What is the closest possible analogue to human cells?
What about Escherichia coli? Every undergraduate lab has easy access to plenty of E.coli culture methods and its stupid easy to culture. There are always HeLa cells as well if you really want the perfect analogue to human cells because, well, they are human cells. Other options would include:

a) HEK 293 cells (Human Embryonic Kidney 293 cells), which are nice for what you're looking at as they don't replicate out of control, but are still extremely easy to grow. However, they're generally used as hosts for gene expression and they're finicky when it comes to contamination and they don't really act like normal cells that would be hit by low level radiation (ie tissues that are closer to the skin's surface)

b) 3T3 cells which would be nice for you as they are contact inhibited cells, meaning they functionally keep growing into a layer one cell thick (a monolayer) which could be nice for you, especially if the low level rad you're testing is alpha emitting in which case you need a homogeneously thick cell layer.