Information Notice No. 86-68: Stuck Control Rod
SSINS No.: 6835
IN 86-68
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
August 15, 1986
Information Notice No. 86-68: STUCK CONTROL ROD
Addressees:
All boiling water reactor facilities holding an operating license or a
construction permit.
Purpose:
This notice alerts addressees to the potential for a stuck control rod and
damaged control rod drive as a result of closed manual isolation valves on
the hydraulic control unit. Recipients are expected to review the
information for applicability to their facilities and consider actions, if
appropriate, to preclude similar problems occurring at their facilities.
However, suggestions contained in this information notice do not constitute
NRC requirements; therefore, no specific action or written response is
required.
Description of Circumstances:
During a full scram preoperational test (before fuel loading) on April 29,
1986, the Clinton nuclear power plant found that rod 08-25 was stuck at
position 06. Attempts to move the rod by gradually increasing drive water
pressure were unsuccessful.
Investigation revealed that the 112 valve (scram discharge riser manual
isolation) on the hydraulic control unit was in the closed position despite
verification that the valve was in the proper (open) position. The closed
112 valve caused a very high pressure to develop in the control rod drive,
crushing the cylinder tube. Because the surface area above the piston is
smaller than that below the piston, a closed 112 valve can cause pressures
above the piston to be several times as large as the 1850 psi applied at the
bottom of the piston by the scram accumulators. The above-the-piston water
surrounds the cylinder and can partially crush the cylinder. The crushed
cylinder tube caused interference with the movement of the piston and thus
the stuck rod.
Subsequent unsuccessful attempts were made to insert the partially withdrawn
control rod into its drive by removing temporary blade guide assemblies and
applying force from above with a specially constructed tool. The rod was
ultimately removed by cutting the index tube into two pieces and removing it
from the drained reactor vessel.
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IN 86-68
August 15, 1986
Page 2 of 2
The licensee has adopted a number of corrective measures. The manual
isolation valves are locked open once they are verified to be in the proper
position. The personnel access control to the hydraulic control units has
been tightened. In addition to the normal independent verification of valve
position, the startup organization has instituted a temporary measure of
having management personnel make additional random checks.
Discussion:
Although not always causing damage to the control rod drive as occurred at
Clinton, there have been several instances of manual isolation valves on the
hydraulic control units causing failure to scram of individual control rods.
Among the other instances when problems with the manual isolation valves on
the hydraulic control unit have prevented a rod from scraming are:
Date Plant Event-Failure to Scram One Rod
October 20, 1984 Dresden Unit 3 Manual isolation valve disc
separated from valve stem
October 24, 1984 Quad Cities Unit 2 Scram inlet isolation valve closed
April 11, 1985 Perry Inadvertently closed 112 valve -
CRDM damaged, index tube cut to
remove
There are other valves on the hydraulic control unit that could prevent a
scram or cause damage to the control rod drive mechanism. Some of these
valves could be discovered by problems with normal rod movement before a
scram. However, a closed 112 valve does not cause problems with normal rod
movement. Because larger pressures are applied to the control rod drive
during the scram, damage caused by closed valves on the hydraulic control
unit is most likely to occur during the scram.
No specific action or written response is required by this information
notice. If you have questions about 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: Eric Weiss, IE
(301) 492-9005
Attachment: List of Recently Issued IE Information Notices
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