ML20010E622
| ML20010E622 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 08/28/1981 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20010E620 | List: |
| References | |
| NUDOCS 8109080062 | |
| Download: ML20010E622 (2) | |
Text
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SAFETY EVALUATI0ft BY THE OFFICE OF NUCLEAR REACTOR REGULATION SUPPORTIllG THE ACTION TAKEN T0 l10DIFY THE CONTROLS FOR THE STEAM DUMP VALVES METROPOLITAN EDIS0N COMPANY JERSEY CENTRAL POWER AND LIGHT C0f1PANY PEllH5YLVAfilA ELECTRIC COMPANY THREE MILE ISLAND NUCLEAR STATI0fi,'UtlIT NO.1 DOCKET NO. 50-289 Introduction Our review of the Crystal River Unit No'. 3 (CR-3) event of February 26, 1980, revealed that failure of the non-nuclear instrumentation (fiNI) or the integrated control system (ICS) power supply can result in spuriously opening the atmos-pheric steam dump valves to a 50% position. Such an anomaly could significantly aggravate an overcooling transient of the primary vstem.
By letter dated Augus t 15, 1980 we informed Met Ed (the licensee) of our concern that on TMI-l upon loss of ICS power supply, the ADVs automatically open to a 50% position, a similar anomaly to the CR-3 occurrence.
In response to our concerns, the licensee by letter dated January 21,1981 (TLL 671) discussed his plant design and proposed modifications to avoid this anomaly.
Discussion & Evaluation _
The licensee's review of his design confirms that upon loss of ICS " Hand Power" the ADVs will open to the midscale positon. To eliminate this situation, the licensee proposed to install a manual control channel for the ADVs which will automatically switch in upon loss of ICS " Hand Power". The manual control channel consists of a " manual loader", a voltage-to-current (E/I) converter, and a lock-out loss-of-voltage relay.
The " manual loader" contains a thumb wheel for the operator to select any position from 0 to 100% open and a meter indicating the, output contrci signal to the ADVs. Administrative controls will require that the manual loader be set to 0% open. Thus, when ICS " Hand Power" is lost, the manual control will switch in automatically and it will he pre-selected to keep the ADVs it' the closed position. The manual loader receives its +24 VDC input power, from an ex! sting 1
DC power supply and the DC power source originates from the 120 VAC " Computer-l Bus".
e The selected voltage-to-current converter receives a it, to +10 VDC signal from the manual loader and provides a corresponding 4-20 mA output current.
This E/I converter is a Moore Type SCT, which dnes not have built-in provisions to test (measure) its output current.
The E/I converter receives 120 VAC input power, from a comon circuit breaker on the " Computer Bus", that pro-vides input power (120 VAC/24 VDC) to both the E/I converter and the manual loader.
8109080062 810828
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PDR ADOCK 05000289 P
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l A basic concern is that loss of control power could result in the ADV opening to the midscale position. The ICS " Hand Power" has many alternate sources but normally obtains power from inverter "l A" which is part of Division "A" of the Class lE power system. The proposed manual ' control channel will obtain its power from the " Computer Cus".
This bus obtains power normally from in-verter "1E", which obtains its 'AC and DC inputt from Division "A" also. Between the inverter "1E" and the Computer Bus is a static transfer switch which pro-vides automatic fast transfer capability to a regulated (Solatron) AC bus in the other Division of the Class lE power system.
Based on our review of the proposed modification that utilizes the " Computer Bus" as an input power source for the manual control channel is sufficiently indepen' dent to reduce the likelihood of loss of both the automatic ICS and the manual control channel for assuring proper operation of the ADVs. On this basis, we find the " Computer Bus" power supply as an alternate power source is acceptable.
Our review also showed that loss of the 24 VDC power supply, failure of the
" manual loader", or failure of the E/I converter could also cause the manual control system to drive the ADVs to the 50% open position. This condition i
arises because the manual loader goes to zero-volts output upon loss of input l
24 volts or upon internal failure. Since a -10v to +10VDC control voltage l
was 'the selected range then zero volts is another midscale failure mode which resulted in opening the valves to the 50% position.
In the absence of reasonable testability of the E/I converter to detect such failures and the absence of i
any periodic surveillance activities being proposed, many of these failures are l
" undetectable". Specifically our concern is that, upon loss of ICS power, the l
control of the ADVs would be automatically transferred to the proposed nanual-control channel which could then send the ADVs spuriously to the 50% open position due to an " undetectable" midscale failure. Further, it is not evident that the operator will know how or be able to easily reclose the ADVs.
To resolve our concern, the licensee has agreed to test the manual backup channel every six months during plant operation by transferring the control of the ACVs temporarily from the ICS to.the new manual control channel to demonstrate that the channel will maintain the ADVs in the closed position. This matter will be included as part of the administrative controls pertaining to this modification.
In addition. depending upon equipment delivery, the licensee expects to have both the preposed modification and administrative controls in place prior to restart.
Since the control power for ADVs and the position indicatar power are fron opposite Divisions of the Class lE power system, there is maximum assurance that both control and indication power will not be lost concurrently. Therefore, at this Unit, no modification is needed in this area.
Based on the above evaluation we concludethe licensee has adequately responded te our request transmitted by letter dated August 15, 1980 and we therefore find the licensee's proposed modifications to the controis of the steam dump valves acceptable.
Oated:
August 28,1981 g-.em
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