ML091900701
| ML091900701 | |
| Person / Time | |
|---|---|
| Issue date: | 05/21/1984 |
| From: | Han J Office of Nuclear Regulatory Research |
| To: | Silberberg M Office of Nuclear Regulatory Research |
| Han, James T. | |
| References | |
| Download: ML091900701 (22) | |
Text
Distribution:
Subj Circ Chron
~
h r/f JHan r/f WMorrison OEBassett MEMORANDUM TO:
M. Silberberg, Branch Chief Fuel Systems Research Branch Division of Accident Evaluation FROM:
J. T. Han Fuel Systems Research Branch Division of Accident Evaluation RESPONSE TO A NRR REQUEST TO REVIEW SNL STUDIES REGARDING SPENT FUEL HEATUP AND BURNING FOLLOWING LOSS OF WATER IN STORAGE POOL
References:
1.
Memo from R. J. Mattson to T. P. Speis, GIMCS Schedule for Generic Safety Issue 82, "Beyond Design Bases Accidents in Spent Fuel Pools," dated February 2, 1984.
2.
A. S. Benjamin, et al., "Spent Fuel Heatup Following Loss of Water During Storage,"
NUREG/CR-0649 (March 1979).
3.
A. S: Benjamin and K. T. Stalker, liThePotential for Propagation of Self-Sustaining Zirconium Oxidation Following Loss of Watpr in a Spent Fuel Storage Pool," Draft Report (January 1984).
4.
Private communications with A. S. Benjamin of SNL (April 1984).
Per NRR request,1 I have reviewed two SNL reports 2-3 studying the spent fuel heatup and burning following loss of water accidents in the storage pool.
As part of the request, Don Hagrman of INEL provided comments on SNL modeling of cladding oxidation and failure, and Ralph Meyer of ASTPO performed a fission product release calculation based on a representative peak clad temperature curve from the SNL reports (Enclosures).
Comments 1.
Models used in both reports2,3 appear to be reasonable prior to fuel clad temperature reaching zircaloy melting at around 1900-2000°C.
Beyond that, unoxidized zircaloy clad will melt and tend to relocate to lower elevations along with some fuel it has dissolved.
- However, the relocation of unoxidized clad and dissolved fuel (fuel liquefaction) is not modeled in SNL studies.
This process can significantly affect the fission product release, and by forming flow blockages in lower elevations it may also affect the fuel rod cooling and oxidation, as shown in PBF severe fuel damage tests.
Therefore, the temperature results above 1900-2000°C in the reports are questionable.
(It is worth noting that PBF and Kfk tests indicated that unoxidized clad relocated at around 1900-2200°C, which is about 0-200°C higher than the zircaloy melting point.)
2.
Uncertainties in SNL temperature calculations are not known because of lack of comparisons with data.
Only one temperature comparison was presented for a heatup experiment with no clad oxidation allowed (in a hellium-flow nine-rod bundle with a 38-cm heated length).
Although the SFUELIW code produced good agreement with the data (see fig. 5.29 and Table 5-3 of Ref. 3), we still do not know how good the code is in predicting temperature rise in a 12-ft. fuel assembly with significant clad oxidation.
(Note that rapid zircaloy-air reaction occurs at around 900°C or higher.)
3.
Results in the first3report2 were calculated by the SFUEL code.
Results in the second report were calculated by a revised and expanded version, SFUELIW, which has additional capabilities to handle burning propagation from one section of the dry pool to adjacent sections.
Peak clad temper-atures calculated by both SFUEL and SFUELIW were compared for PWR cylin-drical storage racks with different baseplate holes (see Tables 2-2 and 2-4 of Ref. 3 and Figs. 2(b) and 3(b) in Ref. 2).
For one case, both codes produced pratically the same results; for another case, the result of SFUEL is higher than that of SFUELIW by a to 150°C.
SNL feels that these comparisons tend t~ indicate that the results in the first report are more conservative.
It should be noted that SFUEL was limited by numeric stability and it could not continue the calculation beyond the occurrence of very rapid oxidation; the limitation has been removed in SFUELlW.
4.
The reaction rate equation used in both reports for zircaloy clad oxidation in air needs to be improved as pointed out by INEL (Enclosure 1).
- However, this improvement is for temperatures below 820°C and should not make a signi-ficant difference.
5.
There is no model to hand13 clad/fuel relocation as mentioned in 1.
- However, the second report presents some temperature calculations by assuminr the clad/fuel (modeled at one temperature) will relocate when an axia node has reached the melting point of ZrO at around 2740°C (this assumption may overestimate the heat release~ due to clad oxidation at temperatures above 1900-2200°C at which significant relocation can occur as indicated in PBF and KfK tests); the axial node and all nodes above it will disappear when this condition is met (there is no tracking of the relocated mass).
Calculations were also made by assuming no relocation and ignoring phase changes to allow the clad/fuel temperature to continue to rise.
Table 2-5 of Ref. 3 presents a comparison of the peak clad temperatures calculated under (a) the relocation assumption mentioned above (Option 4 in the table) and (b) the no-relocation assump-tion (Option 1).
Results in the table are for PWR cylindrical storage racks with 1.5-inch baseplate hole and the hottest spent fuel at 11 kW/MTU (5 kW per fuel assenbly);
note that under Option 4 the clad/fuel relocation was assumed to occur at 42,730 sec when the temperature exceeds ZrO? melting point and after that the temperatures shown are the calculated air temper-ature at that location.
6.
A fission product release calculation was provided by Ralph Meyer of ASTPO using the CORSOR-M correlations based on the peak clad temperatures given in Table 2-5 (Option 4) up to clad/fuel relocation (assumed to occur at around 2740°C) at 42,730 sec. since the loss of water in storage pool (Enclosure 2).
Total releases of Xe, Kr, I, and Cs were estimated at about 100% of the inventory in spent fuel.
The release of Te was estimated in the range of 2 to 100%, where the lower limit was obtained if there was sufficient unoxidized zircaloy in the clad to retain Te in place as observed in PBF 1-1 test and ORNL studies.
Total release rates for Ba were estimated at 2%, and those for Sr and Ru were at 0.2% and 0.002% respectively.
Conclusions Based on SNL Studies Following a loss of water accident in a spent fuel storage pool, the heating up of the fuel rods is generally determined by the fuel decay power, radial power distributions in the pool, and the type of the storage racks holding fuel assemblies.
The higher the decay power or the more restricted air flow in the racks, the faster the temperature rise in the fuel rods.
The spent fuel decay power depends on the decay time after reactor shutdown and the burnup (see Tables III and IV of Ref. 2).
To prevent a runaway temperature rise leading to significant zircaloy-air reaction, a minimum decay time needs to be imposed to certain storage racks.
A.
Well-Ventilated Storage Room (at constant ambient air temperature):
a.
For PWR spent fuel pool following a loss of water accident, the minimum allowable decay time prior to storage was calculated by SFUEL to be about 2 years for high-density closed-frame racks
[depicted in Figs. 2(d) and 3(d) of Ref. 2J with negligible clearance between the pool wall and the adjacent storage racks.
The minimum allowable decay time for the same high-density racks but with a wall clearance of 16 inches or larger is about 10 months.
Figure 17 of Ref. 2 summarizes the results calculated by SFUEL for various storage configurations.
b.
For BWR spent fuel following a loss of water accident, the minimum allowable decay time was calculated by SFUEL to be about 60 days for directional storage racks with fuel channels attached and large baseplate holes (5 inches in diameter or larger) as shown in Fig. 20 of Ref. 2; however, the minimum decay time can be reduced to about 15 days by removing fuel channels before storage.
(See Figs. 2 and 3 for storage rack illustrations.)
The effect of the basepla~e hole in restricting the air flow into each storage rack was also studied.
The minimum allowable decay time was calculated to be about 10 days for cylindrical racks with 3-in. baseplate hole (fuel channels removed), while it was increased to about 200 days for the racks with a 1.5-in. baseplate hole.
c.
The baseplate hole at the bottom of each fuel storage rack should be no smaller than 5 inches in diameter in order to increase the minimum allowable decay time (see Figs. 17 and 20 of Ref. 2).
d.
Insignificant difference exists between PWR 17x17 and 15x15 fuel assemblies.
The difference between BWR 8x8 and 7x7 fuel assemblies is also small.
B.
Inadequately Ventilated Storage Room It was found that the current ANS specifications of one complete room air change per half hour are insufficient to keep the room air temperature below 150°C for PWR spent fuel storage pool in the auxiliary building.
A comparison was made between the case with perfect ventilation and the case with ANS rate of ventilation (producing the same result as no ventilation).
For the former the peak clad temperature was calculated by SFUEL to be about 400°C, and for the latter the peak clad temperature rised beyond the self-sustaining clad oxidation temperature around 900°C (see Fig. 21 of Ref. 2).
No studies were presented for PWR high-density spent fuel storage racks under inadequate ventilation condition.
- However, the ANS specified rate of ventilation was found to be sufficient to maintain the peak clad temperature below 600°C for BWR spent fuel stored in cylindrical racks [depicted in Figs. 2(f) and 3(f)] as shown in Fig. 25 of Ref. 2.
This is because that BWR spent fuel pool is located in the containment building with much larger air space than that above the PWR spent pool.(see Table VII of Ref. 2 for comparison).
C.
Effect of Incomplete Pool Drainage Results discussed in A and B above were obtained by assuming complete pool drainage following a loss of water accident.
- However, it is also possible to have incomplete pool drainage which covers with water the baseplate holes or air entrance to storage racks and thus blocks air flow into storage racks.
As a result, the cooling of uncovered portion of fuel rods may be inhibited.
A calculation was made to study the effect of incomplete pool drainage in which the water level falls below the heated section of fuel rods but not low enough to uncover the entrance to the bottom of storage racks of PWR cylindrical configurations; the peak clad temperature monotonically increases to about 1650°C in two days as shown in Fig. 26 of Ref. 2.
On the other hand, the peak clad temperature for the case of complete pool drainage is less than 200°C.
The condition described above is probably the worst case caused by an incomplete pool drainage.
Under other conditions in which the water level still covers sufficient heated length of fuel rods, enough steam can be generated to cool off the rest of the rods.
Propagation of burning (namely, rapid zircaloy clad oxidation in air) from the hottest3section of the pool to adjacent sections was studied using SFUEL1W.
Dividin~ a half pool into six sections with A7
~
the center section at 30 kW/MTU (equivalent to about 14 kW per PWR
_0.
fuel assembly) and other five sections at the same power in the range of 1 - 6 kW/MTU, it was found that burning propagated from the center section to the adjacent section which was at 5 kW/MTU or greater (see Table 2-6 and Fig. 2.19 of Ref. 3).
Further refinement of the analysis by including pin-to-pin temperature difference in a fuel assembly reduced the decay power of the adjacent section to about 3.5 kW/MTU in order to prevent burning from occuring in the section next to the hottest one where burning has occurred.
This means that the minimum deca time for sent fuel to be stored adjacent to recentl dischar ed fue eca of 9 ass ou e at more t an ears see ppen lX B of Ref. 3.
he resu ts iscussed above are for PWR igh-density storage racks with large baseplate hole (5-inch diameter of larger) in a room maintained at a constant temperature (well-ventilated room)~
b.
Burning propagation in the pool was found to be governed by heat transfer mechanisms of conduction convection, and thermal radiation.
Burning propagation caused by air-borne burning particles or cladding vaporization and subsequent ignition was found to be insignificant (Chapters 3 and 4 of Ref. 3).
E.
Emergency Water Spray Emergency water spray could be supplied by onsite hydrants or storage tanks.
The spray rate required to cool spent fuel was presented in Table IX of Ref. 2.
For PWR spent fuel in high-density storage racks with a 30-day minimum decay time (the hottest fuel at 53 kW/MTU), a water spray of 85 gal/min was found to be sufficient to keep the peak clad temperature at 400°C which is within the safe limit (Fig. 27 of Ref. 2).
1.
The SNL studies have covered a wide range of issues regarding the spent fuel heatup and burning following a loss of water accident in the storage pool.
- However, there are uncertainties in the calculated results.
It is recommended that the SFUEL1W code be validated by comparing calculations with the e~isting small-scale experimental results presented in the second SNL report 2.
Further study_t:S_...needed to repeat the burning propagation ca1culation discussed earlier (under Item D) but under the inadequate room ventilation condition.
Similar calculations should also be performed by varying the hot spent fuel decay power from 20 to 90 kW/MTU.
The end result would be a table summarizing the maximum decay power (or the minimum decay time) allowed for the spent fuel to be stored adjacent to hot fuel (at 20-90kW/MTU)
in order to prevent burning propagation under both the inadequate and adequate room ventilation conditions.
(Recall that the maximum allowable decay power was calculated at around 3.5 kW/MTU for the spent fuel adjacent to hot fuel at 30 kW/MTU under the adequate ventilation room.)
3.
Having com~leted the work specified in Items 1 and 2 above, the second SNL report should be revised and published as a NUREG.-
4.
As an option, the SNL reaction rate equation should be revised according to INEL comments (Enclosure 1).;sensitivity studies should also be performed by varying the zircaloy-air rate correlation.
5.
The SNL results should be compared with German studies for severe accidents in spent fuel storage, which will become available to NRC in late 1984.
6.
If the SFUEL1W code would fail to simulate the data presented in Ref. 3, it is recommended to have INEL to provide SCDAP/RELAP5 calculations for the spent fuel issue in 1985-86.
ORlG1Nl\\L S'GNFD BY:
James T. Han Fuel Systems Research Branch Division of Accident Evaluation cc:
M. Wohl, NRR J. Hulman, NRR W. Pasedag, NRR G. Marino, RES R. Meyer, ASTPO O. E. Bassett, RES D. Ross, RES DAE:FSR~
,I Han/rnd () 1 'I-f 5/21/84
G. A.
Berna ~~/'
P. E. MacDona 1d t(!
V. H. Ransom Centra 1 Fil es D.
L. Hagrman file f)f'"'
~
W. Johnsen file n
EGc..G Id**ho. Inc
<::-.:)
POBOX 1625, IDAHO FALLS.
IDAHO 83415 Mr.
Farrel L. Sims, Chief Reactor Research and Technology Branch Reactor Operations and Programs Division Idaho Operations Office
- Falls, 10 83401 TRANSMITTAL OF COMMENTS ON NUREG/CR-0649
- GWJ-1S-84 As requested by Dr.
James T. Han of NRC-RES, a critical review was made of a report entitled "Spent Fuel Heatup Following Loss of Water During Storage,"
Please find attached a summary of that review.
Very truly
- yours,
("";,..{-6 (', j l.(i-t(i" t ~ <:.----,
L"
.\\",
t
- d. W. Johnsen, Manager LWR Systems and Severe Accident Programs Systems Analysis/Advanced Methods Division
Attachment:
As Stated cc:
R. R. Landry, NRC-RES O. Majumdar, DOE-IO J. O. Zane, EG&G Idaho (w/o Attach)
NUREG/CR-0649, Spent Fuel Heatup Following Loss of Water During Storage,l is a study lito analyze the ~hermal-hydraulic phenomena involved when (fuel) storage racks and their contents become exposed to air, and to determine the conditions which could lead to clad failure due to overheatingll.
Two important parts of this
- analysis, the oxidation rate data and the criterion for cladding failure have been reviewed.
The treatment of the oxidation rate is adequate for the problem being analyzed but the cladding failure criterion overestimates the cladding failure temperature by 250 degrees, an error which is signficant for some of the analysis results.
More detailed comments follow:
1.
The presentation of the computer code listing and the discussion of the data used to derive the expressions on pages 31 to 34 7 -e excellent features.
2.
The authors' zircaloy oxidation data and their use of them for temperatures
(~
toqo K) above the beginning of the alpha-beta phase changeAare acceptable.
Their use of the zirconium oxidation data of Hayes and Roberson with a parabolic rate law for the alpha phase is questionable since there are more applicable data for the oxidation of zircaloy.
The data of Porte et a12 for zirconium with 0.96%
and 1.68~ atom percent tina in air exhibit cubic oxidation followed by a more rapid oxidation due to oxide breakaway after consumption of approximately a milligram of OXYgen per square centimeter of zircaloy surface.
Figure 1 shows the authors' correlation from pages 31-34 of Reference 1 superimposed as a dashed line on some of the data of Reference 2.
The authors' correlation is approximately correct at 1 and 1000 minutes but underestimates the oxidation after 1000 minutes (16.67 hours7.75463e-4 days <br />0.0186 hours <br />1.107804e-4 weeks <br />2.54935e-5 months <br />) at 973 K.
Since oxide breakaway and the associated oxidation rate increase occurs at a given weight
- gain, the authors' correlation will begin to underestimate oxidation at times less than 1000 minutes for temperatures higher a.
Zircaloy-2 and Zircaloy-4 have tin concentrations in this range and should behave like these alloys.
~
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RE~CTlON OF ZIRC::~"UM
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Correlation of Reference 1 superimposed as a dashed line on data from Reference 2.
It should be noted that the errors introduced by the parabolic approxi-mation are probably not serious for the application discussed in Reference 1 because the low temperature alpha phase oxidation is slow and does not represent a major fraction of the heat source.
3.
A more serious problem arises from the authors' treatment of cladding failure.
It is assumed, based on a correlation from R. H. Chapman3 that the cladding will fail only if the temperature exceeds 1173 K (900 oc).a Chapman later found that the cladding burst temperature in the alpha phase region is a function of the heating rate.4 In
- fact, it is now known that one can get burst at any temperature above 750 K simply by waiting long enough because the cladding equation of state for deformation becomes essentially a stress-strain rate law above this temperature.5 One
~eed only apply a stress at any fixed temperature above 750 K and wait for the cladding to creep until the ever-thinning wall experiences a stress equal to the failure stress at the temperature
'selected.
The correlation on page 18 of Reference 3 is correct only for the 280 Cis heating rate used to derive it.
In order to "determine the conditions which could lead to clad failure due to overheating" as the authors state their objective on page 11, one must consider the time-temperature history of the fuel rods with a mechanistic approach like that employed in the BALON-2 computer code.6 A few example results using this code are presented in Table 1, which shows the time-to-failure for fuel rods held at constant temperature and a pressure difference of 4600 KPa (667 psi).
The table shows that fuel cladding can be expected to fail at temperatures as low as 920 K (6470C) if it is allowed to remain hot for the times of up to 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> shown in Figures 12, 14, 15, 16, 18, and 19 of Reference 1.
This long-time failure temperature is 250 degrees below the
~ilure temperature assumed for the corresponding pressure difference in Reference 1.
Telli;J0ratun?
(K) 1033 979 937 920 TIME-TO-FAILURE (min) 5 5.5 22
>150 1.
!~.llan S.
3e:lj:drin, David J.
McCloskey, Dana A.
- Powers, Stephen A.
[J:Jpree,
_~_P'2"'l-!ue1 He~ tup Fa 11 owi ng Loss of Water Duri ng
- Storage, HUREG/CR-0649 Jr~ :~ND 77~1373, March 1979.
2.
H. 'I.
POI',,:e, J.
~chnizle;n, R.
C.
- Vogel, and D.
F.
- Fisher, Oxidation
~~_Zi":i)ni~~~~;d Zirconium
- Alloys, ANL-6046, September 1959.
3.
R. ri.
Ch;:I)'*~"lil*. 1-:liltirod Burst Test Program Quarterly Progress Report Tor Jc:1y-Se()ternl:~
__..l976, ORNL/NUREG/TM-77, February
- 1977, p.
18.
4.
H.
C~~Drn3n,
~ultirod Burst Test Program Progress Report for January-March
}978) ii~KES/C~~-C225 and ORNL/NUREG/Tr~-217, August
- 1978, p.
28.
5.
~). L.
HJgl:~iarl, G.
fl.. Reymann, and R.
E.
- Mason, MATPRO-Vers;on 11 (Revision l) ~ Handbook of Materials Properties for Use in the Analysis 0f Liaht
~!a:=\\.
R~actor Fuel Rod
- Behavior, NUREG/CR-0479 and TREE-1280, Rev.
2,
':;~gust--_~-31, pp.
313-363.
6.
D.:".
rlc.<;r;'1t1'.)
_~i(caloy Cladd;nq Shape at Failure (BALON2),
EGG-CDAP-5379, July Er 1.
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Maximum Clad Node Tem~erature for Different Fuel Relocation Ootions (Modified SFUEL) c.
-' ~ime xl0J sees 0.0 3.6 1
9.0 12.6 16.2 19.8 23.4 27.0 30.6 34.2 37.8 39.2 41.4 42.3 42.54 42.61 42.67 42.73 42.79 42.86 42.98 43.97 43.11 43.13 43.19 43.20
~
10.0 76.6 192.7*
280.4 370.4 458.3 544.7 633.1 723.5 813.5 899.5 942.0 1014 1117 1656 2117 2465 2747
- bRAFT SUgJEC.T T'O c.HA.N,e THE POTENTL\\L FOR EIRGOMOH B~~IUrle PROPAGATION D}-~ '~/J...J"'J+-"*""j
~~t~OLLOWING LOSS OF WATER IN A SPENT FUEL STORAGE POOL r' ~(,
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by I ~
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NICOlA A.
PISANO AND FREDERICK BEST*
tl -'~
MASSACHUSETTS INSTITUTE OF TECHNOLOGY f ri*
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ALlAN S.
BENJAMIN AND K.
TERRY STAl.KER
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SANDIA NATIONAL LABORATORIES u.S.
NUCLEAR REGUlATCRY COMMISSION OFFICE OF NUCLEAR RrACTCR REGUlATION DIVISION OF Sisre:'1S rj.jT~G~TION