Why can't BWRs use dissolved boron to control reactivity?

• A. The moderator boils, causing boron to plate out on internals
• B. Boron would react with steam
• C. Boron would escape from core
• D. Boron would become radioactive

Answer: (A)

In a Boiling Water Reactor, the coolant in the core is boiling and exists as a two-phase mixture. Soluble boron relies on being uniformly distributed in the liquid to absorb neutrons and control reactivity. When the moderator boils, boron tends to deposit (plate out) on the internal surfaces and its distribution becomes highly nonuniform, plus some boron can be carried away by steam. This makes the reactivity control ineffective and unpredictable, which is why dissolved boron isn’t used for control in BWRs.

The other ideas aren’t the primary issue: boron doesn’t react with steam in any meaningful way to drive control; while some boron could leave with the steam, that isn’t the fundamental reason it’s not used. Boron-10 neutron capture doesn’t make boron radioactive in a way that would provide useful control, so that option isn’t correct either.