The U.S. Department of Energy (DOE) and the

National Aeronautics and Space Administration

(NASA) place the highest priority on assuring the

safe use of radioisotope power systems for space

missions. Prior to each mission, the agencies jointly

conduct extensive safety reviews supported by

safety testing and analysis.

Further, an independent safety

evaluation of the mission is

performed as part of the launch

approval process by an ad hoc

Interagency Nuclear Safety

Review Panel (INSRP)

supported by experts from

government, industry and

academia. Safety is of utmost

importance and is reflected in

the overall design of

radioisotope power systems,

their applications, the choice of

radioisotope, and the extensive

testing, analysis and review that

each system undergoes.

Plutonium-238 Dioxide as a

Heat Source and Radioisotope

Power System Fuel Module

Design

The Plutonium-238 isotope used in radioisotope

power systems (RPSs) and lightweight radioisotope

heater units (LWRHUs) produces heat through

natural decay by giving off alpha particles - helium

nuclei - which travel only about three inches in air.

Clothing, skin, or even a sheet of paper stops

external alpha radiation.

The ceramic form of plutonium-238 dioxide that

is used in RPSs and LWRHUs is designed

specifically for safety. The ceramic form of

plutonium-238 dioxide is heat-resistant and limits

the rate of vaporization in fire or reentry conditions.

The material also has low solubility in water. This

material does not disperse or move easily through

the environment, and primarily fractures upon

impact into large particles and chunks that cannot

be inhaled as dust. This is important because

plutonium-238 dioxide particles would have to be

of a sufficiently small size to be inhaled and

deposited in lung tissue, where

they could lead to an increased

lifelong chance of incurring

cancer. Other plutonium-238

dioxide exposure pathways, such

as ingestion, contribute far less to

the potential development of

cancer.

The plutonium in radioisotope

power systems is not

weapons-grade plutonium and its

ceramic form has a low chemical

reactivity (i.e., does not react

with other chemicals). The

radioisotope is enclosed in small,

independent modular units, each

with its own heat shield and

impact shell. This modularity

reduces the likelihood that

significant amounts of the

radioisotope could be released in

launch accidents.

More than 35 years have been invested in the

engineering and testing of RPS systems. Multiple

layers of protective materials, including iridium

capsules (or platinum-rhodium capsules for

radioisotope heater units) and high-strength,

heat-resistant graphite blocks are used to protect

the radioisotope and prevent its release. Iridium is

a metal that is strong, corrosion-resistant, and

chemically compatible with plutonium dioxide.

These characteristics make iridium ideal for

protecting and containing each radioisotope pellet.

Graphite is used because it is lightweight and highly

heat-resistant.

Space Radioisotope Power Systems

Safety

January 2008
Rutger/Dec18_01 Pluto INSRP.ppt (1)

NASA2

Other

Agencies

Mission &

Launch

Vehicle Data

(NASA)

Accident

Descriptions

(NASA)

System Design

& Test Data

(DOE)

Safety

Evaluation

Report

(INSRP)1

Safety

Analysis

Report

(DOE) DOD

DOE

Office of

The

President

Office of

Science &

Technology

Policy

Nuclear Safety Review and Nuclear Safety Review and

Launch Approval Process Launch Approval Process

1 Interagency Nuclear Safety Review Panel

2 Responsible mission agency makes launch recommendation

Visit our web site: nuclear.gov

Safety Testing

Radioisotope power systems are designed to

contain their plutonium dioxide in a wide range of

potential accidents. Rigorous testing is conducted

to demonstrate the radioisotope power system’s

plutonium dioxide containment capabilities in

severe launch vehicle accidents. The battery of tests

that the modular units have undergone include solid

and liquid propellant fires, blasts due to launch

vehicle propellant and motor explosions, re-entry

ablation and thermal environments, Earth impacts,

impacts by shrapnel and large launch vehicle

fragments, and immersion in water. The multi-layer

containment concept employed for the systems is

designed to contain the radioisotope, but even if

the containment is breached, the ceramic

radioisotope pellet has been designed to limit

dispersal of the material into the environment.

Risk Assessment and Launch Approval

A mission that uses radioisotope heaters or power

sources undergoes a safety analysis carried out by

DOE. In that process, DOE prepares a safety

analysis report that provides a comprehensive

assessment of the potential consequences of a

broad range of launch accident scenarios described

by NASA. The potential consequences are based

upon a detailed understanding of the potential

accident environment, characterization of the

radioisotope power system responses based upon

testing and analyses, modeling of how any potential

releases might be transported, and in turn, estimates

of potential public exposure and the consequences

of those exposures. In addition to internal agency

reviews for missions that involve radioisotope

power or heater systems, an ad hoc Interagency

Nuclear Safety Review Panel (INSRP) is

established as part of a Presidential nuclear safety

launch approval process to evaluate the safety

analysis report prepared by DOE. Based upon

recommendations by DOE and other agencies and

the INSRP evaluation, NASA may then submit a

request for nuclear safety launch approval to the

White House Office of Science and Technology

Policy (OSTP). The OSTP Director (i.e., the

President’s science advisor) may make the decision

or refer the matter to the President. In either case,

the normal process for launch cannot proceed until

nuclear safety launch approval has been granted.

Public Safety and Risk

Radioisotope power systems are not a new part of

the U.S. space program. In fact, they have enabled

NASA’s safe exploration of the Solar System for

many years. The Apollo missions to the Moon as

well as the later Viking, Pioneer, Voyager, Ulysses,

Galileo, Cassini, Pluto New Horizons missions all

used radioisotope thermoelectric generators

(RTGs). The RTGs for the Pioneer 10 spacecraft

have operated reliably for three decades and

continue to power the spacecraft as it travels

beyond Pluto. Over the last three decades, the

United States has launched 26 missions involving

45 RTGs. While RTGs have never been the cause

of a spacecraft accident, they have been on board

three space missions that did fail for other reasons.

In all three cases, the RTGs performed as designed.