early on 17 march, tepco requested another attempt by the military to put water on the reactor using a helicopter and four helicopter drops of seawater took place around 10:00 jst. the riot police used a water cannon to spray water onto the top of the reactor building and then were replaced by members of the sdf with spray vehicles. on 18 march, a crew of firemen took over the task with six fire engines each spraying 6 tons of water in 40 minutes. 30 further hyper rescue vehicles were involved in spraying operations. spraying continued each day to 23 march because of concerns the explosion in unit 3 may have damaged the pool (total 3,742 tonnes of water sprayed up to 22 march) with changing crews to minimise radiation exposure. lighting in the control room was restored on 22 march after a connection was made to a new grid power supply, and by 24 march it was possible to add 35 tonnes of seawater to the spent fuel pool using the cooling and purification system. on 21 march grey smoke was reported to be rising from the southeast corner of unit 3 – where the spent fuel pool is located. workers were evacuated from the area. tepco claimed no significant change in radiation levels, and the smoke subsided later the same day.