ML20206M593

Informs of Initiation of Investigation of RWCU Design & Isolation Provisions,Due to Recent Events at BWR Plants Re ESF Actuations Caused by RWCU-generated Isolation Signals

ML20206M593
Person / Time
Site:	Millstone
Issue date:	08/15/1986
From:	Thomasson N NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD)
To:	Rubin S NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD)
References
NUDOCS 8608210267
Download: ML20206M593 (2)

Text

Pg August 15, 1986 MEMORANDUM FOR: Stuart D. Rubin, Chief Reactor Analysis Branch Office for Analysis and Evaluation of Operational Data FROM: Neill Thomasson, Senior Reactor Systems Engineer Reactor Analysis Branch Office for Analysis and Evaluation of Operational Data

SUBJECT:

MILLSTONE 1 RWCU INCIDENT OF JULY 23, 1986 In response to Fred Hebdon's inquiry of July 24, 1986, I have investigated the Millstone 1 reactor water cleanup system (RWCU) isolation signals and those provided in more recent vintage BWRs. Fred questioned whether the RWCU system should automatically isolate in the event of a pipe break, as occurred at Millstone 1. The Millstone 1 event involved a severance of a 1" pipe on a relief valve line from a regenerative heat exchanger. Over 2,200 gallons of reactor coolant was discharged before the reactor operator manually isolated the system based on a high RWCU room temperdttre signal in the control room and visual confirmation of a leak by an auxiliary operator.

Based on information from a Millstone 1 resident inspector, Millstone does not have automatic isolation of the RWCU system by a high flow or differential flow condition; it does isolate on (1) high temperature or high pressure in the coolant downstream of the non-regenerative heat exchanger, (2) low flow to the tilter /denineralizer, (3) high temperature to the auxiliary pump seal, (4) low reactor water level and (5) initiation of the standby liquid control system. These signals, however, do not relate to leakage conditions associated with RWCU system integrity.

According to a Georgia Power Engineer (Hatch Plant), who previously was with GE, Hatch and most other more recent BWRs have an automatic RWCU system isolation by a high differential flow signal. In the Hatch design the flow in the reactor coolant letdown is compared to the flows in the return line to the reactor and the flow to the condenser and the radwaste system downstream of the filter /demineralizer unit. At Hatch, if the flow difference is greater than 79 gpm for more than 45 seconds, automatic isolation occurs by closing the isolation valves on the letdown line. The time delay in the Hatch isolation signal, as well as that provided by the high ambient room temperature isolation signal, is related to allowing operator action to avoid unnecessary isolations.

The other relevant isolation signal at Hatch is high room temperature. This isolation signal is included to provide isolation for leakage that is below technical specification limits, f6002to267 060013 hlDRADOCK 05000245 PDR

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if In the Millstone event, because of the small size of the failed pipe, high differential flows did not occur. Without an automatic isolation by high differential flow in the RWCU system, public health and safety would be provided by isolation of the ventilation system for the reactor building by vent radiation monitors. It is noteworthy that the design flow of roughly 180 gpm in the RWCU system is a small fraction of the reactor coolant makeup capacity and normal coolant flow. Therefore, a break in the RWCU system does not constitute even a sr:11 break LOCA within the context of the FSAR safety analyses. If enough reactor coolant were lost, ultimately isolation would occur due to low reactor water level before the reactor fuel would be uncovered.

Therefore, automatic isolation by high flow is not obviously essential.

During our inquiry into this event, other related recent events were identified at Dresden 2 and Quad Cities. Consequently, as a result of these and numerous recent LERs related to engineered safety feature actuation due to the RWCU generated isolation signals, an investigation of the RWCU design and its isolation provisions has been initiated.

Neill Thomasson, Senior Reactor Systems Engineer Reactor Analysis Branch Office for Analysis and Evaluation of Operational Data DISTRIBUTION:

C RA0B C/F NThomasson SRubin FHebdon d CJHeltemes 4

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DATE :8//S/86  : 8/'F/86  :  :  :  :  :

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