Transport of Beryllium-7 in a Lithium Loop

A ⁷Be transport test was performed in the Beryllium-7 Experimental Lithium Loop with hot leg at 270°C and cold leg at 230°C. The “cold leg” test stringer was in a rising temperature region at 250°C. A total of 108 test coupons were included in the material compatibility and deposition test programs, containing AISI Types 304 and 304L stainless steels, Fe-2¼ Cr-1 Mo, pure iron, molybdenum, beryllium, zirconium, titanium, yttrium, three different aluminide coatings on Type 304 stainless steel substrates, and both tubular and flat butt-welds of Type 304 stainless steel. An average lithium velocity of 1.36 m/s was established during the test, which was terminated after 3718 h. The ⁷Be activity data indicated that the deposition of ⁷Be is a function of temperature. The cold leg positions displayed maximum ⁷Be deposition followed by the intermediate temperature locations (cold leg test stringer) and the hot leg locations. Two chemical forms of ⁷Be are expected in lithium, namely, unbonded ⁷Be at a low concentration, and a fine precipitate of ⁷Be₃N₂ at a high concentration. The deposit in the hot leg region is presumed to be primarily ⁷Be, while the cold leg deposit is mostly ⁷Be₃N₂. The cold leg specimens of a given material showed more ⁷Be deposition than the hot leg specimens of the same material, consistent with the ⁷Be activity data on the piping. Based on the present investigations and previous data, it is concluded that ⁷Be produced in the Fusion Materials Irradiation Test (FMIT) facility will have sufficient nitrogen in the lithium system to form ⁷Be₃N₂. It is expected that the majority of ⁷Be will be transported to the colder parts of the loop and deposited as ⁷Be3N2. However, a small amount of ⁷Be in the unbonded form will be in the hotter parts of the loop and is expected to diffuse into the steel piping. Thus, it is recommended that a cold trap be used to effectively remove most of the ⁷Be from FMIT lithium.