Science 101: How a Chain Reaction Works in a U.S. Nuclear Reactor

Paul Rebstock
Senior Instrumentation and Control Systems Engineer

 

science_101_squeakychalkThe primary active ingredient in nuclear reactor fuel is a particular variety, or “isotope,” of uranium, called U235. U235 is relatively rare — only about 0.7% of uranium as it exists in nature is U235. Uranium must be enriched to contain about 5% U235 to function properly as fuel for a U.S. commercial nuclear power plant.

U235 has 92 protons and 143 neutrons. Protons and neutrons are some of the almost unimaginably tiny particles that make up the nucleus of an atom — see Science 101 Blog #1. All other isotopes of uranium also have 92 protons, but different isotopes have slightly different numbers of neutrons.

Uranium is a radioactive element. Uranium atoms break apart, or disintegrate, into smaller atoms, releasing energy and a few leftover neutrons in the process. This happens very slowly for U235. If you have some U235 today, in about 700 million years you will have only half as much. You will have the remaining U235, plus the smaller atoms. The energy released will have gone into the environment too slowly to be noticed, and the extra neutrons will have been absorbed by other atoms.

While this happens very slowly, the disintegration of each individual atom happens very quickly, and the fragments are ejected at a very high speed. Those high-speed fragments are the source of the heat generated by the reactor. Under the right man-made conditions, the number of U235 atoms that disintegrate each second can be increased.

When a U235 atom disintegrates, it releases some neutrons. Some of those neutrons can be made to interact with other U235 atoms, causing them to disintegrate as well. Those “target” atoms release more neutrons when they disintegrate, and then those neutrons interact with still other U235 atoms, and so on. This is called a “chain reaction.” This process does not work well for other isotopes of uranium, which is why the uranium needs to be enriched in U235 for use as nuclear fuel.

Most of the energy released when a U235 atom disintegrates is in the form of kinetic energy — the energy of physical motion. The fragments of the disintegrated atom collide with nearby atoms and set them vibrating. That vibration constitutes heat. The fuel rods get hot as the reaction progresses. The faster the chain reaction — that is, the larger the number of U235 atoms that disintegrate each second — the faster energy is released and the hotter the fuel rods become.

The uranium in a U.S. commercial nuclear reactor is thoroughly mixed with neutral material and formed into pellets about half inch wide and three-quarters of an inch long. The pellets are stacked tightly in metal tubes, forming “fuel rods” that are several feet long. Each fuel rod is just wide enough to hold a single column of pellets. The fuel rods are sealed, to keep all of the radioactive materials inside. There are thousands of these fuel rods in a typical reactor. They contain around 60 tons of uranium – but only about three tons are U235. (The majority of the uranium in the reactor is in the form of the most abundant naturally occurring isotope of uranium, U238, which cannot sustain the fission process without the help of an elevated concentration of the isotope U235.)

The people in charge of the reactor can control the chain reaction by preventing some or all of the released neutrons from interacting with U235 atoms. The physical arrangement of the fuel rods, the low U235 concentration, and other design factors, also limit the number of neutrons that can interact with U235 atoms.

The heat generated by the chain reaction is used to make steam, and that steam powers specialized machinery that drives an electrical generator, generating electricity. Science 101 will look at how that works in more detail in a later issue.

The author has a BS in Electrical Engineering from Carnegie-Mellon University.

 

NRC Science 101: The What and How of Geiger Counters

Joe DeCicco
Senior Health Physicist
Source Management and Protection Branch
 

In earlier Science 101 posts, we talked about ionizing radiation and different types of radiation. In this post, we’ll look at the Geiger counter, an instrument that can detect radiation.

science_101_squeakychalkJust to recap, the core of an atom (the nucleus) is surrounded by orbiting electrons, like planets around a sun. The electrons have a negative charge and usually cancel out an equal number of positively charged protons in the nucleus. But if an electron absorbs energy from radiation, it can be pushed out of its orbit. This action is called “ionization” and creates an “ion pair”—a free, negatively charged electron and a positively charged atom.

Humans cannot detect creation of an ion pair through their five senses. But the Geiger counter is an instrument sensitive enough to detect ionization. Most of us have heard or seen a Geiger counter. They are the least expensive electronic device that can tell you there is radiation around you—though it can’t tell you the original source of the radiation, what type it is or how much energy it has.

How does it work? A Geiger counter has two main parts—a sealed tube, or chamber, filled with gas, and an information display. Radiation enters the tube and when it collides with the gas, it pushes an electron away from the gas atom and creates an ion pair. A wire in the middle of the tube attracts electrons, creating other ion pairs and sending a current through the wire. The current goes to the information display and moves a needle across a scale or makes a number display on a screen. These devices usually provide “counts per minute,” or the number of ion pairs created every 60 seconds. If the loud speaker is on, it clicks every time an ion pair is created. The number of clicks indicates how much radiation is entering the Geiger counter chamber.

You hear a clicking sound as soon as you turn on the speaker because there is always some radiation in the background. This radiation comes from the sun, natural uranium in the soil, radon, certain types of rock such as granite, plants and food, even other people and animals.

The background counts per minute will vary; the needle will move or the number will change even when there is no know radiation source nearby. Many different things cause this fluctuation, including wind, soil moisture, precipitation (rain or snow), temperature, atmospheric conditions, altitude and indoor ventilation. Other factors in readings include geographical location (higher elevations give higher counts), the size and shape of the detector, and how the detector is built (different chamber material and different gases).

geigercounterDepending on the elevation and the type of Geiger counter, a typical natural background radiation level is anywhere from five to 60 counts per minute or more. Because background radiation rates vary randomly, you might see that range standing in one spot. It is important to understand that the Geiger counter indicates when an ion pair is created, but nothing about the type of radiation or its energy.

Other types of instruments can provide an exposure rate (expressed as milliroentgen per hour or mR/hr). These counters must be calibrated to read a particular type of radiation (alpha, beta, gamma, neutron, x-ray) as well as the amount of energy emitted. The reading will only be accurate for that type of radiation and that energy level. And these instruments need to be calibrated regularly to be sure they are providing correct information over time.

For more sophisticated environmental radiation readings, check out the Environmental Protection Agency’s nationwide system, RadNet. Using equipment far more sensitive than a Geiger counter, it continuously monitors the air and regularly samples precipitation, drinking water and pasteurized milk.

Over its 40-year history, RadNet has developed an extensive nationwide “baseline” of normal background levels. By comparing this baseline to measurements across the U.S. states in March 2011, following the accident at the Fukushima reactors in Japan, the EPA was able to detect very small radiation increases in several western states. EPA detected radiation from Japan that was 100,000 times lower than natural background radiation—far below any level that would be of concern. And well below anything that would be evident using a simple Geiger counter, or even Geiger counters spread across the country.

If RadNet were to detect a meaningful increase in radiation above the baseline, EPA would investigate immediately. With its nationwide system of monitors and sophisticated analytical capability, RadNet is the definitive source for accurate information on radiation levels in the environment in the U.S.

By the way, the Geiger counter is also called a Geiger-Mueller tube, or a G-M counter. It was named after Hans Geiger, a German scientist, who worked on detecting radiation in the early 1900s. Walter Mueller, a graduate PhD student of Geiger’s, perfected the gas-sealed detector in the late 1920s and received credit for his work when he gave his name to the Geiger-Mueller tube.