The script misstates the properties of Cesium-138, possibly deliberately, to prevent information about a hazardous material from being disseminated publicly. First of all, Cesium, regardless of its isotope, has a uniform melting temperature of 28.5 °C (83.3 °F), not 21.1 °C (70 °F) as stated by Barney during the briefing. It will burn, but not explode, when exposed to oxygen or air. It will, however, explode in water. Either reaction occurs whether it is in solid or liquid form, meaning that the requirement for keeping the metal cool is a plot hole as well.
Cesium 138 is a valid isotope, but is complete artificially produced. I has a half-life of just over 1/2 hour. 99% of the stock would decay in 2-1/2 hours. By the time the team arrives, even traces would be impossible (even for the IMF team) to locate.
Although most isotopes of Cesium are radioactive to some degree (Cesium-133 is stable), none of the isotopes have any weapons applications. It is used as a radiation source in laboratories, in the petroleum industry, electronic devices and, perhaps most famously, in atomic clocks (the oscillation period of the cesium-133 atom is now used to define the standard second). Cesium clocks also regulate timing for cell phone networks and internet data flow.
Cesium does not have a cesium-138 isotope.
No isotope of caesium could fuel a nuclear weapon, and would not be an alternative to fissile materials U-235 and Pu-239 in a "cheap nuclear arsenal." Because of the nucleon pairing phenomenon, all fissile materials have even atomic numbers. Caesium's atomic number is odd (55).
When Cinnamon blocks the path of the motorized unit that is being demonstrated in Briggs' apartment with her leg ,you can briefly see the fishing wire used to make the unit turn to the left.
Barney's back is shown to be scarred by flogging in a earlier scene but when he is sent back to a cell later the back is normal looking.
Dan reaches through the prison door grate and unlocks the bolt from the inside. No prison door would be designed this way to allow for such an easy escape.
Dan attaches an adhesive paper to the window he is about to shatter to prevent the glass from scattering that just happens to perfectly match the size and shape of the window.