Introduction:
This document will outline the hazards involved with working inside the Alcator C-Mod vacuum chamber. It will also detail the procedures which will be followed to minimize the risk to Plasma Science and Fusion Center workers.
The Alcator C-Mod vacuum vessel is a toroidally shaped structure with an internal dimension of 49" high by 26" deep with a large diameter of 52". A cross section is shown in Fig. 1. The vacuum vessel is accessed via one of nine racetrack shaped ports which are 24" high, 7.75" wide, and 8" long. The structure is made of stainless steel (no Inconel is involved). It will be mounted on a platform roughly 9' off the floor or on the lower cylinder cover with the mid-plane at 11' 7". Access is obtained from scaffolding surrounding the vessel or from the diagnostic stand. The person entering must slither sideways through the port into the vessel.
Work inside the vacuum vessel includes installation of diagnostics, tiles, divertor hardware and ICRF hardware. Some stud welding or spot welding may be done as well.
The welding of the studs (which are 3/4" long, 5/16" diameter) is accomplished with a stud welding gun which uses up to 200 volts and 20 Amps. The stud is placed in the gun which is then positioned at the desired location, and the trigger is pulled to weld the stud to the wall. The noise of the discharge is very loud. Once these are welded, the area must be scrubbed clean with ethanol.
The installation of the magnetic loops will require spot welding a number of stainless steel strips to the vacuum vessel wall. A small number of arc welds must also be made during this process.
The tile and divertor installation will require bolting the plates to the studs which have previously been installed.
The ICRF antenna and limiter installation is the last procedure which must be done prior to full power operation. Several diagnostics require internal access for installation, adjustment and calibration.
Scope:
This document is designed to provide for safe access to the interior of the Alcator C-Mod vacuum vessel.
Safety Analysis:
Air quality is a primary concern for working inside the Alcator C-Mod vacuum vessel. The air must be monitored for oxygen content and for the buildup of combustible and hazardous fumes. Ethanol will be used for cleaning internal vacuum hardware. Following boronization, fumes from boron products coating the walls are also present.
The potential for electrical shock from the welding machinery exists, as well as ultraviolet radiation. The welding could cause spattering from the welding arc and a buildup of welding related fumes (CO, metal oxides.) The stud welding gun is very loud during the discharge.
Injury could occur from bumping into structures and studs installed inside the experiment.
The experiment is mounted 11' 7" off the floor, so the danger of falling from the scaffolding also exists.
After high power operation, neutron induced activation of the vacuum vessel components could result in workers receiving a radiation dose.
Definitions:
Confined space - means a space that (1) is large enough and so configured that an employee can bodily enter and perform assigned work; and (2) has limited or restricted means for entry or exit, and (3) is not designed for continuous employee occupancy.
Entry - means the action by which a person passes through an opening into a permit-required space. Entry includes ensuing work activities in that space and is considered to have occurred as soon as any part of the entrant's body breaks the plane of an opening into the space. Entry permit means the written or printed document that is provided by the employer to allow and control entry into a permit space and that contains the information specified in this document.
Hazardous atmosphere - means an atmosphere that may expose employees to the risk of death, incapacitation, impairment of ability to self-rescue, injury, or acute illness from one or more of the following causes: (1) Flammable gas, vapor, or mist in excess of 10% of its lower flammable limit (LFL); (2) Airborne combustible dust at a concentration that meets or exceeds its LFL; (3) Atmospheric oxygen concentration below 19.5 % or above 23.5%; (4) Atmospheric concentration of any substance for which a dose or a permissible exposure limit is published in Subpart G, Occupational Health and Environmental Control, or in Subpart Z, Toxic and Hazardous Substances, of this part and which could result in employee exposure in excess of its dose or permissible exposure limit; (5) Any other atmospheric condition that is immediately dangerous to life or health.
Oxygen deficient atmosphere - means an atmosphere containing less than 19.5% oxygen by volume.
Permit-required confined space (permit space) - means a confined space which contains one or more of the following characteristics: (1) Contains, or has a potential to contain a hazardous atmosphere; (2) Contains a material that has the potential for engulfing an entrant; (3) Has an internal configuration such that an entrant could be trapped or asphyxiated by inwardly converging walls or by a floor which slopes downward and tapers to a smaller cross-section; or (4) Contains any other recognized serious safety or health hazard.
Responsibilities:
The Assistant Radiation Protection Officer for the PSFC shall determine if the Radiation Work Authorization Procedure (PSFC-RP-91006) and/or the ALARA Procedure (PSFC-RP-91005) need to be implemented.
The Entry Supervisor, in addition to the responsibilities ascribed to confined space entrants, is responsible for overseeing the permit entry process. This includes:
- Preparation and verification of permits. This includes verification that
all prescribed tests and procedures have been performed, that all required
equipment is available, and that rescue services and a means to summon them are
available. This includes radiation work authorizations and ALARA reviews as
well as any necessary welding permits.
- Monitoring of the entry to see that conditions and procedures are
consistent with the permit.
- Removal of unauthorized persons from the area.
- Termination or cancellation of an entry permit as required.
The Confined Space Entrant is responsible for:
- Understanding the hazards associated with the confined space entry from all
sources which pertain: chemical (from MSDS), reduced O2, electrical,
radiation, engulfment, mechanical, etc.
- Proper use of all required equipment.
- Communication with attendant.
- Recognition of requirements for self-rescue, and proceeding with
self-rescue when necessary.
- Obeying orders from the attendant and the entry supervisor.
- Complying with all applicable safety procedures established for his/her
project, and with the procedures established by the MIT Safety Office and by
the MIT Plasma Fusion Center Safety Committee.
The Confined Space Attendant is responsible for monitoring and protecting confined space entrants. This includes:
- Understanding the hazards associated with the confined space entry from all
sources which pertain: chemical (from MSDS), reduced O2 , electrical,
radiation, engulfment, mechanical, etc.
- Proper use of all required equipment.
- Communication with the entrant.
- Recognition of requirements for self-rescue, and ordering the entrant to
proceed with self-rescue when necessary.
- Ordering evacuation if an uncontrolled hazard is detected, if the entrant
exhibits behavioral effects of hazardous material exposure, if predetermined
entry times are exceeded, if a situation develops outside the space which could
endanger entrants, or the attendant can no longer perform his/her duties.
- Summoning rescue personnel as required.
- Following instructions of the entry supervisor.
Assumptions:
Access to the vacuum vessel is so narrow that only a small number of Alcator employees can pass through the port structure into the machine. These people are required to crawl through a mockup port prior to beginning the qualification process.
A recent medical checkup and respirator evaluation will be required for those who will access the vacuum vessel. Anyone with a heart condition, epilepsy, claustrophobia, or a tendency to dizziness or fainting spells will not be allowed to work inside. Pregnant women will not be able or allowed to access the vacuum vessel.
Personnel allowed access to the vacuum vessel will be trained in confined space entry and rescue procedures on the vacuum vessel. The project supervisor will oversee this training and approve personnel for entry. A person capable of accessing the vessel must be standing by at any time workers are inside the vacuum vessel. A telephone will be available in the Alcator C-Mod cell where this work is being done. No more than 4 people will ever be allowed inside the vessel simultaneously.
All personnel approved to enter the vacuum vessel will be trained in rescue procedures on a mockup of this vacuum vessel. At least one person approved for access will be standing by at any time there is a worker inside the vacuum vessel. This person must maintain frequent verbal communications and, when possible, visual contact with the worker inside the vessel.
The scaffolding around the vessel will be wide enough for comfortable movement around the vessel, and will have railings to prevent falling from the structure.
No person will be required to remain inside the vessel longer than he or she feels comfortable. Personnel working inside the vessel will be required to exit after 4 hours or less.
All required permits, e.g., confined space entry, radiation work authorization, or welding permits have been obtained. All special equipment and protective clothing required by these permits have been obtained.
All external hazards, e.g., RF, LN2, magnet current, and vessel heaters have been locked out.
Required Equipment:
The space will be ventilated at a rate which will provide 2 air changes per minute
(approximately 250 cfm) or as specified by the Industrial Hygiene Office. (It may be necessary to use a smaller blower to prevent a windy feeling inside the vacuum vessel. It will be lighted with low voltage lights, with emergency backup in the event of a power failure. Continuous oxygen and flammable gas monitoring will be done at all times that the space is occupied.
Those working inside the vacuum vessel will wear protective clothing appropriate to the tasks which they are performing. Welding operations will require the wearing of leather gloves, protective eye wear, etc. Use of the stud welding gun will require ear protection. Head protection should be worn. Nomex suits should be worn for stud welding operations instead of Tyvek, which burns too easily. Protective clothing for radiation work will be specified by the Assistant Radiation Protection Officer for the PFC.
Entry into the vessel following boronization of the vacuum vessel walls may require the use of supplied air while boron deposits are cleaned off the wall. An MDA-TLD1 detector is also required to monitor the atmosphere for boron hydride levels.
Procedure:
Only personnel who have had confined space related medical checkups approved by
the MIT Environmental Medical Services and have been trained in entry and rescue techniques will be allowed to enter the vacuum vessel. A list of the approved persons will be posted at the site.
- The ventilation system must be set up on the vacuum vessel and be operating
before entry is attempted.
- An oxygen and flammable gas monitor will be inserted into the vessel and
checked before entry is attempted. This monitor must be operating at all times
that personnel are inside the vacuum vessel. Any one working inside the vessel
must exit immediately if an alarm is registered on this meter. No one may
enter this space if an alarm is registered. A maintenance schedule for this
meter is attached to this procedure.
- Low voltage lighting must be inserted into the vessel, or fluorescent port
lighting fixtures must be turned on before entry is attempted.
- Two ports must be open for every person entering.
- A second person approved for entry into the vacuum vessel (confined space
attendant) must be standing by. This person must maintain frequent verbal
communications and, when possible, visual contact with the worker inside the
vessel. An additional person, capable of assisting with the removal of a
worker from the vessel from outside of the vessel must be in the area as
well.
- Materials to be taken inside the vessel must be reviewed for toxicity by the
Industrial Hygiene Office. Any precautions specified by IHO must be observed.
Vacuum protective clothing must be non-flammable for welding and spot welding
applications.
- The set up of any welding operations inside the vessel must be reviewed by
the welding supervisor and approved for safety. The workers welding inside the
machine must wear the appropriate clothing and protective gear.
- A fire extinguisher must be available on the scaffolding. The fire
extinguisher on the scaffolding shall consist of one of the following three
items: A water hose with spray nozzle; A bicarbonate of soda dry chemical
fire extinguisher; or several fire blankets. The water or dry chemical
extinguisher is preferred.
- An audible alarm must be available for emergencies. The fire pull alarms in
the cell
are deemed acceptable for this purpose. In an emergency situation, the rescuers should pull one of the fire alarm switches.
- Any person entering must be familiar with these procedures. That person
should verify for his or herself that the oxygen-flammable gas monitor is
installed, that the lighting is provided from a low voltage source, that the
blower is operating, that the fire extinguisher is at hand, and that approved
spotters are standing by.
- These procedures must be posted at the site.
In addition the entry supervisor shall:
- Check the entry procedure and make sure that all of the required equipment
has been assembled.
- Make sure that any ventilating equipment has been installed and is operating
satisfactorily.
- Check that all required measurements of atmosphere have been performed and
that levels are in a satisfactory range.
- Satisfy him/herself that the attendants and entrants understand their
responsibilities and the procedure to be followed.
- Fill out the permit (blanks should be obtained from the ES&H office) and
post it at the entry site.
- At completion of the entry the entry supervisor shall return the permit to
the ES&H office.
Procedure for First Entry after Plasma Operation
- If the vacuum vessel was boronized during the run period, then every
attempt should be made to remove boron hydride products from the vacuum vessel
prior to opening the vacuum vessel for entry. These methods can include
combinations of discharge cleaning (both glow and ECDC) as deemed effective by
the operations manager. This should be followed by letting the system up to
atmosphere and flooding it with moist air until boron hydrides no longer
register on the MDA-TLD1 and the characteristic boron hydride odor is gone.
- On first entry, the vessel should be surveyed for activated hot spots, and
these areas must be marked on an in vessel survey sheet. Should a dose rate of
greater than 1 mrem per hour be detected inside the vessel, an ALARA plan
will be prepared to minimize worker exposure.
- Cleaning of residual boron compounds is recommended as soon as feasible
after an opening, and prior to permitting unrestricted access to the vessel.
This is currently accomplished by having a confined space worker enter the
vessel while wearing supplied air and wiping all surfaces with distilled water
until the residues are removed.
Training
- Personnel accessing the vacuum vessel will be trained in CPR. They will also
practice entering a mockup of the vacuum vessel, and will practice removing a
dummy from the vessel. The supervisor must observe this training and verify
that the worker is capable of performing simple rescue techniques. The
supervisor must also verify that the worker is capable of entering and exiting
the vessel under his or her own power.
- Personnel allowed access to the vacuum vessel will be trained in confined space
entry and rescue procedures on the vacuum vessel. The project supervisor will
oversee this training and approve personnel for entry. Additional radiation
safety training shall be provided as necessary.
References: (reserved)
Persons Permitted Access to the Vacuum Vessel
Rejean Boivin
Valerie Censabella
Robert Granetz
Martin Greenwald
Amanda Hubbard
Ian Hutchinson
Brian Labombard
Bruce Lipschultz
Earl Marmar
Alexander Mazurenko
Sam Pierson
Wanda Pina
Spencer Pitcher
Maria Silveira
James Terry
Tom Toland
Rui Vieira
Steve Wukitch
*Some restrictions apply
Oxygen and Flammable Gas Maintenance Schedule
(Extracted from Model 261 Maintenance Manual)
1. The combustible gas sensor of the Model 261 is designed to measure combustible gas or vapor content in air. It will not indicate the combustible gas content in an inert gas background, furnace stack or in mixtures with less than 10% oxygen. Further, this instrument should not be used where the oxygen concentration exceeds that of fresh air (oxygen enriched atmospheres) because the extra oxygen makes any combustible mix easier to ignite and thus, more dangerous.
2. Certain materials such as silicone, silicates and organic lead compounds tend to poison the combustible gas sensor, thereby causing erroneously low readings. Calibration checks should be made frequently if such materials are suspected to be present in the tested atmosphere.
3. The combustible gas sensor detects only combustible gases and vapors in air. It will not indicate the presence of combustible airborne mists or dusts such as lubricating oils, coal dust or grain dust.
4. High or low pressure samples will give erroneous oxygen percent (%) readings. For atmospheric sampling at higher or lower altitudes, the instrument oxygen meter should be calibrated at the elevation where sampling is to take place.
5. Acid gases, such as carbon dioxide will shorten the service life of the oxygen sensor.
6. The oxygen sensor is packaged separately in a sealed container. It must be installed before the Model 261 can be used.
7. Sampling lines increase the response time of the instrument. For example: a 50 foot sample line will increase the initial response time of the Model 261 to approximately 30 seconds and the final response to approximately 3 minutes. Two 50 foot lines connected in series will increase the response time to 60 seconds and 6 minutes, respectively. The use of sampling lines over 100 feet in length is not recommended.
8. Use only genuine MSA replacement parts when performing any maintenance procedures. Failure to do so may seriously impair instrument performance. Repair or alteration of the Model 261 alarm, beyond the scope of these maintenance instructions, or by anyone other than a certified MSA serviceman, could cause the product to fail to perform as designed and persons who rely on this product for their safety could sustain severe bodily injury or death.
9. Combustible gases will burn or explode only when the fuel/air mixtures are within certain proportions. The minimum concentration of a particular combustible gas in air which will burn and continue to burn when ignited is defined as the lower explosive limit (LEL). The maximum concentration that can be ignited is defined as the upper explosive limit (UEL). In some references, the terms used are lower and upper flammable limits (LFL and UFL).
10. Before each day's usage, sensitivity must be tested on a known concentration of pentane in air equivalent to 25% to 50% of full-scale concentration. The indication must be equal to or higher than the actual concentration.
11. Proper readings are obtained only when the battery has a sufficient level of charge and the sample inlet filter is not clogged due to high dust or dirt exposures.
a. The battery charge level should be checked occasionally throughout a testing period.
b. Upon receiving a new Model 261, it is recommended that the battery be charged for at least 14 hours.
c. After each day of use, the battery should be charged for a minimum of 14 hours, and the sample inlet filter should be checked for a dust or dirt coating on the filter element.
d. Recharging must be done in a non-hazardous location known to be free of combustible gases, vapors, or dusts in order to prevent the potential ignition of combustible atmospheres.
e. Do not operate the Model 261 while the battery is being charged.
f. As a regular monthly maintenance item for optimum battery service, the Model 261 battery should be charged for 14 hours. After charging, run the instrument for 8 to 10 hours, and fully charge the battery for 24 to 36 hours.