ML23243B009
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| ML23243B009 | |
| Person / Time | |
|---|---|
| Site: | University of California-Davis |
| Issue date: | 08/30/2023 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| Download: ML23243B009 (1) | |
Text
August 30 UC Davis MNRC th NRC Public Meeting
Outline
- Review of MNRC 2023 fuel inspection
- Overview of issues
- Overview of potential resolutions
- Proposed changes to technical specifications
Previous Inspection Criteria
- These elements were defined as all elements (excluding IFEs) in C ring and surrounding the transient rod.
- This inspection methodology assumed that any issue with the core would be due to issues from the fuel from operating at high temperatures (based on 2.3 MW operation).
- This inspection methodology would not be effective at finding elements with manufacturing defects and issues with other core structures.
Current Fuel Inspection Requirements
- Specification - The reactor shall not be used for operation with damaged fuel. All fuel elements shall be inspected visually for damage or deterioration as per Technical Specifications Section 4.1.4 (100%
core inspection every 5 years).
- A fuel element shall be considered damaged and must be removed from the core if:
- 1. In measuring the transverse bend, the bend exceeds 0.125 inch (3.175 mm) over the full length 23 inches (584 mm) of the cladding, or,
- 2. In measuring the elongation, its length exceeds its initial length by 0.125 inch (3.175 mm),or,
- 3. A cladding failure exists as indicated by measurable release of fission products, or,
- 4. Visual inspection identifies bulges, gross pitting, or corrosion.
- 5. 235U burnup is calculated to be greater than 50% of initial content.
Failed Element (Position D10)
- Insert pic of two elements
Elements in K1 and L1 The Shim Plates The Shim Plates The Shim Plates The Upper Core Ring Position L1 Potential Initiating Event
- In 1998 the fuel transfer cask briefly rested on the chimney structure and may have caused the core barrel or shim plates to shift. Documentation of the event is very limited.
- Observable design flaw in the shim plates/upper core ring and lower grid plate.
- Vibrations from >1.5 MW operation.
- Or a combination of these three potential causes.
Other Elements of Interest
- The remaining elements in G ring were removed from the core for inspection without notable issue. Meaning there was no significant binding when they were removed from the grid plate or reinserted back into the grid plate.
- Several of these elements had visible scratches on the lower section. These scratches mostly date back to the 2 MW initial core loading and dont appear to have worsened significantly over time.
- Based on these scratches almost exclusively found on G ring elements (specifically from corner locations).
Other Elements of Interest Other Elements of Interest Outstanding Issues
- Water filled positions
- Lower grid plate
- Protrusion into element position
- Shim plate contacting an element
Water Filled Positions
- Water filled positions vs. graphite element filled positions (in G ring) do subtly change reactor behavior.
- Water positions tend to over-moderate the system locally.
- Graphite positions provide less moderation and neutron absorption relative to water.
Water Filled Positions in G Ring
- The replacement of graphite elements with water in G ring reduces excess reactivity by about $0.05. This has little impact on reactor safety or performance.
- The substitution of 1 water position for a graphite element increases power on an adjacent element by 1-3%.
- Adjacent element power remains constant as a second graphite element is replaced with water.
- Heavily loading one side of the core with multiple water filled positions reduces adjacent elements power levels and shifts power production away from this region of the core. The hottest element location can move under these conditions.
- Though peaking factors in elements adjacent to water filled positions in G ring may change (increase) slightly, it very unlikely that the peaking factors could change enough to invalidate MNRC current thermal hydraulic analysis as the G ring elements produce ~6-8 kW compared to the 17.69 kW maximum element heat output used as the bounding condition in the analysis.
The Lower Gride Plate Lower Grid Plate
- MNRC is only planning on operating with 3 fueled corner positions in G ring.
- Elements can be positioned such that the bottom triflutes are not located in bottom grid plate gap. Their position can be verified periodically.
- When MNRC receives new fuel elements in 3-5 years, all G ring corner position will likely be retired.
Protrusions
- The existence or absence of protrusions (such as the one found in position L1) can be made as fuel is removed and reinserted during normal fuel inspection.
Shim Plate Position
- The clearance between an element in G ring and the shim plate should be about 0.130 inches.
- MNRCs current thermal hydraulic analysis requires a clearance of 0.065 inches around all elements.
- If this clearance can be verified, MNRC thermal hydraulic analysis is bounding for all elements with this clearance up to 17.69 kW output. Elements in G ring typically have an output of less than half of this value.
- A clearance of less than 0.065 inches may produce a MDNBR of greater than 2.0 for G ring elements, though this would require a new T&H analysis and would result in a new tech spec limit for G ring heat output.
Shim Plate Position Inspection Tool Shim Plate Position Inspection Tool Conclusions
- Disassembly of the core to remove the shim plates/upper core ring would be very difficult for a number of reasons.
- Positions K1 and L1 cannot be used (fueled) due the shim plate/core ring issue. K1 currently has a graphite element inserted while L1 would need to be water filled (license amendment required).
- Since the 2.3 MW upgrade, the G ring positions have not had a fuel element leak and only produced one element that failed the inspection criteria (likely due to a manufacturing defect).
Though other elements have been removed from service as a precaution. This does include a time period of extended operations above 1.5 MW.
Changes to Technical Specifications
- Water filled positions in G ring are permitted.
- (MNRC does not intend to widely utilize this)
- MNRC will inspect all G ring fuel elements every 1-2 years.
- Elements must move in and out of G ring smoothly to indicate there are no protrusions into the grid plate position.
- MNRC will qualify all G ring positions containing fuel every 1-2 years by verifying there is at least 0.065 inches of clearance between a fuel element and all other objects around the fueled section of the element.
Questions?