CREATE TABLE table_17899 ("Unit" TEXT, "Type" TEXT, "Net power" TEXT, "Total power" TEXT, "Construction start" TEXT, "Construction finish" TEXT, "Commercial operation" TEXT, "Shut down" TEXT)

When did the construction of the unit Chinon B1 with net power of 905 MW start?
SELECT "Construction start" FROM table_17899 WHERE "Net power" = '905 MW' AND "Unit" = 'Chinon B1'
SELECT "Construction start" FROM "table_17899" WHERE "Net power" = '905 MW' AND "Unit" = 'Chinon B1'