Passive Safety Systems and Natural Circulation in Water Cooled Nuclear Power Plants

As part of the IAEA’s overall effort to foster international collaborations that strive to improve the economics and safety of future water-cooled nuclear power plants, an IAEA Coordinated Research Project (CRP) was started in early 2004. This CRP, entitled Natural Circulation Phenomena, Modeling and Reliability of Passive Safety Systems that Utilize Natural Circulation, focuses on the use of passive safety systems to help meet the safety and economic goals of a new generation of nuclear power plants. This CRP has been organized within the framework of the IAEA Department of Nuclear Energy’s Technical Working Groups for Advanced Technologies for Light Water Reactors and Heavy Water Reactors (the TWG-LWR and the TWG-HWR) and provides an international cooperation on research work underway at the national level in several IAEA Member States.

The use of passive safety systems was addressed in 1991 at the IAEA Conference on “The Safety of Nuclear Power: Strategy for the Future” [1]. Subsequently, experts in research institutes and nuclear plant design organizations from several IAEA Member States collaboratively presented their common views in a paper entitled ‘Balancing passive and active systems for evolutionary water cooled reactors’

[2] . The experts noted that a designer’s first consideration is to satisfy the required safety function with sufficient reliability, and the designer must also consider other aspects such as the impact on plant operation, design simplicity and costs. The Safety Fundamentals of the IAEA Safety Standards

[3] recommends “an appropriate combination of inherent and engineered safety features” for defence in depth. Design Requirements of the IAEA Safety Standards [4] mentions “following a postulated initiating event, the plant is rendered safe by passive safety features or by the action of safety systems that are continuously operating in the state necessary to control the postulated initiating event”.

The use of passive safety systems such as accumulators, condensation and evaporative heat exchangers, and gravity driven safety injection systems eliminate the costs associated with the installation, maintenance and operation of active safety systems that require multiple pumps with independent and redundant electric power supplies. As a result, passive safety systems are being considered for numerous reactor concepts (including in Generation III and III+ concepts) and are expected to find applications in the Generation-IV reactor concepts, as identified by the Generation IV International Forum (GIF). Another motivation for the use of passive safety systems is the potential for enhanced safety through increased safety system reliability.

The CRP benefits from earlier IAEA activities that include developing databases on physical processes of significant importance to water cooled reactor operations and safety [5,6], technical information exchange meetings on recent technology advances [7-13], and Status Reports on advanced water cooled reactors [14,15]. In the area of thermal hydraulic phenomena in advanced water cooled reactors, recent IAEA activities have assimilated data internationally on heat transfer coefficients and pressure drop [5]; and have shared information on natural circulation data and analytical methods [5], and on experimental tests and qualification of analytical methods [8]. This CRP also benefits from a recent report issued by IAEA [16] on the status of innovative small and medium sized reactor designs.

In order to establish the progress of work in this CRP, an Integrated Research Plan with description of the tasks addressing the objectives of the CRP was defined. These tasks are:

• Establish the state-of-the-art on natural circulation

• Identify and describe reference systems

• Identify and characterize phenomena that influence natural circulation

• Examine application of data and codes to design and safety

• Examine the reliability of passive systems that utilize natural circulation.

The activity under the first task is aimed at summarizing the current understanding of natural circulation system phenomena and the methods used experimentally to investigate and model such phenomena. In November 2005, the IAEA issued a technical document [17], developed by the collaborative effort of the CRP participants and with major contributions from some selected experts in the CRP, aimed at documenting the present knowledge in six specific areas; advantages and

challenges of natural circulation systems in advanced designs, local transport phenomena and models, integral system phenomena and models, natural circulation experiments, advanced computation methods, and reliability assessment methodology.

The activity for the third task is aimed at identifying and categorizing the natural circulation phenomena of importance to advanced reactors and passive safety system operations and reliability. This task is the major link between the second and the fourth tasks. The activities related to the second task and the fourth task including the fifth task are agreed to be published in two different IAEA — TECDOCs by the CRP participants. Since the third task is the backbone for both tasks, inclusion of this task in both IAEA-TECDOCs in an appropriate form is a logical consequence.

The aim of this publication is to describe passive safety systems in a wide range of advanced water — cooled nuclear power plant designs with the goal of gaining insights into the system design, operation, and reliability without endorsement of the performance. This publication has a unique feature which includes plant design descriptions with a strong emphasis on passive safety systems of the specific design. These descriptions of the passive safety systems together with the phenomena identification (including the definitions of the phenomenon to describe in some detail the titles of the phenomenon considered) are given in the Annexes and Appendix of this report, respectively. Based on the passive systems and phenomena, which are considered, a cross reference matrix has been established and also presented in this report. As basis for the phenomenon identification, earlier works performed within the OECD/NEA framework during 1983 to 1997 were considered. These are:

• Code validation matrix of thermal-hydraulic codes for LWR LOCA and transients [24],

• State of the art report (SOAR) on thermo-hydraulic of emergency core cooling in light water reactors [23],

• Separate effects test (SET) validation matrix for light water reactors [19],

• Integral facility tests validation matrix for light water reactors [20],

• Status report on relevant thermal-hydraulic aspects of advanced reactor designs [21].

Since the Generation III and III+ reactor designs contain technological features that are common to the current generation reactors, the phenomena identified during the work performed for first item to fourth item can be used as base knowledge. The fifth item provides the important and relevant thermal hydraulic phenomena for advanced reactor designs in addition to the relevant thermal hydraulic phenomena identified for the current generation of light water reactors (LWR). The list of relevant phenomena established in reference 21 has been taken as basis for the CRP work and has been modified according to the reactor types and passive safety systems considered in this report. It is to be noted that in identifying the relevant thermal hydraulic phenomena in the list which is provided in this report, expert judgement is the main contributor.

IAEA-TECDOC-626 provides definitions for safety related terms as applied to advanced reactors [16]. In that document, the concepts of passive and active safety systems are defined and discussed. The definition of a passive safety system is as follows: Either a system which is composed entirely of passive components and structures or a system which uses active components in a very limited way to initiate subsequent passive operation. Four categories were established to distinguish the different degrees of passivity.

Category A

This category is characterized by:

• no signal inputs of ‘intelligence’

• no external power sources or forces

• no moving mechanical parts, and

• no moving working fluid.

Examples of safety features included in this category are physical barriers against the release of fission products, such as nuclear fuel cladding and pressure boundary systems; hardened building structures for the protection of a plant against seismic and or other external events; core cooling systems relying only on heat radiation and/or conduction from nuclear fuel to outer structural parts, with the reactor in hot shutdown; and static components of safety related passive systems (e. g. tubes, pressurizers, accumulators, surge tanks), as well as structural parts (e. g. supports, shields).

Category B

This category is characterized by:

• no signal inputs of ‘intelligence’

• no external power sources or forces

• no moving mechanical parts; but

• moving working fluids.

Examples of safety features included in this category are reactor shutdown/emergency cooling systems based on injection of borated water produced by the disturbance of a hydrostatic equilibrium between the pressure boundary and an external water pool; reactor emergency cooling systems based on air or water natural circulation in heat exchangers immersed in water pools (inside containment) to which the decay heat is directly transferred; containment cooling systems based on natural circulation of air flowing around the containment walls, with intake and exhaust through a stack or in tubes covering the inner walls of silos of underground reactors; and fluidic gates between process systems, such as ‘surge lines’ of pressurized water reactors (PWRs).

Category C

This category is characterized by:

• no signal inputs of ‘intelligence’

• no external power sources or forces; but

• moving mechanical parts, whether or not moving working fluids are also present.

Examples of safety features included in this category are emergency injection systems consisting of accumulators or storage tanks and discharge lines equipped with check valves; overpressure protection and/or emergency cooling devices of pressure boundary systems based on fluid release through relief valves; filtered venting systems of containments activated by rupture disks; and mechanical actuators, such as check valves and spring-loaded relief valves, as well as some trip mechanisms (e. g. temperature, pressure and level actuators).

Category D

This category is characterized by:

• signal inputs of ‘intelligence’ to initiate the passive process

• energy to initiate the process must be from stored sources such as batteries or elevated fluids

• active components are limited to controls, instrumentation and valves to initiate the passive system

• Manual initiation is excluded.

Examples of safety features included in this category are emergency core cooling and injection systems based on gravity that are initiated by battery-powered electric or electro-pneumatic valves; emergency reactor shutdown systems based on gravity or static pressure driven control rods.

The reader of the present document should consider that:

(a) The information provided shall not be taken as an advertisement for any reactor type.

(b) The description of selected design does not imply a preference relative to other water cooled reactor systems that are not described.

(c) There is no implicit recommendation that passive systems should be preferred to active systems.

(d) Nomenclature in the Annexes may not be consistent with that in the main text. Harmonization was not attempted for the text provided in the Annexes for different reactor designs.

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