ML23258A242
| ML23258A242 | |
| Person / Time | |
|---|---|
| Site: | University of California-Davis |
| Issue date: | 09/15/2023 |
| From: | Frey W McClellan Nuclear Research Center |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| Download: ML23258A242 (1) | |
Text
BERKELEY
- DAVIS
- IRVINE
- LOS ANGELES
- MERCED
- RIVERSIDE
- SAN DIEGO
- SAN FRANCISCO SANTA BARBARA
- SANTA CRUZ UNIVERSITY OF CALIFORNIA, DAVIS McClellan Nuclear Research Center UNIVERSITY OF CALIFORNIA Davis (916)-614-6200 September 15th 2023 Attn: Document Control Desk US Nuclear Regulatory Commission Washington, D.C. 20555-01
Subject:
University of California Davis McClellan Nuclear Research Center (MNRC), Docket Number 50-607, License Number R-130, License Amendment Request 2023-02 Summary of Request:
Based on information discerned from MNRCs 2023 fuel inspection, a change to one of the facilitys technical specifications (4.1.4 Spec 3) and the addition of two new specifications (5.3.1 Spec 4 and Spec 5) are requested. The proposed changes will allow MNRC to continue to operate with greater flexibility and the associated analyses provide reasonable assurance that there is no increased risk to the public or environment.
A brief synopsis of 2023 fuel inspection findings, analyses to support the proposed technical specification changes, and the proposed changes themselves are provided below.
Synopsis of 2023 Fuel Inspection:
During the facilitys routine 2023 fuel inspection, a stainless steel clad standard TRIGA 20/20 element did not pass its visual inspection. The visual inspection of the element showed inward pitting and unusual discoloration in the cladding of the fueled section of the element. The elongation and transvers bend of the element were measured and were within tolerance per the facilitys technical specifications.
Though not required by the facilitys technical specifications, MNRC staff completed the inspection of all adjacent fuel elements and all other standard elements that were not inspected within the last year. This inspection corresponds to approximately 85% of all in-core elements. The inspection did not identify any other elements that failed to meet the inspection criteria called out in MNRCs technical specifications.
During the inspection, it was observed that several elements in G ring (the outer-most fuel ring) were difficult to remove from the grid plate. This trend culminated with two G ring elements that were very difficult to remove from the core. It was observed that in one of these positions there existed a small metal protrusion just below the upper grid plate. This protrusion prevented the reinsertion of fuel elements and graphite elements. At this time, an investigation was opened to determine the cause of the issues associated with G ring. After a review of MNRCs Technical Specifications, it was determined that operating with an empty (water-filled) position where the protrusion was discovered, was not allowed. Thus, preventing restart of the reactor under the current license.
After several weeks of investigation, modeling, and measurements it was determined that a design/construction flaw during the 2.0 MW upgrade in the mid 1990s was the most likely cause of the issues observed in G ring.
During the 2.0 MW upgrade, the core was defueled and the upper and lower grid plates were removed in order to replace the original radial grid plates with new hexagonal grid plates. Six shim plates were added in between the G ring and the core barrel to reduce water bypassing the inner part of the core during >1.0 MW operations (Figures 1 and 2). One remnant of the original 1 MW core that could not be removed during the upgrade was the upper core ring seen in Figure 3. This ring contained 6 tabs that protrude towards the core. A review of records of the 2.0 MW upgrade gave a very strong indication that these tabs should have been machined (underwater) flush with the rest of the ring. This task proved to be difficult and does not appear to have been executed as successfully as was necessary. The consequence of was that several shim plates were pushed out of place towards the core and narrowed the coolant channel gap between the shim plates and elements located in G ring.
A measurement tool was designed and built by MNRC staff to verify which G ring positions had at least the coolant channel gap width used in the thermal hydraulic calculations found in chapter 4 of the SAR (0.065 inches). The results of the inspection showed that 4 out of the 6 shim plates were out of place and none of the grid positions along those shim plates had the required coolant channel gap width. The shim plates could not be moved or shifted when modest force was applied to them by the inspection tool. This leads MNRC staff to conclude the position of the 6 shim plates is likely static and has not changed significantly since the 2.0 MW upgrade approximately 25 years ago.
In some instances, it was determined that the shim plate had essentially no clearance between an adjacent fuel element. This includes the element that showed inward pitting and failed inspection. This conclusion is somewhat encouraging, given that elements with insufficient coolant on one side for 25 years (including operations at up to 2.0 MW) did not result in fuel cladding failure. That said, in at least the instance of the element with inward pitting, cladding failure was likely to occur eventually. These findings warrant a significant reconfiguration of the core to eliminate the use of G ring positions with insufficient cooling channel gaps.
Figure 1 Isometric View of MNRC Reactor Core Structure with Shim Plates (turquois).
Note Upper Grid Plate and Fuel Have Been Removed.
Figure 2 Top-Down View of MNRC Reactor Core Structure with Shim Plates (turquois).
Note Upper Grid Plate and Fuel Have Been Removed.
Figure 3 Depiction of Likely Shim Plate Interference from Upper Core Ring
Analysis for Changing Technical Specification 4.1.4 Spec 3:
This technical specification currently requires that No single element may be operated at a power level above 17.69 kW (as analyzed) at a steady state power level of 1.0 MW. MNRC staff plan to reconfigure the core so that element positions in G ring with insufficient cooling channel gaps are generally filled with graphite elements (no fueled elements). In order to have sufficient excess reactivity, 30/20 elements need to be place in the inner-most fuel rings of the core (C and D ring). This core configuration is very similar to the limiting core configuration previously analyzed in chapter 4 of MNRCs SAR and can be seen below in figure 4.
Figure 4 Planned MNRC Core Configuration. Note Location H8 is the Required IFE Location.
The summary of the analysis of the planned core can be seen in the table below. The overall effective hot channel peak factor is less than that of the limiting core configuration (beginning of life) of the SAR, which has the highest peaking factors of any analyzed core. As the proposed core ages and some burnup is introduced in the new 30/20 elements in C ring, the peaking factors remain essentially unchanged (though the peaking factors do very slightly decrease with burnup).
The hot channel thermal power (hottest single element) was found to be 17.80 kW at 1 MW. This result is significant, because under the current technical specification 4.1.4 spec 3, this core configuration would not be allowed. Under the proposed change to technical specification 4.1.4 spec 3, this core configuration is allowed at a reduced power level, under the condition the hottest element will not exceed a power level of 17.69 kW. In the case of the proposed core, this would limit core power to 993 kW. It is MNRCs responsibility to set the required SCRAM setpoints below this value and administratively limit maximum steady-state power accordingly to comply with this proposed technical specification.
If in the future if the hottest element (as analyzed) was to show an increase in output beyond 17.80 kW at 1 MW, MNRC would use the 50.59 process to adjust (lower) the required SCRAM setpoints and administratively limit maximum steady-state power accordingly. The analysis of the hottest element power output will be performed annually, after a significant core change, or in support of an experiment to be placed in the central cavity of the core.
Hot Channel Power Summary Core Configuration Hot Channel Location Hot Channel Thermal Power [kW]
at 1 MW Hot Channel Peak Factor
[Pmax/Pavg]
Hot Channel Fuel Axial Peak Factor
[Pmax/Pavg]
Hot Channel Fuel Radial Peak Factor
[Pmax/Pavg]
Effective Hot Channel Peak Factor
[Pmax/Pavg]
NEW OCC Core H8 17.80 1.643 1.270 1.620 3.380 LCC BOL Core I6 17.69 1.804 1.218 1.681 3.694 Given the nature of MNRCs mission and somewhat limited excess reactivity, the facility administratively lowered the maximum steady-state power to 800 kW almost a year ago in October of 2022. MNRC has no plans of operating the reactor at power levels above 800 kW.
Analysis for Adding Technical Specifications 5.3.1 Spec 4 and Spec 5:
Shim Plate Clearance:
The proposed changes will require that all positions in G ring where fuel elements will be utilized must have an acceptable shim plate clearance (0.065 inches). The coolant channel width parameter of 0.065 inches is used in MNRCs thermal hydraulic analysis in chapter 4 of the SAR. This thickness provides a departure from nucleated boiling ratio of at least 2 for elements operating at 17.69 kW or less. G ring elements typically operate at power levels of less than half this power level. Clearance verification will be performed prior to introducing an element into a G ring positions as well as when an element located in G ring undergoes the required quinquennial inspection. The inspection itself will be performed utilizing a custom-designed and fabricated precision tool. Critical tool dimensions will be verified using a micrometer prior to performing the G ring shim plate clearance measurements.
At least initially, MNRC staff intend to inspect G ring elements and positions more frequently than the quinquennial requirement. The purpose of this more frequent inspection interval is to verify that the shim plates are not shifting and that the G ring fuel elements do not show any unusual bowing or elongation over the period of the next few years.
Given that the acceptable transverse bend dimension provided in Technical Specification section 3.1.4 specification 1 exceeds the required shim plate clearance, MNRC staff will rotate G ring elements while in core to verify that at no point does the element cladding contact the shim plate. This will be done in addition to the clearance measurement itself. Elements that do contact the shim plate during rotation will not be used in that grid plate position even if the position is measured to have at least a clearance of 0.065 inches.
Ultimately, MNRC intends to submit a future license amendment request to install a fueled B ring in the MNRC core. An internal graphite reflector is currently located in the B ring. This internal reflector supported 2.0 MW operations as fuel could not be safely utilized in the B ring of the 2.0 MW TRIGA reactor. Now that MNRC is a 1.0 MW reactor, there is no reason why a B ring could not be introduced. This would provide MNRC with the ability to potentially discontinue the use of G ring altogether while providing ample excess reactivity.
Use of Empty (water-filled) Positions:
While most TRIGA reactors are licensed to have empty (water-filled) positions in their core configuration, the need for MNRC to utilize this flexibility in core configuration was never envisioned and was therefore never analyzed.
Several combinations of fuel elements, graphite elements, and empty (water-filled) positions in G ring were modeled by MNRC staff. The results showed that generally, the transition of a single graphite element position to an empty (water-filled) position resulted in a very modest increase in adjacent fuel element power. This increase in power was on the order of 1-3% depending on the adjacent fuel element type (20/20 or 30/20) and the burnup level of the element. The transition of a second graphite element to an empty (water-filled) position had mixed results, changing adjacent element power levels by +/- 1-2%. With G ring fuel element power levels on the order of 6-9 kW, these power changes of a few percent would have no significant impact to reactor safety. Each transition of a graphite element to an empty (water) position results in a loss of approximately $0.05 worth of excess reactivity.
Though MNRC would be able to widely utilize empty (water-filled) position in G ring under the proposed technical specification, it is very unlikely MNRC would ever do so. MNRC operates primarily as a neutron radiography center. Empty (water-filled) positions in G ring tend to over-moderate neutrons as they move to the reflector where the beamline inserts are located. The addition of unnecessary, empty (water-filled) positions will increase radiography exposure times. It is expected that only the G ring position where the prostitution was identified will be left empty (water-filled).
Attachments:
- 1) Proposed Technical Specifications Page 18 for change to technical specification 4.1.4 specification 3.
- 2) Proposed Technical Specifications Page 24 for the addition of technical specifications 5.3.1 specification 4 and specification 5, Affirmation: I declare under penalty of perjury that the foregoing is true and correct executed on September 15th 2023.
Wesley Frey PhD CHP MNRC Facility Director
Attachment #1 Proposed Technical Specification Page 18
Specification-
- 1. A daily check of the core shall be made to verify only stainless steel clad 20/20 and 30/20 elements are only located Hex Rings C through G.
- 2. Prior to removal of any fuel element it shall be verified that the core is subcritical by more than the calculated worth of the most reactive fuel element being moved.
- 3. Prior to manual removal of any control rod it shall be verified that the core is subcritical by at least $0.50 with the highest worth control rod in the full-out position.
Basis
- 1. This specification provide verifications that core configuration will not deviate from the core configuration analyzed for in the SAR.
- 2. and 3. These specifications ensure the core will remain shut down during fuel and control rod movements and inspections.
4.1.4 Fuel Parameters Applicability - This specification applies to the surveillance requirements for the fuel elements.
Objective - The objective is to verify the continuing integrity of the fuel element cladding.
Specification -
- 1. All fuel elements shall be inspected for damage or deterioration and measured for length and transverse bend at least at quinquennial intervals.
- 2. An analysis of any irradiation facility installed in the central cavity of this core shall be done before it is used with this core.
- 3. No single element may shall be operated at a power level above 17.69 kW (as analyzed). at a steady state power level of 1.0 MW.
Basis-
- 1. The above specifications assure that the fuel elements shall be inspected regularly and the integrity of the lead fuel elements shall be maintained.
- 2. and 3. Will provide assurance that the thermal hydraulic analysis provided in the SAR is always bounding.
4.2 Reactor Control and Safety Systems 4.2.1 Control Rods Applicability - This specification applies to the surveillance of the control rods.
Objective - The objective is to inspect the physical condition of the reactor control rods and establish the operable condition of the rods.
Attachment #2 Proposed Technical Specification Page 24
positions occupied by in-core experiments, irradiation facilities, graphite dummies, control rods, and startup sources.
- 3. The reflector, excluding experiments and irradiation facilities, shall be graphite. A reflector is not required if the core has been defueled.
- 4. G ring core locations may be empty (water-filled).
- 5. The reactor shall not be operated with fuel elements in G ring core locations that have a shim plate clearance of less than 0.065 inches.
5.3.2 Reactor Fuel Applicability - These specifications apply to the fuel elements used in the reactor core.
Objective - The objective is to assure that the fuel elements are of such design and fabricated in such a manner as to permit their use with a high degree of reliability with respect to their physical and nuclear characteristics.
Specification - The individual unirradiated TRIGA fuel elements shall have the following characteristics:
- 3. Cladding: stainless steel, nominal 0.5mm (0.020 inch) thick.
5.3.3 Control Rods and Control Rod Drives Applicability - This specification applies to the control rods and control rod drives used in the reactor core.
Objective - The objective is to assure the control rods and control rod drives are of such a design as to permit their use with a high degree of reliability with respect to their physical, nuclear, and mechanical characteristics.
Specification -
- 1. All control rods shall have scram capability and contain a neutron poison such as stainless steel, borated graphite, B4C powder, or boron and its compounds in solid form. The shim and regulating rods shall have fuel followers sealed in stainless steel. The transient rod shall have an air filled follower and be sealed in an aluminum tube.
- 2. The control rod drives shall be the standard GA rack and pinion type with an electromagnet and armature attached.
5.4 Fissionable Material Storage Applicability - This specification applies to the storage of reactor fuel at a time when it is not in the reactor core.