ANS is committed to advancing, fostering, and promoting the development and application of nuclear sciences and technologies to benefit society.
Explore the many uses for nuclear science and its impact on energy, the environment, healthcare, food, and more.
Explore membership for yourself or for your organization.
Conference Spotlight
Nuclear Energy Conference & Expo (NECX)
September 8–11, 2025
Atlanta, GA|Atlanta Marriott Marquis
Standards Program
The Standards Committee is responsible for the development and maintenance of voluntary consensus standards that address the design, analysis, and operation of components, systems, and facilities related to the application of nuclear science and technology. Find out What’s New, check out the Standards Store, or Get Involved today!
Latest Magazine Issues
Aug 2025
Jan 2025
Latest Journal Issues
Nuclear Science and Engineering
September 2025
Nuclear Technology
August 2025
Fusion Science and Technology
Latest News
Ho Nieh nominated to the NRC
Nieh
President Trump recently nominated Ho Nieh for the role of commissioner in the Nuclear Regulatory Commission through the remainder of a term that will expire June 30, 2029.
Nieh has been the vice president of regulatory affairs at Southern Nuclear since 2021, though he is currently working as a loaned executive at the Institute of Nuclear Power Operations, where he has been for more than a year.
Nieh’s experience: Nieh started his career at the Knolls Atomic Power Laboratory, where he worked primarily as a nuclear plant engineer and contributed as a civilian instructor in the U.S. Navy’s Nuclear Power Program.
From there, he joined the NRC in 1997 as a project engineer. In more than 19 years of service at the organization, he served in a variety of key leadership roles, including division director of Reactor Projects, division director of Inspection and Regional Support, and director of the Office of Nuclear Reactor Regulation.
Daniel S. Williams, John C. Rommel, Raymond L. Murray
Nuclear Technology | Volume 87 | Number 4 | December 1989 | Pages 1134-1144
Late Paper | TMI-2: Decontamination and Waste Management / Nuclear Safety | doi.org/10.13182/NT89-A27705
Articles are hosted by Taylor and Francis Online.
Criticality safety and adherence to established keff criteria had to be demonstrated for the various defueling operations performed at Three Mile Island Unit 2. This included determination of adequate neutron poisoning for the reactor coolant system (RCS), design of the defueling canisters, the canister-handling devices, the storage racks, and the shipping cask. Tools, equipment, and support systems required for the defueling operation also had to adhere to criticality safety requirements. The keff criterion used to define the poison concentration for the RCS was ≤0.99. This criterion, coupled with an extremely conservative core model, led to a poison concentration that provided an appropriate margin of safety. To define the fixed poison requirements for the defueling canisters, a keff criterion of <0.95 was used for both single canisters and arrays in all credible configurations. For all design analyses, bounding assumptions were made. For each set of analyses performed, the evaluation of keff included an allowance to account for uncertainties in the calculated values. A criticality benchmark study was completed to determine appropriate computer code bias values for both the RCS and canister analyses. A lenticular model, which included the entire fuel inventory, was used to define the RCS boron concentration. Based on the analysis by Oak Ridge National Laboratory, a boron concentration of 4350 ppm was required to meet the keff criterion. Babcock and Wilcox Company performed the design analyses for the defueling canisters. Each type of canister was explicitly modeled and analyzed using the KENO code. The design requirements mandated that the diameter of the canisters be larger than critically safe dimensions; thus, fixed boron poisoning was utilized. The fixed poison used in the canisters was either Boral plates or stacked sintered boron carbide pellets. The quantity and location of the canister poison was determined based on keff limits and operational criteria for the canisters. Analyses were also performed to ensure that keff was ≤0.95 for canisters contained within the canister-handling devices.
