Hfir irradiation facilities improvements — the HIFI project

Journal

of Nuclear

1018

Materials

141-143

(1986) It)18-1(.~4

North-Holland,

HFIR IRRADIATION G.R. HICKS,

FACILITIES

IMPROVEMENTS

B.H. MONTGOMERY

K.R. THOMS

- THE HIFI PROJECT

and CD.

Amsterdam

*

WEST

The High-Flux Isotope Reactor (HFIR) has outstanding neutronics characteristics for materials irradiation. but some relatively minor aspects of its mechanical design severely limit its usefulness for that purpose. In particular. though the flux trap region in the center of the annular fuel elements has a very high neutron flux, it has no provision for instrumentation access to irradiation capsules. The irradiation positions in the beryllium reflector outside the fuel elements also have a high flux; however, although instrumented, they are too small and too few to replace the facilities of a materials testing reactor. To address these drawbacks the HFIR Irradiation Facilities Improvement (HIFI) Project, now under way and scheduled for completion in mid-1987. consists of modifications to the reactor vessel cover, internal structure, and reflector. Instrumentation access will be provided to the flux trap region. and the number of materials irradiation positions in the removable beryllium will be increased from four to eight, each with almost twice the available experimental space of the present ones.

1. Background

The High-Flux Isotope Reactor (HFIR) is a pressurized, light-water-cooled, beryllium-reflected, 100-MW reactor (fig. 1). It was designed for the production of isotopes, particularly transura~um isotopes, which requires high thermal and epithermal neutron fluxes; indeed, the HFIR target region (the cylindrical space inside the two concentric annular fuel elements) has the highest steady state thermal-neutron flux in the world. The high thermal flux also makes the reactor a good source of neutrons for scattering experiments, and a number of beam tubes exist for that purpose {fig. 2). The relatively high reactor power and power density leads to a high fast-neutron flux, so that the HFIR is also used for materials irradiation experiments, It is thus a multipurpose facility, but the fixed loading and conformation of the aluminum-clad, U,O, fuel element ~ consisting of involute plates - is much less flexible than a general purpose materials test reactor in which the loading and arrangement of the fuel elements can be varied within quite wide limits. Small (16-mm-diameter) irradiation capsules can be placed in the target region, but no pressure vessel penetrations provide access to that region so that experiments in the target cannot be instrumented. This total lack of instrumentation is unfortunate because the very high fast- and thermal-neutron flux in the target region make it an extremely desirable facility for engineering materials irradiation. The size and mass of the specimens that can usefully be irradiated using present capsule designs is limited by the very high gamma heating rate. However, the miniature specimens developed by the fusion program can tolerate high gamma heating rates without developing unacceptable temperature gradients, and more than 60 uninstrumented irradi-

* Research

sponsored by the US. under contract DE-AC05-840R21400 Energy Systems, Inc.

0022-3115/86/$03.50 (North-Holland

Department of Energy with Martin Marietta

0 Elsevier Physics

Publishing

Science

Publishers

Division)

ations have been carried out in the HFIR target region for the fusion program Somewhat larger (37-mm-diameter) experiments can be irradiated in four positions in the beryllium reflector surrounding the control plates outside the outer fuel element. This part of the reflector is designed to be readily removable but requires frequent replacement (every 2.5 years) because of radiation damage; thus, the irradiation facilities in it are called the removable beryllium (RIB) positions. The neutron flux at the RB positions, although not as high as in the target region, is two to three times higher than in the Oak Ridge Research Reactor (ORR) and higher than the High-Flux Reactor (HFR) at Petten - two general-purpose reactors used extensively for materials testing (see table 1). There is access for instrumentation and temperature-control gas lines in the RB positions, but the access flanges are not directly above the irradiation facilities so that in situ vertical and rotational adjustments of the instrumented irradiation capsules are difficult. 2. The HFIR Irradiatian Facilities Improvement (HIFI) Project In 1984, an ad hoc committee was established at the Oak Ridge National Laboratory (ORNL) to ‘*. consider und recommend chunges und improvements to the Lubarutoty ‘s facilities for materiols irrudiution testing”. The committee’s report [l], from which much of the material in this paper is drawn, included recommendations for certain modifications to the HFIR that would significantly enhance the number and value of materials irradiation experiments that could be accomodated by the reactor. These recommendations are now being implemented as the HIFI Project with support from the Office of Fusion Energy, the Basic Energy Sciences Program, and other Department of Energy funds. The basic improvements needed to provide better materials irradiation facilities at the HFIR are clearly revealed by table 1. The highest flux positions in the target region cannot be instrumented and are very small. B.V.

1019

G.R. Hicks et al. / HFIR irradiation facilities improvements

‘HERMOCC”“’ LINES

’“Z’__-+

DRY h

WALL

JbNCTlON

IN-POOL

BOX

JUNCTION

BOX

SHIELD PLUG ASSEMBLY

/

VERTICAL

HOLE

-GAS

LINES

I

VERTICAL

HOLE

NO. IO

NO. 16

.

1 r

SHROUD

FLANGE

REACTOR

CORE

REACTOR

VESSEL

&\

Fig. 1. The High Flux Isotope Reactor (HFIR).

The RB positions are few and much smaller than those in the general purpose reactors. Fig. 3 is a diagrammatic side view of the HFIR fuel elements and target region. A structure known as the target tower extends upward from the target region to a quick-access hatch and plug in the pressure vessel lid. At present, the plug is pierced at the center to accommodate a hydraulic rabbit tube. The HIFI Project will make a new plug with three access holes on an equilateral triangle (fig. 4). One will be for the existing rabbit assembly, and the other two, fitted with flanges, seals, and hold-down clamps, will provide instrumentation access to the target region. The target tower assembly provides support for the hydraulic tube, and it must be replaced by a new and different structure. With these modifications, at least two small target capsules of 16-mm diameter may be instrumented. By occupying up to seven target positions, capsules of up

to 25 mm could be accomodated (fig. 5). Multiple position assemblies have already been made for the unmodified HFIR; fig. 6 is a photograph of an aluminum holder that occupies three positions and was built to support an 18-mm-diameter spectral-tailored isotope production capsule. The four 37-mm-diameter irradiation positions in the RB region are shown in fig 7. The HIFI project, timed to coincide with the next scheduled RB replacement in 1987, will involve a new design for the beryllium, having eight holes each with a diameter of 48 mm. This change will increase the total experimental volume available within the capsules by a factor of 3 to 4. These new positions are referred to as the RB Star (RB *) facilities, and they could accommodate most of the irradiation work presently undertaken in the core of the ORR, taking only one-third to one-half the time to accumulate the same damage level.

1020

G.R. Hicks et al. / HFIR irradiation facilities improvements

,THRU

TUBE

REFLECTOR

-TANGENTIAL

Fig. 2. Plan view of the HFIR showing

OUICK HATCH

STKAIQHT ACCESS PENETRATION FOR

TAROET TOWER

SHROUD FIANOE

UPPER TRACKS

reactor

components.

LINER

TUBE

fuel, and beam tubes.

Some other components mounted above the beryllium and the core must also be replaced with new designs. Specifically, the upper track assembly and the shroud flange (which are part of the systems that locate and guide the control plates) must be changed to provide clearance for the new facilities. In addition, a new quick-access hatch (figs. 3 and 4) with more penetrations will provide straight-line access to the RB * positions for instrumentation. The straight-line access will also permit rotation and vertical relocation of irradiation capsules during the course of an experiment and will make experiments interchangeable. Fig. 7b shows the location of the RB * facilities. In addition to the reactor modifications, the HIFI project includes an experiment utilizing the new instrumentation access to the target region to determine more accurately the probable temperature in the uninstrumented target capsules previously irradiated as part of the Japan/U.S. fusion materials program. Also, new capsules will be designed, tested, and built to accommodate the specimens from the two spectral-tailored

Fig. 3. General

arrangement of core target access hatch.

tower

and quick

1021

G.R. Hicks et al. / HFIR irradiation facilities improvements

Table 1 Comparison

of some leading

materials

high-flux

testing

facilities

FFTF

Characteristics

and the HFIR

a

General reactors

purpose

BR-2

HFIR

HFR

ORR

RB

Fast flux b (10” neutrons mm2 s-l) Minimum specimen temperature (O C) Maximum displacements per atom per calendar year, stainless steel (S.S.)

30 365 35

I 70 10

5 70 8

3 70 4

7 70 10

Thermal flux (10” neutrons mm2 SK’) Gamma heating (kW/kg S.S.)

41 4.5

10 15

2 10

2 8

Typical capsule diameter (mm) Typical capsule length (mm) Number of available positions Instrumentation

28 150 60 Yes

52 ’ 600 35 d Yes

14 550 17 Yes

74 550 12

15 17 35 500 4e Yes

a b ’ d ’

f

yes

MOTA - Materials Open Test Assembly facility. Figures relate to the core configuration > 0.1 MeV. There are also two cadmium-shielded positions of 200-mm diameter, Plus two 200-mm-diameter positions, cadmium-shielded for low thermal flwc. Plus four smaller positions, _- 12-mm diameter. Including six peripheral target positions.

experiments

3. Neutron

and MFE7J - that are currently at a much lower damage rate in the

- MFE6J

being irradiated ORR. flux,

fuel cycle length, and dosimetry

mea-

surements

As indicated earlier, the HFIR is a multipurpose reactor, and there has been a natural concern about the possible effects of more and larger materials irradiation experiments on other users. Two important questions concern the effect of neutron absorbers in the RB positions on beam tube thermal flux and on the fuel cycle duration. To answer these questions, a series of experiments and flux measurements have been carried out. RABBIT

RABBIT

/

/

MODIFIED Fig. 4. HFIR

14 70 30 28 55 16 500 14 No

f

in cycle 9. September

1986.

As a worst-case experiment, the effect of a 3.8~mmthick hafnium sleeve placed in each of the four RB positions in turn was investigated. Such a thickness of hafnium would be more than adequate for most spectral tailored irradiation experiments and will absorb almost all of the thermal neutrons falling on it; it is, therefore, a nearly complete absorber and represents the most extreme case to be encountered in practice. The change in the flux at typical neutron-scattering experiment positions at the various beam tubes was measured as the hafnium was placed in each RB position in turn. The results are shown in table 2. The effect is less than about +5% in all the cases likely to be permitted in practice. The increase in flux that is observed in some positions may merit a further explanation. Putting an absorber close to the fuel reduces the thermal flux in

TUBE

PRESENT

beginning

Target

top plug.

TUBE

1022

G.R. Hicks et al. / HFIR

irradiation facrlrtres rmprouements Table 2 Change in flux at the HFIR beam tubes as a 3.Smm-thick, 31.6-mm-diameter hafnium shield is placed in the RB irradiation positions Hafnium position

RBI ’ RB2 RB3 ’ RB4 ’

Fig

5. Instrumented target positions, illustrating accommodating 25-mm capsules.

capability

of

that region and, therefore, the local power level in that part of the core. With the same overall power the local power density on the opposite side of the core must increase to compensate, leading to a higher local flux in that zone. The results are also presented graphically in fig. 8. Measurements of the effect on fuel cycle length are more difficult or at least less accurate, because of the core-to-core variations introduced by manufacturing tolerances and because the loading of experiments and isotope production capsules is changed from cycle to cycle; these and other effects introduce a cycle-to-cycle length variation of up to about 1 d. Reactor time is too

Fig. 6. Holder

for l&mm-diameter

Change a in thermal flux at beam tubes HBl through HB4 h HBl

HB2

._ 3 14 +1 +1

0 d -1 0

HB3 +1 +1 -6 +2

HB4 12

-t 4 +2 -4

” The base case has the usual iridium (isotope production) capsules at all RB positions except RB3. h Measurements taken at a reactor power of 100 kW. ‘ The iridium in each of these positions in turn was replaced by the hafnium sleeve to make the measurements. ’ This combination is considered impractical for heavily absorbing materials irradiation experiments because of the very close proximity of RB3 to the HB2 beam tube (IO-cm sep-

aration). The loss of flux was actually 13%. valuable to perform a controlled experiment with no other experiments allowed to change their loadings over many cycles. Careful measurements and comparison with historical data on the relationship between initial core reactivity and fuel cycle length have resulted in calculations showing that the effect of the hafnium absorber is to shorten the fuel cycle length by 21 f 7 h or 4% of a typical cycle. The corresponding additional fuel costs are only about $lOOOOO/year. Dosimetry experiments were carried out to provide data on the materials damage rate to be expected in the new RB facilities. Analysis of the flux (performed by Larry Greenwood of Argonne National Laboratory) showed that the displacements-per-atom (dpa) rate

HFIR capsule

occupying

three target positions.

1023

G.R. Hicks et al. / HFIR irradiation facilities improvements

Fig. 7. The removable

beryllium

(RB) irradiation

positions

and the planned

RB * positions.

-

Q

-

f SLEEVE

El

-15

I 100

200

300 DISTANCE

Fig. 8. Results of lOO-kW runs with a hafnium

DESlGN REGION

POSITIOh

400 BETWEEN

500

Hf SLEEVE

INSTRUMENTED TARGET AND RB’ POSITIONS

,

POSITIONS

INSTALLATION

RB’

0

700 lmmi

inside the symbols

I

F+,BRlCATlON

INSTRUMENTED

TUBE

The numbers

1 1

RB’

BEAM

sleeve in various RB positions. numbers.

FABRlCATlON -INSTRUMENTED TARGGT REGION INSTALLATlON TARGET REGION

600

AND

1

I d

POSITIONS

w

DESIGN. FABRICATE. AND INSTALL INSTRUMENTATION

I

I

FUSION MATERIALSCAPSULES DESIGN, BUILD. INSTRUMENTED

DESIGN. B”ILD.AND RB’

AND INSTALL TARGET CAPSVLE

I

INSTALL

I I

CAPSVLES

Fig. 9. Schedule

A

INSTRUMENTED

v

RB’

POSITIONS

I TARGET

POSlTlONS

AVAILABLE

FOR

for the HIFI Project.

“SE

AVAILABLE

FOR

USE

designate

beam tube

1024

G. R. Hicks et al. / HFIR

expected in 316 stainless steel in the RB positions is 10.5 dpa/year. Even within the shielding provided by a 3.8-mm hafnium sleeve, the rate is 8.3 dpa/year. 4. Schedule and status of the project The schedule of the project (fig. 9) is closely linked to the RB changeout scheduled for June 1987, the Japan/US. target capsule temperature measurement planned for completion in August 1986, and the completion of the MFE6J and MFE7J IO-dpa irradiations by July 1988 (also part of the Japan/US. program). Engineering design of the reactor components necessary for the instrumented target capsule facilities has been completed, and an engineering design report has been approved and issued. Fabrication of those components is under way with an anticipated delivery date of June 2, 1986, Design of the remaining reactor components associated with the RB * facilities is on schedule.

irradiation facilities

improvements

July 1987, there will be eight RB positions, all with instrumentation access and capable of delivering more than 10 dpa/year; these positions can accommodate spectral-tailored (i.e., shielded) experiments, making them well-suited to fusion materials irradiation. We are most grateful to C.W. Alexander, J.A. Conlin, S.S. Hurt, III, R.M. Moon, Jr., E. Newman, Jr., and A.F. Rowcliffe - members of the ORNL Materials Irradiation Facilities Improvements Committee - for their permission to include excerpts from the Committee’s report. We would also like to thank D.M. McGinty, R.M. Nicklow, and their colleagues who participated in the experiments to measure the effects of hafnium in the RB of the reactor and W.E. Thomas for help in calculating the significance of the fuel cycle length measurements Reference

5. Summary A project to improve the materials irradiation facilities at the HFIR at Oak Ridge is under way. By July 1986, at least two instrumented target positions capable of accepting 25-mm-diameter capsules and delivering a damage rate of 30 dpa/year should be available. By

[l] C.W. Alexander, J.A. Conlin, S.S. Hurt, III, R.M. Moon, Jr., E. Newman, Jr., A. F. Rowcliffe, K.R. Thorns and CD. West, Report of the Materials Irradiation Facilities Improvements Committee, ORNL/TM-9709 (October 1985).