Test of the Core Design Methods for the THTR 3oo with Experimental Results from the Critical Facility KAHTER K. Hofmann, A. Hiibner, S. Brandes Hochtcmperatur-Reaktorbau GmbH, Koln F. Krings Kernforschungsan1age Julich - Institut fiir Re ak to re ntw i ck 1 un g Abstract Results of experiments with a pebble bed critical facility in the . ‘ Kernforschungsanlage Julich which were done for testing design methods for High Temperature Reactors are compared with calculations. Calculational methods, computer programs and cross sections which were used in the design of the THTR 3oo are tested. • ' / Particularly, measured critical masses, control rod worths and neutron flux distributions with Core 1 and Core 2 are compared with theoretical results. Modifications of power reactor design methods which were necessary for use with the critical facility are discussed. INIS DESCRIPTORS THTR-300 REACTOR ZERO POKER REACTORS REACTOR CORES CROSS SECTIONS M CODES C CODES TWO-DIMENSIONAL CALCULATIONS CONTROL ROD WORTHS CRITICAL MASS NEUTRON FLUX SPATIAL DISTRIBUTION
Transcript
Test of the Core Design Methods for the THTR 3oo with Experimental Results from the Critical Facility KAHTER
K. Hofmann, A. Hiibner, S. Brandes Hochtcmperatur-Reaktorbau GmbH, KolnF. KringsKernforschungsan1age Julich - Institut fiir Re ak to re ntw i ck 1 un g
Abstract
Results of experiments with a pebble bed critical facility in the .‘ Kernforschungsanlage Julich which were done for testing designmethods for High Temperature Reactors are compared with calculations.Calculational methods, computer programs and cross sections whichwere used in the design of the THTR 3oo are tested.
• ' /
Particularly, measured critical masses, control rod worths and neutron flux distributions with Core 1 and Core 2 are compared with theoretical results. Modifications of power reactor design methods which were necessary for use with the critical facility are discussed.
INIS DESCRIPTORS
THTR-300 REACTOR
ZERO POKER REACTORS
REACTOR CORES
CROSS SECTIONS
M CODES
C CODES
TWO-DIMENSIONAL CALCULATIONS CONTROL ROD WORTHS CRITICAL MASS NEUTRON FLUX SPATIAL DISTRIBUTION
DISCLAIMER
Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.
-0-
1. Introduction
' In the middle of 1973 the critical facility for High Temperature Reactors / 1constructed at the Institut fur Reaktorentwicklung of the Kernforschungsanlage Julich, came to first criticality. It is a reactor facility similar to the THTR arrangement and designed• particularly for testing calculation methods, calculation programs, and cross section libraries developed for the design of High Temperature Power Reactors. In the initial experimental phase, spherical AVR-type fuel elements and graphite elements are being provided for the facility.
G In the first experiments the highly enriched Th/U-cycle is being investigated. In 1973, critical experiments were carried out with core 1 and core 2 differing in their moderation ratios.
G
. The experiments are suitable for verification of core physicscalculation methods for the THTR 3oo / 2_/: The geometrical arrangement and fuel cycle of the facility are similar to that of the THTR, and core 2 has the same moderation ratio as the THTR initial core. Comprehensive reactivity measurements with control rods were carried out permitting an investigation of the quality of the calculation methods for determining control rod worths.
2. Critical Facility KAHTER ,
2.1 Short Description
The critical facility KAHTER was set up in the Institut fur Reaktorentwicklung in the Kernforschungsanlage Julich.A detailed description is given in / !_/.
Fig. 1 shows a general view. The following details are of interest in connection with the experiments reported:
- Graphite (type AS l-5oo) side reflector of height 3oo cm and width 4o cm.
-3-
- Solid bottom reflector (graphite AS l-5oo) of 45 - 60 cm thickness with conical bottom with an inclination angle of lo°.
- Side and bottom reflectors form a vessel of 216 cm inner diameter and 24o - 255 cm inner height for the pebble bed composed of fuel elements and graphite spheres.
- For the moment there exists no top reflector.
- 8 control rods traveling in axial bore holes (8 cm dia.) in the side reflector at a radius of 115 cm in a uniform azimuthal distribution.
- 1 central control rod in the reactor core moving in an aluminium guide tube (lo cm O.D, 8 cm I.D.).
- Spheres withdrawal tube with means for recirculation of fuelelements. '
- Bottom steel plate.
The reflectors are equipped with radial and axial bore holes for nuclear instrumentation and the neutron source. The facility is shielded by concrete walls.
The fuel elements are spherical AVR elements of 6 cm diameter containing mixed U-Th-oxide particles of approximately 4oo yum diameter coated with layers of carbon. The uranium content per element is l.o75 g, the U235 enrichment is 93%, and the atomic ratio Th/U235 is 5.o3. In addition, there are graphite spheres of the graphite type AL 2-5oo of 6 cm diameter. To achieve differen moderation ratios (S-value = number of C-atoms/number of U235- atoms ) the fuel elements and graphite spheres can be mixed in an arbitrary ratio forming a loose pebble bed with a filling factor of about o.61.
r*\
2.2 Essential Differences of the THTR 3oo Power Reactor
In applying the THTR design methods to the critical assembly which is small compared to the THTR, it is necessary to consider the design differences of both reactors. Specific modifications of the standard calculation methods are required prior to their application to KAHTER.
Whereas in the THTR 3oo £~2_/ reflector thicknesses are at least loo cm, the minimum reflector thickness for KAHTER is only 4o cm. In contrast to the THTR 3oo there is no top reflector. Thus a substantial neutron leakage in the upward direction and an increased lateral leakage must be assumed for KAHTER.
The calculations! handling of the central control rod is complicated by a guide tube and by the geometrical arrangement of the rod itself. This varies from the THTR conditions where the control rods have simple geometrical cross sections and are directly inserted into the pebble bed. The ratio of the azimuthal distance
between control rods in the reflector to the reactor radius is greater than in the THTR 3oo.
The effects of the differences in the plants quoted above on the theoretical considerations will be discussed further below.
3. Standard Calculation Methods for the THTR Design
A common procedure used by HRB for the core physics design of pebble bed reactors has the following principal criteria:
- The calculation of the resonance absorption in Th232 and U238 , takes into account the self-shielding of the fuel. The in
. fluence of the double heterogeneity in the fuel element -- -coated particles in a matrix enclosed by a fuel-free zone -
on the self-shielding is similar to that discussed in / 3_/.
-5-
“5**
- The neutron flux reduction in the fuel matrix is calculated by a cell calculation with the transport program ANISN / 4_/.
' These calculations result in shielding factors for calculatingtrue reaction rates.
- Spectrum calculations are carried out with the zero-dimensional' spectrum program MUPO / 5_7 and the corresponding cross section
library in 4 3 neutron groups. Sucklings are employed to account for neutron leakage.
- Neutron flux distributions, power distributions, and k^^- values result from two-dimensional diffusion calculations with
f ’'i CRAM ^ 6__/ in 3 thermal and 4 fast groups. Due to the voidsbetween the spheres the diffusion constants of the reactor zones are corrected in accordance with the method of Behrens ^ 7_/. Iterations are made between spectrum and diffusion
. calculations to integrate the influence of the zone leakages. • on the shape of the spectrums. x
- - Rod calculations are carried out in two different models:1. Control rods can be described by non-diffusion regions.
The extrapolation lengths are determined as boundary conditions for the neutron fluxes by the calculation of
. first collision probabilities. The exact geometrical rodcross section, a void possibly surrounding it, and, for
Vy *
the central rod in KABTER, the rod guide tube enter intothis calculation.
2. In a R, Z-geometry non-central control rods are replaced by a "grey curtain":
^ Rods arranged on the same radius are azimutally smeared-outand a control poison describes the absorption.
-6-
—6 —
V_v
O
4. Calculations for KAHTER and Comparison Between Theory andExperiment •
4.1 Reactor Core Assemblies Investigated
Calculations were carried out for selected experiments / 8;9_/ on core 1 and core 2. In core 1 the graphite sphere fraction was 25% and the moderation ratio was S = 5ooo, in core 2 a graphite sphere fraction of 5o% resulted in S = 75oo. Critical masses, control rod worths and neutron flux distributions were investigated. . •
4.2 Critical Mass4.2.1 Results .
Critical masses were calculated for the rod-free arrangements of core 1 and core 2.
Fig. 2 shows the classification of regions of the arrangement in the calculation model. The reactor core is subdivided into an inner core, outer core, and a spheres withdrawal region.The reflectors are subdivided into four regions. The guide tube for the central rod is homogenized over the volume it occupies, and in reflector region 6 diluted graphite is assumed due to the bore holes for rods and probes.
In a diffusion approximation, handling of the void above the pebble bed is problematic. As an obvious possibility, assumption of vacuum boundary conditions on the pebble bed surface and inner wall of the reflector can be taken into consideration for the neutron fluxes. This results, however, in an overestimation of the axial leakage,since in reality backscattering occurs at the inner wall of the reflector.
The conditions in core 1 were investigated by transport calculations assuming the void to be an additional region of propagation for neutrons. The transport program DOT-2 lo_/ was applied in -approximation with PQ-scattering. The following cases were calculated both with transport and diffusion theory,' with one thermal group and 3 fast groups:
-7-
-7-
a) Core filled up to lo3 cm cylinder height and side reflector in full height,
b) Core filled up to lo3 cm cylinder height and side reflector- cut-off at this level.
' A comparison of the calculated k^^-values shows that the mult-plication constant calculated by diffusion theory based on the geometry in Fig.2 requires an upward correction Ak/k = 2,5%, a value which coincides also with AVR experience. This value refers to core 1 for a void depth of 137 cm in near-critical conditions. Corrections are estimated for slightly different (about + 3o cm)
r". pebble bed levels from geometrical considerations utilizingthe aperture angle formed between the side reflector and a
point of the core surface.
. The critical numbers of spheres and the gradients, change, per sphere", indicated in Table 1, were obtained by variation of
pebble bed height in diffusion theory (7-group) calculations.- A comparison of calculated values to the experimental results,
■ including corrections, are given below. The critical mass isoverestimated in the calculation by a maximum of 1.5% (core 1),which results in a k ,,-difference of A k = o.65 x lo ^.
ef f •
4.2.2 CommentsC
The reactor core leakages of core 1 and core 2 are 4o% and 43% .compared to 9% in the THTR 3oo. This effect emphasizes the importance of a careful handling of the upper void. Fig.3 shows the . increase of the neutron flux levels in the upper half of the reactor core and in the void when transport theory is employed.
’ For the reactor cut off at the level of the pebble bed surface(case b of the preceding chapter) a of only o,25 x lo ^higher is obtained by the transport theory model compared to that obtained by the diffusion calculation. Thus, in a comparison of the transport calculation with the experiment, the discrepancies for the critical number of spheres are reduced with regard to reactivity by o,25 % Ak.
The calculation values for the critical number of spheres in-8-
-8-/
Table 1 contain the Behrens correction which causes a considerable reactivity effect of -3.8 % A k (core 1) for the small reactor core. ' •
In the voids between the spheres, nitrogen is contained as a constituant of the air. It has been taken into account in the ' calculations and has a negative reactivity effect of about o,5 % A k. Due to the absense of outgassing experiments., adsorbed in the fuel elements and in the rest of the graphite
could not be taken into account. It can be assumed from AVR experiments and e.g. from / 11_7 that Ng adsorption is not completely negligible compared to the contained in air. Additionally, hydrogen is contained in the graphite in the form of moisture and, as a result of the manufacturing process, it is contained in the fuel elements in the form of hydrocarbons, causing a positive reactivity effect in the existing range of
- concentration. Incorporated hydrogen and nitrogen, whoseconcentrations depend on the history of the fuel elements and the graphite, partly neutralize each other in their reactivity effect. They dOjhowever,contribute to the divergence indicated in Table 1.
4.3 Control Rod Worths
4.3.1 Arrangement of Control Rods
In their withdrawn position, the lower ends of the control rods are 4o cm below the upper edge of the side reflector. The rods can be inserted 2oo cm; down to the upper edge of the conical part of the vessel. Fig.4 shows the geometrical rod cross section. While the arrangement (not the dimensions) of the reflector rods with guide bore holes corresponds to THTP. conditions, the central control rod differs from a THTR rod due to the graphite content .and the guide tube.
4.3.2 ResultsThe central control rod with its guide tube was considered in R, Z-geometry as a non-diffusion region and extrapolation lengths were assumed as boundary conditions for the neutron fluxes at the outer surface of the rod guide tube.
-9-
-9-
The worth of control rods in the reflector was first calculated in R, 0-geometry with rod and guide bore holes replaced by a non-diffusion area. Subsequently a grey curtain of the same worth was calculated in the same geometry and included in the Recalculations. • *
n-
The reactivity value is determined from the multiplicationconstants calculated by diffusion theory with (k and
(1) err without (k ££ ) rods according to
5
(i) (2)Lef f - keffkeff(1,x keffU1
Table 2 compares the results to the experimental values and gives the percent deviations from the experiment. Different types of rod measurement methods applied in /"8;9_/ resulted in slightly different control rod worths. Rod data obtained by the method of Inverse Kinetics are considered by the experimenters as the most reliable measured results and have therefore been incorporated in the table (conversion of 0-units in % with Be££ = o.6825xlo .
4.3.3 Comments
Deviations of calculated values from the experimental results are positive for the central control rod and the overall rod system. Control rods in the reflector are underestimated by calculation, especially with the bank of 4 reflector rods inserted. From the adopted model it is obvious that rods with large spaces between each other are unsatisfactorily approximated by a "grey curtain", which explains the greater part of the discrepancy in this case.
In addition, it must be taken into consideration that in the measurements in core 1 and core 2 with inverse kinetics, the falling rods drop about 2 cm lower than the lowest nominal end position after slow insertion. Calculations for the central rod take into account a slippage, whereas the reflector rod calculations are based on the nominal end position. Taking as a basis the minimum
-lo-
—lo—
slippage of 2 era and the measured S-curve for 4 reflector rods of core 1 £ 8_/, the discrepancies for 4 and 8 reflector rodsinserted are reduced by about 2%. However, in that case, the deviations for 9 control rods increase by about 1.5%.
With the data given in Table 2 as a basis, shadowing effects of the control rods can be investigated: Calculation and experiment show negative shadowing effects between the bank of 8 reflector rods and the central control rod (Table 3). The calculation indicates
_ a stronger shadowing effect than observed in the experiment,implying again an inadequacy of the curtain model: The shift of the flux towards the center of the reactor core by the "grey curtain" is more pronounced than in the case of the discrete reflector rods.
4.4 Neutron Flux and Power Distributions
4.4*1 Detectors and Theoretical Analysis "X
Neutron flux distributions have been analyzed in core 2, since for this core, flux measurements are available which are not disturbed by rods £ 9_/. For detectors, wires of Al-Dy(lo %)Al-In (1%), Mn (8o%)-Cu, a BF^-counter and a fission chamber with enriched uranium were available.
The absorption cross section of Dy 164 follows 1/v, so that in this case absorption mainly occurs in the thermal range. The cross section of In 115 shows resonances in the epithermal range, the most important at 1.5 eV. Thus the probe responds strongly to epithermal neutrons. The reaction rates of the probes were determined by activation analysis.The BF^-counter is used with and without cadmium cladding. In the later case it measures the total flux with emphasis to the thermal -range, in the former case it measures the epithermal and fast flux (E > o.4 eV). The counting rate is proportional to the local fission rate. . .
-11-
-11-
For the theoretical analysis, neutron absorption cross sections of Dy, In, and boron were calculated in 7 groups by MUPO 5_/. Two sets each were condensed for both core regions and four sets for the reflector regions (Fig.2). Neutron fluxes were taken from the diffusion calculations in section 4.2.
4.4.2 Results
Fig.5 shows the calculated relative power distribution in an isolinc graph. The power distribution is extremely peaked in the lower portion of the core. The maximum power occurs immediately adjacent to the bottom reflector in a similar way as in the THTR initial core. This is caused by non-existance of a top reflector, nonexistence of an axial gradation of fissile and fertile material and the low H/D-value (Height/diameter of reactor core) of
• approximately o.6. Figures 6-9 show calculated reaction rate • profiles compared to the experiment for the radial channels
U 1 (llo cm above lower edge of bottom reflector), M 1 (16o cm ' • _ above lower edge of bottom reflector) and axial reflector channel
. 2o (on the radius 115 cm). Each graph is normalized to themaximum reaction rate.
For the selected normalization the maximum deviation between theoretical and experimental reaction rate in the core area is O lo% or less for the radial curves with the exception of Fig.6b.In some cases (Fig. 6a, 7b, 8a, 8b). the deviations are only about 6%. This same degree of accuracy, i.e. + lo%, is also achieved for the axial distribution up to about 12o cm reactor height (Fig.9); above 12o cm, the deviation increases,since due
" to the assumption of the vacuum boundary conditions at the coreedge, the calculated flux is too low. The fluxes in the upper
_ region are better represented by transport calculations. Transportcalculations were only carried out for core 1. By adopting a magnification factor from core 1, this factor explains about half of the discrepancy observed in core 2.
-12-
-12-- /■ /
Finally, Fig. lo shows a fission chamber result in channel Ul: Theoretical and experimental power distribution diverge from each other by less than 4% (normalization to the relative maximum in the reactor core).
5. Summary
At the Kernforschungsanlage Jtilich, core physics experiments with core 1 and core 2 of the critical facility for high temperature reactors KAHTER were carried out in 1973. Core 2 corresponds to the THTR initial core in its moderation ratio S = 75oo.
O
Selected experimental results on the critical mass, on control rod worths, and reaction rate distributions were used for testing the most important procedures for the THTR core physics design. The zero-dimensional spectrum program MUPO with its cross section library and the and neutron flux calculations in two-dimensionaldiffusion approximation by CRAM are of central importance.
It proved to be important to introduce modifications specific to the KAHTER plant into the standard models. Thus the void effect (void above the pebble bed) was investigated with DOT-2 by transport theory and a correction was introduced for the critical masses calculated by diffusion theory. Another feature already contained in the standard procedure, the increase of the diffusion constants for the hollow spaces between the spheres, results in a correction of 3.8% <4 k for KAHTER, whereas in the THTR 3oo it only amounts to several tenths % 4k.
Critical masses are predicted with accuracies of 41.5 % or with regard to reactivity <! o. 65 % 4k. The calculated values for the radial neutron flux distributions deviate from the measured values in the core area by approximately lo %. In the case of the axial profiles, deviations are observed at the pebble bed surface which can be explained by the upper void, which cannot be satisfactorily represented by the diffusion theory. Control rod worths are predicted quite well, i.e., to within + 5%. An exception is the bank of 4 reflector rods, where the applied model of the "grey curtain" is not accurate because of the large distances between rods. The calculated control rod worths for that case were found to be too
-13-
low, which does, however, not result in a safety problem.
The authors are grateful to the members of the Institut fur Reaktorentwicklung of the Kemforschungsanlage Jiilich who carried out the experiments on KAHTER for many fruitful discussions In particular we thank Dr.Hecker and Dr.Kirch for their helpful assistance.
/
14
Core 1 Core 2
Number of spheres by 23 69o 27 34odiffusion theory
keffehange/sphere + 1.94 x 10"5 1.68 x 10“5
Correction: - 2.5 x 1CT2 - 2.3 x 10"2number of spheres - 1 300 - 1 370
theor. number of spheres for criticality 22 390 25 970experim. number of spheresfor criticality 22 048 25 760
Discrepancy, theory-experiment:in critical mass 1,5 % 0,8 %in Akeff 0,65 x 10-2 0,35 x 10*"2
Gin the region of the critical number of spheres; averaged over fuel spheres and graphite spheres
++ average over 4 measured values
Table 1:Calculated critical masses and comparison with the experiment
o
S f'J Core 1 Core 2
Rods in Theory Exper. T-E Theory Exper. T-E(T) (E) . E (T) (E) ■ E
Central control rod 2.48 2.44 +1.6*lo-2 3.19 3.12 +2.2* lo"*2
4 control reflector
rods in the side 1.86 2.2o -15.4-lo™2 2.26 2.54 i H H H 0 1 to
8 control reflector
rods in the side 4.19 4.32 - 3. o • lo 2 4.84 4.95 - 2.2-lo™2
= ^(central rod + reflector rods) 7,15 6,85 8,59 8,44
Yj 7,4 x 10”2 1,3 x 10”2 7 x 10“2 4,8 x 10"2
Table 3:Shaddow effects between the bank of 8 reflector control rods and the central control rod
1
2
3
4
5
6
7
8
9
17
KFA-Julich, Institut fur Reaktorentv/i cklung: Sicherheitsbericht fur das kritische Experiment mit Brennelementkugeln in der Warmen Halle des IRS der KFA-Julich GmbH, Bearbeitung: F. Krings (1971)(Safety analysis report for the critical experiment)Harder, H., H. Oehme, J. Schoning, K. Thumher:Das 300-MW-Thorium-Hochtemperaturkernkraftwerk (THTR), Atomwirtschaft-Atomtechnik XVI Nr. 5 (1971) 238-245
Journet, J.: Resonance Absorptions in Materials with Grain Structure - Equivalence Relation, Dragon Project Report 615 (1968)
Fontenay - Aux-Roses Nuclear Research Center:ANISN System: Description and Service Instructions for the ANISN Discrete Ordinate Program and Subroutines, CEA Note N 1358 (1970)
Schlosser, J.: MUPO, Program to Calculate Neutron ' Spectra and Multi-Group Constants, Dragon Project Report 172
Hassit, A.: CRAM, A Computer Program to Solve the Multi^roup Diffusion Equations, TRG Report 229 (R)
Behrens, D.J.: The Effect of Holes in a- Reacting Material on the Passage of Neutrons, Proc. Phys. Soc. (London) 62: Series A, 607 (1949) .
