Nuclear Safety in Light Water Reactors

Severe Accident Phenomenology

Bal Raj Sehgal

• Publisher: Academic Press
• Year: 2011
• Print ISBN: 9780123884466
• eBook ISBN: 9780123919069
• Topic: Tecnología e ingeniería
• Subtopic: Ingeniería general

This vital reference is the only one-stop resource on how to assess, prevent, and manage severe nuclear accidents in the light water reactors (LWRs) that pose the most risk to the public. LWRs are the predominant nuclear reactor in use around the world today, and they will continue to be the most frequently utilized in the near future. Therefore, accurate determination of the safety issues associated with such reactors is central to a consideration of the risks and benefits of nuclear power. This book emphasizes the prevention and management of severe accidents to teach nuclear professionals how to mitigate potential risks to the public to the maximum extent possible.- Organizes and presents all the latest thought on LWR nuclear safety in one consolidated volume, provided by the top experts in the field, ensuring high-quality, credible and easily accessible information- Explains how developments in the field of LWR severe accidents have provided more accurate determinations of risk, thereby shedding new light on the debates surrounding nuclear power safety, particularly in light of the recent tragedy in Japan- Concentrates on prevention and management of accidents, developing methodologies to estimate the consequences and associated risks

Chapter 1. Light Water Reactor Safety

A Historical Review

Bal Raj Sehgal

Chapter Outline

1.1. Introduction2

1.2. The Early Days3

1.3. The Development of Civilian LWRs3

1.4. Early Safety Assesments5

1.5. The Siting Criteria5

1.5.1. Assumptions and Requirements of TID-14844 and 10 CFR 1006

1.6. Safety Philosophy7

1.6.1. The Defense-in-Depth Approach8

1.7. Safety Design Basis10

1.7.1. LOCA and the ECCS Controversies12

1.8. Public Risk of Nuclear Power (WASH-1400)15

1.8.1. The Reactor Safety Study16

1.9. The TMI-2 Accident27

1.9.1. Description of the Accident27

1.9.2. The Aftermath of TMI-2 Accident32

1.10. The Chernobyl Accident33

1.10.1. Background and RBMK Specifics33

1.10.2. How and Why Chernobyl Happened37

1.11. The Difficult Years44

1.12. Severe Accident Research45

1.12.1. In-vessel Accident Progression for a PWR48

1.12.2. In-vessel Accident Progression for a BWR49

1.12.3. Fission Product Release and Transport during the In-vessel Accident Progression50

1.12.4. Ex-vessel Accident Progression51

1.13. Severe Accident Management57

1.13.1. Cooling a Degraded Core58

1.13.2. Management of Combustible Gases60

1.13.3. Management of Containment Temperature, Pressure, and Integrity60

1.13.4. Management of Radioactive Releases61

1.14. The Fukushima Accidents62

1.14.1. Introduction and Plant Characteristics62

1.14.2. Consequences of a Conservative Core-melt Scenario for Fukushima Reactors68

1.14.3. The Actual Progression of the Fukushima Accidents69

1.14.4. Concluding Remarks on the Fukushima Accidents75

1.15. New LWR Plants78

1.15.1. The In-Vessel Melt Retention (IVMR) Strategy80

1.15.2. The Ex-Vessel Melt Retention Strategy82

Conclusions85

References86

In this chapter, a historical review of the developments in the safety of LWR power plants is presented from their inception to date. The chapter reviews the developments prior to the TMI-2 accident, i.e. the siting rule, the concept of defense in depth, the safety design basis, the large LOCA technical controversies, the LWR design-base safety research programs and the public risk estimates made in the landmark probabilistic risk study of WASH-1400.

The TMI-2 accident, which became a turning point in the history of the development of nuclear power, is described briefly. The Chernobyl accident, which terrified the world and almost completely curtailed the development of nuclear power, is also described briefly. The great international effort of research in the LWR severe accidents, which was conducted following the TMI-2 and Chernobyl accidents, is described next. It is followed by the description of the recent tragic accidents at the Fukushima Daiichi plant in Japan, which have raised several new issues on LWR safety performance which will need resolution in near future.

We conclude, however, that in spite of the Fukushima accidents, it can be acknowledged that the science of LWR safety has made great strides and with the knowledge gained and the training of the staff at the presently-installed nuclear power stations, the LWR plants have achieved high levels of safety performance. The Generation III+ LWR power plants, currently being installed, have designed reactor and containment systems which may be able to protect the public fully against the possible radiation hazards of severe accidents.

1.1. Introduction

The light water reactor (LWR) safety that we are concerned with in this book is basically about estimating the risks posed by an individual or a population of nuclear power plants (NPPs) to the public at large and the efforts to reduce these risks. The public of most concern is that which resides in the vicinity of a nuclear power plant but also at other locations, which could be affected by an accident in a NPP located anywhere.

The basic goal of safety is to ensure that a LWR will not contribute significantly to individual and societal health risks. This basic goal translates to the prevention of the release of radioactivity into the environment from the NPP. A complementary aim is to prevent damage to the plant and to protect the personnel at the plant from injury or death in an accident.

Since LWR safety aims to protect the public at large, it is heavily regulated. Each nuclear power country (and even some without NPPs) has regulatory commissions (bodies) that regulate every aspect of a NPP from design and construction to operation and any modifications. They require very extensive analyses, documentation, and quality control. The reactor safety design has to follow definite rules and regulations. Some of these requirements will be described in this chapter.

The reactor performance, on the other hand, is concerned with long-term steady-state operations, since most LWR plants are base-loaded and strive to operate at full power, without interruption, between scheduled outages for maintenance. Reactor performance is also concerned with efficiency, the capacity factor, fuel cycle costs, maintenance costs, and the radiation dose to the operating staff. Thus, it is not regulated. However, it has been found that a well-running LWR plant is, generally, a safer plant with a much lower frequency of incidents, which, generally, are the precursors to more serious events.

1.2. The Early Days

The nuclear era started with the natural uranium-graphite pile built by Fermi and his associates at Stagg Field at the University of Chicago [1]. It did not involve light water as a coolant since only natural uranium was available and criticality could be achieved only with graphite or heavy water. The safety concepts developed there, however, were adopted by the LWR plants that developed several yeas later. Enrico Fermi and his associates recognized that:

• Nuclear fission reactions, which are the basis of nuclear power, emit high levels of radioactivity and thus could be a health hazard to any person exposed to it. This implied shielding, containment, and remote siting.

• The safe operation of the reactor (or pile) would require protective and control measures, as evidenced by the provision of a control rod in the pile that Fermi and his associates built.

Shielding and remote siting were required for the plants that were built for the production of plutonium in the United States and other countries during the years before and after World War II. Remote siting of these plants not only protected the public but also maintained the secrecy surrounding the production of nuclear weapons for a number of years.

The containment aspect of protecting the public from a nuclear accident was not considered or employed for the plants generating plutonium. Those were the years of above-ground nuclear weapons tests, which in any case were releasing considerable amounts of radioactive fission products in the atmosphere. Fortunately, there were no reported accidents of any great significance in the plutonium production plants in either the United States or other Western countries.

Leak-tight containment as a safety system for a civilian NPP was not long in coming. It was proposed in 1947 [2] for a sodium-cooled fast reactor that was the focus of the power reactor development by the U.S. Department of Energy at that time. Later, the LWR plant developers adopted leak-tight containment for their plants.

1.3. The Development of Civilian LWRs

The LWR development started as a military program in the United States and stemmed from the i...

Table of contents

• Cover Image
• Table of Contents
• Front Matter
• Copyright
• Preface
• Contributors
• Chapter 1. Light Water Reactor Safety
• Chapter 2. In-Vessel Core Degradation
• Chapter 3. Early Containment Failure
• Chapter 4. Late Containment Failure
• Chapter 5. Fission Product Release and Transport
• Chapter 6. Severe Accident Management
• Chapter 7. Environmental Consequences and Management of a Severe Accident
• Chapter 8. Integral Codes for Severe Accident Analyses
• Appendix 1. Corium Thermodynamics and Thermophysics
• Appendix 2. Severe Accidents in PHWR Reactors
• Index
• Index