Is chlorella a viable source of Vitamin B-12?

Short answer no.

Long answer: potentially

"Pratt & Johnson (15) (1968, USA) studied numerous batches of chlorella and occasionally found amounts of B12 analogue that were in the range of error for the test method. In other words, they were not able to detect practical amounts. They noted that their extraction processes might not have been adequate though they used many different methods. They also noted that their synthetic medium on which the chlorella was grown might have interfered with B12 analogue synthesis.

Chen and Jiang (17) (2008, Taiwan) used capillary electrophoresis to detect cyanocobalamin and hydroxocobalamin in chlorella. Capillary electrophoresis is a relatively new method that should be able to detect the exact structure of a cobalamin analogue. They found considerable amounts of cyanocobalamin in two samples of Chlorella, with negligible amounts of B12 analogues.

Until chlorella is more thoroughly tested on humans to determine whether it lowers MMA levels, it should not be considered a reliable source of vitamin B12, especially since the study by Pratt & Johnson (15) showed no vitamin B12. "

No, it's not. VEGAN registered dietitian Jack Norris says

"...there are no reliable unfortified plant sources of B12 including tempeh, seaweeds and organic produce."