There seems to be a sentence missing in the question. If the question is as stated, whether what the Ran writes is correct, then the answer is that the words of the Rishonim are always accepted as the truth, as they were great and holy sages who understood the Torah with a profound depth that we cannot begin to fathom.

The Ran is saying that the immersion per se is not a Mitzvah, but rather it is preparatory for other Mitzvos (and to avoid transgressions). In theory, therefore, if a woman does not want to eat Terumah, and she does not want to become permitted to her husband, she does not have to immerse herself and she may remain Tamei, as there is no Mitzvah per se to immerse herself. (Of course, such a decision would qualify as grounds for the husband to divorce his wife.)

Certainly, however, the procedure of immersion *is* Biblical. The Torah says that she must immerse if she wants to eat Terumah, and if she wants to live with her husband. The Torah just does not *command* her to do so. The Torah does prohibit her, of course, from living with her husband and from eating Terumah if she does not immerse herself.

D. Zupnik

DZ:ys