1. A fuel element for a nuclear reactor comprising an elongated sealed enclosure containing nuclear fuel, said enclosure including at one end a pressure-tight non-magnetic extension, an indicator pin within the enclosure and having a portion comprising ferromagnetic material extending within the said extension, said indicator pin being movable in the elongated direction of the enclosure from a first position to a second position, resilient means connected to the indicator pin for biasing the indicator pin for movement toward the second position, pressure differential means within the enclosure for maintaining the indicator pin in its first position against the force exerted by the resilient means, said pressure differential means being disabled from maintaining the indicator pin in its first position when an opening in the enclosure causes a pressure loss within the enclosure whereby the resilient means displaces the indicator pin to its second position indicative of failed fuel, the pressure-tight extension having a reduced width compared with that of the enclosure portion containing the nuclear fuel, and the ferromagnetic material portion being located entirely within the reduced-width extension of both the first and second positions.