Drywell Debris Transport Study: Experimental Work (NUREG/CR-6369, Volume 2)
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Manuscript Completed: February 1998
Date Published: September 1999
D. V. Rao, C. Shaffer, B. Carpenter/SEA
D. Cremer, I Brideau/SEA
G. Hecker, M. Padmanabhan, P. Stacey/ARL
Science and Engineering Associates, Inc.
6100 Uptown Blvd. NE
Albuquerque, NM 87110
Alden Research Laboratory
30 Shrewsbury Street
Holden, MA 01520
Division of Engineering Technology
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
NRC Job Code W6325
This report describes three test programs undertaken as part of the DDTS to provide basic understanding regarding transport of insulation fragments in the drywell following a postulated LOCA. The first two tests focused on transport of debris by blowdown flow. They obtained data related to (a) inertial capture of insulation debris on typical BWR drywell structures while they are transported across the structures by the steam flow; and (b) degradation of large insulation pieces captured on floor gratings when exposed to high velocity steam flow with suspended droplets. These tests clearly established that wet floor gratings would capture significantly more debris than any other BWR drywell structures (e.g., pipes, I-beams and vents). The capture efficiency of all structures was found to be a strong function of debris size and structural wetness, but a weak function of flow velocity and local flow patterns. Floor gratings possess 100% capture efficiency for insulation pieces larger than 6"x4". These large pieces do not degrade or are not forced through the grating clearances (1.5"x4") when subjected to high velocity droplet flow, even though the differential pressure across them is as high as 1 psid.
The third test program addressed the issue of washdown of debris previously captured on floor gratings by break over flow or containment spray flow during ECCS recirculation phase. These tests concluded that majority of the small debris pieces captured on various structures would be washed down by break flow or spray flow. On the other hand, erosion is the only available mechanism by which large pieces deposited on the floor gratings would be transported. In three hours, as much as 25% of the larger pieces can be eroded and transported to the suppression pool.