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BEHAVIOUR OF DIFFERENT BORON RICH SOLIDS XA0053634 AS PROMISING ABSORBERS FOR PWR
D. SIMEONE, X. DESCHANELS, P. CHEMINANT, P. HERTER Centre d'Etudes de Saclay,
Laboratoire d'Etudes des Materiaux Absorbants, Gif-sur-Yvette,
France
Abstract
Absorbing materials are used to control the reactivity of nuclear reactors by taking advantage of nuclear reactions (e g IOB(n,a)7Li) where neutrons are absorbed During such reactions, energetic recoils are produced As a result, radiation damage in absorbing materials originates both from these nuclear reactions, and from elastic collisions between neutrons and atoms This damage eventually leads to a partial destruction of the materials, and this is the main limitation on their lifetime in nuclear reactors Using different techniques, elementary mechanisms responsible of these ceramic destruction are studied The knowledge of such mechanism will help to improve the lifetime of these materials
1. INTRODUCTION
Among pressurized water reactors (PWR) control rods, many of them are made of two kinds of absorbers, an alloy of silver, indium and cadmium, and boron carbide B4C. The isotope 10B can efficiently absorb low and fast neutrons (Fig. 1). This is why boron carbide is also used in Fast Breeder Reactors (FBR). With the future generation of PWR, absorber pins will need to be able to capture more neutrons. Hafnium diboride HfB2 could then be an attractive material for these reactors, because of the high neutron absorption cross sections of both hafnium and 10-boron. In addition, their low cost of fabrication, their high melting point (Tm=2400°C for B4C, and Tm=3380°C for HfB2), and their low neutron activity after irradiation, makes them attractive materials for the nuclear industry.
The evolution of such materials under irradiation is however not enough understood. It is observed that after a typical burnup of 5%, B4C pellets fall apart [1]. The most likely explanation proposed so far is that this destruction is purely caused by the high stresses built up around micrometric penny-shaped He bubbles [1], He coming from the 10B(n,cc)7Li capture reaction. However, radiation damages may also play a role in this degradation process but few calculations of radiation damages have been done for materials undergoing nuclear reactions [2]. In addition to the damage produced by elastic collisions between neutrons and target atoms, energetic recoils are a second source of damage, the
10B(n,a)7Li reaction generates helium and lithium atoms with an average kinetic energy of 1.48 MeV and 0.83 MeV, respectively.
Two different techniques were used to study the evolution of B4C and HfB2 under a neutron irradiation: the Raman spectroscopy permitted to study the B4C [3] and the "Transmittion Electronic Microscopy" was used to analyse the HfB2 [4].
2. DAMAGE STUDY OF B4C AND HfB2
2.1 Raman study of irradiated B4C
To better understand the degradation mechanisms of irradiated B4C, some samples have been submitted to beams like of 1.2 MeV electrons or 180 keV He+. In these experiments, the only damages in the B4C structure are due to the recoil of the boron and carbon atoms knocked out of their equilibrium positions, or the presence of He inside the matrix. Other samples have been irradiated by neutrons, and the damages do not only come from to the recoil of B and C, and the presence of He, but also from the disappearance of boron isotopes from the structure, due to the 10B(n,a)7Li capture reaction.
Arguments for using Raman are provided by comparing peaks between B4C and a-boron. The two structures are identical (the same 12-atoms icosahedron structures) but a C-B-C median chain
exists in B4C and is absent in a-boron (Figure 2a). The Raman spectrum differences between B4C and a-boron is precisely due to the presence or not of this chain. This associated with some modelization allowed to identify precisely two peaks, located at 480 cm"' and 530 cm"1, associated with the C-B-C chain.
Neutrons
Thermal neutrons
E < l e V Rapid neutrons
E> 1 MeV
10B(n,a)7Li reaction dpa
V Li,T, He
V
Heat
FIG. 1. Neutron absorption by 10 boron
Comparison of the C-B-C Raman peaks between B4C samples irradiated only with electrons or He+ (no disappearance of B), and B4C irradiated by neutrons (disappearance of B) shows the material's capacity to restore its structure (C-B-C peaks) when damaged by electrons or He*. This is due to the extraordinary self-healing ability of the icosaedrons which are able to "pump" atoms from the less stable C-B-C chain, to complete their structures. Thus Raman peaks of B4C irradiated by electrons or He" are not degraded or promptly restored.
Neutron irradiation is accompanied by boron isotopes disappearance (Figure 3). To complete their structure, icosaedrons "pump" boron and carbon atoms from the C-B-C chain whose structure disappears (and the Raman peaks too) for lack of atoms. During a neutron irradiation, B4C move progressively to an a-boron structure without C-B-C chain. Only the presence of this chain could enable formation and migration of point defects through the structure, thus its absence would block any defect transfer and consequently the possibility for penny-shaped bubbles in irradiated B4C to relax their surrounding stress field by a defect inflow.
To conclude, arguments have been brought to explain the B4C degradation during a neutron irradiation: the initial damage is due to the building of penny-shaped helium bubbles, associated with
high local stress fields. At the same time an aggravation is occurring inside the matrix, the materials becoming progressively unable to release these stresses, due to the disappearance of point defects and/or their mobility.
FIG. 2a. Crystallographic structure ofB4C
FIG. 2b. Crystallographic structure ofHfB2
Figure 2a shows a schematic description of the rhomboidal unit cell of B4C. C sites refer to carbon atoms of the linear C-B-C chain, B site refers to the inversion center occupied by a boron atom, hi refer to polar sites occupied by boron atoms and h2 sites refer to boron and carbon atoms occupying the equatorial sites.
Figure 2b presents a schematic representation of the HfB, unit cell. Hafnium atoms are whites dots situated at each vertex of the hexagon, and three boron atoms over six are drawn in the hexagon in interstitial positions.
FIG. 2. Crystalographic structures ofB4C and HfB2 2.2. MET study of irradiated HfB2
The symmetry group of HfB2 is P6/mmm (Figure 2b). The unit cell is an hexagone possessing hafnium atoms at each vertex of the structure. Boron atoms occupy vacancies positions inside the hexagon. TEM photographies of HfB2 irradiated under 1073°K at low 10B burnup (about 1020
captures/cm3) show clearly dislocation loops in the basal plans [4]. For high temperature values (above 1073°K) and similar IOB burnup, helium bubbles appear in the grain boundaries of the material. Therefore, helium bubbles are not penny-shaped like in B4C but spherical like in UC>2. No
strain fields are associated to these bubbles in HfB2. The destruction of the material is due to the agglomeration of basal dislocation loops (vacancies loops) which meet each other near the grain boundaries forming micro cracking.
As a conclusion for HfB2, in this material helium bubbles are not damaging thanks to the defects mobility. But the anisotropy of relocation of these defects induces anisotropy of the elementary grain's strain thus a thread to the material's integrity.
ODPA
0.4 DPA
0.7 DPA
C
oo
15 DPA
400 800 1200
Raman shift (crrrl)
1600
FIG. 3. Evolution of Raman spectra ofirradiated B4C samples at different dpa values
3. CONCLUSION
The different unit cell symmetry (R 3 m for B4C and P6/mmm for HfB2) and the different nature of the chemical bonds leads to opposite behaviour under neutron irradiation of these two materials :
In B4C, the Bi2 icosaedric clusters do not permit the appearing of vacancies to diminish strain fields around helium bubbles, which remain damaging for the material.
In HfB2, many vacancies are present and allow helium bubbles to relax in the material.
At low irradiation temperatures (under 1073°K), the high number of vacancies loops (basal loops) are responsible of microcracking apparition in the material.
REFERENCES
[1] T. STOTO, L. ZUPPIROLI, J. PELLISSIER, Radiation Effects, 90, (1985) 160-170.
[2] A. ALBERMANN, D. LESUEUR, Am. Soc. for Test, and Mat, PA 19103, (1989).
[3] D. SIMEONE, C. MALLET, X, DESCHANELS, G. BALDINOZZI, O. KAITASOV, Study of 840 unit cell evolution under neutron irradiation by Raman spectroscopy, submitted in Phys.
Rev. B,( 1998).
[4] P. CHEMINANT, PhD Thesis, Orsay, (1997).
REINFORCEMENT AGAINST CRACK PROPAGATION OF PWR ABSORBERS