Irradiation Embrittlement of Reactor Pressure Vessels (RPVs) in Nuclear Power Plants

Naoki Soneda

• Publisher: Woodhead Publishing
• Year: 2014
• Print ISBN:
  9781845699673
• eBook ISBN:
  9780857096470
• Topic: Technology & Engineering
• Subtopic: Mechanical Engineering
• Index: Technology & Engineering

Reactor Pressure Vessels (RPVs) contain the fuel and therefore the reaction at the heart of nuclear power plants. They are a life-determining structural component: if they suffer serious damage, the continued operation of the plant is in jeopardy. This book critically reviews irradiation embrittlement, the main degradation mechanism affecting RPV steels, and mitigation routes for managing the RPV lifetime. Part I reviews RPV design and fabrication in different countries, with an emphasis on the materials required, their important properties, and manufacturing technologies. Part II then considers RVP embrittlement in operational nuclear power plants using different reactors. Chapters are devoted to embrittlement in light-water reactors, including WWER-type reactors and Magnox reactors. Finally, Part III presents techniques for studying embrittlement, including irradiation simulation techniques, microstructural characterisation techniques, and probabilistic fracture mechanics. Irradiation Embrittlement of Reactor Pressure Vessels (RPVs) in Nuclear Power Plants provides a thorough review of an issue that is central to the safety of nuclear power generation. The book includes contributions from an international team of experts, and will be a useful resource for nuclear plant operators and managers, relevant regulatory and safety bodies, nuclear metallurgists and other academics in this field - Discusses reactor pressure vessel (RPV) design and the effect irradiation embrittlement can have, the main degradation mechanism affecting RPVs - Examines embrittlement processes in RPVs in different reactor types, as well as techniques for studying RPV embrittlement

Part I

Reactor pressure vessel (RPV) design and fabrication

1

Reactor pressure vessel (RPV) design and fabrication: the case of the USA

W.L. Server ATI Consulting, USA

R.K. Nanstad Oak Ridge National Laboratory, USA

Abstract

The general design following the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code and fabrication processes used in the USA for nuclear reactor pressure vessels (RPVs) are described. Also, several RPVs in countries other than the USA were designed and fabricated in the USA using the processes described here. Detailed knowledge of the design and fabrication information is necessary to assure long-term structural integrity and safe operation of the RPVs.

Key words

reactor pressure vessel (RPV) design

reactor pressure vessel (RPV) fabrication

ASME Code

1.1 Introduction

For safety reasons, the reactor pressure vessel (RPV) is generally considered to be the most critical component in the nuclear plant aside from the reactor core. The RPV is also the one major component that may limit the useful life of the nuclear plant, because it is the heart of the nuclear steam supply system and, if it had to be replaced, an extraordinary amount of time and money would be required. Virtually every other component or system could be replaced cost-effectively, including the steam generators.

While good maintenance practices focus on those active components of a nuclear plant that routinely need to be serviced, inspected and replaced, the primary concern for achieving a plant life relates to passive elements such as the RPV that are never replaced or infrequently refurbished. RPV material toughness properties are known to degrade with age because of irradiation damage. While this degradation mechanism was factored into the initial design and considered in the selection of materials of the RPV, a failure of the RPV by rupture or brittle fracture is beyond the design basis of the plant. Therefore, every effort must be made to protect the RPV from brittle fracture by reducing the level of embrittlement or, failing this, by considering even more drastic measures such as RPV thermal annealing or early plant retirement.

Operating safety in nuclear power plants is assured through the integrity of three distinct barriers between the fission products and the environment and through permanent operational availability of the related safety systems. This design philosophy is termed ‘defense in depth’, and the continued safety of a plant is maintained by the integrity of these three barriers:

• Barrier number one is the fuel element cladding, which contains and confines the nuclear reaction products, and whose leak tightness is continually monitored. In the event fuel cladding failures occur, radioactivity thresholds are exceeded, the installation is shut down, and the damaged elements are removed and replaced.

• Barrier number two is the RPV and the reactor coolant system, which must contain the core and primary coolant water under high temperature and pressure. This barrier is crucial because of its potential impact on the integrity of the first barrier. Failure of the RPV could result in overheating and damage to the fuel elements due to loss of core cooling. Since the integrity and reliability of this second barrier is clearly crucial, construction and operation of the RPV are regulated by strict adherence to the ASME Code and NRC requirements governing many aspects of operation and safety.

• Barrier number three is the containment. It is designed to keep all radioactive products within its boundary in the event of any failure of the other barriers, and it has a passive role during normal operation.

In a pressurized water reactor (PWR), there are two cooling systems, termed primary and secondary. In a boiling water reactor (BWR), there is only a primary cooling system. Embrittlement management actions should be taken to assure the integrity of the RPV as the critical component of barrier number two in accordance with the existing regulatory requirements and Codes and Standards governing the RPV.

The design of the RPV has to take into account all functional requirements to provide hot water (in PWRs) or steam (in BWRs) and all possible deviations from normal operating conditions as well as external loads such as seismic events. With regard to neutron irradiation, design considerations are usually limited to the core beltline region of the RPV, which is typically defined as the region where the material accumulates a fluence of more than 10¹⁷ n/cm² (E > 1 MeV). Prevention against failure of the RPV requires that the amount of material degradation, and stresses that occur over its lifetime, for both normal and transient operations, are predicted prior to construction, and these loads and transients are defined as the ‘design basis’. In the USA, this process is in accordance with the requirements in Section III of the ASME Boiler and Pressure Vessel Code (ASME, 2010a).

The initial licensed lifetime of a nuclear power plant in the USA is typically 30–40 years, although its actual useful design lifetime may be much greater as proven by the license renewal of more than half of the plants in the USA to 60 years. Currently, there are activities underway by both the US government and the nuclear industry to stretch the license renewal out to 80 years or more. Plant operators must continually monitor embrittlement and demonstrate that the margins of safety are adequate to prevent brittle and ductile fracture of the RPV. The main issues that could limit plant operation are pressurized thermal shock (PTS), restrictions during heat-up and cool-down, or low upper-shelf energies of RPV materials.

It is the responsibility of each plant owner to op...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Contributor contact details
• Woodhead Publishing Series in Energy
• Preface
• Part I: Reactor pressure vessel (RPV) design and fabrication
• Part II: Reactor pressure vessel (RPV) embrittlement in operational nuclear power plants
• Part III: Techniques for the evaluation of reactor pressure vessel (RPV) embrittlement
• Index