This is a Late reply I know... but I wanted to directly give you and any subsequent readers with similar questions, the proper answer.
NBC Suits are designed to protect against exposure to contaminants. These are designed to prevent liquid contaminants from skin contact, airborne gasses, liquid aerosols, and physical particulate aerosols (think airborne dust) contaminants from being inhaled, and lastly provide a protective barrier from contact against any dangerous physical material that must be handled.
The term NBC suit is more commonly used with regard to Military issue equipment, the term Hazmat Suit tends to be used for civilian equipment. Regardless both are considered standard Personal Protective Equipment with regards to a wide range of environmental hazards from simple chemical protection against acid or other highly toxic vapors to biological protection when dealing with medical waste, and the other major use is as protection against radiological & nuclear hazards. They come in grades for which there are 2 major standards the 4 grade US version and the 6 grade EU version.
Now the thing about the level of protection these suits can offer (not all types will protect from radiation) is in 2 areas, hazmat gear designed specifically to deal with nuclear/radioactive material will likely have some additional layers of shielding which will protect against alpha (if its thicker than paper it shields against alpha), likely protects from beta (tinfoil does a decent job), and may offer some reduction in exposure to lower energy gamma radiation, but this is usually not considered the primary benefit of wearing such gear in a 'hot zone' since skin exposure to alpha and beta radiation are stopped with minimal effort. The biggest reason to wear full hazmat gear which includes its own air supply, (US level A or B) is that there are a number of substances that are radioactive gasses which emit alpha and beta radiation, which if you inhaled them, would be irradiating (in the can cause cancer fashion) AND burning your lungs from the inside due to a type of radiation burns typically called 'beta burns' and they are a lot the kind of serious burns received from heat/fire.
Airborne radioactive gasses & particles, be they already in the air, or more commonly due to radiation induced embrittlement accelerating the creation of fine dust like particles they can be coating surfaces and when disturbed are dispersed into the air surrounding the person that disturbed the material.
By using this kind of protective clothing, they arent exposing themselves to surface and internal burns from radioactive gasses & airborne particulates. Which given the fact it takes 3 inches of lead to stop a gamma ray on average, means that you are providing the workers with as much protection as possible while still letting them do their work.
An area can be hazardously radioactive yet not have sufficient gamma radiation being produced by the contaminants to make humans working there impossible. These suits can block 2 of the 3 types of hazardous ionizing radiation, the ratio of alpha, beta and gamma radiation will depend on the isotopes contaminating the area, and so if the radioactive contamination isn't producing too much gamma radiation then these suits can make working in the area safe enough for people to work.