CREATE TABLE table_12983929_1 (construction_start VARCHAR, net_power VARCHAR, unit VARCHAR)

When did the construction of the unit Chinon B1 with net power of 905 MW start?
SELECT construction_start FROM table_12983929_1 WHERE net_power = "905 MW" AND unit = "Chinon B1"
SELECT "construction_start" FROM "table_12983929_1" WHERE "905 MW" = "net_power" AND "Chinon B1" = "unit"