When did the construction of the unit Chinon B1 with net power of 905 MW start?
CREATE TABLE "reactors" (
  "unit" text,
  "type" text,
  "net_power" text,
  "total_power" text,
  "construction_start" text,
  "construction_finish" text,
  "commercial_operation" text,
  "shut_down" text
);
SELECT "construction_start" FROM "reactors" WHERE "net_power"='905 MW' AND "unit"='Chinon B1';