Compositions – X-ray or neutron shield
Reexamination Certificate
1997-03-24
2002-04-16
Tucker, Philip (Department: 1712)
Compositions
X-ray or neutron shield
C250S515100
Reexamination Certificate
active
06372157
ABSTRACT:
BACKGROUND OF THE INVENTION
This present invention relates generally to radiation shielding materials, radiation shielding containers and methods for preparing the same. More particularly, the present invention relates to radiation shielding materials incorporating uranium dioxide and/or uranium carbide and containers for radioactive materials incorporating these shielding materials. This invention also relates to methods for preparing uranium dioxide and uranium carbide microspheres for use in the radiation shielding materials of the present invention.
Storage, transportation, and disposal of radioactive waste, such as spent nuclear fuel (“SNF”), high level waste (“HLW”), mixed waste, and low level radiation waste is a growing problem in the United States and abroad. In 1995, the Department of Energy (DOE) estimated that the commercial SNF inventory was about 30,000 metric tonnes initial heavy metal (“MTIHM”) and is expected to exceed 80,000 MTIHM within two decades. (1 tonnes=1 metric ton=2,205 pounds). Adding DOE's own inventory of SNF and HLW raises the domestic total to nearly 90,000 MTIHM.
Unfortunately, it appears that many U.S. commercial nuclear power plants do not have sufficient existing storage capacity to accommodate future SNF discharges. Moreover, much of the DOE's SNF and HLW inventory is currently located in unlicensed storage structures. Many of these storage structures will have to be upgraded or replaced, and the SNF and HLW relocated. Thus, there is a need for improved radiation shielding materials and radiation shielding containers incorporating these shielding materials for the storage, transportation, and disposal of radioactive materials, including, in particular, SNF waste.
Two principal types of storage methods are generally used for SNF: wet and dry. In wet storage, the SNF is typically immersed in a lined, water-filled pool which performs the dual functions of shielding and heat removal with the assistance of and reliance on active systems. Wet storage of SNF is generally required for a given period of time (about 5 years) after the SNF has been discharged from a nuclear reactor. Thereafter, the SNF can be placed into long term dry storage. Dry storage encompasses a wide spectrum of structures that house the fuel in a dry inert gas environment, with an emphasis on passive system design and operation. In dry storage, the radioactive material is typically disposed in dry vaults or dry casks. Dry vault installations generally utilize a concrete building or other concrete structure for radiation shielding. Dry cask storage, on the other hand, utilizes prefabricated containers including an appropriate shielding material. Because dry cask storage is usually accomplished more quickly and cheaply, it is generally preferred over vault storage. Dry cask storage is also preferred at sites having an existing infrastructure for receipt, examination, and loading of SNF for economic and scheduling reasons.
The design and manufacture of a suitable container for the dry storage of SNF involves a variety of factors, such as (1) subcriticality assurance, (2) shielding effectiveness, (3) structural integrity (i.e., containment), (4) thermal performance, (5) ease of use, (6) cost, and (7) environmental impact. Other factors that may affect the selection process are whether the design has been previously licensed and actually used to store SNF, or, if the design has not been licensed, its perceived ability to meet applicable regulations and standards.
The first factor in designing a storage container is the maintenance of subcriticality. In dry storage, the subcriticality design relies on controlling the fissile SNF and SNF spacing, and sometimes incorporates the use of neutron-absorbing materials. The subcriticality control design of dry storage containers is generally acceptable and does not typically provide any discriminating factors for selecting one design over another.
The second factor in designing a storage container is shielding effectiveness. Shielding effectiveness affects both onsite worker and public dose rates during the loading and subsequent storage of SNF. Both neutron and gamma ray shielding must be provided and ensured throughout the life of the storage system. Dry storage technology relies on a number of solid shielding materials, sometimes in combination, to reduce gamma and neutron dose rates. The most common solid shielding materials are different forms of concrete (low-density, high-density, or hydrogenated), metal (ductile cast iron, carbon steel, stainless steel, lead), borated resin, and polyethylene (for neutrons). Often, in order to function effectively, metal shielding materials must be combined with additional materials to enhance their neutron absorbing ability.
The third factor in designing a storage container is structural integrity (i.e., containment). Structural integrity ensures that the confinement boundary around the SNF is maintained under all operational and postulated accident conditions. All SNF storage technologies are required to meet the same standards for structural integrity in accordance with appropriate codes. Therefore, the selection of a suitable storage technology will include consideration of the structural integrity of the proposed design.
The fourth factor in container design is thermal performance. With the exception of steel and cast iron, most shielding materials have inherent limiting temperatures (i.e., a maximum allowable temperature that is lower than the fuel cladding temperature limit). Shielding material thermal limits include both absolute values of temperature and, in the case of concrete, temperature gradients that create thermal stresses. Adequate decay heat removal is vital to preventing degradation of the fuel cladding barrier to fission product releases.
Dry storage containers rely on a combination of conduction, convection (natural or forced), and radiation heat transfer mechanisms to maintain fuel cladding temperatures below appropriate long term storage limits. In particular, metal casks rely on a totally passive system for heat removal. The fuel decay heat, in an encapsulating inert gas atmosphere canister, is transferred to the canister's walls by a combination of radiation and conduction heat transfer. The canister walls, which are in contact with the metal cask wall, transfer this heat by conduction. At the outside of the metal cask, the heat is removed by conduction and natural convention to the environment. Metal cask typically are not susceptible to thermal limits, since the metals have a higher temperature limit than that of the fuel cladding. However, in those embodiments where the metal casks incorporate additional neutron shielding materials their favorable heat-transfer properties may be compromised.
As with metal casks, concrete casks use a passive heat removal system. Concrete casks, however, have an inherent vulnerability, because concrete's thermal conductivity is a factor of 10 to 40 lower than that of metal. Thus, in order to remove fuel decay heat and stay below both the fuel cladding and concrete temperature limits, concrete casks must include labyrinthine airflow passages that allow natural convection-driven air to enter the cavity enclosing the canister inside the concrete and then exit through higher elevation passages in the concrete to the enviromnent. The need for these airflow passages introduces the possibility of an accident in which adequate heat removal is reduced or eliminated because of inlets and/or outlets that are blocked by debris, snow, or even nests and hives. As a result, concrete casks require surveillance of their air inlet and outlet flow passages, thereby increasing the associated life-cycle costs and personnel radiation exposures.
The fifth factor in designing a storage container is ease of use, which is defined as the lack of complexity involved in the operation and maintenance of SNF. As noted above, the existence of labrynthine air passages in concrete casks means that additional operation and maintenance is requ
Krill, Jr. Stephen J.
Mirsky Steven M.
Murray Alexander P.
Gottlieb Paul A.
The United States of America as represented by the United States
Tucker Philip
LandOfFree
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