The TRIGA Reactor Facility at the

Armed Forces Radiobiology Research Institute

A Simplified Technical Description

U SU NIFORMED ERVICES NIVERSITY

of the Health Sciences

Armed Forces Radiobiology Research Institute

The TRIGA Reactor Facility at the 

Armed Forces Radiobiology Research Institute

A Simplified Technical Description

AFRRI Technical Report 94-1 

Armed Forces Radiobiology Research Institute

Uniformed Services University of the Health Sciences

8901 Wisconsin Avenue

Bethesda, Maryland 20889-5603

http://www.afrri.usuhs.mil/

Approved for public release; distribution unlimited
 

First printing: 1994 

Second printing: August 2006 

Changes: graphic and layout changes only 

Third printing: March 2009 

On the cover: Electrons surrounding hydrogen and oxygen atoms 

of the reactor’s pool water are stripped away by gamma rays. 

Moving faster than the speed of light in water, the electrons emit 

blue light as they decelerate, an effect known as Cherenkov 

radiation. The effect is most dramatic during a pulse operation.

(Photo: D. Morse, AFRRI)
Contents

Introduction

Mission .........................................................................................................1 

Reactor .........................................................................................................1 

General Principles of Reactor Operation

 Fission process .............................................................................................2 

 Control of the nuclear reaction ....................................................................3 

Description of the TRIGA Reactor

Reactor core .................................................................................................5 

Reactor fuel element ....................................................................................6 

 Control rods .................................................................................................8 

 Neutron source .............................................................................................9 

Instrumentation ............................................................................................9 

 Cooling systems .........................................................................................10 

 Lead shield doors .......................................................................................10 

Unique Features of the TRIGA Reactor

 Modes of operation ....................................................................................11 

 Visible Cherenkov radiation ......................................................................11 

 Exposure facilities ......................................................................................11 

Summary ...............................................................................................................13 

Figures

Figure 1. Cutaway view of AFRRI TRIGA reactor .....................................1 

Figure 2. Nuclear fission chain reaction (gamma rays = γ) .........................2 

Figure 3. Reactor control rod .......................................................................4 

Figure 4. Reactor core ..................................................................................5 

Figure 5. Side view of reactor and exposure facilities .................................6 

Figure 6. Reactor fuel element .....................................................................7 

Figure 7. Overhead view of reactor core (upper grid plate) ........................9 

Figure 8. Pneumatic extractor tube system ................................................12
 
1 

Introduction

Mission

The Armed Forces Radiobiology Research Institute (AFRRI) conducts research in 

the field of radiobiology and related matters that are essential to the operational 

and medical support of the Department of Defense. AFRRI is staffed by members 

of the three military services and by civilian personnel. In support of its mission, 

AFRRI operates a research reactor.

Reactor

The reactor, a medium-sized 1.0-megawatt research reactor that generates 

neutrons and gamma rays for radiation experiments (see figure 1), is a primary 

radiation source at AFRRI. The reactor is General Atomic’s TRIGA reactor; 

TRIGA is an acronym for training, research, isotope, General Atomic. The 

TRIGA reactor is designed to be inherently safe, i.e., the reactor’s design is such 

that there is no possibility of an accident that can produce an unsafe condition for 

the staff or the general public. Since the late 1950s, more than 70 TRIGA reactors 

have been used worldwide for a variety of applications with no accidents. The 

TRIGA reactor at AFRRI has been operating since 1962.

Figure 1. Cutaway view of AFRRI TRIGA reactor
TRIGA REACTOR, AFRRI

2 

General Principles of Reactor Operation

Fission process

At the heart of any reactor is the fission process. Certain elements are likely to 

undergo fission when they absorb a neutron. One such element is uranium-235. 

Uranium-235 is our nuclear fuel, and it is packed into fuel elements in the reactor 

core. When uranium-235 absorbs a neutron, it becomes unstable, breaks up into 

two or more fragments, and releases a large amount of energy. These fragments 

are the nuclei of smaller elements and are called fission products. Most of the 

energy released during fission is manifested in the form of kinetic energy (speed) 

given to the fission products. When the fission products collide with their 

surrounding atoms, heat is produced. In addition to the fission products, several 

extra neutrons and gamma rays are released. Some of the extra neutrons go on to 

be absorbed by other uranium-235 nuclei, thus producing the nuclear fission chain 

reaction shown in figure 2. 

The heat produced in the fission chain reaction is allowed to dissipate in a large 

pool of water. The TRIGA reactor differs from a reactor in a commercial nuclear 

power plant in that the product of interest is the radiation produced; the heat is 

considered a waste product. For a reactor in a commercial nuclear power plant, 

the heat is the product of interest. 

An important factor that influences the fission process is the kinetic energy of the 

neutrons absorbed by uranium-235 nuclei. Uranium-235 nuclei absorb neutrons 

Figure 2. Nuclear fission chain reaction (gamma rays = γ) 
GENERAL PRINCIPLES

3 

when the neutrons are traveling relatively slowly (0.025 electron volts or 2,200 

meters per second). These slow neutrons are called thermal neutrons. Because 

neutrons born from fission are usually traveling fast, they must be slowed down to 

the appropriate speed. This slowing process is called neutron moderation. Neutron 

moderation occurs when neutrons collide with light atoms such as hydrogen and 

lose energy by elastic scatter, similar to how billiard balls slow when they collide.

Most of the fission fragments are radioactive and decay to stable (nonradioactive) 

nuclei by emitting any combination of alpha particles, beta particles, gamma rays, 

and neutrons. The neutrons emitted by fission products are called delayed 

neutrons. Although only about 0.7% of all neutrons are delayed neutrons, they are 

an important component in controlling the nuclear fission chain reaction. 

Control of the nuclear reaction

The nuclear fission chain reaction can grow indefinitely. A reactor is limited only 

by how much heat can be removed from the reactor core. To limit the fission rate 

and thus the heat production rate, neutron-absorbing materials are inserted into the 

reactor core. The neutron absorbers, called control rods, absorb neutrons in the 

reactor core, making the neutrons unavailable for the fission process. If enough 

neutrons are absorbed, there will not be enough neutrons available to sustain the 

fission chain reaction, and the reactor will reduce its power level. 

The neutron absorber used in the TRIGA reactor core is boron carbide. The 

isotope boron-10 readily absorbs neutrons. The boron carbide is packed into the 

control rods as shown in figure 3. The reactor uses four of these control rods to 

control its power level. 

When control rods are pulled out of the reactor, the reactor power increases. The 

controlled increase in power is due to the effects of the delayed neutrons 

discussed in the previous section. If all neutrons were born at the instant of 

fission, reactors would instantaneously increase in power and be difficult, if not 

impossible, to control. As control rods are inserted into the reactor, the power 

level decreases.

The generation of heat during the fission chain reaction affects the reactor core’s 

ability to moderate (slow down) neutrons and decreases the density of the 

uranium-235 fuel atoms and other materials in the core. These effects can be 

harnessed to help control a reactor. Through judicious selection of materials for 

fuel elements and fuel element design, these passive factors in reactor control can 

make the reactor inherently safe. 
TRIGA REACTOR, AFRRI

4 

Figure 3. Reactor control rod
5 

Description of the TRIGA Reactor

Reactor core

To produce a controlled, self-sustaining fission chain reaction, the fuel elements 

containing uranium fuel and the neutron-absorbing control rods must be arranged 

in a special way; this arrangement of fuel elements and control rods is called the 

reactor core. In addition to the fuel elements and control rods, the reactor core 

also contains a neutron source. Figure 4 shows the reactor core. 

The reactor core is suspended under 16 feet of water from a carriage just above 

the reactor pool. The pool is an effective radiation shield, so personnel can safely 

observe the reactor as it operates. The core support structure shown in figure 4 

Figure 4. Reactor core
TRIGA REACTOR, AFRRI

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provides a means to suspend the reactor core. The carriage rides on a track that 

allows movement of the core from one exposure room to the other exposure room, 

as shown in figure 5. Movement of the core carriage along the track from one side 

of the pool to the other is controlled from the reactor console. 

Approximately 5 minutes are required to move the core from one side of the pool 

to the other, a distance of 13 feet. The advantages of a movable core are (a) the 

quantity and character of the radiation reaching the exposure facilities can be 

controlled, and (b) more than one exposure facility can be used during a day of 

reactor operations.

Reactor fuel element

The TRIGA reactor fuel element (figure 6) is designed to capitalize on the 

inherent characteristics of the fuel to provide safe nuclear reactor control. The fuel 

Figure 5. Side view of reactor and exposure facilities
DESCRIPTION

7 

element is made of uranium zirconium-hydride (UZrH) metal. The hydrogen in 

the UZrH metal serves as the primary moderator of neutrons. As the fission chain 

reaction makes the fuel element hotter, the hydrogen atoms vibrate at increasingly 

higher frequencies, and thus move rapidly within the metal structure of the UZrH. 

When a neutron strikes the agitated hydrogen atom, instead of slowing down, it 

actually speeds up and is unable to cause fission. This effect is called the 

zirconium-hydride disadvantage factor and has an extremely strong influence over 

Figure 6. Reactor fuel element
TRIGA REACTOR, AFRRI

8 

controlling the reactor. The zirconium-hydride disadvantage factor makes the 

TRIGA reactor inherently safe.

Specific design features of the TRIGA reactor fuel element are:

• 0.02-inch-thick stainless steel cladding,

• 0.18-inch-diameter solid zirconium rod in the center of the fuel element to 

ensure structural integrity,

• 0.015-inch-thick samarium wafer at each end of the fuel element (a 

samarium wafer is a neutron absorber that helps make the neutron flux 

uniform in the axial direction and thus extends fuel life), and 

• 3.44-inch-long graphite plugs at each end of the fuel element to reflect 

neutrons back into the fuel element.

The TRIGA reactor can accommodate 4 control rods and up to 87 fuel elements in 

the reactor core.

Control rods

Control rods are used to control the rate of the fission chain reaction. Figure 3 

shows a standard control rod used in the reactor core. Some of the neutrons in the 

core that pass into the boron carbide portion of a control rod are absorbed by the 

boron atoms. This reduces the number of neutrons available for fission in the fuel 

elements, so the fission rate is reduced. 

Rod drives operated from the control console move the control rods into or out of 

the core. The amount of a control rod in the core regulates the fission rate. A greater 

length of a control rod in the core results in a greater number of absorbed neutrons 

and a lower fission rate. If neutrons are being absorbed at a greater rate than they 

are being produced, the fission rate decreases, and the production of power and 

radiation ends. If neutrons are being produced at a faster rate than they are being 

absorbed, the fission rate, power level, and corresponding radiation increase. 

Withdrawing just enough of the control rods to match the neutron absorption rate 

with the neutron production rate achieves a steady-state power level.

There are four control rods in the core of the reactor. One control rod is placed in 

the center position—the A ring—and is called the transient rod (figure 7). The 

remaining three control rods are called standard control rods and can be one of 

these types: aluminum followed, air followed, or fuel followed. When the neutron 

absorber section is withdrawn from the core, a section of aluminum, air, or fuel 

follows in its place. These control rods are placed evenly in the D ring (fourth 

ring) of the reactor core. The control rods in the fourth ring are called the safe rod, 

shim rod, and regulating rod. Using the reactor control console, the operator can 

drive these rods slowly up and down. The operator can also “scram” the rods, 

allowing them to drop by means of gravity back into the reactor core. 

The transient rod can be driven up and down in the same manner as the other 

control rods. In addition, the transient rod can be rapidly ejected out of the core by 

means of compressed air. A stopping anvil is positioned above the transient rod to 

limit the length of the transient rod ejected from the core.
DESCRIPTION

9 

Application of compressed air lifts the transient rod from the core until it hits the 

shock-absorbing anvil. The transient rod can be rapidly ejected from the core only 

when the reactor is in the pulse or square wave mode. When the reactor is in the 

pulse mode it is ready for a brief high-power excursion—a sudden dramatic 

increase in power. In the pulse mode, all control rods scram shortly after the 

transient rod is ejected from the core. The time to scram is set by the operator and 

is usually 500 milliseconds. When the reactor is in the steady-state mode, the 

transient rod stays against the anvil and is used like a standard rod. 

Figure 7. Overhead view of reactor core (upper grid plate)

Neutron source

A neutron startup source is necessary to ensure constant availability of neutrons in 

the core when it is in a subcritical configuration: the neutron startup source 

supplies neutrons to start the self-sustaining fission chain reaction as control rods 

are withdrawn from the reactor core. The neutron startup source is composed of a 

mixture of americium, a man-made element, and beryllium, doubly encased in 

stainless steel. This source produces neutrons when americium spontaneously 

decays into neptunium and an alpha particle. The alpha particle reacts with the 

light element beryllium to produce carbon and a high-energy neutron.

Instrumentation

Instruments placed in and around the core monitor important reactor core 

parameters. These parameters include fuel temperature, neutron population levels, 

and gamma ray levels. Fuel temperature is measured directly, using thermocouples
TRIGA REACTOR, AFRRI

10

embedded in at least two fuel elements in the B and C rings of the reactor core. 

Neutron population is determined by a fission chamber and boron-lined gamma 

ionization chambers located above the core. These chambers are calibrated to give 

the steady-state power level. Gamma ray levels are measured by an ionization 

chamber located above the core. Gamma ray levels are also measured during 

pulse operation to determine reactor power. 

In addition to reactor core instrumentation, pool water temperature, radiation 

levels in the reactor room and pool, and the status of various safety interlocks are 

monitored. Pool water temperature is monitored with thermistors located in the 

pool. Radioactivity of the pool water is monitored by a Geiger-Mueller detector to 

ensure that fuel elements do not leak fission products and that dust or other 

particles dropped into the pool does not become activated. The status of various 

safety interlocks throughout the reactor facility are determined by the positions of 

microswitches that indicate core position, status of lead shield doors (open or 

shut), and status of exposure facilities. All parameters of reactor operations are 

displayed on the reactor control console. 

Cooling systems

Heat generated during fission is removed from the reactor core by means of a 

natural convection. The large pool of water acts as a heat sink. The pool water is 

kept cool by circulation through a heat exchanger rated at 1.5 megawatts. The 

cooling water in the heat exchanger is circulated through a cooling tower located 

on the roof of the building. 

Lead shield doors

Two lead shield doors are located at the bottom of the reactor pool (figure 5). The 

doors are aluminum shells 19 inches thick, 5 feet high, and 6 feet wide; they are 

filled with lead shot and transformer oil. The doors must be opened to allow 

movement of the core from one side of the pool to the other. They can be closed 

to provide additional shielding for scientific personnel working in the exposure 

room on the opposite side of the doors from the core. The edges of the doors are 

stepped (designed to overlap) to prevent streaming of radiation from between the 

doors. 
11

Unique Features of the TRIGA Reactor

Modes of operation

The TRIGA reactor is unique because it can operate in a steady-state as well as in 

a pulse mode. When the reactor is in the steady-state mode, control rods are 

withdrawn from the reactor core until the desired power level is achieved. The 

maximum allowed steady-state power level is 1 megawatt. In the pulse mode, one 

of the control rods is rapidly ejected from the reactor core by using a special 

compressed-air control rod drive mechanism. With the rapid ejection of a 

relatively large amount of neutron absorber from the core, the fission chain 

escalates at a rapid rate, called a prompt critical excursion. As the fuel heats 

during the pulse, the inherent effects of the reactor fuel rapidly bring the reactor 

down to a low power level. The maximum allowed peak power from a reactor is 

4,000 megawatts. 

Visible Cherenkov radiation

During a reactor operation, gamma rays born during the fission process strike 

electrons that surround the hydrogen and oxygen atoms of the pool water. The 

electrons are then stripped away from the hydrogen and oxygen atoms. The 

gamma ray energy transferred to the electrons is manifested in the form of high 

kinetic energy of the electrons. The electrons are moving faster than the speed of 

light in water. As these energetic electrons decelerate, they emit blue light. This 

blue light is called Cherenkov radiation. Cherenkov radiation is most dramatic 

during a pulse operation. 

Exposure facilities

Neutrons and gamma rays pass from the reactor core to an exposure facility where 

biological systems are irradiated for studies on the effects of radiation. The 

exposure facilities include two large exposure rooms and a core experiment tube. 

Each exposure facility has unique characteristics.

Exposure room 1 is the largest of the exposure facilities. It is located north of the 

pool and is the most frequently used. The room has a variety of shielding, which 

makes it especially useful to investigators.

The exposure room is 20 feet by 20 feet with an 8-foot-high ceiling. A 

semicylindrical section of the aluminum pool wall projects into the south wall of 

the room. The reactor core can be moved to a position less than 1 inch from the 

aluminum tank wall and is separated from the tank wall by a small amount of 

water. A cadmium-gadolinium shield is positioned on the tank projection to 

absorb the leakage of thermal neutrons from the core into the exposure room. 

The walls and ceiling of exposure room 1 are made of concrete covered with 

wood and painted with gadolinium. The concrete is covered by wood because fast 

neutrons can activate concrete and thus present a hazard to personnel. The wood 
TRIGA REACTOR, AFRRI

12

slows down the fast neutrons emitted from the core, drastically reducing the 

chances of activating the concrete. After being slowed down by the wood, the 

neutrons can bounce back toward the interior of the room, but are absorbed by the 

gadolinium paint before escaping back into the room. 

Exposure room 1 is equipped with a pneumatic extractor tube system, which 

permits quick insertion and withdrawal of samples (figure 8). When in position, 

the extractor tube extends from directly in front of the core, through the west wall, 

and into the prep area outside the exposure room. The section of the tube in the 

wall follows an “S” curve, which prevents the radiation from streaming from the 

exposure room into the prep area.

Figure 8. Pneumatic extractor tube system
UNIQUE FEATURES

13

Samples are placed inside a plastic carrier and positioned in the tube by means of 

a motorized pulley system controlled from the prep area. The quick retrieval of 

samples from the exposure room results in greater precision in evaluating the 

amount of radiation received by the samples, particularly for low-level exposures. 

This eliminates the delays in moving the reactor core, closing the lead shield 

doors, and opening the exposure room doors. Retrieval takes place in seconds 

rather than in the tens of minutes required by procedures for physical entry into 

the exposure room. The rapid extractor system also reduces radiation exposures to 

personnel because there is no need to enter the exposure room. 

Exposure room 2 is similar to exposure room 1 in construction. The room is 

slightly smaller, but the ceiling and walls follow the same design as those of 

exposure room 1—concrete walls are covered with wood painted with 

gadolinium. The pool wall projects through the north wall of exposure room 2, 

but it is not shielded with cadmium-gadolinium. As a result, a greater proportion 

of the neutrons that enter exposure room 2 is thermal. Experiments that need a 

high thermal neutron component are conducted in exposure room 2. 

Samples may also be irradiated in the core itself by using the core experiment 

tube. The core experiment tube is a hollow aluminum tube with an “S” bend to 

prevent streaming of radiation. The samples to be irradiated are placed in small 

polyethylene containers, called “rabbits,” which are then loaded into the tube and 

placed directly into the core. The rabbits may be withdrawn with a modified 

fishing pole. 

The core experiment tube is used primarily to produce isotopes, which are used by 

researchers as biological tags or tracers. The core experiment tube is also used for 

neutron activation analysis that allows for precise identification of materials 

within an irradiated sample and has applications ranging from dating precious 

works of art to forensic analysis of criminal evidence. 

Special setups and custom radiation beams are also available. Setups include inpool portable beam tubes, a pneumatic transfer system, and in-core grid-location 

tubes. These facilities combined with other materials used in the exposure room 

allow the generation of custom radiation beams.

Summary

The TRIGA reactor has been used at AFRRI to conduct radiobiology research 

since 1962. The reactor is unique because it operates in both steady-state and 

pulse modes, its core is visible through a large pool of water, and it includes 

various exposure facilities. The most important feature of the TRIGA reactor is 

that the passive control features of its fuel elements make the reactor inherently 

safe.

____________________________________ 

This publication is a product of the

AFRRI Publishing Division.

Graphic illustrators: Mark Behme and Patrice Bolté

Printing: USU Duplicating Center