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Letter to David Lochbaum, Union of Concerned Scientists, Response to Letter Basis for Yellow Finding Davis-Besse Nuclear Power Station (IR 05000346-2003-015)
	ML032601029
Person / Time
Site:	Davis Besse
Issue date:	09/17/2003
From:	Grobe J; NRC/RGN-III
To:	Lochbaum D; Union of Concerned Scientists
References
	IR-03-015
	Download: ML032601029 (4)
	v • d • e

September 17, 2003

Mr. David A. Lochbaum

Union of Concerned Scientists

1707 H Street, NW, Suite 600

Washington, DC 20006-3919

Dear Mr. Lochbaum:

In your letter to me dated August 4, 2003, you stated that it was not clear from inspection report

50-346/2003-015, whether the preliminary YELLOW finding related to the containment sump at

Davis-Besse accounted for the heightened chances of a medium-size loss of coolant accident

(LOCA) at the plant caused by the damaged reactor vessel head.

The Phase 3 significant determination process (SDP) analysis for the containment sump issue,

which resulted in a YELLOW finding, did not use the increased likelihood frequencies obtained

for the medium and large LOCAs obtained in the evaluation of the RED finding due to the

damaged reactor vessel head. As explained in IMC 0609, Significance Determination

Process, Appendix A,Section III, the manner in which concurrent multiple equipment or

functional degradations are evaluated using the SDP is a function of their cause. If the

concurrent multiple equipment or functional degradations resulted from a common cause (e.g.,

a single inadequate maintenance procedure that directly resulted in deficient maintenance

being performed on multiple components), then a single inspection finding will be written and

characterized for significance by the total increase in core damage frequency. The justification

for existence of a common cause must be a stronger causal relationship than poor

management or cross-cutting programs (e.g., an inadequate problem identification and

resolution program is an inadequate basis to justify a common cause finding). If independent

causes are determined to have resulted in multiple equipment or functional degradations, then

separate inspection findings will be written and individually characterized for significance

assuming none of the other independent findings existed. The SDP is designed this way to

focus NRC inspection resources on licensee performance deficiencies. The SDP is not

intended to be a process which determines integrated plant risk.

Because the causes of the deficiencies that led to the reactor pressure vessel head

degradation were separate from the causes that resulted in the potential clogging of the

containment emergency sump, the NRC staff concluded that separate performance deficiencies

and significance determinations was appropriate.

D. Lochbaum

-2-

Your letter also stated that it appeared that the medium-sized LOCA likelihood used in the RED

finding was significantly different than that used for the containment sump issue. The initiating

event frequencies are different; however, not a million times different as stated in your letter.

The initiating event frequencies for the medium and large LOCAs used for the YELLOW sump

finding are those identified in NUREG/CR-5750, Rates of Initiating Events at U.S. Nuclear

Power Plants, which are 4E-5/reactor-year and 5E-6/reactor-year, respectively. These LOCA

frequencies represent the nominal or base case frequencies used for the analysis. These

values were not changed for the preliminary YELLOW finding for the reasons described above.

Therefore, the actual difference between the likelihood of the medium LOCA for the RED and

YELLOW findings was about 100 times different (4E-5 versus 3E-3). The values you

mentioned in your letter (2.28E-9 and 5.13E-10) are a per hour frequency of the change in the

core damage frequency for the medium and large LOCA, respectively. These values were then

multiplied by 8760 hours0.101 days 2.433 hours 0.0145 weeks 0.00333 months (i.e., number of hours in a year) to yield the significance of the

inspection finding on a per year basis.

Should you have any questions regarding our efforts, please contact Sonia Burgess of the

regional staff at (630) 829-9752.

Sincerely,

/RA/

John A. Grobe, Chairman

Davis-Besse Oversight Panel