|
| ITER Shield Blanket Design |
|
|
| ITER Shield Blanket Design |
The Boeing Company has two contracts that support the design and research and development (R&D) of the shield blanket for the International Thermonuclear Experimental Reactor (ITER). These are:
ITER is an international project to design, construct and operate an experimental fusion tokamak reactor. The European Union (EU), Japan (JA), Russian Federation (RF), and the US are the four participants in the ongoing 6-year Engineering Design Activities (EDA) phase, which is scheduled to be completed in July 1998. Construction of ITER will follow if agreement among participants on siting, design, and funding is reached. The ITER EDA phase is being conducted by a multi-party entity called the Joint Central Team (JCT), together with Home Teams from each of the four participants.
As a member of the US ITER Home Team, Boeing is responsible for the shielding blanket, materials handbook, divertor, and test blanket activities during ITER's EDA phase. All US ITER activities are funded by the Office of Fusion Energy Science (OFES) within the U.S. Department of Energy.
In a fusion reactor, the blanket system is generally defined as the components that surround the plasma, absorbing heat, radiation and neutrons from the plasma, and converting the nuclear energy of the neutrons into thermal energy. Typically, all this energy is removed from the blanket system by a coolant. In a power-producing reactor, the energy in the coolant is removed outside the reactor and converted into electricity through standard energy conversion processes. The blanket system also serves to help protect the vacuum vessel and the superconducting magnets outside the vessel.
There are two types of blankets: breeding and non-breeding. For economics and safety reasons, a reactor producing large amounts of power will be required to breed all of its own tritium, one of the two isotopes of hydrogen needed as fuel for the reactor. (The other isotope, deuterium, is readily obtained from water.) The breeding blanket contains lithium, which transmutes into tritium when hit by a neutron with the right energy level; the tritium is removed from the blanket and recycled into the plasma as fuel. For a reactor producing relatively modest amounts of tritium-integrated power, the tritium can typically be obtained from sources other than the reactor itself. In this case a simpler, non-breeding blanket can be installed, which provides only the shielding and energy-removal functions of the blanket -- this is the "shield blanket."
The planned 20-year operation of ITER will be in two phases:
As a consequence, both types of blankets will be required for ITER, with the shield blanket system needed for the first phase, and the breeding blanket system for the second phase.
Most of Boeing work during EDA has been devoted to the nearer-term shield blanket, although a small amount of design effort is currently being spent on breeding blanket design.
The limiter subsystem consists of a region on the outboard side of the torus, which is subjected to very high surface heat flux values during the start-up and shut-down portions of the plasma burn cycle. Depending on its design configuration and location, the limiter must absorb between approximately 5 and 15 MW/m2, which is equivalent to the surface heat flux range experienced by some rocket nozzles. This value is 10 to 30 times the peak value seen by the primary first wall in the remainder of the blanket system. This heat flux value occurs for 50 seconds during each start-up and shut-down, which makes this virtually a steady-state condition in terms of design requirements. In addition, the limiter plasma-facing surface is subject to much more severe erosion conditions than the blanket during some off-nominal events.
Because the limiter design and R&D involves heat flux values much higher -- and therefore more relevant to power reactors -- than for the primary first wall and shield, the Boeing team and the US ITER Home Team have focused exclusively on the limiter subsystem since September 1995. All present design and R&D efforts are directed toward development of the limiter.
There are a number of major design factors that strongly influence the blanket design:
The shield blanket cross section consists of three main regions having unique design and development requirements. These are briefly discussed below. (The shield blanket design has recently undergone substantial changes. Figures illustrating the latest baseline design will be added to this page at a later date.)
First Wall - This region, only a few centimeters thick, is directly exposed to the burning plasma and absorbs the heat energy generated by the plasma. An armor material, beryllium, is joined to a copper alloy heat sink material and protects it and the stainless steel (SS) shield structure underneath from erosion due to off-normal events and (for limiters) due to startup and shutdown conditions during the burn cycle. The first wall must be cooled to remove the heat and nuclear energy. Finally, the heat sink and armor must be joined to the shield structure.
Shield Block - The shield comprises the bulk of the shield blanket mass. This actively-cooled region, about 30 to 40 cm thick, absorbs most of the neutrons from the plasma, converting them into thermal energy which is removed by the water coolant. The shield uses the first wall coolant; the passages are sized and connected so as to minimize shield thermal stresses and to keep coolant velocity relatively high to avoid flow instability.
Back Plate and Shield Block Attachment - The current back plate design has a total material thickness of over 10 cm of 316 L(N) IG stainless steel. This thickness is needed in order to react the toroidal (hoop) stresses that result from some plasma disruption events. The back plate design and the method of attachment are currently being developed in greater detail with emphasis on mechanical attachment to ease maintenance operations. Options being considered for joining the shield blanket to the back plate are welding and mechanical attachment.
A number of the key issues related to the development of the ITER limiter are being addressed by Boeing through its design and R&D contracts. Those efforts that are generic to the blanket system, e.g. attachment to the back plate, are being addressed through the R&D of other international participants and/or other U.S. Home Team organizations.
G. D. (Dave) Morgan is the Program Manager for Boeing' design and R&D contracts for the ITER blanket. He is a member of the High Energy Systems group in the Materials and Structures Technology organization within Advanced Systems and Technology - Phantom Works. He has nearly twelve years of fusion experience from his participation in programs at Boeing, the majority of which focused on the blanket.