Past, present and future

29 June 2001

In this month’s feature retired nuclear engineer Stanley Thompson tells NEI why, based on his experience and knowledge of the industry, nuclear power should not be a part of the future.

In 1946 I entered the field of peacetime nuclear power, having previously worked as an engineer on steam and gas turbine power plants. I wanted to be part of the development of this new “safe” source of electrical energy “too cheap to meter”.

In 1963, due to safety and economic concerns, I quit the development of nuclear power. Now, after over fifty years of observation, I am convinced that human beings lack the capacity to protect life on our planet from the perils of man-made nuclear devices. To put it bluntly, I have become an anti-nuclear engineer.

A promising career

I studied the reactors that had been built to produce bomb material and analysed engineering methods for the design of reactors to generate thermal power. On the basis of my analysis I taught a design course to reactor engineering students at the Oak Ridge National Laboratory. With my friend, Ole Rodgers, I co-wrote the book, “Thermal Power from Nuclear Reactors”, published in 1956.

I believed that a commitment to the construction of nuclear power plants should and would be preceded by detailed engineering development, including experiments, on each of the components required for a complete nuclear power system, and their interaction with that system. Instead a decision was made to form teams of power plant builders with electric utilities, under the loose technical guidance and firm financial support of the Atomic Energy Commission. Each team would proceed rapidly to build a nuclear power plant based on its own intuitive concepts.

Gradually I realised the lethal load of radioactivity carried in reactors. My investigations showed potential power instabilities capable of spreading these deadly products into the environment. I felt a need for engineering development of any reactor system prior to commitment to its construction. The promoters of nuclear power hadn’t time to listen.

The public trusted the private and public agencies charged with developing nuclear power. The technical “experts”, on whom a complicated society must depend for advice on a complicated subject, warped or hid their opinions to fit their own short-range interests in nuclear power, giving support to corporate and private greed for continuing government contracts. The catastrophic potential of nuclear reactors and their inadequate development were hidden from the public by use of the “secret” stamp.

Government and private nuclear proponents proceeded with plans to build reactors based upon optimism and ignorance, and disdained all negative indications. They proposed to build peacetime nuclear reactors on an a-priori assumption that they were inherently safe and a multipurpose “good”. Besides nuclear electric power plants there would be nuclear aircraft, nuclear space vehicles and nuclear submarines. To prevent skidding on winter ice, our interstate highways would be underlain with nuclear-powered electric wiring. Automobile companies published promotional material, complete with pictures of models of the nuclear powered automobiles of the near future.

My engineering friend, Bill Parrish, terminated his promising nuclear career, stating that: “Ignorance is a hell of a poor basis for nuclear optimism.”

From my own experience I finally concluded that reactors constitute a disasterously uneconomic power source and the most violent and uncontrollable form of accidental and deliberate pollution ever conceived. After 17 years in what I thought was the terminally sick nuclear reactor business, I decided the rest of my life was too short to waste. I became a professor of engineering, first at Robert College in Istanbul, then at Howard University in Washington DC. Then I retired to an abandoned mountain farm bordering Shenandoah National Park in Virginia. I continued to think about reactor instability and the environmental impact of unsafe nuclear reactors. I tried to pass on my concerns to government agencies. My tentative appointments for consultation with staff members of the Nuclear Regulatory Commission were always cancelled.

Inherent stability

The experts say that they can build safe reactors using inherent reactor characteristics to limit any unwanted rise in reactor power. A rise in power increases the temperature of reactor parts, causing them to swell, bend or otherwise distort. A necessary condition for stable reactor power is that the reactor distortion accompanying a temperature rise decreases the reactivity, thus slowing any further rise in power. For instance, the reactor distortion caused by a temperature rise could allow more neutrons to be wasted by leakage, thus decreasing the reactivity. Overly simple equations for reactor power dynamics indicate that this idea should work, and sometimes it does.

The supposition that satisfying the above condition guarantees power stability is based on a false assumption that the reactor distortion occurs simultaneously with the temperature rise which caused it. The distortion lags behind the temperature change by a time determined by the mechanical dynamics of the reactor system. Therefore the change in reactivity also lags behind the temperature change, raising the possibility of unstable power oscillations. This conclusion is supported by actual experience with seriously unstable reactors such as the inoperable Fort St Vrain power reactor.

Experimental evidence

My interest in the subject of reactor instability was aroused by two sets of experiments. I heard of the first set, called “teasing the dragon”, from a physicist who had worked on the atomic bomb project. In a low-level simulation of a bomb test, a sudden pulse of nuclear energy was initiated, sufficient to cause parts of the test assembly to jump apart momentarily (and occasionally to destroy the experimenter, but not quite enough to destroy the apparatus and its surroundings). I wanted to understand how the dynamic mechanism which controlled the energy pulse might be related to the control of reactor power.

The second set of experiments, under the acronym SPERT, was carried out at the National Reactor Test Station to study transients of reactor power. An assembly of thin sheets of aluminium bearing the nuclear fuel was immersed in a pool of water, the surface of the water being open to the atmosphere. Reactor power was slowly increased by gradually pulling out control rods. At a certain low power a spontaneous unstable oscillation suddenly appeared and built rapidly to violent proportions, resulting in a flash of blue light from Cerenkov radiation and the explosive expulsion of water from the reactor pool. The peak of power was greater by a factor of thousands than the level from which the oscillations had started. It was concluded that the unexpected instability was related to coolant boiling.

In the early 1960s I developed an analysis of a possible unstable mode of power oscillation which I thought could lead to reactor explosions. I developed mathematical criteria to try to predict the threshold of reactor power at which such unstable oscillations would occur. Based on my analyses, I concluded that coolant boiling in the SPERT reactor set up a dynamic configuration in which a heavy mass of water bounced at a low natural frequency on the soft spring provided by vapour bubbles. The mechanical oscillation was coupled with the nuclear power oscillation in an unstable combination. Water-moderated reactors, the type most commonly used for the generation of power, might be susceptible to catastrophic nuclear power oscillations if the pressure of the water in the reactor suddenly decreased, creating vapour bubbles in the core.

I wrote several reports predicting that various power reactors could be susceptible to power oscillations of the type observed in the SPERT experiments. One preliminary report entitled “Study of Reactor Kinetics” was published by the American Society of Mechanical Engineers as Paper Number 62-WA-218 late in 1961. In this, there is an example of a water-based reactor with local boiling, and also an example of a solid reactor. I presented the report at the annual meeting of the Society in New York in November 1962. I received encouragement from engineering friends, but was unable to get any attention from the nuclear establishment and was denied publication in the nuclear journals.

The internal study and public scrutiny of potential instabilities of reactor power was (and is) not popular with reactor proponents. Even when instabilities cause accidents, they are not acknowledged as such.

Unstable reactors

The SPERT experiments seemed to us pertinent to our claim that mechanical effects contribute to reactor instability. Subsequent reactors have demonstrated instabilities similar to our predictions.

A non-nuclear model of a nuclear reactor, for a space propulsion project named ROVER, was composed of large graphite blocks through which a gas was circulated in a flow test. The blocks were restrained against the radial pressure of the gas by springs at their outer periphery. A motion picture taken during a flow test showed the blocks banging to pieces as the pressure of the circulating gas bounced them against the restraining springs in a self-excited oscillation, equivalent to a negative mechanical damping in the system. No nuclear catastrophe occurred only because there was no nuclear power to couple with the mechanical oscillation of the graphite blocks.

A high-temperature, gas-cooled reactor for electrical power was built for Colorado Public Service at Fort St Vrain. On completion it could not be brought to a useful level of operating power because persistent power oscillations worsened as power was increased. Because of the oscillations and other problems the reactor was abandoned. A successful ratepayers’ suit removed the useless power plant from the rate base.

Other examples of unstable reactor behaviours exist.

Reactor kinetics

Twenty-five years after my failure to achieve publication in nuclear journals, my son, Bruce Thompson, volunteered to help me write a straight technical article about stability, based on a computer programme which we had developed together. Our programme solves by finite difference mathematics a non-linear, fourth-order differential equation involving nuclear, mechanical and thermal characteristics of reactors. “A Model of Reactor Kinetics” was finally published in Nuclear Science and Engineering, the technical journal of the American Nuclear Society in September 1988 (Vol 100, No 1, P 83). It demonstrates a mechanism for catastrophic instability.

Our paper demonstrates that mechanical friction in a reactor core structure, like the shock absorber in an automobile, is necessary to limit oscillations of reactor power. Without adequate internal friction, a nuclear power driven mechanical oscillation increases toward destruction of the reactor core. Design engineers in many fields have found to their sorrow that any given level of mechanical friction is difficult to guarantee. A small perturbation in power could cause an initially small oscillation which builds rapidly to destruction, either blow up or melt down.

In the design, construction and operation of nuclear reactors an attempt is made to maintain steady operation at any desired power level from fission of the nuclear fuels, uranium or plutonium. This attempt can fail, sometimes catastrophically. A nuclear power plant is a nuclear system and a mechanical system. It is also a heat-transfer system, tied in with controls, boilers, turbines, human operators and a multiplicity of other complicating factors. The possibilities for instability are myriad. This fiercely complicated set of systems is such as to preclude any possibility of the formation of adequate analytical equations and their solution to guarantee the stability of power. An experimental programme sufficient to eliminate the possibilities of power instability in reactors would be expected to be ruinous of both the economy and the environment. Our programme covers only a small part of the complicated possibilities, and demonstrates only one type of power instability.

The future

The decision whether to cease and desist from nuclear power should not be left to the nuclear “experts”. They, and their supporting military nuclear adventurers, have a vested professional interest in its continuation. That crucial decision can be made only by a citizenry as aware as possible of military and civilian nuclear perils, and with a primary vested interest in the continuation of grandchildren and their progeny. People who are not nuclear experts must trust and use their own power of observation, noting that:

•Commercial nuclear power is an economic failure without the government subsidy due to its potential support for military ventures.

•Nuclear reactors, like all complicated technical devices, will occasionally fail.

•Failure of a reactor, as demonstrated at Chernobyl, inflicts radioactive damage on populations and their environment.

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