Evaluation of Neutron Poison Materials for DOE SNF Disposal Systems Page: 4 of 8
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reactivity would be excessive, and that poisons would
be required in the canister to meet criticality
requirements. Analyses of several scenarios for
degradation of both the DHLW and Al-SNF within the
waste package have been conducted using the EQ3/6
geochemistry computer program . Criticality
calculations indicate that the resulting configurations
are less reactive than the case of a flooded, intact
canister with degraded fuel.
The product of the melt-dilute technology option was
evaluated using the SCALE 4.3 family of computer
codes, by Westinghouse Safety Management solutions
. Preliminary results for degraded configurations of
Al-SNF that has undergone the melt-dilute treatment to
produce an SNF forn of U-Al alloy at 20% 211U
enrichment indicate that criticality is precluded by
moderator exclusion in a flooded DOE SNF canister.
Analysis of the melt-dilute SNF form in a waste
package, in which the DOE SNF canister has degraded,
has not yet been completed.
Poisons are needed to maintain keff < 0.95 in desired
packaging configurations of Al-SNF directly loaded in
canisters. Candidate poison materials include isotopes
and compounds of boron, gadolinium, hafnium,
europium, and zirconium. The following is a brief
summary of the current understanding of potential
poison materials as described by Anderson and
Theilacker  and by McDonell and Parks .
The most predominantly utilized poison materials are
boron and borated metals. Boron used in a boron-
stainless steel alloy has been used for discreet burnable
poison and control rod applications. This is primarily
due to the availability and excellent nuclear
characteristics associated with boron. An advantage of
boron is that its reaction products, helium and lithium-
7, are not radioactive. Further, the cross section of the
neutron absorbing isotope of boron, '"B, is inversely
proportional to the velocity of the incident radiation,
which simplifies boron burnable poison physics
calculations when compared with those for silver,
hafnium, europium, or indium which have relatively
complicated resonance absorption cross sections.
While the reaction products of boron are not
radioactive, the production of helium and lithium-7 can
cause metallurgical problems in a metal matrix. Boron
has low solubility in structural materials such as
stainless steel or aluminum. The result of irradiation of
metal-boron alloys is accelerated embrittlement of the
alloys. The helium gas generated when "0B captures a
neutron is extremely mobile and tends to accumulate at
points of stress concentration. The lithium generated
by neutron capture in 10B puts additional stress on the
Gadolinium has the highest absorption cross section of
any element. Gadolinium, however, has very poor
corrosion resistance and high solumbility. Typically, it
is used as an oxide dispersion in a metal matrix,
however, Gadolinia (Gd2O3) dispersions are limited for
practical purposes to about 40 percent by volume.
Gadolinia has been used in transport/shipment casks as
a criticality control material.
Silver-base alloys were first considered for power
reactor application in 1950. Early work was based on
alloys of silver containing 20 to 40 percent cadmium,
which has good corrosion resistance, plus small
amounts of other elements, such as copper, to add
strength. These early alloys exhibited poor corrosion
resistance in high temperature water and tended to lose
weight during corrosion and, thus, release their high
activity nuclides to the coolant. Europium has also
found use as a control material in power reactors.
Europium has the unique quality that capture of a
neutron by 51Eu begins a chain of four daughter
isotopes that each have relatively high absorption cross
sections. This quality provides for a relatively long-
lived absorber material. Problems with europium
include cost and scarcity. Further, due to its high vapor
pressure and relatively low melting point, europium is
difficult to alloy with common reactor materials such as
stainless steel and zirconium. Zirconium alloys have
found extensive use in light water reactors based
entirely upon their excellent high temperature
mechanical properties and their good corrosion
resistance. Zirconium has a very low absorption cross
section, and this is its major disadvantage with respect
to criticality control. Of the materials considered for
control rod application, hafnium is the least
complicated metallurgically. In its pure form, hafnium
has about the strength of Zircaloy-2 and about one third
its corrosion rate in 500 to 600 0F water. Hafnium,
therefore, requires no cladding or protective plating for
water-cooled reactor application. The disadvantages of
hafnium are primarily economic rather than difficulties
with performance. The only significant source of
hafnium is from zirconium ores that contain only about
2 percent hafnium by weight. However, the absorption
cross section of hafnium is more than 500 times that of
its sister element, zirconium.
Poison Viability Considerations
The governing regulations forbid the inclusion of
pyrophoric, combustible, explosive, or chemically
reactive materials in waste packages to be disposed at
Yucca Mountain. Therefore, it is important to consider
the chemical behavior of candidate neutron absorbing
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Vinson, D.W.; Caskey, G.R. Jr. & Sindelar, R.L. Evaluation of Neutron Poison Materials for DOE SNF Disposal Systems, article, September 1, 1998; Aiken, South Carolina. (digital.library.unt.edu/ark:/67531/metadc676162/m1/4/: accessed September 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.