A New Look at the Nuclear Energy Option
The United States is faced with a serious dilemma as it wrestles with a war against terrorism sparked in large part by our long-term involvement with Saudi Arabia and other Middle Eastern nations. We are gravely energy dependent, and the source of a significant part of our energy comes from Saudi Arabia and Kuwait.
The current energy situation has critical national security and economic implications for the future of the United States. In light of the recent terrorist events it is time to reexamine the nuclear option for U.S. domestic energy needs. In this article, I will examine the basics of nuclear power in the perspective of the 1979 accident at Three Mile Island in Pennsylvania – a mishap that led to a political over-reaction that all but killed off nuclear power as an energy source for the United States. In subsequent weeks, I will examine other facets of nuclear energy.
First, we need to review some basics of nuclear reactors.
Uranium occurs naturally in nature as a pitchblende ore. Pitchblende is mildly radioactive, which means that it spontaneously emits alpha particles. The level of radioactivity is very low, however, so there is no threat from mining and transporting the ore, and in any case, alpha particles, which are nothing more than Helium atoms stripped of their two electrons, pose no threat outside the body.
The Uranium normally extracted from pitchblende typically takes two different forms, called isotopes: Uranium-235 and Uranium-238. About 99.3 percent of Uranium in the Earth's crust is Uranium-238; only about 0.7 percent is Uranium-235, along with a vanishingly small percentage of four other isotopes. The 235 isotope of Uranium is the basis of most current nuclear power generation.
Uranium-235 is fissile, meaning that it can fission thermally by absorbing a slow neutron and splitting into two pieces plus two or three neutrons plus energy. About eighty-five percent of the energy released in a nuclear reactor is carried in the motion of these fission byproducts. About seven percent is generated by their radioactive decay.
Uranium-238 is not fissile but it can capture a thermal neutron, becoming—after a complicated process—Americium-241, which forms the heart of the modern household smoke detector.
So when you fuel a nuclear reactor with a mix of Uranium-235 and Uranium-238, you get a lot of energy in the form of heat, and some radioactivity in the form of beta particles and gamma rays. In current American reactors, the final products are a mixture of various isotopes of Barium, Krypton, Strontium, Cesium, Iodine, Xenon, Plutonium, Neptunium, and Americium. The latter three emit alpha particles for thousands of years.
In the final analysis this complicated process just produces a lot of heat, that in turn is used to create steam to drive a turbine to generate electricity.
Nuclear reactor fuel consists of Uranium mix formed into small pellets loaded into Zirconium tubes. These tubes are then bundled into an assembly called a fuel rod. Fuel rods are placed inside the reactor core where the fissile Uranium-235 atoms begin to fission, producing increasing numbers of neutrons until the core goes critical – the reaction becomes self-sustaining. In order to control the reaction process so that the number of neutrons produced equals the number absorbed, control rods made of Boron or Cadmium are distributed among the fuel rods. These control rods can be adjusted so that the amount of rod material inside the reactor exactly controls the level of neutron production – like a burner knob on a gas stove.
The primary coolant filling the core absorbs the energy released by the fissioning Uranium-235 atoms and carries this heat to a heat exchanger where it is transferred to the secondary coolant. The secondary coolant flashes to steam which then drives the turbine. The steam is condensed back to water in the cooling towers that are characteristic of nuclear power plants.
If something goes wrong at any point, the worst that can happen is the reactor shuts itself off, and it then sits there stewing in its own heat. This can damage internal components of the reactor, but that's it. A nuclear reactor really is nothing more than a device to boil water into steam to drive a turbine. As with any boiler, the steam it creates is under a lot of pressure.
Can a nuclear reactor explode?
Not like a nuclear bomb – that's impossible, completely, utterly impossible. A nuclear bomb is a runaway chain reaction in fissile material that is tightly contained for several microseconds until the internal pressure builds up sufficiently to cause a gigantic explosion. In a nuclear reactor, the fissile material is not tightly contained. If a runaway chain reaction were somehow to happen, the very worst possible result would be for the material to get so hot that it would melt and flow around inside the reactor, or perhaps melt out the bottom of a badly-designed reactor vessel. That's it.
Once it starts flowing around on the floor, it cannot maintain its criticality: no more fission, no more heat, and it all stops. The so-called China Syndrome – the idea that a run-away nuclear reactor could somehow melt through the Earth's crust and sink right through the Earth to China – is a myth. The physical laws of the universe prevent it.
A reactor is as likely to explode in a non-nuclear way as any other pressurized steam device. If you build it correctly, it won't happen. But if something you didn't plan for goes wrong and it does explode, you get a bunch of hot steam and pieces of pipe and boiler, and – unfortunately – unwanted high-energy emissions, and scrap that emits alpha and beta particles. This is why reactors operate inside containment buildings, which are reinforced concrete structures specifically designed to remain intact should there be an explosion of the pressurized reactor core. When properly designed, they contain the products of the explosion – hence the term, "containment."
The 1979 Three Mile Island accident was caused by a sequence of errors – mechanical and human. Nevertheless, everything remained inside the containment building. There was essentially no release of harmful emissions. Had people reacted properly to the initial problem, the plant could be in production today. More importantly, should a similar sequence of events happen in a nuclear power plant today, the outcome would be a minor local problem, and the plant would be back on line in a few days.
The Three Mile Island incident commenced with a mechanical or possibly an electrical failure of the secondary coolant pumps. One of the routine safety precautions used in nuclear power plants is to maintain a strict separation between the water used to cool the core directly (the primary coolant), and the water used to drive the generating turbine (the secondary coolant). Heat collected by the primary coolant is transferred to the secondary coolant in a heat exchanger, which essentially is a boiler wherein the secondary coolant is circulated around metal tubes containing the primary coolant. The two coolants never directly contact each other.
The result of failure of the secondary coolant pumps at Three Mile Island was that both the turbine and the reactor automatically shut down, just as they were designed to do.
The normal consequence of such a shutdown is that pressure inside the reactor increases, because the heat is no longer being removed. A reactor vessel contains a pressure relief valve that functions similar to the pressure relief valve on a household pressure cooker. At Three Mile Island, this valve opened to decrease internal reactor pressure. Unfortunately, the valve failed in the open position – it was stuck open; and to complicate matters, when it failed, it also broke the transmitter that indicated the valve's position to the operator. Consequently, the valve remained open, so that the internal reactor pressure continued to drop.
Unrelated to this problem, two days earlier reactor operators had tested the emergency feedwater system. This is a routine test to verify that the backup cooling system functions correctly. This backup system is designed to supply emergency cooling of the reactor in the event of failure of the secondary cooling system. In effect, this system supplies a secondary source of water to extract heat from the primary coolant. Part of this routine test requires that a valve be shut at the beginning of the test, and reopened at the end of the test. Somebody goofed and this valve was not reopened. Consequently, when the secondary pumps failed, emergency feedwater was not available to take up the slack. Eight minutes into the incident, someone discovered the shut valve and opened it.
In the meantime, however, under-pressure in the primary coolant system caused by the stuck open pressure relief valve created steam bubbles throughout the primary coolant system. These bubbles caused the reactor vessel to fill with water, giving a false reading that the primary system was filled to capacity, and the operator stopped adding additional water to the primary coolant system, even though such additional water was critically necessary – another big mistake. The end result of all this was that a gas bubble formed at the top of the reactor vessel, and the fuel rods sustained significant heat damage because they were not cooled adequately.
Furthermore, some of the radioactive gas was released into the containment building, carrying with it alpha and beta particle-emitting nuclear byproducts, contaminating the air in the building.
To complicate matters further, some primary coolant carrying radioactive debris from the damaged fuel rods leaked from the system and found its way into the basement of the reactor containment building. There it evaporated and condensed on the walls of the building, adding additional alpha and beta emitters to the already contaminated interior.
After it was all said and done, the actual release of radioactive material into the environment from Three Mile Island was miniscule, about the amount produced by nine thousand luminous exit signs.
The average radiation dose received by residents near Three Mile Island was only 1.4 millirems, compared to a typical annual background dose of 360 millirems that each individual receives from the natural environment. This is about the same amount you would receive from cosmic radiation on a flight from New York to Los Angeles.
Clearly the Three Mile Island incident did not hazard anybody. It took the court system ten years to validate this conclusion, but it stands now as nearly absolute as anything can be. The worst nuclear accident in United States history caused absolutely no harm.
Generating power using nuclear reactors is safe and efficient, despite what we have been led to believe by some environmentalists with private agendas. Next week we will examine the reactor accident at Chernobyl in 1986 to see what really happened, and what impact this has had on our ability to generate our power using nuclear reactors.