Containers for a fluid sample containing a radionuclide for measurement of radiation from the radionuclide include an outer shell having one or more ports between and interior and an exterior of the outer shell, and an inner shell secured to the outer shell. The inner shell includes a detector receptacle sized for at least partial insertion into the outer shell. The inner shell and outer shell together at least partially define a fluid sample space. The outer shell and inner shell are configured for maintaining an operating pressure within the fluid sample space of at least about 1000 psi. Systems for measuring radioactivity in a fluid include such a container and a radiation detector received at least partially within the detector receptacle. Methods of measuring radioactivity in a fluid sample include maintaining a pressure of a fluid sample within a Marinelli-type container at least at about 1000 psi.
 This invention was made with government support under Contract Number DE-AC07-05ID14517 awarded by the United States Department of Energy. The government has certain rights in the invention.