From the Brussels airport site:
Brussels Airport remains open.
We advise all passengers holding Sabena tickets, not to come to the airport. Passengers of other airlines are requested to check in as usual.
Flights handled by AviaPartner are not hampered. As far as flights of airlines handled by Sabena are concerned, the situation might be different. Please check our website or teletext.
We advise all passengers to come to the airport by train.