Nitrogen-16 Generation and Transport and Associated Shielding Requirements in a Supercritical-Water-Cooled Reactor

As a water-cooled nuclear system with a direct thermal cycle, the supercritical-water-cooled reactor (SCWR) shares with the boiling water reactor (BWR) the issue of coolant activation and transport of the coolant activation products to the turbine and balance of plant (BOP). Consistent with the BWR experience, the dominant nuclide contributing to the SCWR coolant radioactivity at full power is ¹⁶N, which is produced by an (n,p) reaction on ¹⁶O. In this paper the production and decay of ¹⁶N in the SCWR coolant circuit along with the shielding requirements imposed on the BOP are analyzed and compared with those in a BWR with a similar thermal power rating. A simple control-mass approach is adopted in which the ¹⁶N inventory in a unit mass of coolant is tracked as the coolant flows in the SCWR and BWR primary systems, which are divided into several compartments (e.g., core, lower plenum, downcomer, etc.) of known volume, mass flow rate, and neutron flux. The values of the neutron flux and (n,p) cross section in the SCWR and BWR cores are calculated by means of full-length radially reflected Monte Carlo eigenvalue models of the SCWR and BWR fuel assemblies. The results are as follows: The ¹⁶N activities in the steam lines of the BWR with normal water chemistry, in the BWR with hydrogen water chemistry, and in the SCWR are about 40, 180, and 380 Ci/g, respectively. The calculated BWR values compare well with the trends and ranges found in the literature. The SCWR ¹⁶N concentration is significantly higher than that in the BWR for the following four reasons:

1. The coolant transit time in the SCWR core is about twice that in the BWR core.
2. The neutron flux is higher in the SCWR because of the higher power density.
3. The slow coolant pass in the water rods produces a significant ¹⁶N activity at the SCWR core inlet.
4. In the SCWR all the ¹⁶N generated in the core is sent to the steam lines because there is no recirculation within the vessel.

A simple gamma attenuation model shows that the higher ¹⁶N activity in the SCWR results in shielding requirements only up to 10% higher than for the BWR with hydrogen water chemistry. However, because of the higher SCWR electric power, the specific shielding costs per unit of electric power associated with ¹⁶N are expected to be similar to or better than that for BWRs.