Preliminary Siting, Operations, and Transportation Considerations for Licensing Fission Batteries in the United States

: Nuclear energy is currently in the spotlight as a future energy source all over the world amid the global warming crisis. In the current state of miniaturization, through the development of advanced reactors, such as small modular reactors (SMRs) and micro-reactors, a ﬁssion battery is inspired by the idea that nuclear energy can be used by ordinary people using the “plug-and-play” concept, such as chemical batteries. As for design requirements, ﬁssion batteries must be economical, standardized, installed, unattended, and reliable. Meanwhile, the commercialization of reactors is regulated by national bodies, such as the United States (U.S.) Nuclear Regulatory Commission (NRC). At an international level, the International Atomic Energy Agency (IAEA) oversees the safe and peaceful use of nuclear power. However, regulations currently face a signiﬁcant gap in terms of their applicability to advanced non-light water reactors (non-LWRs). Therefore, this study investigates the regulatory gaps in the licensing of ﬁssion batteries concerning safety in terms of siting, autonomous operation, and transportation, and suggests response strategies to supplement them. To ﬁgure out the applicability of the current licensing framework to ﬁssion batteries, we reviewed the U.S. NRC Title 10, Code of Federal Regulations (CFR), and IAEA INSAG-12. To address siting issues, we explored the non-power reactor (NPR) approach for site restrictions and the permit-by-rule (PBR) approach for excessive time burdens. In addition, we discussed how the development of an advanced human-system interface augmented with artiﬁcial intelligence and monitored by personnel for ﬁssion batteries may enable successful exemptions from the current regulatory operation stafﬁng requirements. Finally, we discovered that no transportation regulatory challenge exists.


Introduction 1.Background
Nuclear energy is one of the eco-friendly and low-carbon energy sources for our world currently struggling with pollution, severe climate change, and the resulting natural disasters.Historically, the power of nuclear energy was recognized and started to be used in the 1940s, and through continuous development, it has become a major energy source, accounting for 10% of the global electricity production and 20% of the United States' (U.S.) electricity production [1].
However, historical accidents from the previous generation, large-scale nuclear power plants (NPPs), have taken away trust in nuclear energy and instilled fear.As a result, the United Kingdom, France, South Korea, and Japan declared a gradual reduction in NPPs, although some have reconsidered their position due to recent global events and climate goals.In the U.S., cost considerations are forcing the early retirement of NPPs and weakening the national nuclear supply chain [2].
In this trend, nuclear experts are conducting research on the miniaturization of NPPs to reduce huge damage in the event of an accident and the economic burden from the large capital cost per plant of the current NPPs.Accordingly, advanced reactors, such Eng 2022, 3 374 as SMRs and micro-reactors, are under development, where SMRs are expected to be commercialized in 2029 [1].Going one step further, Idaho National Laboratory (INL) took the idea from batteries and established the fission battery initiative to make nuclear energy accessible to the public in any location with the vision of "plug and play", just like in chemical batteries, without the need for licensed operators.
Meanwhile, the commercialization of reactors is regulated by the U.S. Nuclear Regulatory Commission (NRC), and the safe and peaceful use of nuclear energy in terms of safety, security, and safeguards are supervised by the International Atomic Energy Agency (IAEA).However, current regulations focusing on current NPPs are facing significant regulatory gaps of applicability to advanced reactors.Therefore, this research investigates the regulatory challenges of the licensing of fission batteries concerning safety in terms of siting, autonomous operation, and transportation, and suggests potential response strategies to supplement it.

Fission Batteries 1.2.1. Fission Battery Attributes
Five attributes, economical, standardized, installed, unattended, and reliable, support the vision and suggest the direction for development [3].The fission battery attributes are defined as follows: • Economical: Fission batteries will have cost competitiveness, compared to energy sources that operate only on a specific platform, through a wide range of use and multiple deployments.

•
Standardized: Fission batteries will be developed in standardized sizes, power outputs, and manufacturing processes for extensive use, and will be fully assembled in the factory to ensure low-cost and quality assurance.

•
Installed: Fission batteries will be ready for deployment to implement "plug-and-play".

•
Unattended: Fission batteries will be operated without the need for on-site operators based on a resilient and autonomous system.

•
Reliable: Fission batteries will have high reliability during their lifetime based on a robust, resilient, fault-tolerant, and durable system to achieve fail-safe operation.

Fission Battery Design Features
The fission battery design is expected to follow the micro-reactor design features, mainly gas-cooled reactors with tri-structural isotropic (TRISO) fuel or heat-pipe reactors with metal, oxide, or TRISO fuels.Fission batteries will be designed to be used for less than 1 year with an output of less than 25 MWth and cheaper than 0.1 billion USD to meet midsize customer energy demands [4], such as isolated grids, military bases, and electricity supply to electric vehicles [5].A design example of an autonomous micro-reactor currently is the eVinci design, currently under development by Westinghouse [6,7].
The most notable feature of this design is that it aims for a dramatically enhanced safety performance compared to the current large light water reactors.This is achieved by active and passive safety features for reactivity control, heat removal, and containment for redundancy and diversity [7].For reactivity control, three strategies were designed: control drum subsystem, emergency shutdown subsystem, and passive release of hydrogen from the moderator.The design includes two strategies for decay heat removal, one using heat channels through the power conversion system and the other through the reliance on the conduction of heat through the core block to the canister with natural air convection heat removal from the outside surface of the canister to an air duct system that channels air to the surrounding environment.For the containment function, the eVinci design includes three barriers to prevent the release of radioactive material: a monolith encapsulation of fuel, a solid core block, and a canister containment subsystem.
Fuel with high-assay low-enriched uranium (HALEU), enriched from 5% to 20%, is the most typically considered fuel type for advanced reactors, such as the eVinci microreactor design described above [8].TRISO fuel is one of the representative fuels using HALEU with the uranium form of uranium oxycarbide (UCO) or uranium dioxide (UO 2 ).The TRISO particles are encapsulated with three layers of carbon and ceramic-based materials that prevent the release of most radioactive fission products and withstand very high temperatures without melting, ensuring good fission product retention even under extremely severe conditions, including temperatures of 1600 • C for hundreds of hours [9].
Moreover, since the thermal power level is hundreds of times smaller compared to light water reactors, the number of fission products that are produced and could potentially be released to the environment is also much smaller.This can be seen from the radiological consequence evaluations performed for the eVinci micro-reactor design configurations having thermal power output levels of 1 MWth and 14 MWth [10].When considering three barriers, the maximum total effective dose equivalent (TEDE) to a dose receptor within 1 m was between 6.33 × 10 −12 rem and 6.84 × 10 −8 rem for a 1 MWth reactor, and for a 14 MWth reactor, it was between 6.33 × 10 −12 rem and 9.58 × 10 −7 rem in all accident scenarios [10].This shows that the released doses are essentially zero for all practical purposes when compared to the average U.S. resident's annual background radiation from natural and anthropologic sources of approximately 6.2 × 10 −1 rem [11].Even when assuming only one barrier, the maximum total effective dose equivalent (TEDE) to a dose receptor within 1 m was between 5.11 × 10 −4 rem and 5.59 rem for a 1 MWth reactor, and for a 14 MWth reactor, it was between 5.11 × 10 −4 rem and 78.3 rem in all accident scenarios [10].To put it into perspective, this is about the same order of magnitude of doses below which we have no data to establish a firm link between radiation exposure and cancer [11] and thousands smaller than the high doses 134 workers received while on the site during the early morning of 26 April 1986 after the Chernobyl accident, of which 28 were confirmed dead in the first three months due to radiation exposure [12].
In addition, the eVinci micro-reactor design supports the fission battery unattended attribute by including only one operator action of tripping the control drum system [10].However, during emergency conditions, the reactivity control function is achieved without operator actions through the automatic emergency shutdown subsystem and the passive release of hydrogen from the moderator.

Regulatory Review Methodology
The regulatory review methodology used in this study is shown in Figure 1.
high temperatures without melting, ensuring good fission product retention even under extremely severe conditions, including temperatures of 1600 °C for hundreds of hours [9].
Moreover, since the thermal power level is hundreds of times smaller compared to light water reactors, the number of fission products that are produced and could potentially be released to the environment is also much smaller.This can be seen from the radiological consequence evaluations performed for the eVinci micro-reactor design configurations having thermal power output levels of 1 MWth and 14 MWth [10].When considering three barriers, the maximum total effective dose equivalent (TEDE) to a dose receptor within 1 m was between 6.33 × 10 rem and 6.84 × 10 rem for a 1 MWth reactor, and for a 14 MWth reactor, it was between 6.33 × 10 rem and 9.58 × 10 rem in all accident scenarios [10].This shows that the released doses are essentially zero for all practical purposes when compared to the average U.S. resident's annual background radiation from natural and anthropologic sources of approximately 6.2 × 10 rem [11].Even when assuming only one barrier, the maximum total effective dose equivalent (TEDE) to a dose receptor within 1 m was between 5.11 × 10 rem and 5.59 rem for a 1 MWth reactor, and for a 14 MWth reactor, it was between 5.11 × 10 rem and 78.3 rem in all accident scenarios [10].To put it into perspective, this is about the same order of magnitude of doses below which we have no data to establish a firm link between radiation exposure and cancer [11] and thousands smaller than the high doses 134 workers received while on the site during the early morning of 26 April 1986 after the Chernobyl accident, of which 28 were confirmed dead in the first three months due to radiation exposure [12].
In addition, the eVinci micro-reactor design supports the fission battery unattended attribute by including only one operator action of tripping the control drum system [10].However, during emergency conditions, the reactivity control function is achieved without operator actions through the automatic emergency shutdown subsystem and the passive release of hydrogen from the moderator.

Regulatory Review Methodology
The regulatory review methodology used in this study is shown in Figure 1.INSAG-12 "Basic Safety Principles for NPPs" [13] and the U.S. NRC Title 10, Code of Federal Regulations (CFR) [14] were reviewed to evaluate the specific safety principles essential for the licensing of NPPs and obtain regulatory information on the licensing process of 10 CFR Part 50 and 52 (Table 1.) INSAG-12 "Basic Safety Principles for NPPs" [13] and the U.S. NRC Title 10, Code of Federal Regulations (CFR) [14] were reviewed to evaluate the specific safety principles essential for the licensing of NPPs and obtain regulatory information on the licensing process of 10 CFR Part 50 and 52 (Table 1).
The next step was to apply the current regulatory licensing framework to fission batteries in terms of siting, operation, and transportation, and figure out regulatory challenges considering the characteristics of fission batteries.Finally, response strategies were presented to support the licensing of fission batteries against the challenges.In this step, the non-power reactor approach was cited from the "Regulatory review of microreactors-Initial considerations" [15] and the permit-by-rule approach was referenced from "Key regulatory issues in nuclear microreactor transport and siting" [16].In addition, an advanced human-system interface (HSI) for autonomous operation approach was derived from "Human-system interface to automatic systems: Review guidance and technical basis" [17] and "A method to select human-system interface for NPPs" [18].

Basic Safety Principles for NPPs (INSAG-12)
Internationally, the IAEA oversees the safe and peaceful use of nuclear energy, ensuring the protection of people and the environment from the harmful effects of radiation.INSAG-12 [13], written by the International Nuclear Safety Advisory Group (INSAG), provides three safety objectives, three fundamental safety principles, and eight specific principles.The specific principles present eight types of safety principles applied during the main design phases, from the early conceptual design phase to the decommissioning phase (Figure 2).Above all, siting and operation are recognized as top research priorities for licensing issues considering the features of fission batteries that would be used anywhere without on-site licensed operators.Licensing and regulations of advanced nuclear reactors The next step was to apply the current regulatory licensing framework to fission batteries in terms of siting, operation, and transportation, and figure out regulatory challenges considering the characteristics of fission batteries.
Finally, response strategies were presented to support the licensing of fission batteries against the challenges.In this step, the non-power reactor approach was cited from the "Regulatory review of microreactors-Initial considerations" [15] and the permit-by-rule approach was referenced from "Key regulatory issues in nuclear microreactor transport and siting" [16].In addition, an advanced human-system interface (HSI) for autonomous operation approach was derived from "Human-system interface to automatic systems: Review guidance and technical basis" [17] and "A method to select human-system interface for NPPs" [18].

Basic Safety Principles for NPPs (INSAG-12)
Internationally, the IAEA oversees the safe and peaceful use of nuclear energy, ensuring the protection of people and the environment from the harmful effects of radiation.INSAG-12 [13], written by the International Nuclear Safety Advisory Group (INSAG), provides three safety objectives, three fundamental safety principles, and eight specific principles.The specific principles present eight types of safety principles applied during the main design phases, from the early conceptual design phase to the decommissioning phase (Figure 2).Above all, siting and operation are recognized as top research priorities for licensing issues considering the features of fission batteries that would be used anywhere without on-site licensed operators.

The Regulatory Licensing Framework of the U.S. NRC
Title 10 of the CFR, established by the U.S. NRC, contains the requirements that need to be met by organizations using nuclear materials or operating nuclear facilities in the U.S. Currently, there are two ways to achieve a commercial license regulated by the U.S. NRC; 10 CFR Part 50 dividing construction permit (CP) and operating license (OL) or 10 CFR Part 52 supporting a combined process of construction and operating license (COL) [19].
According to Figures 3 and 4 describing the process of Part 50, initial public hearings, an environmental report, and an NRC review of the preliminary design for a CP are required, which is a pre-requisite for obtaining an OL issued with final mandatory public  According to Figures 3 and 4 describing the process of Part 50, initial public hearings, an environmental report, and an NRC review of the preliminary design for a CP are required, which is a pre-requisite for obtaining an OL issued with final mandatory public hearings and final safety and environmental requirements [20].Through this process, obtaining a license normally takes more than 10 years [21].
hearings and final safety and environmental requirements [20].Through this process, obtaining a license normally takes more than 10 years [21].In order to reduce the various economic and regulatory risks that may arise during the long period of the Part 50 process, Part 52 was introduced [21], combining CP and OL steps.As seen in Figure 5, the Part 52 process is conducted with an early site permit (ESP) and a design certification (DC) together prior to issuing a COL [20].Through this streamlined process, it shortens the period to within 10 years.Figure 6 graphically shows, for power reactors, how the two kinds of licenses can be obtained with the process of Part 50 and Part 52, that is, prototype license and license through analysis and test [22].hearings and final safety and environmental requirements [20].Through this process, obtaining a license normally takes more than 10 years [21].In order to reduce the various economic and regulatory risks that may arise during the long period of the Part 50 process, Part 52 was introduced [21], combining CP and OL steps.As seen in Figure 5, the Part 52 process is conducted with an early site permit (ESP) and a design certification (DC) together prior to issuing a COL [20].Through this streamlined process, it shortens the period to within 10 years.Figure 6 graphically shows, for power reactors, how the two kinds of licenses can be obtained with the process of Part 50 and Part 52, that is, prototype license and license through analysis and test [22].In order to reduce the various economic and regulatory risks that may arise during the long period of the Part 50 process, Part 52 was introduced [21], combining CP and OL steps.As seen in Figure 5, the Part 52 process is conducted with an early site permit (ESP) and a design certification (DC) together prior to issuing a COL [20].Through this streamlined process, it shortens the period to within 10 years.Figure 6 graphically shows, for power reactors, how the two kinds of licenses can be obtained with the process of Part 50 and Part 52, that is, prototype license and license through analysis and test [22].
hearings and final safety and environmental requirements [20].Through this process, obtaining a license normally takes more than 10 years [21].In order to reduce the various economic and regulatory risks that may arise during the long period of the Part 50 process, Part 52 was introduced [21], combining CP and OL steps.As seen in Figure 5, the Part 52 process is conducted with an early site permit (ESP) and a design certification (DC) together prior to issuing a COL [20].Through this streamlined process, it shortens the period to within 10 years.Figure 6 graphically shows, for power reactors, how the two kinds of licenses can be obtained with the process of Part 50 and Part 52, that is, prototype license and license through analysis and test [22].However, the current licensing framework of Part 50 and Part 52 does not fully consider the features of advanced reactors, and so the U.S. NRC is taking the process for 10 CFR Part 53 "Licensing and Regulations of advanced nuclear reactors" mandated by the Nuclear Energy Innovation and Modernization Act (NEIMA).Currently, the preliminary rule language consists of 10 subparts based on a risk-informed and performance-based regulatory approach [23].

Siting Regulations
Siting is used to select an appropriate location for a safe operation, including the process of analyzing natural and anthropogenic hazards, such as the radiological impact on the public and the environment [13].Accordingly, the NRC requires an environmental report (ER) during the licensing process for CP, which takes several years for the site investigation and requires detailed site-specific information, including impacts on area populations and surrounding environmental conditions.To minimize the impacts on the site, regulations and guidance are strictly stipulated by the U.S. NRC and IAEA.Table 2 shows in detail the current regulations and regulatory guides related to siting.

Applicability of the Current U.S. Licensing Framework to Fission Batteries 4.1. Siting Regulations
Siting is used to select an appropriate location for a safe operation, including the process of analyzing natural and anthropogenic hazards, such as the radiological impact on the public and the environment [13].Accordingly, the NRC requires an environmental report (ER) during the licensing process for CP, which takes several years for the site investigation and requires detailed site-specific information, including impacts on area populations and surrounding environmental conditions.To minimize the impacts on the site, regulations and guidance are strictly stipulated by the U.S. NRC and IAEA.Table 2 shows in detail the current regulations and regulatory guides related to siting.

RG. 1.7
A reactor should be located away from population centers of more than 25,000 people.

NUREG-0800
A reactor should be located more than 10 miles away from an airport and 5 miles from a hazardous site.

Applicability of Siting Regulations to Fission Batteries
Considering the expansive use of target electricity markets by military bases, isolated grids, and electricity supply to electric vehicles, fission batteries are expected to be developed to enable multi-site deployment with the concept of "plug-and-play".However, the current regulations and guidance presented above, directly contradict the vision for fission batteries, designed to be used anywhere, due to numerous prescriptive rules on the site selection.
In response to performance rules, such as doses at the exclusion areas under normal operation and emergency conditions, technology suppliers are designing enhanced passive safety systems for advanced reactors.Fission batteries are expected to have additional attributes such that any abnormal events will result in a significantly reduced source term and limit any radioactive materials release to within the site boundary or be limited to within a short distance of the exclusion area boundary [24].Therefore, site restrictions may not be suitable for the universal use of fission batteries equipped with enhanced passive safety systems.
Additionally, the long duration, on the order of multiple years, for site evaluation in the current licensing process would interfere with the multi-site deployment and expedient site transfer required by user needs.
As a result, we conclude that the current site regulations and licensing process do not apply to the characteristics of fission batteries in terms of site restrictions and excessive time burdens for on-site evaluations.

Response Strategy to Site Restrictions: Non-Power Reactor Approach
The IAEA suggests an emergency planning zone (EPZ) where preparations are made to promptly shelter in place to perform environmental monitoring and to implement urgent protective actions.Table 3 shows the represented EPZ size [25].Therefore, it can be inferred that the EPZ size of fission batteries whose output is [4] less than 25 MWth would be within 1.5 km.However, it seems to be reasonable to re-analyze the EPZ size of fission batteries that are expected to be equipped with enhanced passive safety systems, so those doses could be under the regulatory limit of 1 rem for non-power reactors [15].Accordingly, applying the rules to non-power reactors is appropriate, and Table 4 shows the EPZ size of non-power reactors [26].If it is applied to fission batteries, the EPZ size of the fission batteries would be reduced to approximately 400 m for power levels up to 20 MWth and even the operation boundary for power levels below 2 MWth.Meanwhile, SMR developers insist that the EPZ size for SMRs with an output of 300 MWth should be within 300 m or less to replace fossil fuel power plants located near big cities [27].In Tables 3 and 4, we can see the relationships between EPZ size and power output.In Table 3, it shows that when power output increases 10 times from 10 MWth to 100 MWth, EPZ size also becomes 10 times larger, from 0.5 km to 5 km.Therefore, if we assume that the power level is proportional to the fission products that are produced and potentially released and that the relationship above could apply to advanced reactors, such as SMRs and fission batteries, we could expect that the EPZ size of fission batteries whose power output may be less than 25 MWth would be 25 m.These assumptions need to be confirmed by analysis; however, since the sudden request for a big regulatory change can be burdensome to the regulatory authorities, starting with the officially proven non-power reactor approach, it is desirable to request gradual deregulation, as the design of fission batteries is materialized, and its safety systems are verified and validated.Finally, the zero-EPZ concept should be applicable in the future [28], such that fission batteries can be widely used in highly populated areas.

Response Strategy to Excessive Time Burdens: Permit-By-Rule Approach
A permit-by-rule is a pre-construction permit issued by a reviewing authority that may be applied to a number of similar emissions units or sources within a designated category [29].It is widely used for safety-guaranteed facilities, for example, on-site power generation.Sites for fuel-burning equipment are applied to permit-by-rule in Georgia State [30].The key to applying permit-by-rule is to prove high levels of safety and reliability at the design stage.Therefore, considering the enhanced safety features of fission batteries, permit-by-rule would be a fast and reliable regulatory approach for achieving multi-site deployment and expedient site transfer by reducing the time for siting to a few days or weeks instead of several years within the current regulation [16].
The permit-by-rule approach would be conducted with the analysis of the plant parameter envelope and site parameter envelope.Major steps for it may include [16]: Defining a safety design plant parameter envelope of mandatory requirements for construction and operation under permit-by-rule.

2.
Defining a site parameter envelope related to plant design safety parameters.

3.
Safety assessment with a plant parameter envelope and a site parameter envelope.

4.
Plant parameter envelope site acceptance criteria would be created based on the above steps.
Therefore, defining a well-developed plant parameter envelope and a hypothetical site parameter envelope are essential for the permit-by-rule approach.A plant parameter envelope may be analyzed in the design process, and a site parameter envelope may be analyzed by modeling, applying simulation tools, and applying unsupervised machine learning technology for expected areas.
As a result, when fission battery design is mature enough and a high-quality enhanced safety system is verified and validated, permit-by-rule could be a reasonable approach that would sufficiently replace or complement the current years-long siting evaluation process for fission batteries that may require hundreds or thousands to be deployed simultaneously.

Operations Staffing Regulations
Operation is the key phase in the lifecycle of NPPs.Once NPPs begin operation, their safety performance depends on the reliability and capability of the facility equipment and human personnel, especially during abnormal conditions.As shown in Table 5, therefore, the composition of the control room and related staffing regulations are specified in 10 CFR Part 50 and Part 55.What stands out is that regulations prescriptively set the minimum required number of licensed operators on site during normal operation and emergencies.Even the preliminary language of 10 CFR Part 53 for advanced reactors still requires licensed human operators.As such, the operation and response to emergencies for NPPs are highly dependent on licensed human personnel.
A senior operator shall be present at the facility during initial start-up and approach to power, recovery from an unplanned or un-scheduled shut-down or significant reduction in power, and re-fueling.Each licensee must establish and implement a facility safety program (FSP) that routinely and systematically evaluates potential hazards, operating experience relate to human actions and programmatic controls affecting the safety functions.

NUREG-0654
Addresses the minimum staffing requirements for emergencies, 10 staff on-site, and 11 additional staff within 30 min, and 15 additional staff within 60 min.

Applicability of Operations Staffing Regulations to Fission Batteries
According to the un-attended fission battery attribute, fission batteries are expected to be developed for un-attended operation through resilience and automation.Investigations in the aftermath of the Three Mile Island and Chernobyl accidents showed that human errors resulted from equipment design and human factor deficiencies [31].Therefore, the development of automation should be attained with the advanced design of passive safety systems, simplicity of operation, and limited important human actions based on innovative un-supervised machine learning technology [3].
However, the current operations staffing regulations covering licensed operators seem to be highly dependent on personnel and do not fully capture current automation capabilities.The exemption process for control room staffing requirements shows some benefits.For example, NuScale Power successfully obtained an exemption to reduce the number of staff for its SMR light-water design, but it was not a complete elimination [22].
Nevertheless, the designer community is still expected to develop fission batteries with high automation and remote monitoring and with no operator control or at least partial control [32].This is because the un-attended operation is the most important attribute of fission batteries, that is, aiming to enable their use by ordinary people, such as chemical batteries.Therefore, current regulations related to operators cannot be applied to unattended operations of any advanced reactor, including fission batteries, for which a change in regulations is necessary.

Response Strategy to Operations Staffing Regulations: Advanced Human-System Interface
Human-system interface technology is defined as the part of the nuclear reactor through which personnel interact to perform their functions and tasks with the systems.The primary purpose of the human-system interface is to provide the operator with a means to monitor and control the nuclear reactors and to restore them to a safe state when adverse conditions occur [18], and so it is widely used at present.
In advanced human-system interface systems with improved telecommunication technologies, an off-site space equipped with a set of computer displays and input devices may replace the control rooms and make it feasible for remote monitoring and control.Moreover, the enhanced safety systems and simplified design may allow one controller to manage multiple reactors.In the current state, human-system interface technology still requires minimum human functions.However, for fission batteries equipped with un-supervised machine learning technology, un-supervised machine learning could replace human functions.Therefore, an advanced human-system interface managed by un-supervised machine learning would be the core technology required for autonomous operation of fission batteries.
Meanwhile, the advancements in automation systems and the development of computer performance have had a tremendous impact on the deployment of automation technologies and systems in many industries over the past years, such as a nearly autonomous management and control (NAMAC) system [33], where the digital twin (DT) and advanced machine learning algorithms play key roles in replacing human personnel.
Therefore, optimistic expectations on the progress of a complete remote-control system with advanced telecommunication technologies and human-system interface operated by un-supervised machine learning could make it possible for fission batteries to be exempted from current regulations related to operations staffing.

Transportation Regulations
Transportation in the nuclear industry means moving radioactive materials to the desired places.Related regulations are co-managed by 10 CFR 71 of the U.S. NRC and 49 CFR 173 of the U.S. Department of Transportation (DOT).Table 6 shows the currently regulated packaging types for the transportation of radioactive materials.The packaging type is determined by the quantities of radioactive materials, and each package is required to resist certain conditions.In order to verify the safety performance of each package, the U.S. NRC requires specific tests on the normal conditions of transport (NCT) and hypothetical accident conditions (HAC).Especially, tests on the HAC for Type B packaging assuming possible severe transportation accidents are specified in 10 CFR 71.73 [34].The need for the safety performance test is to prevent the leakage of radioactive material or to control it below a prescribed regulatory dose limit as described in Table 7 under all conditions.  1  10 mSv/h at 1 m from the external surface of the package on the HAC 1 Additional requirements for Type B packaging.
Therefore, even in the most severe transportation accidents, packages should be able to maintain their containment function and prevent the release of radioactive materials under regulatory the dose limit.

Applicability of Transportation Regulations to Fission Batteries
According to the installed fission battery attribute, fission batteries are expected to be installed at the factory, delivered to multiple users, and decommissioned with the fuel loaded.Considering this design goal, three transportation phases could be analyzed:

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Transporting fresh fuel to the manufacturing factory.

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Deploying new fission batteries to users with fresh fuel.

•
Transferring the fission battery's location after an operation with a used fuel either to a different user or for decommissioning.
These new transportation phases for mobile reactors equipped with fuel pose technical and regulatory challenges.The third phase is especially critical for fission batteries since the used fuel will contain highly radioactive fission products.
When it comes to technical challenges, the key for fission battery transportation is to achieve complete safety reliability for radiation shielding, decay heat removal, and maintaining subcriticality, and capability for preventing the release of radioactive materials.However, when fission batteries are fully developed and commercialized, the technical challenges are expected to be addressed.Therefore, this study assumed that fission batteries would have adequate safety systems to meet the technical challenges associated with fission battery transportation.
For regulatory considerations, the current regulations for transporting radioactive materials stipulate the packaging type and the dose limit for packages.Currently, one of the most hazardous radioactive materials is used or spent nuclear fuel, which requires the use of Type B packaging as seen in Table 6.
Moreover, since fission battery transportation will include used fuel during the third transportation phase, it seems reasonable to assume the designers may try to meet the Type B packaging requirements, which is the safest and most robust cask in use nowadays.Accordingly, if fission battery designs meet the Type B packaging requirements at a reasonable cost and, implicitly, meet the regulatory dose limits, safe fission battery transportation is possible under current regulations without any foreseeable burdens due to the performance-based nature of transportation regulations [35].

Discussion on the Applicability of the Current U.S. Licensing Framework to Fission Batteries
This research indicates that the current regulatory framework is facing considerable challenges in terms of its applicability to fission batteries for siting and operations staffing; however, under certain design constraints for the fission batteries, it is feasible to meet the current transportation rules.
For siting regulations, strict site restrictions and excessive analysis and review-time burdens were presented as a limitation for the deployment of fission batteries.Thus, suggesting that the non-power reactors approach to resolve siting regulatory limitations.However, before applying the results of the non-power reactor approach, the fundamental difference should be considered, that is, a fission battery is a power reactor, unlike nonpower reactors.Nevertheless, since the safety features of fission batteries would be more adequate than non-power reactors, the non-power reactors approach may be reasonable considering that site inspection is focused on the safety aspects.Next, we proposed the permit-by-rule approach as a countermeasure to excessive analysis and review-time burdens.Similar to the non-power reactors approach, the permit-by-rule approach requires a reliable safety performance.Therefore, if regulatory authorities accept a permit-by-rule approach for fission batteries, multi-site deployment could be achievable.
In the case of operations staffing regulations, autonomous operation is an essential feature for fission batteries, thus, fission battery developers are working on applying unsupervised machine learning technologies to fission batteries to replace licensed operators.Therefore, for fission batteries controlled by un-supervised machine learning, human personnel will have the role of only monitoring the un-supervised machine learning control systems as necessary.As a result, an advanced human-system interface was suggested to support the role of an off-site un-supervised machine learning specialist for the successful exemption from regulations on control room operations staffing.If an advanced humansystem interface is applied to fission batteries controlled by un-supervised machine learning, the regulatory authorities will need to change their rules to accommodate the new role of human personnel from licensed operators controlling the fission batteries directly to the certified personnel monitoring un-supervised machine learning control systems.
Finally, for transportation regulations, we have identified no regulatory gaps for the licensing of fission batteries, but it is necessary to consider whether to use Type B packaging requirements for fission batteries, which have their own safety features unlike used nuclear fuel, which may be too conservative.Therefore, using Type B packaging may be un-economical for nuclear vendors.As a result, it is necessary to carry out dedicated studies on the development of specific packaging for fission batteries and figure out which packaging is more reasonable to use in terms of safety and economic production of fission batteries.

Conclusions
In the development of innovative technologies, numerous regulatory barriers exist in all industries.Now that the transformation of nuclear reactors is taking place, fission batteries are at the peak of innovation, and accordingly, many challenges are expected.For this reason, this research was aimed at identifying possible regulatory challenges for the licensing of fission batteries and suggesting countermeasures to support their successful development and licensing.Among the many licensing topics, siting, operations staffing, and transportation were intensively studied, considering the five attributes of fission batteries.
For siting challenges, strict site restrictions to prevent impact on the public and several years of site inspections were presented.Considering the expansive use of fission batteries equipped with enhanced safety systems, the non-power reactor approach to relax site limitations and the permit-by-rule approach to shorten review time periods were proposed for site inspections to support simultaneous multi-site deployment.
Regarding operations staffing issues, un-attended operation is the core attribute of fission batteries.Currently, nuclear reactors are highly dependent on control room operators.However, fission batteries are envisioned to be operated by un-supervised machine learning control systems without the need for on-site staff.Therefore, the development of an advanced human-system interface supporting remote monitoring for fission batteries controlled by un-supervised machine learning may enable successful exemptions from the current regulatory requirements.
In terms of transportation regulations, fission batteries have the characteristic of needing to be transportable without removing the used fuel after an operation.If the fission battery designs meet the regulatory dose limits by adopting the certified Type B packaging requirements used for the transportation of used fuel from current reactors, fission battery transportation is achievable within the current regulatory framework.
Overall, the development of fission batteries in the U.S. is facing other regulatory challenges than the three discussed above.However, the status of present regulations should not hinder the development of innovative technologies in the future.Therefore, the necessary regulatory changes and the development of fission batteries should evolve in parallel through an open regulatory engagement process for the safe and practical deployment of advanced nuclear energy.

Figure 1 .
Figure 1.Regulatory review process in this study.

Figure 1 .
Figure 1.Regulatory review process in this study.

3. 2 .
The Regulatory Licensing Framework of the U.S. NRC Title 10 of the CFR, established by the U.S. NRC, contains the requirements that need to be met by organizations using nuclear materials or operating nuclear facilities in the U.S. Currently, there are two ways to achieve a commercial license regulated by the U.S. NRC; 10 CFR Part 50 dividing construction permit (CP) and operating license (OL) or 10 CFR Part 52 supporting a combined process of construction and operating license (COL)[19].

Figure 5 .
Figure 5. COL process of 10 CFR Part 52.However, the current licensing framework of Part 50 and Part 52 does not fully consider the features of advanced reactors, and so the U.S. NRC is taking the process for 10 CFR Part 53 "Licensing and Regulations of advanced nuclear reactors" mandated by the Nuclear Energy Innovation and Modernization Act (NEIMA).Currently, the preliminary rule language consists of 10 subparts based on a risk-informed and performance-based regulatory approach[23].

Table 1 .
List of U.S. NRC 10 CFR Part related to licensing commercial nuclear reactors.

Table 1 .
List of U.S. NRC 10 CFR Part related to licensing commercial nuclear reactors.

Table 2 .
Regulations and regulatory guides related to siting.
RG. 1.7A reactor should be located away from population centers of more than 25,000 people.

Table 2 .
Regulations and regulatory guides related to siting.

Table 5 .
Regulations and regulatory guides related to operations staffing.

Table 6 .
Classification of packaging type for transportation of radioactive materials.

Table 7 .
Regulated radiation dose limits for transportation of radioactive materials.