unreviewed safety issue determinationlh2targ/npdg/supporting documents/usid npd-gamma... · cubic...

SNS 102030102-ES0029-R01

of 13 USID for NPD-Gamma Liquid Hydrogen Target

Unreviewed Safety Issue Determination

I. Title of USID:

USID for NPD-Gamma Liquid Hydrogen Target

II. Description of Proposed Activity (or discovered condition):

The purpose of this USID is to consider whether the installation and operation of the NPDGamma (NPDγ) hydrogen target in the SNS instrument hall constitutes an unreviewed safety issue. The emphasis is on whether the hydrogen in the NPDγ target can significantly affect the previously reviewed analyses of the safety of the target mercury hazard, as documented in the Final Safety Assessment Document for Neutron Facilities (FSAD-NF).

The NPDγ experiment is an ongoing joint project between the following major partners: UT-Battelle (ORNL Physics and Neutron Sciences Divisions), Indiana University (IU, Physics Department & Cyclotron Facility), and Los Alamos National Laboratory (LANL, LANSCE Facility). ORNL has operational and project management responsibility for the experiment, IU has component design and fabrication responsibilities as well as data interpretation responsibility, and LANSCE provided a location for the assembly, integral testing and initial operation of the experiment. Now that the initial operational phase of the project has been completed, the experiment is being moved from LANSCE to Oak Ridge for the proposed installation at SNS beam line 13 (BL 13). The purpose of the experiment is to measure γ-ray asymmetry in the capture reaction between neutrons and protons.

N + P D + γ (2.2. MeV)

To achieve adequate statistics in the desired measurement, a 16-liter liquid H2 target is required and the cold neutron flux levels achievable at SNS are necessary to complete the data taking in a reasonable time. The ORNL Physics Division will operate the NPDγ experiment for about 7 months with full SNS production power before it is removed to make way for other nuclear physics experiments on BL 13.

Successful operation of the NPDγ experiment at the LANL LANSCE accelerator facility took place between 2004 and 2006. It included both in-beam and out of beam operation and operational tests. Special tests, including the following, were completed at LANSCE to establish the safety characteristics of the NPDγ system. For example, an integral venting test with the target prototype outside the LANSCE beamline was conducted and then repeated with the experiment installed in the LANSCE ER-2 instrument hall. Other significant tests included a thorough cryogenic testing of the target system without beam and a thorough testing of the target interlock system before having beam on target.

The BL 13 experiment is essentially the same as the one operated at LANSCE, with adaptations and improvements for installation at SNS. For example, a new ASME code stamped liquid H2 vessel has been designed and fabricated to hold the 16 liters of liquid H2. Items such as the closed loop cryo-refrigerator needed to maintain the H2 at ~20 K are slated to be utilized in the installation at SNS. An external ortho-to-para converter (OPC) has been added to the design. Its purpose is to decrease the overall setup time required to fill the target with para LH2 to prepare it for experiment data collection. During target fill, the incoming gas flows through a LN2 cold trap at the OPC inlet and then through a mechanically cooled catalyst bed that accomplishes the conversion. The H2 gas does not condense in the external OPC.

Figure 1 shows a view of BL 13 and the NPDγ experiment. The roof of the instrument enclosure has been omitted for clarity in showing the location of the experiment inside. Figure 2 shows the cryostat and closely related components.

Major parts of the NPDγ experiment will be located in or near the SNS north instrument hall, as follows:

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• Outdoors

o H2 cylinders: connection station will be located outdoors on the northwest wall of the BL 11 external building. Everything else is inside the target building

• Inside Target Building but outside the BL 13 Instrument Enclosure

o Gas panel cabinet: this cabinet contains the valves needed to supply gas or vacuum as needed to prepare for and perform the H2 filling operation. The cabinet is mounted outside the BL 13 instrument enclosure on the outermost shield wall (~4 ft high, for ease of operator observation). The cabinet is vented by natural circulation to a rooftop outdoor location.

o External OPC with LN2 cold trap and mechanically cooled catalyst bed.

o H2 vent isolation cabinet contains the vent isolation box and the rest of the valves and demountable joints. The cabinet is outside the instrument enclosure on the outermost shield wall (~10 ft high). The vent isolation box is a sturdy chamber to which all four of the H2 relief devices discharge upon actuation. The discharge line from the vent isolation box is argon purged and routes all relief discharges to an outdoor location on top of the north instrument hall. The cabinet is vented outdoors by natural circulation to the rooftop.

o The compressor of the closed-loop helium cryo-refrigerator is mounted on top of the instrument enclosure. The refrigerator condenses the H2 during the fill operation and maintains the H2 in liquid form during experiment data collection.

o Blower for instrument enclosure ventilation: installed ~10 ft high, just outside the outer wall of the instrument enclosure. The supply duct for the ~1200 cfm ventilation flow penetrates the enclosure, blowing chilled air into the enclosure. The corresponding outflow of air from the enclosure is primarily through the chain-link type, PPS-controlled access door.

o Experiment personnel station: a person trained in the operation of the experiment will be present during operation of the experiment. CRTs and other support equipment will be installed in the area between the BL 13 instrument enclosure and the north wall of the target building instrument hall.

o Access control gate into the instrument enclosure: the Instrument PPS controlled chain-link gate is located in the 4 ft wide labyrinth walkway that leads into the enclosure. [Note: plans also allow for the BL 13 Instrument PPS, in addition to controlling access to the enclosure, to monitor enclosure oxygen concentration.]

• Inside the BL 13 Instrument Enclosure

o H2 target: inside the instrument enclosure. All liquid hydrogen is inside the enclosure. A gaseous H2 line from the H2 vent isolation cabinet to the target has two functions; it serves as the H2 supply line, which is valved-off except during the H2 condensation phase of the target fill operation, and then as the H2 relief line, which extends to the H2 vent isolation cabinet.

The hydrogen inventory of the experiment is about 1.2 kg of H2. Most of this is accounted for by the liquid inventory of the 16-liter H2 target vessel. The primary hazard associated with the use of hydrogen is its flammability in air. Hydrogen can burn in air at concentrations between 4% (lower flammability limit) and 75% by volume. Downward flame propagation is possible at concentrations above 9%.

Both inherent and intentionally incorporated design features of the NPDγ experiment contribute to the overall safety of the experiment and to the answers to the USID evaluation criteria (Section V, below). The limited quantity of H2 present, its initially cryogenic state, and its high degree of buoyancy limit the amount that could accumulate in a flammable configuration in the event of an inadvertent release. Edeskuty (Ref.1) lists

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the buoyant velocity of H2 as between 1 and 9 m/s. The initially cryogenic state ensures that there are two boundaries against release of the hydrogen (outer and inner vacuum boundaries) and that a leak in either boundary would lead to venting of the hydrogen outside the target building. The vacuum layer is a major part of the thermal insulation of the liquid hydrogen and when the vacuum is degraded, hydrogen pressure increases until it reaches the actuation pressure of the relief valve that discharges to the outdoors. Monitoring of the vacuum (e.g., by vacuum pressure sensor and residual gas detector (RGD), as planned for NPDγ) allows small leaks in either the H2 or vacuum boundaries to be detected early. Although no credible operational scenarios involving leakage of H2 into the BL 13 instrument enclosure have been identified, it should be noted that the 4 ft wide access labyrinth (not significantly impeded by the PPS-controlled wire mesh access door) and slots between shielding beams in the enclosure ceiling would provide an open path for the leaking H2 to be dispersed into the large volume of the instrument hall. As discussed in Section V.3, below, accident release of H2 to the instrument enclosure has been evaluated as a result of a postulated severe earthquake.

The industrial safety properties of hydrogen are well known and described extensively in the literature. Design and operational features of the NPDγ installation ensure the safety of workers in the instrument hall. These features , listed below, make inadvertent H2 releases inside the target building very unlikely and, should accidental leakage occur, minimize the chances for combustion.

• The H2 supply cylinders are located outdoors, arrayed along the exterior west wall of the BL 11 external building. Positions are provided for connection of 3 “A” size H2 cylinders (196 standard cubic ft capacity each). Only one cylinder is valved-in at a time so that a cylinder can be emptied before starting on the next. The line from the H2 cylinders to the BL 13 gas panel cabinet is about 200 ft length of ¼” OD (0.18” ID) stainless steel tubing.

• A flow limiting orifice between the H2 cylinder connection station and the gas panel cabinet limits maximum flow of H2 into the building to 25 liters/minute. Should the flow limitation device be valved-out (there is a bypass around it which is for cleaning and vacuum testing only), the ~200-ft length and ¼” diameter of the tubing would provide an upper limit on maximum H2 flow rate into the building should the line be severed.

• Gas and vacuum valves are located inside the gas panel cabinet that is vented to outdoors. Venting is by natural circulation, taking advantage of buoyancy of H2 and eliminating dependence on an active blower. No ignition sources are located inside the cabinet.

• The H2 boundary is protected by two relief devices (a relief valve and a burst disc), either of which is capable of preventing over-pressure in the event that vacuum is lost.

• The vacuum boundary is protected by two burst discs either of which if capable of preventing excess pressure in the event that breakage of the H2 boundary occurs.

• The relief devices are located inside, or attached to, the vent isolation box that is located in the vent isolation cabinet. The vent isolation box vents outdoors (to the target building roof) through the argon purged discharge line (the line is argon purged to remove the possibility of air ingress into the hydrogen or vacuum). The cabinet itself is vented to the outdoors by natural circulation. No sources of ignition are located inside the vent isolation cabinet.

• Demountable connections (e.g., flange type connections) of the H2 boundary and its enclosing cryogenic vacuum system are designed with helium pressurized interspace that serves two purposes: (1) it prevents air ingress and (2) it allows RGD detection of helium leakage into the cryogenic vacuum space.

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• An information panel is provided to alert the experiment personnel of conditions that could indicate a developing problem with the cryogenic confinement of H2 in the experiment. Different levels of alarm or warning are provided, and the evacuation alarm is tied into the building fire alarm system. Sensor output signals fed to the alarm logic include atmospheric H2 concentration as sensed in the instrument enclosure and in the two cabinets (gas panel cabinet and H2 vent isolation cabinet).

References for this USID

1. F. J. Edeskuty and W. F. Stewart, Safety in the Handling of Cryogenic Fluids, 1996 Plenum Press, New York.

2. R. M. Harrington, Post Earthquake Hydrogen Combustion Effects in the BL 13 NPDγ Experiment, SNS document 102030102-TR0012-R00.

3. Description of specifications of NPDγ Hydrogen target and hydrogen related systems at SNS are presented in NPDGamma SNS Liquid Hydrogen Target Engineering Document, version 2.00, June 05, 2008.

III. Does the proposed activity or discovered condition affect information presented in the FSAD-NF or the FSAD-PF, e.g. regarding equipment, administrative controls, or safety analyses. If so specify the applicable FSAD and relevant sections.

The use of significant quantities of flammable gas in the instrument halls is not covered in the FSAD-NF –either in Section 3.3.13, Instrument Systems, or in Chapter 7, Instrument System Hazards. Cryogenic and oxygen deficiency hazards associated with neutron instrument operation are covered in FSAD-NF Section 7.2.

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End of beam guide

Spin flipper

Vacuum cryostat (target containing part inserted into detector array)

Vacuum vent line (H2 vent line runs inside until close to vent isolation box)

Beam stop

Vent isolation box (enclosing cabinet not shown)

Enclosure air supply blower and chiller

H2 vent line (from vent isolation box). Note: neither the vent isolation box cabinet vent line nor the gas panel vent line are shown.

Building support columns

Gamma detector array (H2 target protrudes into center of array)

Figure 1 Schematic 3D layout diagram for the NPDγ experiment at BL 13 (enclosure roof omitted)


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Figure 2 The LH2 target, cryostat, fill-vent line, and vent isolation box outside BL13 enclosure (The boundary of the cabinet that encloses the vent isolation box is not shown.)

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IV. Does the proposed activity or discovered condition affect any of the requirements of the ASE? If so, list the affected sections

No.

V. USI Determination Evaluation Criteria:

1. Could the change significantly increase the probability of occurrence of an accident previously evaluated in the authorization basis? Yes__ No Justification: The numerous safety features of the NPDγ, both inherent and designed-in, combine to prevent its being a likely source of fire in the instrument hall. As summarized on Table 1 the use of H2 in the NPDγ experiment does not significantly impact the frequency of any of the previously evaluated accidents.

2. Could the change significantly increase the consequences of an accident previously evaluated in the authorization basis? Yes__ No Justification: It is not reasonable to postulate that hydrogen from the NPDγ experiment could burn or explode in the presence of target mercury. Therefore, consequences of previously identified accidents should not increase. As summarized in Table 1 the use of H2 in the NPDγ experiment does not significantly impact the consequences of any of the previously evaluated accidents.

3. Could the change significantly increase the probability of occurrence of a malfunction of equipment important to safety previously evaluated in the authorization basis? Yes__ No Justification: The limited amount and cryogenic state of the NPDγ H2 and its distance from any of the SNS credited engineered controls prevent it from being able to significantly increase the probability of failure of any of the SNS credited engineered controls. See summary in Table 1.

4. Could the change significantly increase the consequences of a malfunction of equipment important to safety previously evaluated in the authorization basis? Yes__ No Justification: The limited amount and cryogenic state of the NPDγ H2 and its distance from any of the SNS credited engineered controls prevent it from being able to significantly increase the consequences of failure of any of the SNS credited engineered controls.

5. Could the change create the possibility of a different type of accident than any previously evaluated in the authorization basis that would have potentially significant safety consequences? Yes__ No Justification: For the NPDγ H2 to cause a new type of accident would require that the H2 be transported into the monolith or target service bay. This is not possible because the BL13 instrument enclosure is separated from the target-mercury-containing areas by distance and by massive steel and concrete structures. The only possible connection is the neutron guide tube which provides a curved path, through neutron beam windows and chopper cavities, connecting the BL 13 enclosure to the outer edge of the BL 13 main shutter. The following scenario involving the neutron guide tube was considered but discarded as physically unrealistic. An earthquake could possibly cause release of hydrogen from the NPDγ system into the BL 13 instrument enclosure and at the same time cause failure of the neutron guide tube, which is kept under a vacuum during scientific operation. If the guide tube outermost beam window failed inside the BL 13 enclosure and no other guide tube (or chopper cavity) failures occurred at points outside the enclosure, the initial vacuum could suck some of the BL 13 atmosphere into the guide tube. The small amount of BL 13 atmosphere that could be drawn into the guide tube would not have a significant concentration of H2 because, as demonstrated in accident analyses (Ref. 2), any H2 released into the enclosure would rise to the ceiling as it flows out of the enclosure through the 4 ft wide access labyrinth (not significantly impeded by the PPS-controlled wire mesh entrance gate) and slot-like openings between the shielding beams that form the ceiling of the enclosure. The beam tube termination with the failed beam window is more than 6 ft below the ceiling, so it is not located near a natural collection point for escaping hydrogen. Thus, in addition to requiring the postulation of a very

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improbable combination of seismic failures, this scenario does not provide a physically realistic way to transport significant H2 into the beam tube.

6. Could the change increase the possibility of a different type of malfunction of equipment important to safety than any previously evaluated in the authorization basis? Yes__ No Justification: As summarized by Table 1, even worst case accidents involving the NPDγ H2 would not be able to cause failure of any kind to occur to SNS credited engineered controls.

7. Could the change significantly reduce the margin of safety as defined in the Accelerator Safety Envelope (list any affected part(s) of ASE in the Justification)? Yes__ No Justification: The NPDγ experiment does not require any new SNS Credited Engineered Controls and does not modify any existing CECs or ASE requirements.

SNS 102030102-ES0029-R01 Table 1 Potential Impacts NPDγ Hydrogen on SNS Design Basis Accidents (See Table A-1 in FSAD-NF; only accidents with conceivable link to NPDγ are listed)

Event Number & Description Quoted from FSAD-NF

Initiating Event Frequency from FSAD-NF & Impact of NPDγ

Unmitigated Consequences from FSAD-NF & Impact of NPDγ (see Note a)

Credited Controls for This Event (CECs and CACs)

NPDγ Impact on Credited Controls

TS1—2

“Medium Size Fire- Fire starts outside of the Target Service Bay and propagates to Transfer cell and Target Service Bay (Air intake for the Transfer and Target Service Bay is located in the Decon Room.) Release of Hg and activated water from the systems in the Target Service Bay caused by the fire.”

Unlikely

Uncontrolled release of H2 from the NPDγ is made unlikely by its inherent features and its several provided safety features. Therefore the rate of medium fires is not significantly increased.

High

The amount of H2 that could be released (1.2 kg) and the lack of any credible way for it to burn in the presence of mercury prevents the NPDγ from increasing medium fire consequences, which are already listed as High for all on-site receptors.

“*Fire detection / suppression system (NFPA-13) outside fire barrier”

*2-hour fire barrier enclosing the Target Service Bay and core vessel.

*Combustible Material Control (CMC) Program outside the Target Service Bay.

*Primary Confinement Exhaust System (PCES) [Design Feature].

*Mercury inventory control on PCES charcoal adsorbers (for medium fire in charcoal adsorber room).”

CEC fire suppression system: Since the BL 13 enclosure does not incorporate a large quantity of hydrocarbon neutron shielding, the BL 13 sprinklers are not necessary for the credited safety function of the instrument hall sprinklers. The instrument hall sprinklers (above the 30-ton crane) are of sturdy metal construction that would not be adversely impacted by a H2 burn.

2-hour fire barrier: It would take a massive H2 combustion event to cause structural failure of the credited service bay/core vessel fire barrier. There is not sufficient H2 in NPDγ to cause such a combustion event.

CMC Program (SNS OPM 2.J-3): The instrument hall is a high fire loading area; the 1.2 kg of H2 is a small fraction of the amount of transient combustibles allowed without a special permit—e.g., 20 gallons of flammable liquid.

The PCES has no connection to the NPDγ experiment or to the BL 13 instrument station.

Mercury inventory control: not applicable to NPDγ.


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CM2—1a

“Breach of Cryogenic Moderator vessel allows hydrogen to escape from the moderator vessel (large leak) into the surrounding area within the core vessel; Hydrogen accumulates in concentrations greater than the LEL in air, is inadvertently ignited and explodes releasing mercury and activated cooling water. Follow-on fire results.”

Unlikely

No connection to NPDγ except the BL 13 beam guide, which provides a curved pathway from the BL 13 enclosure nearly to the outer edge of the main shutter, thru multiple chopper cavities and neutron beam windows. The beam guide is kept under a vacuum during scientific operations. This connection is not sufficient to allow NPDγ H2 to impact the cryogenic moderators in the core vessel (see column 5 entry, at right).

High

No impact: The BL 13 neutron beam guide would not provide a pathway for the NPDγ H2 to reach the core vessel and burn in the presence of mercury (see col. 5 entry).

“*Robust hydrogen barrier design including relief path.

*Restrained / externally protected hydrogen equipment.

*Robust vacuum barrier design including relief path.”

Robust H2 barrier: No impact because a severe seismic event is the only credible way that H2 might be released inside the BL 13 instrument enclosure. Analysis (Ref. 2) shows that the peak pressure from a worst case combustion event inside the enclosure would not be sufficient to fail neutron beam windows. If mechanical stresses from the seismic event failed only the outermost window, enclosure air could be drawn into the beam guide, but it would not include significant H2 because H2 is buoyant and, if released inside the enclosure, would rise to the ceiling which is about 6 ft higher than the beam guide.

Restrained/externally protected hydrogen equipment: these are design features in a different location and unchanged by the NPDγ.

Robust vacuum barrier: No impact. Same considerations as for H2 barrier.

BG1—1

“Facility wide fire results in release of hazardous material (fire originates outside the target service bay).”

Extremely Unlikely

Uncontrolled release of H2 from the NPDγ is made unlikely by its inherent features and its several provided safety features. Therefore the rate of facility wide fires is

High

The amount of H2 that could be released (1.2 kg) and the lack of any credible way for it to burn in the presence of mercury prevents the NPDγ from increasing facility wide fire

Credited Controls for this event are essentially the same as for the Medium Fire (TS1—2). See table entries above.


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not significantly increased.

consequences, which are already listed as High for all on-site receptors.

BG6—11

“External crane drops load on Target Building or impacts building resulting in release of radiological material (Hg).”

Unlikely

NPDγ experiment cannot affect the frequency of external load drop accidents.

High

External crane load drop onto the target service bay or monolith would not affect the NPDγ experiment in the instrument hall. A load drop directly onto the NPDγ could cause release of H2 but the limited quantity and rate of release of H2 and the open spaces to which it would be released prevent it from being able to significantly affect the confinement of mercury

*Hoisting & rigging program for external crane.”

The credited control is an administrative program (H&R controls) that is not affected by any of the instrument hall experiments. No impact on this administrative program.

BG7—1

“Damage to target building and subsequent release of hazardous material due to NPH event followed by an explosion and follow-on fire.”

Unlikely

The experiment cannot affect the frequency of earthquakes.

High

The amount of H2 that could be released (1.2 kg) and the lack of any credible way for it to burn in the presence of mercury prevents the NPDγ from increasing medium fire consequences, which are already listed as High for all on-site receptors.

“*Mercury Inventory Control on the PCES charcoal adsorbers.

*2-hour seismically qualified equivalent fire barrier enclosing the Target Service Bay and core vessel

*Combustible Material Control Program outside the Target Service Bay.

Mercury inventory control: not applicable to NPDγ.

2-hour fire barrier: It would take a massive H2 combustion event to cause structural failure of the credited service bay/core vessel fire barrier. There is not sufficient H2 in NPDγ for such a combustion event.

CMC Program (SNS OPM 2.J-3): The instrument hall is a high fire loading area; the 1.2 kg of H2 is a small fraction of the amount of transient combustibles allowed without a special permit—e.g., 20 gallons

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BG—7-1 (continued)

*Combustible Material Control Program inside the Target Service Bay.

*Robust hydrogen barrier design (PC-3)

*Seismically qualified / restrained / protected hydrogen equipment to PC-3.”

of flammable liquid.

CMC inside TSB: not applicable to NPDγ.

Robust H2 barrier & equipment: No impact because events in BL 13 cannot affect equipment inside the core vessel. Analysis shows that the peak pressure from a worst case, post-seismic combustion event inside the enclosure would not be sufficient to fail neutron beam windows. If mechanical stresses from the seismic event failed only the outermost window, enclosure air could be drawn into the beam guide, but it would not include significant H2 because H2 is buoyant and, if released inside the enclosure, would rise to the ceiling which is about 6 ft higher than the beam guide.

Note a: Consequences listed are the radiological consequences and are for onsite (worker) groups Onsite-1 and Onsite-2, which means workers inside the facility versus workers outside the facility.

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