Sentence:  The <WSD>caravans</WSD> were the demarcation between the non-radioactive areas and the radioactive areas. There were two main <WSD>caravans</WSD>, one for people going into the forward area, and the other <WSD>caravan</WSD> was for people returning. 
Meaning of the word marked with <WSD> tags:  A furnished vehicle towed behind a car, etc., and used as a dwelling when stationary.