Fusion Power Update
By Gerard Hillenbrand, P.E.
That was the subject of the joint dinner meeting held on Thursday, April 19, 2007 at Con Edison Headquarters in Manhattan. More specifically the subject presented was “The Engineering and Construction of the National Compact Stellarator Experiment” (NCSX). The featured speaker was Larry E. Dudek, P.E. who is head of fabrication at the Princeton Plasma Physics Laboratory. This meeting was co-sponsored by the Power Engineering group of IEEE, the Edison Engineering Society, and ASME’s Metropolitan Section. Approximately sixty engineers attended this detailed and highly technical presentation and attendees qualified for 2.0 hours of professional development credit toward the mandates recently established by the N.Y. State Department of Education for Continuing Engineering Education.
The NCSX is a new experimental facility under construction at the Princeton Plasma Physics Laboratory in partnership with Oak Ridge National Laboratory in Tennessee. The objective of this program is to develop the physics principles and to determine the suitability of the Compact Stellarator Mechanism as a basis for construction of a future cost effective commercial Fusion Power Reactor. Magnetic Fusion Energy Scientist are researching the optimum shape for the hot reacting plasma and the magnetic fields that hold the plasma in place to produce energy. Recent advances in magnetic confinement physics have resulted in an advanced reactor configuration that is the Compact Stellarator.
The Princeton Plasma Physics Laboratory is directly funded under contract with the U.S. Department of Energy with an Annual Budget of $75.0 Million and more than 400 Employees. The Princeton Facility has been working on Magnetic Fusion research since 1951 and has conducted a series of experiments, which have progressively defined the parameters of a practical reactor. The fusion process consists of a reaction created by the high temperature (more than 100 million degrees Celsius) fusing of the light isotopes of hydrogen (Tritium and Deuterium) resulting in a huge energy release consisting of electrons forming an ionized gas, or plasma. Magnetic fields are used to confine the plasma at the density and temperature necessary for sustained production of power. Plasma currents as high as 1.4 million amperes have been achieved. The advantages of fusion generation of power include the availability of low cost fuel (hydrogen), no combustion products and resulting pollution, minimum radioactive hazards, no possibility of runaway reactions, and by products which are unsuitable for weapons production.
A series of experiments over the years have concentrated on the development of optimized plasma confinement configurations. For example, in the 1950’s tests were made on a Figure-eight-shaped tube confining the plasma in an externally generated magnetic field. This shape has evolved into a so-called spherical torus configuration. In elementary terms think of the classical donut shape, then twist this donut configuration into a warped configuration and the progressive tests have produced a plasma that is shaped like a sphere with a hole through its center. Mr. Dudek’s computerized slide presentation summarized his laboratory’s efforts to manufacture and construct the Compact Stellarator machine and bring this configuration to practical power generation in early 2009. The machine has 1.4-meter major torodial radius and a height of about 8 meters and will produce a plasma current of 350,000 amperes. Modular electrical coils are wound directly onto a multiple number of steel winding forms comprising the torodial shape. The coil windings are cooled to 80 Kelvin by liquid Nitrogen.
The steel winding forms are fabricated from “Stellaloy”, and advanced type of CF8M stainless steel having low magnetic permeability, good welding properties, good fracture mechanics, good fatigue characteristics, and a tensile strength of 655 MPa (95,000 psi). The winding forms are first cast using the classic sand casting method from accurate wood patterns modified with added stock for machining and shrinkage allowances. Each casting weighs about 2,700 kg. Computerized flow solidification analysis is employed to optimize the number and location of the risers and gating in the casting mold assembly which can weigh as much as 30 tons when multiple winding forms are cast simultaneously. Preformed billets of “Stellaloy” are melted in induction furnaces and 22,000 lbs. Of liquid material is poured within 1-½ minutes through multiple inlets in the mold assembly. The castings are air-cooled and then carefully inspected using dimensional analysis, radiographic and laser scanning, liquid penetration tests and a final magnetic permeability inspection before and after stress relieving operations.
The castings are then machined using several muti-axis-milling machines employing custom designed cutting tools necessary to achieve the complex winding form geometry. The final dimensions are measured and inspected on a gantry type, multi-axis coordinate measuring machine and all deviations are subject to rigorous scrutiny before acceptance for use in the final assembly. Maximum machining tolerances are ± 0.010 inch. Each final machined winding form weighs about 6,000 pounds and can be as tall as 110 inches. The forms are designed to support loads from the surrounding electromagnetic coils of 7,000 psi.
These modular coils consist of a conductor composed of 3420 strands of 0.006 inch diameter oxygen free copper wire compacted into a rectangular shape of 0.35×0.44 inches. This conductor is then insulated with two layers of dry S-2 0.004-inch thick glass cloth and then wound in multiple layers around the winding forms. The winding location is controlled to a positional tolerance of 1 mm after which the conductor is locked in position using lacing strips. Wrap insulation is then applied to the conductor assembly and outside layer of chill plates added containing cooling tubes for the liquid nitrogen coolant. After a 500-volt test, the coil assembly (still somewhat flexible at this point) is surrounded by a silicone rubber bag which is air evacuated and tested for leaks. A structural epoxy shell is applied to the bag and the entire structure is placed in an autoclave for epoxy impregnation and curing. The cured coil assembly (now rigid) is tested for inductance at 7500 volts and finally clamped into permanent position, using type 316 stainless steel clamps.
The coil assembly is then surrounded by a vacuum vessel, which resembles a twisted donut in shape. The vessel is fabricated in three sections from 0.375 inch thick Inconel 625, a material having high electrical resistivity to suppress random electrical currents and aid in confinement of the plasma. The vessel sections are press formed from individual panels and are jig assembled and checked. 300-ton capacity forming dies are employed for the press operation. The three sections will be welded together during final assembly of the Stellarator. The vacuum vessel weighs 18,000 lbs., can withstand operating temperatures varying from 20 to 350 Centigrade, and maintaining an operating vacuum of 2×10-8 Torr (4×10-10 psi). Stress analysis of the assembled vacuum vessel is completed using the computerized “ANSYS” program and final dimensions are also checked to within ± 0.010-inch accuracy via computer, and laser scanning. The outside of the vacuum vessel will contain 90 ports providing access for instrumentation monitoring the temperatures, pressures and magnetic characteristics of the plasma and its resulting current generation. Heating and cooling hoses are welded to the outside of the vessel to interconnect the various measuring instruments for the purpose of maintaining steady-state operational control.
Currently this project is at the 5-stage assembly sequence with completion and initial operation scheduled for 2009. Total cost of this development is $100 million expended over many years and these ongoing program results in the Princeton Plasma Physics Laboratory consuming the most electrical power of any user in the state of New Jersey.
The sponsoring organizations wish to thank Mr. Dudek and his excellent scientific and technically detailed explanation of Fusion Power development and for the advances made in Mechanical Engineering in pursuit of his goals.