Is it permissible for a person in a state of ritual impurity (junub) to enter a bathtub full of water and sealed with a stopper, and what is the ruling on that or its expiation?

What you mentioned, that a person in a state of janabah (major ritual impurity) should not immerse himself in a full, enclosed basin of water, is correct; due to the Prophet’s (peace be upon him) prohibition of a person in a state of janabah bathing in standing water. Scholars have differed regarding the reason for this prohibition. Some said that the water becomes musta’mal (used) and loses its purifying quality, while others said that repeated bathing contaminates it.

The majority of scholars restricted the prohibition to water that is less than two qullatayn (a specific volume). As for abundant water exceeding two qullatayn, they hold that the immersion of a person in a state of janabah does not harm it. The hadath (ritual impurity) of the person in janabah is removed if he immerses himself in standing water, and this is the more sound of the two scholarly opinions.

Therefore, it is not permissible to perform ghusl (ritual bath) from janabah in the aforementioned basin due to the Prophet’s (peace be upon him) prohibition of it. There is no expiation for someone who immerses himself in standing water; rather, he must repent and seek forgiveness if we consider the bathing to be forbidden.

However, if we interpret the prohibition as indicating karahah (dislike) and not tahreem (prohibition), then there is no sin upon him; rather, he should refrain from this action. Many scholars hold that the prohibition indicates dislike, not prohibition.