ML053630287

NRC IR 05000390-05-013; Preliminary Greater than Green Finding, Additional Information Regarding Conservatisms in Pressure-Temperature Curves

ML053630287
Person / Time
Site:	Watts Bar
Issue date:	12/27/2005
From:	Pace P Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
IR-05-013
Download: ML053630287 (6)

Text

Tennessee Valley Authority, Post Office Box 2000, Spring City, Tennessee 37381-2000 DEC 2 7 2005 10 CFR 50.4 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20555-0001 Gentlemen:

In the Matter of ) Docket No. 50-390 Tennessee Valley authority WATTS BAR NUCLEAR PLANT (WBN) - NRC INSPECTION REPORT NO.

05000390/2005013; PRELIMINARY GREATER THAN GREEN FINDING; ADDITIONAL INFORMATION REGARDING CONSERVATISMS IN PRESSURE-TEMPERATURE CURVES In the regulatory conference concerning the subject inspection report on October 25, 2005, TVA presented information crediting secondary plant cooling. Attached for NRC use in evaluating TVA's presentation is information provided by TVA's NSSS vendor regarding the conservatisms included in WBN's pressure - temperature limit curves.

There are no commitments in this submittal. Should there be any questions on this information please contact me at 423 365-1824.

Sincerely, P. L. Pace Manager, Site Licensing and Industry Affairs Enclosure cc: See Page 2

-D5 6O)

PMIed on Pyced apbA

U.S. Nuclear Regulatory Commission Page 2 M.f 27 2005 Enclosure cc (Enclosure):

NRC Resident Inspector Watts Bar Nuclear Plant 1260 Nuclear Plant Road Spring City, Tennessee 37381 Mr. D. V. Pickett, Project Manager U.S. Nuclear Regulatory Commission MS 08G9a One White Flint North 11555 Rockville Pike Rockville, Maryland 20852-2738 U.S. Nuclear Regulatory Commission Region II Sam Nunn Atlanta Federal Center 61 Forsyth St., SW, Suite 23T85 Atlanta, Georgia 30303

Enclosure Conservatismns in P-T Limit Curves There are significant conservatisms in the ASME P-T limit curve development process, and this technical note will describe them in summary form. First there are three key safety margins built into the process:

• Assumption of a large flaw (25% of the RV wall thickness)

• Application of a safety factor of 2 on the primary, or pressure stresses

• Use of a lower bound fracture toughness An illustration of the impacts of these margins was given some years ago in a paper by Chirigos and Meyers, as shown below. Here we have a typical P-T curve, shown at the bottom of the figure, and then a series of curves above it with the various margins removed. We will pay special attention to the lower portion of the curve, which allows 400 psi at room temperature. This curve was generated using the same margins as the Watts Bar curves.

Removing the safety factor on pressure obviously doubles the allowable pressure to 800 psi in the low temperature range. Next, using a realistic lower bound toughness, Kic, instead of the code-required Kia at the time, improves the allowable to about 1000 psi. The next step, using a postulated flaw that is still very large (I inch, not 25% of the thickness) raises the allowable to 1425 psi. Changing the postulated flaw to 0.5 inches deep brings us to nearly full pressure (2200 psi) at room temperature. The paper goes on to illustrate other margins, which add up to a factor of approximately nine in the low temperature range.

Therefore, it can be clearly seen that the P-T curve process is very, very conservative.

It is worthy of note that the Code was changed recently, as a result of a Westinghouse action, to allow the use of the more realistic toughness, Kic, instead of Kia. This alone would add about 200 psi to the allowable pressure, so there is additional margin over and above your existing curves, which were generated using the old rules.

Therefore there should be no concern with respect to the structural integrity of the reactor vessel, as a result of the recent event at WBN. In fact, the event demonstrates the usefulness of the overpressure mitigation system, which has reduced the probability of a violation of the P-T curve significantly.

During a recent simulation conducted on the Watts Bar simulator, the worst case that occurred was a pressure of 841 psi, which occurred at a steady state condition, and a temperature of 126F. This condition was the result of a relatively low probability event, but it will be instructive to assume that it happened at the plant.

The P-T curve for the Watts Bar plant is provided in figure 2, shown below. As described above, there are three clearly specified margins inherent in these P-T curves: a large postulated flaw(25% of the vessel wall, or 2.15 inches deep), use of a lower bound fracture toughness, and a factor of two on the pressure. If we keep the first two margins, and eliminate the third one, the allowable pressure for Watts Bar in this scenario is 11 80 psi. This seems to be a reasonable assumption, given that the initiating event is a low probability to start with. Therefore there was clearly no concern about the integrity of the reactor vessel in this case, even though the allowable P-T curve was postulate to be exceeded.

Original Record Electronically Approved in EDMS 2000

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Figure 2: P-T Limits for Watts Bar Unit I [WBN Reactor Coolant System Description, N3-68-4001, Revision 24, 41712005]

Original Record Electronically Approved in EDMS 2000