ORNL-TM-7207 - the Molten Salt Energy Technologies Web Site

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..WY used in the MSRE amd contributed significantly to the success sf the ex-
periment. HQW~WX, two problems were noted with Hastelloy-N which needed
further attention before more advanced reactors could be builto First,
Haste1loy-N was found to be embrittled by helium produced directly from
traces of ’QB and indirectly from nickel by a two-step reactiom.
type of radiation embrittlement is common to most iron- and nickel-base
alloys. The second problem arose from the fission-product tellurium dif-
fusing a short distance into the metal along the grain Bs~undaries and em-
brittling the boundaries.
nis
Considerable success was encountered in modifying the composition of
Bastelloy-N ts sbtafn better resistance to embrittlement by irradiation.
The key factor was to modify the carbide precipitate from the coarse type
found itl Standard Bastelloy-N to a Very fine type. ‘Ehe p%eSeKXe Of 16%
wolybdemaurn and 0.5% silicon led to the formation of a coarse carbide that
had little benefit. Reduction sf the m~lybdenum concentration to 12X and
the SiliCSrt content to 0.1% and addition of 8 reactive carbide former such
as titanium or niobium led to the formation of a fine carbide precipitate
and an alloy with good resistance to embrittlement by helium. Consider-
able progress was macle in the scale-up of an alloy containing 2% titanium,
but this alloy does not have sufficient resistance to intergranular crack-
ing by tellurium. An alloy containing B to 2% niobium was fasted to be
very resistant to cracking by tellurium and was produced in small commer-
cia% IIleltS. The cOmpoSit%Qn Of the niobfWll-modif$ed alloy iS ShQwsl in
Table 28. This alloy majntains good ductility up to the 46-ppm max3mm
helium content anticipated in the wall of a molten-salt reactor vessel.
In studying the tellurium embrittlement problem, considerable effort
was spent in seeking better methods of exposing test specimens to tellu-
rium. 1x1 the MSRE, the flux of the tellurium atoms reaching the metal was
about atoms m”2 s-’, and this value would be 10n4 atoms m-2 s-l for a
high-performance breeder. Even the value for a high-performance breeder
is very mall from the experimental standpoint. For example, this flux
would require that a total of 7.6 x 10-6 g of tellurium be transferred to
a sample having a surface area of IO cm2 in 1000 h.
Electrochemical
probes were immersed directly in salt melts kn~~m to contain tellurium,