FABRICATION OF TARGETS FOR NEUTRON IRRADIATION OF NEPTUNIUM DIOXIDE R . E. M Y R I C K AND R. L. FOLGER Savannah River Labortory. E. I. du Pont de Nemours 3 Co., AikPn,
S.C.
Neptunium targets for neutron irradiation in a nuclear reactor were fabricated by a powder metallurgy process. Blends of neptunium dioxide and aluminum were compacted a t ambient temperature to about 90% of theoretical density, and the compacts were jacketed by sintering in an aluminum can a t 550” to 625” C. under 19 tons per square inch. Metallurgical bonding of the core and cladding occurred primarily by grain growth across the core-cladding interface. Process parameters were explored with neptunium dioxide and also with thorium dioxide and uranium dioxide as metallurgical substitutes. Bond strength generally increased with sintering temperature and varied inversely with actinide concentration in the core. Solid state reduction of each actinide oxide by aluminum to form aluminides was also observed. The strengths of core-cladding bonds across ThAla and across NpAld were also measured.
E P ~ ~ I - S X U targets M - ~ ~ ~for
irradiation Lvith neutrons to form plutonium-238, as described by Vondy, Lane, and Gresky( 9 ) , are fabricated by distributing the target atoms in a matrix which is relatively transparent to neutrons and which has good heat transfer characteristics. At the Savannah River plant, neptunium dioxide from the oxalate process described by Porter (7) is blended with aluminum po\vder and pressed into compacts for loading into a n aluminum can. T h e compacts are bonded to the can by sintering under pressure, and the r a n is sealed by ~veldingso that the finished slug presents a continuous aluminum surface to the environment. Operations through the Xvelding step are performed in glove boxes sealed to prevent the spread of radioactive contamination. General Process Description
Slugs are fabricated tvith a fixed concentration of N p O ? so that the interaction of neptunium atoms with the neutron flux pattern in the reactor gives the desired isotopic composition of the plutonium product in a n irradiation time consistent with reasonable yield. Self-shielding of the neptunium and multiple capture to give PuZ34are factors to be minimized in choosing target composition. T h e crystal densities of N p O ? and aluminum are used to calculate the composition of the powder blend Xvhich will give the desired concentration of neptunium within the core volume of the finished slug. Powder Blending. Target atoms in the compact must have a uniform distribution to prevent local overheating and failure during irradiation. NpO, is blended with Alcoa Type 101 aluminum powder in a standard one-gallon jar mill containing porcelain balls. L-niformity of the dispersion is determined by a statistical analysis of the gamma activity of five random samples from each ball-mill batch. T h e gamma radiation. bvhich cornes from the Pa233daughter of the SpZ3’ alpha decay. is proportional to the neptunium content of the samples within a gilven batch. Compact Pressing. “Green” compacts. 3 inches long and 0.86 inch in diameter, are formed by pressing the blended poLvder in a tool-steel die at 19.8 t.s.i. (tons per square inch) at ambient temperature to give 90 to 92% of the theoretical density of a fully compacted mixture of NpOr and A1 crystals. T h e die is lubricated u i t h n-dodecanol to prevent the aluminum from cold-tvelding to the die surface. A double-acting press transmits equal force to both ends of the compact to give more
uniform compaction than could be obtained by applying force to only one end of a cylinder of loose powder. Handling of the target material in the form of compacts avoids excessive travel of the sealed ram in the vacuum hot-pressing operation. and also helps to control the spread of radioactive contamination during loading of the slug can. Excessive densification is undersirable during compacting because some travel of the compact surface relative to the can wall is needed during hot pressing to provide a fresh metallic surface for bonding. Slug Loading. Two compacts are loaded into a n impactextruded aluminum can (Alcoa Type 1245) which is then closed by a n aluminum cap. T h e assembled slug is loaded into a n Inconel X die lubricated with Aquadag (a colloidal dispersion of graphite in water). T h e assembly is placed in a vacuum furnace consisting of a floating Inconel X backup die inside a stainless steel sheath wrapped with resistance heaters (Figure 1). T h e slug is heated to a n estimated 600 to 620 C.
RAM EXTENSION
R I M E I S STEEl FURNACE BODY
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PLUG
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RESISTANCE HE4TER
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s0TTOU-j PLUG
Figure 1.
\SPRING
Hot-press-bond furnace
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OCTOBER 1 9 6 4
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-=Length
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Figure 2. Effect of pressure on density of Np02-AI compacts
average
assure cladding integrity throughout the irradiation cycle. Radiographs verify that there is sufficient cladding thickness to prevent exposure of the core to the reactor coolant in spite of the normal erosion and corrosion which occur during irradiation. and surface quality is controlled to avoid nicks and scratches which might act as stress raisers to accelerate corrosion. Process parameters were selected to assure an average bond strength of at least 5000 p.s.i. betw'een the core and the aluminum cladding, to prevent mechanical separation during thermal cycling in the reactor. Areas of zero bond strength (nonbond areas) are limited to less than 0.06 square inch per 1.5 inches of slug length to avoid localized overheating which could lead to rupture of the cladding. Nonbond areas are detected nondestructively by measuring the attenuation of a beam of ultrasonic vibrations passed through the slug by a water-coupled transducer. Bond strength is improved and surface blemishes and weld defects are often cured by repressing the slugs in the furnace; about 270 of the slugs are re-pressed. Over-all yield of acceptable slugs is greater than
99%. Process Development
56 0
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Pressure, I I i.
Figure 3. Effect of pressure on average density of various actinides
(backup die temperature is 620' to 635" C.) under a vacuum of
