CNA-74-205

SLOWPOKE AT THE :

THREE YEARS OF PROGRESS

by - •"-

li- + + R.E. Jervis*, J.W. Hilborn1, R.E. Kay1, R.G.V. Hancock*

* University of Toronto

Atomic Energy of Canada Limited Chalk River Nuclear Laboratories

INTRODUCTION Three years ago, arrangements were made with Atomic

Energy of Canada Limited to test the prototype SLOWPOKE reactor at the University of Toronto. The central, downtown campus was chosen as the site of the installation because of its proximity to established research groups which were making extensive use of either commercially available radioisotopes or neutron activation analysis.

SLOWPOKE is an acronym for Safe Low Power Critical

Experiment. It is a new type of laboratory developed by Atomic Energy of Canada Limited and suitable for isotope pro- duction and neutron activation analysis at universities, hospitals and research institute. It provides a higher (useable thermal-neutron flux of 10l2n/cmz.s at 20 kW thermal power) than is available from small accelerators or radioactive sources, while avoiding the complexity and high operating costs of conventional -2-

nuclear reactors. Two SLOWPOKE reactors have been operating

in Toronto and Ottawa since 1971 and are licensed to operate

without conventional automatic shut-down devices and without

an operator in attendance.

The SLOWPOKE-1 reactor installed at the University of

Toronto is illustrated in Figures 1 and 2, and described in detail

in References 1 and 2. It is a pool type reactor containing about

760 g of 2 3 ^ in the form of highly enriched 2 3 5U aluminum alloy

fuel pins moderated by light water. The reactor core is surrounded

by,a 4 inch thick reflector in which sites for the

irradiation of samples are located. The top and bottom reflectors

are slightly separated from the side reflector to provide passages

for coolant circulation by natural convection.

ONCISUAL FEATURES

Inherent reactor safety is guaranteed by a combination of

the negative temperature and void coefficients of the undermoderated

reactor core, a limited maximum excess reactivity of 3.4 mk, and

restricted access to the reactor core by users. As a result, con-

ventional electro-mechanical safety devices are deemed unnecessary

and are not incorporated into SLOWPOKE reactors. The reactor has

neither an" automatic safety system nor a duplicate control system.

Automatic control of the reactor is exercised by a single motor driven absorber rod. The control rod motor is activated by a simple "on-off" controller, responding to a signal generated by -3-

a self-powered neutron detector. No ion chambers or neutron

counters are required, and the control system incorporates no

electrical limit switches. If the auto-control system fails, the

maximum credible reactivity insertion will result in a power

transient limited to safe levels by the reactor's inherent negative

feedback characteristics. A second absorber is available to shut-

down the reactor should the auto-control system fail.

Start-up to a pre-set power level is automatically initiated

by a single pushbutton and takes about 3 minutes. No critical

approach is required and there is no low power start-up instrument-

ation. In essence, this reactor is an "on-off" device. Reactor

power is displayed on a 0 to 5 kW or 0 to 20 kW indicator using

the signal from a second self-powered neutron detector. Once operating,

the reactor is licensed to be left unattended for periods up to

18 hours, thus permitting convenient overnight irradiations. During

unattended operation the reactor is monitored remotely at the

University's 24 h Emergency Control Centre; i.e. the centre dealing with routine security, fire alarms, etc. Shutdown is initiated by

a manual pushbutton.

Long-term reactivity compensation is effected by adding

thin beryllium plates to the top reflector at intervals of about

6 months. This beryllium plate addition takes about 2 hours and is performed by a qualified roactor engineer while the reactor is maintained critical at low power. Replacement of a fuel charge -4-

will be required after about 20 kW years of operation.

Because the reactor is so simple to operate, users of

the facility can be licensed as operators without formal training

in reactor technology. They must, of course, be fully qualified

in radiation protection procedures. At the University of Toronto

the persons licensed to operate the reactor are radiochemists;

one licence holder is a Ph.D student in the Department of Chemical

Engineering.

OPERATING EXPERIENCE

The CRNL construction and commissioning crew installed the

reactor in the prepared pool during the month of May 1971, and

completed all essential tests prior to fuel loading by June 3rd.

Operating licences were received from the Atomic Energy Control

Board on June 4th, fuel loading commenced immediately and criticality

was achieved the same day. Commissioning continued for two weeks

during which the control rod was calibrated; low power operation,

and high power automatic operation were proved satisfactory;

radiation surveys were conducted; and self-limiting power excursions were performed. Operation of the reactor was formally transferred

from AECL to the University on June 25, 1971. Since that date the reactor has been in use as a neutron scarce almost daily an-î to date has generated about 30,000 kWh of energy.

A few statistics which illustrate the progress in reactor operation over the three year period are presented in Table 1. -5-

From this data it is apparent that the use of the reactor has

increased dramatically over the period, and that with the present

schedule of over 20 days per month operation at over 10 hours

operation each day, the reactor is very heavily utilized. These figures

also illustrate that the efficiency of using the reactor has also

significantly increased over the three year period: presently

about 95% of the time when the reactor is in operation, samples

are being irradiated.

The present energy generation of about 1,000 kWh per

month requires reactivity adjustment to be made about every six

months and will require a replacement core at about 10 years. To

date reactivity adjustment by beryllium plate addition has been

performed on four occasions and as expected has proved to be an

extremely simple operation.

As might be expected during three years operation we have

had a number of operational problems. During the first year we

experienced frequent spurious radiation monitor alarms, the control

rod position indicator developed an intermittent fault, minor leaks

developed in the water purification system, and a number of small

- problems occured which presented no safety hazard and which did not

interrupt reactor operation. In fact,we have experienced only two

problems which necessitated brief reactor shutdown. During a routine

maintenance test in January 1972 the outer absorber plate jammed

when fully inserted and in July 1973 a mechanical failure in the auto-

control rod system prevented normal reactor shutdown. Over the three

year period only six days operation have been lost due to reactor

malfunctions. -6-

KEQUIREMENTS FOR A REACTOR AT TORONTO

Although University of Toronto personnel have always

had good access to the McMaster reactor, in 1970 there was con-

siderable interest in having a modest installation, such as

SLOWPOKE-1, available on the central campus of the University.

This site was chosen so that the reactor would be close to interested

researchers in pure and applied science, medical departments, and

to the nearby teaching hospitals. Of interest were the possibilities

of utilizing a wider variety of short-lived radioisotopes than were

available commercially, and of opening up the field of instrumental

neutron activation analysis, through solid-state germanium gamma

spectrometry of short and intermediate half-life nuclides.

The growth in SLOWPOKE uses detailed below, dramatically demonstrate the need for ready access to such a facility.

REACTOR USAGE

Reactor usage has increased significantly over the first three years. The average number of irradiations performed per month has increased each year by over fifty per cent, as is shown in Table 2. It should be noted that Table 2 has been prepared on the basis of the SLOWPOKE-1 annual reporting years.

Although the average irradiation time is just a little over 1 h per sample at SLOWPOKE-1, the total number of irradiations compares quite favorably with the number of sample irradiation requests received at other reactors, for example, those that were processed at the McMaster Reactor 2008 (1972), 2259 (1973) . A — 7 —

direct comparison cannot be made, though, since the number of

multiple irradiations at McMaster is not known, nor is the average

irradiation time.

Table 2 also shows the distribution of the number of

irradiations performed on a month-by-month basis. The undergraduate

teaching year is well defined by SLOWPOKE-1 irradiations which

are concentrated more in the spring term than in the autumn. Most

of these irradiations arise so far from interest generated in

Engineering and Physics.

The granting, in January , 1973, of a new licence, which

allowed higher power and unattended overnight operation of the reactor,

has greatly enhanced the versatility of service irradiations

performed at SLOWPOKE-1, and has encouraged usage by a number of

different groups.

A summary of the distribution of reactor usage by various

sectors of the university community is shown in Table 3.

Before comparisons are made between work loads of each

reporting year, it should be noted that the usage times listed

for the year 1971/72 are based on estimates of the time the reactor was tied up by various groups, rather than by straight irradiation time usage, as is listed for the subsequent two years. As can be seen from the total time involved for 1971/72, the total irradiation time involved 736 hours over the year, but this tied up the facility for a total of 886 hours. This extra 150 hours would be spread non-uniformly through the different sectors, with undergraduate -8-

and graduate student research claiming the bulk of it, because of

the nature of the work.

With this point in mind, it can be seen that undergraduate

usage has involved approximately sixteen percent of the total usage.

However, even though the graduate usage has increased steadily

ovor the three years, the rate of increase has not kept pace with

the average increase, and so the overall percentage use has declined.

Faculty usage has increased the most significantly. This

includes all research projects where graduate and undergraduate

students are not involved. The major reason for this increase has

come from a growing awareness of SLOWPOKE-1's capabilities and

reliability that is now becoming apparent to medical researchers.

Irradiations involving SLOWPOKE-i staff for such matters

as materials and standards testing have declined, as expected, with

increased operating experience.

Even though no attempt has been made to turn the SLOWPOKE-1

programme into a semi-commercial venture, the number of occasional

non-University of Toronto users has risen slowly, thus maintaining

a constant, though low, use factor.

PROJECTS

Each year over fifty percent of undergraduate usage has come from B,A. Sc. thesis research projects. These have included

«.frjidies of: corrosion products obtained from Pickering and

Doii%/>s Point generating stations; the fate of impurities in high tapy-jsrature Baï"iO3 production; the distribution of cadmium -9-

in foods; trace elements in atmospheric participates; the

adsorption of mercury compounds onto clays and clay minerals; and sewage sludges.

The remainder of undergraduate usage, though not so

involved or glamorous, has brought large numbers of students into

contact with neutron activation analysis and simple nuclear reactor

kinetics.

Graduate research follows a similar pattern. Major

research projects such as studies of calcium metabolism, environ-

mental investigations, air particulate work, and continued study

of mercury compound adsorption on clay and clay minerals, have

been interspersed with a number of metal alloy analyses, and the

determination of trace amounts of in rocks.

Faculty research was initiated by programmes such as

the analysis of ancient ceramic materials, the production of

small quantities of *8F, and general environmental stidies. It has now added a regular weekly production of llZK for medical research, periodic 2 "*Na production for gastroenteritis studies, and an investigation of the extracellular fluid space in babies by the measurement of bromide concentrations in blood.

Recent non-University of Toronto usage has included the analysis of bromide- and chloride-doped sodium iodide crystals, a zinc analysis in hair from patients with Possibly abnormal zinc concentrations, and analyses of assorted samples including -10-

coal and fly ash made in conjunction with members of the analytical

research group at Ontario Hydro.

On the whole, then, the versatility of the SLOWPOKE-1

reactor is proving to be adequate in many quite diversified

teaching and research fields. From a very small group of active

supporters back in 1971, the reliability and accessibility of the

SLOWPOKE-1 reactor on the St. George campus has swelled the ranks

of users to nearly two dozen research groups. The ready avail- ability of this small nuclear facility, with the necessary equipment to allow researchers to perform neutron activation analyses using short-lived radioisotopes, has been of major importance to the success of the project. The fact that SLOWPOKE-1 complements the much larger McMaster Reactor, at Hamilton, without undue competition or rivalry also testifies to the ingenuity of researchers willing to utilize SLOWPOKE as it was first conceived at Chalk River

Nuclear Laboratories.

Therefore, given reasonable cooperation by government agencies and Atomic Energy of Canada Limited itself, there appear to be no real reasons (apart from the normal ones of financing and operational competence) for the commercial model of the

SLOWPOKE reactor not to be extremely useful in both university and medical research environments. -11-

REFERENCES

1. SLOWPOKE at the University of Toronto: A Laboratory

Reactor for Neutron Irradiation; R.E. Kay, J.W. Hilborn,

P.D. Stevens-Guille, AECL-4212 (1972).

2. SLOWPOKE: A New Low-Cost Laboratory Reactor;R.E. Kay,

J.W. Hilborn, P.D. Stevens-Guille, R.E. Jervis, Int. J.

of Applied Radn. and Isotopes Vol. 24, pp 509-518 (1973),

Also available as AECL-4541. TABLE 1

SLOWPOKE-1 OPERATING STATISTICS

Year 1971 1972 1973 Jan-March 1974

Av. no of days/ month operation 12 18 20 22

Av. no. of oper- ating hours/day 5.9 6.8 10.2 12.1

Av. energy kWh/ month 377 622 967 1308

% operating time when irradiatinc samples 44 66 84 95 TABLE 2 RESUME OF SLOWPOKE IRRADIATIC'-'S 1971/72 1972/73 1973/74 Month Irrad. Grad.+ Undergrad. Irrad. Grad.+ Undergrad, Irrad. Grad.+ Undergrad.

June 6 6 — 149 149 — July 29 29 - 55 55 - 100 100 - Aug. 14 14 - 80 80 - 145 145 -

Sept. 25 25 •- 99 99 — 75 73 2

Oct. 52 43 9 115 109 6 175 170 5 Nov. 68 37 31 117 108 9 112 82 30 Dec. 54 45 9 38 32 6 87 86 1 Jan. 120 76 44 85 50 35 249 158 91

Feb. 138 46 92 133 63 70 302 228 74 Mar. 110 61 49 120 86 34 286 248 38 April 105 104 1 80 72 8 May 138 123 15 96 89 7 June 165 165 -

Total 859 609 250 1332 1157 175 1531 1290 241

Av./Mth. 72 51 21 102 89 13 170 143 27 TABLE 3 DISTRIBUTION OP USAGE AMONG SLOWPOKE USERS

1971/72 1972/73 1973/74

Irradiations Time*(h) Irradiations Time(h) Irradiations Time(h)

Undergraduate 250 220 179 235 245 402 Graduate 301 349 551 603 594 615 Faculty 110 117 431 531 377 1145 SLOWPOKE Staff 157 181 139 121 139 55 Non U.of T. 61 19 32 76 176 90

Totals 859 886* (736) 1332 1566 1531 2307

Av./month 72 74* (61) 102 120 170 256

* Time based on involvement of the reactor rather than by additive irradiation times alone. 3536-K

OUTER CONTROL GAS PURGING CENTRAL CONTROl P1ATE MECHANISM INLET ' ROD MECHANISM cqmtoiCAiimi GUIDE TUBE \

,SAMfL£ STATION REACTOR TO SAMPLE CONTAINER STATION

CONTAINER CONCItTE SHII1DS WATER LEVEL

•iACTOIt CONTAINER

SAMPLE TUBE IVARIABIE RADIUS)

URANIUM/AIUMINUM FUEL ELEMENTS SAMPLE TUBES

PIG. It SL0WP0KE-1 Installation at University of Toronto. FIG. 2: Section through SLOWPOKE-1 Reactor.