3. DESCRIPTION OF PROGRAM OR FUNCTION
ZEBRA, the zero energy breeder reactor assembly, was used to study the neutron physics of fast reactors (R D Smith, Nuclear Engineering Vol. 7 No. 76, Sept 1962, "ZEBRA, a Zero Power Fast Reactor"). It operated from 1962 until 1982. Mock-up type assemblies representing PFR, MONJU and the proposed European Fast Reactor, EFR, were studied, as well as simple assemblies for nuclear data validation. Some experiments were designed to validate the methods used to treat the heterogeneity of the composition and to calculate control rods.
Most ZEBRA assemblies were built using 2 inch square plates (51 mm square) stacked in stainless steel tubes, with the tubes, or elements, being located in holes in the reactor base plate. Blocks of 5x5 elements are grouped inside a mild steel superlattice and so the spacing of elements is not completely uniform.
This size of plates was chosen to be compatible with the components in the ZPR facilities in Argonne National Laboratory and is also compatible with the MASURCA and SNEAK components, permitting these to be combined to build large assemblies. Minicalandria containing sodium (or voided of sodium) and with uranium oxide and mixed uranium-plutonium oxide fuel in pin form (4x4 pins per minicalandria) are also used in the later assemblies.
The ZEBRA control rods use smaller components. In addition, special assemblies simulating power reactor control rods, rod followers and other singularities have been studied. These replace groups of elements.
The compositions of the components are detailed in PLATEDATA. The arrangement of these components to form cells and the loading of the cells in elements and special assemblies is detailed in the documents listed below, together with the arrangement of the elements in the reactor.
Control rod studies and sodium voiding studies have been carried out on several assemblies. Spectral indices, small sample reactivity worths and reaction rate distributions were measured on most assemblies. Neutron spectrum measurements have also been made on some assemblies. In the later assemblies a multichamber scanning system was used to measure fission rates at a large number of points in the core.
Various simplifications were made in developing calculational models to analyse the measurements, although information is also provided giving the full details of the core loadings. Simplifications include corrections to replace the ZEBRA control rods by standard core elements and replacing the slightly different plutonium plates by a single one. These differences result in small differences in core height for the different elements and a correction is also made for this. In the earlier cores equivalent homogeneous compositions were derived and cylindrical (and even spherical) equivalent models derived. Later the plates were represented explicitly in the cell calculations used to derive homogeneous cell-averaged cross-sections for whole reactor calculations, including representation of the cladding materials (in a three dimensional, XYZ representation).
1- The startup assembly, designed to test the measurement techniques and make comparisons with measurements made in ZPR-III. It was a small (89 elements) uranium fuelled core having an average enrichment of 12% and with a nat. U reflector. (A repeat of ZPR-III Assemblies 11 and 22). (AEEW-R315, AEEW-R372, AEEW-M525)
2- A 173 element uranium fuelled core of 14% average enrichment and with graphite diluent to give a softer spectrum than Assembly 1, similar to that calculated for a fast power reactor. (AEEW-R410, AEEW-R372, AEEW-R415, AEEW-M525)
3- A small 57 element Pu/Unat assembly with a Pu/U ratio of about 0.11 and a critical mass of 81kg of Pu-239. (AEEW-R461, AEEW-M525)
4- Similar to Assembly 2 (AEEW-R433)
4A - a radial blanket study for PFR. (AEEW-R405)
4B and 4C - a heterogeneous axial reflector study for PFR
5- A mixed critical assembly used primarily for heated loop Doppler coefficient measurements. (AEEW-R481, AEEW-R465)
6- A Pu-fuelled fast critical assembly with uranium, graphite and sodium diluents, critical mass 190kg (Pu239 + Pu241) (AEEW-R567)
7- A full scale PFR mock-up (Pu239 + U235) (AEEW-R591, AEEW-R636)
8- A series of seven test zones in the central region of the core, 8A to 8G, consisting of plate cells with k-inf, close to unity and fast/intermediate neutron-spectra. 8G/2 was built to examine pin-plate differences. Spectrum measurements were also performed. Int. Handbook of Evaluated Criticality Safety Benchmark Experiments, Vol.VI, MIX-MET-FAST-008, AEEW-R646, AEEW-R721, AEEW-R888, AEEW-R893, AEEW-R2245, ZERS 8 Notes.
9- A simple two region assembly. Pu + UO2 core with steel + graphite diluent and nat. U reflector. 217 and 221 core element versions. Buckling measurements and foil irradiations. (ZERS 9 Notes)
221 element version -3l0kg (Pu239 + Pu241) K-eff = 1.0046 +/- 0.0008
10- Pu+UO2 core with graphite diluent and nat. U reflector (ZERS 10 Notes). Buckling measurements. Small sample reactivity worths for polythene, D2O, Li-6, B-10 and core constituents. Reaction rate ratio measurements using several different techniques and intercomparisons with Cadarache (discrepancies for F8/F5 and C8/F5). TOF spectrum measurements.
241 element version ~303kg (Pu239 + Pu241) K-eff = 1.0031 +/- 0.0009
Interpretation of the buckling measurements, to derive a single buckling, is complicated by the different spatial variations of the different fission rates, F5, F8, F9, F7, and inhomogeneities in the core composition.
The Japan/UK Mozart programme. Studies in support of the design of MONJU.
Described in the MTN series of notes and summarised in Proc.Int Symp. on Physics of Fast Reactors, Tokyo (1973).
11- MZA - a single zone core with sodium diluent Reaction rate distributions, neutron spectrum measurements.
318kg (Pu239 + Pu241)
12- MZB - a representation of the clean MONJU reactor. Neutron and gamma ray scans were measured through the blanket and shield regions. Vapour explosion and fuel meltdown simulations were studied.
794kg (Pu239 + Pu241)
MZC - MONJU mock-up with control rods. The mock-up control rods contained 19 pin clusters (1+6+12 inside a cylinder). The absorber pins were of natural B4C, 80% enriched and 90% enriched B4C, and tantalum. Reactivity worth measurements were made for different types and configurations of rods and sodium filled rod followers, together with fission rate distributions within the rods and in the core. The effect of sodium voiding on rod worths was also measured.
960kg (Pu239 + Pu241)
The second PFR mock-up core
13- Equilibrium PFR mock-up with singularities (nine different core loadings, including changes to the breeder regions).
Reaction rate scans in breeder regions, gamma energy deposition, sodium voiding. Reaction rates in and near PFR simulated control rods and the effects of sodium voiding on rod worth. (AEEW-R1388, ZTN-13 series.)
14- Assembly 13 with modified singularity loadings (two different loadings) (1974-75) (ZTN-14 series, AEEW-R1035, NEACRP-U-75.)
The relative reactivity worths of 21 potential absorber materials.
The effects of hydrogen moderation on control rod performance.
Gamma ray activity induced in sodium, steels, cobalt, molybdenum and tantalum. The fission rate distribution near to an Sb-Be source.
The Bizet Programme (Belgium, Germany, The Netherlands, UK).
Described in the BTN series of notes.
15- BZA - a large conventional two zone reactor with 19 simulated control rod channels. Measurements include: reactivity worth measurements for sodium and core materials, spectral index measurements
BTN/21 to /32, /35, /51, /53, /85
Inner core enrichment (Pu/Pu+U) 16%;
outer core enrichment 21%
16- BZB - extension of BZA with 27 simulated control rod channels and part inserted B4C absorbers (4 versions)
Measurements include: sodium voiding; effects of hydrocarbons; Pu-239 fission rate scans with different arrays of absorbers; reaction rates in absorbers.
BTN/39 to /45, /52, /55, /57, /59, /60, /66, /67, /73, /77, /82, /90, /91, /93, /95, /101, /102, /109, /110, /127, /135
17- BZC - large heterogenous design with 15 UO2-Na fertile-elements in a "salt-and-pepper" arrangement. Fissile enrichment (Pu/Pu+U) 24%. 16 simulated control rod positions with followers in BZC/1 and B4C absorbers in BZC/2. Measurements include: reaction rate distributions; large scale sodium voiding reactivity; absorber worths and interactions.
BTN/65, /68, /74, /84, /86, /88, /89, /103, /120, /122, /130, /140
18- BZD/1 - extension of BZC with fertile elements collected together to form central fertile zone. 12 simulated control rod positions containing 1/2 inserted B4C absorbers. Measurements included large scale sodium voiding effects.
BTN/79, /80, /123, /124
19- BZD/1-A - derived from Assembly 18 with a reduced central fertile zone (241to 121 lattice positions) and relocating 90 fertile elements in a thin ring at approximately mid-radius of the fissile annulus.
BTN/92, /98, /131
19- BZD/2 - as Assembly 18 but with followers in simulated control rod positions. Reactions rate measurements in core and breeder regions; reactivity worths of fuel and sodium.
BTN/111, /114, /118
21- BZD/3 - as Assembly 18 but with a uniform fissile region without singularities. The measurements included some long irradiations of materials to obtain activation data.
BTN/108, /113, /118, /125, /129, /133, /137
The Cadenza cores (1980 -1982).
Intercomaprison of the heterogeneity effects of plutonium/uranium fuel in plate and pin geometry form (NEACRP A 445). (The ZTN-22 series of notes.)
22- Normal plate cell core.
The compositions of the core sections of the fissile elements are the same as those of the elements used in the heterogeneous assemblies BZC and BZD but the axial blanket regions are of natural uranium. In version 22B there was a rearrangement of the axial orientation of cells because sodium voiding experiments had shown a strong axial asymmetry. The core contained 215 fissile elements. Reactivity perturbation measurements were made for constituents of the cells, including oxygen, steel, graphite and aluminium, and the effects of changing the heterogeneity of the geometry, using thinner plates. The standard spectral indices were also measured, together with reaction rate fine structure through the plates. Although several different plutonium metal plates are used the simplification is usually made that they are equivalent.
23- Normal pin cell core.
The plate elements of Assembly 22A were progressively replaced with minicalandria pin fuelled elements occupying up to 168 lattice positions, with the remaining 49 plate elements (in the standard version) being at the core radial boundary. Measurements were also made of the reactivity effect of interchanging small groups of elements. Reactivity perturbation measurements were made in pin geometry for the constituent materials.
24- Sodium voided plate cell core
This contained 231 elements with the sodium plates of assembly 22 replaced by "steel dummies". A number of reactivity perturbation measurements were made.
25- Sodium voided pin cell core
A 243 element core