ZPPR-20 phase D : a cylindrical assembly of polyethylene moderated U metal reflected by beryllium oxide and polyethylene. Page: 4 of 259
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SP-100 fuel type, was not available. ZPPR Assembly 20D was designed to simulate a water
immersion accident. The water was simulated by polyethylene (CH2), which contains a similar
amount of hydrogen and has a similar density.
A very accurate transformation to a simplified model is needed to make any of the ZPPR assemblies
a practical criticality-safety benchmark. There is simply too much geometric detail in an exact
model of a ZPPR assembly, particularly as complicated an assembly as ZPPR-20D. The
transformation must reduce the detail to a practical level without masking any of the important
features of the critical experiment. And it must do this without increasing the total uncertainty far
beyond that of the original experiment. Such a transformation will be described in a later section.
First, Assembly 20D was modeled in full detail-every plate, drawer, matrix tube, and air gap was
modeled explicitly. Then the regionwise compositions and volumes from this model were converted
to an RZ model.
ZPPR Assembly 20D has been determined to be an acceptable criticality-safety benchmark
1.2 Description of Experimental Configuration
1.2.1 The ZPPR Facility - At the heart of the ZPPR fast critical facility was a horizontal split-
table type machine consisting of a large, cast-steel bed supporting two tables, one stationary and the
other movable. An artist's rendering of the split-table machine, with components labeled, is shown
in Figure 1. Figure 2 shows a photograph of the ZPPR matrix being loaded. Each table was 14 feet
(4.3 m)a wide and 5 feet (1.5 m) long. During loading operations, the tables were separated by 5 feet
(1.5 m). In operation, the movable table was driven against the stationary table by three electric
motors with a nut and lead screw mechanism. "Mechanical operation of the matrix is that of a very
large, high precision industrial machine. Up to 120 tons of reactor materials can be loaded into the
matrix, causing a deflection of no more than 0.0040 in. The top/bottom and left/right alignment of
the matrix faces is precise within a few millimeters, and is reproducible within a fraction of a
millimeter."b Stainless steel square tubes, nominally 0.040 inches (1 mm) thick, 2.175 inches
(55 mm) on a side (outside dimension) and 5 feet (1.5 m) long, were stacked horizontally on both
tables to form a 77-row and 77-column square "honeycomb" matrix. (To be precise, an 11x11 set
of tubes is missing from each lower corner, as seen in Figure 2.) A matrix position is specified by
three parameters: matrix half (1 for stationary or 2 for movable), row number (starting from the top
with number 11) and column number (starting from the left, again with number 11, looking from the
movable half towards the stationary half). For example, the central position in the stationary half
according to this numbering convention is 149-49. The matrix tubes were supported by massive
cast-iron, L-shaped structures known as the bed and knees. A plenum region, which included
control-rod drives and experimental apparatus, existed beyond the matrix at the outer end of each
a Almost all of the references give dimensions in English units and some also give metric equivalents. We
display the metric equivalent in parentheses when practical, as a courtesy to international readers.
b H. F. McFarlane, personal communication (1986).
' Imperfect alignment of the matrix bundles produces a very small gap at the interface when the tables are driven
to the "closed" position.
Date: September 30, 2006 Page 2 of 253
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Lell, R.; Grimm, K.; McKnight, R.; Shaefer, R.; Division, Nuclear Engineering & INL. ZPPR-20 phase D : a cylindrical assembly of polyethylene moderated U metal reflected by beryllium oxide and polyethylene., report, September 30, 2006; United States. (digital.library.unt.edu/ark:/67531/metadc899618/m1/4/: accessed February 17, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.