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Asset Retirement Obligations | 12 Months Ended
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Sep. 30, 2023
Asset Retirement Obligation Disclosure [Abstract] |
Asset Retirement Obligations | 13. Asset Retirement Obligations During the year ended September 30, 2023, TVA's total ARO liability increased $327 million. To estimate its decommissioning obligation related to its nuclear generating stations, TVA uses a probability-weighted, discounted cash flow model which, on a unit-by-unit basis, considers multiple outcome scenarios that include significant estimations and assumptions. Those assumptions include (1) estimates of the cost of decommissioning; (2) the method of decommissioning and the timing of the related cash flows; (3) the license period of the nuclear plant, considering the probability of license extensions; (4) cost escalation factors; and (5) the credit adjusted risk free rate to measure the obligation at the present value of the future estimated costs. TVA has ascribed probabilities to two different decommissioning methods related to its nuclear decommissioning obligation estimate: the DECON method and the SAFSTOR method. The DECON method requires radioactive contamination to be removed from a site and safely disposed of or decontaminated to a level that permits the site to be released for unrestricted use shortly after it ceases operation. The SAFSTOR method allows nuclear facilities to be placed and maintained in a condition that allows the facilities to be safely stored and subsequently decontaminated to levels that permit release for unrestricted use. TVA also has decommissioning obligations related to its non-nuclear generating sites, ash impoundments, transmission substation and distribution assets, and certain general facilities. To estimate its decommissioning obligation related to these assets, TVA uses estimations and assumptions for the amounts and timing of future expenditures and makes judgments concerning whether or not such costs are considered a legal obligation. Those assumptions include (1) estimates of the costs of decommissioning, (2) the method of decommissioning and the timing of the related cash flows, (3) the expected retirement date of each asset, (4) cost escalation factors, and (5) the credit adjusted risk free rate to measure the obligation at the present value of the future estimated costs. TVA bases its decommissioning estimates for each asset on its identified preferred closure method. The revisions in non-nuclear estimates increased the liability balance by $362 million for the year ended September 30, 2023. During the year coal combustion residuals ("CCR") closure liabilities at Bull Run, Johnsonville, and Cumberland increased $458 million due to revised cost estimates for final closure activities based on TVA's current approved closure strategies at these sites. Partially offsetting these increases, expected reductions in CCR post-closure costs for long-term monitoring at Gallatin resulted in a decrease of $60 million. In addition, CCR closure liabilities at Cumberland decreased $15 million due to identified changes in the projected timing of certain asset retirement activities, and CCR closure liabilities at Paradise decreased $9 million based on refined project cost estimates. The revisions in non-nuclear estimates increased $186 million for the year ended September 30, 2022. During the year, TVA completed an engineering review of its cost estimates for closure of certain areas containing coal fines at Paradise Fossil Plant ("Paradise"), which resulted in an increase of $119 million due to expected cost increases for necessary changes in activities associated with proper completion of the closure. CCR closure liabilities at Paradise and Cumberland increased $82 million due to new vendor bids, modified closure designs, and revised estimates for construction costs. Refined project cost assumptions and scope changes related to TVA's AROs for the closure of certain coal yards at its fossil plants resulted in an increase of $57 million. In addition, effective October 1, 2022, TVA implemented revised depreciation rates applicable to its completed plant due to the results of a new depreciation study. The study included a decline in the service life estimates of TVA's coal-fired plants based on current planning assumptions to potentially retire the remainder of the coal-fired fleet by 2035. As a result of the change in service life estimates reflected in the depreciation study, TVA performed an assessment of the assumptions used in the timing of cash flows related to its non-nuclear AROs. Based on the assessment, TVA increased AROs by $47 million due to identified changes in the projections of timing of certain asset retirement activities. Partially offsetting these increases, expected reductions in CCR post-closure costs for maintenance and monitoring at Paradise, Shawnee, and Colbert Fossil Plant ("Colbert") resulted in a decrease of $53 million. CCR closure liabilities at Gallatin decreased $30 million due to selection of a new vendor bid and changes to anticipated timing of certain asset retirement activities. In addition, CCR closure liabilities at Allen Fossil Plant decreased $15 million as a result of changes in cost estimates and anticipated timing of various retirement activities. The revisions in nuclear estimates increased the liability balance by $61 million for the year ended September 30, 2022. This was primarily due to approval and implementation of the most recent site-specific cost study in September 2022 resulting in an increase of $58 million. Additionally, during the years ended September 30, 2023 and 2022, both the nuclear and non-nuclear liabilities were increased by periodic accretion, partially offset by settlement projects that were conducted during these periods. The nuclear and non-nuclear accretion amounts were deferred as regulatory assets. During 2023, 2022, and 2021, $188 million, $137 million, and $72 million, respectively, of the related regulatory assets were amortized into expense as these amounts were collected in rates. See Note 10 — Regulatory Assets and Liabilities. TVA maintains investment trusts to help fund its decommissioning obligations. See Note 16 — Fair Value Measurements — Investment Funds and Note 22 — Commitments and Contingencies — Contingencies — Decommissioning Costs for a discussion of the trusts' objectives and the current balances of the trusts.
Asset Retirement Obligation Activity (in millions)
| Nuclear |  | Non-Nuclear |  | Total |
Balance at September 30, 2021 | $ | 3,428 |  |  | $ | 3,574 |  |  | $ | 7,002 |  |
Settlements | (2) |  |  | (309) |  |  | (311) |  |
Revisions in estimate | 61 |  |  | 186 |  |  | 247 |  |
Accretion (recorded as regulatory asset) | 156 |  |  | 68 |  |  | 224 |  |
Balance at September 30, 2022 | 3,643 |  |  | 3,519 |  |  | 7,162 |  | (1)
Settlements | (8) |  |  | (271) |  |  | (279) |  |
Revisions in estimate | 7 |  |  | 362 |  |  | 369 |  |
Accretion (recorded as regulatory asset) | 166 |  |  | 71 |  |  | 237 |  |
Balance at September 30, 2023 | $ | 3,808 |  |  | $ | 3,681 |  |  | $ | 7,489 |  | (1)
Note (1) Includes $272 million and $275 million at September 30, 2023 and 2022, respectively, in Current liabilities.

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- Definition The entire disclosure for an asset retirement obligation and the associated long-lived asset. An asset retirement obligation is a legal obligation associated with the disposal or retirement from service of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 410 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481850/410-20-50-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 410 -SubTopic 20 -Name Accounting Standards Codification -Publisher FASB -URI https://asc.fasb.org//410-20/tableOfContent Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section 50 -Paragraph 2 -SubTopic 20 -Topic 410 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481850/410-20-50-2 + Details
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