a b s t r a c tHelium behavior in Al/B 4 C metal matrix composite with two different sets of ion irradiation conditions has been investigated by transmission electron microscopy. Helium bubbles in Al were found to be much larger than those in B 4 C after a helium fluence of 1.5 Â 10 17 ions/cm 2 at the room temperature. Also, bubbles at grain boundaries and their vicinity in aluminum are faceted. With additional proton irradiation, a bubble denuded zone along the aluminum grain boundary appears. The results are discussed in terms of the energetics of the material system.Ó 2015 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.Al/B 4 C metal matrix composite (MMC) is an important neutron absorbing material used in both wet storage pools and dry storage casks of spent nuclear fuel for preventing criticality. Al/B 4 C MMC can effectively absorb fast and thermal neutrons, because of the high neutron absorption cross-section of 10 B through the 10 B(n, a) 7 Li transmutation reaction for a wide energy range of neutrons.The damage produced by elastic collisions between neutrons and other energetic particles generated by transmutation reactions with the target atoms along with the build-up of helium concentration in the MMC may lead to the precipitation of helium bubbles. The gamma and neutron doses received in the MMC depend on a number of factors including fuel burnup, storage time, and the self-shielding of the fuel assemblies. In spent fuel fool storage, the neutron flux can be as high as 2.6 Â 10 5 neutron/cm 2 s and the gamma dose rate is in the range of 0.1-10 kGy/h [1,2], corresponding to several thousand appm of transmuted helium build up and 1-2 dpa (displacement per atom) of damage during the estimated storage period of 60 years in spent fuel pool storage and about 200 years in the dry storage casks.As a control rod material, the defect structure in neutron irradiated B 4 C was investigated by Ashbee [3,4], Jostsons [5,6], Hollenberg [7] and Donomae [8] et al. He + implantation of B 4 C has been performed by Stoto [9] and Maruyama [10]. Irradiation effects and helium bubble formation in aluminum after high energy proton irradiation were reported by Singh and Victoria [11,12]. The precipitation of helium causes not only changes in microstructure but degradation in the physical, chemical and mechanical properties of Al/B 4 C MMC, such as, reduced thermal conductivity, decreased corrosion resistance and volume swelling. The property change in Al/B 4 C MMC affects the long-term performance of the MMC in its working environment. In fact, irradiation induced microcracking in B 4 C that leads to increased boron release was reported by Copeland [13] and Stoto [14].This paper focuses on helium bubble formation upon helium implantation and bubble evolution during additional proton irradiation in the Al/B 4 C MMC. Ion irradiations were conducted to simulate the effects of radiation and helium build-up in Al/B 4 C MMC as neutron absorbers in wet storage pool and dry storage casks. Angular shape B 4 C particles w...

Laves-type intermetallic phases have been observed to be the dominant phases in a series of alloy compositions being designed for the immobilization of technetium in a metallic waste form. The dominant metals in the alloy compositions were Fe-Mo and Fe-Mo-Zr. The alloy composition, Fe-Mo-Zr, also contained Pd, Zr, Cr, and Ni. Both non-radioactive rhenium-containing and radioactive technetium-bearing alloy compositions were investigated. In the Fe-Mo series, the phases observed were Fe2Mo (C14 Laves phase) and ferrite in agreement with predictions. Both Tc and Re resided predominantly in the Laves phases. In the Fe-Mo-Zr system, the phases included hexagonal C14 with the composition (Fe,Cr)2Mo, cubic C15 phase with a (Fe,Ni)2Zr composition, and the hcp phase Pd2Zr. The observation of these phases was in agreement with predictions. Re was found in the C14 intermetallic, (Fe,Cr)2Mo. Technetium was also observed to be partitioned preferentially into the (Fe,Cr)2Mo phase; however, this phase exhibited a cubic structure consistent with the C15 structural type. The composition of Laves phases is influenced by both the atomic size and electro-negativity of the constituent elements. The long-term release behavior of technetium under nuclear waste disposal conditions may be more dependent on the corrosion characteristics of these individual Laves phases containing Tc than the other metallic phases.

Al/B 4 C metal matrix composite (MMC) is an important neutron absorbing material used in both wet storage pools and dry storage casks of spent nuclear fuel for preventing criticality. Because of the high neutron absorption cross-section of 10 B, the material can effectively absorb fast and thermal neutrons, but at the same time suffers neutron irradiation damage from the nuclear spent fuel. Moreover, interactions of fast and thermal neutrons with boron lead to the production of several transmutation species such as helium, lithium and others, according to the transmutations reactions induced by neutron irradiation, including the 10 B (n, 4 He) 7 Li reaction and others. The most abundant product of these reactions is energetic helium (~1.47MeV), which may induce further radiation damage and be precipitated out in the material as helium bubbles. The formation and coalescence of the helium bubbles in boron carbide and on boundaries, especially phase boundaries, would affect the corrosion performance of the material in its working environment. The degradation of the material may lead to boron loss which is the critical element to control the criticality of spent fuel. To evaluate the radiation and corrosion effects for interim storage of two hundred years accelerated He + irradiation and corrosion tests were carried out.

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