Information Notice 2010-16, Failure to Disable Unsafe Geometry Bandsaw Reservoir Results in Criticality Safety-Related Alert
| ML100540070 | |
| Person / Time | |
|---|---|
| Issue date: | 08/13/2010 |
| From: | Marissa Bailey NRC/NMSS/FCSS |
| To: | |
| Dennis Morey, NMSS/FCSS 301-492-3112 | |
| References | |
| IN-10-016 | |
| Download: ML100540070 (8) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS
WASHINGTON, D.C. 20555
August 13, 2010
NRC INFORMATION NOTICE 2010-16:
FAILURE TO DISABLE UNSAFE GEOMETRY
BANDSAW RESERVIOR RESULTS IN
CRITCALITY SAFETY-RELATED ALERT
ADDRESSEES
All licensees authorized to possess critical mass of special nuclear material.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert
addressees to a concern arising from inadequate verification of new items relied on for safety
(IROFS) during an engineered change to equipment. NRC expects that recipients will review
the information for applicability to their facilities and consider actions to avoid similar problems.
Suggestions contained in this IN are not new NRC requirements; therefore, no specific action or
written response is needed.
BACKGROUND
B&W Nuclear Operations Group, Inc. (licensee) manufactures high-enriched uranium fuel, reactor core components and reactor cores for public and private sector customers at its
Lynchburg, Virginia, facility. The licensee uses a large horizontal band saw, shown in Figure 1,
Figure 1 Band saw to cut fissile components during preparation of samples which may be requested by customers
for destructive analysis. The licensee had performed cutting operations with the band saw for
many years in its original configuration shown in Figure 2. In this original configuration, the
band saw re-circulated cutting fluid through a 60 liter (15 gallon unsafe geometry) reservoir to
lubricate and cool the saw blade and component during cutting operations.
Figure 2 Original Band Saw Configuration
In 2003, based on the need to prepare samples for evaluation, the licensee decided to move the
band saw to its current location. Criticality safety analysis of the proposed relocation resulted in
the requirement that the band saw cutting fluid system include favorable geometry components.
This resulted in the installation of a new external cutting fluid reservoir as shown in Figure 3. To
facilitate use of the new favorable geometry cutting fluid reservoir, a new chip tray was designed
and substituted for the previous chip tray that had a screen on top of the old cutting fluid
reservoir as shown in Figures 4 thru 7 on the next page.
As shown in Figure 4, the previous chip tray with screen was replaced with a sloped tray which
directed the cutting fluid out of the body of the band saw to the new external cutting fluid
reservoir on the floor. The new chip tray did not have a screened bottom and fissile material
chips were filtered out of the cutting fluid by a small round screen in front of the tube at the exit
of the new chip tray. The modified band saw configuration is shown in Figure 8. During cutting Figure 4 Modified Chip Tray
Figure 5 Chip Tray Location
Figure 7 Chip Tray in Position
Figure 3 External Safe Geometry Column
Figure 6 Chip Tray Pulled Out of Saw operations with the modified band saw, the cutting fluid is pumped from the new external cutting
fluid reservoir back to the spray nozzles, collected by the removable cutting fluid tray and drained
back to the new external cutting fluid reservoir. Only a limited amount of cutting fluid remains in
the new chip tray. When this modification was implemented, the licensee safety organization, using a procedure known as a criticality safety release, directed that the old cutting fluid
reservoir, formed by the body of the band saw, be disabled. The nuclear safety release for the
modified band saw specifically stated If each machine has a built in coolant [cutting fluid]
reservoir it shall be disabled such that it is not usable. No specific instructions were provided in
the release for disabling the old cutting fluid reservoir on the band saw and no actions were
taken by the licensee during installation to disable the old cutting fluid reservoir.
As shown in Figures 5 and 6, the old cutting fluid reservoir could not easily be seen even with
the new chip tray removed and the operators had no reason or motive to look during routine
operations. The licensee always used the same two operators with the band saw and these
operators were trained and experienced enough to know that solution in the old cutting fluid
reservoir was an upset condition.
Figure 8 Modified Band Saw Configuration
DESCRIPTION OF CIRCUMSTANCES
On July 15, 2009, while the band saw was in operation cutting a fissile component, a band saw
operator observed cutting fluid leaking from a screw hole in the band saw body and the licensee
shut down the cutting process. Licensee safety personnel subsequently removed the new chip
tray from the band saw and found that the old cutting fluid reservoir was nearly filled with cutting
fluid. The reservoir had a base of approximately 12 inches by 24 inches and was filled with
cutting fluid to a depth of nearly 14 inches. These dimensions are unsafe for highly enriched U-
235 solutions and mixtures. The licensee declared an Alert and took actions to restore control of
the band saw including evacuating the areas near the saw, preparing a sampling procedure, and
performing sampling to assure that criticality would not occur during removal of the cutting fluid
from the reservoir. Subsequent to the sampling, the licensee drained the solution from the
reservoir and estimated that it contained 51 liters of cutting fluid and 13.76 grams of U-235.
The licensee subsequently determined that it was possible for cutting fluid to pass by the new
chip tray and enter the old cutting fluid reservoir because the new chip tray was removable and
the old cutting fluid reservoir was not sealed. It was possible during operations for cutting fluid to
splash upward, pass around the new chip tray and enter the old cutting fluid reservoir. Also, one
to two liters of water were used to clean the band saw every day and cleaning water could also
enter the old cutting fluid reservoir. The licensee noted that, during operations, an average of a
liter of cutting fluid was added to the system each day to make up for losses including losses to
the old cutting fluid reservoir. The licensee also determined, based on analysis of the cutting
fluid found in the old cutting fluid reservoir and the likely mode of entry for fluid, that the old
cutting fluid reservoir was probably filled at least halfway with fresh cutting fluid during relocation
as would be normal if the band saw had not been modified. NRC determined that the old cutting
fluid reservoir would likely become critical at approximately 9 to 10 inches depth with sufficient
mass.
Prior to adding the external safe geometry column, the licensee assured criticality safety for the
operation by estimating the maximum mass that could be removed in a single cut on a
component and then using a mass log to track the number of cuts performed on that component.
When 350 grams was approached or cutting on the component was complete, the licensee
removed the cutting fluid, cleaned the old cutting fluid reservoir and replaced the cutting fluid.
Based on the operations of the band saw, cutting any single component could not result in more
than 780 grams (the licensees safe mass limit) of fissile material accumulating in the old cutting
fluid reservoir before the cutting fluid was replaced. This method had provided criticality safety
protection for the band saw for many years1.
Corrective Actions
The licensee performed extensive corrective actions including:
Facility extent of condition review walkdowns.
Facility nuclear safety release requirements review for the last 10 years to identify similar
problems.
1 Additional discussion and analysis of the band saw event is contained in Inspection Report 70-
27/2009-006 (ML092870702). *
Nuclear safety release procedure revisions to include specific instructions for verification.
Nuclear safety release peer checking system development and implementation.
DoAll saw modification design and implementation.
Integrated safety review of the sectioning facility including development of a new DoAll saw
criticality safety analysis.
Favorable geometry coolant system failure mode analysis.
DISCUSSION
NRC considered what safety margin remained in the above upset condition. Based on sampling
of the cutting fluid found in the old cutting fluid reservoir, many years would have been required
before a number of cuts were performed that would have made 1800 grams of U-235 available
to the old cutting fluid reservoir. Also, the intended function of the new chip tray was to remove
chips from the cutting fluid through screening or settling and NRC determined that the tray had
worked as designed. As a result of these two factors, NRC concluded that criticality in the
reservoir remained unlikely during the upset but that double contingency protection had been
lost. For normal and upset conditions, NRC expects that criticality will be highly unlikely or that
double contingency protection will be provided.
NRC noted that the amount of cuts made during the upset was fortuitous and that the new chip
tray design was not implemented as a criticality control. Therefore, changes to the number of
requested cuts, the new chip tray design or the band saw operating procedure could have led to
a different result. The above event represented an unacceptable loss of control.
The licensee had a system for ensuring implementation of new and changed criticality safety
controls called a nuclear safety release. This system required the responsible criticality safety
engineer to place pre-operational requirements in the engineering change package which would
be verified before the safety signature was received approving use of the new or changed
equipment or process. This system had worked well for many years but can be seen to depend
on the judgment and thoroughness of criticality safety engineers in establishing and verifying
pre-operational requirements.
NRC is concerned about licensee management measures used to assure that IROFS are not
compromised during the change process. In the situation described above, either more detailed
pre-operational requirements or a more thorough review of the stated requirement may have
prevented the event. NRC expects management measures at fuel cycle licensees to maintain
the availability and reliability of IROFS.
Failure to adequately verify planned equipment modifications affecting criticality safety can result
in failure to establish necessary controls or the compromise of established controls. NRC safety
inspections will routinely review licensee facility operations to ensure that plant changes are
adequately evaluated, implemented and verified. These inspections will include review of
licensee change management procedures to ensure that analytical assumptions are not
compromised during or after implementation.
CONTACT
This information notice requires no specific action, nor written response. If you have any
questions about the information in this notice, please contact the technical staff listed below.
/RA/
Marissa G. Bailey, Acting Director
Division of Fuel Cycle Safety
and Safeguards
Office of Nuclear Material Safety
and Safeguards
Technical Contact:
301-492-3112
E-mail: dennis.morey@nrc.gov
CONTACT
This information notice requires no specific action, nor written response. If you have any
questions about the information in this notice, please contact the technical staff listed below.
/RA/
Marissa G. Bailey, Acting Director
Division of Fuel Cycle Safety
and Safeguards
Office of Nuclear Material Safety
and Safeguards
Technical Contact:
301-492-3112
E-mail: dennis.morey@nrc.gov
OFC
FCSS/TSB
FCSS/TSB
FCSS/TSB
FCSS/SPTSD
FCSS
NAME
DMorey
CFisher
PSilva
MBailey
DDorman
DATE
3/3/2010
3/24/2010
6/16/2010
7/14/2010
8/13/2010
OFFICIAL RECORD COPY