This invention generally relates to fissionable fuel material for
use in nuclear reactors. In particular, the invention is directed to an improvement
in the process for fabricating mixed-oxide fuel.
The need to dispose of plutonium derived from the dismantlement of
nuclear weapons is a serious problem. One step which can be taken to alleviate
this problem is to recycle weapons-grade plutonium as fuel for a nuclear reactor
power plant. This solution requires that the plutonium be transported from the
weapons site to the fuel fabrication facility. However, this prospect raises the
concern about the best method for plutonium shipment and storage from both the
safety and diversion standpoints.
Historically, plutonium has been shipped as the metal, nitrate or
oxide. The metal form must be shipped within an inert atmosphere to prevent oxidation.
In a postulated accident, the plutonium metal would be difficult to disperse into
the environment because of its dense form. However, if the plutonium metal comes
into contact with air, it can oxidize or burn and thus form the oxide. The oxide
is a fine powder which could be readily dispersed into the environment. Furthermore,
plutonium metal is ideal for fabrication of nuclear weapons and is thus highly
susceptible as a target for diversion. The nitrate is a liquid which is slightly
more resistant to diversion but can be easily dispersed into the environment in
the event that the storage container is breached during an accident or act of
sabotage. The oxide is generally shipped as a fine powder which is more resistant
to diversion than is the liquid nitrate. However, the oxide can also be easily
dispersed into the environment if the storage container breaks open during an
accident or act of sabotage. Thus, there is a need for a safe and secure method
for transporting weapons-grade plutonium to the fuel fabrication facility.
Previous mixed-oxide fuel fabrication has relied on fine plutonia
feed powders usually converted from plutonium nitrate solutions to the oxide by
direct precipitation processes or as a co-precipitated compound with uranium.
These conversion processes result in a very fine powder that is blended with urania
to produce mixed-oxide feed material such as described in FR-A2148174 and FR-A-2049034.
In some cases, this blended material has been milled by a high-energy process to
improve the plutonium dispersion. Statistical sampling and metallographic examination
techniques are utilized to qualify each power blend. One hundred percent inspection
techniques are utilized to verify that gross plutonium dispersion did not occur
in any portion of a completed fuel rod.
According to the invention, there is provided a method for fabricating
mixed-oxide fuel characterized by the steps of:
- converting plutonium metal into plutonia powder at a site remote from said
fuel fabrication facility;
- pressing said plutonia powder into the shape of a pellet at said remote site;
- firing said pressed powder to form a plutonia pellet at said remote site;
- shipping said plutonia pellet to said fuel fabrication facility;
- breaking up the plutonia pellet into pieces;
- grinding said pieces into plutonia powder;
- segregating fine particles of plutionia from said plutonia powder to form a
fine plutonia powder; and
- blending said fine plutonia powder with urania powder.
Embodiments of the present invention provide a process fabricating
weapons-grade plutonium into mixed-oxide fuel for use in a nuclear reactor. The
plutonium is converted into plutonia powder at a site remote from the fuel, fabrication
facility and then the plutonia powder is pressed and fired into high-density pellets
for transport to the fuel fabrication facility. Since the pellets are relatively
insoluble, it is difficult to separate the plutonium from the high-density pellets
which serve as a diversion-resistant and environmentally sound method of shipping
the plutonia for use as a feed material.
Plutonia pellet form provides the greatest fabrication challenge
from the standpoint of plutonium homogeneity. Comminution methods must be employed
to reduce the plutonia pellets to a fine powder for blending with urania feed
to produce mixed oxide acceptable for reactor operation. In this process the plutonia
pellets are ground into a fine powder and screened to segregate the fines. The
fines are blended with urania to form a mixed-oxide powder blend which can be fabricated
into fuel pellets by standard techniques. Since the fine plutonia powder produced
by comminution methods does not tend to agglomerate like powder produced by chemical
processes, a homogeneous blend of mixed oxide can be more easily produced.
The invention will not be described in greater detail, by way of example.
We will now describe a process for fabricating weapons-grade plutonium
into mixed-oxide fuel for use in a nuclear reactor. The preferred form of plutonium
feed material for processing in the fuel fabrication plant is plutonia, PuO2.
Therefore, the metallic weapons-grade plutonium must be converted into ceramic
grade plutonia, which is delivered to the site of the fuel fabrication facility.
Plutonia made from weapons-grade plutonium is of sufficiently high purity that
no further refining to further remove impurities is necessary, with the possible
exception of gallium.
There are several alternative for converting the plutonium into plutonia:
(1) controlled burning of the metal to form the oxide and followed by a mechanical
milling step; (2) controlled burning of the metal followed by dissolution and
precipitation process steps; and (3) controlled hydriding, dehydriding and oxidation
The preferred method for converting plutonium into plutonia is controlled
burning and subsequent milling. The potentially reduced activity of plutonia may
tend to limit the mixed oxide composition to a maximum of about 10% plutonia and
allows pellet densities in the low 90s range. This process provides the lowest
cost approach and is expected to produce plutonia which meets the fuel performance
requirements for plutonium disposition. If plutonia of higher activity or finer
particle size is required, then the second and third alternative processes may
be used. However, the hydride-dehydriding process coupled with a controlled oxidation
operation would also provide plutonia with higher activity or finer particle size.
The hydriding step allows better control of the final oxidation process.
After the plutonium has been converted into plutonia powder, the
plutonia powder is pressed into high-density pellets and fired in a sintering furnace
for transport to the fuel fabrication facility. Conventional comminution techniques
are used to reduce the plutonia pellets to a fine powder for blending with urania
powder to produce mixed oxide feed material acceptable for reactor operation. More
specifically, the high-density plutonia pellets were broken up and ground to produce
a fine powder which does not tend to agglomerate. The plutonia was ground until
it passed through a 325 mesh screen (<44 microns). Then urania powder was blended
with the ground and sized plutonia to produce a mixed oxide powder blend in accordance
with conventional mechanical blending techniques.
The process for fabricating fine plutonia feed powder into oxide
fuel pellets has been utilized to fabricate both urania and mixed-oxide fuel for
light water reactors, mixed oxide fuel for liquid metal reactors and uranium nitride
fuel for space reactor applications. The basic mill-slug-granulate process has
been well developed for over 20 years. In this process the feed powders are blended
together in the desired concentration, milled to improve inhomogeneity and pre-pressed
and granulated to form free-flowing powder for pellet pressing. The resulting
pellets are sintered in a reducing atmosphere for 1 to 5 hours at 1600°C to 1750°C,
resulting in pellets which are acceptable from the plutonium homogeneity standpoint.
The pellets are then ground to size, inspected and loaded into fuel rods. The
fuel rods are sealed by welding, inspected and assembled into fuel bundles. The
completed fuel bundles are stored at the fabrication facility until they are needed
to provide initial or reload assemblies for a reactor.
The relatively low radioactivity associated with weapons-grade plutonium
will allow the fuel fabrication activities to be performed in a glove box arrangement.
The requirement for minimized worker exposure is handled by mechanizing all unit
processing operations, which will reduce the time the operator is in close proximity
to the nuclear material. Also, localized shielding will be installed and lead
glass and sheet added as needed to the exterior of the glove boxes.
The feed materials for the fuel fabrication process which utilizes
powder blending are the oxide forms of uranium, plutonium and gadolinium. The urania
(either natural or depleted) will be supplied as an active powder capable of sintering
to high density. The plutonia must meet the requirements of ASTM C757, Nuclear-Grade
Plutonium Dioxide Powder, Sinterable.
In the fuel pellet fabrication process the feed oxides are transferred
from storage to geometrically safe holding bins. Weight feeders are used to load
the appropriate amount of materials into a vibromill where the powders are blended
and milled together. The resulting blend is split into as many as eight increments
and the subsequent eight increments are cross blended to form a large uniform
feed lot of powder. The blend splits are stored in a geometrically safe arrangement
and removed for continued processing one at a time. During reblending or cross-blending,
each blend split is vibromilled to smooth blend differences and to produce an
agglomerated powder for feed to pellet pressing. Small hydraulic pellet presses
developed for glove box use are utilized to press pellets. Each press can optionally
be provided with multi-cavity dies, depending on the throughput requirements.
The green pellets are loaded into molybdenum sintering boats. Specially
designed sinter furnaces with removable refractor sections are utilized to sinter
the mixed-oxide pellets. The oxygen atmosphere of the sintering furnace is controlled
to obtain fuel pellets of the desired stoichiometry. All of the sintered pellets
are passed through a centerless grinding station after inspections for sintering
characteristics to control the pressing operations. the grinder has an automatic
control feedback loop which will automatically adjust the grinding to maintain
the pellet within limits. The column of pellets which exit the grinding operation
pass directly to the automated pellet inspection station. At the station, the pellets
are inspected and sorted for geometric properties (diameter, length, density,
surface finish, cracks and pits). After sampling for chemical characteristics,
the pellets are placed into storage to await certification approval for fuel loading.
The certified fuel pellets are aligned into columns, where final column geometry
is obtained prior to loading into fuel cladding (tubes with one end plug welded