NRC Event Number 52744 – Event Date 05-10-2017 APPENDIX J LOCAL LEAKAGE EXCEEDED ACCEPTANCE CRITERIA "On Wednesday May 10, 2017, at 1411 EDT, with the reactor at 0 percent core thermal power (CTP), Pilgrim Nuclear Power Station (PNPS) was in a Refueling Outage, performing a review of Local Leak Rate Testing results, when it was concluded that PNPS had exceeded its Title 10 Code of Federal Regulations Part 50, Appendix J, Option B, Type B and C Local Leak Rate Test (LLRT) leakage criteria. "Previously, on April 22, 2017, when PNPS was performing LLRT of the High Pressure Coolant Injection (HPCI) steam exhaust line check valves, both valves failed to meet their LLRT acceptance criteria specified in plant procedures. Neither of the check valves seated acceptably. Based on the ongoing evaluation of these test exceedances, it was concluded that these test results cause the plant to exceed the overall as-found minimum path Appendix J acceptance criteria of 0.6 La (126.3 SLM [Standard Liters per Minute]). "Further investigation is ongoing. "This event has no impact on the health and safety of the public. "The licensee has notified the NRC Senior Resident Inspector. "This notification is being made in accordance with 10 CFR 50.72(b)(3)(ii)(A), any event or condition that resulted in the condition of the nuclear power plant, including its principle safety barriers, being seriously degraded. "The licensee will notify the Commonwealth of Massachusetts Emergency Management Agency."

The High Pressure Coolant Injection (HPCI) system is one of the Emergency Core Cooling Systems (ECCS) in Boiling Water Reactors (BWR). HPCI uses steam produced by the reactor core’s decay heat to spin a turbine connected to a pump that transfers makeup water from the suppression pool (or Condensate Storage Tank) to the reactor vessel.

Pilgrim has a Mark I containment design Pilgrim has a Mark I containment design. It’s primary containment consists of the drywell (inverted lightbulb shaped) and the wetwell or suppression chamber (donut shaped). Most of the HPCI equipment is located outside primary containment inside secondary containment, also called the reactor building.

The primary and secondary containments are designed to minimize the amount of radioactivity that is released into the environment during an accident. Primary containment is supposed to contain most of the radioactivity. Secondary containment is designed to collect leakage from primary containment and minimize how much then escapes to the environment. HPCI Room

To help primary containment hold up most of the radioactivity, all penetrations through primary containment walls have isolation devices (e.g., valves, doors, dampers, etc.) that restrict the flow of radioactivity contaminated air and water through the walls.

(The main steam lines each have two isolation valves not shown on this drawing.) For increased reliability, each penetration through primary containment walls has two isolation devices in case one fails to close or closes but leaks excessively.

This event involved the isolation devices in the pipe that carries steam leaving the HPCI turbine to the suppression pool. The steam is discharged from the pipe below the surface of the water in the suppression pool. The steam gets cooled down and converted back into water.

For this HPCI system exhaust line, the isolation devices installed were check valves. Check valves permit flow in one direction (HPCI turbine to suppression pool in this case) and automatically block flow in the reverse direction (suppression pool to HPCI turbine).

A check valve has a metal plate or disc attached to a hinge pin. Flow in the proper direction swings the disc on the hinge pin open to allow passage. Flow in the opposite direction swings the disc on the hinge pin closed to block flow.

Federal regulations, specifically Appendix J to 10 CFR Part 50, require that containment isolation devices be periodically tested for leakage. Some leakage is permitted. But excessive leakage could mean that more radioactivity gets into and then out of secondary containment than allowed during an accident.

Both check valves in the HPCI steam line failed their individual leak rate tests. Because both isolation devices failed, a pathway was created for radioactivity to get outside primary containment during an accident in larger amounts than assumed in the safety studies of radiation exposure to workers and the public.