College of Engineering
University of Wisconsin - Madison
UWNR

University of Wisconsin Nuclear Reactor Tour

Welcome to the UWNR Quick Tour.

In the first part of the tour we will discuss nuclear energy, fission, fusion, and nuclear reactors and how they work. This will give you a background for understanding the components that are present in the UWNR and why they are necessary. In the second part of the tour we will discuss the parts of the UWNR, such as the reactor core, the fuel, and the cooling system, among other things.

Nuclear Energy

Atoms are the building blocks from which matter is formed. Everything around us is made up of atoms. Nuclear energy is contained within the center of the atom in a place known as the nucleus. Particles within the nucleus are held together by a strong force. If a large nucleus is split apart (fission), generous amounts of energy can be liberated. Small nuclei can also be combined (fusion) with an accompanying release of energy. Using this strong force that holds the nucleus together to produce energy is essentially what the field of nuclear power generation is about.
In the fission process certain heavy elements, such as some forms of Uranium, are split when a neutron strikes them. When they split, they release energy in the form of kinetic energy (heat) and radiation. The process not only produces energy but also additional neutrons that can be used to fission other Uranium nuclei and start a chain reaction.
In fusion, nuclei of light elements are brought together under conditions of high pressure and temperature, causing them to combine and produce new elements and energy.
 

Fission

Fission is a nuclear reaction in which an atomic nucleus splits, or fissions, into fragments, usually two fragments of comparable mass, with the release of large amounts of energy in the form of heat and radiation. In the UWNR, Uranium-235 is the fuel and it is struck by a moving neutron, which combines with the U-235 to become U-236. Because of the mass and energy imparted to the nucleus by the neutron, the nucleus has enough energy to fission and breaks down into two (or more) smaller nuclei and two or three new neutrons which together have less mass than the original U-236 nucleus. This missing mass, sometimes known as the mass defect, is changed into energy.

Fusion

Energy can also be produced by combining light nuclei in a process is called nuclear fusion. As an energy source, fusion has several advantages over fission: the light nuclei are plentiful and easy to obtain, and the end products of fusion are usually light, stable nuclei rather than heavy radioactive ones. There is one considerable disadvantage: before light nuclei can be combined however, their mutual repulsion must be overcome due to the fact that the positively charged protons of the nuclei repulse each other. Because of this problem, fusion reactors are not yet producing electrical power. This is an area of great research interest in the field of nuclear engineering and physics.

Nuclear Reactors

In most electric power plants, water is heated and converted into steam, which drives a turbine-generator to produce electricity. Fossil-fueled power plants produce heat by burning coal, oil, or natural gas. In a nuclear power plant, the fission of Uranium atoms in the reactor provides the heat to produce steam for generating electricity.
Several commercial reactor designs are currently in use in the United States. The most widely used design consists of a heavy steel pressure vessel surrounding a reactor core. The reactor core contains the Uranium fuel. The fuel is formed into cylindrical ceramic pellets about one-half inch in diameter, which are sealed in long metal tubes called fuel tubes. The pins are arranged in groups to make a fuel assembly. A group of fuel assemblies forms the core of the reactor.

How They Work

Heat is produced in a nuclear reactor when neutrons strike Uranium atoms causing them to fission in a continuous chain reaction. Control elements, which are made of materials that absorb neutrons, are placed among the fuel assemblies. When the control elements, or control rods as they are often called, are pulled out of the core, more neutrons are available and the chain reaction speeds up, producing more heat. When they are inserted into the core, more neutrons are absorbed, and the chain reaction slows or stops, reducing the heat.
Most commercial nuclear reactors use ordinary water to remove the heat created by the fission process. These are called light water reactors. The water also serves to slow down, or "moderate" the neutrons. In this type of reactor, the chain reaction will not occur without the water to serve as a moderator. In the United States, two different light-water reactor designs are currently in use, the Pressurized Water Reactor (PWR) and the Boiling Water Reactor (BWR).
In a PWR, the heat is removed from the reactor by water flowing in a closed pressurized loop. The heat is transferred to a second water loop through a heat exchanger. The second loop is kept at a lower pressure, allowing the water to boil and create steam, which is used to turn the turbine-generator and produce electricity. Afterward, the steam is condensed into water and returned to the heat exchanger.
  In a BWR, water boils inside the reactor itself, and the steam goes directly to the turbine-generator to produce electricity. Here, too, the steam is condensed and reused.

The University of Wisconsin Nuclear Reactor (UWNR)

Most nuclear reactors use the energy (heat) of the fission fragments to produce steam which is then used to spin a turbine and produce electricity. The UWNR is a research reactor primarily used to instruct students, train operators and perform scientific studies using the neutrons in the reactor core. A major difference between the UWNR and the nuclear power reactors is the UWNR is not a pressurized system and it does not produce electricity. It is also about one tenth the size of an average commercial reactor.  The reactor core is covered by water in an open pool. Because our reactor operates at power levels significantly below that of power producing reactors, the water does not boil. The reactor core (fuel and control elements) resides at the bottom of a deep pool of water. Because of the large amount of water between the reactor core and the top of the pool and also because the water serves as a shield against radiation, personnel can go to the top of the pool during operation of the reactor.

Reactor Shield

As mentioned before, the radiation coming from the reactor core is shielded by concrete and water. The shield, which also serves as a vessel for the pool water, is made of ordinary concrete. The shield surrounds the pool of water, changing in thickness the lower you get. The sheild extends to above the pool top and is shaped roughly octagonally if viewed from the top.

Reactor Bridge and Core

The core is suspended from an all-aluminum frame which extends from the grid box at the bottom of the pool to above the pool surface. The grid box at the pool bottom is an "egg crate" type structure that holds the individual fuel bundles and shrouds for the control blades. The reactor bridge (mounted over the pool) supports the core suspension frame. A diffuser pump and jet above the core but within the reactor pool helps keep water circulating throughout the reactor pool.

Grid Box

The core elements, such as the fuel and control elements, are supported and enclosed on four sides by the grid box. The bottom is an aluminum grid plate spaced to conform with the reactor's fuel bundles. This is the "egg crate" type structure previously mentioned. The sides of the grid box help to maintain a current of cooling water through the core. The grid box does not rest directly on the bottom of the pool, but is suspended from above. The grid box is supported by the alumium frame above the pool top.

Fuel

The fuel we use at the UWNR is Uranium. Our fuel contains Uranium-235 and Uranium-238. Uranium  is mined out of the ground and is about 1% U-235 and 99% U-238. The Uranium in our fuel is processed and enriched to have a higher concentration of U-235. Each fuel element is about a meter long, which includes a top and bottom end fitting. Inside the center of the fuel pin is a mixture of Uranium, Zirconium, Erbium, and Hydrogen. On either end of this fuel section is a plug of graphite which helps to reflect neutrons back toward the fuel, thereby increasing the efficiency of the reactor. Four of these fuel pins are put together to form a fuel bundle. A fuel bundle has a square bottom fitting so that it can be positioned in the grid box. It also has a top fitting which has a handle on it so the bundle can moved around as needed for different tests and experiments. There are several dozen fuel bundles in the reactor arranged in a nearly square shaped pattern with the control elemets mized in.
 

Control Elements

The reactor is controlled by four safety blades and one transient control rod. The scrammable safety blades are held up by magnets, energized by a current. The magnets are attached to a motor-operated drive, which controls the height of the rods. The neutron absorbing section of the blade is made from standard boral. Boral is a material that loves to absorbs neutrons. The transient control rod is held up by pneumatic (air) pressure and its position can be adjusted by a drive mechanism similar to the other control elements. The transient control rod is borated graphite contained in a 1-1/4 inch diameter aluminum tube located inside the core where one fuel pins would have been located.

Neutron Source

A neutron source is provided to always maintain a neutron population in the reactor, because in order to start a chain reaction you need to have neutrons. The neutron source is a radium-beryllium source irradiated to give an out at least 10 million neutrons per second.

Radiation

The most common forms of nuclear radiation can be classified into the following three following categories- alpha particles, beta particles, and gamma rays.

Alpha Particle

An Alpha Particle consists of 2 neutrons and 2 protons, and it is identical to the nucleus of a helium atom. It is the most massive of the common radiation particles and it therefore carries more energy and can do more damage than the other two main types of radiation. However, because it is so heavy and it is also charged, it has a very short range. It travels less than 5 cm in air before expending its energy. It is generally not a hazard unless it gets inside the body, where it can cause more damage than a beta particle or gamma ray.

Beta Particle

A Beta particle is essentially a high-speed electron. However, it can also come in the form of a positively charged electron called a positron. Because a beta particle is a small particle, it has a much greater range than an alpha particle. It can travel from anywhere between 6 millimeters to 7 meters (20 feet) in air, depending on the nucleus it is emitted from. Beta particles can be a hazard to the body because they can penetrate the outer layer of dead skin and cause damage to living tissue underneath.

Gamma Rays

A Gamma ray is an electromagnetic ray, or light, emitted from the nucleus of an excited atom following radioactive decay. Gamma rays are emitted at discrete energies and provide a way for the nucleus to rid itself of the energy which was not carried off by the particle emitted in decay. Due to the fact that certain isotopes emit gamma rays of a certain energy, it is possible to determine the composition of a given object by subjecting it to activation, or irradiating it, and measuring the gamma rays coming from it. Gamma rays are similar to light but have a higher energy. They are highly penetrating and can easily pass through a considerable amount of shielding and still pose a hazard to the human body.
 

Radiation in the Reactor

Although you cannot see the radiation mentioned above directly, you can see its effect on certain materials. When the UWNR is operated at full power, a blue glow surrounds the core as shown in the icon at the top of these tour pages. The blue glow is known as Cherenkov Radiation and is quite pleasing to the eye.

UWNR Irradiation Facilities

Here at the UWNR we perform a number of irradiation experiments, and in order to carry out these experiments we have special facilities in the reactor lab. One of these facilities, called the beam port, is used to irradiate objects with neutrons. It is a metal pipe that runs from the core to the outside of the reactor and it is used to carry a stream (or beam) of neutrons to the outside of the reactor.
One of the beam port experiments in progress at the UWNR is the neutron radiography experiment. In this experiment, neutrons are used to take pictures of the inside of objects in a similar manner as you would use an x-ray to take a picture of the inside of your body. X-rays penetrate light elements well and are absorbed by heavy elements, while neutrons penetrate many heavy materials well while being absorbed by light elements. Neutron radiography is particularly useful for imaging light elements inside components made of heavy elements. An example in which neutron radiography could be used is to visualize water or oil flow inside a thick metal object such as a pipe or engine.

UWNR Thermal Column

Another source of neutrons for experiments is the thermal column. The thermal column, like the beam port, runs from the inside of the reactor to the outside of the reactor and has a thick shielded door which is closed during operation of the reactor. The thermal column is different from the beam port in that it is used to produce thermal, or slow, neutrons whereas the beam port uses fast neutrons. The thermal column can do this because it contains graphite, which slows down neutrons.

Neutron Activation Analysis (NAA)

In another radiation experiment known as neutron activation analysis, samples, such as soil samples or biological samples, are irradiated, and the type of radiation coming from the irradiated object is measured to determine what elements were present in the samples. Samples are placed in plastic vials (we use two different vial sizes, one typically holding 0.5 ml of material and the other holding up to 5 ml of material). Samples are placed near the reactor core by pneumatic or hydraulic irradiation systems. After irradiation, the sample is sent  to a radiation detector to measure the radiation coming from the sample. The data coming from the detector is fed to a computer which counts the energy and frequency of the gamma radiation coming from the sample and plots it on a computer screen. If you were to look at such a plot, you will notice that there are some pronounced peaks in it. These peaks can be matched with certain elements, and the height (or intensity) of the peaks can show how much of a given element is present in a sample.

Applications for NAA

Due to the fact that Neutron Activation Analysis is very sensitive for many trace elements, it is useful for a wide variety of applications. Some of the applications done here at the UWNR are as follows- A few of the many substances where Neutron Activation Analysis has been applied are as follows: alloys, animal samples, blood, bones, bullets, chemicals, chemical compounds, chemical solutions, chemical tracers, crystals, enzymes, fingernails, fish, food, fossils, gems, glass, gunshot residues, hair, minerals, metals, meteorites, moon samples, obsidian, ocean sediments, oil, oxides, paint, plastic films, rocks, salts, shells, skin, soil, sugar, teeth, tissue samples, tree leaves, tree needles, urine, water, wheat spores, and wood.

Reactor Console

The reactor console is in the control room where reactor operators monitor the nuclear reactor. There are various displays that an operator uses to gauge the condition of the reactor and maintain control of the reactor. Switches are positioned directly in front of the operator that allow control element movement. This is required to change operating conditions of the reactor (i.e., to make the nuclear chain reaction self sustaining and make power level adjustments). Other analog and digital readouts are provided that allow the operator to monitor power level, system temperatures and radiation levels throughout the reactor lab. From this room all pertinent information about the reactor and its condition during its operation is provided, so that the operator is in complete control of the reactor.

Demineralizer

The demineralizer system continuously removes water from the pool and runs it through a mixed resin bed to reduce mineral content in the water, which therefore reduces corrosion of the components of the reactor and the radioactivity of the water. The water is then returned to the reactor pool. The PH is maintained between 6 and 7, with the resistivity of the water kept at a high level to ensure low levels of mineral buildup in the water. Any radioactive material taken out of the water is held in the resin bed in the demineralizer, where it can be monitored.

Pool Top

The bridge along with the various drives that are attached to the control elements and the neutron source are visible from the pool top. The use and description of these devices are provided below.

Safety Blade Drives

The first structures noticed at the top of the pool are the control blade drives. The drive mechanism for the safety blades includes a reversible electric motor. Each blade is coupled to its mechanism through an electromagnet that allows the blades to undergo a free fall by gravity when it loses electricity to ensure the reactor turns off, also known as a SCRAM. Instrumentation in the drive mechanism gives the operator indication of the height of the drive and therefore the height of the blade and whether or not the drive is attached to the blade by the magnet. The safety blade drive mechanism operates under a range of only 16 inches.

Transient Rod Drive

The transient rod drive can be used to control the reactor as the safety blade drives are used. In order to perform certain experiments, a control element which can be quickly withdrawn from the core is necessary. The transient rod can be used as such an element. The transient rod has a piston within a cylinder that is attached to the transient rod by means of a connecting rod. When the transient rod is ejected from the core, compressed air is to drive the piston upward. As the piston rises, the air being compressed above the piston is forced out through vents at the upper end of the cylinder. When the transient rod needs to be lowered, the compressed air is allowed out of the cylinder and the rod falls back down.

Fission Counter Drive

The fission counter is a nitrogen filled instrument with a U-235 lining. The purpose of the fission counter is to show the number of neutrons in the core while the reactor is being started. The fission counter counts the number of fissions that occur from the neutrons in a shut down or subcritical reactor striking the U-235 in the counter, which gives an idea of how many neutrons are present in the core.

Cooling System

The reactor cooling system is composed of three circulating coolant loops. These loops carry reactor heat away from the pool and dissipate it to the atmosphere. The cooling system will dissipate 1 MW (1,000,000 Watts) of energy during year round operation. (To give you an idea of how much energy that is, most light bulbs used your home are between 60 and 100 Watts.)
  The primary system continuously circulates pool water through a heat exchanger. The secondary side flow is maintained at higher pressure to ensure that if a heat exchanger leak were to occur it would not allow possibly contaminated primary water to flow into the secondary side. The secondary side flow is pumped to a sump tank where it is then pumped through a cooling tower on the roof of the reactor building.

Primary System

The primary system is composed of a pump, a heat exchanger, isolation valves, an orifice plate (for flow measurements), and interconnecting piping. All system components are of stainless steel to maintain pool water purity.
  The primary system inlet line extends about 6 feet below the pool surface; holes in the lower 5 feet of the suction line diffuse water being drawn into the inlet line. Water from the inlet line passes through an isolation valve, the primary pump, and then is discharged into a heat exchanger. Pressure gauges mounted on the heat exchanger inlet and outlet lines provide indication of heat exchanger pressure drop. Thermocouples, which are devices used to measure temperature, are mounted in piping to and from the heat exchanger for temperature readout at the console. Water leaving the heat exchanger is discharged at the pool bottom. Holes have been drilled into the lower 6 feet of the return piping to diffuse the cooler return water. Valves in the inlet and outlet lines to the cooling system provide isolation in case of  leakage and for system maintenance when required.

Secondary System

The secondary system consists of two separate 8 inch thick carbon steel pipe loops that begin and end in the 3,000 gallon sump tank. Circulation in the first loop is maintained by a pump drawing a suction on the sump tank and discharging to the heat exchanger back into the sump tank. Water flow through the secondary side of the heat exchanger removes reactor heat from the heat exchanger to the sump tank.
  A tower pump removes heated water from the sump tank and sends it through the cooling tower (on the Reactor Lab roof above the control room) and back to the sump tank. In the tower the water is air cooled before being returned to the sump tank. Baffles installed within the sump tank provide a thorough mixing of water from both the secondary loops. A cement-like insulation called "precrete", applied to the tank walls, stops rust while maintaining sump tank temperature and minimizing heat loss to the room. To prevent winter freeze-up of the coolant tower when not operating the system, tower water is automatically drained to the sump tank when the pump is off. Make-up water, as well as additives to control pH and corrosion inhibitors, is controlled by a sensing unit installed in the sump tank. A manhole, used for maintenance and tank cleaning, is accessible by way of a cut-out in the floor near the console.

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