It has been surmised that an effective
way to coat the walls of the Alcator C-Mod vacuum vessel with a thin layer
of boron is to create a gaseous discharge in diborane gas inside the vacuum
vessel. Diborane is highly toxic and flammable, and presents an explosion
hazard. The gas delivery system must be of high integrity and leak
free. Leak detection and remote control are essential. Procedures for
handling, maintenance, process, and emergency response are
required, as
is training of any personnel directly or peripherally involved with
using this material.
Scope/Purpose
This document covers the maintenance
requirements for the Alcator C-Mod Diborane facility.
Consequences of procedural failure.
Failure to adequately maintain the diborane system could result
in leakage of diborane gas into the cell or into the diborane housing,
which could lead to fire or explosion, or to personnel exposure to toxic
gas. Diborane is immediately dangerous to life and health at 40 ppm, and
an explosion hazard at 9000 ppm.
Definitions
- MDA-TLD-1-Gas monitor for the diborane system. One is located
at the diborane gas cabinet. The other is located on the igloo at
B-port underneath the diagnostic stand.
Responsibilities
- Maintenance of the system is the responsibility of
Frank Silva, Joe Bosco, Chris Reddy, Catherine Fiore, and Bob Childs.
- Installation of the diborane gas cylinder: Two persons, trained
and approved to wear SCBA, trained in diborane hazards, and trained in
all of these procedures will install the SCBA bottle.
- These procedures will be reviewed and approved by a committee of
no less than 3 knowledgeable persons, appointed by the Safety Officer.
Required Equipment
- Wrenches for changing gas bottles.
Required Materials
- Diborane monitor Chemcasettes.
Assumptions and Prerequisites
Procedure for maintenance of the diborane monitors
Please refer to the "TLD-1 Toxic Gas Detector: Operating
Instructions" for more information.
Change chemical tape on diborane monitors. This must be done
every two weeks during operation of the diborane monitors.
- Turn the Tape Load Lever to the "open" (9 o'clock) position,
and remove the Chemcassette from the locator post. Pull the Take-up Reel cover
off the Take-up Reel, slip off the used Chemcassette tape and discard.
- To load in a fresh tape, make sure that the Tape Load Lever
is in the "open" (9 o'clock) position. Pull approximately 12 inches of tape
out of the fresh Chemcassette. Place the end of the tape in the slot on the
Chemcassette Take-up Reel cover. Thread the Chemcassette tape through the
Detector Head, Capstan Assembly, and adn over the Guide Post. Push the fresh
Chemcassette (with the raised lettering facing you) on the Chemcassette Locator
Post and secure with the screw cover. Also push the Take-up Real cover into
place. Rotate the assembled Take-up Reel on or two turns to take up any slack.
Turn the Tape Load Lever to the "closed" (6 o'clock) position.
Check function of the diborane monitors. (Perform prior to
installation of diborane bottle, and every 2 weeks while diborane is in
the cell.)
- Turn the Tape Load Lever to the "open" (9 o'clock) position,
remove the Chemcassette, and press the Alarm Test Key. The green
System Status Indicator will flash.
- Insert the test card with position#1 centered in the Detector
Head; be sure that the colored chip on the test card faces up and that the
card is inserted fully into the Detector Head.
- Close the Tape Load Lever and press the Alarm Test Key. The
instrument will beep once.
- Open the Tape Load Lever and reverse the Test Card, centering
position #2 in the Detector Head.
- Close the Tape Load Lever and press the Alarm Test Key. If
all electronics and optical systems are operating properly, the instrument will
go into an alarm condition, with both the audible and visual alarms activated
and the alarm relay energized. Check that the process gas valve has
closed.
- Open the Tape Load Lever and press the Alarm Reset Key. Rethread
the Chemcassette Detection Tape and turn the Tape Load Lever to the closed (6
o'clock) position. The test is now complete.
- Repeat steps 1-6 for the second monitor.
Change Internal Flow Filter (Yearly). Consult Manual.
Procedure for Purge System Maintenance
Replace Venturi Nitrogen Gas. To be done when the gas bottle
pressure falls to 500 PSI.
- This should be done only when the diborane is not present
in the process gas lines, and the diborane bottle valve is closed.
- Close the valves on the nitrogen bottles and open PV1
to release the pressure from the system. Close the regulator REG3 and
MV3 and close the
valve on the nitrogen bottle.
- Disconnect the nitrogen bottles (outside the gas cabinet on the right)
and remove them.
- Replace the bottles, and open the bottle valves. Set the
REG3 to 100 Psi and open MV3.
Replace Helium Purge Gas. To be done when the gas bottle
pressure falls to 500 PSI.
- This should be done only when the diborane is not present
in the process gas lines, and the diborane bottle valve is closed.
- Close the valve on the helium bottle. Open PV1, PV2 and PV4
to release the pressure, then close the regulator REG2 and MV2.
Open the gas cabinet.
Disconnect the helium bottle (on the right) and remove it.
- Replace the bottle, and open the bottle valve. Set the
REG2 to 30 Psi and open MV2.
Test procedure
Refer to the "Test Procedure for the Diborane Gas System" for
performing these tests.
- Helium leak check the diborane gas lines. Perform prior
to beginning of any boronization campaign, and whenever an opening in the
diborane gas lines has occurred.
- Test the diborane safety system performance. Perform prior
to beginning of any boronization campaign.
- Test the system performance in helium. Perform prior
to beginning of any boronization campaign.
Training
Those responsible should review these procedures in detail, and
should receive training in toxic gas hazards, and SCBA usage.
Cross references
Distribution
E. Marmar, J. Irby, C. Reddy, R. Clifford, J. Bosco, B. Childs,
References
