Information Notice No. 86-06: Failure of Lifting Rig Attachment While Lifting the Upper Guide Structure at St Lucie Unit 1
SSINS No: 6835
IN 86-06
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
February 3, 1986
Information Notice No. NO 86-06: FAILURE OF LIFTING RIG ATTACHMENT WHILE
LIFTING THE UPPER GUIDE STRUCTURE AT ST
LUCIE UNIT 1
Addressees:
All nuclear power facilities holding an operating license (OL) or a
construction permit (CP)
Purpose:
This notice is provided to advise licensees of a potentially significant
problem that occurred during the movement of a heavy load over the reactor
core It is expected that recipients will review this information for
applicability to their facilities and consider actions, if appropriate, to
preclude similar problems at their facilities However, the suggestions
contained in this notice do not constitute NRC requirements; therefore, no
specific action or written response is required
Description of Circumstances:
On November 6, 1985, while lifting the upper guide structure from the St
Lucie Unit 1 reactor vessel, licensee personnel noticed the lifting rig
tilt The lift was immediately stopped, with the lifting rig canted upward
about 6 inches and the guide structure canted downward about 6 inches at one
of the three attachment points An attempt was promptly made to lower the
load back to its installed position, but the load cells indicated binding so
the attempt was terminated after lowering the load a few inches The 50-ton
load was left suspended about 8 feet above the reactor core
The licensee declared an Unusual Event Core alteration containment
integrity was enhanced by resuming full use of the airlocks Temporary
primary manway covers were installed on both hot and cold legs to enhance
the nozzle dams Survey transits were set up and procedures implemented to
monitor the rig and load for any shifts in their positions
The licensee and the nuclear steam system supplier, Combustion Engineering,
designed and tested a supplementary lifting rig to support the upper guide
structure from the upper portion of the normal rig, using a cable and J-hook
system On November 9 with the supplementary rig installed, the load was
jacked to a level orientation and moved to its normal parking position in
the refueling pool
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IN 86-06
February 3, 1986
Page 2 of 3
Damage caused during the incident included bending the lifting rig and one
of the two guide pins that align the rig with the reactor vessel
Discussion:
The upper guide structure is shown in Figure 1 It is supported in the
reactor vessel by its upper flange It is aligned by eight alignment keys,
four at the top and four at the bottom The structure fits down inside the
core support barrel, just above the fuel assemblies (see Figure 2) The fuel
assembly alignment plate is the bottom component of the upper guide
structure
The lifting rig is attached to the upper guide structure by three vertically
oriented bolts These bolts are attached from above the water line by torque
tools that run down the hollow columns of the rig (see Figure 3) Combustion
Engineering's procedure for attaching the rig calls for checking for thread
engagement and torquing each bolt to 50 ft-lbs The licensee's procedure
omitted the step concerning the check for thread engagement Subsequent
inspection of the bolt that had pulled loose indicated that part of the last
thread was stripped It is assumed that this bolt cross-threaded or bound
due to rig to guide structure misalignment during attachment and reached the
50 ft-lb torque requirement with only part of one thread engaged During the
lift, the few inches of unengaged bolt shaft were pulled through the lifting
rig until the bolt head rested on the rig's surface at the bottom of the
column, resulting in an imperceptible tilt The resulting lateral load was
initially supported by the guide pins When the rig and guide structure were
lifted about 8 feet, where the guide bushings on the lifting rig reached the
tapered portion of the guide pins, it is surmised that sufficient lateral
motion was permitted to allow the thread of the improperly engaged bolt to
slip free This caused the observed motion and tilt
After the guide structure was supported by the supplemental lifting rig and
leveled, it was moved to its normal parking position in the refueling pool
The short attachment bolts and torque tools were then replaced with
full-length bolts The long bolts are designed with heads that rest on
surfaces at the top of the three hollow columns of the lifting rig This has
the advantage of making any lack of full thread engagement more apparent to
the personnel attaching the rig The guide structure was subsequently
returned to its installed position using the long attachment bolts
The licensee has not yet decided whether to permanently modify the
attachment bolts The licensee plans to review all reactor-related lifts for
adequacy of the procedures to ensure proper lift rig attachment, including
provisions for measuring thread engagement
The potential consequences of dropping heavy loads into the open reactor
vessel were addressed by Unresolved Safety Issue (USI) A-36, "Control of
Heavy Loads Near Spent Fuel" The concern for a UGS drop is that fuel
assemblies might be sufficiently damaged to release the radioactive gases
and iodines held within the fuel-clad gap Under the reduced containment
integrity requirements for the refueling mode, damage to several fuel
assemblies might cause the radiation dose limits of 10 CFR 100 to be
exceeded
IN 86-06
February 3, 1986
Page 3 of 3
Plant specific calculations were not made for a UGS drop at St Lucie
because the NRC determined that further calculations were not required after
reviewing initial calculations previously submitted by other reactor
facilities in response to Phase II of USI A-36 Some indication of the
consequences of a UGS drop at St Lucie can be gained from calculations
performed by Combustion Engineering for a reactor vessel head drop at
Waterford 3 The head drop calculations assumed the reactor vessel head was
sufficiently tilted at impact to directly strike the UGS with the UGS at
rest in its normal installed position The calculated response velocity of
the Waterford UGS was 28 feet per second, and the resulting vertical
stresses imposed on the fuel were not sufficient to rupture the cladding
If the St Lucie UGS had dropped from an 8 foot elevation, its striking
velocity would have been substantially less than the UGS response velocity
calculated for the Waterford head drop However, the potential for
misalignment of the recesses in the bottom of the UGS (ie, the fuel
assembly alignment plate) with the fuel assembly upper end fitting posts was
not addressed by the Waterford scenario If substantial misalignment
occurred, the fuel could be subjected to additional axial loading
Significant misalignment could not occur without substantial impact damage
to the eight keys and keyways, which would also result in a reduced striking
velocity of the UGS as it reached the fuel On this basis, significant
radioactive gas release is considered to be unlikely, although it has hot
been shown to be impossible
No specific action or written response is required by this notice If you
have any questions regarding this matter, please contact the Regional
Administrator of the appropriate NRC regional office or this office
Edward L Jordan Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: S M Long, IE
(301) 492-7159
D E Sells, NRR
(301) 492-9735
Attachments:
1 Figure 1, Upper Guide Structure Assembly
2 Figure 2, Reactor Internals Assembly
3 Figure 3, Upper Guide Structure Lifting Rig
4 List of Recently Issued IE Information Notices
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