Orme des Merisiers
CEA Saclay is around 23 km South of Paris. The astrophysics division (SAp) is located at the CEA site at Orme des Merisiers, which is around 1 km South of the main CEA campus.
See this page for hotels near Saclay. It is advisable to get accommodation along the RER B line (e.g. Gif-sur-Yvette, Orsay, Massy), or in Paris on the South bank (most convenient is 14th arrondissement).
By public transport
You can reach the SAp by subway train (RER) and bus, from Paris downtown (45 min travel time) and the airports Charles-de-Gaulle (1h30) and Orly (1h). Check this page for directions and time-tables.
Take the RER B towards Saint Rémy les Chevreuses until the stop Le Guichet. (Note that not all trains stop at Le Guichet, as indicated by the lights on the panel at every station.) From Le Guichet, there are public buses to take you to Orme des Merisiers.
To reach the bus stop, exit the RER station at exit #2, rue de Versailles. Pass by the café La Terasse, cross the road towards the day-care and go down a little path (there are signs "Gare routière") until you reach another road. The public bus stand is on the opposite side of the road from where the coach leaves. The line 9 goes every 15-20 minutes and takes about 10 minutes to reach Orme des Merisiers.
Another possibility is to take the RER B to Massy-Palaiseau and from there take the bus 91.06B, 91.06C, or 91.10 (Make sure the bus goes "via Polytechnique"). One bus of any of those lines leaves every 5-10 minutes and take 20 minutes to Orme des Merisiers.
After you exit the bus at the stop Orme des Merisiers, keep walking along the direction of the bus, and take the first road to the left to enter the campus. Go straight through the roundabout, and take the second road to your left (after ~ 50m). The SAp is the reddish building across parking lot, building number 709 (see map).
Getting to Paris
Paris has two airports, Orly [ORY] and Charles-de-Gaulle [CDG]. From both airports, you can easily reach Saclay by public transport. From Orly it's about 1h, from CDG 1:30h.
By train you will most likely arrive at one of the train stations in Paris downtown. From there you can take the metro to get to the RER B which takes you towards Saclay. Go to vianavigo.fr to find how to reach the nearest RER B station.