Helium effects on void formation in 9Cr-1MoVNb and 12Cr-1MoVW irradiated in HFIR

Abstract

Up to 2 wt % Ni was added to 9Cr-lMoVNb and 12Cr-IMoVW ferritic steels to increase helium production by transmutation during HFIR irradiation.The various steels were irradiated to ~39 dpa. Voids were found in all the undoped and nickel-doped steels irradiated at 400°C, most of them at 500°C, but not in any of them at 300 or 600°C. Bubble formation, however, was increased at all temperatures in the nickel-doped steels. Maximum void formation was found at 400°C, but swelling remained less than 0.5% even with up to 440 appm He. Irradiation at 300 to 500°C caused dissolution of as-tempered M23C6 precipitates and coarsening of the lath/subgrain structure in the 9-Cr steels, whereas the microstructure generally remained stable in the 12-Cr steels. Irradiation in this temperature range also caused compositional changes in the as-tempered MC phase in all the steels, and produced combinations of fine H(P, G, and M2X precipitates in various steels. The subgrain boundaries appear to be strong sinks that enhance resistance to void formation. Higher helium production during irradiation appears to shorten the incubation period for void formation. The effects of helium on steady state void swelling behavior, however, remain unknown.

Elevated temperature ferritic and martensitic steels and their application to future nuclear reactors

“…32 Irradiation-induced precipitate changes can also affect properties. [30][31][32]34,35 Precipitates formed in the 9-12% Cr steels during irradiation include α΄, 30,35 G-phase, 35 M 6 C, 31,34 and chi-phase. 32,35 For most of the 9-12% Cr Cr-Mo steels investigated, Laves phase, which forms during thermal aging at ≈400 to 600ºC, 30,32,[34][35][36] can cause embrittlement; 36 it does not form if irradiation is above ≈600ºC.…”

“…However, ion 37 and proton 38,39 irradiations have produced evidence for retention of considerable amounts of the hydrogen, but it is not expected to be an issue in martensitic steels above about 250ºC. 39 For a fusion reactor, where the helium:dpa ratio is about 10, indications are that helium can affect swelling, 31 although the 9-12% Cr steels still remain low swelling. The helium:dpa ratio for ferritic/martensitic steels in most fission reactors is about two ordersof-magnitude lower and has a minimal effect on swelling.…”

“…Loop size increases and loop number density decreases with increasing temperature, eventually becoming unstable. [30][31][32][33][34] In ferritic/martensitic steels, agglomeration of vacancies can lead to void swelling up to about 500ºC.…”

Irradiation Effects on Precipitation in Reduced-Activation Ferritic/Martensitic Steels

“…This irradiation does not produce any transmutation-formed helium in F82H+2Ni, as the thermal neutrons are shielded by Eu 2 O 3 (Large amounts of helium can be formed from 58 Ni when thermal neutrons are not shielded. 7) ). Specimens selected for microstructure analyses were 1/3-size Charpy specimens that fractured in a brittle mode (less than 1.0 J) on the lower shelf near the DBTT.…”

“…[6][7][8] This method is very effective to find and analyze the detailed structure of precipitates, or to find minor or very fine precipitates in the matrix. On the other hand, analyses of extraction residue have the advantage of determining the major trend of precipitation from larger specimen volume.…”