Molten Salt Reactors and Integrated Molten Salt Reactors: Integrated Power Conversion

By Bahman Zohuri

Understanding the evolution and advances of energy conversion is critical to meet today’s energy demands while lowering emissions in the fight against climate change. One advancement within nuclear plants that continues to gain interest is molten salt reactors and integrated molten salt reactors, which are the new proposed generation IV small modular reactors. To get up to speed on the latest technology, Molten Salt Reactors and Integrated Molten Salt Reactors: Integrated Power Conversion delivers a critical reference covering the main steps for the application of these reactors. Creating a more environmentally friendly energy production methodology, the reference reviews the past, current, and future states of the reactors including pros and cons, designs and safety features involved, and additional references. Included in the reference is a new approach to energy conversion technology, including coverage on material, economic, and technical challenges towards waste heat recovery, power conversion systems, and advanced computational materials proposed for generation IV systems. Advanced nuclear open air-brayton cycles are also included for higher efficiency. Rounding out with guidance on avoiding salt freezing and salt cleanup for fission and fusion reactors, Molten Salt Reactors and Integrated Molten Salt Reactors: Integrated Power Conversion provides today’s nuclear engineer and power plant engineer with the impactful content of rising efficiency in molten salt reactors, ultimately leading to more efficient and affordable electricity.

• Gain the latest applications and steps to implement modular reactors, including safety and technical considerations
• Learn an innovative approach to nuclear air combined cycles (NACC), bringing down the costs of producing electricity in nuclear power plants
• Practice techniques and computer modeling with additional appendices that include experimental validation methods and computer code results

• Language: English
• Publisher: Academic Press
• Release date: Jun 29, 2021
• ISBN: 9780323918466

Bahman Zohuri

Dr. Bahman Zohuri is currently an Adjunct Professor in Artificial Intelligence Science at Golden Gate University, San Francisco, California, who runs his own consulting company and was previously a consultant at Sandia National Laboratory. Dr. Zohuri earned his bachelor’s and master’s degrees in physics from the University of Illinois. He earned his second master’s degree in mechanical engineering, and also his doctorate in nuclear engineering from the University of New Mexico. He owns three patents and has published more than 40 textbooks and numerous journal publications.

Book preview

Chapter 1

Molten Salt Reactor History, From Past to Present

Abstract

Traditionally these reactors are thought of as thermal breeder reactors running on the thorium to ²³³Uranium cycle and the historical competitor to fast breeder reactors. However, simplified versions running as converter reactors without any fuel processing and consuming low-enriched uranium are perhaps a more attractive option. Molten-salt reactors were first proposed by Ed Bettis and Ray Briant of Oak Ridge National Laboratory (ORNL) during the post-World War II attempt to design a nuclear-powered aircraft. The attraction of molten fluoride salts for that program was the great stability of the salts, both to high temperatures and to radiation. An active development program aimed at such an aircraft reactor was carried out from about 1950 to 1956.

Keywords

Atomic Energy Commission; Molten Salt History; Nuclear Energy for the Propulsion of Aircraft; Uranium and Thorium Fluorides in Lithium and Beryllium Fluorides as Fuels

Molten salt reactors (MSRs) are one of six next-generation designs chosen by the Generation IV (GEN IV) program. Traditionally these reactors are thought of as thermal breeder reactors running on the thorium to ²³³Uranium cycle and the historical competitor to fast breeder reactors. However, simplified versions running as converter reactors without any fuel processing and consuming low-enriched uranium (LEU) are perhaps a more attractive option. Molten-salt reactors were first proposed by Ed Bettis and Ray Briant of Oak Ridge National Laboratory (ORNL) during the post-World War II attempt to design a nuclear-powered aircraft. The attraction of molten fluoride salts for that program was the great stability of the salts, both to high temperatures and to radiation. An active development program aimed at such an aircraft reactor was carried out from about 1950 to 1956.

1.1 Introduction

MSRs are liquid fuel reactors that use solution of uranium and thorium fluorides in lithium and beryllium fluorides as fuels. They operate at high temperature and low pressure and have excellent nuclear characteristics. They offer promise as breeders of fissionable material and as producers of low-cost electricity in large central power stations and recently selected by Department of Energy (DOE) is one of the Six Generation IV power plants and with their high temperature operational level are an excellent candidate as advanced Small Modular Reactor (AdvSMR) utilizing an innovative technology nuclear open air-Brayton combined cycle (NACC) [9].

The origination of an MSR starts with the aircraft reactor experiment (ARE), a small reactor using a circulating molten fuel salt, operated for several days in 1954 and reached a peak temperature of 1620 °F (See Section 1.2 for more details). The aircraft nuclear propulsion (ANP) program and the preceding Nuclear Energy for the Propulsion of Aircraft (NEPA) project worked to develop a nuclear propulsion system for aircraft. The United States Army Air Forces initiated Project NEPA on May 28, 1946 [1]. NEPA operated until May 1951, when the project was transferred to the joint Atomic Energy Commission (AEC)/USAF ANP [2]. The USAF pursued two different systems for nuclear-powered jet engines, the Direct Air Cycle concept, which was developed by General Electric (GE), and Indirect Air Cycle, which was assigned to Pratt & Whitney. The program was intended to develop and test the Convair X-6 but was cancelled in 1961 before that aircraft was built. The total cost of the program from 1946 to 1961 was about $1 billion [3].

In 1956 interest in the airplane began to fall off, and Alvin Weinberg, an American nuclear physicist who was the administrator at ORNL during and after the Manhattan Project, wished to see whether the molten fluoride fuel technology that had been developed for the aircraft could be adapted to civilian power reactors. Part of his interest stemmed from the fact that all of the other materials and coolants being suggested for reactors had been anticipated by the reactor design group at the Metallurgical Lab oratory in Chicago during World War II. This was new event.

Considering ANP for a civilian program, this type of reactor for this purpose a lot head start information based on historical data was collected during period of its study for military applications for aircraft propulsion purpose, although Division of Reactor Development of the US AEC of the time never showed much enthusiasm for MSR Program for this purpose.

However, with ORNL effort behind the effort of pushing the MSR Program concept, was that when AEC eliminated a few reactor concepts, the decision was to establish task forces of outside experts to evaluate the reactor concepts and, especially, to point out their weaknesses. After a couple of other reactor concepts had been eliminated by this process, the AEC formed the fluid fuels reactors Task Force to evaluate and compare three different fluid fuel reactors: the aqueous homogeneous, the liquid bismuth, and the MSRs. The task force met In Washington for about 2 months early in 1959. In this meeting representative from ORNL were presenting the two reactors concepts one being molten salt system and the aqueous homogeneous, while someone from Brookhaven National Laboratory represented the bismuth-graphite reactor.

Task force members came from other AEC laboratories, from electric utilities, from architect engineering firms, and from the AEC itself. The first sentence of the Summary of the Task Force Report (TID-8505) was, The Molten Salt Reactor has the highest probability of achieving technical feasibility.1

This conclusion arose from the fact that the molten fluoride salts (1) have a wide range of solubility of uranium and thorium, (2) are stable thermodynamically and do not undergo radiolytic decomposition, (3) have a very low vapor pressure at operating temperatures, and (4) do not attack the nickel-based alloy used in the circulating salt system.

As a result of the task force deliberations, the other two concepts were abandoned, and the molten salt system continued its precarious existence. The reactor considered by the Task Force was a converter reactor, not a breeder, and was described as follows [4].

The reference design MSR is a nickel-molybdenum-chromium-iron alloy-8 (now called Hastelloy-N) vessel containing a graphite assembly 12.25 feet in diameter by 12.25 feet high, through which molten salt flows in vertical channels. The fuel salt is a solution composed of 0.3 mole percent UF4, 13 mole percent ThF4, 16 mole percent BeF2, and 70.7 mole percent ⁷LiF. The fuel salt is heated from 1075 °F to 1225 °F in the core and is circulated from the reactor vessel to four primary heat exchangers by four fuel pumps. A barren coolant salt is used as the intermediate heat exchange fluid [5].

In 1972 ORNL proposed a major development program that would culminate in the construction and operation of a demonstration reactor called the molten salt breeder experiment (MSRE). The program was estimated to cost a total of $350 million over a period of 11 years.

However, the MSRE was a very successful experiment, in that it answered many questions and posed but a few new ones. Perhaps the most important result was the conclusion that it was quite a practical reactor. It ran for long periods of time, and when maintenance was required, it was accomplished safely and without excessive delay. Also, it demonstrated the expected flexibility and ease of handling the fuel. As mentioned above, it was the first reactor in the world to operate with ²³³U as the sole fuel, and the highly radioactive ²³³U used would have been extremely difficult to handle if it had had to be incorporated into solid fuel elements. In preparation for the run with ²³³U, the ²³⁵U was removed from the carrier salt in 4 days by the fluoride volatility process. This process decontaminated the 218 kg of uranium of gamma radiation by a factor of 4 × 10⁹ so that it could be handled without shielding. As an aside, this equipment used for the MSRE was sufficiently large so that it could satisfactorily handle all of the fuel processing needs for a 1000-MWe molten salt converter reactor.

Moreover, three problems did arise during the construction and operation of the MSRE that required further research and development (R&D) and they were as follows:

1. The first was the Hastelloy-N used for the MSRE was subject to a kind of radiation hardening, due to accumulation of helium at grain boundaries. Later, it was found that modified alloys that had fine carbide precipitates within the grains would hold the helium and restrain this migration to the grain boundaries. Nevertheless, it is still desirable to design well-blanketed reactors in which the exposure of the reactor vessel wall to fast neutron radiation is