t-U
uLU
AAEC/E574
AUSTRALIAN ATOMIC ENERGY COMMISSION
RESEARCH ESTABLISHMENT
LUCAS HEIGHTS RESEARCH LABORATORIES
REDUCED ENRICHMENT FUEL AND ITS REACTIVITY EFFECTS IN
THE UNIVERSITY TRAINING REACTOR MOATA
by
D.J. WILSON
August 1983
ISBN 0 642 59777 4
AUSTRALIAN ATOMIC ENERGY COMMISSION
RESEARCH ESTABLISHMENT
LUCAS HEIGHTS RESEARCH LABORATORIES
REDUCED ENRICHMENT FUEL AND ITS REACTIVITY
EFFECTS IN THE UNIVERSITY TRAINING REACTOR MOATA
by
D.J. WILSON
ABSTRACT
Concern for nuclear proliferation is likely to preclude future supply of
highly enriched uranium fuel for research reactors such as the University
Training Reactor Moata. This study calculates the fuel densities necessary to
maintain the reactivity per plate of the present high enrichment (90 per centpoc
U) fuel for a range of lower enrichments assuming that no geometry changes
are allowed.
The maximum uranium density for commercially available aluminium-type
?3research reactor fuels is generally considered to be about 1.7 g cm . With
this density limitation, the minimum enrichment to maintain present reactivityO O C
per plate is about 35 per cent U. For low enrichment (max. 20 per cento
U) fuel, the required U density is about 2.9 g cm , which is beyond the
expected range for UA1 -Al but within that projected for the longer term
development and full qualification for U,0R-A1. Medium enrichment (nominallyO o
(Continued)
oo c45 per cent. U) UA1
X~A1 would be entirely satisfactory as an immediate
replacement fuel, requiring no modifications to the reactor and operating
procedures, and minimal reappraisal of safety issues.
Included in this study are calculations of the fuel coefficients at
various enrichments, the effect of replacing standard fuel plates or complete
elements with 45 per cent enriched fuel, and the reactivity to be gained by
replacing 12-plate with 13-plate elements.
National Library of Australia card number and ISBN 0 642 59777 4
The following descriptors have been selected from the INIS Thesaurus to
describe the subject content of this report for information retrieval
purposes. For further details please refer to IAEA-INIS-12 (INIS: Manual for
Indexing) and IAEA-INIS-13 (INIS: Thesaurus) published in Vienna by the
International Atomic Energy Agency.
DENSITY; FUEL PLATES; HIGHLY ENRICHED URANIUM; MOATA REACTOR; MODERATELY
ENRICHED URANIUM; REACTIVITY; SOLID FUELS
CONTENTS
1. INTRODUCTION
2. MOATA AND ITS FUEL
3. CALCULATIONAL METHOD
1
1
2
4. METHODS AND RESULTS 2
4.1 Most Appropriate Replacement Fuel 2
4.2 The Addition of Standard Fuel to a Vacant Plate Position in a
12-plate Element 3
4.3 The 13-plate Element with Standard 90 per cent Enriched Fuel 4
4.4 The Substitution of 45 per cent Enriched Plates for 90 per cent
Enriched Plates in 12-plate Elements 4
4.5 The Substitution of 12-plate 45 per cent Enriched Elements for
12-plate 90 per cent Enriched Elements 4
5. CONCLUSIONS 5
6. ACKNOWLEDGEMENTS
7. REFERENCES 5
Table 1 Element and tank dimension 7
Table 2 Fuel details for critical cores 8
Table 3 Reactivity changes when replacing fuel plates 9
Table 4 The variation in reactivity when substituting a 45 per cent 10235
enriched fuel element containing 22.9 g of U
for a 90 per cent enriched fuel element containing
22.0 g of 235U
Figure 1 Moata research reactor 11
Figure 2 Moata fuel element 2
Figure 3 Fuel cell I, individual plates and coolant channels 13
Figure 4 Fuel cell II, variation from element to element 13
Figure 5 Dimensions for the reactor calculation 14235
Figure 6 Mass of U per plate 15
(Continued)
Figure Fuel densities required to replace standard fuel 16
(90% enriched) with no changes in fuel geometry
Figure 8 Replacement of 90% enriched fuel in Moata at constant 17
reactivity. Variation of fuel coefficient with fuel
on enrichment
Figure 9 Fuel plate positions for replacement 18
1. INTRODUCTION
The Moata reactor has been operated at Lucas Heights for some 20 years,
initially with a maximum power of 10 kW but upgraded to 100 kW in 1972. For
flexibility with safety, the excess reactivity of the system has been kept to
a minimum, but burn-up has now reached the stage where it is necessary to add
fuel. The 12 fuel elements (Figure 1) are currently built up to a maximum of
12 plates each, except that in one core position, 11 fuel plates and 1 dummy
(aluminium) plate are used, and in another position, 10 fuel plates and two
dummies are used. Initially these dummy plates will be replaced with fuel
plates, but subsequent increases in core fuel content will be made by
substituting 13-plate for 12-plate elements.
To increase the core loading, as insurance against fuel plate damage and
to ensure future requirements, new fuel is required. From the operational
point of view, it would be most satisfactory to use additional or replacement
fuel similar in all respects to that in current use. This would ensure that
the only difference experienced would be due to small manufacturing
tolerances.
Concern with nuclear proliferation, however, means that there is now
considerable resistance to the supply of highly enriched uranium (HEU) to
research reactors, and it is most unlikely that 90 per cent enriched fuel will
be available for future Moata fuel supplies.
This paper examines the possibility of using lower enrichments and
calculates the various fuel coefficients and their spatial variations.
2. MOATA AND ITS FUEL
The University Training Reactor (UTR) Moata (Figure 1) was designed and
built by the Advanced Technology Laboratories, USA, and first went critical in
April 1961. It is a thermal heterogeneous reactor fuelled with UA1 -Al alloyX
clad in aluminium (Figure 2). The twelve fuel elements are cooled and
moderated by light water and reflected by graphite (horizontally) and water
(vertically).
The design is based on the Argonaut reactor developed at the Argonne
National Laboratory, USA, the main difference being the two slab cores of the
UTR and the annular core of the Argonaut. Table 1 gives details of the fuel
elements, plates and core tanks of Moata.
3. CALCULATIONAL METHOD
All calculations were made using AUS, the Australian modular scheme for
reactor neutronics calculations [Robinson 1975], which is a method of
dynamically linking the required module or set of modules. The modules used
are MIRANDA, EDIT, ICRP, POW3D.
The MIRANDA module [Robinson 1977] is used for cross-section preparation.
It includes a multi-region resonance calculation and a cell-averaged flux
solution for preliminary group condensation. The 128-group cross-section
library is an AUS data pool, AUS.ENDFB, based on the ENDFB/IV cross-section
library [Honeck 1964].
The EDIT module provides editing and flux group condensing facilities.
ICRP [based on the work of Doherty 1969a-c, 1970] calculates many group, few
region fluxes within a cell using first collision probability routines. POW
[Pollard 1974] and POW3D [AAEC/E report, in preparation] are general purpose
diffusion codes. POW and POW3D can be used in the two-dimensional mode, but
POW3D is much faster in operation.
The models used in the calculation are shown in Figures 3 to 5. Details
of the fuel mixtures are given in Table 2.
4. METHODS AND RESULTS
4.1 Most Appropriate Replacement Fuel
A calculation is made first with a complete core fuelled with 90 per cent
enriched uranium as an aluminium alloy clad in aluminium. The calculated
effective multiplication coefficient k ~ is used as the datum for all other
calculations.
A set of three calculations at different uranium densities is then made
at each enrichment and the results (Figure 6) are fitted to the equation
keff = k0 + Am + Bm2 (1)
pop-where m is the mass of U in the reactor core. This equation is then used
to determine the core U mass or uranium density necessary to produce a core
of the same reactivity as the standard versions (Figure 7 and Table 2).
Although, in the long-term, uranium densities in UA1 fuel are expected to
reach about 2.8 g cm , the present fully proved maximum is 1.7 g cm [IAEA
1980], which will allow enrichments down to about 35 per cent to be used.
Differentiation of Equation 1 results in the mean core fuel coefficient
Cfuel = ? = A + 2Bm (2)
235defined as the change in reactivity produced by an increase of 1 gram of U
in the reactor core; the values are listed in Table 2. The variation in mean
core fuel coefficient with density or enrichment is shown in Figure 8.
4.2 The Addition of Standard Fuel to a Vacant Plate Position in a 12-plate
Element
To ensure a proper coolant flow, 11-plate elements have the missing fuel
plate substituted by an aluminium dummy plate of the same dimensions. To
calculate the effect of replacing a dummy plate with a fuel plate, the mesh
regions of the calculation are made to coincide with the fuel plates and the
surrounding coolant. The material in each region is then defined as a fuel
and coolant mixture or an aluminium and coolant mixture. Because the model is
a quadrant of the core reflected on two boundaries, the calculated reactivity
difference is that due to the replacement of four plates, i.e. one each in
each of the four symmetrically positioned elements.
These results also give the fuel coefficient of reactivity, i.e. the
reactivity change due to the addition of a whole fuel plate.
The effect of fuel /dummy substitution is calculated for the two outer
plate positions (which would be the normal substitution positions) and for a
central plate in each of the three different element positions in the core
quadrant. The fuel plate positions are shown in Figure 9 and the results
given in Table 3.
Replacing dummy plates with 45 per cent enriched fuel plates could result
in slightly different reactivity changes - see, for example, the changes
produced when substituting whole elements (Table 4). These differences,
however, would be much smaller than those caused by variation in fuel content
due to manufacturing tolerances. For the latter, the standard deviation is ?
1.34 g which is worth about ? 11.5 x 10 in reactivity.
4.3 The 13-plate Element with Standard 90 per cent Enriched Fuel
In this method, interplate separation is reduced and the calculation
described in Section 4.1 is then carried out with the standard fuel atomic
number densities. This gives the reactivity for the twelve 13-plate elements,
and the reactivity change due to each of the three different 13-plate element
positions can be determined by weighting with the fuel coefficients reported
in Section 4.2. The mean gain in reactivity obtained by replacing one 12-
plate element with a 13-plate element of 90 per cent enriched fuel is 229.4 xio-5.
4.4 The Substitution of 45 per cent Enriched Plates for 90 per cent Enriched
Plates in 12-plate Elements
This calculation is carried out in two parts: first, a 45 per cent
enriched fuel is calculated as shown in Section 4.1, the the resulting cross
sections being stored on a library tape. A second calculation is then made
for a core of 90 per cent enriched fuel in which mesh regions coinciding with
specified fuel plates have been filled with material detailed from the 45 per
cent fuel library tape. This gives a reactivity change due to the
substitution of four equivalent plates. The results are given in Table 3, the
magnitude being of the same order as that due to variations in the fuel
contents of the plates.
4.5 The Substitution of 12-plate 45 per cent Enriched Elements for 12-plate
90 per cent Enriched Elements
This calculation is carried out in case there is some physical or
administrative reason for not mixing the enrichment within an element. The
method is similar to that decribed in Section 4.4 except that a whole element
is substituted. The results are given in Table 4; the variation is less than
that due to variation in the total weight of 235U in the elements.
5. CONCLUSIONS
To avoid the cost and inconvenience of modifying Moata, the constraint of
current established fuel technology (max. U density ^ 1.7 g cm~") necessitates235
the use of fuel of minimum enrichment % 35 per cent U.
235If fuel of the standard medium enrichment of 45 per cent U is used,
there appear to be no reactivity-related problems and the fuel can be
substituted on a plate for plate or element for element basis. Although there
are some spatial reactivity effects, these are not larger than the variations
due to the manufacturing tolerances in the fuel content of the plates and will
not be noticed in practice.
Projected uranium density limits for UA1 -Al and U700-A1 development
A O Oprograms (Figure 7) suggest that only with the latter may it be subsequently
possible to change to low enrichment (_< 20 per cent U) on a similar
equivalent reactivity basis. The required U density would be about 2.9 g
cm" .
6. ACKNOWLEDGEMENTS
It is a pleasure to record the instruction in the manipulation of the
various codes given by Dr J Pollard and Mr G Robinson - without their
assistance this report would not have been written.
7. REFERENCES
Doherty, G. [1969a] - Some methods of calculating first flight collision
probabilities in slab and cylindrical lattices. AAEC internal
report.
Doherty, G. [1969b] - Solution of the multigroup collision probability
equation. AAEC/E197.
Doherty, G. [1969c] - Solution of some problems by collision probability
methods. AAEC/E199.
Doherty, G. [1970] - Collision probability calculations in cluster geometry.
AAEC/E205.
Honeck, H.C. [1964] - ENOF Evaluated nuclear data file description and
specifications. BNL-8381.
IAEA [1980] - Research reactor core conversion from the use of highly enriched
uranium to the use of low enriched uranium fuels guidebook. IAEA-
TECDOC-233. IAEA, Vienna.
Pollard, J.P. [1974] - ADS module POW - A general purpose 0, 1 and 2D
multigroup neutron diffusion code including feedback - free
kinetics. AAEC/E269.
Robinson, G.S. [1975] - AUS - The Australian modular scheme for reactor
neutronics computations. AAEC/E369.
Robinson, G.S. [1977] - AUS module MIRANDA - A data preparation code based on
multiregion resonance theory. AAEC/E410.
TABLE 1
ELEMENT AND TANK DIMENSION
c; rsrl TJ
Core containment
Fuel elements
Total number
Fuel plates/element
Overall fuel plate
dimensions
Fuel alloy dimensions
Alloy composition
Cladding
*Interplate coolant gap
Reflectors
Two aluminium tanks:
Height 1470 mm
Length 506 mm
Width 148 mm
Wall 6.35 mm
Parallel plate
12 (6 in each tank)
Normally 12
660 x 76.2 x 2.03 mm
584.2 x 69.85 x 1.016 mm
90% enriched U as 18.6wt % alloy
(22 g US/plate)
Aluminium, 0.508 mm thick
10.16 mm (12 plate element)
Water above and below core
tanks, graphite on sides.
*For the 13-plate elements the overall size of the
element is unchanged but the plate separation is
reduced to 9.144 mm to accommodate the 13 plates.
TABLE 2
FUEL DETAILS FOR CRITICAL CORES
Enrichment
%
10
20
30
45
60
75
90
235U per Plate
g
26.47
24.20
23.48
22.90
22.50
22.24
22.00
Element
235U
0.6385
0.5837
0.5663
0.5524
0.5427
0.5364
0.5306
Densiti
238U
5.7455
2.3353
1.3217
0.6746
0.3413
0.1796
0.0596
es g cm"3
Total U
6.384
2.919
1.888
1.227
0.884
0.716
0.590
Atomic Nu
2350
0.001636
0.001496
0.001451
0.001416
0.001392
0.001375
0.001360
rriber Densit
238U
0.01454
0.005909
0.003343
0.001709
0.0009163
0.0004526
0.0001490
24
ies x 10
Al
0.03994
0.05095
0.05423
0.05633
0.05734
0.05796
0.05835
Fuel Coefficient
x ID"5 g'1 735U
6.451
7.742
8.179
8.584
8.879
9.128
9.405
OD
TABLE 3
REACTIVITY CHANGES WHEN REPLACING FUEL PLATES
Plate Position
(Figure 9)
A
B
c
D
E
F
G
H
I
Ak per Plate x 10~5
Replacing Dummy*
With Standard Fuel
386.3
298.5
175.0
382.7
289.3
161.0
321.8
244.7
143.3
Replacing Standard Fuel*
With 45% Enriched Fue?
2.78
2.06
1.16
-2.59
-2.14
-0.92
2.06
1.43
0.58
The standard fuel is enriched to 90% 235U and contains 22.0 g
of 235U per plate. The 45% enriched fuel contains 22.9 g
235U per plate.
10
TABLE 4
THE VARIATION IN REACTIVITY WHEN SUBSTITUTING A
45 PER CENT ENRICHED FUEL ELEMENT CONTAINING 22.9 g?1R
OF U FOR A 90 PER CENT ENRICHED FUEL ELEMENT
CONTAINING 22.0 g OF
Fuel Element
Position (Figure 5)
Change inReactivity for 1 Element
-3.5 x 10 5
-0.9 x 10~5
+4.4 x 10~5
(T) Graphite Core
Core Tanks
Fuel Element
FIGURE 1. MOATA RESEARCH REACTOR
12
Lifting Boss
Stainless Steel
Scroll Pin
Stainless Steel
End Spacer AJ
Stainless ~t
Steel Screw
Fuel Width
Cladding
Thickness
Spacers
Allen Screw
All Dimensions in mm
FIGURE 2. MOATA FUEL ELEMENT
13
0.508
1.016
10.16
Aluminium Cladding
Fuel Mixture
Fuel Cell Reflective Boundaries
All Dimensions in mm
FIGURE 3. FUTL CELL I. INDIVIDUAL PLATES AND
COOLANT CHANNELS
LT>
C--
na
c\j
cb
LTi
LO
Fuel Cell
Reflective Boundaries
6.35 Aluminium Tank
of a Core Tank
All Dimensions in mm
FIGURE 4. FUEL CELL II, VARIATION FROM
ELEMENT TO ELEMENT
14
Y
Reflective Boundary
CO
COi_n
in
225.85
CSl
enin
Graphite
J i.
FE3 -
FE2
148.4
? 6.35
Aluminium
Tank
324.28
X
Reflective Boundary
All Dimensions in mm
FIGURE 5. DIMENSIONS FOR THE REACTOR CALCULATION
15
1.05
1.04 -
1.03 -
0.97
22 23 24 25 26 27 28 29
235MASS OF "bU PER PLATE (g!
FIGURE 6. MASS OF 235 U PER PLATE
ca
a
o
Eu
en
i/i
Z
LUa
Long-term
-Maximum
U30,-Al
Near-term
Maximum U,QB-Al
J 0
Availability
Long-term Maximum UAlx~Al
Near-term Maximum likely UAlx-Al
Current Qualified Fuel
I
0 20 30 40 50
ENRICHMENT (%
60
235U)
70 80 90 100
FIGURE 7. FUEL DENSITIES REQUIRED TO REPLACE STANDARD FUEL
(90% ENRICHED) WITH NO CHANGES IN FUEL GEOMETRY
10
17
inm
CM
cn
t_
OJ
fa-
o^?
X
LO
LJ
8
O
L_J
LU
I
I 1 I I 1
1 23456
FUEL DENSITY ( grams of total uranium per cm3)
i i i i i i i
90 60 45
75
30 20 10
ENRICHMENT (%)
FIGURE 8. REPLACEMENT OF 90% ENRICHED FUEL IN MOATA AT
CONSTANT REACTIVITY. VARIATION OF FUEL
COEFFICIENT WITH FUEL DENSITY ON ENRICHMENT
18
FE3
FE2
Inner Graphite
Reflector
FE1
C
?
A
F
?
D
I
H
G
Outer Graphite
Reflector
12
FIGURE 9. FUEL PLATE POSITIONS FOR REPLACEMENT