March 2011 – NRC INFORMATION NOTICE NO. 88-92: POTENTIAL FOR SPENT FUEL POOL DRAINDOWN

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Purpose:

This information notice is being provided to alert addressees to potential problems resulting from the failure of the fuel transfer canal door seal.

It Is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar
problems.

However, suggestions contained in this information notice do not constitute NRC requirements; therefore, no specific action or written response
is required.

Description of Circumstances:

On October 2, 1988, with Surry Unit 1 in cold shutdown, the licensee was preparing to test the fuel transfer system (see attached figure) before fuel
off-load. The transfer canal door was in place and the single door seal was inflated. The fuel transfer canal was dry. The fuel transfer tube was open,
the blind flange was removed on the containment side, and the gate valve was open on the spent fuel pool side. The refueling cavity seal was not in place.

An accidental pinhole puncture of the single air supply line to the transfer canal door pneumatic seal was promptly detected and the air leak quickly
stopped before it could lead to a loss of seal integrity.

Discussion:

A review of this event by the licensee showed that, given the configuration of the transfer canal, the transfer tube, and the refueling cavity existing at the time of the event, an inadvertent draindown of the spent fuel pool could occur to a height of only 13″ above the top of the fuel assemblies (see attached figure). This postulated draindown assumes no operator action and a loss of instrument air or pneumatic seal failure.

Increased radiation levels in the spent fuel pool building would have limited stay time in the building and impeded recovery. The licensee estimated that the dose rate, based on the spent fuel inventory at the time of the event, could have reached 50 R/hour on the operating deck. The licensee also calculated that with the refueling cavity seal assembly in place, the spent fuel pool could only draindown to 14 feet above the top of the fuel assemblies.

If newly discharged fuel had been placed in the spent fuel pool, a postulated draindown of the spent fuel pool could have led to even higher radiation levels in the spent fuel pool building than the radiation levels postulated by the licensee. If a seal failed and spent fuel pool water were lost while a fuel assembly was lifted, fuel could be uncovered and fuel cladding could fail.

The licensee is considering several actions based on the review of this event. The short-term actions include: (1) revising procedures to require that the refueling cavity seal assembly be installed before opening the transfer tube gate valve for dry testing the fuel transfer system, (2) reviewing and upgrading a procedure for the loss of spent fuel pool inventory, (3) upgrading the material of the plastic air hose in which the pinhole occurrede and (4) providing an emergency escape system for personnel in the transfer canal. The long-term actions include: (1) evaluating the canal door and seal design including the need for a backup air supply, a low air pressure alarm, and a backup seal, (2) evaluating other pneumatic seals used at Surry, (3) developing a procedure for installing, inspecting, and testing the seals in accordance with the manufacturer’s recommendations, (4) revising procedures to ensure the transfer tube blind flange is installed whenever the transfer canal is drained for maintenance on the transfer tube gate valve, and (5) reviewing further the procedures for loss of spent fuel pool and refueling cavity water level, after the short-term modifications to these procedures.

In 1981, a related event occurred at Arkansas Nuclear One, Unit 2. At the end of a refueling outage with the transfer canal door closed and the door
seal inflated, the transfer canal was drained in preparation for performing maintenance on the fuel upender. Concurrent maintenance on the air system
resulted in a loss of air pressure to the seal, and water leaked from the spent fuel pool into the fuel upender pit. The transfer gate valve was closed
and acted as a barrier to the flow. The leakage stopped when the water levels equalized. The spent fuel pool level had decreased by 7 feet. If the seal
had leaked while the maintenance on the upender was in progress with the fuel transfer tube gate valve open and the fuel transfer tube blind flange removed, the spent fuel pool could have drained down to a level just above the top of the fuel assemblies.

Pneumatic seals are also used in the refueling cavity seal assembly. In many cases, the failure modes of the refueling cavity seals (IE Bulletin 84-03,
“Refueling Cavity Water Sealu and Information Notice 84-93, “Potential for Loss of Water From the Refueling Cavity”) apply to the spent fuel pool gate
seals.

These events show that a door equipped. with a single seal and/or a single air supply is subject to complete loss of function from a single failure. Therefore, the seal must be properly installed, and the seal and air supply system must be carefully maintained. The scenarios discussed also point out the need for carefully coordinating such activities as maintenance and testing to avoid undesirable interactions between systems which could result in fuel uncovery, personnel injury, and personnel contamination.

On September 23, 1991, Wolf Creek Generating Station (WCGS) was in cold shutdown in preparation for refueling when it experienced a .SFP draindown. The
gate between the SFP and the fuel transfer canal was in place with the dual boot seals inflated (Figure 1). The fuel transfer canal was partially filled (about half full) with borated water and the fuel transfer tube which connects to the refueling cavity was closed. The air supply for the SFP gate seals comes from the nonsafety-related service air system. The event was initiated by the loss of a nonsafety-related electrical bus which caused the service air to isolate from its source. The loss of this bus also caused a loss of the instrumentation that provided SFP level indication. The SFP gate seals subsequently depressurized through leaks in the service air system (Figure 2). The leaks occurred at isolation valve packings, check valves, and at “Chicago” quick-connect fittings that were not in accordance with design drawings.

To mitigate the risk of the second scenario, the licensee committed to either have all three fuel building cavities i.e. the SFP, the cask loading pool, and the fuel transfer canal, full or to notify management and establish other compensatory measures when performing fuel inspections or reconstitutions.

Potential for Spent Fuel Pool Draindown – Pages From c142449-02g-3


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