Status of Development, ANS Standard 53.1, Nuclear Safety Criteria for the Design of Modular Helium Cooled Reactor Plants

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1 Status of Development, ANS Standard 53.1, Nuclear Safety Criteria for the Design of Modular Helium Cooled Reactor Plants M. LaBar General Atomics 3550 General Atomics Court San Diego, CA Abstract A new American Nuclear Society (ANS) standards development subcommittee, Identified as ANS 28, has been organized to develop ANS Standards for High Temperature Gas Reactors. The first standard the subcommittee is developing is a safety design criteria standard entitled Nuclear Safety Criteria for the Design of Modular Helium-Cooled Reactor Plants and is identified as ANS Standard The scope of ANS 53.1 has been developed to explicitly apply to the type of passively safe, modular helium cooled reactor plants currently under development by several organizations in the world. The intent of ANS 53.1 is to provide safety criteria for the design of modular helium reactors based on the use of performance based, risk informed methodologies. The goal of the ANS 28 subcommittee is to complete a first draft of ANS 53.1 during calendar year The status of the standard development work is described in this paper. I. INTRODUCTION Many ANS standards have been developed and are successfully being used by nuclear power plant designers and suppliers, plant owners and operators, as well as State and Federal regulatory bodies. Almost all of these standards were originally developed for application to light water reactor (LWR) nuclear power plants. A new generation of advanced gas cooled reactors is currently being developed. There are several projects being conducted by various organizations throughout the world on development of advanced gas reactors that have in common fuel, moderator and coolant. These plants have been evaluated to have attractive key performance characteristics (safety, economics, proliferation resistance, etc). A high temperature variant is planned for a Next Generation Nuclear Plant (NGNP) demonstration project to be built at the Idaho National Laboratory (INL) If the performance characteristics are successfully demonstrated in appropriate prototype plants, such as the NGNP project, significant numbers of these plants are expected to be deployed to help meet future energy requirements. In view of the pending NGNP project and the expected commercialization of the technology, the ANS Nuclear Facilities Standards Committee (NFSC) implemented the formation of a subcommittee, identified as ANS 28, specifically for development of standards for gas reactors of the type currently under development. This paper provides a status of the work to-date by the ANS 28 subcommittee. II. BACKGROUND ON ANS GAS REACTOR STANDARDS The current generation of gas reactors being developed has evolved from the High Temperature Gascooled Reactor (HTGR) technology developed in the 1960s and 70s in the United States and Germany. In the United States, two HTGR plants were designed, constructed and operated: Peach Bottom I, a 115 MW(t) development plant constructed on the Philadelphia Electric Company system that started up in Fort St. Vrain, an 842 MW(t) demonstration plant constructed on the Public Service Company of Colorado that started up in These early US HTGRS led to the development of large HTGR reactors for the US market in the MW(t) range intended to be deployed starting in late 1970s. Contracts were signed for the supply of several of these large HTGR plants. During the large HTGR development phase, an effort was initiated to develop HTGR ANS standards. Development of ANS standards for HTGRs was taken to the point of completing, in January 1974, Draft 9, Revision 2 of a standard entitled Nuclear Safety Criteria for the Design of Stationary Gas Cooled Reactor Plant. This standard carried the number N213 and was produced by an ANS 23 subcommittee. Subsequently, the draft was renumbered to ANS 53.1, an identification number 1

2 that corresponds to ANS 51.1, Nuclear Safety Criteria for the Design of Stationary Pressurized Water Reactor Plants and ANS 52.1, Nuclear Safety Criteria for the Design of Stationary Boiling Water Reactor Plants. The safety design criteria of N213 (53.1) were intended to be specifically applicable to gas cooled reactors having the following characteristics: 1. Helium cooled, 2. Solid ceramic fuel enclosed in a graphite matrix, 3. Fixed fuel-moderator-reflector geometry, 4. Operational control of reactivity by remotely actuated mechanisms which move control elements, 5. Core and coolant enclosed with a high integrity prestressed concrete reactor vessel (PCRV), 6. PCRE enclosed with a containment barrier of high integrity, 7. Graphite moderated. Effort on development of ANS standards for HTGR plants stopped subsequent to 1974 due to the cancellation of all the HTGR plants on order, along with the cancellation of approximately 100 other nuclear power plants due to the impacts of the oil embargo in the mid 1970s. Further work on the development of ANS standards for HTGR laid dormant until recently, circa III. ANS 28: STARTUP PROCESS AND MEMBERSHIP COMPOSITION The primary motivation for the NFSC to reinaugurate a subcommittee for the development of gas reactor standards was the emerging worldwide interest in gas cooled reactors and the announced intention to deploy the NGNP in Idaho. Membership in this new subcommittee was solicited from various stakeholders and a subcommittee kick-off meeting was held at the November 2003 ANS annual meeting in New Orleans. At the November 2003 meeting, the process for development of ANS standards was reviewed. Between the first subcommittee meeting held in November 2003 and the next one held at the following ANS annual meeting, efforts focused on filling out the subcommittee membership with representatives from stakeholder organizations. Volunteers from the following organizations were found to serve on the subcommittee: General Atomics (Reactor vendor) Framatome (Reactor vendor) Westinghouse Electric Co. (Reactor vendor) PBMR Ltd (Reactor vendor) Bechtel Power (A-E) Core Inc (Nuclear consultant) Nuclear Regulatory Agency (Regulation) Nuclear Management Co (Utility operator) Idaho National Laboratory (National lab) The type of organization for each of the stakeholders is also shown in the above list. ANS standards are intended to become ANSI standards and to qualify as an ANSI standard, the standard must be a consensus standard. The initial membership of ANS 28 was specifically developed to have representation from the various types of stakeholder organizations to satisfy the consensus requirement. III. ANS 28 FIRST ACCOMPLISHMENTS The first real progress by ANS 28 on standards development was made at the second meeting of the ANS 28 subcommittee held at the June 2004 ANS annual meeting in Pittsburgh, PA. The key accomplishments made at that time were as follows: 1. The Subcommittee decided the first gas-cooled reactor standard to be developed would be the Safety Criteria Standard designated as ANS This standard would serve as the top level gas-cooled reactor safety criteria standard equivalent to ANS 51.1 and 52.1 for PWRs and BWRs, respectively. 2. Following completion of ANS 53.1, the Subcommittee would endeavor to form working groups as required for the development of lower level gas-cooled reactor ANS standards. The working groups would report to Subcommittee as is generally done on other subcommittees. 3. The subcommittee decided the scope of 53.1 would cover both prismatic block and pebble bed modular gas-cooled reactors having passive safety characteristics. 4. For time being, the Subcommittee would focus on safety criteria for satisfying US requirements. 5. However, internationalization of ANS standard(s) is being evaluated. As a first step, for ANS 28 members would develop recommendations for participation by international organizations. 6. The subcommittee decided a first work item needing to be accomplished on ANS 53.1 was development of a new draft for the scope of the standard. 7. A second work item was agreed to be development of an outline for the standard. 2

3 8. The Subcommittee decided to hold monthly teleconferences to discuss work on the standard and to hold quarterly face-to-face meetings. Two of these each year would be held during the ANS annual meetings and one approximately mid-term between each of the ANS annual meetings. 9. The starting point for development of 53.1 would be to review: The safety criteria first developed for the Modular High Temperature Gas-cooled Reactor (MHTGR) as submitted to the NRC in a Preliminary Safety Information Document (PSID) and, The Pebble Bed Modular Reactor (PBMR) topical report on licensing submitted by Excelon to the NRC. 10. The technical document developed by the IAEA, IAEA TECDOC-1366, that deals with development of safety requirements for modular high temperature gas reactors was also identified as a reference ANS 28 should use. IV. ANS 53.1 SCOPE, PURPOSE AND APPLICABILITY Primary purposes of the third ANS 28 meeting held September 2004 in Las Vegas was to agree on the scope, purpose and applicability of ANS Other meeting objectives were to agree on a title, a draft outline for ANS 53.1 and to identify process for preparation of the next sections to be developed. V.A ANS 53.1.Scope and Purpose The draft scope of ANS 53.1 was agreed to be to establish the nuclear safety design criteria and functional performance requirements consistent with established risk objectives for modular helium reactor (MHR) plants. Operations, maintenance, and testing requirements are to be covered only to the extent that they affect design provisions. Included within the scope of the standard is: A risk-informed process for identifying and categorizing events as frequent, infrequent, and rare. The identification of a risk informed safety classification methodology for classifying all SSC equipment into safety classes according to their importance to nuclear safety, and Identification of specific requirements for Systems, Structures and Components (SSCs) important to safety. These requirements are to be related to other, more specific design standards and intended to amplify the criteria given in the Code of Federal Regulations, Title 10, "Energy," and other top-level regulatory requirements and guidance. The purpose of ANS 53.1 was agreed to be to identify requirements that provide a degree of assurance that MHR plants are designed and constructed such that they can be operated without undue risk to public health and safety while not imposing undue burden upon the development of such plants. V.B.Applicability of ANS 53.1 The purpose of the standard was agreed to be specifically for MHR plants that may consist of one or more reactor modules. MHR modules are standard configurations that consist of a nuclear reactor coupled to a direct or indirect power conversion system and/or a process heat utilization system, and which can be replicated as necessary to produce the required output. In all cases, an MHR module utilizes helium primary coolant to transport the thermal energy produced in the reactor core to the energy utilization element. From the onset, there was general agreement amongst the ANS 28 subcommittee members that ANS 53.1 would be developed for the type of the passively safety MHR plants currently under development by several organizations throughout the world. The subcommittee agreed safety design criteria would be developed intended to be specifically applicable to MHR plants having one or more reactor modules, with each module having the following characteristics: a) Helium primary coolant b) Graphite moderator c) Ceramic coated particle nuclear fuel enclosed in a graphite matrix d) Defined core geometry to be maintained in order to assure adequate core cooling e) Core contained within a metallic reactor pressure vessel f) Reactor vessel contained within a robust building structure 3

4 g) Capability for maintaining radioactive releases within public health and safety requirements solely by passive SSCs and/or inherent characteristics (e.g., high integrity ceramic fuel particles, passive heat removal, strong negative temperature coefficient of reactivity). Some of these characteristics are basically the same as the gas reactor characteristics given in Section II for the original 1974 draft. There are, however, important differences. A characteristic of defined core geometry is used in lieu of fixed fuel-moderator-reflector geometry (for application to pebble bed reactors); A metallic reactor pressure vessel is identified in lieu of a PCRV, No mention is made of operational control rods (also for application to pebble bed reactors), The capability is identified for control of radioactive releases solely by passive means. In addition to these differences, the standard is identified to be applicable to plants having one or more reactor modules. V.C. ANS 53.1Title While seemingly not a major issue, considerable discussion was required before agreement was reached on the title for the standard. These discussions resulted in selecting the title to be Nuclear Safety Criteria for the Design of Modular Helium Cooled Reactor Plants. Important differences between this title and the titles for the corresponding PWR and BWR ANS standards are, (1) no mention is made of the plants being stationary and (2) the term Modular is included in the title. The term Stationary was left out of the title because the subcommittee could not identify a reason for including the term. The term Modular was specifically included in the title because of the intent for the standard to apply to plants having one or more reactor modules. In passing, it should be noted the selected title was not unanimously accepted by all members of the ANS Standards Board. One member objected to including the term Modular in the title. The subcommittee s review of his position concluded that the objector did not recognize the implication that the standard was intended for plants containing multiple reactor modules and that there was valid reason for including the term Modular. As indicated above, the purpose of the standard makes this intention clear. VI. ANS 53.1 STATUS OF DEVELOPMENT In conjunction with development of the scope and purpose, a draft outline of the standard was developed based upon the standard implementing risk-informed, performance based safety criteria. The current draft outline is as shown in Table 1. Table 1 ANS 53.1 Draft Outline 1. Introduction 1.1 Scope 1.2 Purpose 1.3 Approach to Standard 2. Definitions 3. Safety Criteria 3.1 Risk Informed, Performance-based Approach [philosophy section on approach] Performance-based Criteria for Limiting Risk to the Public Frequency versus Consequence Limit Curve 3.2 Top Level Regulatory Bases Criteria Frequency-Consequence Acceptance Criteria Basis for Selection of Event Region Quantitative Frequency and Dose Ranges Reactor vs. Plant Risk Criteria 3.3 Process for Selection of Licensing Basis Events (LBEs) Performance of PRA Establishing Event Families Selection of LBEs for each Region 3.4 Process for Risk Informed Safety Classification of SSCs Functional Analyses and Risk Insights to Determine Safety Functions for LBEs Safety Classes for SSCs Safety Class Interfaces Safety Class Requirements Correlations Special Treatment Requirements for SSCs Principles and Processes for Identifying Special Treatment Requirements 4

5 3.5 Deterministic Safety Analyses General Requirements Requirements for LBEs Requirements for Other Plant Conditions 3.6 Defense in Depth Defense in Depth for Modular Helium- Cooled Reactor Plants Application of Defense in Depth for LBEs Defense in Depth SSCs Role of the Operator Emergency Planning 3.7 Industry SSC Codes and Standards Mechanical Equipment (Safety) Electrical Equipment (Safety) Plant Structures (Safety) Non-Safety Equipment Quality Assurance 4. Design Criteria 5. References Appendices (to be identified) As noted above, Section 1, Scope and Purpose, is essentially done (in rough form). Drafting efforts are now in process for several parts of Section 3. Section 3.1, the philosophical approach (risk-informed, performance based) has been drafted. Initial drafts of Sections 3.2 and 3.3 have been prepared and the draft form of the frequency versus consequence limit curve has been identified (Figure 1). reached a consensus position. Agreement on the content of this section is expected to be accomplished in the next meeting of the subcommittee currently scheduled in the later part of September VII LESSONS LEARNED On the bright side, work on Section 3.4 has identified a productive approach to the preparation of standards. Most ANS standards appear to have been drafted by a small number of experts (1,2 or 3). The subcommittee started out with a much larger number of people, all interested in helping to develop ANS Initially, drafting efforts of various sections went to individuals. This process tended to thwart the consensus process and resulted in additional inputs by other individuals. Combining the inputs into a composite draft was problematic. For Section 3.4 (equipment classification) a 3 man working group of experts on the subject was established to prepare the draft. This approach proved to be a very productive way for developing a composite draft (even though in the case of Section 3.4 getting agreement on the requirements has not yet been achieved). This same approach, use of working subgroups consisting of 2 or 3 people, is planned to be used for the preparation of draft sections for the balance of ANS The other key lesson learned to-date is that it is not easy to reach consensus, but once reached, there is likelihood the result will have staying power. A prime example is the title of the standard. Even though subcommittee spent considerable effort on deriving the title, a follow-on reviewer was unwilling to accept it. But, because the subcommittee had carefully deliberated the title, all the subcommittee members were willing to stand behind it and requested the title remain unchanged. VIII. SUMMARY AND CONCLUSIONS The salient points and conclusions of this paper are summarized as follows: Advanced HTGR plants are being developed by several organizations throughout the world. All of these plants have in common the use of coated particle fuel, graphite moderator and helium coolant. Fig 1. Frequency-Consequence Limit Curve (Draft) The subcommittee is in the process of preparing a draft of Section 3.4 on safety classification of SSCs. Section 3.4 has turned out to be the most contentious section attempted to date and the subcommittee has not The NFSC concluded the growing interest in advanced HTGR plants and the prospect of a demonstration plant of a very high temperature variant at the INL warranted the development of ANS standards for these reactor plants. A subcommittee identified as ANS 28 was established for development of ANS standards for 5

6 gas reactors and populated with membership from representative stakeholder organizations. The first standard undertaken for development by ANS 28 is the top level safety criteria standard for gas reactors identified as ANS 53.1 intended to be equivalent to ANS 51.1 for PWR plants and ANS 52.1 for BWR plants. The scope of the standard has been defined to establish the nuclear safety design criteria and functional performance requirements for MHR plants, consistent with established risk objectives. The purpose of the standard has been defined to be specifically for MHR plants that may consist of one or more reactor modules where MHR modules are standard configurations that consist of a nuclear reactor coupled to a direct or indirect power conversion system and/or a process heat utilization system. The standard is specifically intended for MHR plants with reactor modules having the following characteristics: a) Helium primary coolant b) Graphite moderator c) Ceramic coated particle d) Defined core geometry e) Metallic reactor pressure vessel f) Reactor vessel contained within a robust building g) Capability for containing radioactive releases solely by passive SSCs and/or inherent characteristics An applicable title for the standard has been concluded to be: Nuclear Safety Criteria for the Design of Modular Helium Cooled Reactor Plants An outline for the standard has been developed and various sections for the standard are in the process of being prepared. A risk-informed, performance based approach has been selected for the standard and a draft frequencyconsequence limit curve has been established. The best approach for the drafting of individual sections has been concluded to be for a small working group (2 to 3 people) to prepare a draft for subsequent review by the composite subcommittee. 6