ML20129E955
| ML20129E955 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 10/23/1996 |
| From: | Wadley M NORTHERN STATES POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| GL-96-04, GL-96-4, NUDOCS 9610280214 | |
| Download: ML20129E955 (6) | |
Text
.
Northern States Power Company Prairie Island Nuclear Generating Plant 1717 Wakonade Dr. East Welch. Minnesota 55089 October 23,1996 Generic Letter 96-04 j
U S Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 PRAIRIE ISLAND NUCLEAR GENERATING PLANT Docket Nos.50-282 License Nos.DPR-42 50-306 DPR-60 Response to Generic Letter 9604, Boraflex Dearadation in Spent Fuel Fool Storaae Racks This letter is in response to Generic Letter 96-04: Boraflex Degradation in Spent Fuel Pool Storage Racks. The Generic Letter requests a written response to the information requested within 120 days from the date of the Generic Letter. Our response to the information requested by Generic Letter 96-04 is attached to this letter.
In this letter we have made new Nuclear Regulatory Commission commitments, indicated as the statements in italics.
Please contact Gene Eckholt (612-388-1121) if you have any questions related to our response to Generic Letter 96-04.
gg YC Michael D Wadley Plant Manager Prairie Island Nuclear Generating Plant I
c:
Regional Administrator - Region Ill, NRC Senior Resident inspector, NRC i
NRR Project Manager, NRC J E Silberg Attachments:
- 1. Affidavit 2800Sg
- 2. Response to Generic Letter 96-04 9610280214 961023 PDR ADOCK 05000282 P
e i
e
]
UNITED STATES NUCLEAR REGULATORY COMMISSION NORTHERN STATES POWER COMPANY PRAIRIE ISLAND NUCLEAR GENERATING PLANT DOCKET NO. 50-282 2
50-306
(;ENERIC LETTER 96-04, Boraflex Degradation in Spent Fuel Pool Storage Racks a
Northem States Power Company, a Minnesota corporation, with this letter is submitting information requested by NRC Generic Letter 96-04.
This letter contains no restricted or other defense information.
NORTHERN STATES POWER COMPANY BY b
[
l Michael D Wadley j['
Plant Manager v
Prairie Island Nuclear Generating Plant On this, ay of _
before me a notary public in and for said County, personally appeared Michael D Wadley, Plant Manager, Prairie Island Nuclear Generating Plant; and being first duty swom acknowledged that he is authorized to execute this document on behalf of Northern States Power Company, that he knows the contents thereof, and that to the best of his knowledge, information, and belief the statements made in it are true and that it is not interposed for delay.
I 6
1x e
s v
(,
^
LWICIA Il LACC E NOTARY MRXM5llICOTA HDelEP9dCOUNTY
!!y C** Ex;*u h. 31,2:c2 J
RESPONSE TO GENERIC LETTER 96-04 i
Requested Action:
Provide an assessment of the physical condition of the Boraflex, including any deterioration, on the basis of current accumulated gamma exposure and possible water ingress to the Boraflex and state whether a subcritical margin of 5 percent can be maintained for the racks in unborated water.
i Response to the Reauested Action:
The Prairie Island Boraflex spent fuel storage racks were put into use in late 1981 and were filled to near capacity by 1995. As a result, most of the individual spent fuel cells have a high accumulated gamma dose. A Boraflex coupon surveillance program was established when the racks were installed to monitor long term performance of the Boraflex material. At five year intervals, an irradiated Boraflex test coupon is removed from its location in the spent fuel pool and sent for analysis of physical dimensions, hardness and B10 areal density. The last coupon was j
analyzed in 1994 and was found to not be significantly degraded, with the B10 areal density exceeding original specification.
Due to Prairie Island's high spent fuel pool silica levels relative to the rest of the industry, EPRI and the EPRI Boraflex Working Group shifted its focus from gap formation in Boraflex to the potential for Boraflex dissolution under long term exposure to spent fuel pool conditions. EPRI sponsored studies have shown that the combination of gamma exposure and water ingress into the Boraflex panel cavity significantly degrades the silica polymer matrix. The Prairie Island rack design allowed for a large amount of water ingress to alleviate the potential for cell wall bulging as a result of the Boraflex material offgassing. The Prairie Island sample coupons were not designed to allow for significant water ingress and are therefore not believed to be accurate indicators of rack Boraflex degradation.
The EPRI sponsored calculational model RACKLIFE is being used to estimate the condition of the Boraflex panels. The RACKLIFE model utilizes spent fuel pool chemistry parameters (including silica levels) and cell-by-cell exposure histories to estimate each Boraflex panel's potential for degradation. Additional developmental work is being performed on the model, specifically for Prairie Island, to accurately simulate the rapid increase in the Prairie Island spent fuel pool silica levels. Once this work is complete, the information will be used to determine if 5% subcriticality margin is being maintained. The evaluation of the rack subcriticality margin is expected to be completed by November 30,1996 and a supplemental response to Generic Letter 96-04, including the results of the subcriticality evaluation, will be submitted to the NRC at that time.
~ -.
i i
j l
Requested Action:
Submit a description of any proposed actions to monitor or confirm that this 5%
l subcriticality margin can be maintained for the lifetime of the storage racks and j
describe what corrective actions could be taken in the event it cannot be
{
maintained.
Response to the Reauested Action:
Beyond the surveillance coupon program, which is believed to be unrepresentative in Prairie Island's case, there are no practical ways to directly monitor or confirm i
that the 5% subcriticality margin is being maintained (see blackness testing discussion below). The design of the racks at Prairie Island make direct inspection
]
of the Boraflex impossible without destroying the rack itself. A management i
strategu based on the EPRI sponsored RACKLIFE program is being developed to ensure that cells that are modeled to have been subjected to the highest i
degradation receive and store only low reactivity fuel assemblies.
I in addition to the management strategy, Prairie Island is the lead plant for the proposed Westinghouse methodology by which cred;t could be taken for soluble i
boron in the spent fuel pool. If approved, the spent fuel pool would contain a minimum boron concentration in combination with other neutron poisons and checkerboarding schemes to ensure 5% subcriticality margin under normal circumstances and to ensure that the racks would remain subcritical in the event of
?
complete dilution to zero ppm boron. The Prairie Island specific criticality analysis
{
submitted as part of the lead plant submittal for soluble boron credit in the spent i
fuel pool takes no credit for Boraflex.
Prairie Island is currently maintaining an administrative limit of 2000 ppm soluble boron in the spent fuel pool as a conservative measure to ensure 5% subcriticality margin. The 2000 ppm limit was developed in-house and verified to be conservative by subsequent Westinghouse analysis. This administrative limit will be maintained in place until the license amendment allowing credit for soluble baron in the spent fuel poolis approved by the NRC.
Requested Action:
Describe the results from any previous post operational blackness tests and state whether blackness testing, or other in-situ tests or measurements, will be periodically performed.
Response to the Reauested Action:
There have been no past operational blackness testing on the Prairie Island Boraflex racks and none is planned. Due to the small number of empty cells in the
~
)
l Prairie Island spent fuel pool, a large number of fuel moves would need to be made into temporary racks in order to clear sufficient space to make any blackness testing data meaningful (i.e., eliminate gamma background). Typically, blackness testing is a go/no-go technology that indicates the presence of neutron absorber without the ability to accurately quantify the results. The BADGER System (Boron Areal Density Gage for Evaluating Racks) developed by EPRI is capable of quantifying the B10 density in c~tain rack designs but has not been tested with a flux-trap design like Prairie S -
s and is assumed to have prohibitively high measurement errors for fit.s 4 o,lications. Given these facts, it is believed that 4
currently available in-sito te. % /
Id not provide useful information for Prairie Island's situation.
Requested Action:
Provide chronological trends of pool reactive silica levels, along with timing of significant events such as refuelings, pool silica cleanups, etc.
Response to the Reauested Action: gives a chronological trend of spent fuel pool reactive silica from the time silica was first measured at Prairie Island. As can be seen, an aggressive effort was undertaken in 1987 to reduce the spent fuel pool silica levels using reverse osmosis. At that time it was not yet known that Boraflex was the source of j
the silica. The silica concentration was reduced from approximately 40 ppm to 4
<1 ppm. Since that time, the silica concentration has increased rapidly to a peak of 107 ppm in 1995. There are no plans to reduce silica levels further until the 1
Boraflex issue is resolved.
Requested Action:
Describe the implications of how pool silica levels relate to Boraflex i
performance.
Response to the Recuested Action:
EPRI studies indicate that the concentration of silica in borated spent fuel pools directly relates to the level of degradation of Boraflex in the spent fuel pool racks.
As indicated above, the EPRI sponsored calculational model RACKLIFE can be used to estimate the condition of the Boraflex panels. The RACKLIFE model i
utilizes spent fuel pool chemistry parameters (including silica levels) and cell-by-cell exposure histories to estimate each Boraflex panel's potential for degradation.
I
1 i
4 l
l
[ Silica](ppm) i o
5 8
8 8
8 8
8 k
o o
1 Jan-85 4
4 I
Jun-85 4
Nov-85 li e
Apr-86 l
Sep 86 j
Feb-87 i
Start Silica Clean-up i
Jul-87 Dec-87 May-88 End Silica Clean-up j
Oct-88 4
i Mar-89 m
m' Aug-89 s
t/b
=1 Jan-90 5'g i
m-a 8
Jun-90 O o' m
{
o_
t 3 m F Nov-90 l
nF 0
To
' $s l
Apr-91 g"
o
~
n Sep-91 F
i 3
i Feb 92 Jul-92, Dec-92 May-93 m
cr.
Oct-93 E
[
O i
8
_i Mar-94
$5 I
Aug-94 g
1 Jan-95
~
Jun-95 Nov-95 i
Apr-96 j
N
._