ML20195G553
| ML20195G553 | |
| Person / Time | |
|---|---|
| Site: | South Texas, 05000000 |
| Issue date: | 06/24/1988 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20151V292 | List:
|
| References | |
| FOIA-88-497 NUDOCS 8811280063 | |
| Download: ML20195G553 (8) | |
Text
-
)
[pa a r eg*'o, UNIT E D ST AT E S l'
'i NUCLEAR SEGULATORY COMMISSION A
.i
.u ssiN o 7 O N, D. C. 20555
{
%,.....f ENCLOSL'Et 1 SAFETY EVALUATION BY THE OFFICE OF NurLEAR PEACTOR REGUL ATION FEl ATING TO SPENT FUEt. POOL PODirlCATIONS HOUSTON LIGHTING A P_0WEd COMPANY SOUTH TEXAS PROJECT UNITS 1 AND ?
DOCKET NOS. 50 498 & 50-499 1.0 INTR 000CT10N 1
By letter dated March 8,1988, Houston Lighting and Power Company (FLP) applied for an amendment to increase the storage capacity of the spent fuel pools for Sout5 Texas Proiect l' nits 1 and 2.
This expansion i', to be accomplished by installirg new storage racks in the existing fuel cool which can store spent (irradiated) fuel assemblies in a high density array. The proposed trcdificatters will increase the spent fuel storage capacity of each spent fuel pool to 1969 fuel assemblies, thus extending the full core reserve storage capacity until the year 2020.
The spent fuel storage pools will be divided into two regions. Region 1 contains 288 storage cells with a nceninal center to-center spacing of 10.96 inches and is designed to accomodate non-irradiated fuel assemblies with U 235 enrichments up to 4.5 weight percent.
Region 2 will contain 1681 storage cells with a nominal center-to-center spacing of 9.15 inches.
Placement of fuel in Region 2 is restricted by burnup and enrichtent limits.
In Region 1 the Boraflex assembly consists of a thin rectangular stainless steel water box with a Boraflex sheet affixed on one side of the box and another Boraflex sheet on the other. A thin stainless steel plate is welded over each of the two Boraflex shtets on the water box. The entire assembly is removable from the storane cell.
In Region 2, a Boraflex sheet is positioned 112 BOO 63 001103 i
,_.~
2 between two adjacent walls of the square storage cells. The Boraflex sheets in Region 2 are not removable, in both regions, single sheets of Boraflex are used, and the Boraflex sheets are not physically f astened to or permanently glued onto any surface or structure.
2.0 Evo t'ATION Calculation Pethods The calculation of the effective multiplication factor, keff, makes use of the P0Q-7 twc-dirensional four-group diffusion theory computer code with neutron cross sections generated by the LEOPAFD code. These ccdes were benchmarked against a ser'es of critical experiments with characteristics similar to the South Texas spent fuel pool racks. These cocparisons resulted in a model bias of + 0.0067 and a 95/95 prcbability/ confidence uncertainty of z 0.0077 for the Fepion i racks anc
.1 model bias of + 0.0057 and a 95/95 uncertainty of a 0.0026 for the fegion 2 racks.
In order to calculate the criterion for acceptable burnup for storage in Pegion 2, calculations were made for fuel of several different initial enrichments and, at each enricheent, a limiting reactivity value, which included an additional factor for untertainty in the burnup analysis, was established. Burnup values which yielded the limiting reactivity values were then determined for each enrichment from which the acceptable burnup demain for storage in Region 2, as shown in figure 5.6-1, was obtained. The staff finds this pre;edure acceptable.
I Treatment of Uncertainties A correction for the reactivity effect of pool temperature is included as well as a geometric modeling effect bias to account for mesh spacing and seeared st.inless steel-water composition effects.
l
i I
I l
For the Pegion 1 analysis, the total uncertainty is the statistical combination of the calculational uncertainty and manufacturing and rechanical uncertainties due to variations in Boraflex thickness, inner stainless steel storage box dimension, stainless steel thickness, and fuel enrichment and density.
In Recion 2 analysis, the sare uncertainties are considered.
In addition, an uncertainty due to the burnup analysis is estimated and combined statistically i
with the other uncertainties.
The staff concludes that the appropriate uncertainties have been considered and have been calculated in an acceptable ranner.
In addition, these uncertainties were determined at least at a 957 probability 95% confidence level, thereby meeting the NRC requirerents, and are acceptable.
i f
Pesults of Analysis for Fegion 1, the rack rultiplication factor is calculated to be 0.9250, including uncertainties at the 95/95 probability / confidence level, when fuel t
having an enrichrent of 4.5 weight percent U-235 is stored therein. Although the pool is nnrrally flooded with water berated to 2500 ppm, unborated water was assumed in the analysis.
For Pegion 2, the rack multiplication factor is calculated to be 0.9478 for the rost ieactive irradiated fuel permitted to be stored in the racks, i.e.,
fuel with the mintrum burnup permitted for each initial enrichment as shown in Figure 5.6-1.
The design will accept fuel of 4.5 weight percent U-235 initial enrichment burned to 40.0 WD/kgU. The analysis of the Region ? racks also assured full ficoding by unborated water.
Therefore, the results of the criticality analyses eet the staff's acceptance criterion of k,ff no greater than 0.95 includir.9 all uncertainties at toe 95/95 probability / confidence level.
i a
4 Accident Analvses 1
Of Most abnormal storage conditions will not result in an increase in the keff the racks. For example, loss cf a cooling system will result in a decrease in the k,77 value since reactivit, decreaset with decreasing water density. This also allows new (unirradiated) ~uel to be stored in the Region 1 racks in a dry condition prior to initial cs i loading of Unit 2 since inadvertent partial or full flooding would mai tain k,77 less than 0.95, it is possible to postulate events, such as an inadvertent misplacement of a fresh fuel assembly either into e Pegion 2 storage cell nr outside and adjacent to a rack module, which could lead to an increase in pool reactivity. However, for such events credit may be taken for the Technical Specification requirement of at least 2500 ppm of boron in the refueling canal durine refueling operations. The reduction in the k value caused by the boron more than l
eff l
offsets the reactivity addition caused by credible accidents.
I Boraflex Decradation The staff has expressed concerns regarding the possibility of irradiation l
]
induced axial shrinkage of the Borafley panels as documented in NRC Information I
Notice l'o. 87 a2, Rased on thir. the licensee has performed analyses to l
determine the reactivity effects of potential Boraflex shrinkane on the South Texas spent fuel pool.
i f
i l
Several scenarios were evaluated ranging from shrinkage of the top and bottom of each Boraflex panel with a corresponding exposure of active fuel at each l
end, to a single tear in each panel at the active fuel mid-plane. The results indicate that sufficient margin is available in both the Region 1 and Region I
2 rack design to accomodate at least 8 inches of shrinkage at aach end.
In
[
addition, for Region 1, a nid-plane gap of up to 4.5 inches o every panel would still maintain k,ff less than 0.95.
For Region 2 mid plane gaps in every panel of up to 3.2 inches could be accomodated.
If the mid-plane gaps i
f I
5 are assumed to occur in only two of the four panels in each Region 2 cell, gaps es large as 10 inches could be accommodated and still maintain k,ff less than 0.95.
Therefore, although it is not likely that significant gap formation will occur in the Boraflex panels, the sta'f feels that there would be sufficient time to detect such anomalies and provide appropriate a tiens before any significant adverse reactivity ef fects are realized.
Technical Specification Changes The following Technical Specification (TS) changes have been made as a result of the proposed spent fuel pool storage modifications.
The staff finds these changes acceptable.
1.
TS 5.6.1.1 and 5.6.1.2 are combined into one Specification (5.6.1).
The new TS 5.6.1 correctly acccunts for the uncertainties and tolerances assured in the criticality analyses as well as the nominal center-to-center fuel assembly spacing, the maximum allowable U-235 enrichrent, and the installation of Boraflex between spent fuel assemblies.
2.
Figure 5.6-1 has been added to specify the initial enrichment vs. burnup requirerents to be ret prior to storage of fuel assemblies into Region 2.
3.
The spent fuel pool storage capacity has been increased from 196 to 1969 fuel assemblies in TS 5.6.3.
3.0 CONCLUSION
Based on the review described above, the staff finds the criticality aspects of the design of the South Texas Units 1 and 2 spent fuel racks to be acceptable and meet the requirements of General Oesign Criterion 62 for the prevention of criticality in fuel storage and handling. The staff concludes 4
1 6
l that fuel from Unit 1 or Unit 2 may be safely stored in Region 1 provided that the enrichment does not exceed 4.5 weight percent U-235. Any of these fuel assemblies may also be stored in Region 2 provided it meets the burnup and enrichment limits specified in figure 5.6-1 of the South Texas Technical Specifications.
4 50 40 9
Acceptsble 30 Mi Unacceptab!e E
E 20 E
10 i
0 1.4 2.0 3.0 4.0 5.0 l
Initla! Enrichment (w/o) l Figurt 5.61 SOUTH TEXA5 PROJECT SPENT FUEL RACK 5 REGION 2 REQUIRED BURNUP A5 A FUNCT10N OF INmAt ENRICHMENT
4 r
F ENCLOSURE 2 S0llTH TEXAS PROJECT i
SYSTEFATIC ASSESSPLNT OF LICENSEE PERFORPAt!CE t
i Functional Areas _
I l
1.
Management involvement in Assuring Quality.
t 4
Technical review of the submittal indicates that the canagement reviews i
are tirr.ely and technically appropriate.
Rating: Category 1 t
2.
Approach to Pesolution of Technical Issues from a Safety Standpoint.
The licensee shewed a general understanding of the technical issue and 1
)
used acceptable approaches.
J j
Rating: Category 1 1
3.
Responsiveness to NRC Initiatives.
The licensee responded favorably to NRC initiatives.
t i
Rating: Category 1
(
2
(
i 4.
Enforcement History.
E N/A i
l 5.
Operational and Construction Events.
j N/A r
j i
j 6.
Staffing (including Management).
l N/A I
7.
Training and Qualification Effectiveness.
N/A I
1 i
t
)
I
.