Nuclear Power in Space
Submitted to the University of Colorado Space Grant Consortium's
Undergraduate Space Research Symposium
By
Terrell R. Gropp
March 23, 2001
When I found out about the opportunity to write a paper for the Symposium this year, I was unsure at first. Never before had I presented a paper of this scope on any subject and quite frankly did not know what to research. Upon reflection, I decided to write about something that I had experience in, nuclear power. Completing training only two years ago as a certified reactor operator by the Department of Energy has provided me with a unique insight into the world of nuclear power. I have found that there are many public myths as well as practical fears about nuclear power. There are also myriad possibilities for applications of nuclear power. During my time as a student in the Navy, I often thought about nuclear power in space applications. The goal of this paper is to dispel some of the myths, address legitimate concerns, offer a logical explanation of why nuclear power is needed in space exploration, and outline guidelines set forward by the United Nations.
Human fears develop out of ignorance of a subject. Racism is the result of ignorance about other peoples and cultures. So, it is the same with nuclear power. I cannot even begin nor will I attempt to list the amount of myths I have encountered about nuclear power and radioactivity. Instead, I feel that education about the subject is a better means of dispelling any myths about the subject. First, nuclear power involves the use of radioactive decay to generate heat. This heat is in turn transformed into electrical energy using different methods. For large reactors, a cooling system and turbine generators are used. So, how does radioactive decay work? Below is a brief mathematical explanation of the process:
Half Life of Radioactive Species
Law of Radioactive Decay:(1)
Integrating:(2)
Where the decay constant, (3)
Units:1 Curie = 3.7 x 1010 disintegrations per second
The above mathematical calculation can be used to understand the amount of energy generated by the following relationships:
Maximum energy released from the decay parent AXZ to its daughter(s)
Einstein’s Energy Equation: E = m*c2
E = (mass AXZ - mass)*c2
Take for example the radioactive decay of plutonium 238.
238Pu94234U92 + 4He2
The mass changes from the decay are listed below:
Mass 238Pu94 = 238.049555 amu
Mass 234U92 = 234.040947 amu
Mass 4He2 = 4.002603 amu
m ~ 0.006005 amu
Therefore,
E = 5.59MeV
This is enough energy to power nearly two million Game Boys!! Just from the decay of one plutonium-238 atom! Just imagine the amount of energy you could gain from a plutonium source the size of two AA batteries!!
When placed in a reactor system, the heat is transferred via coolant to a secondary fluid that in turn is used to turn a turbine in the form of high-pressure steam. Other than the intense heat developed, two things make this a dangerous process.
First, the process of radioactive decay is exponential. The particles emitted from a decay process strike other unstable elements. This transfer of kinetic energy is all that is needed to help the next element decay, which in turn emits several more particles, which then help several more elements decay. This chain reaction when uncontrolled is the basis behind nuclear weapons. By controlling the chain reaction, we have nuclear power. Therefore, if the decay process becomes uncontrolled, there is a meltdown of the Figure 1- Radioactive Decay
reactor, thus releasing a major amount of energy and radioactivity. Figure 1 depicts the radioactive decay of Uranium 235. Notice the different particles emitted from the decay process.
The second dangerous part of radioactive decay is the fission byproducts. These particles come in different shapes and sizes. In Figure 1 above, neutrons and high-energy gamma rays are shown. Beta and alpha particles are also possible depending upon the isotope. These particles affect the human body in different ways. Depending on how they enter the human body, all can cause damage. This damage is done directly to living body cells. Four things can happen when a particle emitted from radioactive decay strikes a living cell. One, nothing happens, the cell is unaffected. Two, the cell is damaged, but has sufficient strength to regenerate to its original form. Three, permanent damage is done to the DNA structure of the cell, and it continues to reproduce (CANCER!!!) Four, the cell is completely destroyed. Now, while complete cell destruction might seem better than cancer, remember that when exposed to high levels of radiation, all of the above four could occur all over the body at the same time. Too much cell destruction can result in death anywhere from minutes to months.
Just from the above discussion, it is obvious why nuclear power must be given immense respect, as well as strictly regulated. It is this respect that has insured the successful operation of nuclear reactors within the Navy for many years. When I first thought about entering the Naval service to become a reactor operator many images came to mind, none of which were good. Many disastrous memories as a young boy came fluttering back. The largest looming in my mind was that of the disaster of Chernobyl. With it came other childhood cold war fears of nuclear destruction. You could say that I first associated nuclear power with death and destruction. Why not? Most of the public old enough to remember surely remembers Chernobyl as well as Three Mile Island. It was these two accidents as well as other minor accidents worldwide, which led to the decline of public opinion during the early 80’s. Prior to the Chernobyl accident, 45% of Americans were opposed to nuclear power, and after about 50%. So, as far as Americans were concerned, there wasn’t a major shift in public opinion. For the most part Americans have mostly been 50/50 on the subject. However, for European countries closer to the disaster at Chernobyl, opposition increased up to 80%. The major opposing countries during this time were the United Kingdom, Austria, Yugoslavia, and West Germany.
With such a lethal energy source in the backyard of America, many people oppose the use of nuclear power. The main concern of the public has to do not with reactors in space, but the process by which we deliver them. With a mishap from a launch vehicle, an explosion could send radioactive material into the atmosphere similar to Chernobyl. A major opponent to nuclear power even prior to the disaster at Chernobyl, Ralph Nader stated in a speech at Syracuse University on 6 April, 1975,
“There are over two thousand times more radioactive materials in a nuclear power plant than the fallout from the Hiroshima weapon.”
Due to such an international opposition, as well as a desire for public health, the United Nations (U.N.) developed guidelines for radioactive material and power generation in space operations. During the 35th session of the U.N., principles relevant to the use of nuclear power sources in outer space were discussed. Below is an excerpt from the 35th session of the U.N. general assembly concerning safe use and guidelines for radioactive material in space.
Principle 3. Guidelines and criteria for safe use
In order to minimize the quantity of radioactive material in space and the risks involved, the use of nuclear power sources in outer space shall be restricted to those space missions which cannot be operated by non-nuclear energy sources in a reasonable way.
1. General goals for radiation protection and nuclear safety
(a) States launching space objects with nuclear power sources on board shall endeavour to protect individuals, populations and the biosphere against radiological hazards. The design and use of space objects with nuclear power sources on board shall ensure, with a high degree of confidence, that the hazards, in foreseeable operational or accidental circumstances, are kept below acceptable levels as defined in paragraphs 1 (b) and (c).
Such design and use shall also ensure with high reliability that radioactive material does not cause a significant contamination of outer space.
(b) During the normal operation of space objects with nuclear power sources on board, including re-entry from the sufficiently high orbit as defined in paragraph 2 (b), the appropriate radiation protection objective for the public recommended by the International Commission on Radiological Protection shall be observed. During such normal operation there shall be no significant radiation exposure.
(c) To limit exposure in accidents, the design and construction of the nuclear power source systems shall take into account relevant and generally accepted international radiological protection guidelines.
Except in cases of low-probability accidents with potentially serious radiological consequences, the design for the nuclear power source systems shall, with a high degree of confidence, restrict radiation exposure to a limited geographical region and to individuals to the principal limit of 1 mSv in a year. It is permissible to use a subsidiary dose limit of 5 mSv in a year for some years, provided that the average annual effective dose equivalent over a lifetime does not exceed the principal limit of 1 mSv in a year.
The probability of accidents with potentially serious radiological consequences referred to above shall be kept extremely small by virtue of the design of the system.
Future modifications of the guidelines referred to in this paragraph shall be applied as soon as practicable.
(d) Systems important for safety shall be designed, constructed and operated in accordance with the general concept of defence-in-depth. Pursuant to this concept, foreseeable safety-related failures or malfunctions must be capable of being corrected or counteracted by an action or a procedure, possibly automatic.
The reliability of systems important for safety shall be ensured, inter alia, by redundancy, physical separation, functional isolation and adequate independence of their components.
Other measures shall also be taken to raise the level of safety.
To prevent a large-scale disaster similar to Chernobyl from occurring during transport of radioactive material to space, the United Nations in their 35th general assembly outlined specific guidelines.
2. Nuclear reactors
(a) Nuclear reactors may be operated:
(i) On interplanetary missions;
(ii) In sufficiently high orbits as defined in paragraph 2 (b);
(iii) In low-Earth orbits if they are stored in sufficiently high orbits after the operational part of their mission.
(b) The sufficiently high orbit is one in which the orbital lifetime is long enough to allow for a sufficient decay of the fission products to approximately the activity of the actinides. The sufficiently high orbit must be such that the risks to existing and future outer space missions and of collision with other space objects are kept to a minimum. The necessity for the parts of a destroyed reactor also to attain the required decay time before re-entering the Earth's atmosphere shall be considered in determining the sufficiently high orbit altitude.
(c) Nuclear reactors shall use only highly enriched uranium 235 as fuel. The design shall take into account the radioactive decay of the fission and activation products.
(d) Nuclear reactors shall not be made critical before they have reached their operating orbit or interplanetary trajectory.
(e) The design and construction of the nuclear reactor shall ensure that it cannot become critical before reaching the operating orbit during all possible events, including rocket explosion, re-entry, impact on ground or water, submersion in water or water intruding into the core.
(f) In order to reduce significantly the possibility of failures in satellites with nuclear reactors on board during operations in an orbit with a lifetime less than in the sufficiently high orbit (including operations for transfer into the sufficiently high orbit), there shall be a highly reliable operational system to ensure an effective and controlled disposal of the reactor.
3. Radioisotope generators
(a) Radioisotope generators may be used for interplanetary missions and other missions leaving the gravity field of the Earth. They may also be used in Earth orbit if, after conclusion of the operational part of their mission, they are stored in a high orbit. In any case ultimate disposal is necessary.
(b) Radioisotope generators shall be protected by a containment system that is designed and constructed to withstand the heat and aerodynamic forces of re-entry in the upper atmosphere under foreseeable orbital conditions, including highly elliptical or hyperbolic orbits where relevant. Upon impact, the containment system and the physical form of the isotope shall ensure that no radioactive material is scattered into the environment so that the impact area can be completely cleared of radioactivity by a recovery operation.
Mankind has always looked out to the unknown, wanting to explore. Exploration has always come at the cost of men’s lives. Manifest destiny is not something that belongs in the history books. Still today, with a world completely discovered, man looks outward. The U.N. realized that mankind will continue to explore and that innovations will be made to travel further and further into space. To accomplish this, the need for energy is obvious.
How far we can go depends mostly on how long our energy source(s) will last. For unmanned exploration, less energy is required. For the most part, solar arrays can generate enough energy for earth orbit satellites and space stations. But what of satellites sent to deep space, or even large scale manned space stations in orbit around the earth, or on other planets? The simple fact is manned stations require much more energy than unmanned stations, and the further from the sun or another star, the less energy that is obtainable in the form of solar energy. That brings us to chemical energy. While chemical energy can surely help to sustain where solar energy cannot, how much space must be used? An efficient way to outfit a space vehicle with a large amount of stored energy must be used. If there is anything that our current exploits to space have taught us is that every single resource, tool, and structure needs to be confined to a small space. The most feasible energy source is nuclear power. 1 kg of nuclear fuel in a nuclear reactor can contain up to 10 million times the amount of energy found in 1 kg of chemicals.
In summary nuclear power is an incredibly large responsibility. Extremely large amounts of energy can be provided in a relatively small area. There are also known disastrous consequences if nuclear power is improperly maintained or operated. However, mankind has always pushed the frontier of discovery. The power per area required to go where we want to go today comes in the form of nuclear power. The United Nations realized this in their 35th general assembly and set aside specific guidelines concerning the use of radioactive material in space applications.
References:
Lapp, Dr. Ralph E., Nader’s Nuclear Issues, University of Oxford, 1975.
Van der Pligt, Joop, Nuclear Energy and The Public, Chapters 1,4,7, Cambridge, Massachusetts, 1992.
Figure 1 – Radioactive Decay,
University of Wisconsin's Geology 376 Page - Nuclear Power in Space, Lecture 25, Professor G. L. Kulcinski, March 25, 1996,
United Nations 35th general assembly,