Fusion Engineering and Design 58–59 (2001) 535–541

High dose neutron irradiation damage in berylliumas blanket material

V.P. Chakin a,*, V.A. Kazakov a, A.A. Teykovtsev a, V.V. Pimenov a,G.A. Shimansky a, Z.E. Ostrovsky a, D.N. Suslov a, R.N. Latypov a,

S.V. Belozerov a, I.B. Kupriyanov b,1

a SSC RF RIAR, Ulyano�sk region, 433510 Dimitro�grad, Russiab SSC RF VNIINM, Rogo� street, 5, 123060 Moscow, Russia

Abstract

The paper presents the investigation results of beryllium products that operated in the SM and BOR-60 reactorsup to neutron doses of 2.8×1022 and 8.0×1022 cm−2 (E�1 MeV), respectively. The calculated and experimentaldata are given on helium and tritium accumulation, swelling, micro-hardness and thermal conductivity. Themicrostructural investigation results of irradiated beryllium are also presented. It is shown that the rate of helium andtritium accumulation in beryllium in the SM and BOR-60 reactors is high enough, which is of interest from theviewpoint of modeling the working conditions of the DEMO fusion reactor. Swelling of beryllium at irradiationtemperature of 70–150 °C and neutron fluence of 2.8×1022 cm−2 (E�1 MeV) makes up 0.8–1.5%, at 400 °C andfluence of 8×1022 cm−2 (E�1 MeV) – 3.2–5.0%. Irradiation hardening and decrease of thermal conductivitystrongly depend on the irradiation temperature and are more significant at reduced temperatures. All results presentedin the paper were analyzed with due account of the supposed working parameters of the DEMO fusion reactorblanket. © 2001 Published by Elsevier Science B.V.

Keywords: Beryllium; Irradiation; DEMO fusion reactor

www.elsevier.com/locate/fusengdes

1. Introduction

At present beryllium is widely used in nuclearreactors as a neutron reflector and moderatormaterial. Usage of this material is especially effec-tive in research reactors that were developed to

realize the special neutron-physical requirementsfor their core parameters. Two research reactorsof this type are operating in Russia now. They arethe SM and MIR reactors that are situated inSSC RF RIAR. Long experience of berylliumblocks operation in these reactors gives valuableinformation on beryllium behavior at high neu-tron doses that can be claimed and useful ininvestigations for the fusion reactor.

Now beryllium is considered as a promisingmaterial for the first wall, divertor and blanket in

* Corresponding author. Tel.: +7-84235-32021; fax: +7-84235-35648.

E-mail addresses: fae@niiar.ru (V.P. Chakin),vniinm.400@g23.relkom.ru (I.B. Kupriyanov).

1 Tel.: +7-95-1908015; fax: +7-95-9255972/9252896.

0920-3796/01/$ - see front matter © 2001 Published by Elsevier Science B.V.

PII: S0920 -3796 (01 )00226 -5

V.P. Chakin et al. / Fusion Engineering and Design 58–59 (2001) 535–541536

the international thermonuclear experimental re-actor (ITER) and DEMO fusion reactor designs.Degradation of beryllium properties under high-energy fusion neutron flux irradiation is the mainproblem that requires the greatest attention and,thus, experimental investigations. A neutron spec-trum of the fission reactor is softer as comparedto that of the fusion reactor. However, spectra ofthe SM high-flux research reactor and the BOR-60 fast reactor (also situated at SSC RF RIAR)have a substantial fraction of high-energy neu-trons with energies more than 1 MeV. Therefore,irradiation of promising materials in these reac-tors allows rather effective modeling of the work-ing conditions of the fusion reactor.

The most powerful planned neutron effect willbe on beryllium during its usage as the DEMOreactor blanket material. Due to high temperatureand damage doses there are changes in berylliumrelated to accumulation of large quantities oftransmutant-gases and other radiation-induceddefects. This causes considerable deterioration ofphysical–mechanical characteristics of thematerial.

The present paper includes the investigationresults of beryllium products, that operated in theSM and BOR-60 fission reactors up to the servicelife parameters, and analysis of the obtained re-sults as applied to the working conditions of theDEMO fusion reactor blanket.

2. Materials and constructions, experimentconditions

The neutron trap of the SM reactor is locatedin the SM reactor core center (Fig. 1(a)) and isdesigned for establishing the maximum thermalneutron flux density in the central channel. Theneutron moderator is a beryllium insert of theneutron trap (Fig. 1(b)), which during operationis surrounded by the coolant, primary circuit wa-ter. The insert has a complicated geometricalshape, considerable overall dimensions and ismade of hot-pressed TE-56 grade of berylliumwith the grain size �56 �m (average grain sizeabout 25 �m) produced by the hot extrusiontechnology and containing 1.5% of BeO. After

in-reactor operation up to the planned service lifeduring 1.5–2 years the insert is sent for destruc-tive material science investigations. The specimensare mechanically cut out from different parts ofthe irradiated product. The most damageable partof the insert is the core central plane area, wherethe maximum neutron flux density in the reactoris provided.

In the BOR-60 reactor beryllium is not a mate-rial of the core structural elements. However, withthe special aim the reactor periodically uses theberyllium-filled neutron source that is subject tostrong radiation-induced damage in the process ofreactor operation. The neutron source consists ofseven stainless steel capsules (Fig. 2(c)), insideeach of them there are five cylinders of the TE-400 grade hot-pressed grade of beryllium pro-duced by the hot extrusion technology. Thisberyllium is an obsolete grade, has a large grainsize (�400 �m) and high content of foreign im-purities. In the process of irradiation beryllium isisolated from the primary circuit sodium and is inhelium medium. Seven capsules with berylliumwere collected into the standard hexahedral as-sembly of the BOR-60 reactor (Fig. 2(a) and (b)).This neutron source operated in the seventh rowof the reactor reflector during 12 years.

Fig. 1. Insert of the neutron trap, the SM reactor: (a) centralregion of core; (b) appearance of beryllium insert.

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Fig. 2. Neutron source, the BOR-60 reactor: (a) appearance ofneutron source; (b) across section of neutron source; (c) cap-sule with beryllium filling.

radiation resistance at its boundary operatingparameters.

3. Experimental results and discussion

3.1. Gas accumulation

The main consequence of beryllium neutronirradiation is accumulation of great heliumamount (and significantly less tritium amount)which causes degradation of the physical–me-chanical properties. The helium accumulationdata are summarized in Table 2. Measurementswere conducted by the mass-spectrometricmethod. The ultimate parameters of gas accumu-lation in the DEMO reactor beryllium blanket arealso presented here. For the SM reactor there isquite good agreement of the calculated and exper-imental values of the produced helium amount,for the BOR-60 reactor the experimental valuesare considerably less than the calculated ones.Refs. [3,4] present the investigation results of he-lium accumulation in beryllium performed at theBelgian research high-flux reactor BR-2analogous to the SM reactor. At fast neutronfluence of 3.39×1022 sm−2 in beryllium accord-ing to the experimental data of the SCK�CENhelium production made up 12 719 and 12 830appm and according to the data of the RA Amer-ican laboratory—12 490 appm. In this case, thecalculated value was 10 832 appm. Comparison ofthese results with the calculated and experimentalvalues of helium accumulation in beryllium in theSM reactor allows statement of their satisfactoryagreement. As to tritium accumulation in bothRIAR’s reactors and helium accumulation in the

The main reactor irradiation parameters of theabove beryllium structural elements are summa-rized in Table 1. The planned parameters of theDEMO reactor beryllium blanket are shown too[1,2]. Comparison of the presented parametersshows that the irradiation temperature of thereactor blocks (obtained by calculation) has asomewhat less value than the planned workingtemperature of the fusion reactor blanket. How-ever, their fast neutron fluence and damagedose considerably exceed the planned level for thefusion reactor. Thus, the investigation resultsindicated below are of obvious interest fromthe viewpoint of estimation of beryllium

Table 1Reactor irradiation parameters of beryllium products

Status Product fromReactor Temperature Neutron fluence Damage doseberyllium (°C) (cm−2) (E�1 MeV) (dpa)

High-flux research reactor, situated atSM 3770–150Insert of the 2.8×1022

SSC RF RIAR neutron trapFast reactor, situated at SSC RF 93BOR-60 Neutron source 350–440 8.0×1022

RIARBlanket 500–640DEMO 2.2×1022Fusion reactor design 30

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Table 2Helium and tritium accumulation in reactor products from beryllium under neutron irradiation up to the service life parameters

Product from beryllium Content of T (appm)Content of 4He (appm)

ExperimentCalculation Calculation Experiment

11 416 1700Insert of the neutron trap, the SM reactor, center plate of core 54412 0005742 2859800 20Neutron source, the BOR-60 reactor, center plate of core

DEMO fusion reactor blanket 16 300 210

BOR-60 reactor, the specifying experimentalinvestigations are being performed at present.

3.2. Swelling

Swelling was obtained by measuring the hydro-static density of the irradiated beryllium productfragments and comparing it with beryllium initialdensity. The swelling data are presented in Table3. It follows that irradiation in the SM reactorcauses considerably less swelling than in the BOR-60 reactor. Likely this is effect of the irradiationtemperature. The effect of vacuum annealing atelevated temperatures on swelling of berylliumirradiated in the SM reactor was investigated. Ifannealing at 500 °C practically does not changethe geometrical dimensions of irradiated speci-mens, then above 700 °C there is further growthof swelling, that in some cases can achieve �10%.

The cause of swelling is helium formed as theresult of threshold nuclear reactions of berylliumatoms and high-energy neutrons. However, thedegree of gas atoms effect on its value stronglydepends on helium atoms state in the crystallattice. At reduced temperatures (70–150 °C) thegas atoms diffusion is slowed down, and they arenear the formation place. The diffusive mobility ofhelium atoms grows with increase of the irradia-tion temperature, they are re-distributed over thestructure with formation of aggregates of atoms,gas pores and bubbles. The analogous phe-nomenon is observed at high-temperature anneal-ing of beryllium pre-irradiated at reducedtemperatures. Formation of gas pores and bubblesin the structure and further growth are the causeof significant increase of swelling with increase ofthe irradiation temperature (or high-temperature

annealing after low-temperature irradiation) to500–700 °C [5–7]. The results of the present pa-per evidence that at least up to the irradiationtemperature of 440 °C swelling does not exceed5% at fast neutron fluence of 8×1022 cm−2. Thisvalue can be a characteristic of beryllium dimen-sional stability in the area of the low workingtemperature boundary of the DEMO reactorblanket.

3.3. Mechanical characteristics

Neutron irradiation leads to hardening and em-brittlement of beryllium. The results of micro-hardness measurement by pricking the specimenwith a diamond pyramid are presented in Fig. 3,from which it follows that the greatest increase ofmicro-hardness with growth of neutron dose oc-curs under low temperature irradiation, when itachieves �200% for the maximum investigateddose about 3×1022 cm−2. Irradiation at 400 °Ccauses the material hardening to a considerablyless degree, the growth of micro-hardness does notexceed 70%.

The earlier published papers [8,9] present theresults of mechanical tests of beryllium irradiatedin different temperature-dose ranges. Practicallycomplete absence of permanent elongation is char-acteristic of the mechanical properties of irradi-ated beryllium. Therefore, due to strongirradiation embrittlement in massive berylliumblocks of the SM and MIR reactors during opera-tion up to service life doses there is cracking. Anadditional factor promoting crack formation andpropagation includes thermal stresses appearing inthe blocks because of non-uniformity of the tem-perature fields.

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Table 3Swelling of beryllium

Swelling (%)Product from beryllium

Irradiation+annealing Irradiation+annealing atIrradiationat 500 °C, 1 h 700 °C, 1 h

Insert of the neutron trap, the SM reactor, center plate 0.8–1.5 0.7–1.3 2.6–13.6of core

– –Neutron source, the BOR-60 reactor, center plate of core 3.2–5.0DEMO fusion reactor blanket �8

3.4. Thermal conducti�ity

The thermal-physical properties of berylliumunder neutron irradiation are important for sub-stantiation of future construction of blanket. Dis-tribution of the temperature fields in aconstruction and, thus, the thermal stresses leveland diffusive mobility of helium and tritiumatoms depend on the thermal conductivity coeffi-cient quantity.

Fig. 4 presents the investigation results of neu-tron irradiation effect on the beryllium thermalconductivity coefficient. In the initial state beryl-lium has high thermal conductivity, which greatlydepends on the metal quality, foreign impuritiescontent, grain size. The thermal-physical proper-ties deteriorate with increase of the measurementtemperature. The irradiation effect strongly de-pends on the temperature, at which the specimenwas irradiated. So, thermal conductivity coeffi-cient of the specimen irradiated at 70 °C de-creases over four times, and of the specimenirradiated at 400 °C–only by �10–15% (despitethe considerably large neutron dose). In this case,there is smoothing of the irradiated berylliumthermal conductivity coefficient versus the mea-surement temperature.

The authors know comparatively few worksdevoted to investigations of the thermal conduc-tivity of irradiated beryllium. Ref. [10], e.g., pre-sents the data on thermal conductivity coefficientof cast and hot-pressed beryllium irradiated at 50,100, 800 °C up to fluences of 6×1020–3.7×1021

cm−2, from which it follows that the thermalconductivity decreases to a greater degree afterirradiation at reduced temperatures. However,

this low neutron doses do not allow judgement ofsuch effects value at the working parameters ofthe DEMO reactor blanket.

3.5. Microstructure

Fig. 5(a) presents microstructure of berylliumafter low temperature irradiation by means oftransmission electron microscopy (TEM). Thereare only dislocation loops of interstitial type, withthe 20 nm average size and volume density of3.1×1015 cm−3. There are no visible gas poresand bubbles. Apparently theirs sizes are verysmall and probably below the detection limit ofmicroscope. Annealing at temperatures exceedingthe irradiation temperature speeds up the diffu-

Fig. 3. Micro-hardness of irradiated beryllium.

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Fig. 4. Effect of neutron irradiation on thermal conductivity ofberyllium.

5(b)), which diameter does not exceed 2 nm,density – 1.9×1017 cm−3. The first microscope-resolved helium bubbles appear only starting atthe annealing temperature – 500 °C. Further in-crease of the annealing temperature causes inten-sive growth of bubbles, their diameter achieves 12nm (Fig. 5(c)). Irradiation at 400 °C leads toformation of flat pores, their maximum diameterachieves 80 nm, the minimum size is 5 nm (Fig.5(d)). The pores are located on the boundariesand in the grain body. Extensive depleted areascan be seen along the boundaries. Ref. [11] givesthe microstructure of beryllium pebbles irradiatedin the EBR-II reactor at 380 °C up to 4.31×1022

cm−2 (E�0.1 MeV), i.e. in the analogous neu-tron spectrum and at close temperatures. Alongwith other peculiarities in the investigated mi-crostructure there are gas pores as thin flat platesoriented along the basal plane. Such flat pores canbe formed due to segregation of helium atoms ondislocation loops.

sion of gas atoms, leads to growth of bubbles andappearance of first visible finest bubbles (Fig.

Fig. 5. Microstructure of irradiated beryllium: (a) TE-56, Tirr=100 °C, F=1×1022 cm−2 (E�1 MeV), the SM reactor; (b) TE-56,Tirr=100 °C+annealing 500 °C, 1 h, F=1×1022 cm−2 (E�1 MeV), the SM reactor; (c) TE-56, Tirr=100 °C+annealing700 °C, 1 h, F=1×1022 cm−2 (E�1 MeV), the SM reactor; (d) TE-400, Tirr=400 °C, F=8×1022 cm−2 (E�1 MeV), theBOR-60 reactor.

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4. Conclusions

Investigations of beryllium as product frag-ments irradiated by high neutron doses in the SMand BOR-60 reactors allow the following to benoted:1. Rate of helium and tritium transmutant-gases

accumulation in beryllium products of thesereactors is high enough and comparable withthe rate of accumulation in the DEMO fusionreactor blanket.

2. Swelling of beryllium at 400 °C and fluence of8×1022 cm−2 (E�1 MeV) does not exceed5%.

3. Radiation hardening and decrease thermalconductivity of beryllium strongly depends onthe neutron irradiation parameters, especiallyon the exposure temperature. In particular, theirradiation effect at low temperature (70–150 °C) achieves a few hundreds of percents,at 400 °C – only a few tens of percent.

4. Under low temperature (70–150 °C) irradia-tion of beryllium there is microstructural for-mation of dislocation loops and helium andtritium gas atoms. Visible bubbles are onlyafter annealing at 500 °C. Increase of the irra-diation temperature up to 400 °C causes for-mation of gas pores as flat plates oriented inspecific crystallographic directions.

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