From: Eugen Leitl (eugen@leitl.org)
Date: Mon Oct 21 2002 - 08:13:21 MDT
Apropos of a recent discussion. 6 kg + 60 kg enrichened uranium.
http://www.spacedaily.com/news/energy-tech-02s.html
Neptunium Goes Critical During Experiment At Los Alamos
The nickel-clad neptunuim sphere is shown resting in the lower half of the
assembly surrounded by the shells of enriched uranium that make up the
upper half of the experiment. Photo Credit: Los Alamos National Laboratory
Los Alamos - Oct 18, 2002
A full-controlled criticality of the element neptunium was achieved in
late September at Los Alamos National Laboratory's Technical Area 18 using
a six kilogram nickel-clad neptunium sphere in combination with
approximately 60 kilograms of enriched uranium.
The experiment was conducted using the "Planet" assembly device at the Los
Alamos Critical Experiments Facility or LACEF. The neptunium and enriched
uranium assembly was constructed at TA-18's Critical Assembly and Storage
Area-One, and mounted on the "Planet" device. The actual criticality was
controlled remotely to assure the safety and security of the experiment.
The experiment has yielded preliminary data that show the critical mass of
neptunium is actually less than previously predicted. Following additional
experimentation, the data will eventually pinpoint the element's exact
critical mass, something that has not been determined before in the United
States.
Neptunium is an element produced as a by-product of nuclear power
generation. Prior to this experiment, the critical mass of neptunium was
only estimated with computer models from data based on earlier experiments
using much smaller amounts of the element in less than optimal
configurations.
"The results of this experiment are of interest to scientists working in
the fields of nuclear safeguards, nuclear nonproliferation and criticality
safety," said Steve Clement of the Laboratory's Advanced Nuclear
Technology group, part of the Nonproliferation and International Security
Division.
"While the actual criticality was achieved in about four days, this
experiment has been in the works for 12 years, so on many levels, it's a
major accomplishment."
Neptunium is a man-made actinide metal, grayish in color, which lies on
the periodic table of elements between uranium and plutonium. The isotope
of neptunium used in this first criticality experiment was neptunium-237.
The element has other isotopes that are very short lived, but
neptunium-237 has an extraordinary long half-life of two million years.
The International Atomic Energy Agency approved monitoring neptunium in
1999.
Rene Sanchez and David Loaiza, both of Advanced Nuclear Technology, were
primarily responsible for the successful criticality, along with a team
that included Clement, Robert Kimpland, David Hayes, Peter Jaegers,
Charlene Cappiello, Bill Myers, Ken Butterfield, Charles Hollas, Charles
Goulding, Joetta Goda, Eric Sorensen and a support team of special nuclear
materials custodians and others.
"Fabrication of the sphere was completed about 18 months ago here at Los
Alamos," said Sanchez. "Since then we've been in planning, getting
permission from the Department of Energy to do the experiment and taking
care of security issues.
The lower half of the neptunuim critical assembly is flanked by David
Loaiza, left, and Rene Sanchez, both of Advanced Nuclear Technology
(NIS-6). Loaiza and Sanchez credit the hard work and tenacity of the whole
NIS-6 team involved in the neptunium criticality experiment with its
ultimate sucess. Photo Credit: Los Alamos National Laboratory
"Once all that was in place, it took about four days to do the technical
operations of the experiment. It could not have been accomplished without
the hard work and determination of the whole team, NIS management and the
NNSA Office of Los Alamos Site Operations."
"After working together as a team for so many years doing something and
wondering 'Is it ever going to happen?' it was such a great feeling to
actually bring it together," said Loaiza. "The people here at TA-18 are
just amazing, excellent people."
The work was done to support the DOE's Criticality Safety Program and the
National Nuclear Security Administration's Nonproliferation Program and
Emergency Response Program.
So-called "bare" criticality is achieved when sufficient mass of fissile
material is present to sustain a nuclear chain reaction without any
reflective materials. The neptunium criticality was achieved in a "low
power" state, where the overall radioactivity is kept relatively low, at
about 300 millirem per hour, and no significant heat or fission byproducts
are created.
Since the neptunium sphere alone was not of sufficient mass to sustain the
nuclear reaction, it was placed in the center of several thin nested
shells of enriched uranium configured in an upper and lower half, with the
neptunium sphere located in the lower section.
The uranium in this case helps drive the system aiding in the neptunium's
ability to sustain the chain reaction. The two halves of the assembly were
placed in the "Planet" device, one half above the other, a safe distance
apart.
During the experiment, measurements are taken as the lower section is
raised and brought closer and closer to the upper section until sufficient
mass of fissile materials is present and criticality is achieved. Since
the reactivity of enriched uranium is well established the critical mass
of neptunium can be readily calculated from the experiment's resultant
data.
"LACEF is the only operational general purpose critical mass laboratory in
the Western Hemisphere. It is the only facility in the United States where
this kind of experiment could be done," said Clement.
The team plans to continue with further experiments using a variety of
reflecting materials. This will enable the scientists to determine the
critical mass of neptunium under a variety of conditions, as that mass can
vary depending on the amount of neutrons reflected or absorbed by
materials surrounding the nuclear material.
Los Alamos National Laboratory is operated by the University of California
for the National Nuclear Security Administration (NNSA) of the U.S.
Department of Energy and works in partnership with NNSA's Sandia and
Lawrence Livermore national laboratories to support NNSA in its mission.
Los Alamos enhances global security by ensuring the safety and reliability
of the U.S. nuclear stockpile, developing technologies to reduce threats
from weapons of mass destruction, and solving problems related to energy,
environment, infrastructure, health and national security concerns.
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