Surface Effects on Tritium Diffusion in Materials in a Radiation

18 Surface Effects on Tritium Diffusion in Materials in a Radiation Environment

Downloaded by UNIV OF ARIZONA on August 21, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0158.ch018

G. R. CASKEY, JR. Savannah River Laboratory, Ε. I. du Pont de Nemours and Co., Aiken, S.C. 29801

Tritium transport and distribution in a material are con trolled by chemical potential gradients, thermal gradients, and cross-coupling to impurities and defects. Surfaces in fluence tritium diffusion by acting as sources and sinks for defects and impurities. Surfacefilmsrestrict tritium transfer between the solid and surrounding fluids. Radiation directly affects boundary processes such as dissociation or adsorp tion, may erode a surface film or the surface itself, and introduces defects and impurities into the solid by radiation damage, transmutation, or ion implantation, thereby modi fying tritium transport within the solid and its transfer across external interfaces. There have been no definitive investigations of these effects, but their practical significance has been demonstrated in tritium release or absorption studies with stainless steel, Zircaloy, niobium, and other materials. / C u r r e n t interest i n tritium migration i n solids arises from technical, ^ economic, and environmental considerations associated with operat ing fission reactor plants and projected fusion reactor systems. I n the first case, the interest is almost entirely caused b y the possible environ mental contamination b y tritium leakage to plant effluents ( I , 2 ) . Small quantities of tritium are generated by both ternary fission and reaction of neutrons w i t h boron and lithium. There are many ways that this tritium may eventually escape to the environment; for example, through permeation through the steam generator system. Fusion reactors, on the other hand, are designed to process continuously large (kilogram) quan tities of tritium both i n the feed and exhaust from the deuterium-tritium ( D - T ) plasma and i n the tritium breeding, extraction, and purification 366 In Radiation Effects on Solid Surfaces; Kaminsky, M.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

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Surface Effects on THtium Diffusion

367

systems (3, 4, 5, 6, 7). Efficient containment and recovery are essential i n this case. Potential problems associated w i t h fusion systems have been described i n detail (4, 5, 6 ) , and several alternative approaches to m i n i mize tritium release have been advanced (4). I n a l l cases, knowledge of the diffusion and permeation characteristics of tritium i n structural materials of principal interest is helpful i n analyzing containment, breeding, and handling problems. Potential materials include: stainless steel; alloys of niobium, vanadium, or molybdenum; Zircaloy-2; silicon carbide; beryllium oxide; alumina; oxides commonly formed as surface films on metals; tritides, such as T i T w h i c h may be used i n targets for neutron generation (8) and i n extraction systems; and various grades of steels for steam-generating systems. A small amount of experimental data is available on tritium diffusion i n these materials. There is much more information for protium and deuterium (9,10,11,12) which is applicable to tritium, if the data are corrected for the effects of isotope mass.


2

Figure I .

Diagram of interrelations among tritium diffusion, surface effects, and radiation

Tritium diffusion occurs i n response to gradients i n chemical potential and temperature i n solids and is influenced by interaction with defects, impurities, and w i t h surfaces. These interactions may involve: (1) Boundary processes such as dissociation and chemisorption. (2) External surfaces (such as those between two solids or between a solid and a fluid) and internal surfaces (such as grain boundaries) which act as sources and sinks for defects and impurities. (3) Surface films, such as oxides on metal surfaces, which may change the rate of tritium transfer between phases.

In Radiation Effects on Solid Surfaces; Kaminsky, M.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

368

RADIATION E F F E C T S O N SOLID SURFACES


Potential interrelations among these factors are represented diagrammatically i n Figure 1. Several possible effects of radiation on the processes affecting tritium diffusion are indicated, including: ( 1 ) Catalytic effect on dissociation or adsorption. (2) Erosion of the surface or of surface films. (3) Increased defect and impurity fluxes to or from the surfaces. (4) Increased defect and impurity concentrations within the solid. The effects of these processes on tritium diffusion i n solids have not been investigated. Several related problems, however, have been discussed or are under investigation (13). For example, the effect of oxide films on tritium release and permeation i n stainless steel (14), trapping effects i n niobium (11), and photodesorption (15, 16) have all been reported i n some detail. This chapter w i l l focus attention on those aspects of tritium absorption and transport where information is available and where experimental investigation is i n progress. W e w i l l also try to identify areas where additional investigation is needed. Interaction of Tritium Atoms with Defects and Impurities Tritium diffusion and its ultimate distribution throughout a solid is strongly affected by lattice defects and impurities because of repulsive or attractive interactions such as: trapping of tritium at immobile defects or impurities, diffusion of tritium i n response to gradients i n the chemical potential of impurities, migration of tritium with moving defects. These processes are related to radiation effects because radiation damage, transmutation, and ion bombardment change the number and distribution of defects and impurities and may implant tritium within the lattice. Tritium Solution in Solids. Dissolved tritium may occupy either octahedral ( O ) or tetrahedral ( T ) interstitial positions i n metals depending on the crystal lattice of the host structure. Neutron diffraction, ion channeling, and nuclear magnetic resonance techniques have identified the equilibrium interstitial sites i n several metals (17, 18, 19, 20). Generally, the T-site is occupied i n metals with a body-centered cubic ( B C C ) lattice, and the O-site is occupied i n metals with a face-centered cubic ( F C C ) or hexagonal close-packed ( H C P ) lattice. Chromium ( B C C ) appears to be an exception, as an ion-channeling study indicates that the O-site is occupied (21). Atomic and molecular solutions of tritium have been observed i n nonmetallic solids (22-29), but there is very limited direct evidence on the specific location of tritium in such materials. Generally, interstitial solution is assumed, although substitutional replacement may be anticipated i n some cases, as i n polymers. Tritium solution and diffusion i n silica and silicate glasses is normally molecular (22, 28). However,


18.

Surface Effects on Tritium Diffusion

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atomic diffusion of tritium ions has been observed in quartz and fused silica (24), and electrolysis apparently introduces atomic hydrogen i n vitreous silica (25). Tritium solution i n polymers is normally molecular as indicated by the direct dependence of permeation rate on the first power of the hydrogen pressure (26). In this instance, isotopic exchange of tritium with the hydrogen i n the polymer could alter the diffusion kinetics and eventually degrade the polymer by breaking the bonds where replacement tritium decayed. Tritium solution i n metal oxides may be atomic i n the few cases where it has been studied, e.g., ZnO (27) and T i 0 ( 28). However, the pressure dependence of the permeation rate of hydrogen i n Z r 0 suggests a molecular solution (29). Tritium diffusivity has been measured i n various materials by gas phase charging (28), electrolytic charging (25), ion bombardment (24), and recoil from L i (14). Results for several types of solid are shown i n Figure 2. Generally, tritium diffusivities correlate w i t h protium or deuterium diffusivities by the inverse-square-root-of-mass relation where data exist to make the comparison. Silver and aluminum are apparent exceptions because of effects associated with ion implantation or radiation, as discussed later. Protium and deuterium diffusivities i n metals have been summarized previously (9, 10, 11, 12). Little work has been done with oxides, but this area is being investigated currently on materials of interest i n fusion reactor development (30). Trapping by Immobile Defects or Impurities. A n attractive interaction between diffusing tritium and lattice defects or impurity atoms is not accounted for i n the simple diffusion equation:


2

2

a

dC/dt — V D V C

(1)

Therefore, this equation is not applicable under such conditions. More complex formulations are required to incorporate additional processes into the analysis. Formal representation of trapping has been approached in two ways: incorporation of additional terms i n the continuum diffusion equation to account for trapping (31, 32, 33, 34, 35) and analysis based on the concept of mean-free passage time taken from the theory of stochastic processes (36). The former approach has been used extensively but may not be appropriate when considering singularities of atomic dimension. The latter approach takes into account details of the lattice, trap site, and shifting of the saddle point energy caused by the perturbation of the lattice by the defect, but it has not been applied to analysis of hydrogen diffusion experiments. Trapping sites that have been identified or suspected i n metals include: dislocations in type 304L stainless steel, nickel, vanadium, niobium, iron, and molybdenum (37-42); interstitial or vacancy clusters


370

RADIATION E F F E C T S O N SOLID SURFACES

Temperature, °C


1000

500

Figure 2.

100

20

Diffusivity of tritium in various materials

i n copper, silver, and gold (43); oxygen i n niobium and copper (44, 45); and voids or gas bubbles i n iron, silver-lithium, and aluminum-Hthium alloys (46, 47, 48). Potential trapping sites i n nonmetallic solids include Schottky or Frenkel defects and impurity atoms. General representation of diffusion w i t h trapping at only one type trap has been developed (33) by replacing the simple differential equa tion w i t h : dC/dt + Ndn/dt — D V C 2

(2)

where dn/dt = kC ( 1 — η) + pn, Ν is the number of traps per unit vol ume, η is the fractional occupancy of trap sites, and k and ρ are capture



18.

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Surface

Effects

on THtium

371

Diffusion

and release probabilities, respectively. The resulting equations are non linear and have no general analytic solution. Finite difference methods yielded approximate solutions for boundary and initial conditions corre sponding to permeation and evolution from a plane sheet (49). Exact solutions have been obtained for certain limiting cases where the equation reduces to a linear form (33, 34, 50). Characteristics of trapping effects on permeation and evolution (Table I) show that: apparent diffusivity is less than the true lattice diffusivity, actual solubility is greater than the lattice solubility, and steady state processes are unaffected. The significance of trapping on tritium diffusion within a solid depends on the concentrations of both tritium and trapping sites i n the solid (33, 34). I n the case of a Tokamak fusion reactor w i t h a molten lithium blanket, the effective tritium pressure throughout the blanket region is very low; therefore, tritium concentrations are low. F o r example, in the University of Wisconsin Tokamak fusion reactor, U W M A K - I ( 7 ) , the tritium content of the lithium ( 10 kg) is estimated at 4.5 Χ 10" mole fraction which corresponds to an effective tritium gas pressure of 10" Pa ( 10" atm). The quantity of tritium i n solution i n the large mass (6 X 10 kg) of type 316 stainless steel alloy is only ~11000 cm . If the void volume arising from radiation damage were 0.1% throughout the struc tural framework of the blanket, the additional tritium trapped i n the voids at 10 Pa ( 10" atm ) would be ~ 10" cm . The effective tritium diffusivity w i l l not be altered significantly by such a small void volume. Internal Surfaces. Internal surfaces, such as grain and subgrain boundaries, may serve as trap sites or act as short circuit diffusion paths. F o r example, hydrogen solubility i n polycrystalline nickel is distinctly greater than i n single crystals over a wide temperature range. Further, 5

5

4

9

6

3

-4

9

Table I.

3

3

Effects of Trapping and Surface Films on Permeation" Reversible Trapping

Irreversible Trapping

Continuous Film

S t e a d y state p e r m e a t i o n rate Poo =

(Poo)

Ρ » (ideal)

P* =

P

(ideal)

x

P„

(ideal)

< P»

A.pparent diffusivity ( D * ) Instantaneous p e r m e a t i o n rate (Pt)

L

D*

Pt < Pt

(ideal)

P

E

(ideal)

E

D*

Surface Effects on Tritium Diffusion in Materials in a Radiation

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