Can a Annulment be granted after the marriage received a Sanatio in Radice (Radical Sanation)? If yes, what justification is needed/valid?
A marriage may be declared null (granted an "annulment," although Canon Law does not use that term) after a sanatio in radice for the same reasons that any marriage is declared null: either because of impediments or because of a defect in consent. (See my answer to Is a Catholic annulment divorce in a Catholic way?.)
However, the process would be, so to speak, doubly difficult, because the invalidity of the marriage would have to be established both at the time of the exchange of consent (that is, at the wedding) and at the time of its convalidation (in this case, by means of a radical sanation).
(There is, of course, reason to suspect that the marriage was invalid until the time of its convalidation: that is why a radical sanation was sought. The case of nullity would probably have to rest on some other factor that the radical sanation did not take into consideration.)
A little background for the benefit of readers
If a Catholic couple discovers that there could be something rendering their marriage invalid (some impediment or a defect in consent), the Church offers various ways to regularize their situation. (These are found in the Code of Canon Law, can. 1156–1165.) Such a regularization is needed, because if the marriage is, in fact, invalid, the couple does not benefit from the graces that come from the Sacrament of Matrimony.
The simplest way is what is called a simple convalidation: the couple simply exchange their vows in the usual way before an authorized witness (usually a priest or deacon). It is effectively a new, albeit simple, wedding ceremony.
In some cases, however, a simple convalidation is not necessary, because the Church can take advantage of the consent already exchanged. In that case, the proper procedure to use is the sanatio in radice, which is essentially a decree on the part of the competent authority (the local ordinary) that all impediments to the marriage have been removed. Once the sanatio is given, the consent originally exchanged is sufficient for the marriage to be valid.