Will gamma radiation cause "thermal" decomposition of sodium bicarbonate?
There is an equilibrium between decomposition and recombination, i.e. as $\ce{CO2}$ is given off, it joins back to the  $\ce{Na2CO3}$ to form $\ce{NaHCO3}$. 


  $\ce{2NaHCO3 &lt;--&gt;Na2CO3 + CO2}$


So for moderate doses, given at moderate rates, it's unlikely great pressure would develop, but you'd need to test to see if the drug is degraded by decomposition products.