ML19220C594
| ML19220C594 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 04/30/1979 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| NUDOCS 7905110279 | |
| Download: ML19220C594 (5) | |
Text
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TMI-2 II;CIDENT Ac f'/;'f
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The following is the sequence of significant events that occurred at TMI-2.
a.
The turbine tripped due to loss of main feedwater to the steam generators, b.
The reactor tripped, c.
The auxiliary feedwater pumps started but ficw was to the steam generators was not automatically established, d.
The pressurizer relief valve apparently stuck open, e.
The high pressure emergency core was turned off, and f.
The reactor coolant pumps were turned off.
Our preliminary evaluation indicates that the incident may ha.e been compounded by misleading indication of primary system water level.
In addition, the consequences of the incident were increased because reactor containment was not designed to isolate on ECCS actuation.
The initiating event, turbine trip and subsequent reactor trip, are anticipated events; i.e., they are expected to occur during the plant lifetime and the system is designed to respond safety.
In fact, other B&W designed operating plants have experienced these kinds of transients and have responded safely.
As a result of cur preliminary evaluaticn of the TitI incident, however, we have preliminarily identified several human errors and a mechanical failure.
They are all essentially related to the loss of feedwater (item c above), the sticking open of the relief 97 240 790511027%
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valve (item d above), the turning off of the high pressure injection system, (item e above), and the turning off of the reactor coolant pumps (item f above).
To ensure that the potential human and mechanical failures experienced at TMI-2 do not result in a similar accident at other operating facilities having B&W designed reactors, we have directed (via IE Sulletins) owners of these facilities to take several steps to ensure that safety margins are maintained.
In addition, we have formed an flRC Task Force to review I
the design considerations related to the TMI-2 actions and upon completion [
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of these efforts will be completed about the end of this month.
Several f4RC actions are also being taken at this time because of the preliminary nature of our evaluation. Additional information will be developed by the Task Force which will provide additional insights into the actual causes and consequences of the various actions during the event. At this time, hcwever, our preliminary understanding of the event is sufficient to enable us to define the immediate actions required of operating facilities with B&W reactors to prevent such an occurrence at these plants, and provide us with an adequate basis to allcw continued operation of these facilities.
First, the accident sequence at-TMI-2 began with a loss of auxil.iary feedwater following a turbine trip (item c above).
Since PWRs are not designed and evaluated for the complete loss of all feedwater, we have taken steps to ensure that the auxiliary feedwater system will be available to inject water if main feedwater is lost. At TMI-2, the block valves in the discharge lines frca the auxiliary feedwater pumps were closed.
'le have required that coeraging facilities with B&W reactors ensure that W
these valves are always open oirecting their specific examination of these
valve positions.
The position indication will be further verified by a full-time NRC IE inspector at each of these plants.
Another generic aspect of this event and a significant contributor vias the apparent sticking open of the electrcmatic relief valve en the pressurizer.
Licensees are being directed to examine their procedures to assure that operators are aware of all valve positions, including tha backup block valve to the relief valve, and have information available to pennit its use.
The sticking open of the relief valve was a significant contributor to this event and could be considered an important event regardless of whether auxiliary feedwater were available or not.
Another significant concern, which also has generic considerations, concerns
/
the turning off of the high, pressure injection system.
Iw.-9dirah /!e have requested all operators of plants to exere.ise extreme caution before Mn turning off any safety system. g!pecifically, we have taken steps to require operators to maintain high pressure injection for a minimum of 20 minutes if it is automatically actuated or until low pressure injecticn has been functioning for 20 minutes and conditions are stable.
We believe such actions may cause operational inconveniences, but they are not sifnificant when compred with the gain to be made should a severe transient occur. We also require that if high pressure injection is relied upon, it continue until both the hot and cold leg temperatures 0
are at least 50 before the saturation temperature.
These conditions apply for all cases including the case where a relief valve inadvertently opens and sticks open.
[b
Finally, we are directing that for B&W reactors, if the resctor coolant pumps are in operation should a feed. cater transient occur, the reactor coolant pumps should be kept in operation if at all possible.
Furthermore, if possible, one pump should be kept running in each loop.
This require-ment provides an extra level of safety to cover a broad variety of transients.
In developing this requirement, it was recognized that operation of the reactor ccolant pumps under certain conditions may damage the pumps due to cavitation; however, it is believed that such operation is appropriate to ensure adequate response to a wide variety of transients and that damage to the pumps is of lesser concern.
The above mentioned staff requirements, in addition to our requirement that all licensees with B&W reactors review their designs and procedures in light of the TMI-2 event h increase margins regarding/ ave been imposed to mainta their response to feedwater and other transients.
Such actions will, we believe, compensate for any remaining generic concerns regarding B&W reactors response to such transients.
Because the TMI accident appears to have been further complicated by the containment not being isolated upon actuatian of energency core cooling (in this high pressure injection), we have also taken steps to ensure that the containment is isolated to the extent possible whenever emergency core cooling is initiated.
The above mentioned considerations have been directed towards B&W reactors because they appear to be the ones most directly affected.
The described acticns are irrediate interim actions and may well be modified as a result of the 3C Task Force review 28
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of B&W reactor transients.
Certaijoftheseinterimactionsmayalso later be shown to be applicable to other pressurized water reactors, i.e.,
those designed by Westinghouse and Ccabustion Engineering.
These facilities have significant design differences.
Two significant differences between the B&W primary system design and those of ',lestinghouse and Combustion 'ngineering are more directly measured.
The operators of those plants would, therefore, have had a more direct indication of primary system water level during high pressure injection flow.
- Secondly, the steam generator volumes are larger for Uestinghouse and Combustion Engineering plants, and therefore are less sensitive to feedwater transients; that is, more time is available to detect and correct any deficiencies in auxiliary feedwater ficw.
Therefore, we have not required any actions of licensees with Westinghouse and Combustion Engineering designed plants at this time, although we have sent them information copies of all actions required of owners of B&W reactors.
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