ML20057C521
| ML20057C521 | |
| Person / Time | |
|---|---|
| Issue date: | 09/22/1993 |
| From: | Taylor J NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| To: | |
| References | |
| SECY-93-265, NUDOCS 9309290066 | |
| Download: ML20057C521 (6) | |
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POLICY ISSUE September 22, 1993 SECY-93-265 (Informat. ion) f_QR:
The Commissioners FROM:
James M. Taylor Executive Director for Operations
SUBJECT:
ISSUES IN THE REVIEW 0F A DUAL-PURPOSE CASK FOR TRANSPORTATION AND DRY STORAGE OF SPENT FUEL l
PURPOSE:
l To inform the Commission of the technical issues associated with the review of a dual-purpose cask design for spent fuel transportation and dry storage.
SUMMARY
l This paper discusses the differences in the U.S. Nuclear Regulatory Cormission l
requirements for certifying a spent fuel cask (1) for storage and (2) for I
transportation, and the technical issues that arise when reviewing a cask l
design proposed for a dual-purpose use.
BACKGROUND:
For reasons of economy and greater efficiency in spent fuel handling operations, there is a move by industry to use a spent fuel cask designed for both transportation and storage.' Staff has received one application for a dual-purpose cask and expects to complete its review in early 1994.
An application for another dual-purpose design is expected soon.
In addition, the U.S. Department of Energy (DOE) is considering a cask design, referred to as a Multi-Purpose Canister (MPC), for NRC certification for transportation, storage, and, ultimately, disposal in the high-level waste geologic repository.
Contact:
M. Lusardi, STSB/NMSS NOTE:
TO BE MADE PUBLICLY AVAILABLE 504-2704 AT COMMISSION MEETING ON j
SEPTEMBER 30, 1993 i
' Industry initiatives on a dual-purpose cask for storage and transportation are also supported by HRC dry-storage regulations that require consideration, to the extent practical, in dry cask storage design, of the cask's " compatibility with removal of the stored spent fuel from a reactor site, transportation, and ultimate disposition" by DOE.
10 CFR 72.236(m) ms 3
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Because the requirements for transportation, storage, ana disposal are
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different, a separate, independent, review is required for each type of cask design. DOE and industry have recently inquired about the similarities and differences in the requirements for each purpose, particularly in the areas of l
storage and transportation.
In addition, some questions have been raised concerning the coordination of the different reviews. This paper addresses the technical issues associated with the differences in the transportation and storage requirements and the NRC review associated with each.
DISCUSSION:
Under NRC transportation and storage regulations, a spent fuel cask design generally must provide four primary functions.
First, adequate shieldino must be provided to ensure that external radiation dose rates do not exceed applicable limits.
Second, adequate containment or confinement of the radioactive material must be provided to prevent harmful releases and dispersion of the material. Third, the radioactive contents must be packaged in such a manner that subcriticality is maintained.
Finally, adequate ken e
removal must be provided to dissipate the decay heat produced by the radioactive contents and to protect the integrity of the fuel.
Performance requirements are specified for both transportation and storage to ensure that a cask provides these four primary safety functions in the 2
environments to which it may be subjected to under both normal and accident t
conditions. However, because the operating environments and use of a cask are very different for transportation and storage, the performance requirements for each are not the same. These differences are reflected in the regulations and evaluations performed for transportation and storage spent fuel casks.
For example, a transportation cask must provide the safety functions offsite in an uncontrolled environment, predominantly in the public domain. The cask is only loaded for a relatively short period, but it must provide protection against dynamic loads in severe transport accidents. As a result, the transportation regulations (10 CFR Part 71) include a series of postulated accident conditions which a cask must withstand.
For storage, a cask must provide the safety functions on a licensee's controlled site. A storage cask experiences a generally static environment,
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but must provide protection against potential environments and loads to which it may be subjected during its life. A storage cask has a long period of operation.
Therefore, the storage regulations (10 CFR Part 72) specify a series of external environmental conditions a cask is required to withstand.
In addition, storage systems must be designed to adequately protect against degradation of their contents to ensure the integrity of the spent fuel when j
it is retrieved for further processing, storage, or disposal.
Under the regulations for both transportation and storage, an applicant must show that the performance requirements are met by full-scale testing, scale-model testing, engineering analysis, or a combination of these methods.
No full-scale tests have been conducted on current NRC-licensed spent fuel casks for transportation or storage.
In practice, engineering analysis, j
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supplemented with scale-model testing, has been used to demonstrate that cask designs meet NRC regulations.
NRC has certified six transportation spent fuel casks.
It is important to note that in the 35-year history of NRC regulating radioactive materials transportation, there has never been a release of radioactive materials from an NRC-certified transportation spent fuel cask either during normal operations or during an accident.
There have been seven spent fuel dry storage systems licensed or certified by NRC.
Spent fuel storage casks have never been involved in an accident, and there has never been a release of radioactive materials from an NRC-approvid spent fuel cask during normal storage operations.
Recent industry and DOE interest in proposing dual-and multi-purpose casks has led the staff to begin a careful look at the differences in requirements for storage and transportation.
In reviewing these differences, three key issues emerge:
(1) the role of water ingress in criticality analyses; (2) consideration of credit for burnup of the fuel in criticality analyses; and (3) the consideration of nodular cast iron as a cask material.
Each of these issues is discussed below.
Water Inaress NRC regulations (10 CFR Part 71) require that water ipgress be assumed in l
evaluating the criticality safety of transport casks.
This design criterion has been codified in international and United States transportation regulations for many years, and is based on the premise of water ingress (cask i
flooding). Cask flooding could result from a cask falling into a body of i
water or from water spray from firefighters responding to an accident.
Although transport casks are designed with lid seals to limit the leakage of
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radioactive material during normal or accident conditions, it cannot be assured that water ingress will be precluded under all conditions.
For example, potential movement and shifting of the cask during transport could j
loosen the seals, diminishing their effectiveness.
The regulations for storage casks do not specifically require that water ingress be assumed for assessing criticality safety. Some of the currently approved licensed storage facilities have relied on an assumption of water ingress in its criticality analysis. However, water ingress need not be i
assumed where it can be shown that the arrangement of the casks on the site provides assurance against cask flooding.
If prevention of cask flooding cannot be ensured, an applicant must consider water ingress in demonstrating 2
Under NRC's transportation regulations (10 CFR 71.55(c)) a cask could be approved without assuming water ingress, provided the applicant can demonstrate that the package incorporates special design features, such that no single packaging failure would permit leakage. However, demonstrating the adequacy of such features in a transportation environment is extremely difficult, and no applicant has ever asked for such an exception.
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i criticality safety.
In addition, storage casks are designed with seals capable of maintaining a helium atmosphere within the cask.
These seals are subject to continuous monitoring by the licensee and give additional assurance that water will not leak into the cask.
Both applications for dual-purpose casks (the one currently under review, and the one expected shortly) assume water ingress in evaluating criticality i
safety to satisfy the transportation function of the cask. Staff believes the existing regulations and guidance on this issue for transportation casks are adequate and appropriate for use in evaluating this safety aspect (i.e. water ingress) for dual-purpose casks.
Fuel Burnup Credit Under the high-level waste program, DOE is considering relying on credit for burnup of the fuel in the criticality safety design of its containers for the transport and storage of spent fuel. Allowing credit for fuel burnup would provide for more spent fuel to be transported in a single cask, resulting in fewer shipments. The present analytical practice is to assume that the fuel is unirradiated.
The staff continues to use this conservative assumption because it has identified a number of issues that need to be addressed and resolved to quantify justified credit.
l The net effect on reactivity from fuel burnup is difficult to determine accurately. This difficulty is especially acute at the ends of the fuel rods; the amount of burnup decreases rapidly toward the ends of the rods. The ability to evaluate end effects properly must be improved. Other areas needingbettgr,UandthebuildupofPuandfissionproductneutronpoisons, understanding are:
(1) the ability to calculate accurately the decrease of and (2) the detailed structures of the neutron absorption cross sections for key fission product poisons and their effect on the reactivity of the spent fuel. Also, there are no benchmark experiments which can be used to verify j
adequately the accuracy of criticality calculations for spent fuel, i
In spite of these difficulties, the staff remains open to examining proposals from applicants with sufficient technical bases that would account for burnup credit, and has contracted with Lawrence Livermore National Laboratory (LLNL) to investigate the technical basis for burnup credit in transport casks and independent storage facilities. Staff will use the results of LLNL's work as j
input in making a decision concerning burnup credit. However, based on the difficulties in accurately determining fuel burnup, and, at present, based on its own work and preliminary discussions with LLNL, the staff believes that if the technology can be developed to verify the effects of burnup, such as uranium, plutonium, and fission products-remaining, this would be preferable to relying solely on calculation. Staff has identified these issues to DOE in several meetings, and DOE has acknowledged the need to address these issues.
j Cask Materials More stringent criteria are applied to materials used for transportation casks than for storage casks because of the more potentially demanding loads and 1
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conditions to which transport casks may be subjected.
In addition, it is desirable to have greater margins in material behavior for transportation casks because these casks are used in environments where public access cannot be controlled, and where the potential consequences of a cask failure are greater than for storage.
The transportation regulations encompass the conditions expected in most transport accidents. NRC has performed safety studies of transportation casks under severe accident conditions that are not encompassed by the regulations.
These studies have shown that casks provide a high degree of safety, even in accidents that exceed the conditions in the regulations. One reason is that transportation casks are constructed of materials that behave in a ductile, plastic manner when subjected to high levels of stress and strain.
If i
overloaded, the degree of failure would likely be limited and characterized by potential cracks and localized leaking.
In contrast, non-ductile failure can be characterized as a sudden fracture and potentially total failure.
Nodular cast iron is a material that does not have the degree of ductility and toughness of those materials presently accepted.
It has been used in Europe for transportation casks for several years and consideration of its use in the United States has been raised by industry. Casks constructed from this i
material could possibly pass the requirements in 10 CFR Part 71, depending on the size of potential internal flaws and the effectiveness of cask impact limiters in controlling stresses. However, the staff believes that the experience and data available for this material does not provide the assurance of sufficient margin, given the uncertainties of potential loads, uncertainties of the existence of flaws and the temperature to which it may be subjected. Also, nodular cast iron casks would not have the large tolerance for overload that is exhibited by present casks.
The material properties of nodular cast iron are sensitive to the fabrication i
process and are difficult to reproduce. As a result, a high level of quality assurance is required to control adequately the fabrication process. Nodular 4
cast iron is not authorized in the ASME code for use in nuclear vessels, or for use in non-nuclear vessels that contain lethal substances.
The potential loads and conditions to which a storage cask may be subjected are less severe and more predictable than for transportation casks.
Therefore, the potential for the type and severity of impact loadings that would place stress / strain demands on cask materials that might result in brittle fracture are less in storage than transportation.
As a consequence nodular cast iron has been approved by NRC in applications for use in individual storage casks under Part 72.
NRC has published technical criteria for preventing brittle failure in transportation casks in regulatory guides. Staff is reviewing the appropriateness of. adopting these fracture criteria in Part 71 to exclude, specifically, by regulation, non-ductile materials for use as structural components in transportation casks. These criteria would also apply for the transportation function of dual-or multi-purpose casks.
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CONCLUSIONS Because of the different requirements for storage and transportation, two separate, independent, but coordinated reviews are conducted for a cask designed for a dual-purpose use and two separate certifications are issued.
As industry moves to develop dual-purpose casks, and DOE possibly moves toward an MPC, staff must (and will) be prepared to adequately integrate the reviews for each functional area.
The staff believes the current regulatory scheme is adequate to address dual-purpose cask initiatives.
Industry is developing at least two different designs to serve the dual purpose of transportation and storage.
l Transportation and storage staff have been organized under the same management and are currently coordinating their independent reviews in support of certification in each area, with the ultimate goal of combining the reviews when justified.
Staff will continue to review the technical issues and data associated with water ingress, fuel burnup credit, and nodular cast iron.
However, at present, staff believes a technical basis does not exist to change the casks for transportation and storage of spent fuel.
The review criteria for the disposal portion related to the multi-purpose canister must await further development of the repository design by DOE. Once the repository design is established, NRC can determine the technical review criteria needed to approve the canister for disposal.
C0 ORDINATION:
This paper has been coordinated with the Office of the General Counsel, and it has no legal objection.
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