There is no prebooking, please book on your own, via your favourite booking site. If you want to stay in downtown Paris you can also book via the Paris Tourist Office.
Note that there exist a number of accommodation possibilities in local hotels (see below), and that the journey to CEA from downtown Paris can take some time (see location and travel). For your convenience, if you stay in Paris downtown, select a hotel close to RER line B (Denfert-Rochereau, Port-Royal, Luxembourg, Saint-Michel...).
Paris downtown
There is a large number of offers for accommodation on the official website "Paris Tourism Office"
You should book a hotel nearby a connection with RER B-line. For reference Le Guichet is about 35 min by RER B from the station “Denfert-Rochereau” (Paris 14).
See maps on https://www.ratp.fr/plan-rer
Another useful link is the tourism information on http://en.parisinfo.com/
Closer to CEA
Hôtel Mercure Paris Massy Gare TGV
https://www.accorhotels.com/fr/hotel-1176-hotel-mercure-paris-massy-gare-tgv/index.shtml
RER B station: Massy-Palaiseau
21 Avenue Carnot,
91300 Massy
Hotel d'Orsay
https://www.orsay-hotel.com/?lang=en
RER B station: Le Guichet
5 minutes by walk from the station
2 rue François Leroux
91400 Orsay
Hotel Le Village
http://www.hotellevillage.fr/en/
RER B station: Gif-sur Yvette (from Paris, 4 stations after Le Guichet)
4 rue Neuve
91190 Gif-sur-Yvette
On the same plateau as CEA
These hotels are very close to Orme des Merisiers, meaning they are just a few bus stops away, and also you have to take a buss from Massy-Palaiseau or Le Guichet to reach them.
B&B HOTEL Saclay
https://www.hotel-bb.com/en/hotel/saclay
Plateau du Moulon N118
91190 Gif-sur-Yvette
+33(0)298 337 529
Hôtel Campanile Paris - Saclay
https://paris-saclay.campanile.com/fr-fr/
9 ter RUE JOLIOT CURIE
91192 GIF SUR YVETTE
