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APPLICABILITY OF IAEA SAFETY STANDARD SSR-2/1 TO WATER-COOLED SMALL MODULAR REACTORSMADDEN Kristine MarieInternational Atomic Energy AgencyEmail: K.Madden@iaea.org

MAGRUDER StewartInternational Atomic Energy AgencyEmail: S.Magruder@iaea.org

SUBKI HadidInternational Atomic Energy AgencyEmail: H.Subki@iaea.org

Abstract

In 2016, the International Atomic Energy Agency (IAEA) conducted a study on the graded approach application of the IAEA design safety requirements contained in SSR-2/1, Safety of Nuclear Power Plants: Design, to small modular reactors (SMRs) with a focus on water-cooled and high temperature gas reactors. Each of the eighty-two SSR-2/1 design safety requirements was evaluated for their applicability to SMR designs by a team of international experts that included vendors, regulators and internal personnel. The approach utilized was to bin the requirements into one of the following five categories: applicable as is; applicable with modification; applicable with interpretation; new criteria; and not applicable. For water-cooled SMRs, proposed changes were made to thirteen SSR-2/1 design safety requirements and one new design safety requirement was suggested. The results of the study will be used as an input to the development of future IAEA safety standard review processes in order to reflect practices in this area and foster their practical applicability to SMRs.

1. INTRODUCTION

A continuously increasing interest in nuclear power has been obvious over the past several years in several Member States with already established nuclear power programmes, as well as in Member States at various stages of preparation or initiation of a nuclear power programme. At present, there are at least 50 SMR designs for which research and development (R&D) work is being undertaken. The following three industrial demonstration SMRs are in advanced stages of construction: CAREM, a 30 MW(e) PWR in Argentina; HTR-PM, a 250 MW(e) high temperature gas cooled reactor in China; and KLT-40S, a floating power unit in the Russian Federation. These designs are scheduled to begin operation between 2018 and 2020. Dozens of SMR designs are also being prepared for near term deployment, including the NuScale reactor, which is currently being reviewed for design certification in the United States, the SMART reactor developed by Republic of Korea and the ACP100 being developed in China. As a result of these different design approaches, technologies and safety features, Member States must establish new design safety requirements or apply their current design safety requirements to advanced reactor designs.

A study on the current practices of applying the IAEA Safety Standard SSR-2/1 design safety requirements to SMR technologies was organized by the IAEA and contributions were made by a team of international experts. The participants evaluated the application of SSR-2/1 design safety requirements to at-large SMR designs expected to be deployed in the near-term. This included the water-cooled reactor, water-cooled floating reactor and high temperature gas reactor SMR designs. The results of the study were then compiled and distributed to the team members prior to the consultancy meeting held at the IAEA headquarters in Vienna, Austria in February 2017. The consultancy was divided into two working groups representing the gas-cooled and water-cooled SMR designs. Each working groups developed a common understanding for each of the eighty-two design safety requirements in SSR-2/1 which is described here within. The participants developed contributions to a project report as a part of a home-based assignment to document the results of the consultancy meeting. The contributions related to water-cooled SMRs are documented within this paper. The contributions related to high temperature gas reactor SMRs are expected later this year. This input will be used in the development of future IAEA safety standards review process in order to reflect practises in this area and foster the practical applicability to SMRs.

2. WATER-COOLED SMALL MODULAR REACTOR GENERIC DESIGN DESCRIPTION

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A SMR is generally defined as an advanced reactor that generates equivalent output capacity of up to 300 MW(e) and is designed in modules aiming for the economy of serial production. SMRs are derived from designs used in the design of standard nuclear power plants, including water-cooled, high-temperature gas cooled, liquid metal cooled with fast neutron spectrum, and molten salt-cooled reactors.

For water-cooled SMR designs, many of the designs adopt the integrated pressurized water reactors (iPWR) concept, for which due to the lower thermal power of up to 1000 MW(th), the components within the primary reactor coolant system (e.g. steam generators, pressurizer) can be installed within the reactor vessel together with the core. This integration of the primary cooling system is an approach to enable modular deployment. From a safety point of view, the potential for large and medium breaks, such as hot/cold leg, pressurizer surge line, and primary pump suction/discharge line breaks are eliminated by a function of design. While most of SMR designs are land-based, some countries are also pioneering in the development and application of floating and marine-based nuclear power plant powered by PWR-type (pressurized water reactor) SMRs. Innovative features allow SMRs to converge to provide safe, reliable and affordable plants and are described as follows:

a) Design Simplification and CompactnessThe integral configuration results in a lighter weight, better transportability and compact reactor. This integration yields substantial reduction in the size of the nuclear steam supply system (NSSS). Some iPWR SMR designs adopt natural circulation for the primary heat removal from the core. The need for primary pumps can then be eliminated; hence, loss of flow event due to pump failure is practically eliminated. Natural circulation also reduces mechanical complexity. Other designs adopt conventional forced convection either using horizontally or vertically mounted primary pumps at the reactor vessel through nozzles. In-vessel steam generator (SG) cartridges are adopted for all iPWR SMR designs. The once-through helical coil steam generator is one of the types being utilized and offers a larger heat transfer area in a compact geometry.

b) Enhanced SafetyMost if not all PWR-based SMRs adopt passive safety systems, which are based on natural laws, such as gravity-driven and natural circulation (e.g. buoyancy force). The integral design of NSSS module eliminates external coolant loop piping, which eliminates the large-break loss-of-coolant accident (LBLOCA). The passive engineered safety features (ESFs) eliminate the need for external power under accident conditions. With these passive safety systems, small-break LOCAs do not significantly challenge the safety of the plant. The expected core damage frequency (CDF) is expected to be of the order of 10-6 to 10-8 per year; however, this needs to be confirmed by further detailed probabilistic safety analysis (PSA) as the designs evolve..

c) Economic CompetitivenessSMRs are intended for specific markets in which large reactors would not necessarily be applicable or competitive. SMRs may provide an attractive and affordable nuclear power option for developing countries with small electrical grids and limited investment capability. The adverse impacts of the economy of scale is compensated by offering the economy of mass production of prefabricated modules, a simplified and standardized design, shorter construction time, less operation and maintenance cost, the option of incremental capacity increase, and cogeneration of electricity and process heat.

3. PROCESS

In 2016, the IAEA conducted a study on the applicability of the IAEA Safety Standard SSR-2/1, Safety of Nuclear Power Plants: Design, to SMR technologies; the study focused on water-cooled and gas-cooled SMRs. First, a home-based assignment verifying the applicability of SSR -2/1 to SMRs was completed by Member State participants. The Member States were asked to grade the applicability based on the following criterion:

- Applicable as is- Applicable with modification- Applicable with interpretation- Not applicable

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- New criteriaThe difference between applicable with modification and applicable with interpretation should be noted.

Applicable with modification infers that the text needs to be modified for the requirement to be applicable to the design; whereas, applicable with interpretation infers that the definition of a pre-existing word must be modified for that specific technology and therefore does not result in changes to the text. After determining the applicability of a requirement, participants were asked to provide input into what modification or interpretation was required, if any. Further, to aid in the understanding of the specific change, participants were also requested to provide rationale for said change. Following receipt of the participant’s comments, the IAEA compiled said requirements into a common document for review at a consultancy meeting from 20 – 24 February 2017 in Vienna.

Based on the differences in technology, the February 2017 consultancy meeting was divided into two working groups; one for water-cooled SMRs and another for gas-cooled SMRs. Each group completed an intensive review of the received comments and developed a common understanding among participants for each of the comments requiring modification and/or interpretation. The working groups focused on addressing comments required as a result of direct changes required for their specific SMR technology. The working groups avoided changes where changed were solely related to the improvement of the document as a whole rather than a focus on the direct impact from a technology-specific viewpoint.

When reviewing the requirements, the working groups utilized a liberal interpretation of qualifiers, such as necessary and appropriate. Participants regarded these qualifiers as options allowing one to opt-out of said requirement should it be deemed unnecessary or inappropriate where required.

4. AUDIENCE

The intended audience for this project report is Member State design organizations, technical support organizations, regulators and operators interested in developing a nuclear power plant programme or with an existing nuclear power plant programmes.

5. SUGGESTIONS TO SSR-2/1 APPLICABILITY TO WATER-COOLED SMALL MODULAR REACTORS

Changes were recommended to about 15% of the SSR-2/1 design safety requirements. Note, the changes were developed with the assumption that standard water-cooled nuclear power plants and water-cooled SMRs would be included together in a future document; rather, than requiring a new design safety requirement standard to be developed that is exclusive to SMRs. Figure 1 provides the distribution of changes required in SSR-2/1 based on the suggestions made.

A common theme was identified among the recommended changes to the SSR-2/1 design safety requirements. For a number of these suggestions, the contributing factor was related to the fact that several SMRs can be part of a single plant; thereby, allowing a nuclear power plant to have several cores. As such, the terminology “core,” “module,” “unit” and “plant” needs to be consistently applied throughout SSR-2/1. As a result of SMRs allowing for multiple cores within one plant, some requirements related to the location,

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FIG. 1. SSR-2/1 Distribution for Recommended Changes

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segregation and independence of safety systems were suggested for change. In addition, consideration for compactness of design had to be considered as this may affect access to containment at full power. Further changes related to emergency power supply systems were evaluated and augmented to consider that in the case of some SMR designs, safety features may not be dependent on electrical power. One of the benefits of SMRs is that these plants can be manufactured almost completely at a manufacturing facility as a result of simplified systems and smaller footprints. Other changes were suggested based on the option for a fleet solution to waste management given that several units may be considered for a site.

Table 1 provides a detailed list and explanations for the changes suggested to SSR-2/1 design safety requirements for water-cooled SMRs.

TABLE 1. Suggested Changes and New Design Safety Requirements for Consideration to SSR-2/1

Requirement No. Suggested Changes and New Design Safety Requirements for Consideration11, Provision for Construction

Proposed changes, suggestions or consideration to text: In 4.19, change wording “in a provision of construction and operation” to “in a provision of manufacture and construction.”Interpretation required: None.Justification for proposed modification and/or interpretation: An industry viewed best practice is to manufacture SMR components and modules in a factory setting rather than to construct the entire plant at the final plant site. In this view, SMR power plants are in many cases being designed to optimize offsite manufacture of major portions of the nuclear power plant to leverage the value of this best practice. From this practice, multiple advantages are gained, including the:

Ability to build more of the plant as modules in factories; Factory packaging of modules can ensure they arrive on site in an as expected state so

that quality can be assured; Use of modules enables easier inspection to ensure that equipment and components are in

an as expected state prior to installation; thus, ensuring quality and hence safety.With the implementation of factory manufacturing, there is a need for the inclusion of manufacturing as one of the provisions associated with this specific safety requirement. Additionally, Requirement 11 provides for, and is bounded by the aspects associated with the manufacture, construction, assembly, installation and erection activities and specifically does not include any necessity of recognizing the impacts of operations. To this end, “operation” is proposed to be removed from this requirement as proposed to ensure consistency of application of this safety requirement.

12, Features to Facilitate Radioactive Waste Management and Decommissioning

Proposed changes, suggestions or consideration to text: In paragraph 5.15A, change the word ‘located’ to ‘located and/or segregated.’Interpretation required: Broaden the interpretation of sources of internal and external hazards to include those that could arise from any connection to process heat facilities coupled directly to SMRs.Justification for proposed modification and/or interpretation: The interpretation of the use of the word ‘located’ in the current requirement implies separation by distance. As SMRs are intended to be more compact nuclear power plants, protection against zonal effects can be provided by appropriate barriers as much as through separation by distance.Future applications of SMRs include the direct use of process heat from the power plant, e.g. for district heating, heat processing or water desalination. It is important to note that these additional connections are also potential sources of hazards.

17, Internal and External Hazards

Proposed changes, suggestions or consideration to text: Add a new sub-item:‘4.20A. Where there are multiple units of the same type, consideration should be given to the derivation of a fleet solution for waste management and the decommissioning process’.Interpretation required: None.Justification for proposed modification and/or interpretation: The requirement as currently written can be interpreted as applying only to a single nuclear power plant. SMRs by design could be built in larger numbers in a geographic area, and therefore, could enable a fleet solution to be derived

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for the effective and safe management of waste and the decommissioning process. This may enable consideration to be given to the construction of a single facility that is built solely for that purpose. Such a fleet facility would have greater throughput of waste thereby offering a greater opportunity for the application of advanced processing technology to reduce environmental impact.

33, Safety Systems, and Safety Features for Design Extension Conditions, of Units of a Multiple Unit Nuclear Power Plant

Proposed changes, suggestions or consideration to text: Each unit, which may be comprised of one or more reactor cores, of a multiple nuclear power plant shall have its own safety systems and shall have its own safety features for design extension conditions.Interpretation required: The term “unit” is interpreted to mean a single large reactor core and dedicated secondary and auxiliary systems. If the term “unit” can be interpreted (or defined via a glossary) as a collection of smaller reactor modules, then the wording of the original requirement is acceptable as written.Justification for proposed modification and/or interpretation: SMRs introduce an additional level of reactor grouping than has been traditionally the case. Existing experience is to have a plant (or site) comprised of one or more single reactor units and associated secondary and auxiliary systems. SMRs introduce the possibility of units being comprised of multiple smaller cores with shared secondary or auxiliary systems. The safety requirements must be clear as to whether they pertain to an individual reactor primary system (module), a grouping of modules (unit) or the entire plant/site.

45, Control of the Reactor Core;46, Reactor Shutdown;51, Removal of Residual Heat from the Reactor Core; 52, Emergency Cooling of the Reactor Core

Proposed changes, suggestions or consideration to text: None.Interpretation required: The term ‘the reactor core’ can be one core or several cores.Justification for proposed modification and/or interpretation: In case of an SMR, there may be several cores in one plant. Each of the cores in the plant shall be inherently stable. In a large NPP, we use ‘control of the reactor core’ even when there are more than one reactors located in the same plant. In the case, means for the removal of residual heat need to be provided for each core. The terminology of “core”, “module”, “unit” and “plant” needs to be consistent throughout SSR-2/1.

57, Access to the Containment

Proposed changes, suggestions or consideration to text: Replace: “airlocks equipped with doors that are interlocked to ensure that at least one of the doors is closed during reactor power operation and in accident conditions” with “controlled and monitored access ways to ensure containment functionality during reactor power operation and in accident conditions.”Interpretation required: None.Justification for proposed modification and/or interpretation: Maintaining at least one door closed during reactor power operations and in accident conditions ensures reactor containment integrity is maintained during the time personnel access to the interior of containment is desired. Many SMR containments are not designed for any human habitation during power operations and are not equipped with large doors or equipment access hatches. Some, such as the NuScale Power Module, with its small volume containment vessel, utilizes by design, appropriately sized manway-like access hatches and openings rather than doors or airlocks.To ensure consistency in the intent of this safety requirement in its application to SMRs, it is appropriate, as proposed, to modify the wording to ensure the access ways to containment are “controlled” and “monitored” to ensure containment functionality during reactor power operations and in accident conditions. With this proposed change, reactor designers will be afforded the opportunity to employ containment access means and methods that are not limited to “doors and airlocks” and which are directly appropriate for the containment system associated with their particular design, all the while, remaining in compliance with the intent of the requirement.

68, Design for Withstanding the Loss of Off Site Power

Proposed changes, suggestions or consideration to text: Change the requirement to: “If a system important to safety at the nuclear power plant is dependent upon power, the design of the nuclear power plant shall include an emergency power supply capable of supplying the necessary power in anticipated operational occurrences and design basis accidents, in the event of a loss of off-site power. If required, the design shall include an alternate power source to supply the necessary power in design extension conditions.”

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Interpretation required: Add a new paragraph of interpretation: 6.45B. The design of a power plant may incorporate safety features that are not dependent on electrical power. In this case, consideration should be given to requirements 6.44A and 6.44C for alternate power sources to further strengthen the adopted defence in depth against design extension conditions.Justification for proposed modification and/or interpretation: The current requirement as written can be interpreted that an emergency power supply is required to maintain safety in the event of a loss of off-site power. SMR may be designed with passive or non-power dependent safety features and therefore are not reliant on power in order to maintain safety. Therefore, inclusion of extensive emergency power supplies could be unnecessary.In addition, alternate power sources may not be required to make the design of the SMR robust against Design Extension Conditions.

73, Air Conditioning and Ventilation Systems

Proposed changes, suggestions or consideration to text: Change sub-item 6.49 to: “6.49. The design should minimize spread of contamination from areas of high contamination to areas of low contamination.”Add sub-item 6.49A: “6.49A: The design of any air conditioning, air heating, air cooling or ventilation should consider margin for dealing with foreseeable future extension in capacity of the plant.”Interpretation required: None.Justification for proposed modification and/or interpretation: For 6.49: Even though the negative pressure differential (partial vacuum) has been utilized for the minimization of contamination spread for the existing nuclear plants, other mechanisms can be utilized to achieve the aim. The original requirement describes the negative pressure differential as only measure for the minimization of contamination spread, and as such requires generalization for SMR application. Especially for the SMR with passive safety system, alternative means can be employed for isolating areas of contamination from clean areas when an accident occurs.For 6.49A: Some SMR designs adopt extension of power capacity during plant lifetime through additional module installation. Changes in requirements or capability may result in the addition of new equipment which could increase the load on HVAC systems. Therefore, consideration should be given to including margin in the design capability of HVAC to allow for the potential addition of new equipment at a later date. This will reduce the impact on equipment important to safety.

76, Overhead Lifting Equipment

Proposed changes, suggestions or consideration to text: Remove “overhead” from the title and the body of the requirement. Also, replace “crane” with “lifting equipment”.Interpretation required: None.Justification for proposed modification and/or interpretation: Limiting the lifting equipment to overhead equipment only may have two undesirable effects: Some SMRs designs do not allow for the use of overhead lifting equipment because of a lack of overall volume to handle the items. Requirement 76 as it is currently written is not applicable for such designs.Due to the compactness of some SMRs designs, overhead lifting equipment may have a negative impact on safety by increasing the risk of dropping loads.The proposed change removes this limitation, allowing for more adapted lifting equipment when appropriate, such as jacks, fork lifts, etc.It should be noted that the new formulation will most likely have very little impact on the lifting equipment used in large scale plants: the items to be lifted are heavy enough to necessitate overhead lifting.

No. 78 Systems for Treatment and Control of Waste

Proposed changes, suggestions or consideration to text: Change sub-item 6.59 to include the following text: “6.59. Systems and facilities shall be provided for the management and storage of radioactive waste on the nuclear power plant site, or at a suitable dedicated site, for a period of time consistent with the availability of the relevant disposal option.”Interpretation required: None.Justification for proposed modification and/or interpretation: Similarly to Requirement 12, this requirement as currently written can also be interpreted as applying only to a single nuclear power plant. SMRs by design could be built in larger numbers in a geographic area and therefore could enable a fleet solution to be derived for the effective and safe management of waste and the

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decommissioning process. This may enable consideration to be given to the construction of a single facility that is built solely for that purpose. Such a fleet facility would have greater throughput of waste and therefore would offer a greater opportunity for the application of advanced processing technology to reduce environmental impact.In addition, alternate power sources may not be required to make the design of the SMR robust against Design Extension Conditions.

New Requirement 41A, Interactions between the Heat Delivery System and the Plant

Proposed changes, suggestions or consideration to text: The functionality of items important to safety at the nuclear power plant shall not be compromised by disturbances in the heat delivery system between the plant and a coupled process heat facility if present.Justification for addition: SMRs provide greater opportunity for nuclear power plants to support industrial facilities that utilize heat or both heat and electricity. Analogous to Requirement 41, which addresses the potential impact of grid disturbances on reactor safety, this proposed requirement addresses potential disturbances from the process heat user. An example is that the shutdown of a coupled desalination plant would represent a loss of load to the reactor plant.

6. CONSIDERATIONS FOR FUTURE WORK

In regards to future work, the participants recommended the following:- A future revision of SSR-2/1 to incorporate considerations outlined in Section 5 of the report;- Further review of the SSR-2/1 requirements to high temperature gas reactors through the

implementation of additional meetings;- A review of the U.S. NRC 10CFR50 Appendix A requirements for advanced non-light water

reactors;- Development of an IAEA technical report to document the outcomes of this project.

7. ACKNOWLEDGEMENTS

FLAUW, Yann FranceFLOWER, Alison United KingdomINGERSOLL, Daniel United States of AmericaKANG, Han Ok Republic of KoreaMOOR, Steve United KingdomSONG, Danrong People’s Republic of ChinaRICKMAN, Robin United States of America

8. REFERENCES

[1] Safety of Nuclear Power Plants: Design, SSR-2/1, IAEA, Vienna (2016).

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