Pradip Saha - Nuclear Reactor Thermal-Hydraulics: Past, Present and Future

Dr. Pradip Saha has been a Principal Engineer at GE Hitachi Nuclear Energy (GEH [2005present]), Wilmington, North Carolina, USA, since 2005. In 1974, he received his Ph.D. degree in Mechanical Engineering with specialization in Fluid and Thermal Sciences from Georgia Institute of Technology, Atlanta, USA. Since then he has been active in research and development of thermal hydraulics and safety of Boiling Water Reactor (BWR), Pressurized Water Reactor (PWR), Pressurized Heavy Water Reactor (PHWR), Supercritical Water-cooled Reactor (SCWR), Gas-cooled Reactor (GCR) and Sodium-cooled Fast Reactor (SFR) at various organizations including Massachusetts Institute of Technology (20032005), Flotherm Consultants Private Limited (in India) as the Founder and Managing Director (19862002), Brookhaven National Laboratory (19761985), and General Electric Company (19741976). His major accomplishments have been in the areas of two-phase flow in thermal non-equilibrium, density wave flow instabilities at subcritical and supercritical pressures, mixed convection heat transfer in gas-cooled reactors, reactor safety analysis code development, assessment and improvement, development of real-time engineering simulation software for PHWR, BWR scaling methodology, development of core power-feedwater temperature operating domain for ESBWR, improvement of BWR sub-channel analysis code and various aspects of GEHs SFR design PRISM.

Dr. Saha has authored more than 150 technical reports and papers, many published in reputed international journals. He is a Fellow of both American Society of Mechanical Engineers (ASME) and American Nuclear Society (ANS), and received Best Paper Award of ANS Thermal Hydraulic Division in 2008. He has lectured extensively at various academic institutions and professional society meetings, and mentored early-career engineers in USA and India.

1. Introduction

Nuclear power plants, which produce electricity with almost zero greenhouse gas or carbon emissions at stable and competitive costs, are operating in more than thirty countries throughout the world. According to International Atomic Energy Agency (IAEA) Reference Data Series No. 1 2015 Edition []. Percentage of electricity from nuclear is also increasing in Asia, particularly in China, Korea and India. Thus nuclear power constitutes an element of the solution to global warming and a means of delivering electricity to both developed and emerging countries.

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Figure 1-1 Nuclear share of total electricity generation in 2014 (reproduced from Ref. 1 with permission by the IAEA).

Figure 1-2 Forecast of U.S. electricity generation by fuel type (reproduced from Ref. 9 with permission by the USGAO).

There are many types of nuclear power reactors operating in the world today to generate electricity. However, the basic principles of these operating reactors are the same. In simple words, heat is generated in the reactor core by sustaining a fission chain reaction in nuclear fuel. That heat is extracted by a coolant flowing through the reactor core to produce steam and drive a steam turbine to generate electricity.

1.1 Nuclear fission and heat generation

Nuclear fission reactions can occur when a neutron strikes the nucleus of a large fissile atom, typically the fissile isotopes uranium-235 or plutonium-239, causing that nucleus to split or fission. The result of a fission reaction is typically two fission fragments or smaller nuclei, two or more fast-moving high-energy neutrons, and significant heat. The new neutrons produced by a fission reaction initiate new fission reactions, resulting in a sustained fission chain reaction. This is depicted in .

Figure 1-3 Depiction of nuclear fission process.

Nuclear reactors typically fall into one of two types Thermal or Fast reactors based on the neutron spectrum or neutron energies at which the fission reactions occur:

• Thermal reactors optimize the fission reaction rate in their fuel by slowing down, or moderating, the high-energy fast neutrons that are produced as a result of fission reactions. This moderation of the fast neutrons increases the likelihood that a new neutron will initiate fission to sustain the chain reaction. Most of the operating nuclear power reactors today are thermal reactors and the process of neutron moderation is depicted in .
• Fast reactors do not moderate the fission neutrons, instead leaving them fast at high energy. Fast neutrons allow these reactors to be more effective than thermal reactors at creating or breeding new fuel through neutron absorption in uranium-238, creating more fissile material plutonium-239.

Figure 1-4 Neutron slowdown by a moderator.

For both thermal and fast reactors, the fission reaction occurs in the central region of a reactor called the reactor core, which typically contains the following components:

• Nuclear fuel: Nuclear reactors need fissile isotopes, such as uranium-235 or plutonium-239 to sustain a chain reaction and generate heat. Most of the commercial power reactors are light water reactors (LWRs), which need slightly enriched (<5%) uranium-235 as their fissile fuel, the rest of the fuel being non-fissile uranium-238, some of which is converted to fissile plutonium-239 during reactor operation. Some reactors, such as the pressurized heavy water reactor (PHWR), can use natural uranium containing around 0.7% of fissile uranium-235, the rest being non-fissile uranium-238, as fuel and some reactors can utilize thorium-232 to produce fissile uranium-233. Nuclear fuel used in most of the operating reactors is in the uranium oxide form, although metallic or other forms are being considered for some reactor designs.