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Spent Fuel Storage
Published in Kenneth D. Kok, Nuclear Engineering Handbook, 2016
The criticality safety analysis must show that the effective multiplication factor (keff) does not exceed 0.95, including all biases and uncertainties, evaluated with a 95% probability at the 95% confidence level, under all credible normal, off-normal, and accident events. At least two unlikely and independent changes or accidents must occur to modify the conditions that ensure criticality safety before a nuclear criticality accident is possible.
The Medical Implications of Nuclear Power Plant Accidents
Published in W.A. Crosbie, J.H. Gittus, Medical Response to Effects of Ionising Radiation, 2003
The potential hazards associated with operations at the Capenhurst and Springfields plants are mainly of a chemical nature, due to toxic chemicals handled in various parts of the process, including hydrogen fluorides and uranium hexafluoride, which decompose in the atmosphere yielding hydrogen fluoride and uranyl fluoride. Whilst there is an associated radiological risk, it is of a very much lower magnitude and comes from the low levels of radiation naturally emitted in the spontaneous radioactive decay of uranium in uranyl fluoride. The only significant potential radiological hazard at these sites which could lead to major doses to the workforce, is from the inadvertent bringing together of a critical mass of uranium such that fission can take place (a criticality incident/accident). As discussed later in the paper, the plants are designed to minimise this possibility and operating rules further reduce the chance of such untoward occurrences. A criticality accident would be extremely unlikely to have any off-site consequences for the general public, but is the major radiological hazard to the workforce.
Evaluations of the Effect of Heterogeneity in HALEU Systems Using Modified Critical Benchmarks
Published in Nuclear Science and Engineering, 2022
Joseph A. Christensen, R. A. Borrelli
As discussed in our previous work,1 it was explained that the study of heterogeneity in high-assay low-enriched uranium (HALEU) systems is of paramount importance for a number of applications. Work in this area has direct impact on the readiness of the U.S. nuclear fuel industry to safely handle, manufacture, and transport advanced nuclear fuel with higher enrichments than those already in service. The nuclear criticality safety implications of a transition from “true” low-enriched uranium (LEU) fuels (5 wt% or less 235U) to HALEU fuels are not trivial, as demonstrated by the Japan Nuclear Fuel Conversion Company Tokai-mura criticality accident in 1999. One significant shortcoming in the methods currently available for use in the evaluation of heterogeneity for nuclear criticality safety was discussed: the lack of available critical experiment data for uranium of the appropriate enrichment. As a first step to improving the methods available for the evaluation of heterogeneity, this work demonstrates a way to use existing critical benchmark data to improve the quality of the overall data set available to nuclear criticality safety practitioners.
A New Era of Nuclear Criticality Experiments: The First 10 Years of Radiation Test Object Operations at NCERC
Published in Nuclear Science and Engineering, 2021
Jesson Hutchinson, John Bounds, Theresa Cutler, Derek Dinwiddie, Joetta Goda, Travis Grove, David Hayes, George McKenzie, Alexander McSpaden, James Miller, William Myers, Ernesto Andres Ordonez Ferrer, Rene Sanchez, Travis Smith, Katrina Stults, Nicholas Thompson, Jessie Walker
Understanding the critical mass of nuclear material was of great importance during the Manhattan Project. In order to address this, integral experiments were performed at Los Alamos. Of primary interest were critical mass data on highly enriched uranium (HEU) and weapons-grade plutonium (WG Pu). These early experiments2–4 were the first measurements ever performed on large (kilogram) quantities of HEU and plutonium metal and are similar to many of the RTO experiments discussed in this work. On August 21, 1945, shortly after the end of World War II, Harry Daghlian was conducting hands-on experiments with a 6.2-kg sphere of -phase WG Pu reflected by tungsten carbide bricks.5 During the experiment, a brick slipped from his hand onto the assembly, resulting in a criticality accident that yielded a lethal dose of radiation to Daghlian. This resulted in moving these operations from Omega Canyon in Los Alamos to TA-18, a more remote area of the laboratory. A second criticality accident occurred on May 21 of the following year using the same plutonium core reflected by beryllium.5 These two accidents led to a requirement that any systems that have a reasonable chance of reaching criticality must be controlled remotely,6 ultimately leading to construction of several critical assembly machines and the establishment of LACEF (Ref. 7).
Nuclear Criticality Safety Aspects for the Future of HALEU: Evaluating Heterogeneity in Intermediate-Enrichment Uranium Using Critical Benchmark Experiments
Published in Nuclear Science and Engineering, 2021
Joseph A. Christensen, R. A. Borrelli
The details of the criticality accident at the JCO fuel fabrication facility in Tokai-Mura, Japan, in 1999 are described in A Review of Criticality Accidents.5 At the summary level, a criticality accident took place due to a series of procedural violations by operators that allowed a critical mass of 18.8 wt% enriched fuel solution into an unfavorable geometry container. The operators’ procedural violations involved the use of unauthorized portable equipment—a bucket and flasks—to bypass the installed favorable-geometry dissolver and storage vessel and a large, nonfavorable-geometry mixing vessel to reduce the process time necessary to process a batch of solid uranium oxide into liquid uranyl nitrate solution (see Fig. 1). Under the conditions to which the operators had become accustomed, working with 5 wt% enriched uranium, these procedural deviations were “safe” in that they were not sufficient to cause a criticality accident, although they were completely outside the analyzed safety basis. The introduction of higher-enrichment fuel resulted in an accident due to these procedural deviations, killing two operators and exposing a third to dangerous levels of radiation.