Utility requirements for advanced LWR passive plants

Nuclear Engineering and Design 136 (1992) 187-193 North-Holland

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Utility requirements for advanced LWR passive plants J.M. Y e d i d i a a n d W.R. S u g n e t Electric Power Research Institute, Palo Alto, California, USA

Received 1 September 1990

LWR Passive Plants are becoming an increasingly attractive and prominent option for future electric generating capacity for U.S. utilities. Conceptual designs for ALWR Passive Plants are currently being developed by U.S. suppliers. EPRIsponsored work beginning in 1985 developed preliminary conceptual designs for a passive BWR and PWR. DOE-sponsored work from 1986 to the present in conjunction with further EPRI-sponsored studies has continued this development to the point of mature conceptual designs. The success to date in developing the ALWR Passive Plant concepts has substantially increased utility interest. The EPRI ALWR Program has responded by augmenting its initial scope to develop a Utility Requirements Document for ALWR Passive Plants. These requirements will be largely based on the ALWR Utility Requirements Document for Evolutionary Plants, but with significant changes in areas related to the passive safety functions and system configurations. This work was begun in late 1988, and the thirteen-chapter Passive Plant Utility Requirements Document will be completed in 1990. This paper discusses the progress to date in developing the Passive Plant requirements, reviews the top-level requirements, and discusses key issues related to adaptation of the utility requirements to passive safety functions and system configurations.

1. Introduction

2. Program participants

For several years, the U.S. utility industry, with the support of utility organizations in Europe and Asia, has been sponsoring a major program, managed by the Electric Power Research Institute (EPRI), to develop design concepts for the next generation of light water reactors. This effort has become known as the Advanced Light Water Reactor (ALWR) Program. The purpose of this paper is to provide a perspective on the ALWR Program - in particular, on that part of the program that relates to the newly proposed designs of light water reactors - BWRs and PWRs - which rely primarily on passive means, such as gravity, natural convection or stored energy, to remove decay heat from the reactors, provide emergency core cooling, and contain fission product activity even in the unlikely event of a core damaging accident.

The ALWR Program involves a broad spectrum of participants, including U.S. and international utilities, NSSS vendors, architect engineer and consultants, National Labs and Universities, as well as support from U.S. DOE. The Utilities, represented by a Steering Committee and supported by the EPRI staff, provide direction and guidance to the program. The utilities have accumulated a substantial body of nuclear experience over the past thirty years and understand very well what they want and need in the next generation of nuclear power plants. More importantly, the utilities are the ones that bear the responsibility to their customers, their stockholders and to the public for t.he safety, environmental compatibility as well as financial viability of their power plants. Therefore it is only natural that they seek to have a major role in defining the path which nuclear power plant design will take in the future. The Steering Committee representing the utilities and directing the program consists of senior members of U.S. and foreign companies which run and operate nuclear power plants, and these members vote

Correspondence to: Mr. J. M. Yedidia, Electric Power Research Institute, 3412 Hillview Avenue, P. O. Box 10412, Palo Alto, CA 94303, USA.

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and decide on all the key issues which arise during the development of the Requirements Document. The role of the reactor vendors, architect engineers and consultants, who participate in the program as contractors, is to provide the engineering input required for the development of the detailed technical requirements which describe the advanced reactors' systems. Their work is coordinated and directed by the EPRI Staff. Government agencies are closely involved and interact with the program on a continuous basis. This interaction involves both the Department of Energy and the Nuclear Regulatory Commission. The DOE's A L W R Program supports the design and certification of advanced concepts which are based on principles defined by the A L W R Program. The NRC interacts with the A L W R Program directly, by reviewing the Utility Requirements Document and resolving key safety and licensing issues, and indirectly by providing certification to the designs mentioned above. The international involvement in the program continues to grow. Presently there are six foreign entities, four of them national utilities, who are participating in the program.

performance requirements for the advanced LWR, as established by U.S. utilities in cooperation with their overseas partners. The document will reflect agreements between the utilities and the NRC on A L W R licensing requirements, but will, as stated above, contain primarily requirements which describe the utilities needs, mostly in terms of enhanced reliability and margin. The Requirements Document is intended to be the starting point for subsequent detailed engineering and will be a basis for development of standard plant designs of the future. These plant designs will embrace a family of A L W R variations, including both PWRs and BWRs, both evolutionary and innovative designs. The main difference between the evolutionary and the innovative designs will be in the extent to which the plant safety systems rely on passive, self-sustaining features rather than conventional engineered safety features. The Requirements Document, as presently structured, consists of three volumes (see fig. 1), one which provides the policy and top-tier requirements, one for the Evolutionary Plant A L W R requirements and one for the Passive Plant A L W R Requirements. Volume I is designed for the readers who are less interested in the technical details contained in the two companion volumes. It contains such top-tier requirements as safety targets, availability, lifetime, constructibility, radwaste or occupational exposure. It also provides some major policy statements concerning simplification, design margin, human factors, plant types to be pursued,

3. ALWR requirements document The A L W R Utility Requirements Document is an extensive compilation of the design, construction and

VOLUME I - ALWR TOP-TIER REQUIREMENTS • EXECUTIVE SUMMARY • ALWR POLICIES • ALWR KEY REQUIREMENTS

I VOLUME III - PASSIVE PLANT ALWR REQUIREMENTS VOLUME II - EVOLUTIONARY PLANT ALWR REQUIREMENTS CHAPTER 1 :

OVERALL PERFORMANCE AND DESIGN REQUIREMENTS FOR EVOLUTIONARY ALWR PLANTS CHAPTER 2-13: REQUIREMENTS FOR SYSTEMS AND STRUCTURES

i

CHAPTER 1 :

OVERALL PERFORMANCE AND DESIGN REQUIREMENTS FOR PASSIVE ALWR PLANTS CHAPTER 2-13: REQUIREMENTS FOR SYSTEMS AND STRUCTURES

Fig. 1. ALWR requirements document structure.

I

J.M. Yedidia, W.R. Sugnet /Advanced LWR passive plants the regulatory stabilization, proven technology, economics, etc. Beyond that, Volume I also addresses the issue of A L W R implementation scenarios, for both the Evolutionary and Passive designs. Volumes II and III present much more detailed requirements for specific A L W R design concepts. Volume II covers the BWR and PWR versions of the Evolutionary ALWR. These are simpler, improved versions of existing LWRs, nominally 1200 MWe, and they employ conventional, active safety systems. Volume III covers Passive ALWRs, greatly simplified, smaller plants which employ primarily passive means for essential safety functions. Two passive design concepts are addressed in Volume III, the Passive BWR with pressure suppression containment and the loop-type Passive PWR with dry containment. Preparation of Volume II, Evolutionary Plant requirements, is complete. Detailed design, construction and performance criteria have been incorporated into a 13 chapter Requirements Document, all of which have already been submitted to the Nuclear Regulatory Commission for review and comment. Draft Safety Evaluation Reports for these chapters have already been received and a Final Safety Evaluation Report is due by the middle of 1992.

4. The

Passive

ALWR

As early as 1986, when the work on the requirements for the Evolutionary Plant was just beginning,

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the A L W R Program team also began to explore a new LWR concept, which was called the "Passive Plant". This Passive Plant was envisioned as a smaller reactor which would employ primarily passive means - gravity, natural circulation and stored energy - for its essential safety functions. The Passive A L W R design concept was considered to be potentially attractive to utility investors, for several reasons: - Due to the fundamental simplicity of the passive safety concept, it could offer an opportunity to effect wholesale simplification (in the form of reduction of many valves, pumps, tanks, instruments, etc.,) with attendant improvement in construction cost and schedule, plant operability, and maintainability. - By eliminating reliance on active components and human intervention, the Passive Plant could accommodate a wide range of upset conditions and internal and external plant threats, such as loss of all electrical power. A reference size of 600 MWe was selected for the Passive Plant studies. In theory, the Passive Plant could be of any size, but it is likely that ratings significantly higher than 600 MWe will prove impractical or not cost effective, because of the relatively large component sizes (such as reactor vessel, cooling water tanks) involved. Furthermore, a smaller plant size may prove to be an advantage in its own right in that plants of 500 to 600 MWe may fit more easily into the capacity planning schemes of most U.S. utilities. Also, smaller plant sizes offer a potential for shorter construction time,

PRELIMINARY CONCEPT PHASEIf I INITIALSTUDIES~I ~

FEASIBLE CONCEPT

IUTILITYREQUIREMENTS I

INREADY VESTMENT

PHASE{ STANDARD PLANTDESIGN~ [ &NRCCERTIFICATION ! III

[

FIRSTPLANT IMPLEMENTATIOI N I

CONSTRUCTI O&N I SITE SPECIFIC DESIGN DETAIL I

Fig. 2. Passive Plant implementation steps.

CONSTRUCTION

FIRST UNIT

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J.M, Yedidia, W.R. Sugnet / Adt,anced L WR passit,e plants

more extensive modularization of plant equipment, replication learning curve and other factors that can improve overall plant economics. Phase 1 (see fig. 2) of ALWR Passive Plant work was a design competition from which two promising conceptual 600 MWe passive designs, a PWR and a BWR, were selected for further development. These concepts have been described in some detail in several publications (ref. [1,2,3]). Both concepts employ passive safety systems and offer fundamental advances compared to existing plants. The most important of these is the design criterion that no operator action shall be required for a period of three days following a design basis event to protect the plant or public. Phase 2 of the ALWR Passive Plant development, conducted in collaboration with the U.S. Department of Energy, involved fleshing out the details of these concepts through extensive technical studies and equipment and system development activities. In parallel with the development effort of the Passive Plant, Phase 2 has more recently incorporated the preparation of requirements to be included in Volume III of the ALWR Requirements Document. The description of this effort and some of its results are the subject of the following sections of this paper. A follow-on program - Passive Plant Phase 3 - is planned to take the Passive Plant to the point that it is "investment ready". By that we mean that the design of the plant includes sufficient detail, and its components and systems have been sufficiently tested, to demonstrate that it can meet the needs of utilities, regulators and the public, and can provide a sound basis for utility investment.

more certified, investment ready ALWR Passive Plant designs by the mid-1990s. Volume III was submitted to the NRC during calendar year 1990.

6. Passive Plant - Basic Objectives

The objective of the Passive Plant design is to use passive systems to replace the active engineered safety systems presently used in light water reactors. The benefits derived from such an approach are multiple: First, it is expected that a passive design approach will significantly simplify the overall plant design, including a reduction in the number of components and simpler operation and maintenance. Second, it is expected that the overall reliability of the systems will be improved and consequently a significant reduction in the risk to public safety may be achieved. Third, the need for operator action will be strongly reduced, minimizing the uncertainty associated with human behavior. Finally, it is expected that reliance on passive safety features will lead to a recognition by the public that a visible improvement in safety has been achieved. Utilization of passive systems in the ALWR includes both prevention and mitigation of core damaging sequences. For prevention, the systems should be able to provide passive core cooling (decay heat removal) and passive makeup of reactor coolant inventory in the case of a loss of coolant event. For mitigation, the passive safety features should be able to contain radioactive releases from the corc, rcmove containment heat and submerge or flood any core debris which might be released into the containment as a result of the core damaging event.

5. The Passive ALWR Requirements Document

The Requirements Document for the Passive Plant is being prepared along the same lines as those for the Evolutionary Plant. The 13 chapters of Volume II Evolutionary Plant, are being used as the basis for the preparation of the 13 chapters of Volume I I I - Passive Plant. The objective here is to have two completely independent documents which can be referred to by the future user, be it a utility organization or a plant designer, without any need for cross reference. The preparation of Volume III is being conducted at an accelerated pace, to ensure that these requirements will be used by the plant designers who are going to participate in Phase 3 of the program, which, as noted above, is being planned by EPRI and DOE as a major new work phase, targeted at achieving one or

7. Definition of passive safety features

The definition of passive safety has recently evolved into an important discussion issue among members of the technical community, as well as the public in general. The general public is looking for improvements in nuclear plant safety that are undesirable to them. Passive plant designs may be able to meet this need. Within the technical community, it has been recognized that there might be various degrees of "passivity" of a system, and that there might be a need to develop definitions for a spectrum of passive systems designs. Various types of reactors might use somewhat different definitions. For the ALWR, passive safety systems are defined as shown in fig 3. These systems do

J.M. Yedidia, W.R. Sugnet /Adcanced L WR passive plants

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Passive Safety Systems MAY Eml~lov: • Natural forces: gravity, natural circulation, evaporation • Stored energy: spring, compressed fluid, battery, chemical, etc. • Check valves, and valves that move once to initiate a system function • DC powered valves (as above) and inverters for instruments

Passive Safety Systems SHALL NOT Employ: • Continuously rotating machinery (e.g. pump, turbine, etc.) • Multiple acting or modulating valves • AC powered devices other than inverter supplied instruments

Operator Action: • Design shall not rely on operator action for necessary safety functions in licensing design basis events before 72 hr (however operator shall not be locked out) Fig. 3. Passive safety systems - Definition.

not require pumped cooling water, cooled air or electric power from rotating machinery to perform their safety function. They rely on natural forces, such as gravity or natural circulation or stored energy such as compressed air or batteries, or on sources of energy within the system itself, such as steam. Passive systems do not include or rely on any components which are rotating or reciprocating, such as pumps or turbines. They do not rely on modulating valves or other types of valves which would operate repeatedly during the performance of the safety function. Passive systems may, however, include features such as check valves, instruments and single-action valves which initiate systems operation, if these features are activated by stored energy such as compressed gas, springs or battery-supplied direct current (dc). During the three days following initiation, the passive systems must be able to perform their safety function independent of operator action. In order to keep passive safety systems in a state of preparedness at all times, the infrequent, intermittent use of active support systems, using for example ac-powered pumps and valves, is permissible. Operator action before 72 hours should be possible but not required. In other words, the operator should not be "locked-out" from taking action during an accident. Still, the operator should not be allowed to interfere or stop the operation of a passive safety system unless the system level actuation signal has cleared.

Over the past years, the IAEA has established an international working group to work on the development of passive safety definitions, and a draft paper [4] has been prepared. The above definition provided by the ALWR Requirements Document appears to fall within the range of options for passive systems defined in the draft IAEA paper.

8. Passive systems related issues During the initial preparation of requirements for passive plant systems, it became apparent that there are several important issues which are unique to such plants and which have to be resolved as part of the overall development of the concept. In this paper, we will discuss some of the most significant of those issues.

8.1. Depressurization system actuation and recovery The passive ALWR employs a safety injection system that does not rely on pumps or other rotating machinery to maintain adequate reactor coolant inventory for any of the licensing design basis events, including a loss-of-coolant accident (LOCA). In concept, the passive safety injection system consists of means to automatically depressurize the reactor coolant system (RCS) to near atmospheric pressure, to allow water to drain into the system by gravity from storage pools

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located inside the reactor containment building. Depressurization of the RCS is accomplished by release of fluid from the RCS to the containment space. The initial release is quenched within large pools of water in the containment, but the final release of the RCS fluid needs probably to be directed to the containment atmosphere. This final stage of depressurization of the RCS has the potential for adverse effects on in-containment equipment. Also, consideration must be given to the fact that such release of RCS coolant to containment might be inadvertent, i.e., unrelated to any real need for system actuation. Requirements have been included in the ALWR Requirements Document which will ensure that (a) the frequency of inadvertent actuation will be extremely low and (b) the recovery from such inadvertent actuation is feasible without compromising ALWR plant availability requirements. At this stage of the development of the requirements, we intend to seek a level of reliability which will limit the likelihood of inadvertent actuation to less than 10% chance during the 60-year lifetime of a plant. For recovery time, we plan on requiring that all provisions be made in the plant, such that if an inadvertent actuation did occur, the time to recover will not exceed 30 days, and will not reduce the plant lifetime availability by more than 0.1 percent (assuming a 60-year lifetime). 8.2. Control room habitability in the passiL,e A L W R Elimination of the need for safety-grade on-site ac electric power supply is considered to be one of the major improvements provided by the passive plant designs. However, this improvement carries with it the issue of how much credit can be taken, for licensing purposes, for heating, ventilation and air conditioning (HVAC) systems which would ensure the cooling of equipment and personnel in a post-accident control room environment. According to the rules spelled out above, no operator action is needed for 72 hours. Nevertheless, a plant license would require that the operator be able to monitor the plant condition during the accident period, and that he be able to eventually stabilize the situation. The issue is still being investigated. One option would be to rely completely on the main control room for such events, arguing that (a) the non-safety ac power supply is highly reliable, albeit not safety related and (b) in the remote event that the normal HVAC is unavailable, the main control room itself will have passive backup systems, such as bottled air, to provide habitability to equipment and operators.

Another option is to suggest that the plant license be obtained on the basis of the availability of an auxiliary control room, with minimal monitoring equipment, which would be able to maintain habitable conditions for 72 hours, using passive means only, such as bottled air. This option has not been accepted by the Utility Steering Committee.

8.3. Definition of the safe cold shutdown temperature NRC regulations require that both PWRs and BWRs be able to achieve a cold shut down state within a reasonable time using safety grade equipment only. This has been interpreted to date as a requirement that the RCS reach a temperature of 200°F (for PWR) or 212°F (for BWR) at the end of a 36 hour cooldown period. Passive plant designs for both BWR and PWR employ a natural circulation full-pressure system that transfers decay heat from the RCS to water pools inside or outside the containment. Ultimately, the heat is removed from these pools by evaporation. Since water pools will be at 212°F or more when boiling, it is not feasible to expect the cold shutdown temperature of 200°F for 212°F to be reached with passive means only. It is expected that this issue, as well as several other ones which are unique to the ALWR passive plant designs, will have to be resolved by an "optimization" of NRC requirements. The ALWR Requirements Document includes "optimization issue papers" which presents the case for regulatory change to the NRC and requests NRC agreement on this matter. To summarize, the new concepts of passive ALWR designs provide great promise for system simplification and improved plant safety. At the same time, they provide the designers and the future users with some new challenges, new questions, new concerns. The process of putting in place a set of requirements, thoroughly explored by all parties involved, is an ideal vehicle for bringing many of the new, unexplored issues to the fore. We believe that this process is going to work. Thomas Edison once said "Nothing is as strong as an idea whose time has come." The time has come for a revival of new nuclear power in the United States and around the world. The motivations are compelling growing need for electricity, combined with diminishing supplies of fossil fuels and growing concern over the implications of their use. And at the same time the technical concept of simpler and better new reactors is becoming a reality. The Passive Plant is an idea whose time has come.

J.M. Yedidia, W.R. Sugnet / Advanced LWR passive plants References [i] J.J. Taylor, Improved and safer nuclear power, Science 244 (April 21, 1989). [2] K.E. Stahlkopf, J.C. DeVine, Jr. and W.R. Sugnet, U.S. ALWR program sets out utility requirements for the future, Nuclear Engineering International (Nov. 1988).

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[3] R. Livingston, The next generation, Nuclear Industry (July/August 1988). [4] Draft Description of Passive Safety Related Terms, IAEA (October 1988; 622-13-TC-633).