Nuclear reactor fuel elements have burnups that are an order of magnitude higher than can currently be achieved by conventional design practice. Elements have greater time integrated power producing capacity per unit volume. Element design concept capitalizes on known design principles and observed behavior of nuclear fuel

Mcdonald, G. E.

Rom, F. E.

English

NASA Technical Reports Server

May 1969	 Brief 69-10154 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information; Springfield, Virginia 22151. 
Fuel Element Concept for Long Life High Power Nuclear Reactors 
Tube	
Thin Fuel Layer 
.	
End Closure	
Void Central 
It is possible to conservatively design nuclear reac-
tor fuel elements with burnups that are an order of 
magnitude higher than can currently be achieved by 
conventional design practice. At the same time, these 
fuel elements have greater time-integrated power pro-
ducing capacity per unit volume than has been 
achieved in the best conventional practice. 
The proposed reactor fuel element design concept 
capitalizes on known design principles and previously 
observed behavior of nuclear fuel. The basic principles 
have been used in the design of various other fuel ele-
ments and tests have verified their validity. These 
principles and knowledge of behavior have not, how-
ever, been previously applied altogether in a fuel ele-
ment design procedure. 
This fuel element concept has the potential for: 
(a) increasing the operating life of reactor fuel, thus 
decreasing the operating cost of nuclear power plants; 
(b) increasing the power density of power reactors for 
a given core life with a consequent reduction in capi-
tal cost due to a more compact installation; and (c) 
making possible very compact high power long life
reactors for mobile reactors where minimization of 
shield weight is important. 
The life of almost all high power long life reactors 
is limited by the life of the fuel elements. The lifetime 
of fuel elements is generally limited by failure due to 
swelling of the fuel material or release of fission gases 
which generate high pressures within the fuel. The 
swelled fuel or high fission gas pressure then causes 
excessive growth or rupture of the fuel-clad material. 
In practice, excessive growth or rupture occurs when 
fuel burnup (in atoms fissured per heavy atoms pres-
ent) reaches 1 to 3 percent. 
The proposed fuel element design concept is simply 
a tube sealed at both ends that contains a layer of fuel 
and a large void space as shown in the figure. The 
fuel coats the inside surface of the tube with a layer 
that is thin enough so that it cannot exert significant 
forces on the tube due to thermal expansion or swell-
ing. The only major force that the tube experiences is 
that due to the difference in pressure across the tube 
wall. (continued overleaf) 
This document was prepared under the sponsorship of the National	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States	 information contained in this document, or warrants that such use 
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The large void volume allows room for fission prod-
uct gases that escape from the fuel. The burnup limit 
of the fuel element is determined when the tube can 
no longer contain the pressure that is built up by the 
fission gases. The larger the void volume fraction and 
the stronger the tube wall, the higher the burnup 
limit. 
An important feature of the design is that it per-
mits vapor fuel transport. By design, the fuel thick-
ness for any particular application is selected to pro-
duce an inside fuel surface temperature high enough 
to yield significant vapor pressure. The inside fuel 
surface will therefore tend toward operation at a con-
stant temperature independent of all operating condi-
tions external to the fuel. Hot spots in the fuel, for 
example, are automatically eleminated by fuel vapori- 
zation from the hot zone. Like.vise, cold spots are 
eliminated by selective condensation of fuel vapor 
onto the cold regions. The equilibrium fuel thickness 
distribution automatically reverts to that which pro-
duces a uniform inside fuel surface temperature. 
Elimination of hot spots means greater safety margins 
or increased fuel reliability and life. 
Fuel vapor transport also results in lower fission 
gas pressure because the fission products are uni-
formly distributed over the entire fuel element length. 
The gas pressure is therefore the average pressure 
regardless of the burnup distribution along the length 
of the fuel. This increases fuel element life.
An additional feature is that the control of nuclear 
reactors using this fuel element concept may be sim-
plified. Reactor fuel tends to be used up in the center 
portion of the pin at a more rapid rate than at the 
ends. The change in reactivity due to fuel usage must 
be compensated for by a control system. The vapor 
transport phenomena used in this concept moves fuel 
from the ends to the center to replace the used up 
fuel. This then reduces the reactivity change and hence 
less control system compensation is required. In addi-
tion, for any given allowable reactivity change over 
the lifetime of a reactor, the vapor transport fuel ele-
ment will allow longer reactor life. 
Notes: 
1. The proposed fuel element design concept can be 
applied to any tube material, fuel, and fuel form. 
2. Documentation is available from: 
Clearinghouse for Federal Scientific 
and Technical Information 
Springfield, Virginia 22151 
Price $3.00 
Reference: TSP69-10154 
Technical questions may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Refeence: B69-10154 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may he made to NASA, Code 
GP, Washington, D.C. 20546. 
Source: Frank E. Rom and Glen E. McDonald 
(LEW- 10309)
. 
. 
Brief 69-10154	 Category 03


Fuel element concept for long life high power nuclear reactors

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