 Only emergency officials will be testing the alert system of the Salem-Hope Creek nuclear power plants tonight. DEMA will set off 37 sirens within 10 miles of the plants, which include Delaware City, Middletown, Odessa and Townsend. Residents hearing the siren, which will last three to five minutes, are then advised to tune to WDEL for a message in case of emergency. The sirens are tested four times a year.