Exposition of the Blessed Sacrament is only permitted during Mass when it takes place in a chapel that is completely sealed off from the body of the church and where some of the faithful remain in adoration.
The celebration of Mass is prohibited within the body of the church during exposition of the Blessed Sacrament . . . If exposition of the Blessed Sacrament goes on for a day or for several successive days, it should be interrupted during the celebration of Mass, unless it is celebrated in a chapel separate from the area of exposition and at least some of the faithful remain in adoration . . . (Eucharisticum Mysterium: Instructions on Worship of the Eucharistic Mystery 61).