Thermal-hydraulic analyses of steamline break accidents with reduced boric acid concentration in the safety injection system

In 1983 Virginia Power performed a series of analyses to justify elimination of the minimum boric acid concentration requirement for the Boron Injection Tank (BIT) at Surry Units 1 and 2, identical 2441 MWt 3-loop Westinghouse PWR's which have integral flow restrictors installed in the steam generator outlet nozzles. The BIT, a component of the Safety Injection System (SIS), originally contained 900 gallons of boric acid solution, a neutron poison, at a minimum concentration of 11.5 wt% (approximately 20,000 ppm). Upon Safety Injection, the BIT is discharged into the RCS, providing a large negative reactivity insertion. This minimum BIT boron concentration requirement historically led to a severe maintenance burden due to the corrosive nature of concentrated boric acid and the requirement for local heating of the acid to maintain solubility