1.1. BACKGROUND
Typical light water reactor (LWR) fuel is UO2 with a 235U enrichment in the range of 3% to just below 5% and operates with an average discharge burnup below approximately 60 MW·d/kgU.
Indeed, extending fuel discharge burnup has been pursued over the last decades in order to benefit in various ways. Economic benefit can be attributed to maximized fuel utilization, operational benefit to increased operational flexibility with various options on in-core management schemes. Economic benefit is also expected from a reduction of spent fuel as well as from extended cycle lengths that in turn contribute to higher capacity factors.
Previous studies [1–3] have demonstrated such benefits through theoretical case studies of operating UO2 fuel to extended discharge burnups. For example, one study [2] indicated that, for both boiling water reactors (BWR) and pressurized water reactors (PWR), fuel costs continued to decline with increasing batch average discharge burnups, in this case up to 65.2 MW·d/kgU for BWR with 24-month cycles and 70.3 MW·d/kgU for PWR with 18-month cycles. The economic calculations included a cost penalty caused by the higher enrichment cost of fuel, in excess of the current limit of 5% 235U, which was necessary to achieve the high discharge burnups considered.
The maximum discharge burnup of fuel in a reactor can be determined by taking account of various factors that include but are not limited to:
• 5% 235U enrichment limit;
• Cycle length (12, 18 or 24 months);
• Limits on oxydation thickness and hydrogen pickup of the cladding;
• Radial power distribution across the core;
• Fuel performance in the reactor;
• Economics.
Extended discharge burnup of LWR fuels can be obtained simply by using higher 235U enrichment (Fig.1), although other methods are available to some extent, such as, subdivision of fuel rods in a fuel assembly to reduce the maximum thermal load imposed on the fuel rod, reconstitution of spent fuel assemblies, and reshuffling of spent fuel assemblies. Use of mixed oxide (MOX) fuel (a combination of plutonium and uranium oxides) is also an alternative.
The 5% 235U enrichment limit has existed since the establishment of the first requirements for nuclear fuel design and manufacturing and has become the industry standard for LWR fuel operation. Although no real justification can be found for this 5% 235U enrichment limit, it has remained unchanged. However, LWR fuel enrichments of 235U have gradually increased and in recent designs the 235U enrichment effectively reaches the 5% limit, with 4.95% used to allow for a manufacturing uncertainty to the 5% limit.
The limitation of 235U enrichment to 5% can also affect Accident Tolerant Fuel (ATF) development programmes. New fuel cladding materials1 developed for ATF may require an increased enrichment of fuel pellets to compensate for their higher thermal neutron absorption than for Zircaloy cladding.
FIG. 1. Initial 235U enrichment versus estimated average discharge burnup in LWRs, Reproduced courtesy of the IAEA [5].
Therefore, the limitation of 235U enrichment to 5% has become a new concern among Member States.
In response to the request of Member States, the IAEA organized two technical meetings on Light Water Reactor Fuel Enrichment beyond the 5% Limit: Perspectives and Challenges in 2015 and in 2018, in order to facilitate a review of national and international programmes focusing on opportunities and implementation issues for the use of High Assay Low Enriched Uranium (HALEU), whose 235U enrichment ranges from 5% up to 19.75%, in LWRs and to facilitate the exchange of relevant knowledge and experience among R&D organizations, utilities, regulatory authorities and governmental agencies.
This publication is intended to compile the results and conclusions of these technical meetings especially in terms of the benefits to be obtained from the use of HALEU fuel and consideration of safety issues that arise.
Similar publications [2,3,6,7] made by other organizations mainly focus on specific aspects, such as economic benefit, domestic fuel cycle infrastructure and licensing practice.
1.2. OBJECTIVES
The objective of this publication is to document perspectives and challenges with the use of HALEU fuel in LWRs, based on the results of the two technical meetings on Light Water
1 These include coated cladding, FeCrAl steel cladding, lined Mo alloy cladding, SiC/SiC composite cladding;
see [4] for detailed information.
Reactor Fuel Enrichment beyond the 5% Limit: Perspectives and Challenges, held in Vienna, 2015 and in Moscow, 2018. Specifically, this publication is intended to:
• Present various benefits, in qualitative and quantitative manners, attainable through the use of HALEU fuel in LWRs;
• Identify safety concerns that would need to be overcome for the use of HALEU fuel in LWRs; and propose methods to mitigate critical challenges based on practice in Member States;
• Present collected materials (papers and presentations) from the aforementioned two technical meetings.
1.3. SCOPE
The scope of this publication covers:
• Feedback from national and international R&D programmes on the use of HALEU;
• Technological options and corresponding issues regarding fuel and core design, safety analysis and assessments and other aspects related to the use of nuclear fuels for LWRs having enrichments higher than 5% in 235U, such as: manufacturing, handling, transportation, storage, irradiation, and performance in normal and accident conditions;
• Assessment of accident tolerant fuel (ATF) iron-based cladding design options that require higher enrichment and their compatibility with existing reactor designs.
Some design options for the ATF and for innovative fuels to enhance thermal margins in the reactor may need to use HALEU to accommodate for a large absorption of neutrons by the cladding and structural materials. This publication addresses their impact on the 235U enrichment of the fuel and their nuclear compatibility in the existing reactors. Subsequently, the selection of burnable absorber materials and their performance investigation in the material test reactors are of interest. This publication does not address required activities for the design and design verification of the ATF and innovative fuels.
1.4. STRUCTURE
In Section 1, the justification of this project is described.
In Section 2, a worldwide overview on using HALEU in LWRs is described.
In Section 3, benefits that can be achieved from the use of HALEU fuel in power reactors are described from various aspects: high burnup, cycle length, power uprating, economics, adaptability of innovative fuels, waste generation, use of reprocessed uranium and use of ATF.
In Section 4, impacts of ATF iron-based cladding design options on their compatibility with existing reactor designs are described.
In Section 5, additional safety considerations and challenges that come from the use of HALEU fuel are described from viewpoints of international standards, front-end fuel cycle facilities,
reactor safety, back-end fuel cycle facilities, accident analysis and validation of computer codes used for safety analysis.
In Section 6, R&D activities on burnable absorbers to facilitate the use of HALEU are described.
In Section 7, conclusions are provided.
Papers presented at the two technical meetings are contained in supplementary material (see Annex I).