user: Question: When did the construction of the unit Chinon B1 with net power of 905 MW start?
Columns: ['Unit', 'Type', 'Net power', 'Total power', 'Construction start', 'Construction finish', 'Commercial operation', 'Shut down']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Chinon A1', 'Magnox-Reactor', '70 MW', '80 MW', '01.02.1957', '14.06.1963', '01.02.1964', '16.04.1973']
SQL:

assistant: SELECT "Construction start" FROM "Table" WHERE "Net power" = '905 MW' AND "Unit" = 'Chinon B1';