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Absorber materials for control rod systems of fast breeder reactors
185
Journal of Nuclear Materials 124 (1984) 185-194
North-Holland, Amsterdam
ABSORBER MATERIALS FOR CONTROL ROD SYSTEMS OF FAST BREEDER REACTORS * Ph. DijNNER,
H.-J. HEUVEL
and M. HORLE
INTERA TOM, 5060Berg&h Gladbach, Fed. Rep. Germany Received 10 January 1984; accepted 20 January 1984
The physical, metallurgical and chemical properties of absorber materials for Fast Breeder Reactors (boron carbide, europium boride, europium oxide) and results on their behaviour under neutron irradiation (swelling, H-release) are reported. Especially the functional correlations are presented which describe the influence of reactor conditions (temperature and neutron dose) and product characteristics (porosity, density) on the material behaviour. These correlations are the base of the design calculations for absorber pins of SNR 300 and KNK-II.
1. Introduction
The reference absorber material for the control rods of the Fast Breeder Reactor SNR 300 is pellet&d boron carbide (B4C) with a pellet density of W-92% TD. The objective of the R&D-work in the field of absorber material development for SNR was to ensure and to complete the knowledge of the physical, metallurgical and chemical properties and especially to get experience on the behaviour of BqC under irradiation of fast neutrons. For the control rod design the following criteria have to be regarded: no absorber material melting, limited stresses and strains induced by He gas pressure, differential thermal strains, absorber material/clad mechanical and chemical interactions. The material properties related to these criteria were the main subject of the R&D work. The results presented in this paper are the base for design, fabrication and licensing procedure of a functionable absorber pin with a lifetime as long as possible. The reference absorber pin concept for the first core of SNR 300 is a tightly welded pressurized pin filled with B,C-pellets. Advanced absorber pin concepts with higher bumup potential foreseen for SNR 300 reloads and SNR-2 are double vented pins with B4Cpellets and Na-bonding (“wet” pins). As alternative FBR absorber materials europium boride (Eu$) and europium oxide pellets (Eu,O,) have
been investigated. Eu$ has special advantages with respect to nuclear worth whilst Eu,O, can be combined with metal hydrides in moderated control systems.
2. Physical and material properties The behaviour of absorber materials under irradiation is characterized by the basic properties, the chemical and mechanical interaction with the surrounding absorber pin components and by changes in material properties related to irradiation. The investigated B_,C material has been manufactured by a hot pressing procedure. Eu$ pellets were produced by three different processes, hot pressing, pressureless sintering and hot isotatic pressing. Eu,O, pellets were prepared by cold pressing followed by sintering. 2. I. Thermal conductivity
The considered absorber materials are characterized by a strong temperature dependence of the thermal conductivity. In general it decreases with increasing temperature. At higher temperatures (T> 1OOO’C) the temperatures influence is reversed. 2.1.1. Thermal conductivity of B,C The thermal conductivity and density of the material
* Dedicated to Prof. F. Thttmmler on the occasion of his 60th birthday.
0022-3115/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
is defined
by temperature
and the prevailing irradiation conditions, the latter being characterized by the irradiation temperature and the “B bumup. From liter-
B.V.
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Ph. Diinner et al. / Control rod systems
ature data [1,2] and own measurements correlations were evaluated:
the following
- Temperature dependence: For non-irradiated BqC with 100% theoretical density the temperature dependence is described by X, = 0.4256 - 3.7657 x 1O-4 T+ 1.3234 x lo-’ T2 x lo-”
-7.3688
- Influence ofporosity: For the correction of thermal conductivity due to porosity the following equation is recommended [l]:
h=h,(:;.IJ. 1
K)
p (-)
X = 0.537 - 1.122 x 1O-3 T+ 9.964 x lo-’ -3.759 x lo-”
T3.
Range of validity: 473 - 2273 K, with = thermal conductivity, X0 (W/cm K) = absorber material temperature. T (K)
where A, (W/cm
2.1.2. Thermal conductivity of EuB~ Results on measurement of the temperature dependence of the thermal conductivity for non-irradiated material and 90% TD Eu$ have been reported in [4]. The temperature dependence is described by:
= thermal conductivity for material with porosity P; = porosity fraction.
T3 + 5.273 x lo-l4
T*
T4
Range of validity: 473 - 1273 K, where X (W/cm. K) = thermal conductivity; = absorber material temperature. T (K) Since up to the present time only 90% TD Eu$ has been investigated, no value for the porosity dependence of this material can be given. Similarly, the effect of irradiation is still unknown. 2.1.3. Thermal conductivity of Eu,O, Measurements of the thermal conductivity of nonirradiated Eu,O, have been published in [4], [5] and [6]. From these results the following temperature dependence has been derived: X0 = 0.0404 - 3.8943 x 1O-5 T
- Influence of irradiation : According to [3] the influence of burnup and irradiation temperature on the thermal conductivity is taken into account by X,,=X;K where Aa, (W/cm
K)
The irradiation
= thermal conductivity ated B4C. factor K is given by:
of irradi -
+2.0218 x lo-’ T2 - 1.2962 x lo--” T3 Range of validity: 473 - 1673 K, where X0 (W/cm K) = thermal conductivity for non-irradiated Eu,O, with 100% TD; = absorber material temperature. T(K) According to [6] the porosity of the material is taken into account via the correction
h,=X, K=(K,-K,)exp
with ABu (10” capt/cm3) = burnup increment in the actual time interval (tz - 1,); = 1 for the first calculation step; K, = j( T) where T is the irradiation temperature K2 in K; and j(T) = min{l; 7.59 x 10e4 T -
(
1
:,.g.
The influence of irradiation is still unknown. A comparison of the thermal conductivities of the three absorber materials (90% TD, non-irradiated) is shown in fig. 1. Whereas B4C and EuB, have similar heat conductions, the thermal conductivity of Eu,O, is lower by about a factor of 10. 2.2. Linear thermal expansion
0.2536).
For the estimation of the central B4C temperature in the respective bumup ranges the following mean thermal conductivities are used: 0.12 W/cm K for Bu < 0.5 x =
X
10” capt/cm3,
0.08 W/cm K for Bu = 0.5.. .l
x
102’ capt/cm3,
i 0.07 W/cm K for Bu > 1 X 102t capt/cm3,
Own measurements of non-irradiated B4C, Eu$ and Eu,O, up to 1000°C yield the following temperature dependence for the linear thermal expansion: - B4C c=
-0.1306 x 1O-3 + 0.3711 x 1O-5
T
lo-’ T2 - 0.1632 x 10-l’ Range of validity: 293-2273 K. +0.3579x
T3.
187
Ph. Diinner et al. / Control rod systems
I
0 ,2\
1’
\ ?
e
5 2 0
W, 0A2010
400
__--600
800
1000
1200
1400
__ -- _-1600
1800
2000
2200
2400
T I'%1
Fig.
1. Thermal
conductivity
of the absorber
materials
B4C, EuB,
and
Eu20,
for non-irradiated
90% dense material; -
ascertained by experimental data, - - - - - - extrapolated up to the melting point.
-
E,;
where E (N/mm*)=
e = 0.2909 x 1O-4 + 0.6647 x 1O-5 T +0.1288
x lo-*
Range of validity: -
T*. 293-1773
K.
-
E!!*% c =
0.2424 x 1O-6 + 0.9954 x 1O-5 T +0.1250x
lo-*
T*.
Range of validity: 293-1773 K, where c (-) = linear thermal expansion; T (“C) = absorber material temperature. 2.3. Young’s modulus
of elasticity;
The
2.4. Poisson’s ratio -
Temperatureand porosity modulus have been derived non-irradiated material [7,8]; - &C: E = (0.46 x lo6 - 7.38 T-
dependence of Young’s from measurements on
3.96 x 1O-3 T*)
293-2273
B‘& Data from literature range between 0.14 and 0.25 for Poisson’s ratio [10,11,12]. Recent own measurements of the sound velocity of longitudinal waves in BqC (90% TD) yielded p= 0.22. -EEUB, Measurements at KIK [9] yielded p = 0.23. -
1-P x 1 + 2.13. P ’ Range of validity:
modulus
= absorber material temperature; T (“C) = porosity fraction. P (-) w, and Eu2& KfK measurements on 90% T materials at temperature yielded the following values [9]: ELI&: E = 207000 N/mm*. Eu,O,: E = 133000 N/mm*. These values are valid at room temperature. influence of porosity is not known until now.
K,
Eu293
From Young’s modulus and the shear modulus [12] the Poisson’s ratio was determined to f.~= 0.25. These results are room temperature values.
188
Ph. Diinner et al. / Control rod systems
2.5. Theoretical density The theoretical density of BqC and Et& is dependent on “B enrichment, for EuzO, the crystallographic structure is determinative. Own investigations by X-ray diffraction analysis yielded the following correlations for the theoretical density at room temperature: - B,C
pro = 2.553 - 1.823 x 1O-3 a. -
E, pro = 4.931-
-
1.359 X 10P3 a.
Eu2_03 pTD = 7.94
(monoclines),
pro = 7.29 (bee), where pro (g/cm3) = theoretical density; a (at%) = lo B enrichment. 2.6. Melting point The following melting points of unirradiated are known from literature [13]: - B& TM = 2450°C -
material
E&S, T, = 258O”C,
-
&2!?3
TM = 2050°C.
Neither influences of irradiation nor of stoichiometry, density or l”B enrichment are known. 2.7. Chemical behaviour of the absorber material/ system
cladding
The chemical behaviour of absorber materials with stainless steel cladding has been studied in a series of in-pile and out-of-pile investigations. The out-of-pile studies have been carried out in a broad parameter field including simulation of in-pile chemistry of absorber pins (doping of Li, C, B). Supplementary investigations have been performed in presence of sodium, to provide reliable data for the design of vented absorber pins with Na-bonding. Moreover, the possibility to reduce or eliminate absorber material cladding chemical interaction (ACCI) has been studied by incorporating a chromium or a niobium layer on the inner surface of the cladding.
The experimental work involved isothermal anneals of stainless steel capsules containing absorber material and a helium- or sodium bonding. The capsules consisted of X10 CrNiMoTiB 1515 (W.-Nr. 1.4970) or X8 CrNiMoNb 1616 (W.-Nr. 1.4981) type tubes. A chromium or niobium layer was deposited on the inner tube surface electrolytically or by chemical vapour deposition. BqC and Eu$ pellets were produced in a hot pressing sintering technique, their bulk density ranged between 88 and 93% TD. Eu,O, was fabricated by cold pressing and sintering to pellets with a bulk density of 90% TD with a monoclinic structure. Reactor grade sodium was used with a B-content below 0.2 ppm and a C content of about 15 ppm. To simulate reactor conditions; lithium which is generated by (n, a) reaction from t”B has been added to some of the BqC containing capsules in a quantity of 1 wt%. The capsules were sealed under helium and heat treated at 600, 700 and 800°C for times between 750 and 8640 hours. After heat treatment absorber- and cladding material went in for a series of examinations, the results are described subsequently: B&/stainless steel Chemical interaction between BqC and cladding material has been studied with BqC powder in an inert atmosphere. ACCI was negligible at 6OO’C after 8600 h test duration. By adding sodium to the B,C/cladding system chemical interaction is considerably enhanced. During heat treatment the BqC pellets remain intact, they suffer a small diameter- and weight loss, the rate of mass transfer from BqC to the sodium and/or cladding follows a parabolic time law. Metallographic cuts of the uncoated cladding material reveal a pronounced chemical attack on the inner capsule surface. The zone of interaction is characterized by two or three uniform reaction layers with a considerably higher microhardness than the matrix, separated from the unattached cladding material by a dark transition zone (fig. 2). The overall penetration depth reaches from 50 pm at 6OO”C/8600 h to 160 pm at 8OO”C/750 h. In the investigated temperature range the penetration rate corresponds to a tl/’ rule. X-ray diffraction and microprobe analysis showed evidence of preferred boron diffusion in the stainless steel with Ni,B, FeB and Fe,B as main products of interaction. Chemical analysis of the penetrated capsule material revealed a considerable increase in boron content but insignificant change in carbon content. Capsules containing lithium showed reduction of B penetration at 600 and 8OO’C. By incor-
Ph. Diinner et al. / Control rod systems
189
Fig. 2. Metallographic section of stainless steel cladding after heat treatment with B,C pellets and sodium at 600°C for 8640 hours.
porating a niobium or chromium layer in the capsules chemical attack was eliminated at 600°C (Cr, Nb-coating) and reduced at 75O’C (Cr-coating). Based on the experimental results in the pure B,C/Na/stainless steel system, a correlation between growth of interaction wne and temperature was developed. By analogy with a thermal activated process, the available data have been fitted by the following equation k=Aexp(-Q/RT),
where k is the growth of the interaction zone, A is a constant, Q is an activation energy, R is the gas constant and T the Kelvin temperature. The regression analysis of experimental data yields the following temperature dependence of penetration rate K = 0.522 exp( -97,2OO/RT),
with K = growth of interaction R = 0.00831 kJ/mol K T = cladding temperature
zone in cm/s’/*,
in K. This correlation coincides well with results reported in
[14], especially in the important temperature range between 600 and 7OO’C (fig. 3). The activation energy of 97.2 kJ/mol is comparable with that one for boron diffusion in y-Fe, if FeB and Fe,B [15] or a solid solution with boron is formed [16]. The kinetics of ACCI thus seems to be controlled by the diffusion of boron in austenite or through the reaction layer. The out-of-pile chemical behaviour of the B,C/Na/ stainless steel system is confirmed by the results from the irradiation experiments RAMBEX [17] and DFR 510 (181; the in-pile penetration depth does not exceed the level defined by the out-of-pile correlation. From the test results it is concluded, that in a Na-bonded vented absorber pin with B& pellets as absorbing material ACCI amounts to 48 microns within one year residence time at 6OOT, the maximum cladding temperature in SNR absorber pin design. Relative to ACCI a residence time of at least 420 efpd is therefore justified (corresponding to 2 cycles of SNR 300-reloads). Furthermore it could be demonstrated that incorporation of a niobium or chromium inner layer will be an appropriate measure to reduce ACCI.
190
Ph. Diinner et al. / Control rod systems
500 .+I-
600
-
700
900
lO-+
i
o . 0 A p,
ul i
l.C970/Na/B,C 1.4970/Na-Li/B,C l.L970#WNa/B,C l.49wNdB4c 1.49H/Cr/NaIB~C
10-S-
13
12
t
11
10
9 y
2
I K-l1
Fig. 3. Growth of an interaction zone on stainless steel at contact with B4C pellets and sodium.
EuB,/stainless steel The chemical behaviour between Eu$ and stainless steel has been investigated in a sandwich arrangement at 600 and 700°C for test durations of 4320 and 8640 h. By metallographic examinations no interaction between Eu$ and stainless steel has been observed. Microprobe analysis indicated Fe-diffusion into the Eu$ matrix and a small increase in Ni concentration. The presence of sodium again enhances chemical interaction. From the few out-of-pile results available, it was found that type and scale of interaction are very similar to the BqC containing system, main product of interaction is Fe,B. The temperature dependence of penetration depth is well described by the correlation developed for the B,C/Na/stainless steel system, the level of penetration depth is even lower. These preliminary results indicate a sufficient low rate of chemical interaction between Eu$ and stainless steel in sodium environment for a residence time up to - 730 efpd, the target value for SNR-2 control rods.
indicated
insignificant
chemical
interaction
below
900°C. Beyond 900°C europium and oxygen penetrate the cladding material particularly along grain boundaries to react with cladding components. Main products of interaction consist of Eu, M (metal)-double oxides. With respect to chemical interaction, Eu,O, would be an reasonable absorber material, since the threshold temperature for ACCI in the Eu,O,/cladding systems lies far beyond the actual temperatures for control rods.
3. Llehaviour under irradiation Gas production and -release, and swelling from the absorber material are the most important irradiation effects for control rod performance. Moreover, there is a known influence of irradiation on thermal conductivity of B4C, which is described in chapter 2.1.1. 3. I. Swelling of absorber materials
~&/stainless steel Investigations of Eu,O, pellets with cladding material heat treated in the temperature range 600 to 1OOO“C
B‘&
A detailed investigation
of BqC swelling has been
191
Ph. Diinner et al. / Control rod systems
conducted in the course of the last years. To put the functional description on a basis as wide as possible, all clearly specified swelling data in the literature on SNR specific BqC as well as own irradiation results from the RAMBEX [17] and the DFR 510 experiment [18] were brought into the analysis. The maximum “B bumup amounted to 8 X lo’* capt/cm3 and the irradiation temperatures lie between 450 and 98O’C. The swelling of B.+C is based essentially on two processes, which proceed together, micro-swelling and macro-swelling. Hereby, the micro-swelling supplies the main amount of the swelling by forming defect structures in the BqC lattice. Macro-swelling provides the second contribution by formation of cracks. These become effective particularly at higher burnup and high linear power above 100 W/cm according to results reported in [19]. The B,C swelling curve derived from literature data and own experiments [20,21] is shown in fig. 4. This curve is the base of the design calculations for SNR 300 and KNK-II absorber pins. As can be seen, BqC swelling increases above 1.5 X 102’ capt/cm3 linearly with increasing “B bumup. The corresponding correlation is given by: A D/D = 0.551 (Bu + 0.614[exp( - Bu/0.614) - 11)
where = diametrical swelling; AD/D (W) Bu (102’ capt/cm3) = bumup. EuB, At present the swelling behaviour of Eu$ has only been investigated on a few samples in the RAMBEX irradiation experiment. The irradiation results allow the conclusion that the swelling of Eu$ is nearly comparable to that of BqC with a tendency to a lower swelling rate. El!293
Swelling of Eu,O, exhibits a wide range of experimental results. Whereas American investigations [22] show scarcely measurable expansions ,which were related to the formation of cracks, Eu,O, swelling was demonstrated in the RAMBEX experiment. However, under the RAMBEX conditions, Eu,O, pellets swelled at least 40% less than B4C. The results of the DFR 510 irradiation experiment have shown that Eu,O, has a considerably greater resistance to swelling than 90% TD BqC pellets. In the low temperature range (< 525’C swelling of Eu,O, can almost be ignored. The data are
7.00 [WA 6.00
RAMBEX OFR OFR
0
0
0.00
1.00
2.00
-
510, &C-temperature 510, B&temperature
3.00
l 2a
result&r
4.00
/
525 *C 755 OC
5.00
6.00
7.00
/-
/
8.00
9.00
10 B burn up I l(r" CAPTURES/d Fig. 4. Swelling of BqC as function of “B bumup.
10.00
I
Ph. Dinner et al. / Control rod systems
192
not sufficient to derive an appropriate swelling correlation, therefore it is recommended to describe the swelling of EuzO, in a conservative manner by the B4C correlation.
with falo (-)= portion of captures in ‘*B related to the quantity of total captures. 3.2.2. Helium release
3.2. Production and release of helium 3.2.1. Pr~ucti~n of helium Helium is produced as a result of the capture of neutrons via the (n, a)-process of ‘*B. In the case of B4C, the number of neutrons absorbed is the same as the number of He atoms produced if the small percentage of the competing (n, 2T) reaction is ignored. Thus the quantity of produced He per cm3 BqC is directly correlated with the ‘OB burnup M,, = 37.191. Bu where M,, (cm3 He/c&
B&) = quantity of produced He, = bumup.
Bu (10” capt/cm3) !%!!!6
In Eu$, neutrona are captured by l”B- and Euatoms. Therefore, the produced helium is reduced by the portion of captures in europium: M,, = 37.191 * Bu -fa,,,,
B,C To get a reliable basis for the prediction of He release from B& all available results in the literature and results from our own irradiation experiment RAMBEX [15] and DFR 510 [16] were taken into account in the performed data analysis. Hereby most emphasis was placed on the investigations of SNR type material. However, He release values from highly compacted BqC granulate were also taken into account. Since an exact characterization of the samples was not possible in all cases, parameters were not set for density, grain size, stoichiometry and methods of production. In terms of the burnup dependence of the He-release, the analysis yielded the following picture. At low burnup (< 1.5 X 102’ capt/cm3) the release of He is relatively high (about 60%). With increasing burnup, however, the percentage of He released decreases, so that at high bumups (> 4 x 102’ capt/cm3) only less than 30% of the integrally produced He contributes to the buildup of pressure in a closed absorber rod. The release of gas is temperature dependent. The percentage of released He increased with increasing irradiation temperature.
% KNKII/l
-
*O
$
70
-
60
r 0
ZF RAHBEX
0
-z 500 oc
A
500-600
:
600-700 * 700 *c OC >
Y
lit.
ronsrrvative envelope
dab
I
:
ink@ mvllopc envrlopc < 600 lC
sp J
;I
2
20
y
10 0 0
t;0
2;o
3,O
LO
5.0
6,O
10 3 burn up Fig. 5. Helium release from B4C as function of “B burnup.
7.0 I
6,O
9,O
CAPT / cm3xlQn 1
Ph. Diinner et al. / Control rod systems
CWVI I : illteQd cucvc II :
193
burn up I CAPT / cm3x10'2'1 MVdOpL
cnvrlopcfor
1 < 600 OC
Fig. 6. Integral helium quantity released from BqC as function of “B bumup. For the calculation of the release from B4C the following concept was employed (fig. 5): Curve I represents the conservative envelope of all measured values in relation to burnup; curve II demonstrates the en-
velope of all measured values at irradiation temperatures below 600°C. Experimental results on the release from RAMBEX and DFR 510, recent results from KNK II/l and intermediate inspections on absorber pins of irradiation experiment PFR 13/12 were all found within these envelopes and confirm their conservativity. From the curves in fig. 5 the released He quantity per cm3 B4C has been calculated. The correlation is shown in fig. 6; it can be seen that, following an initial rapid increase in the helium quantity, above 2 x lo*’ capt/cm3 saturation is reached to a certain degree. Further He release within the experimentally covered range does not exceed additional 50% of the already released quantity. EuB, The experimental findings from the RAMBEX experiment are at present the only irradiation results on Eu$. A He release of almost 100% was found. In accordance with the correlation for He production in chapter 3.2.1 the He release is characterized by V ,zREE= 37.191. Bu .feio, VFREE(cm3 He/cm3 Et&)
= released quantity of He. 4. Conclusions The reported results on absorber materials, referring to physical, chemical and material properties as well as
to the behaviour under neutron irradiation, led to a complete qualification of B4C pellets as reference absorber material for the absorber systems of the first core of SNR 300 and of KNK-II-reloads with a maximum i”B bumup of - 7 x lo*’ capt/cm3 and ensure the design of these absorber pins under the licensing requirements. Furtheron, a sufficient extrapolation base for the design of advanced absorbers for SNR 3@0-reloads (“wet”, vented pins) with an extended target-‘OBbumup of - 15 X lO*l capt/cm3 is given. The design of a “high bumup”-absorber system with a target ‘OBbumup of - 20 X lo*’ capt/& as it is foreseen for SNR-2 under economical aspects requires a completion of the experimental background in the future. Acknowledgements
The R&D-work, sponsored by the Bundesministerium fur Forschung und Technologie (BMFT), was performed in cooperation with the Kernforschungszentrum Karlsruhe GmbH (KfK).
References [1] D. Mahagin, J.L. Babes and D.E. Babes, report HEDLTME 73-78 (1973). [2) A. Strasser and W. Yario, report NP-1974, EPRI contract TPS-79-708. [3] F. Gestermann, Reaktortagung Mannheim (1977) DAtF, Proc. symp., p. 518. [4] M. Harle and B. Schulz, Reaktortagung Dusseldorf (1976). DAtF, Proc. symp. p. 782. [5] A.E. Pasto, report ORNL-TM-4226 (1973). (61 A.E. Pasto and M.M. Martin, Eu,O,: report ORNL-5291 (1977).
194
Ph. Dibmer et al. / Control rod systems
[7] W.E. Roake and T.T. Claudson, report HEDL TME 77-33 (1977). [8] A. Lipp, Techn. Runds. 14, 28, 33 (1965); 7 (1966). [9] B. Schulz, P. Misaelidis, Kemforschungszentrum Karlsruhe, private communication (1978). [lo] F. Hashin and W.B. Rosen, J. App. Mech. 31 (1964) 223. [ll] S. Ihara, K. Tanaka, M. Kojima and Y. Akimoto, Meeting of the Intern. Working Group for Fast Reactors, Dimitrovgrad, USSR (1973), p. 201. [12] B.T. Kelly, UKAEA Springfields, private communication (1976). (13) K. Schwetz and A. Lipp, Atomwirtschaft 18 (1973) 531. [14] R.E. Dahl: Intern. Working Group for Fast Reactors, Dimitrovgrad, USSR (1973), p. 123. [15] G.V. Samsonov and I. Vinitskii, Handbook of Refractory Compounds, (IFI/Plenum, New York, 1980).
(161 P. Busby, N. Warga and C. Wells, J. Metals, Sect. I, 5 (1953) 1453. (171 M. Hiirle, H. Enderlein, R. Pejsa and Ph. Dtnner, Reaktortagung Dusseldorf (1976), Proc. symp., p. 778. 1181 M. Hbrle, to be published. [19] M. Horle and C. Sari, Reaktortagung, Berlin (1980) Proc. symp., p. 602. [20] Ph. Dhnner, R. Habel, M. Horle and H. Zimmermann, IWGFR-specialists’ Meeting on Fast Reactor Absorber Materials and Control Rods, Obninsk, USSR, (1983). (211 Ph. Dtnner, R. I-Babel,H.J. Heuvel and M. HBrle, report BMFT 4-015 (1983), ch. I. [22] M.M. Martin, report ORNL/TM-5400 (1976).
Journal of Nuclear Materials 124 (1984) 185-194
North-Holland, Amsterdam
ABSORBER MATERIALS FOR CONTROL ROD SYSTEMS OF FAST BREEDER REACTORS * Ph. DijNNER,
H.-J. HEUVEL
and M. HORLE
INTERA TOM, 5060Berg&h Gladbach, Fed. Rep. Germany Received 10 January 1984; accepted 20 January 1984
The physical, metallurgical and chemical properties of absorber materials for Fast Breeder Reactors (boron carbide, europium boride, europium oxide) and results on their behaviour under neutron irradiation (swelling, H-release) are reported. Especially the functional correlations are presented which describe the influence of reactor conditions (temperature and neutron dose) and product characteristics (porosity, density) on the material behaviour. These correlations are the base of the design calculations for absorber pins of SNR 300 and KNK-II.
1. Introduction
The reference absorber material for the control rods of the Fast Breeder Reactor SNR 300 is pellet&d boron carbide (B4C) with a pellet density of W-92% TD. The objective of the R&D-work in the field of absorber material development for SNR was to ensure and to complete the knowledge of the physical, metallurgical and chemical properties and especially to get experience on the behaviour of BqC under irradiation of fast neutrons. For the control rod design the following criteria have to be regarded: no absorber material melting, limited stresses and strains induced by He gas pressure, differential thermal strains, absorber material/clad mechanical and chemical interactions. The material properties related to these criteria were the main subject of the R&D work. The results presented in this paper are the base for design, fabrication and licensing procedure of a functionable absorber pin with a lifetime as long as possible. The reference absorber pin concept for the first core of SNR 300 is a tightly welded pressurized pin filled with B,C-pellets. Advanced absorber pin concepts with higher bumup potential foreseen for SNR 300 reloads and SNR-2 are double vented pins with B4Cpellets and Na-bonding (“wet” pins). As alternative FBR absorber materials europium boride (Eu$) and europium oxide pellets (Eu,O,) have
been investigated. Eu$ has special advantages with respect to nuclear worth whilst Eu,O, can be combined with metal hydrides in moderated control systems.
2. Physical and material properties The behaviour of absorber materials under irradiation is characterized by the basic properties, the chemical and mechanical interaction with the surrounding absorber pin components and by changes in material properties related to irradiation. The investigated B_,C material has been manufactured by a hot pressing procedure. Eu$ pellets were produced by three different processes, hot pressing, pressureless sintering and hot isotatic pressing. Eu,O, pellets were prepared by cold pressing followed by sintering. 2. I. Thermal conductivity
The considered absorber materials are characterized by a strong temperature dependence of the thermal conductivity. In general it decreases with increasing temperature. At higher temperatures (T> 1OOO’C) the temperatures influence is reversed. 2.1.1. Thermal conductivity of B,C The thermal conductivity and density of the material
* Dedicated to Prof. F. Thttmmler on the occasion of his 60th birthday.
0022-3115/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
is defined
by temperature
and the prevailing irradiation conditions, the latter being characterized by the irradiation temperature and the “B bumup. From liter-
B.V.
186
Ph. Diinner et al. / Control rod systems
ature data [1,2] and own measurements correlations were evaluated:
the following
- Temperature dependence: For non-irradiated BqC with 100% theoretical density the temperature dependence is described by X, = 0.4256 - 3.7657 x 1O-4 T+ 1.3234 x lo-’ T2 x lo-”
-7.3688
- Influence ofporosity: For the correction of thermal conductivity due to porosity the following equation is recommended [l]:
h=h,(:;.IJ. 1
K)
p (-)
X = 0.537 - 1.122 x 1O-3 T+ 9.964 x lo-’ -3.759 x lo-”
T3.
Range of validity: 473 - 2273 K, with = thermal conductivity, X0 (W/cm K) = absorber material temperature. T (K)
where A, (W/cm
2.1.2. Thermal conductivity of EuB~ Results on measurement of the temperature dependence of the thermal conductivity for non-irradiated material and 90% TD Eu$ have been reported in [4]. The temperature dependence is described by:
= thermal conductivity for material with porosity P; = porosity fraction.
T3 + 5.273 x lo-l4
T*
T4
Range of validity: 473 - 1273 K, where X (W/cm. K) = thermal conductivity; = absorber material temperature. T (K) Since up to the present time only 90% TD Eu$ has been investigated, no value for the porosity dependence of this material can be given. Similarly, the effect of irradiation is still unknown. 2.1.3. Thermal conductivity of Eu,O, Measurements of the thermal conductivity of nonirradiated Eu,O, have been published in [4], [5] and [6]. From these results the following temperature dependence has been derived: X0 = 0.0404 - 3.8943 x 1O-5 T
- Influence of irradiation : According to [3] the influence of burnup and irradiation temperature on the thermal conductivity is taken into account by X,,=X;K where Aa, (W/cm
K)
The irradiation
= thermal conductivity ated B4C. factor K is given by:
of irradi -
+2.0218 x lo-’ T2 - 1.2962 x lo--” T3 Range of validity: 473 - 1673 K, where X0 (W/cm K) = thermal conductivity for non-irradiated Eu,O, with 100% TD; = absorber material temperature. T(K) According to [6] the porosity of the material is taken into account via the correction
h,=X, K=(K,-K,)exp
with ABu (10” capt/cm3) = burnup increment in the actual time interval (tz - 1,); = 1 for the first calculation step; K, = j( T) where T is the irradiation temperature K2 in K; and j(T) = min{l; 7.59 x 10e4 T -
(
1
:,.g.
The influence of irradiation is still unknown. A comparison of the thermal conductivities of the three absorber materials (90% TD, non-irradiated) is shown in fig. 1. Whereas B4C and EuB, have similar heat conductions, the thermal conductivity of Eu,O, is lower by about a factor of 10. 2.2. Linear thermal expansion
0.2536).
For the estimation of the central B4C temperature in the respective bumup ranges the following mean thermal conductivities are used: 0.12 W/cm K for Bu < 0.5 x =
X
10” capt/cm3,
0.08 W/cm K for Bu = 0.5.. .l
x
102’ capt/cm3,
i 0.07 W/cm K for Bu > 1 X 102t capt/cm3,
Own measurements of non-irradiated B4C, Eu$ and Eu,O, up to 1000°C yield the following temperature dependence for the linear thermal expansion: - B4C c=
-0.1306 x 1O-3 + 0.3711 x 1O-5
T
lo-’ T2 - 0.1632 x 10-l’ Range of validity: 293-2273 K. +0.3579x
T3.
187
Ph. Diinner et al. / Control rod systems
I
0 ,2\
1’
\ ?
e
5 2 0
W, 0A2010
400
__--600
800
1000
1200
1400
__ -- _-1600
1800
2000
2200
2400
T I'%1
Fig.
1. Thermal
conductivity
of the absorber
materials
B4C, EuB,
and
Eu20,
for non-irradiated
90% dense material; -
ascertained by experimental data, - - - - - - extrapolated up to the melting point.
-
E,;
where E (N/mm*)=
e = 0.2909 x 1O-4 + 0.6647 x 1O-5 T +0.1288
x lo-*
Range of validity: -
T*. 293-1773
K.
-
E!!*% c =
0.2424 x 1O-6 + 0.9954 x 1O-5 T +0.1250x
lo-*
T*.
Range of validity: 293-1773 K, where c (-) = linear thermal expansion; T (“C) = absorber material temperature. 2.3. Young’s modulus
of elasticity;
The
2.4. Poisson’s ratio -
Temperatureand porosity modulus have been derived non-irradiated material [7,8]; - &C: E = (0.46 x lo6 - 7.38 T-
dependence of Young’s from measurements on
3.96 x 1O-3 T*)
293-2273
B‘& Data from literature range between 0.14 and 0.25 for Poisson’s ratio [10,11,12]. Recent own measurements of the sound velocity of longitudinal waves in BqC (90% TD) yielded p= 0.22. -EEUB, Measurements at KIK [9] yielded p = 0.23. -
1-P x 1 + 2.13. P ’ Range of validity:
modulus
= absorber material temperature; T (“C) = porosity fraction. P (-) w, and Eu2& KfK measurements on 90% T materials at temperature yielded the following values [9]: ELI&: E = 207000 N/mm*. Eu,O,: E = 133000 N/mm*. These values are valid at room temperature. influence of porosity is not known until now.
K,
Eu293
From Young’s modulus and the shear modulus [12] the Poisson’s ratio was determined to f.~= 0.25. These results are room temperature values.
188
Ph. Diinner et al. / Control rod systems
2.5. Theoretical density The theoretical density of BqC and Et& is dependent on “B enrichment, for EuzO, the crystallographic structure is determinative. Own investigations by X-ray diffraction analysis yielded the following correlations for the theoretical density at room temperature: - B,C
pro = 2.553 - 1.823 x 1O-3 a. -
E, pro = 4.931-
-
1.359 X 10P3 a.
Eu2_03 pTD = 7.94
(monoclines),
pro = 7.29 (bee), where pro (g/cm3) = theoretical density; a (at%) = lo B enrichment. 2.6. Melting point The following melting points of unirradiated are known from literature [13]: - B& TM = 2450°C -
material
E&S, T, = 258O”C,
-
&2!?3
TM = 2050°C.
Neither influences of irradiation nor of stoichiometry, density or l”B enrichment are known. 2.7. Chemical behaviour of the absorber material/ system
cladding
The chemical behaviour of absorber materials with stainless steel cladding has been studied in a series of in-pile and out-of-pile investigations. The out-of-pile studies have been carried out in a broad parameter field including simulation of in-pile chemistry of absorber pins (doping of Li, C, B). Supplementary investigations have been performed in presence of sodium, to provide reliable data for the design of vented absorber pins with Na-bonding. Moreover, the possibility to reduce or eliminate absorber material cladding chemical interaction (ACCI) has been studied by incorporating a chromium or a niobium layer on the inner surface of the cladding.
The experimental work involved isothermal anneals of stainless steel capsules containing absorber material and a helium- or sodium bonding. The capsules consisted of X10 CrNiMoTiB 1515 (W.-Nr. 1.4970) or X8 CrNiMoNb 1616 (W.-Nr. 1.4981) type tubes. A chromium or niobium layer was deposited on the inner tube surface electrolytically or by chemical vapour deposition. BqC and Eu$ pellets were produced in a hot pressing sintering technique, their bulk density ranged between 88 and 93% TD. Eu,O, was fabricated by cold pressing and sintering to pellets with a bulk density of 90% TD with a monoclinic structure. Reactor grade sodium was used with a B-content below 0.2 ppm and a C content of about 15 ppm. To simulate reactor conditions; lithium which is generated by (n, a) reaction from t”B has been added to some of the BqC containing capsules in a quantity of 1 wt%. The capsules were sealed under helium and heat treated at 600, 700 and 800°C for times between 750 and 8640 hours. After heat treatment absorber- and cladding material went in for a series of examinations, the results are described subsequently: B&/stainless steel Chemical interaction between BqC and cladding material has been studied with BqC powder in an inert atmosphere. ACCI was negligible at 6OO’C after 8600 h test duration. By adding sodium to the B,C/cladding system chemical interaction is considerably enhanced. During heat treatment the BqC pellets remain intact, they suffer a small diameter- and weight loss, the rate of mass transfer from BqC to the sodium and/or cladding follows a parabolic time law. Metallographic cuts of the uncoated cladding material reveal a pronounced chemical attack on the inner capsule surface. The zone of interaction is characterized by two or three uniform reaction layers with a considerably higher microhardness than the matrix, separated from the unattached cladding material by a dark transition zone (fig. 2). The overall penetration depth reaches from 50 pm at 6OO”C/8600 h to 160 pm at 8OO”C/750 h. In the investigated temperature range the penetration rate corresponds to a tl/’ rule. X-ray diffraction and microprobe analysis showed evidence of preferred boron diffusion in the stainless steel with Ni,B, FeB and Fe,B as main products of interaction. Chemical analysis of the penetrated capsule material revealed a considerable increase in boron content but insignificant change in carbon content. Capsules containing lithium showed reduction of B penetration at 600 and 8OO’C. By incor-
Ph. Diinner et al. / Control rod systems
189
Fig. 2. Metallographic section of stainless steel cladding after heat treatment with B,C pellets and sodium at 600°C for 8640 hours.
porating a niobium or chromium layer in the capsules chemical attack was eliminated at 600°C (Cr, Nb-coating) and reduced at 75O’C (Cr-coating). Based on the experimental results in the pure B,C/Na/stainless steel system, a correlation between growth of interaction wne and temperature was developed. By analogy with a thermal activated process, the available data have been fitted by the following equation k=Aexp(-Q/RT),
where k is the growth of the interaction zone, A is a constant, Q is an activation energy, R is the gas constant and T the Kelvin temperature. The regression analysis of experimental data yields the following temperature dependence of penetration rate K = 0.522 exp( -97,2OO/RT),
with K = growth of interaction R = 0.00831 kJ/mol K T = cladding temperature
zone in cm/s’/*,
in K. This correlation coincides well with results reported in
[14], especially in the important temperature range between 600 and 7OO’C (fig. 3). The activation energy of 97.2 kJ/mol is comparable with that one for boron diffusion in y-Fe, if FeB and Fe,B [15] or a solid solution with boron is formed [16]. The kinetics of ACCI thus seems to be controlled by the diffusion of boron in austenite or through the reaction layer. The out-of-pile chemical behaviour of the B,C/Na/ stainless steel system is confirmed by the results from the irradiation experiments RAMBEX [17] and DFR 510 (181; the in-pile penetration depth does not exceed the level defined by the out-of-pile correlation. From the test results it is concluded, that in a Na-bonded vented absorber pin with B& pellets as absorbing material ACCI amounts to 48 microns within one year residence time at 6OOT, the maximum cladding temperature in SNR absorber pin design. Relative to ACCI a residence time of at least 420 efpd is therefore justified (corresponding to 2 cycles of SNR 300-reloads). Furthermore it could be demonstrated that incorporation of a niobium or chromium inner layer will be an appropriate measure to reduce ACCI.
190
Ph. Diinner et al. / Control rod systems
500 .+I-
600
-
700
900
lO-+
i
o . 0 A p,
ul i
l.C970/Na/B,C 1.4970/Na-Li/B,C l.L970#WNa/B,C l.49wNdB4c 1.49H/Cr/NaIB~C
10-S-
13
12
t
11
10
9 y
2
I K-l1
Fig. 3. Growth of an interaction zone on stainless steel at contact with B4C pellets and sodium.
EuB,/stainless steel The chemical behaviour between Eu$ and stainless steel has been investigated in a sandwich arrangement at 600 and 700°C for test durations of 4320 and 8640 h. By metallographic examinations no interaction between Eu$ and stainless steel has been observed. Microprobe analysis indicated Fe-diffusion into the Eu$ matrix and a small increase in Ni concentration. The presence of sodium again enhances chemical interaction. From the few out-of-pile results available, it was found that type and scale of interaction are very similar to the BqC containing system, main product of interaction is Fe,B. The temperature dependence of penetration depth is well described by the correlation developed for the B,C/Na/stainless steel system, the level of penetration depth is even lower. These preliminary results indicate a sufficient low rate of chemical interaction between Eu$ and stainless steel in sodium environment for a residence time up to - 730 efpd, the target value for SNR-2 control rods.
indicated
insignificant
chemical
interaction
below
900°C. Beyond 900°C europium and oxygen penetrate the cladding material particularly along grain boundaries to react with cladding components. Main products of interaction consist of Eu, M (metal)-double oxides. With respect to chemical interaction, Eu,O, would be an reasonable absorber material, since the threshold temperature for ACCI in the Eu,O,/cladding systems lies far beyond the actual temperatures for control rods.
3. Llehaviour under irradiation Gas production and -release, and swelling from the absorber material are the most important irradiation effects for control rod performance. Moreover, there is a known influence of irradiation on thermal conductivity of B4C, which is described in chapter 2.1.1. 3. I. Swelling of absorber materials
~&/stainless steel Investigations of Eu,O, pellets with cladding material heat treated in the temperature range 600 to 1OOO“C
B‘&
A detailed investigation
of BqC swelling has been
191
Ph. Diinner et al. / Control rod systems
conducted in the course of the last years. To put the functional description on a basis as wide as possible, all clearly specified swelling data in the literature on SNR specific BqC as well as own irradiation results from the RAMBEX [17] and the DFR 510 experiment [18] were brought into the analysis. The maximum “B bumup amounted to 8 X lo’* capt/cm3 and the irradiation temperatures lie between 450 and 98O’C. The swelling of B.+C is based essentially on two processes, which proceed together, micro-swelling and macro-swelling. Hereby, the micro-swelling supplies the main amount of the swelling by forming defect structures in the BqC lattice. Macro-swelling provides the second contribution by formation of cracks. These become effective particularly at higher burnup and high linear power above 100 W/cm according to results reported in [19]. The B,C swelling curve derived from literature data and own experiments [20,21] is shown in fig. 4. This curve is the base of the design calculations for SNR 300 and KNK-II absorber pins. As can be seen, BqC swelling increases above 1.5 X 102’ capt/cm3 linearly with increasing “B bumup. The corresponding correlation is given by: A D/D = 0.551 (Bu + 0.614[exp( - Bu/0.614) - 11)
where = diametrical swelling; AD/D (W) Bu (102’ capt/cm3) = bumup. EuB, At present the swelling behaviour of Eu$ has only been investigated on a few samples in the RAMBEX irradiation experiment. The irradiation results allow the conclusion that the swelling of Eu$ is nearly comparable to that of BqC with a tendency to a lower swelling rate. El!293
Swelling of Eu,O, exhibits a wide range of experimental results. Whereas American investigations [22] show scarcely measurable expansions ,which were related to the formation of cracks, Eu,O, swelling was demonstrated in the RAMBEX experiment. However, under the RAMBEX conditions, Eu,O, pellets swelled at least 40% less than B4C. The results of the DFR 510 irradiation experiment have shown that Eu,O, has a considerably greater resistance to swelling than 90% TD BqC pellets. In the low temperature range (< 525’C swelling of Eu,O, can almost be ignored. The data are
7.00 [WA 6.00
RAMBEX OFR OFR
0
0
0.00
1.00
2.00
-
510, &C-temperature 510, B&temperature
3.00
l 2a
result&r
4.00
/
525 *C 755 OC
5.00
6.00
7.00
/-
/
8.00
9.00
10 B burn up I l(r" CAPTURES/d Fig. 4. Swelling of BqC as function of “B bumup.
10.00
I
Ph. Dinner et al. / Control rod systems
192
not sufficient to derive an appropriate swelling correlation, therefore it is recommended to describe the swelling of EuzO, in a conservative manner by the B4C correlation.
with falo (-)= portion of captures in ‘*B related to the quantity of total captures. 3.2.2. Helium release
3.2. Production and release of helium 3.2.1. Pr~ucti~n of helium Helium is produced as a result of the capture of neutrons via the (n, a)-process of ‘*B. In the case of B4C, the number of neutrons absorbed is the same as the number of He atoms produced if the small percentage of the competing (n, 2T) reaction is ignored. Thus the quantity of produced He per cm3 BqC is directly correlated with the ‘OB burnup M,, = 37.191. Bu where M,, (cm3 He/c&
B&) = quantity of produced He, = bumup.
Bu (10” capt/cm3) !%!!!6
In Eu$, neutrona are captured by l”B- and Euatoms. Therefore, the produced helium is reduced by the portion of captures in europium: M,, = 37.191 * Bu -fa,,,,
B,C To get a reliable basis for the prediction of He release from B& all available results in the literature and results from our own irradiation experiment RAMBEX [15] and DFR 510 [16] were taken into account in the performed data analysis. Hereby most emphasis was placed on the investigations of SNR type material. However, He release values from highly compacted BqC granulate were also taken into account. Since an exact characterization of the samples was not possible in all cases, parameters were not set for density, grain size, stoichiometry and methods of production. In terms of the burnup dependence of the He-release, the analysis yielded the following picture. At low burnup (< 1.5 X 102’ capt/cm3) the release of He is relatively high (about 60%). With increasing burnup, however, the percentage of He released decreases, so that at high bumups (> 4 x 102’ capt/cm3) only less than 30% of the integrally produced He contributes to the buildup of pressure in a closed absorber rod. The release of gas is temperature dependent. The percentage of released He increased with increasing irradiation temperature.
% KNKII/l
-
*O
$
70
-
60
r 0
ZF RAHBEX
0
-z 500 oc
A
500-600
:
600-700 * 700 *c OC >
Y
lit.
ronsrrvative envelope
dab
I
:
ink@ mvllopc envrlopc < 600 lC
sp J
;I
2
20
y
10 0 0
t;0
2;o
3,O
LO
5.0
6,O
10 3 burn up Fig. 5. Helium release from B4C as function of “B burnup.
7.0 I
6,O
9,O
CAPT / cm3xlQn 1
Ph. Diinner et al. / Control rod systems
CWVI I : illteQd cucvc II :
193
burn up I CAPT / cm3x10'2'1 MVdOpL
cnvrlopcfor
1 < 600 OC
Fig. 6. Integral helium quantity released from BqC as function of “B bumup. For the calculation of the release from B4C the following concept was employed (fig. 5): Curve I represents the conservative envelope of all measured values in relation to burnup; curve II demonstrates the en-
velope of all measured values at irradiation temperatures below 600°C. Experimental results on the release from RAMBEX and DFR 510, recent results from KNK II/l and intermediate inspections on absorber pins of irradiation experiment PFR 13/12 were all found within these envelopes and confirm their conservativity. From the curves in fig. 5 the released He quantity per cm3 B4C has been calculated. The correlation is shown in fig. 6; it can be seen that, following an initial rapid increase in the helium quantity, above 2 x lo*’ capt/cm3 saturation is reached to a certain degree. Further He release within the experimentally covered range does not exceed additional 50% of the already released quantity. EuB, The experimental findings from the RAMBEX experiment are at present the only irradiation results on Eu$. A He release of almost 100% was found. In accordance with the correlation for He production in chapter 3.2.1 the He release is characterized by V ,zREE= 37.191. Bu .feio, VFREE(cm3 He/cm3 Et&)
= released quantity of He. 4. Conclusions The reported results on absorber materials, referring to physical, chemical and material properties as well as
to the behaviour under neutron irradiation, led to a complete qualification of B4C pellets as reference absorber material for the absorber systems of the first core of SNR 300 and of KNK-II-reloads with a maximum i”B bumup of - 7 x lo*’ capt/cm3 and ensure the design of these absorber pins under the licensing requirements. Furtheron, a sufficient extrapolation base for the design of advanced absorbers for SNR 3@0-reloads (“wet”, vented pins) with an extended target-‘OBbumup of - 15 X lO*l capt/cm3 is given. The design of a “high bumup”-absorber system with a target ‘OBbumup of - 20 X lo*’ capt/& as it is foreseen for SNR-2 under economical aspects requires a completion of the experimental background in the future. Acknowledgements
The R&D-work, sponsored by the Bundesministerium fur Forschung und Technologie (BMFT), was performed in cooperation with the Kernforschungszentrum Karlsruhe GmbH (KfK).
References [1] D. Mahagin, J.L. Babes and D.E. Babes, report HEDLTME 73-78 (1973). [2) A. Strasser and W. Yario, report NP-1974, EPRI contract TPS-79-708. [3] F. Gestermann, Reaktortagung Mannheim (1977) DAtF, Proc. symp., p. 518. [4] M. Harle and B. Schulz, Reaktortagung Dusseldorf (1976). DAtF, Proc. symp. p. 782. [5] A.E. Pasto, report ORNL-TM-4226 (1973). (61 A.E. Pasto and M.M. Martin, Eu,O,: report ORNL-5291 (1977).
194
Ph. Dibmer et al. / Control rod systems
[7] W.E. Roake and T.T. Claudson, report HEDL TME 77-33 (1977). [8] A. Lipp, Techn. Runds. 14, 28, 33 (1965); 7 (1966). [9] B. Schulz, P. Misaelidis, Kemforschungszentrum Karlsruhe, private communication (1978). [lo] F. Hashin and W.B. Rosen, J. App. Mech. 31 (1964) 223. [ll] S. Ihara, K. Tanaka, M. Kojima and Y. Akimoto, Meeting of the Intern. Working Group for Fast Reactors, Dimitrovgrad, USSR (1973), p. 201. [12] B.T. Kelly, UKAEA Springfields, private communication (1976). (13) K. Schwetz and A. Lipp, Atomwirtschaft 18 (1973) 531. [14] R.E. Dahl: Intern. Working Group for Fast Reactors, Dimitrovgrad, USSR (1973), p. 123. [15] G.V. Samsonov and I. Vinitskii, Handbook of Refractory Compounds, (IFI/Plenum, New York, 1980).
(161 P. Busby, N. Warga and C. Wells, J. Metals, Sect. I, 5 (1953) 1453. (171 M. Hiirle, H. Enderlein, R. Pejsa and Ph. Dtnner, Reaktortagung Dusseldorf (1976), Proc. symp., p. 778. 1181 M. Hbrle, to be published. [19] M. Horle and C. Sari, Reaktortagung, Berlin (1980) Proc. symp., p. 602. [20] Ph. Dhnner, R. Habel, M. Horle and H. Zimmermann, IWGFR-specialists’ Meeting on Fast Reactor Absorber Materials and Control Rods, Obninsk, USSR, (1983). (211 Ph. Dtnner, R. I-Babel,H.J. Heuvel and M. HBrle, report BMFT 4-015 (1983), ch. I. [22] M.M. Martin, report ORNL/TM-5400 (1976).