Production and Release of Positron Emitters from Water-Moderated Power Reactors

A significant amount of radioactivity is pro-duced via secondary nuclear reactions in the water which circulates through the core of a power reactor. The most important reactions are those which produce positron emitters, namely ¹⁶(p,α)¹³N and ¹¹B(a,α)¹³N which are endoergic, and ¹⁸O(p,n)¹⁸F which is exoergic. The production of positron-emitting nuclides is of particular significance in the case of boiling water reactors. In such reactors, noncondensibles and steam, which are continually vented from the condenser, may contain appreciable amounts of ¹³N. The production rates for ¹⁸N and ¹⁸F have been calculated in the case of a 3250-MW(th) reactor using a simple model for the energy dependence of the neutron and proton fluxes and literature values of the cross sections. The resultant production rates are 2.39 × 10¹² at./sec for ¹³N, and 7.65 × 10¹¹ at./sec for ¹⁸F. These productions are in good agreement with measured values of the production rates when the latter are scaled up to 3250 MW(th). Nitrogen-13 release rates scale to be ∼3 mCi/sec, whereas the calculated production rate corresponds to 75 mCi/sec. The difference is probably due to the efficient removal of ¹³N by the anion exchanges.