Reduce Risk of Baffle Jetting by Modifying Core Coolant Flow Path - MTA-NF-002
| Administrative Items | |
|---|---|
| Date | 12/15/2020 |
| Functional Area Where Benefits Will Be Realized | Nuclear Fuels |
| Reference Implementation Guidance |
2017 NEI TIP Awards – Submittal 12 (ID: 10053033) PWR Owners Group – “Baffle Former Bolt Overview of Industry Experience” |
| Industry SME | EPRI – Kyle Amberge
Contact: NuclearPlantMod@epri.com |
| Previous Implementation | Please contact EPRI for implementation examples and contacts. |
| Implementation Enablers | N/A |
| SWEEP Score |
|
| Applicability |
Westinghouse plants with downward flow between baffle plates and core barrel All geographic regions |
| Keywords | Westinghouse; baffle jet impingement; fuel failure; baffle bolt; upflow; downflow |
| Business Case Analysis Cross-Reference | N/A |
Description
Baffle jet impingement caused by gaps between the core baffle and former plates in conjunction with high differential pressures between the outside (barrel side) and inside (core side) of the baffle plates can lead to fuel failures. Westinghouse plants in which the coolant flow path from the hot leg nozzle splits to flow between (1) the baffle‑former assembly and core barrel and (2) between the core barrel and RPV have relatively large differential pressures across the baffle‑former assembly, which can lead to plate deformation and jetting. More recent designs eliminate the downward flow path between the baffle‑former assembly, routing the entire inlet flow through the barrel‑RPV annulus. At the core inlet, some of the coolant flow is routed upward between the baffle‑former assembly and barrel to provide cooling. This configuration, called “upflow”, significantly reduces the differential pressure across the baffle plates as well as the resulting jetting. Later generations of Westinghouse nuclear power plants were designed with the upflow configuration and baffle jetting has not been observed in these plants. Likewise, baffle jetting has been eliminated in plants that were converted from downflow to upflow.
This MTA summarizes the process of converting a plant from downward to upward flow. The modification is performed by machining of holes on the topmost former plate and installation of plugs in existing holes in the core barrel. The modification may also reduce the risk of baffle bolt failures due to stress corrosion cracking by reducing the tensile stress in the baffle bolts.
To gain time for modification planning when baffle jet impingement is initially discovered, hardened fuel assemblies featuring low‑enrichment fuel and stainless steel rods may be used at peripheral locations for one fuel cycle. Hardened fuel assemblies are not the focus of this MTA.
Benefits
Benefits Estimate
Level 1 – The direct benefit of avoiding a fuel failure of a single, peripheral fuel assembly is typically less than $1 million. Significant additional savings (Level 3) may be realized if baffle bolt failures are avoided.
Benefits Description
- Reduced risk of baffle jetting and associated fuel failures.
- Reduced need for replacement fuel assemblies and disposal of old fuel assemblies.
- Reduced concentration of radionuclides in primary system by avoiding fuel failures leads to reduced coolant clean‑up and reduced waste from the clean‑up system.
- Reduced tensile stress in baffle former bolts, which may prevent baffle bolt failures. Reducing the differential pressure between the core and the baffle/barrel annulus reduces the risk of baffle bolt failure but may not prevent failures in all cases.
Costs and Schedule
Cost
Level 1 – Cost for the modification, including engineering (contractor and plant), material and implementation ranges from $8 million to $12 million depending on the PWR design type.
Schedule
One to three years (design, engineering and planning can be conducted within one 18‑month refueling cycle).
Scope Context
Per reactor
Risks
- Implementation of the plant‑specific modification extends the critical path of the outage. Contingency planning is required to ensure implementation is completed on time.
- Implementation of the plant‑specific modification requires utility planning of the plant‑specific design change package (DCP) several years in advance of the outage. Use of capital project funds (as applicable) for execution of the modification may also be considered and justified, since the modification represents a proven physical improvement to the PWR unit.
- Plant‑specific planning is required to provide adequate shielding while modifications are made to the former plates and the core barrel flow holes. Improved shielding can reduce dose by 40%.
- The modification requires cutting holes in the top former plate. Care should be taken to ensure foreign material exclusion best practices are followed.
- The modification requires plugging of existing holes in the core barrel. Pre‑ and post‑installation testing of the hydraulically expanded plugs should be conducted to verify proper manufacturing and installation.