensure nuclear flight safety for all other missions.

C.1.2.2 While the term spacecraft is used in both NSPM-20 and this directive, any radioactive material on the integrated launch vehicle aside from the spacecraft would need to be considered (acknowledging that this would be atypical for a NASA mission).

C.1.2.3 The lower bound of Tier I is treated to equate to an A2 mission multiple of 1,000. This is effectively the lower end of historical SNS flown and comports with NSPM-20's reporting requirement bounds codified in Section 6 of that document.

C.1.2.4 There is a possibility that an SNS (that is not a fission reactor system) with an A2 mission multiple of less than 1,000 could surpass the NSPM-20 Tier III criterion associated with the probability of an exposure in excess of 25 rem being greater than 1 in 1 million. The tiering approach taken in this directive only addresses this possibility to the extent that it can be reasonably foreseen at the Preliminary Tier Determination stage. Nevertheless, this is not a situation that is anticipated to occur in practice based on contemporary SNS designs, mission profiles, and radiological risk state-of-knowledge.

C.2 The following additional information is provided pertaining to nuclear flight safety, relative to the requirements contained in Chapter 4. . This is clarifying information and does not contain any additional requirements.

C.2.1 The following passages provide contextual information about safety analysis preparation practices.

C.2.1.1 While the details of the SAS or mission SAR schedule will be informed by other guidance, the mutually agreed upon schedule would typically address: the planned analysis schedule; a technical interface document between NASA and the safety analysis preparer; base assumptions, analysis limitations/bounds, and model descriptions associated with the mission SAR or SAS development; and the development of a draft or initial mission SAR well in advance of (e.g., one year prior to) the final mission SAR. This, and related information, can be captured in a Safety Design Strategy, Safety Architecture, Safety Case, or equivalent product.

C.2.1.2 While the details of the SAS or mission SAR will be informed by other guidance, the scope of the SAS or mission SAR activity would typically include pre-launch and launch activities, as well as all operational phases where the SNS or other radioactive material could result in exposure of a member of the public in the event of an accident. The consideration of accident impacts would typically be sufficiently broad to support the nuclear safety review, the nuclear launch or reentry authorization or concurrence process (comparison to NSPM-20's safety guidelines, the specific items described in Section 5(b) of NSPM-20, and mission tiering), range safety uses, mishap preparedness and contingency planning activities, and public risk communications. The mission SAR or SAS would typically utilize a recognized standard or precedent (NRC or DOE guidance, INSRB guidance, a NASA Technical Standard, a consensus standard) or an appropriate precedent (e.g., the mission SAR from a contemporary mission with a similar payload), adjusted as necessary to address the specifics of the SNS context and the mission at-hand, for baselining the contents of the mission SAR.

C.2.1.3 The level of detail and content of the mission SAR or SAS will be commensurate with the mission radiological risk. Per NSPM-20, "a mission SAR may incorporate a system-specific SAR that establishes a safety basis for the space nuclear system," and NSPM-20 goes on to describe this relationship. In cases where launch vehicles, configurations, mission characteristics, and SNS are similar and it is determined by the safety analysis and review stakeholders that a comparative analysis will appropriately estimate the radiological risk of the mission, a comparative analysis can be utilized. Radioisotope heater unit (RHU) and radioisotope thermoelectric generator (RTG) risk assessments have demonstrated over time that a fairly mature understanding of mission phase radiological risk contributions exists for these devices, while also demonstrating shifts in the relative importance of phenomena associated with both changes to state-of-knowledge (e.g., breach modeling, dispersion modeling) and mission characteristics (e.g., clad temperature, launch window climatological conditions).

C.2.1.4 Where nuclear or radioactive materials are being provided from multiple sources, MDAAs may provide a single or multiple safety analysis document(s) to best meet this requirement (e.g., in a case where an SNS relies heavily on an existing system-specific SAR while other radioactive material requires a completely new SAS). Depending on the specifics of the circumstances, it may be necessary to justify why the radiological risks can be treated in an additive fashion when using multiple mission SARs. Some consequence metrics do not scale linearly with the activity of released material.

C.2.2 Regarding insurance and indemnification in the context of radiological incidents, in general, Federal funding is made available for resulting damages and response costs in accordance with Federal law through one or more of the Price-Anderson Act, the Space Act, and the Stafford Act. The final determination of which of these, or other authorities, would be used to provide Federal funding in the event of a launch or reentry accident can only be made after-the-fact, based on the specific facts of the actual incident and the claims in question.

C.2.3 Regarding SMA oversight following launch authorization, two approaches are suggested below for accomplishing this best practice.

C.2.3.1 Alternative #1 - The NFSO, in coordination with the NASA Program or Project Manager, the Payload Safety Working Group (PSWG) Chair, the Project-Level SMA TA, and the INSRB (when applicable) can use the results of the nuclear safety analysis and nuclear safety review to construct a nuclear-specific and mission-specific critical analysis assumptions list that would be formally issued to the PSWG Chair and relevant SMA TAs, for use within the routine launch services and mission execution processes. This list would, in effect, serve as an amendment to the NPR 8719.24 compliance matrix and would serve to facilitate the monitoring of anticipatable issues, if any are identified, that would cause the mission to likely exceed (or further exceed by a significant amount) NSPM-20's safety guidelines, or that would cause a significant degradation of defense-in-depth (e.g., fission product barriers). In this way, the list serves as a tool to codify nuclear flight safety-specific factors into these stages of NASA's routine risk management processes.

C.2.3.2 Alternative #2 - Barring adoption of the above, the NFSO could periodically monitor available information streams and/or consult with the applicable SMA interface for that phase of the mission in order to identify events and conditions that significantly deviate from the assumptions of the SAR during periods leading up to launch and subsequent operation (that are within scope of the SAR) for Tier I, II, and III missions, and which are also prior to the spacecraft entering inter-planetary flight with no plan for return or Earth gravity assist. In cases where identified events could reasonably be expected to cause the mission execution to exceed (or further exceed by a significant additional quantity) NSPM-20's safety guidelines or requirements (e.g., Department of the Air Force risk constraint), the NFSO would perform a simple, scoping-level qualitative or semi-quantitative assessment of the impact of the event or condition, and document this assessment in a note to file. In cases where a specific event or condition is found to indeed exceed (or further exceed by a significant additional quantity) the safety guidelines or a quantitative requirement, this would be captured in a memo and discussed with the relevant SMA interface, such that it can be factored in to safety and mission success activities (e.g., a PSWG Safety Review, the Safety and Mission Success Review (SMSR)).

C.2.3.2.1 Regarding Alternative #2, depending on the circumstances, the appropriate interface might be the Project-level SMA TA, a Program-affiliated SMA TA, the SMA Launch Services Division Mission Safety Engineer, the Payload Safety Program Executive, the Range Safety Program Executive, or the PSWG Chair. In some cases, the monitoring of information sources (e.g., the Launch Services Portal where the PSWG posts spacecraft non-compliances after the Payload Safety Compliance has been signed) may be a suitable replacement to contacting the interface.

C.2.3.2.2 Also regarding Alternative #2, the appropriate periodicity of monitoring the mission will vary greatly depending on the phase of the mission execution (e.g., more frequently during the period following SNS integration into the integrated launch vehicle and prior to launch versus less frequently during spaceflight). Examples of events and conditions that would be relevant are: (a) a two-fold increase in the time window in which the SNS is being cooled by ground support equipment relative to that assumed in the SAR, (b) a significant deficiency identified in the performance of a safety-relief valve that cannot be mitigated and effectively results in an increase in launch vehicle unreliability, and (c) a spacecraft malfunction while in Earth orbit that significantly increases the likelihood of not being in a sufficiently high orbit when the mission reaches end-of-life.

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