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Section 4.5.8, Reactor and reactor coolant system

The application should demonstrate that the reactor and reactor coolant system meet the requirements in sections 8.1, 8.2 and 8.4 of REGDOC‑2.5.2, Design of Reactor Facilities: Nuclear Power Plants [9].

The applicant should provide relevant information concerning the reactor, including a summary description of:

·       mechanical, nuclear, thermal and hydraulic behaviour of the designs of the various reactor components

·       fuel, reactor internals, and reactivity control systems

·       related instrumentation and control systems in place to demonstrate the capability of the reactor to perform its design safety functions in all operational states throughout its design life

The applicant should ensure that the nuclear criticality safety program meets the requirements in REGDOC‑2.4.3, Nuclear Criticality Safety [29] and section 8.12 of REGDOC‑2.5.2, Design of Reactor Facilities: Nuclear Power Plants [9].

Design of fuel system

The applicant should provide the following information concerning the thermal, mechanical, thermal-hydraulic and material design of all fuel systems and components, including a description of the fuel manufacturing and a summary of the in-core fuel management:

·       the design documents of all fuel systems to be used, including the fuel design drawings

·       the fuel design basis requirements

·       evaluations of the fuel design

·       a description of the methods and computer codes used to assess the fuel behaviour under normal and accident conditions

·       testing, inspection and surveillance plans

·       the manufacturing process

Design of the reactor internals

The application should describe the design of the reactor internals and their design basis requirements, specifically:

·       structures into which the fuel has been assembled (for example, the fuel assembly or fuel bundle)

·       related components required for fuel positioning

·       all supporting elements internal to the reactor, including any separate provisions for moderation and fuel location

The information provided should link to and complement other sections that cover related aspects of the reactor fuel and its handling and storage, such as:

·       physical and chemical properties of the fuel components including:

·       thermalhydraulic, structural and mechanical aspects

·       the expected response to static and dynamic mechanical loads and their behaviour

·       a description of the effects of irradiation on the ability of the reactor internals to perform their safety functions adequately over the design life of the reactor facility

·       any significant sub-system components, including any separate provisions for moderation and fuel location (corresponding design drawings should be provided)

·       consideration of service effects on the performance of safety functions, including surveillance and/or inspection programs for reactor internals to monitor the effects of irradiation and aging on them

·       program to monitor the behaviour and performance of the core, which should include provisions to monitor the neutronics, dimensions, and temperatures of the core

Nuclear design and core nuclear performance

The application should describe how the design meets the design basis requirements for:

·       nuclear design of the fuel

·       reactivity control systems (including nuclear and reactivity control limits such as excess reactivity, fuel burn-up, reactivity feedbacks)

·       core design lifetime

·       fuel replacement strategies

·       reactivity coefficients

·       stability criteria

·       maximum controlled reactivity insertion and removal rates

·       control of power distributions

·       shutdown margins

·       rod speeds and stuck rod criteria

·       chemical and mechanical shim control

·       neutron poison requirements

·       all shutdown provisions

The description should also include any of the following areas of the design if applicable:

·       fuel enrichment distributions

·       burnable poison distributions

·       physical features of the lattice or assemblies relevant to nuclear design parameters

·       delayed neutron fractions and neutron lifetimes

·       core lifetime and burn-up

·       plutonium build-up

·       soluble poison insertion rates

·       xenon burnout or any other transient requirements

Further detailed information should be provided on the following topics, as appropriate:

·       power distributions

·       reactivity coefficients

·       reactivity control requirements

·       reactivity devices

·       criticality during refuelling

·       reactor core stability, irradiation issues

·       analytical methods used (with verification and validation information and uncertainties)

·       testing and inspection plans

·       operational limits and conditions

Core thermalhydraulic design

The applicant should provide information concerning the reactor and reactor coolant system thermalhydraulic design, including:

·       design basis requirements, the thermal and hydraulic design for the reactor core and attendant structures, and the interface requirements for the thermal and hydraulic design of the reactor coolant system

·       analytical tools, methods and computer codes (with codes for verification, and validation information and uncertainties) used to calculate thermal and hydraulic parameters

·       flow, pressure, void and temperature distributions, and the specification of their limiting values and a comparison with design limits

·       justification for the thermalhydraulic stability of the core; for example, stability in forced or natural circulation flow against:

·       neutronic/thermalhydraulic feedback

·       flow oscillations

·       parallel channel instabilities

Reactivity control systems

The design of the reactivity control systems should provide the means for detecting levels and distributions of neutron flux. Information provided on the reactivity control systems should include, but not be limited to:

·       design basis requirements for the systems

·       demonstration that the reactivity control systems, including any essential ancillary equipment, are designed to provide the required functional performance and are properly isolated from other equipment

·       description of the qualification and commissioning tests that have been carried out, in order to ensure that the equipment and system performance comply with the design requirements and meet the claims for their performance made in the safety analysis

·       description on how separation and diversity have been achieved

·       description of the rate of reactivity insertion and the depth of each reactivity control system in accordance with section 8.4 of REGDOC‑2.5.2, Design of Reactor Facilities: Nuclear Power Plants [9]

Taken together, the SSCs important to safety instrumentation and control systems and the reactivity control systems should meet the expectations for shutdown means, in accordance with section 8.4 of REGDOC‑2.5.2 [9].

Reactor materials

The application should describe the materials used for the components of the reactor (including the materials for the reactor coolant system pressure boundary, the materials for the core support function and the materials for in-core components such as control rods and instrumentation). The application should include information on the material specifications, including:

·       chemical, physical and mechanical properties

·       resistance to corrosion

·       dimensional stability, strength, toughness, hardness and crack tolerance

·       where important, microstructure and material fabrication details

The application should describe the properties and required performance of seals, gaskets and fasteners in the primary pressure boundary.

The application should describe a material surveillance program that will address potential material degradation for all components, particularly for components operated in high radiation fields, in order to determine the metallurgical or other degradation effects of factors such as irradiation, stress corrosion cracking, flow-accelerated corrosion, thermal embrittlement, vibration fatigue, and other aging mechanisms.

The application should describe how neutronic properties of control rod materials are addressed in the nuclear design and core nuclear performance section.

Design of the reactor coolant system and reactor auxiliary system

The application should provide the design basis requirements for the reactor coolant system and its major components. The application should describe the system design performance and features to ensure that its various components and its interfacing subsystems meet the safety requirements for design.

The application should demonstrate that the reactor coolant SSCs are designed, manufactured and installed in a manner to allow periodic inspections and tests during their operating lifetime.

Where applicable, the information provided should cover:

·       reactor coolant pumps

·       steam generators or boilers

·       depressurization system

·       reactor coolant system piping

·       main steamline isolation system

·       isolation cooling system for the reactor core

·       main steamline and feedwater piping

·       pressurizer

·       pressure-relief discharge system

·       provisions for main and emergency cooling

·       residual heat removal system and its components, such as pumps and valves

·       supports for piping, vessels and components

The application should indicate the location of specified inspection information in the design documentation, including the volumetric or visual examination and testing.

The application should describe any additional systems associated with the reactor that are not described elsewhere in the application. For example, for CANDU reactors, such information would include the moderator system and its auxiliaries, the end shield cooling system and the annulus gas system. For light water reactors, an example would be the primary leak detection system.

Integrity of the reactor coolant system pressure or fluid boundary

The application should include the results of the detailed analytical and numerical stress evaluations, and of the engineering mechanics and fracture mechanics studies for all components comprising the reactor coolant system pressure or fluid boundary.

The application should take into account the entire range of operating and postulated accident conditions in all operating and shutdown states. The description should directly refer to the detailed stress analyses for each of the major components, to permit further evaluations to be made, if necessary.

The information should be detailed enough to demonstrate that the materials, fabrication methods, inspection techniques, loading conditions and load combinations used conform to all applicable regulations, codes and standards. The pressure or fluid boundary materials, the pressure-temperature or fluid-temperature limits and the integrity of the reactor pressure or fluid boundary – including embrittlement considerations – should all be taken into account in this information.

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