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NRC Science 101 – About Spent Nuclear Fuel Part II

Greg Casto
Branch Chief
Division of Reactor Safety Systems
science_101_squeakychalkOur last post talked about the fuel that powers nuclear reactors. Today, we’ll talk about what happens to that fuel when it’s removed from a reactor.

You’ll recall that fuel becomes very hot and very radioactive as it is used in the reactor core to heat water. After about five years, the fuel is no longer useful and is removed. Reactor operators have to manage the heat and radioactivity that remains in the “spent fuel” after it’s taken out of the reactor. In the U.S., every reactor has at least one pool on the plant site where spent fuel is placed for storage. Plant personnel move the spent fuel underwater from the reactor to the pool. Over time, as the spent fuel is stored in the pool, it becomes cooler as the radioactivity decays away.

These pools contain an enormous quantity of water—enough to cover the fuel by about 20 feet. The water serves two purposes: it cools the fuel and shields workers at the plant from radioactivity. Having 20 feet of water above the fuel means there is a lot more water than is needed for cooling and shielding the workers. Also, because of the extra water and the simple design of the pool, there is a lot of time for plant personnel to add water to the pool if needed for any reason.

fuelpoolThe pools are built to meet strict NRC safety requirements. They have very thick, steel-reinforced concrete walls and stainless-steel liners, and are protected by security personnel. There are no drains that would allow the water level to drop or the pool to become empty. The plants have a variety of extra water sources and equipment to replenish water that evaporates over time, or in case there is a leak. Plant personnel are also trained and prepared to quickly respond to a problem. They keep their skills sharp by routinely practicing their emergency plans and procedures.

When the plants were designed, the pools were intended to provide temporary onsite storage. The idea was for the spent fuel to sit in the pool for a few years to cool before it would be shipped offsite to be “reprocessed,” or separated so usable portions could be recycled into new fuel. But reprocessing didn’t end up being an option for nuclear power plants and the pools began to fill up.

In the early 1980s, nuclear plants began to look for ways to increase the amount of spent fuel they could store at the plant site. One way was to replace spent fuel storage racks in the pools with racks containing a special material that allowed spent fuel to be packed closer together. Another way was to place older, cooler and less radioactive fuel in dry storage casks that could be stored in specially built facilities at the plant site. We’ll talk more about dry spent fuel storage in future blog posts.

Most plants today use both re-designed storage racks and dry storage facilities to store spent fuel. All storage methods must be reviewed in detail and approved by the NRC before a plant is allowed to change storage methods.

Science 101 – What is Nuclear Fuel?

Kevin Heller
Reactor Systems Engineer, Division of Safety Systems

science_101_squeakychalkIn earlier Science 101 posts, we told you about nuclear chain reactions and how they are used to generate electricity in reactors. This post focuses on the fuel that reactors use to create those chain reactions.

You may recall that nuclear fuel rods get hot because of the nuclear reaction, and that heat is key to generating electricity. But what exactly are these fuel rods?

Nuclear fuel starts with uranium ore, which is found in the ground throughout the world. For now, we’ll just say that uranium ore goes through several steps to be processed and manufactured into nuclear fuel. In a future Science 101 post, we’ll talk more about the process of turning uranium ore into fuel pellets.

fuelpelletEach pellet is about the size of a pencil eraser. These pellets are stacked inside 12-foot long metal tubes known as fuel cladding. The tubes are sealed on each end to form a fuel rod, and between 100 and 300 fuel rods are arranged in a square pattern to form a fuel assembly. The number of fuel rods used to make a fuel assembly depends on the type of reactor the assembly will be used in and the company that makes the fuel.

fuelrodsWhile the assemblies are very long (about 12 feet), they are less than 1 foot wide. The assemblies have special hardware at the top and bottom and at intervals in between to keep the fuel rods firmly held and evenly spaced. Fuel assemblies are only slightly radioactive before they are placed into a reactor core. Typically, a reactor core will have between 150 and 250 fuel assemblies.

We talked before about the form of uranium that is important in commercial nuclear reactors. It is an “isotope,” or an atom with a very specific number of neutrons, known as U-235. Part of the process of turning uranium ore into nuclear fuel is enrichment—which increases the amount of U-235 relative to the other isotopes naturally found in uranium. Under the right conditions in a reactor, neutrons will cause U-235 atoms to fission, or split. This leaves two new, different atoms and a couple of neutrons. These new neutrons will then cause other U-235 atoms to fission, forming a chain reaction.

As U-235 atoms fission, energy is released in the form of heat. That heat creates steam which turns a turbine to create electricity. After a few years, there is considerably less U-235 in the fuel. If the amount of U-235 were to drop too low, there would no longer be enough to keep a chain reaction going. So every 18-24 months about one-third of the fuel in a reactor core is removed and replaced with new, fresh fuel. The used fuel is often called “spent fuel.”

Spent fuel is very hot and very radioactive. The atoms created by the fission process are unstable at first and emit particles that create heat. Therefore, spent fuel must be handled and stored carefully, and under controlled conditions. We’ll talk more about spent fuel and how it is managed in a future Science 101 post.

NRC Science 101: How a Nuclear Reactor Generates Electricity

Paul Rebstock
Senior Instrumentation and Control Systems Engineer
 

science_101_squeakychalkHow does a nuclear reactor generate electricity? Well — it doesn’t, really. Let’s begin at the end and see how it all fits together.

We begin by looking at an electric motor. A motor consists primarily of two major components: a stator, which stands still, and a rotor, which rotates within the stator. When electricity is applied to the motor, electromagnets within the stator and the rotor push and pull on each other in a way that causes the rotor to rotate. The magnets in the stator pull magnets in the rotor toward them, and then, as the rotor magnets pass by reverse themselves and push the rotor magnets away.

The parts are arranged so the pulling and pushing are all in the same direction, so the rotor spins inside the stator. The electrical energy applied to the motor results in mechanical energy in the rotor.

But that same machine can be used in reverse: If some outside force causes the rotor to spin, the interaction of the magnets causes electricity to be produced: the “motor” is now a “generator,” producing electrical energy as a result of the mechanical energy applied to its rotor. That’s the most common way to make large quantities of electricity.

So how do you make the rotor spin? That’s where the nuclear reactor comes in, although still indirectly. Recall that a nuclear reactor generates heat. The fuel rods get hot because of the nuclear reaction. That heat is used to boil water, and the steam from that boiling water is used to spin the rotor. As we have seen, when the rotor spins, electricity comes out of the stator.

When water boils, the steam that is produced occupies much more physical space than the water that produced it. So if you pump water through some sort of a heat source — like a nuclear reactor, or a coal‑fired boiler — that is hot enough to boil the water, the exiting steam will be travelling much faster than the water going in. That steam runs through a machine called a turbine, which acts something like a highly‑sophisticated windmill. The physical structure is vastly different from a windmill, and a large turbine can be far more powerful than any windmill that has ever been made, but the effect is somewhat the same: the steam, or wind, causes part of the machine to spin, and that spinning part can be connected to a generator to produce electricity.

The steam leaving the turbine is collected in a device called a condenser — essentially a metal box the size of a house, with thousands of pipes running through it. Cool water flows through the pipes, and the steam from the turbine is cooled and condenses back into water. Then the water is pumped back through the heater and the cycle continues.

Now, back to the nuclear reactor . . . We have seen how the reactor generates heat, and we have seen how heat is used to generate steam and how the steam then powers the turbine, which spins the generator that produces electricity. The final piece in the puzzle is how the heat from the nuclear reaction generates the steam.

bwrThe fuel rods are suspended in a water bath contained in a large metal container somewhat like a gigantic pressure cooker. A typical “reactor vessel” might be 15 feet in diameter and 20 feet high, and some are much larger than that. In some types of reactors, the water is allowed to boil, and the heat generated in the fuel rods is carried away in steam. These are called “boiling water reactors” (or “BWR”).

In others, the water is held at a very high pressure — on the order of 2000 pounds per square inch. (By the way, that is more than 60 times the pressure in the tires of a typical car.) In that situation, the water cannot expand and cannot boil. The water in that type of reactor carries the heat away while remaining liquid, and that heat is then transferred to another water system where the boiling occurs. This transfer takes place in a device aptly named a “steam generator.”

These are called “pressurized water reactors” (or “PWR”). A small PWR might have two steam generators. A large one might have four. Some have three. The steam from all of the steam generators is typically combined into a single “main steam line” that carries the steam to the turbine, so the reactor and all of the steam generators act together as a single steam source.

The water from the condenser is pumped directly into the reactor vessel for a BWR, or into the steam generators for a PWR.

So there you have it: the nuclear reaction heats the fuel, the fuel heats the water to make steam, the steam spins the turbine, the turbine turns the generator, and the generator makes electricity.

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

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.

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