In Fallout 4, you will often come in areas which are radioactive. To protect you from RAD you will need a Hazmat radiation protection suit.
To protect you with the Radiation Protection Suit against RAD, you have three options:
1 You can buy radiation protection suit.
2 You will find more of these suits during the "Mass Fusion" quest.
3 You can get one near the Hugo’s hole.
The easiest way is of course to buy radiation protection suit. Go to the Diamond City market and enter the Fallon's Basement. The owner of the clothes shop, Becky Fallon is selling you the suit for 300 bottle caps. With a correspondingly dismounted Charisma- attribute and the Perk "Cap Collector", you can reduce the price.
Radiation Protection Suit in Mass Fusion
As part of the Institute's Quest in skyscraper "Mass Fusion" you will enter a reactor chamber. You can find several radiation protection suits in the locker room or in one of the control rooms.
Radiation Protection Suit in Hugo’s hole
West of the Museum of Witchcraft you will find Dunwich Borer. Look for the excavator concrete blocks. To get to the Radiation Protection Suit, you must now jump on the concrete blocks with a little skill. Jump first on the leaning stone, from which you can easily jump on the excavator.
Climb onto the concrete block in the arm of the excavator and jump with a run-up to the concrete column to the left of you. Follow the column and in between blue fabric roofs jump down. You will find radiation protection suit on the table next to Hugo's bed.