The State of
Utah reported that one of their licensees, Chevron
USA, Inc., reported a source disconnect on a custom made Ronan level density
gauge at their refinery. The
gauge was located in a dry storage tank with two pressure vessels inside. The source was inside the storage tank and was retrieved by a health physics contractor. The source is currently stored at the licensee's facility in a locked storage shed, surrounded by
lead bricks. The licensee is awaiting the arrival of the Ronan representative on-site. There were no excessive exposures to either workers or the public.
The source was 100 mCi of Cs-137 and is a Model SA-4.
The following information was submitted by the state by fax:
Approximately 3:00 PM on December 22, 2008, Chris Crossman RSO from Chevron called to report that a Ronan Model SA-4 source holder containing a Cesium-137 source with approximately 100 mCi (s/n: 2231 CM) became unattached from a belt that the source holder was clamped to and fell to the ground and landed inside the storage silo it was mounted to.
Upon discovery, the RSO was notified and the area was immediately cordoned off, and perimeter access was maintained. A service licensee was called to the facility to perform surveys of the area and retrieve the source. The source was sequestered and placed into a lead brick cave in a secured storage shed on the licensee's premises. Ronan Engineering, who built this custom device for Chevron, has been contacted and they indicated that they will be out sometime after the first of the year to re-install the source.
On January 13, 2009, a reciprocity inspection and investigation by the Utah DRC was performed at the Chevron Refinery with a Ronan representative, [DELETED], and [DELETED] the Chevron instrument technician who initially reported the incident.
The source holder is comprised of an 18 inch stainless steel rod with a clamp at one end. This clamp, the 'tape grabber,' clamps to the end of a guide belt. Over the clamp is a stainless steel threaded sleeve, held in place with a small tension screw. At the opposite end of the source rod is the Cs-137 source mounted inside the stainless steel rod. Over the source is a cylindrical collimator which can open to fine tune the exposure field. The source rod is mounted inside a dry 'guide tube' that is raised and lowered inside the tank, via an electronic pulley mechanism. The pulley also controls in tandem, at the same level as the source, a detector mounted on the outside of the tank, also in a dry guide tube.
By using a dummy source holder for comparison, the source holder and clamp were briefly examined while inside the lead cave. The source holder was found intact and a previously done leak test indicated no leakage from the source. Closer scrutiny of the clamp end (the Tape Grabber) revealed that the set screw that holds the tape grabber together had partially loosened. Furthermore, small tension screw that holds the outer sleeve in place had worked itself loose and somehow sheared off. With the inner screw loose and the outer screw the weight of the source holder is all that was needed to pull the guide belt out of the clamp.
Root Cause: Failure of equipment to perform as designed. Poor design and testing of design contributed to the failure of this device.
Corrective Actions: The licensee called Rocky Mountain Health Physics [RMHP] who came out and relocated the source and source holder to a secure storage shed on the licensee's premises. The area where the source had fallen was roped off and all personnel in the immediate area were escorted away. RMHP wipe tested the source holder and there was no evidence of a leaking source. The licensee then called Ronan Engineering to report the failure of the device and Ronan indicated they would send a representative to the refinery in January 2009.
Utah Incident Number: UT 08-0006
Notified the R4DO (Farnholtz) and
FSME (Chang).