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MEMORANDUM FOR:
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Licensing ranch 3, DL FROM:
Brian W. Sheron, Chief Reactor Systems Branch, DSI
SUBJECT:
DIABLO CANYON UNIT 2 - STEAM GENERATOR TUBE RUPTURE The Steam Generator Tube Rupture (SGTR) design basis event has been histori-cally reviewed by the staff from the radiological conscquences standpoint.
l Therefore, emphasis was placed on the conservativeness of radiological releases I
and environmental conditions at the time of these releases.
The Diablo Canyon l
Units 1 and 2 safety evaluation of that event was conducted by the staff under l
these premises.
However, in the last few years SGTR events have taken place at l
several operating plants (e.g. Ginna, Prairie Island Unit 1 and Surry Unit 2).
The experience gained from these events proved that some of the assumptions made by the applicant for the SGTR event analysis are not as conservative as was earlier perceived, nor did tie analysis provide assurance that applicable regulations were met.
Furthermore, this event has typically been analyzed as a limiting fault, i.e. of a very low probability of occurrence.
However, the operating experience suggests otherwise.
We realize the. fact that Diablo Canyon Units 1 and 2 are replicate plants. We also realize that unit I has been granted a license without undergoing a SGTR event evaluation to the same degree of depth as that exercised with all the Near Term OL (NTOL) applicants.
However, we believe that Diablo Canyon Unit 2, being an NT0L, should properly address the SGTR event consistent with l
requirements we have placed on all other NTOL plants designed by Westinghouse.
Therefore, we provide for your consideration the enclosed request for additional information so that we may fully evaluate this event.
Please note that the requested evaluation is considered continuing for the other Westinghouse plants recently licensed, and this would be the case for Diablo as well, and would not hold up licensing of the plant.
1% t1/w Brian Sheron, Chief Reactor Systems Branch Division of Systems Integration cc:
H. Schierling R. Houston B. Bernero RSB S/Ls T. Novak CONTACT:
S. Diab, RSB X-29440
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ENCLOSURE REQUEST FOR ADDITIONAL INFORMATION STEAM GENERATOR TUBE RUPTURE Steam Generator Tebe Rupture:
The accident scenario examined in the FSAR involves a complete severance of a single steam generator tube. The applicant's description of.the accident was reviewed, including the sequence of events.
If offsite power were not available, the turbine bypass valves would close and the steam would discharge to the atmosphere via the steam generator atmospheric relief and/or safety valves.
If offsite power was available the turbine bypass valves would discharge steam to the condenser.
For either case, the FSAR assumes that the break flow is terminated within 30 minutes after initiation of the event by operator action to equalize the primary and secondary pressures. However, SGTR events at operating reactors generally indicate that a time period longer than 30 minutes is required for pressure equalization (e.g., 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> at Ginna).
Therefore, the staff' requires the following information to fully evaluate this event analysis:
1).
FSAR Section 15.6.3 indicates equalization of primary and secondary pressure 30 minutes after the SGTR event, with consequent termination of steam generator tube leakage. Justify that this can be achieved under loss of offsite power / natural circulation conditions under which a steam bubble might form in the reactor vessel head.
< 2).
Discuss w'hether as a result of possible modification to your analysis including consideration of. longer leak times, liquid can enter the main steam lines. 'If so, discuss the effects on the integrity of the steam piping and supports. Consider both the liquid dead weight and the possibility of water hammer. Also discuss whether the steam generator safety valves would function properly if their actuation pressures are
. reached with the main steam lines filled with liquid and whether they would reseat at the proper pressure.
3). Describe the sequence of events which includes an identification of all operator actions and when these actions are expected to occur. Justify f
the analytical assumptions regarding operator' action to (1) open steam line atmospheric relief valves and (2) open a pressurizer PORV. Also include descriptions of the automatic initiations and actuation as they occur chronologically.
s 4).
Provide the following parameters as a function of time, until releases from the ruptured steam generator are terminated:
(a) the primary system pressure; (b) intact and ruptured hot leg fluid temperature; (c) the secondary liquid water mass and level in each steam generator; (d) the primary system liquid mass; (e)' the secondary system pressure in each steam generator;
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(f) the integrated mass released out of the atmospheric relief
. valves or safety valves for the intact ~ steam generators and for the ruptured steam generator; (g) pressurizer level; (h).the tube rupture flow rate and integrated tube rupture flow (i).the extent of upper head voiding if predicted; and (j) the steam flow rate for all steam generators.
5). Describe or reference the computer code utilized to calculate the primary and secondry system response. Justify that the code is appropriate for
'SGTR analysis.
6). Justify that sufficient noding is provided in the analysis to predict head bubble formation or loss of natural circulations in loops for which the steam and feedwater flow has been isolated.
7).
Include in the analysis of the SGTR accident the most limiting single active failure.
If the most limiting single active failure is failure of an atmospheric relief valve to close, operator action to close the block valve may be assumed if justified. The analysis should assume that the accident begins with the primary coolant iodine concentrations at the technical specification limit and that an iodine spike occurs as a result of the primary system-depressurization.
8).
Identify all equipment which is relied upon to mitigate a design basis SGTR event.
If reliance on the pressurizer PORVs and/or steam generator ADVs is essential for the SGTR mitigation, the applicant should develop
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and propose appropriate Technical Specification limits to ensure the continued operability of this equipment.
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