ML20052E248
| ML20052E248 | |
| Person / Time | |
|---|---|
| Site: | University of Illinois |
| Issue date: | 04/30/1982 |
| From: | Beck G VILLANOVA UNIV., VILLANOVA, PA |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8205100235 | |
| Download: ML20052E248 (3) | |
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' University of Illinois at Urbana-Champaign Nuclear Engineering Program G
214 Nuclear Engineering Laboratory 4-30 D
8' Urbana, Illinois 61801 (217) 333-2295 Director, Division of Reactor Licensing RECLNVED 9
U. S. Nuclear Regulatory Commission p.
MAY 7 1982>
Washington, D. C.
20555
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Dear Sir:
TmG Report: Abnormal Occurrence, Stuck Fuel Element
/f Violation of a limiting condition for operati 15 Event During a routine inspection of the fuel elements for changes in length and diameter, the fuel element in an hexagonal position designated as B-2 could not be removed through the top grid plate. Repeated efforts showed that the maximum distance that the element could be moved was approximately 12-inches. This would be about 1.3-inches above the vertical center line of the element. Adjacent elements in the B-hexagonal were removed with no difficulty. These were checked for length and diameter. The element in B-1 showed little change. However, the element in B-3 would not clear a go-no-go tube that it had cleared in a previous check.
It did clear a go-no-go tube with a diameter of 1.53-inches.
From the Technical Specifications, Section 4.1.c., there is a requirement that an element shall be considered to be damaged and shall not be used in the core for further operations if a lateral bending greater that 1/16 of an inch is detected.
Since the element had obviously been used previous to the check, this could be considered as a violation of a limiting condition of operation.
Note: The go-no-go tubes are made of aluminum with a central opening and a length of 30-inches.
Two of these tubes, with respective central openings of 1.50" and 1.53", are used to check any bending or swelling of the fuel elements.
Since the original diameter of a fuel element is 1.47", a failure of the element to clear the 1.53" tube means that it is very close to reaching the Technical Specification limit.
If it fails the clear the top grid plate opening (1.535"), it has exceed he limit.
The core is hexagonal in shapc with a series of hexagonal openings for locating the fuel elements. A central opening is designated as position A.
The remaining hexagonals are designated as B, C, D, E,. F, and G.
The numbering of the elements starts at one of the vertices and then continues in a clockwise direction.
Later indications of the position of an element will just give the actual location on the above baris.
Procedure after Occurrence The first attempt to free the element consisted of changing the temperature of the water in the reactor tank.
It was hoped that the difference in the expansion or contractio.1 between the grid plate and the fuel element might be sufficient to free the element. The water temperature was initially lowered to 24 C and then later heatml to 38 C.
Attempts to free the element at these extremes were unsuccessful A this time, discussiol.s were held with a number of individuals. This included a call to General Atomic for any advice.
From these discussions, 8205100235 820430
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y page 2 several alternatives were considered as possibilities.- These are given in the sequence:
A.
Remove the center section of the top grid plate.
i To accomplish this under normal conditions elements in B and C would be 3
removed. The ce ter section is then rotated 30 and lifted from the grid.
It was felt that this could be accomplished with the single element in the B-hexagonal. The element would be tilted to allow this to occur.
With the grid removed, the handling tool could then be attached to the element for i
movement to one of the underwater racks.
B.
Attempt to lower the element through the top grid plate.
This would be attempted by attaching a tool to the lower section of the J
element and then tilting the-element until the top grid plate is cleared.
. Before doing this, a mock set-up would be made to assure adequate clearance and to avoid the possibility of wedging the fuel element.
C.
Apply more force to the handling tool to see if the element could be squeezed through the opening in the top grid plate.
A calculation showed that up to 2 tons of force could be applied before rupturing the.20 mil stainless steel cladding.
It was further determined that the aluminum handling tool would pull apart with much less force.
If this method were to be used, constraints would be placed on the grid plate to avoid any buckling or stripping the threads of the hold-down screws.
j D.
Remove the top grid plate.
'Ihis would be a last resort since it would be necessary to remove all of the fuel-elements and graphite dummy elements from the core. Several underwater racks would have to be built or altered.to accomodate the 102 fuel elements, j
The stuck element was found on Thrusday (April 22). Most of Friday was I
spent in studying the different plans.
Late Friday and early the following Monday, the other elements in B and C were removed. Each element was checked for length and diameter and then placed in underwater racks.
I After the fuel was unloaded, efforts were made to rotate the center section.
This was successful, however, it was found that a special tool would probably be necessary to lift the section.
By pulling on the section with a hook in one of the empty fuel openings, it did appear that the section moved slightly l
upward, but was probably wedging on the sample tube that is located in the central position.
After tapping on parts of the section that were slightly elevated, the section suddenly returned to its original position. The force of the element probably caused this to occur.
a l-One of the individuals who was helping with this work made another. attempt-to free the element. He was told not to worry about the upward force that he could exert in using the handling tool. After a short series of rotating the element followed by a quick pull, the element suddenly came free..It should not be difficult to visualize the feelings of relief that resulted.
The element appeared to have a blister slightly above the vertical center line.
In checking the element with.the go-no-go tubes, about 40% went into the 1.50" tube and 60% could be lowered in the 1.53" tube.
It is hypothesized that the blister stopped the element in the larger tube.
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page 3 Other B-C Elements Except for the change that was earlier indicated on the element in B-3, only one other distinct change was noted in the remaining 16 elements. The The element in B-4 stopped in the 1.50" tube with about 50% inserted.
In the previous measurement it had been inserted to-70% in this tube. The same is true for elements in the other hexagonals that have been measured to this time.
Past Histcry Since 1969, when operations were started, there have been 12 complete checks on the fuel elements. Three additional checks have also been made on the B-C elements. Changes have been noted in both parameters. However, with one exception, they have been within the specified limits. One element was removed in 1970 when it failed to insert through the 1.53" tube. This element easily cleared the'1" thick top grid plate and the damage was attributed to bowing.
As expected, the change in an element is related to its position in the core with larger changes occurring as one moves to the center of the core.
A number of elements have been shifted since the start of operations.
Typically, when new elements are idded to increase the reactivity, these are placed in the B or C hexagonal and the removed element is transferred to the G-hexagonal.
In early movements, the selection of the element to be moved was based on the change in length.
This parameter can be measured within about 5 mils and thus is more accurate that the diameter check. Recent changes of fuel elements have primarily been based on burn-up of U-235 in the element.
Previous to the last measurement, none of the elements in the B-hexagonal were in this location at the start of operations in 1969. Three of the elements in this location, B-2, B-3, and B-4 were placed in this location in 1972 and have been there since that time.
These are the three elements where changes were noted in the check that was just completed. However, the change in the length of these elements is below the normal that has been noted in the B-hexagonal.
Although changes in the diameter of elements has been noted, this was never considered in moving an element.
During the checks, the number of elements that fail to completed insert in the 1.50" tube has shown a gradual increase.
Most of this has been attributed to bowing rather than swelling.
In the measurements, previous to reactor operation, there were 8 element-that either railed or were tight in the 1.50: tube. Excluding the damaged element (B-2),
there are presently 27 elements that either fail to clear or are tight in the 1.50" tube.
Of this group,17 elements are stopped with 80% inserted where the basic change is attribtuted to bowing, 5 elements will clear up to 70% and the change could be a combination of bowing and swelling, and the 5 remaining stop at 50-60% where the primary change would be attributed to swelling near the center of the fuel element.
Future Efforts to Avoid Recurrence When the elements were reloaded in the core, all of the elements in B-C that failed to clear the 1.50" tube were moved to the F-hexagonal. One element ia the D-hexagonal,which only cleared to 60%,was also moved to the F-hexagonal.
This practice will continue following future checks. The only exception that might be made would be cases where the element clears 90% or more of the 1.50" tube.
University of ILLINOIS TRIGA Y rs trul License No. R-115
/A Docket No. 50-151 erald P. Beck, Reactor Supervisor Copy to Region III, Off. of Insp. 6 Enf.
3 Goodwin Urbana, Ill.
61801