With a special mass and a rare, if not unprecedented press conference, six Poor Clare nuns formally entered a cloister in Fairfax County yesterday committing themselves to a lifetime or poverty, abstinence, celibacy and almost total silence.

The nuns were transferred here to be cloistered from Roswell. N.M., where the Poor Clare Order gained 25 new adherents in 10 years and outgrew its farmhouse styled monastery. There are between 450 and 500 Poor Clare sisters in the United States and their numbers are increasing.

Temporarily secluded in a section of the convent of St. Louis Parish in Alexandria while their spartan monastary is being constructed on Stonehedge Drive. the

"People think we come in and lock the door on the world," said Mother Mary Francis, abbess of the Roswell Poor Clare Order. "but it is for the world that we pray and do penance."

Sister Mary Maragret, one of the six nun formally cloistered yesterday, said that she once dreamed of being a doctor. But when she was 16, she decided that "there were too many needs in the world: (and) this (entering the cloister to pray) would be one way to reach them all."

The nuns are temporarily secluded in a section of the convent of the St. Louis Parish south of Alexandria while waiting for their new monastery to be completed nearby on Stonehedge Drive.

In the year before the new monastery is complete, their cloister will be somewhat makeshift as they nevertheless maintain the Poor Clare way of life begun in the 13th century under St. Francis of Assist.

Work for the Poor Clares will be interspersed among seven daily prayer periods, including one from 12:30 a.m. to 2 a.m. The sisters rise at 5:30 a.m. and retire at 8 p.m. They support themselves, with the help of donations by gardening and making craft items to be sold by volunteers in the monastery.

The sisters eat no meat and supplement their diet with donated seafood or egg products. Their primary meal is at noon, when they eat potatoes, soup and a vegetable. Sweets donated to them by supporters are saved for special feast days.

The new monastery will be simple - no carpet, nonesential furniture or decorative items. Meals will be served at special counters where the sisters can stand and eat.

The nuns speak to one another only during a daily recreation period. They wear no shoes and sleep on straw mattresses.

The contemplative life means separation from the world, but persons seeking advice from the sisters will be able to visit them at the convent (and later in the monastery) and talk through a large screened partition. Articles can be passed from one side to the other by means of a device similar to a Lazy Susan.

Sister Margaret Mary will be less cloistered than the other nuns and is known as an extern sister. She will take such duties as answering the door and telephone.

By special permission of Mother Mary Francis, the Poor Clare sisters of Fairfax discussed their life with the press yesterday - an event the abbess thought was unpecedented.

The sisters helped explain why a girl graduating from high school or college - "full of dreams of being a doctor, teacher or ballerina" - would choose a life of prayer, fasting and abstinence.

Sister Clare said she thought "one attraction was living in a community and praying in a community, doing what we want to when we all want to do it."

For Sister Clare, the call to the monastery came when she was reading an article about the Roswell cloister "completing a little wing" and being ready for a few vocations. "It appealed to me because I knew I would be poor." she said.