“Neutronics” at Wisconsin, ORNL advances ITER shielding and international collaboration

A neutronics model of ITER is behind (left to right) Ed Marriott, Tim Bohm, Paul Wilson, Mohamed Sawan and Ahmad Ibrahim, US ITER researchers at the University of Wisconsin.
A neutronics model of ITER is behind (left to right) Ed Marriott, Tim Bohm, Paul Wilson, Mohamed Sawan and Ahmad Ibrahim, US ITER researchers at the University of Wisconsin. Photo: US ITER

Computer codes calculate nuclear heating, neutron radiation damage and activation of fusion reactor materials.

―Lynne Degitz

US ITER researchers at the University of Wisconsin and Oak Ridge National Laboratory are developing advanced processes to assess ITER’s unique tokamak components and materials in the presence of the tremendous amount of neutron flux and energy released by fusion reactions. The process, called neutronics analysis, involves a palette of complex computational codes and libraries for predicting neutron impacts.

“The neutrons penetrate the materials all around the machine,” explained US ITER chief engineer Brad Nelson, who is based at ORNL. “Neutrons produce heat that can lead to damage. They also create isotopes in the steel and other materials of the reactor. We need to know what happens to all these materials in the specific ITER geometry.”

In a fusion reaction two isotopes of hydrogen, deuterium and tritium, are smashed together at temperatures of 150 million to 200 million degrees Celsius, releasing high energy neutrons and alpha particles. ITER, an international project supported by the United States, is an experimental fusion facility under construction in France with the mission of demonstrating the feasibility of fusion energy for massive energy production. 

“Neutronics analysis tells us where the heat of the neutrons is being deposited and where we need cooling water to carry away heat efficiently. That is a very iterative process,” said Nelson. “Every time the model changes, you must do the neutronics analysis again, until we arrive at a workable solution.”

“The effort to improve the neutronics tools (both data and codes) has been on-going for some time and great progress has been achieved already,” said Mohamed Sawan, a research professor who leads the neutronics team at the University of Wisconsin. Sawan and his team worked with ITER designers to introduce changes in the configuration of the blanket, the gaps, manifolds and in-vessel coils in order to reduce the streaming of neutrons through to the vacuum vessel. The vacuum vessel design now has sufficient steel for shielding the magnets. With the added water for cooling, shielding for the magnets is maximized. The vacuum vessel design is now frozen and has gone to procurement.

Shielding at the port plugs is also advancing. This material will protect the diagnostics and other sensitive components located at the port plug. Shielding is also needed so that maintenance staff can access ports safely.

“The neutronics calculations require significant effort, due to the complex geometry and different components used in the ports,” Sawan said. “Much significant additional work is needed, once the port designs become better defined.”

In neutron shielding, the first line of defense in the tokamak is the blanket. The energetic neutrons emanating from the plasma strike the blanket at full velocity. There is actually an energy multiplication effect when the neutrons are fired into the blanket, Sawan explained.

“The 400 MW of neutrons transported in the blanket and other components around the plasma produce a significant amount of gamma radiation. When both neutrons and gammas are absorbed there, they deposit roughly 600 MW. The total thermal power then is actually about 700 MW, with the 100 MW deposited by the alpha particles. This power is removed by the cooling system, particularly in the blanket and in the divertor (at the bottom of the reactor); these have to be carefully designed so that the temperature of the structural material does not exceed design limits, and to avoid excessive stresses.”

All of ITER’s members—the United States, China, the European Union, India, Japan, the Republic of Korea and the Russian Federation—must perform neutronics for their own contributions to the project. However, the members are also working together to make their neutronics compatible wherever interfaces occur. In addition to efforts at Wisconsin and ORNL, teams at Princeton Plasma Physics Laboratory and the University of California Los Angeles are also contributing to US ITER neutronics work.

The main neutronics code is an enhanced Monte Carlo N-Particle (MCNP) Transport Code, a software package for simulating nuclear processes. The software uses a class of algorithms that relies on repeated random sampling of data sources. ORNL manages and distributes the code and data through its Radiation Safety Information Computational Center.

An international neutronics data library, FENDL-3 has been finalized and a version is available for testing and validation. FENDL-3 holds data for all of the materials used in ITER. It was compiled from libraries developed by the United States, Japan, Europe and Russia and will be officially released by the end of 2012.

The work on the codes and library has taken 5 years. “We are currently performing validation calculations using ITER-relevant benchmarks,” explained Sawan. 

Code development for the enhanced, Direct Accelerated Geometry MCNP (DAG-MCNP) is continually advancing. “We are in contact with other developers in China, Germany and Japan,” Sawan said, “and perform benchmark calculations to compare capabilities and enhance performance.” 

Wisconsin developed the DAG-MCNP code, which preserves original design details by performing the neutronics calculations directly in the CAD (computer-assisted design) geometry. This allows for rapid design iterations. The team will couple the enhanced code with activation calculations and other engineering design analysis codes to streamline the design process.

At ORNL, radiation transport group leader Robert Grove and postdoctoral research associate Ahmad Ibrahim in the Reactor and Nuclear Systems Division are developing another accelerated version of MCNP called ADVANTG. This code will speed up a calculation by four orders of magnitude, making it a factor of 10,000 times faster to get a good answer than the original Monte Carlo code.

Ibrahim, a University of Wisconsin graduate student who spent summer internships at ORNL working on the code, is now calculating the neutron dose around the ITER bio-shield during operation.

“We are making a lot of progress in advancing the state of the art of neutronics analysis, with some experimental and calculation validation” Sawan said. “We build some conservatism into our calculations. Using tools that preserve the actual geometrical details helps reduce the uncertainty significantly.”

US ITER is a DOE Office of Science project managed by Oak Ridge National Laboratory in Tennessee with contributions by partner labs Princeton Plasma Physics Laboratory and Savannah River National Laboratory. As an ITER member, the United States receives full access to all ITER-developed technology and scientific data, but bears less than 10% of the construction cost. Initial operations, or “first plasma”, of the ITER facility is planned for 2020.

Media Contact: Lynne Degitz or @US_ITER