ML24136A255
| ML24136A255 | |
| Person / Time | |
|---|---|
| Site: | Hermes, 99902069 File:Kairos Power icon.png |
| Issue date: | 05/15/2024 |
| From: | Kairos Power |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24136A253 | List: |
| References | |
| KP-NRC-2405-006 | |
| Download: ML24136A255 (1) | |
Text
KP-NRC-2405-006
Enclosure 1 Changes to Hermes 2 PSAR Chapter 9 (Non-Proprietary)
Preliminary Safety Analysis Report Auxiliary Systems
in the processing system moves pebbles from the off-head conveyance to the inspection area. After inspection, the pebbles are directed for re-insertion into the core, or to pebble storage for removal from the circulating pebble inventory, based on inspection results.
9.3.1.5 Pebble Inspection An automated inspection system provides information to the processing portion of the PHSS for determining pebble health. This includes inspection of the physical condition of the pebble for unacceptable wear or damage, identifying moderator and fuel pebbles, as well as an evaluation of the burnup of the fuel relative to a maximum burnup limit using the burn up measurement sensor (BUMS).
The burnup measurement is done by means of a gamma spectrometer. Further details pertaining to inspections for wear and damage will be provided with the application for an Operating License.
9.3.1.6 Pebble Insertion Pebbles are received from the processing system and placed in a buffer storage until required for reinsertion. The pebble buffer storage is sized and orientated to prevent a critical configuration.
Individual pebbles are fed into the step feeder insertion machine from this pebble buffer storage as shown in Figure 9.3-2. The pebbles are inserted into the top of the reactor vessel head, then pushed through the insertion line and enter the reactor core via the in-vessel fueling chute at the bottom of the core (see Section 4.3). There is a single active insertion line into the vessel and is designed with overflow protection cutouts to limit coolant loss from the reactor vessel in the event the insertion line breaks.
9.3.1.7 PHSS Inert Gas Boundary The components of the PHSS are designed to maintain an inert gas boundary outside of the reactor vessel for pebble handling. The function of the inert gas environment is to prevent absorption of moisture and oxygen into pebbles for pebble handling during normal operations. The inert gas boundary within the PHSS (see Figure 9.3-2) is created by a mechanical structure that encloses the aforementioned components with penetrations for motor shafts, storage outlets, inspection viewport, data channels, electrical power, and pebbles from the off-head conveyance mechanism and for insertion. Portions of the inert gas boundary that are adjacent to personnel access areas have the appropriate radiation shielding.
9.3.1.8 Pebble Storage Pebble storage is provided for pebble debris, damaged pebbles, spent fuel, and end of life moderator pebbles. The storage portion of the system is composed of a stainless steel storage canister and fuel canister transporter device. Individual storage canisters are sized to hold approximately 1,900-2,100 pebbles. The dimensions of the canister and quantity of pebbles are sized to maintain a non-critical configuration. A transporter device is used to transfer canisters to either the spent fuel storage area during normal operation or the full core offload area in the event of a periodic maintenance full core offload or an emergent full core offload. Storage canisters are designed to maintain their integrity in the event of a drop from the fuel canister transporter into the water-cooled spent fuel storage pool and air-cooled spent fuel storage bay. The canisters are also designed to preclude interference with the racks during insertion and removal from the full core offload and spent fuel storage racks and the air-cooled spent fuel storage racks.
9.3.1.8.1 Spent Fuel Storage Spent fuel is discharged from service in the core under normal operating conditions, placed in sealed storage canisters, and moved to the spent fuel storage area as shown in Figure 9.3-2. The initial storage area is a cooling pool designed to hold spent fuel canisters in water-cooled spent fuel storage racks
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while the decay heat of the pebbles drops. The pool is designed to limit radiation exposure to personnel.
After cooling in pool storage, the canisters are moved to air-cooled spent fuel storage racks in a concrete storage bay with radiation shielding and forced air cooling. The pool is actively cooled by the CCWS using an in-pool heat exchanger. Water is re-circulated in the pool by the SFCS and make-up water is provided by the treated water system (see Section 9.7.2). The pool and concrete storage bay are designed to prevent a critical configuration. The storage bay sizing is sufficient to store spent fuel and moderator pebbles generated during the 11 year operating lifetime of the reactor. Criticality calculations will assume that the canister storage bay and canister interiors are flooded for conservatism. The air cooled storage bay is cooled by the SFCS. Both air-cooled and water-cooled storage areas are sized and spaced to passively cool the spent fuel and moderator pebbles using storage racks and concrete structures under normal conditions and postulated events.
Full Core Offload and Spent Fuel Storage Racks The water-cooled spent fuel storage rack design consists of circular support structures that maintain the relative position of the canisters and associated lattice structural framing to support the circular supports. The racks are constructed of 304 SS that meets ASTM A240. Each circular support structure holds one canister and the canister rests on the bottom of the rack. The canister is also held by the circular support structure at multiple elevations to preclude movement during a seismic event. The full core offload and spent fuel storage racks are designed as safety-related structures that meet AISC N690-18 and AISC 370-21 code requirements. The safety function of the racks is to preclude criticality in the event of an earthquake by maintaining a safe geometry. The racks also maintain the geometry to support heat removal. The concrete and stainless-steel structures surrounding the racks are part of the reactor building and provide support to the full core offload and spent fuel storage racks.
Air-Cooled Spent Fuel Storage Racks The air-cooled storage racks are 304 SS that meets ASTM A240. The air-cooled storage racks consist of 1) vertical supports for maintaining the canisters elevation 2) circular supports to maintain the canisters orientation, and 3) associated lattice structural supports to support the canister mass. The vertical supports are designed for the canister cylinder to rest on, supporting the canister mass. The cylindrical supports maintain the canister orientation. The canisters are stacked two high in the storage bay area, so vertical support structure of the rack is designed to allow the lower canister to pass through to the bottom of the bay. The air-cooled storage racks are designed as safety-related structures that meet AISC N690-18 and AISC 370-21 code requirements. The safety function of the racks is to preclude criticality in the event of an earthquake by maintaining a safe geometry. The racks also maintain the geometry to support heat removal. The concrete and stainless-steel structures surrounding the racks are part of the reactor building and provide support to the air-cooled spent fuel storage racks.
9.3.1.8.2 Fill, Sealing, and Movement The storage canister interior is maintained in an inert environment while attached to the processing portion of the system for canister filling of pebbles and preparing the filled canisters for storage. Filling is performed by attachment of a storage canister to pebble processing via a chute and pebbles identified for storage are routed to the canister. Once the canister is filled with pebbles, the fill valve is closed and the canister is moved via an automated transfer systema fuel canister transporter for sealing or welding.
The canister is then moved within a the canister transporter to the cooling pool for initial spent fuel storage. The canister has two seals in series to prevent accidental ingress of oxygen. The fill environment remains within the inert gas boundary in argon gas as shown in Figure 9.3-2. The sealing and welding processes are performed in a shielded and recessed concrete bay.
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Fuel Canister Transporter A crane-supported, shielded fuel canister transporter is used to move a single canister from the fill station to the full core offload and spent fuel storage racks. The transporter is also used to move canisters from the full core offload and spent fuel storage racks to the air-cooled spent fuel storage racks. The transporter complies with ASME BTH-1-2017 and is moved using a transporter crane which is single failure proof and is designed as a Type I crane per ASME NOG-1. This precludes a drop of the shielded fuel canister transporter onto the full core offload and spent fuel storage racks and the air-cooled spent fuel storage racks. A canister lifting device is used to lift and lower the fuel canister from inside the transporter into the storage rack. Analysis will consider a failure of the canister lifting device resulting in a load drop over both the full-core offload and spent fuel storage racks and the air-cooled spent fuel storage racks.
9.3.1.8.3 Full Core Offload The PHSS has the capability to fully offload pebbles from the core in the event periodic maintenance requires complete removal of all the fuel within the reactor or if an emergent issue requires a full core offload. During a full core offload, the pebbles are directed to storage canisters for filling. During this process, pebbles are not sorted based on burnup level or pebble type (i.e., moderator, fuel, etc.). Once the canister is full, the fill, sealing, and movement operations are performed, and the canister fill valve is closed. The canister is not welded shut but rather sealed via a valve to allow reintroduction of the pebbles into the core.
Full core offload is functionally similar to spent fuel storage but has a different cooling demand due to the increased decay heat production rates of the removed pebbles. The canisters are stored in a pool.
The pool is sized and the canister spacing is such that during a loss of power condition there is sufficient thermal mass to prevent overheating of pebbles in the storage canisters. The concrete structure surrounding the pool and storage bay, as well as the support restraints304 SS storage racks in the pool and the storage bay which holdingthe canisters in place, are designed as seismic design category (SDC) 3 structures. The storage pool is cooled by the CCWS and is designed to ensure a subcritical configuration.
9.3.1.9 New Fuel Pebble Introduction New fuel pebbles are received from shipment and stored in their shipping containers in a new fuel storage area until required. The new fuel pebble storage area is sized and arranged such that a subcritical geometry is maintained under all conditions.
New pebbles are moved into the preconditioning and introduction area when desired for use. Pebbles are first removed from the shipping container and placed into a new pebble canister. New pebbles are pre-conditioned by baking them to remove moisture and oxygen. A vacuum is also pulled to remove the contaminants from the gas space, followed by an argon purge.
The preconditioned pebbles are then inserted into the PHSS inert gas boundary, and ultimately the insertion system. The insertion point precedes the inspection system to allow for pebble inspection, if deemed necessary. This same insertion point is also used for reintroduction of pebbles after a full core offload. In the full core offload scenario, inspection and burn-up measurements are conducted to exclude pebbles that would not meet physical condition and burnup limits. The pebble introduction process is done via two sequential valves to prevent introduction of contaminants to the PHSS inert gas boundary or new pebbles. The interstitial space between the valves is purged prior to opening of either valve to limit the ingress of oxygen.
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9.3.2 Design Bases Consistent with PDC 2, the PHSS is designed to withstand the effects of natural phenomena without exceeding the offsite dose consequences of the MHA, compromising decay heat removal, or criticality as a result of a system failure or breach.
Consistent with PDC 3, the PHSS is designed and located within the facility to minimize the probability and dose consequences of fires and explosions.
Consistent with PDC 4, the PHSS is designed to accommodate environmental conditions associated with normal operation, maintenance, testing and postulated events.
Consistent with PDC 33, The PHSS is designed to limit the loss of reactor coolant from the reactor vessel due to potential breaks in the system.
Consistent with PDC 61, the PHSS is designed to permit periodic inspection and testing and is suitably shielded for radiation protection. The PHSS design includes appropriate confinement and adequately accounts for decay heat and a reduction in fuel storage cooling under postulated events.
Consistent with PDC 62, the PHSS is designed to prevent criticality.
Consistent with PDC 63, the PHSS is designed to detect conditions that may result in excessive radiation levels and initiates appropriate safety actions.
Consistent with 10 CFR 70.24(a)(1), the PHSS design includes a monitoring system capable of detecting criticality.
Consistent with 10 CFR 20, the PHSS is designed to be shielded to support worker occupational dose limits and adhere to a radiation protection program.
Consistent with 10 CFR 20.1406, the PHSS is designed, to the extent practicable, to minimize contamination of the facility and the environment, and facilitate eventual decommissioning.
9.3.3 System Evaluation The concrete structures associated with the storage bay, pool, water-cooled and support restraints in the pool, the air-cooled racks in the are designed as SDC 3 structuresThe geometry of fuel in the storage area of the PHSS is maintained to preclude an inadvertent criticality during a design basis earthquake and in the event of a single canister drop onto the storage racks from the canister lifting device. This is accomplished by the design of three components of the storage area: 1) the concrete and stainless-steel structures surrounding the storage bay and pool, 2) the full core offload and spent fuel storage racks and 3) the air-cooled spent fuel storage racks. The concrete and stainless-steel structures associated with the storage bay and pool, are part of the building and maintain their integrity during a design basis earthquake. The concrete and stainless-steel structures of the building meet the codes and standards described in Section 3.5. Both the safety-related full-core offload and spent fuel storage racks and the safety related air-cooled spent fuel storage racks meet AISC N690-18 and AISC 370-21 code requirements. The canisters are designed to maintain their integrity during a postulated earthquake and canister drop. However, in the event the dropped canister fails to maintain its integrity, a sub-critical configuration is still maintained in the storage area due to the geometry set by the safety-related storage racks. Criticality analyses will demonstrate subcriticality assuming the dropped canister does not maintain its integrity. A summary of the criticality analyses will be provided with the application for an Operating License.
to ensure the geometry of the storage area is maintained to preclude an inadvertent criticality during a design basis earthquake. The design of the support restraintsair-cooled storage racks in theand storage
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bay also ensures that adequate spacing is maintained for air cooling between each canister. During a postulated earthquake, the fuel particles prevent radionuclide release. The particles are supported in their safety function during a postulated earthquake by the pool and rack design and by the canister transporter, both of which provide passive cooling and spacing to restrict pebble movement thereby preventing recriticality. Other portions of the PHSS that do not perform a safety function are designed to be either seismically mounted or physically separated to preclude adverse interactions with other safety-related SSCs during a design basis earthquake. These design features satisfy the requirements of PDC 2.
The PHSS is designed to minimize the probability of a fire or explosion by limiting the accumulation of potentially combustible material such as graphite dust and debris within the system. Grinding of pebbles which contribute to graphite dust generation is precluded by system design. The small amount of graphite dust that might be generated is directed through pebble motion to the storage canisters for removal from the system. The PHSS is not located near nor interfaces with pneumatic systems with the potential for air in-leakage. The system is filled with an inert gas operated at a slightly positive pressure to further prevent air ingress in the event of a PHSS breach. Locations where pebbles are not submerged in coolant, such as the PEM, will either not exceed temperatures that would induce oxidation of the graphite or are expected to cool quickly such that oxidation, if any, would be minimal and not affect the acceptability of the pebble for reuse. These design features satisfy the requirements of PDC 3 for the PHSS. Fire protection systems are further discussed in Section 9.4.
The pebble handling portion of the PHSS is protected from the effects of discharging fluids. There are no pressurized piping systems in or around the PHSS thus precluding the design from pipe whip hazards. A hypothetical water line break in the area of the storage system does not pose a criticality risk as the analyses supporting the storage system assume complete submergence and internal flooding of the storage canisters in water. The PHSS is designed in consideration of the high radiation environment where equipment will be functioning. The PHSS design also considers and accounts for the temperature within the system to preclude oxidation of graphite pebbles. The stainless steel PHSS storage canisters are designed to accommodate pressure due to the accumulation of radionuclides and thermal loads associated with the amount of spent fuel loaded in each canister during normal and postulated event conditions. The canisters are also designed to accommodate the tensile stress exerted during transfer and are compatible with handling equipment. The interior of the stainless steel canisters is also designed to account for radiolysis products from spent nuclear fuel and ensures the integrity of the canister, seal, and weld thus precluding the potential release of radionuclides from the canister. Analysis of the loads on both types of storage racks is consistent with load combinations provided in Table 3.5-1 apart from loads on both rack designs upon canister removal/insertion and impact loads due to a dropped canister.
The storage canisters are designed to preclude interference with the full core offload and spent fuel storage racks and the air-cooled spent fuel storage racks therefore loads on the racks due to canister removal are not considered. Only impact loads due to a load drop from the canister lifting device are considered for both rack designs. These design features demonstrate that the PHSS satisfies the environmental and dynamic effects in PDC 4.
The PHSS interfaces with the reactor vessel at the PEM and the pebble insertion line. The elevation of the PEM relative to the coolant free surface is such that coolant inventory loss from the reactor vessel is limited in the event the PEM breaks. The pebble insertion line is designed to limit inventory loss to an elevation no lower than the primary salt pump elevation, in the event of a break in the insertion line.
The pebble insertion line uses overflow protection cutouts to direct any coolant in the insertion line back down into the reactor vessel. Cover gas fills the line to break the siphon. These design features of the PHSS satisfy the requirements in PDC 33.
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PDC 61 requires that the safety-related portions of the PHSS that contain radioactivity be designed to ensure (1) capability to permit appropriate periodic inspection and testing of components, (2) suitable shielding for radiation protection, (3) appropriate containment, confinement, and filtering, (4) residual heat removal capability, and (5) significant reduction in fuel storage cooling under postulated event conditions is precluded. The design features which address PDC 61 for the PHSS are discussed below:
The TRISO fuel particle provides a functional containment as described in Section 6.2. Radioactive material and fission products are contained within the particle unless the TRISO layers are compromised or defective (see Section 4.2.1). The fuel pebble, as described in Section 4.2.1, is designed to preclude physical damage or changes in geometry to the TRISO particle during anticipated loads from normal operation, storage, shipping and handling. Therefore, the TRISO particle is credited for the confinement of radioactive materials rather than the PHSS. The pebble can experience thermal and mechanical loads while being handled, inspected, operated, and stored; however, such loads do not introduce incremental failures of TRISO particles. Furthermore, the PHSS design precludes pebble damage from overheating and oxidation. Heat removal mechanisms within the system, such as thermal radiation and convection via natural circulation, are sufficient to remove the decay heat produced by individual pebbles during their transit through the PHSS. Also, oxidation associated with air or moisture ingress into the PHSS is negligible for pebbles at temperatures experienced in the system. The system also minimizes pebble wear. The limiting PHSS malfunction event, which is discussed in Section 13.1.5, does not cause temperature excursions, oxidation, or mechanical stresses on the TRISO particles. Therefore, containment and confinement of radioactivity is maintained by the TRISO particles.
Fuel and moderator pebbles are manufactured to specifications as described in Section 4.2.1 and are baked prior to introduction to the reactor to remove residual moisture. After the pebbles exit the core, the inspection system, as described in Section 9.3.1.5, is used to inspect the physical condition of the pebble and measure the fuel burnup. The inspection is performed to identify abnormal wear, cracking, and missing surfaces due to pebble chipping. Gamma spectrometry is also used to determine the burnup by measuring gamma ray activity from fission products. Pebbles at or approaching the burnup limit are sent to storage in lieu of being returned to the core. Pebbles that show indications of wear, cracking, or missing surfaces are also removed from service and placed into storage.
The PHSS is adequately shielded to limit worker dose, in accordance with 10 CFR 20 and the radiation protection program, as described in Chapter 11.
The storage part of PHSS is designed to transfer ex-vessel decay heat to the CCWS and the SFCS from a full core offload and pebble offload due to normal operation. The PHSS is designed to ensure decay heat loads from pebbles in the spent fuel storage pool are passively cooled by the water of the pool and spacing of the storage canisters in the event of a loss of power. The pebbles are also passively cooled by the spacing of the storage canisters, maintained by the full core offload and spent fuel storage racks and the air-cooled spent fuel storage racks, in the event of a load drop from the canister lifting device. The canisters in the storage bay are cooled during postulated events by natural convection due to the spacing maintained by the air-cooled spent fuel storage rack which allows sufficient air flow.
PDC 62 requires criticality in a fuel storage and handling system be prevented by physical systems or processes, preferably by use of geometrically safe configurations. The design features which address PDC 62 for the PHSS are described below:
The PHSS is designed to preclude criticality by maintaining a subcritical geometry during handling.
The PHSS removes pebbles from the core at a rate that prohibits the formation of a critical
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configuration of fuel pebbles outside the reactor. In the event of a PHSS line breach, the number of spilled pebbles is limited and a critical geometry is precluded by design. The off-head conveyance, processing, inspection, pebble insertion, storage areas, and inert gas boundary maintain an inert gas environment precluding moisture intrusion into those handling areas, further reducing the risk of criticality. Fuel handling equipment maintains a subcritical geometry via physical constraints and/or system interlocks.
The spent fuel storage area consists of a water-cooled pool, an air-cooled storage bay, seismic restraints and racks maintaining the canisters physical location (i.e., spacing), and the surrounding concrete structure. The preliminary criticality analysis determining the spacing requirements for each canister in the spent fuel storage area conservatively assumes the storage containers are flooded and completely submerged under water.
The transport configuration, in which a storage canister is being moved using a canister transporter to either the storage bay or the full core off-load system (i.e., fuel pool), will be analyzed to ensure a subcritical geometry is maintained in the event of a load drop from the canister lifting device. A summary of the criticality analyses confirming the system design maintains a geometrically safe configuration will be provided with the application for an Operating License.
PDC 63 requires detection of conditions that could result in excessive radiation levels in handling areas and a means by which to initiate appropriate safety actions. The PHSS is designed to assure that mechanical and thermal loads to the fuel pebble as well as oxidation during handling, inspection, and loading into canisters do not exceed pebble design limits. Therefore, operations in the PHSS do not introduce TRISO particle failures that would result in excessive radiation levels in the handling area. The pebble inspection and sorting functions performed by the PHSS ensure that damaged pebbles removed from the reactor core are removed from use. Monitoring of the cover gas and reactor coolant radioactivity provides early indication of a potential TRISO particle failures. This satisfies the requirements of PDC 63.
The PHSS contains radiological contaminants; therefore, the system is designed to minimize contamination and support eventual decommissioning, consistent with the requirements of 10 CFR 20.1406.
9.3.4 Testing and Inspection The fuel pebble inspection portion of the system is periodically calibrated to provide assurance that limits on the physical condition and burnup of the pebbles to be reinserted into the core are within specified Technical Specification limits. Temperature of the spent fuel storage pool, water level of the spent fuel pool, and air temperature and flow in the storage bay are also monitored to confirm adequate cooling of storage canisters. The plant control system (see Section 7.2) is capable of shutting down the system such that additional pebbles do not enter the PHSS line upon a PHSS line breach.
Criticality monitoring alarms throughout the PHSS are tested periodically to confirm functionality.
9.3.5 References None
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