Most of these advantages result from the fact that an increased concentration of the fissile isotope uranium-235 makes it easier to build reactors with a high neutron output or a high neutron flux, which are useful in research or isotope production applications. In naval reactors, the use of HEU facilitates longer service life of a reactor core and may help achieve better reactor performance. Since, until recently, the availability or cost of HEU was rarely a significant constraint, most applications moved toward using uranium with higher enrichments whenever it was practical.