ML19326E020
| ML19326E020 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 07/18/1980 |
| From: | Crouse R TOLEDO EDISON CO. |
| To: | Novak T Office of Nuclear Reactor Regulation |
| References | |
| 632, NUDOCS 8007250427 | |
| Download: ML19326E020 (9) | |
Text
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Docket No. 50-346 TOLEDO License No. NPF-3 ggg Serial No. 632 RCHI.ao P. CACUSE July 18, 1980 7/l,7"'
?413) 259-E22 I Director of Nuclear Reactor Regulation Attention: Mr. Thomas M. Novak, Assistant Director Operating Reactors Division of Licensing U.S. Nuclear Regulatory Commission Washington, D.C.
20555
Dear Mr. Novak:
This is-in response to your letter of July 1,1980 (Log No. 573) concerning the failure of fuel assembly holddown springs at the Davis-Besse Nuclear Power Station Unit 1 (DB-1).
On June 10,.1980, Toledo Edison and Babcock & Wilcox met in your Bethesda offices to discuss the status of the spring failures and inspections. Also described at the meeting was a proposed repair process to be utilized at DB-1.
On July 10, 1980, Toledo Edison submitted a supplement to the Cycle 2 Reload Report (Serial No. 627) identifying that a total of 19 broken springs were found at our facility and that all springs in Cycle 1 fuel assemblies returning to the core had been replaced. This removed from further operation all the spring material that exhibited heightened susceptibility to fatigue and stress cortosion cracking.
The attachment provided here responds to the questions in your July 1, 1980 letter. The responses are provided recognizing specifically that the material with heightened susceptibility has been removed from further service.
Very truly yours,
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THE TCLEDO EDISCN COMPANY EDISCN PLAZA 300 MACISCN AVENUE TOLECO, CHIO a3652 a o onsoy17
Docket No. 50-346 License No. NPF-3 Serial No. 632 July 18, 1980 Attachment Responses to Questions Concerning Fuel Assembly Holddown Springs For Davis-Besse Nuclear Power Station, Unit 1 (NRC Letter July 1,1980)
Item 1 (If the reactor is down for refueling and the reactor vessel head is off)
Examine all fuel assembly holddown springs in the core and in the spent fuel pool and report the number and extent of damage on the springs and affected assembly components.
Response
A total of 19 failed holddown springs were identified as a result of a complete inspection of all Cycle 1 fuel assemblies. Single and multiple breaks were found among the 19 springs. The breaks are characterized in the response to Item 2a.
All springs and spring parts were totally captive in the upper end fitting area of the fuel assembly.
All holddown springs on Cycle 1 fuel assemblies returning to the core were replaced.
Item 2a
" Provide a discussion of the safety significance of operating with one or more broken springs in the core. Your discussion should include, but not necessarily be limited to the following:
a.
Assume the holddown spring is broken, provide an estimate of the flow conditions under which the assemblies would be levitated.
(Provide the value of the force required to lif t the assembly, the flow conditions under which that force would be supplied, the number of coolant pumps that would be in operation under such conditions, and the schedule of reactor operations under which such conditions might have been achieved.)
Contrarily, demonstrate the margin between the assembly weight and the calculated maximum applied lif t-off force, if there is such margin."
Response
A break occurring in a holddown spring will result in a decrease in holddown force which is a function of the location of the break and the degree to which the coils become misaligned. To quantify this decrease, tests were run on springs cut at typical break locations. The test fixture was an upper end fitting complete with guide tube nuts, a holddown spider and a simulation of the upper grid plate pads.
Docket No. ' 50-346 License No. NPF-3 Serial No. 632 July 18, 1980 :
4 Two springs were prepared 'for the test:
one cut at the location of the transition from the dead coil to active coil, the second was cut at a location k i
-coil towards the mid-coil of the spring. A series of six tests were run using the two springs. Each was tested with: 1) no breaks, 2) one break at the upper i
location, and 3) breaks at both upper and lower locations.
The retained holddown force.(at 100*. power, BOL) exceeded 300 pounds for three of the four break configurations. - Only the configuration with two breaks, each k coil f rom i
the transition, resulted in a significant loss. The retained holddown force for j
this extreme configuration was 64 pounds.
i The broken springs observed were characterized by breaks within k coil of the 4
top or bottom transition point. Two springs have contained breaks at both top and bottom. There was a single exception to this pattern where one spring was broken at the' lower transition and again approximately 1/3 coil towards the spring mid-coil. The broken piece was wedged between the dead coil and the upper coils. From the test results, it is estimated that this spring would have l
retained approximately 100 pounds holddown force without the wedged piece (which should increase holddown).
Thus, this test encompasses the observed spring breaks. Based upon these results,- it can be concluded that a broken spring is likely to retain from 64 to 500 pounds holddown force. The break most frequently observed would provide a retained holddown force near the upper end of this range.
It is significant to note that each of the broken holddown springs observed to date has held. the spring spider against the retaining plugs. This pinned
. condition is only possible due to some retained preload on the spring. Springs in this condition are expected to develop a minimum of 100 pounds retained holddown force when ; extrapolated to operating conditions due to additional preload f rom the reactor internals.
The flow of coolant water through the core during normal operation produces large hydraulic forces on the fuel assemblies. The actual forces imposed during 2
operation will depend -on'the total flow through-the core and the distribution of coolant flow to the various assemblies. The total mass flow is a function of the
- coolant temperature and the number of reactor coolant pumps (1-4) in operation.
The flow distribution is affected by:
- 1) the power distribution, 2) the
- assembly geometry (i.e., control rod, orifice rod, BPR, open guide tube), and
- 3) -the location within the core (peripheral / interior).
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Docket No. 50-346 License No. NPF-3 Serial No. 632 July 18, 1980 Counteracting these large hydraulic forces are the fuel assembly weight (approximately 1510 lbs. in air), the supplemental force supplied by the preloaded holddown spring and frictional forces exerted by the reactor internals and adjacent fuel assemblies. The holddown spring is sized to provide a minimum force under the most adverse conditions (coolant) temperature, irradiation exposure, dimension tolerances, etc.), without consideration of frictional forces.
The force required to lift a fuel assembly is assumed, for this evaluation, to be equal to the weight of the fuel assembly in water (that is, it is assumed that there is no holddown force available frcm the holddown spring or from frictional forces).
Based on a nominal system flow rate of 114% of design (where the design rate is 352,000 GPM) the maximum net lif t force on any assembly is +58 pounds. That is,
the net vertical (upward) force on the fuel assembly, taking credit for only the wet weight (no spring force) is 58 pounds. The net force on fuel assemblies in control rod locations varies between -143 pounds to -236 pounds indicating that all of these assemblies have significant margin to lift even if no credit is taken for the spring's holddown force. Of the core locations not occupied by control rods, 52 have a net positive lif t force with no spring force considered.
Recalling that all broken springs observed to date have retained at least 100 pounds holddown force it can be concluded that no fuel assembly lift would be predicted for normal operation with broken holddown springs.
The hydraulic forces on the fuel assembly generally increase with decreasing temperature.
The phenomenon is due to the increased fluid density at the reduced temperature. Therefore, the most severe lift condition is the lowest temperature at which four reactor coolant pumps are in operation. The holddown spring is sized to accommodate this limiting condition --the fourth pump startup.
The maximum net lift force at the fourth pump startup temperature of 500 F is
+134 pounds.
For this condition all control rod locations maintain positive holddown without the benefit of the spring force. Lif t forces on assemblies in control red locations vary from -62 pounds to -163 pounds. Assuming a minumum retained holddown force of 100 pounds for a broken spring, only 20 assemblies would be predicted to lift for this extreme temperature condition; however, lif ting under this transient condition is not a significant concern and will not cause significant fuel assembly wear or damage.
r Docket No. 50-346 License No. NPF-3 Serial.No. 632 July 18, 1980 !-
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Due to the increase in holddown requirements with decreasing temperature, transients which cause an ' overcooling of the primary system are the most limiting with respect to fuel assembly lift. Such transients will, in general, be terminated before reaching a condition analyzed for the fourth reactor coolant pump startup. However, if the primary coolant temperature were to go'below.500 F and all four reactor coolant pumps were inadvertently left on, the required holddown force would continue to. increase at a rate of approximately 120 lbs. for each 100 F the prima ry coolant - temperature drops below 500 F.
Without the force from i
holddown springs,-a significant number of fuel assemblies'would be expect 2d to lift under this condition; however, lifting under these transient conditions is not a significant concern and will not cause significant fuel assembly wear or fuel damage.
Operation with less than four reactor coolant pumps is expected to produce no fuel assembly lift regardless of the spring holddown force
'available. The maximum net lift force (with no credit for spring force) 4 for three pump operation at 100 F is -73 pounds, indicating significant margin to lift. This temperature was chosen for the evaluation to conservatively accommodate all possible three pump operation. Due to the demonstrated. conservatism shown for 3 pump operation, no ' evaluation i
-of 1 or 2 pump operation is required; fuel assembly lift will not occur
- for these pump operating conditions regardless of the spring force available.
i Item 2b "Have any loose assembly parts (i.e., broken springs, pieces of cladding) been observed anywhere in the primary system? Describe your methods for loose part detection. Are there installed noise detectors capable of detection of broken springs, pieces of cladding, or vibrating assemblies?"
Response-There have been no loose fuel assembly parts observed in the primary system.
There are two methods at DB-1 to detect loose parts. The first is by a Vibration and Loose Parts Monitor System (VLPM)..The second is by audio monitoring'of.the VLPM.
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Docket No. 50-346 License No. NPF-3 Serial No. 632 July 18, 1980 The VLPM is a twenty-seven channel system that continuously monitors all channels. Eight of the channels have loose parts detection capabilities. Two channels each are located in the upper and lower steam generator as well as the upper and lower reactor vessel areas. The detection of a loose part results in the lighting of a red indicator light on the VLPM panel.
The sensitivity of the system is 0.05 ft. pounds at the sensor with an alarm setting of 0.5 f t. p'ounds +0, -0.25 f t. pounds, Therefore, the system is able to detect a loose part of a mass of 0.007 pounds.
The audio monitoring is performed at the VLPM panel via a speaker provided in the system. The eight loose parts channels are monitored at least once a shif t by operations personnel and at least five times a week by the system's cognizant individual. The method used is listening to each channel for unusual noise characteristics of loose parts.
The system would be capable of detecting broken springs and pieces of cladding if the mass of the part is 0.007 pounds or greater.
The VLPM is not capable of detecting an assembly vibrating vertically, and there is a very small possibility of detecting horizontal assembly vibration.
Item 2c "Have there been any excore or in-core neutron detector indications of levitated assemblies? Describe the expected reactivity effects that would result from lif t-of f or reseating of assemblies with broken holddown springs. What efforts are being utilized to detect those assemblies by either nuclear or mechanical monitoring devices?"
Response
A check of power and intermediate range logs and printouts of in-core detector output shows only routine power variations at steady state of several tenths of a percent of full power.
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Docket No. 50-346 License.No..NPF-3
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Serial No. 632 July 18, 1980' ll.
Normal steady state operation with lifted fuel assemblies does not represent a safety concern. If a lifted assembly were to reseat during operation, a small
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increase in core reactivity would occur due to the relative motion between the i
fuel assembly and a partially inserted control rod. Conservative calculations have predicted that a" fuel assembly lifting 1.5 inches (the maximum possible) would change the core reactivity 0.002% 4 k/k at hot full power and 0.006%
4 k/k at hot zero power. The limiting reactivity insertion would occur if the fuel assemblies in all 61 control rod locations were lifted the maximum-distance. As discussed in the response to question 2(a), assemblies in control rod locations retain positive holddown during normal operation even with no spring force. Thus, this limiting reactivity insertion is a hypothetical event.
4 For this condition a maximum reactivity insertion of only 0.1% A k/k at hot full power is predicted. The resulting transient would, at worst, be characterized by a small, rapid increase in neutron power, tripping the plant on high flux in the first few seconds of the transient. The transient would also result in a small increase in reactor coolant system pressure with no change in core inlet temperature 'for approximately 10 seconds (one loop transit time). Thus, even j
this hypothetical reactivity insertion does not significataly affect the steady.
state and transient safety analysis; the potential reactivity insertion from a small number of spring failures, if lif ting were to occur, is shown to be of no consequence.
Items 2d and 2e'
- 2d -
"Have there been any observed indications of lateral repositioning of loose assemblies? Describe the methods used to detect lateral assembly motion.
Describe the degree of lateral repositioning that is physically
'(dimensionally) possible after lift-off. What are the postulated worst-case. effects of a laterally displaced assembly?"
2e
"(i) Describe the degree of " worst-case" mechanical damage that would be expected as~a result of movement of a " loose" assembly (one with a broken
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spring). against adjacent assemblies, core baf fle, or other core
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-components."
"(ii)-Discuss the' results of flow tests or other experiments that have provided measurements of axial or lateral vibratory motion of an assembly after lift-off or that would otherwise support the response to Question 2. e (i ) '. "
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Docket No. 50-346 License No. NPF-3 Serial No. 632 July 18,1980 Response (2d and 2e)
As discussed in the response to Question 2a, fuel assemblies with broken holddown springs would not be predicted to lif t-of f d tring normal operation.
Fu rthe rmo re, there have been no indications that any of these assemblies did lift-off.
Three fuel assemblies containing broken holddown springs were visually examined.
No evidence of lift or of wear from lift or lateral displacement was found. No fuel assembly damage of any kind was found.
A fuel assembly suddenly experiencing a loss of holddown could move upward a maximum of 1.5 inches, with a cocresponding impact energy level of less than 50 ft.-lbs.
This level of impact is far below the energy necessary to damage the fuel assemblies. For example, LOCA analysis has shown that the fuel assembly can withstand impact energies in the range of 500 f t.-lbs.
Thus, gross impact of fuel assemblies can be eliminated as a cause for concern, but there is the possibility of lower level vibrations which could cause some wear. Also, there is the possibility of spacer grid mismatch due to lifting of one assembly while its neighbor remains seated. The fuel assembly can lift up to 1.5 inches at beginnit+ of life, whereas 1.2 inches lif t will rest.t in the spacer grids outside st ips no longer matching up.
Long term operation under this condition would, at worst, result in damage to some peripheral fuel rods. There is no possibility of damage resulting in non-insertion of control rods, because the guide tubes are protected by two rows of fuel rods.
Horizontal vi ration of the fuel assembly while in the lifted condition may be more pronouncsJ at the lower end fitting, because it may not be held tightly by the grid pads. Lateral motion in which two adjacent ass olies contact at the lower end fitting is possible and could cause wear on t ae lower end fitting.
However, the lower end fitting has thick cross sections which can withstand significant wear without loss of function. Peripheral assemblies might contact the core baffle plates but again wear would not be a significant problem. The lower end fitting of a fuel assembly, which is postulated to lif t 1 inches, can raise up onto the chamfered lead in surfaces of the guide blocks such that 0.4 inches c f lateral repositioning could theoretically occur. However, lateral is nominally limited to the clearances between the lifted assembly and adjacent seated issemblies or baffle plates (which are 0.05 inches and 0.1 inches, respectively).
The upper end fitting will remain closely aligned by the upper grid pads at all times.
Lateral vibration would not be expected to increase. For this reason tpper end fittings wear.or control component wear would not be expected to be any greater than the !ow lsvels experienced during normal operation.
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Docket No. 50-346 License No. NPF-3 Serial No. 632 July 18, 1980 There have been several tests run to determine the flow required to cause fuel assembly lift. These tests also provide an indication of assembly vibration levels in the lifted condition. They were run in the Controi Rod Drive Line Test Facility (Alliance Research Center), which is a single fuel assembly test loop simulating reactor flow, temperature and pressure. A displacement transducer was used in determining fuel assembly lif t.
During these tests, the holddown spring remains uncompressed, because since the maximum loop flow is incapable of lif ting the assembly with the spring compressed. The flow is increased in small increments until the assembly lifts, at which point the flow is then varied to determine the lift velocity as accurately as possible. There has been no indication of vertical oscillation of the assembly during these tests. Also, the fuel assemblies were examined af ter each test and no evidence of impact or wear has been found. These results indicate that severe vibration will not result for a lif ted assembly.
Item 3
" Provide a description of the cause of the failures and corrective action to reduce the likelihood of future failures at your facility."
Response
The cause of the holddown spring failures was an improper material condition characterized by a coarse outer grain structure. Coarse grain.ctructure is indicative of less fatigue resistance. The coarse grain material precipitated fatigue crack initiation. The mechanism of failure was then fatigue propogation followed by the secondary effects of stress corrosion cracking and final fracture.
Corrective action consisted of replacing the springs from this heat of material.
Replacement springs were made to a current specification, which controls grain size to obtain uniform fine grain to provide increased fatigue resistance.
i TC 10/2-9
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