N-16 in reactor coolant

[george.j.vargo@ccmail.pnl.gov (George J Vargo)]

Coming from a BWR that implemented hydrogen water chemistry, I had to get up to 
speed on the behavior of N-16 a few years ago.

In reactor coolant, N-16 exists in several chemical species and the equilibrium 
between these species is affected by several factors including pH and the 
presence of free radical scavengers such as ppt concentrations of Cu.  The 
relative volatility of these species determines the partition on N-16 in liquid 
and gaseous phases.  Since the PWR is closed cycle and mostly in a single phase,
shifts in these factors (e.g., pH) do not produce readily observable effects.

In the BWR, however, changes in pH can significantly change the fraction of N-16
carried into the gaseous phase (and cause big changes in radiation levels in 
main steam lines, turbines and moisture separator/reheaters).  Under normal 
water chemistry in the BWR most of the N-16 is in nitrate species that tend to 
remain dissolved in the liquid phase.  As H2 is added to the feedwater to 
suppress free oxygen in the reactor, there is a shift from an oxidizing to a 
reducing environment.  The equilibrium of N-16 is shifted toward ammonia species
that are more volatile and carry over into the gaseous phase more readily.

What's the net effect?  At H2 concentrations high enough to adequately protect 
the recirculation piping from IGSCC, radiation levels around the main steam 
system components can increase by a factor of 3 to 5.  Some BWRS that 
implemented HWC needed to add shielding around the turbine and raise the 
setpoint on the main steam high radiation scram in the reactor protection 
system.

The N-16 concentrations quoted (~100-120 uCi/g in coolant and ~50-60 uCi/g in 
steam) are reasonable for normal water chemistry conditions.  There have been a 
few attempts made at directly measuring N-16 concentrations at various points 
(Burns, et al. 1969).  For the most part the uncertainty of the measurements is 
within analytical estimates.

I don't know if this answers any of your questions, but it was fun thinking back
~12 years ago when we first did some of the hydrogen water chemistry tests at 
FitzPatrick.  Some of this was written up in a paper of mine on source term 
control that was published in the June 1991 issue of Health Physics.  For those 
of you with access to it there is some dark grey literature published by EPRI in
addition to the very dark grey reports by NSSS vendors.


George J. Vargo, Ph.D., CHP
International Nuclear Safety Program
Pacific Northwest National Laboratory
george.j.vargo@pnl.gov