The present publication responds to growing awareness of a need for open discussion and concerted action to promote effective improvements in nuclear fuel reliability, as was formulated in more general terms at the 9th biennial general meeting of the World Association of Nuclear Operators (Chicago, September 2007) by WANO Chairman William Cavanaugh: “Meeting the unprecedented demands of the nuclear renaissance will require operators not only to take on the individual responsibility to guarantee the safety of their own fleet, but also to assume a collective responsibility to work together to continually upgrade the safety of operating nuclear power stations worldwide. The public demands no less from us.”
The current study covers about 96% of water reactor fuels, thus providing the most representative picture of fuel reliability available in open literature and presumes the opportunity for a number of conclusions.
Statistics for fuel failures (Section 3) seem to reveal a contradiction in incentives: on one hand, fuels should operate in more challenging conditions, with up-rate power outputs but increased failure probability, and on the other hand there is an aspiration to have fully reliable fuels with ‘zero failure rates’ to a practical ‘ppm’ extent. This balance varies from country to country and is dependent both on achieved levels of technological maturity and on local levels of economically and publicly acceptable risk.
Fuel rod failure rates in LWRs have been significantly (but not monotonically) reduced since 1987, on average to levels of 10–5 between 2003 and 2006. In CANDUs, element failure rates have been near 10–5 and have remained at this low level over the reporting period, reaching 5 × 10–6 between 2003 and 2006. However, the fuel failure rate has not markedly decreased during the last decade, with a relatively large number of failures still occurring in a few plants. Moreover, signs that new failures are increasing were observed in the early 2000s. It is interesting to note that a majority of the mechanisms causing earlier fuel failures are still prevailing, mostly in combination with new contributing factors like higher burn-ups and power ramps.
In a few cases, adequate design and manufacturing solutions have led to considerable improvements. Clad collapse (PWRs) and failures due to excessive rod bow or differential length growth have practically disappeared.
Failures from baffle jetting (PWRs) temporarily increased during the 1980s, but have been reduced now to nearly zero. Hydriding caused by moisture in fuel, which had earlier been a major source of failure in all LWR types, has been essentially eliminated. Other manufacturing related failures are low in absolute numbers, accounting for 5%.
Important current issues (2003–2006) in fuel performance include grid to rod fretting in PWRs (52% of all identified root causes) and corrosion by itself or in combination with crud deposits in BWRs (46%). Debris fretting continued at significant levels (28% in BWRs and 9% in PWRs), as well as PCI/SCC in BWRs (12%). Despite all efforts to find adequate remedies, there seem to be problems that continue to challenge the industry. Besides traditional rod failures, new fuel assembly related issues have appeared that can seriously affect plant operations:
fuel assembly bow and its consequences on incomplete control rod insertion (IRI), axial offset anomaly (AOA), and crud deposits on fuel. Handling damage of PWR fuel is often related to assembly bow with the consequence of spacer damage during loading or off-loading. A variety of incidents involving fretting wear have been reported.
Failure rates
(1) The existing non-uniformity in approach regarding fuel failure data and rate calculations can lead to misinterpretation of statistical results. The current study proposes a methodology of failure rate assessment on the basis of fuel reloads which more realistically reflect fuel reliability than calculations based on fuel inventories.
(2) Despite continuous fuel material upgrades, and design and quality assurance procedures implemented within the last decade aiming at improving fuel reliability, failure rates have oscillated in most countries (with the only exception being Japan, which has a stable and very low failure rate).
Failure mechanisms
(3) No new failure mechanisms have been revealed during the last decade, but experimental simulation and theoretical modelling indicate the possibility of material degradation in more challenging operating conditions, e.g. hydriding of BWR fuel rods in power ramp tests.
(4) The latest design improvements in LWR fuels have effectively tackled, but in some cases still not fully resolved, problems related to structural behaviour of fuel assemblies.
Failure analysis and management
(5) In the current study, the methods of data collection and methodology used for statistical calculations were applied mainly to fuel leakage failures. At the same time, it was demonstrated that non-leaker failures can cause serious problems for nuclear unit operations. Deeper analysis of assembly and core related issues is needed.
(6) Incidents involving massive failures should be specifically considered and taken into account.
(7) An approach to the ambitious ‘zero rate’ target will require complex technical and organizational measures including deep analysis of feedback at all stages of the fuel quality assurance circle (R&D, design, fabrication, and operation).
ABBREVIATIONS
ASTM American Society for Testing and Materials
AUC ammonium uranyl carbonate
CSNI Committee on the Safety of Nuclear Installations (OECD/NEA) DAD defective assembly detection
GW◊d/t U burnup, measured in gigawatt days per tonne uranium
HGC hydrogen gas cracking
INPRO International Project on Innovative Nuclear Reactors and Fuel Cycles (IAEA) IRI incomplete (control) rod insertion
LHGR linear heat generation rate, in W/cm or kW/m or kW/ft LOCA loss of coolant accident NFCIS Nuclear Fuel Cycle Information System (IAEA) NGS nuclear generating station (used in Canada) NMCA noble metal chemical addition
NPP nuclear power plant
NRC (United States) Nuclear Regulatory Commission
OECD/NEA Organisation for Economic Co-operation and Development Nuclear Energy Agency
OS outer surface
RCCA rod cluster control assembly RIA reactivity initiated accident
RIAR Research Institute of Atomic Reactors (Dimitrovgrad, Russian Federation)
RTP ramp terminal power
SCC stress corrosion cracking
SCIP Studsvik Cladding Integrity Programme
SCRs spacer capture rods
SEM scanning electron microscopy
SEU slightly enriched uranium
SG spacer grid
SPLIT split propagation laboratory investigation test
SPP secondary phase particles
SRA stress relieved annealed
SRP standard review plan
SS stainless steel
TD theoretical density
TEM transmission electron microscope TIG tungsten inert gas (welding process)
TQM total quality management
TSSd terminal solid solubility for dissolution TSSp terminal solid solubility for precipitation
TWGFPT Technical Working Group on Water Reactor Fuel Performance and Technology (IAEA)
UCL upper control limit
USW upset shape welding (process)
UT ultrasonic testing
WANO World Association of Nuclear Operators
CONTRIBUTORS TO DRAFTING AND REVIEW
Alvarez, L. Comision Nacional de Energia Atomica, Argentina
Daniels, T. Ontario Power Generation, Canada
Dangoulème, D. Electricite de France, France
Doria, F. Atomic Energy of Canada Ltd, Canada
Edsinger, K. Energy Power Research Institute, United States of America Fujiwara, S. Federation of Electric Power Companies of Japan, Japan Inozemtsev, V. International Atomic Energy Agency
Ip, M. Bruce Power, Canada
Kamimura, K. Japan Nuclear Energy Safety Organization, Japan Killeen, J. International Atomic Energy Agency
Kucuk, A. Electric Power Research Institute, United States of America
Lewis, B. Royal Military College, Canada
Novikov, V. All-Russia Scientific Research Institute of Inorganic Materials, Russian Federation Onufriev, V. Private consultant, Russian Federation
Pageau, F. Hydro Quebec, Canada
Prasad, P. N. Nuclear Power Corporation of India Ltd, India
Reid, P. Atomic Energy of Canada Ltd, Canada
Sun, A. Atomic Energy of Canada Ltd, Canada
Suk, E. Atomic Energy of Canada Ltd, Canada
Tayal, M. Atomic Energy of Canada Ltd, Canada
Tran, T. New Brunswick Power, Canada
Consultants Meetings Vienna, Austria: 13–15 February 2007;
Vienna, Austria: 27–29 November 2007;
Vienna, Austria: 27–29 May 2008.
Examples Principles NG-G-3.1:Nuclear General (NG), Guide, Nuclear Infrastructure and Planning (topic 3), #1 NP-T-5.4:Nuclear Power (NP), Report (T), Research Reactors (topic 5), #4 NF-T-3.6:Nuclear Fuel (NF), Report (T), Spent Fuel Management and Reprocessing, #6 echnical Reports NW-G-1.1:Radioactive Waste Management and Decommissioning (NW), Guide, Topic designations Radioactive Waste (topic 1), #1 Guide or Report number (1, 2, 3, 4, etc.)
St ructure of the IAEA Nuclear Energ y Serie s
Radioactive Waste Management and Decommissioning Objectives NW-O Nuclear Fuel Cycle Objectives NF-ONuclear Power Objectives NP-O
Nuclear General Objectives NG-O
Nuclear Energy Basic Principles NE-BP 1. Management Systems NG-G-1.# NG-T-1.# 2. Human Resources NG-G-2.# NG-T-2.# 3. Nuclear Infrastructure and Planning NG-G-3.# NG-T-3.# 4. Economics NG-G-4.# NG-T-4.# 5. Energy System Analysis NG-G-5.# NG-T-5.# 6. Knowledge Management NG-G-6.# NG-T-6.#
1. Technology Development NP-G-1.# NP-T-1.# 2. Design and Construction of Nuclear Power Plants NP-G-2.# NP-T-2.# 3. Operation of Nuclear Power Plants NP-G-3.# NP-T-3.# 4. Non-Electrical Applications NP-G-4.# NP-T-4.# 5. Research Reactors NP-G-5.# NP-T-5.#
1. Resources NF-G-1.# NF-T-1.# 2. Fuel Engineering and Performance NF-G-2.# NF-T-2.# 3. Spent Fuel Management and Reprocessing NF-G-3.# NF-T-3.# 4. Fuel Cycles NF-G-4.# NF-T-4.# 5. Research Reactors — Nuclear Fuel Cycle NF-G-5.# NF-T-5.#
1. Radioactive Waste Management NW-G-1.# NW-T-1.# 2. Decommissioning of Nuclear Facilities NW-G-2.# NW-T-2.# 3. Site Remediation NW-G-3.# NW-T-3.#
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IAEA Nuclear Energy Series
Technical Reports
Review of
Fuel Failures in
Water Cooled Reactors
No. NF-T-2.1
IAEA Nuclear Energy Series No. NF-T-2.1Review of Fuel Failures in Water-Cooled Reactors