MSIV failed definition
MSIV failed line means that the inboard MSIV in one of the shortest MSL fails to close and remains open during the accident, which instantly extends the well mixed volume boundary from the reactor pressure vessel nozzle to the outboard MSIV. This MSIV failure complies with a single active component failure requirement that results in the most limiting radiological consequences (RG 1.183 Rev. 0, Section 5.1.2). All MSLs in the MSIV leakage release pathways are seismically designed and supported to withstand the Safe Shutdown Earthquake (SSE) and thereby comply with the RG 1.183, Appendix A, Section 6.5 requirement. As the MSLs remain intact, the horizontal pipe surface area is credited for the aerosol deposition and volume for dilution in the MSL upstream of the inboard MSIV that failed to close. All four MSL headers are Seismic Class I and QA Category I from the reactor pressure vessel nozzle to the seismic boundary break at the turbine stop valve. Therefore, they are qualified to withstand the SSE which complies with the RG 1.183, Appendix A, Section 6.5 requirement to be credited for aerosol deposition. Deposition credit in the outboard section of the MSL (between the outboard MSIV and the turbine stop valve) is determined in the same manner as the inboard section.