Atomic Energy of Canada Limited

SLOWPOKE AT THE

A LABORATORY REACTOR FOR NEUTRON IRRADIATION

by

J.W. HILBORN, R.E. KAY, P.D. STEVENS-GUILLE Atomic Energy of Canada Limited

and

R.E. JERVIS University of Toronto

Chalk River, Ontario

August 1972

AECL-4212 SLOWPOKE AT THE UNIVERSITY OF TORONTO:

A LABORATORY REACTOR FOR NEUTRON IRRADIATION

by

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

Atomic Energy of Canada Limited

and

R.E. Jervis, University of Toronto

Chalk River Nuclear Laboratories Chalk River, Ontario August 1972

AECL-4212 SLOWPOKE AT THE UNIVERSITY OF TORONTO:

A LABORATORY REACTOR FOR NEUTRON IRRADIATION

bv

J.W. Hilborn, R.E. Kay, P.D. Stevens-Guille Atomic Energy of Canada Limited

and

R.E. Jervis University of Toronto

ABSTRACT

SLOWPOKE is a new type of reactor developed by Atomic Energy of Canada Limited for use by universities, hospitals and research institutes. It is a safe, reliable, low-cost source of neutrons for isotope production and neutron acti- vation analysis. The prototype reactor, built and tested at Chalk River Nuclear Laboratories, was subsequently dis- mantled and installed at the University of Toronto, where it has successfully completed its first year of operation.

This reactor has a number of novel features including low , inherent safety, and unattended operation. Irradiation facilities allow samples to be irradiated in a thermal up to 2.6 x 10l l n/cm2.s at a reactor power of 5 kW. A summary of the first year's operation shows the high availability of the reactor and its increasing use within the University. An outline is given of future plans to uprate the thermal flux to 1 x 1012 n/cm2.s, and extend the period of unattended operation to 18 hours.

Chalk River Nuclear Laboratories Chalk River, Ontario August 19 72 AECL-4212 Le SLOWPOKE à l'Université de Toronto:

Un réacteur de laboratoire pour l'irradiation neutronique

par

J.W. Hilborn, R.E. Kay, P.D. Stevens-Guille L'Energie Atomique du Canada, Limitée

et

R.E. Jervis Université de Toronto

Résumé

Le SLOWPOKE est un nouveau type de réacteur développé par l'Energie Atomique du Canada, Limitée pour les universités, les hôpitaux et les instituts de recherches. C'est une source de neutrons sûre, fiable et peu coûteuse qui permet de produire des radioéléments et de faire des analyses par activation neutronique, Ce réacteur prototype, construit et mis à l'essai à Chalk River, a par la suite été démonté et installé à l'Université de Toronto où il vient de terminer avec succès sa premiere année de fonctionnement.

Ce réacteur possède un certain nombre de caractéristiques nouvelles y compris une faible masse critique, une sécurité inhérente et un fonctionnement sans surveillance. Ses dispositifs d'irradiation permettent aux échantillons d'être irradiés dans un flux de neutrons thermiques allant jusqu'à 2.6 x 1011 n/cm2.s pour une puissance de réacteur de 5 kW.

Le sommaire du fonctionnement de la premiere année révèle la haute disponibilité de ce réacteur et son emploi accru à l'Université. On projette de porter le flux thermique à 1 x 10 n/cm .s et de prolonger jusqu'à 18 heures la période de fonctionnement sans surveillance.

L'Energie Atomique du Canada, Limitée Laboratoires Nucléaires de Chalk River Chalk River, Ontario

ALCL-4212 TABLE OF CONTENTS

Page

1. INTRODUCTION 1

2. NOVEL FEATURES 3 2.1 General 3 2.2 Low Critical Mass 3 2.3 Inherent Safety 5 2.4 Restricted Access and Unattended Operation 6

3. DESCRIPTION OF SITE, REACTOR AND AUXILIARY SYSTEMS 7 3.1 Site 7 3.2 Reactor 7 3.3 Irradiation Facilities 12 3.4 Fuel 13 3.5 Reflectors 16 3.6 Control Systems 16 3.7 Automatic Shutdown 18 3.8 Radiation Monitoring 20 3.9 Water Purification 21 3.10 Gas Purging 21

4. SAFETY ANALYSIS 22 4.1 Reactivity Transient 22 4.2 Radiolytic Hydrogen Production 26 4.3 Radionuclide Release 26 4.4 Loss of Electric Power to the Reactor 27 4.5 Remote Monitoring During Unattended Operation 28 4.6 Loss of Pool Water 28

5. THE FIRST YEAR'S OPERATION 30 5.1 Commissioning 30 5.2 Operating Experience 30 5.3 Utilization of SLOWPOKE 32

6. UPRATING THE REACTOR TO EXTEND ITS USEFULNESS 36

ACKNOWLEDGEMENTS 37 FIGURES

Page

1. SLOWPOKE Reactor 2

2. Reactor Room at the University of Toronto 8

3. Sample Station 14

4. Sample Loading 15

5. Fuel Pin Loading 17

6. Reactor Control Panel 19

7. SLOWPOKE Power Transient 2 3

8. SLOWPOKE Temperature Transient 23

9. Peak Fission Power Versus Reactivity 24

10. Peak Coolant. Outlet Temperature Versus Reactivity 24

TABLES

1. Summary of Reactor Specifications - 1 -

1. INTRODUCTION

SLOWPOKE is a new type of developed by Atomic Energy of Canada Limited at the Chalk River Nuclear Laboratories and sold by the•Commercial Products division in Ottawa. The name is an acronym for Safe LOW POwer Critical Experiment. This report describes SLOWPOKE-1, the prototype reactor which was operated at Chalk River and subsequently moved to the University of Toronto. The reactor was designed for isotope production and neutron activation analysis at universities, hospitals and research institutes. Samples can be irradiated in a thermal neutron flux of 2.6 x 10 *x n/cm2.s at 5 kW thermal power and are transferred to and from the reactor in pneumatic tubes. Development of the concept at Chalk River during 1968-1970 led to the commissioning of the prototype reactor in May 1970. From May 1970 to March 1971 the characteristics of the reactor were investigated in detail. In April 1971 the reactor was dismantled and reassembled at the Environ- mental Science and Engineering Institute of the University of Toronto. Operation of the reactor is licensed by the Atomic Energy Control Board of Canada (AECB). Since its recommissioning in June 1971 the reactor has had consid- erable use, operating over 1300 hours during the first 12 months. Figure 1 shows a cross section through the reactor. OUTER CONTROL GAS PURGING CENTRAL CONTROL CONCRETE SAMPLE STATION INLET PLATE MECHANISMf\ ROD MECHANISM SHIELDS

GUIDE TUBE FROM FLUX DETECTOR

REACTOR CONTAINER

CONTAINER WATER LEVEL \

POOL WATER LEVEL

BERYLLIUM SHIM PLATES REACTOR CONTAINER

OUTLET ORIFICE

FUEL REGION HEIGHT 10 IN. REFLECTOR DIAMETER 9 IN.

SAMPLE TUBE (VARIABLE RADIUS) CROSS-SECTION OF REACTOR POOL

INLET ORIFICE CROSS-SECTION OF REACTOR

URANIUM/ALUMINUM NEUTRON SAMPLE TUBE SLOWPOKE REACTOR FUEL ELEMENTS SOURCE

FIGURE 1 - 3 -

2. NOVEL FEATURES

2.1 General

The novel features of the SLOWPOKE reactor concept are low critical mass, inherent safety and unattended operation. Low critical mass is achieved by surrounding a small 235U-aluminum-water core with a beryllium reflector, The ratio of fuel to water is chosen to give a negative temperature coefficient of reactivity. By limiting excess reactivity to a small value and relying on the inherent negative temperature coefficient as the primary safety mechanism, it has been demonstrated that conventional electromechanical safety devices are unnecessary. More- over, the skilled tradesmen normally required to test and maintain the safety devices are unnecessary; and because of the inherent safety, an operator need not be always present in the reactor room during operation. In summary, SLOWPOKE provides a higher neutron flux than is available from small accelerators or radioactive sources, while avoiding the complexity and high operating cost of conventional nuclear reactors.

2.2 Low Critical Mars

Early in 1967, experiments at Los Alamos showed that a 2 35 U-polyethylene core surrounded by a thick beryllium reflector gave a surprisingly low critical mass . For

(1) G.A. Jarvis, C.B. Mills, "Critical Mass Reduction", LA-3651, Los Alamos, March 1967. - 4 - practical reasons SLOWPOKE uses -aluminum alloy instead of uranium metal fuel, water in place of poly- ethylene, and a thinner beryllium reflector. Low critical mass configurations are characterized by a low ratio of fission power to thermal neutron flux. In fact, the power required to produce a given flux is approximately proportional to the mass of -35U in the core. Low power is desirable in a neutron-producing reactor because of the reduced consumption of 235U. In addition, less shielding is required, less cooling is required and less fission products are produced. Another characteristic of these hydrogen-moderated low critical mass systems is their small physical size and relatively high leakage of fast neutrons from the core region. As a result, the thermal neutron flux in the beryllium reflector is almost as high as at the centre of the core. Therefore, sample irradiation tubes can be located in the reflector instead of in the core, and the entire fuel assembly can be designed as an easily replace- able unit. A distinguishing feature of the SLOWPOKE reactor is the ease with which compensation can be made for long-term reactivity changes, due to fuel burnup and poison buildup. After every 6000 kW.h period (about 6 month to 1 year of normal operation) the excess reactivity of the system will be readjusted to its initial maximum value by the addition of a beryllium plate to the top reflector. This operation is very simple and can be accomplished within a few hours. The initial fuel charge has a life of about 20 kW.year. On the basis of 5 kW operation for 8h/day, refuelling is required only every 12 years. — 5 —

2 .3 Inherent Safety

The SLOWPOKE core is designed to have a negative temperature coefficient. That is, heating of the fuel and moderator causes the reactivity to decrease. There- fore, if the reactor is just critical and reactivity is then added, the fission power will increase until the excess reactivity is balanced by the loss of reactivity from the negative temperature coefficient. A direct consequence of this self-stabilizing characteristic is an upper limit on equilibrium power, equivalent to the heat removal capacity of the cooling system. If the cooling system is turned off, the reactor power will slowly decrease to a level equal to the natural leakage of heat to the surroundings. Because the reactor core is undermoderated it has a large negative . Therefore, if during a reactivity excursion boiling of the moderator water occurs, this immediately decreases the system reactivity and promptly lowers the reactor power. The maximum reactivity is established during con- struction of the reactor by adding fuel and beryllium in small controlled steps. Then the reactor container is bolted and sealed in such a way that the user is physically prevented from making any changes which could increase the excess reactivity beyond that originally built into t':e reactor. An exception is the possible insertion of in a sample irradiation tube. This pos- sibility can be averted only by administrative controls. No hazardous conditions have been envisaged which require immediate corrective action by a human operator. -"6

Integrity of the reactor container and fuel and strict control over the irradiation of samples containing fissile material are such that radioactive gases will never escape to the atmosphere so suddenly, or in such quantities, that persons in the reactor room or adjacent rooms will be endangered. A more detailed safety analysis is presented in Section 4.

2.4 Restricted Access and Unattended Operation

There are only three specified persons from the University of Toronto, approved by AECL and licensed by the AECB to operate the reactor; these three persons are designated "licensed users". At all times when a licensed user is absent from the facility the reactor room is locked. The inherent safety of SLOWPOKE and restricted access to the reactor container led to the conclusion that it is not essential for a reactor operator to be in constant attendance. The reactor is licensed to operate unattended for periods of up to 4h, and over the first years' operation the reactor has frequently operated at 5 k%T whilst unat- tended. During periods when no licensed user is present in the reactor room, limited remote monitoring of the facility is available at a 24h emergency service; by this monitoring service it is possible to bring a licensed user to the reactor to investigate any malfunction, usually within a period of less than 1 hour. Licensed users are qualified to start up and shut down the reactor, operate the water purification systems and irradiate samples. They are not permitted to open the reactor container or repair faulty equipment. - 7 -

DESCRIPTION OF SITE, REACTOR AND AUXILIARY SYSTEMS

Site

SLOWPOKE is a modified "swimming pool" type reactor. The core, reflectors and control mechanisms are housed in a 2 ft diameter container, 14 ft long. The container is suspended in a pool which provides cooling and shielding as shown in Figure 1. Table 1 summarizes the reactor specifi- cations. A pool was constructed at ground level in an existing building on the central St. George campus of the University, from a galvanized steel liner, a base of reinforced concrete 12" thick and an inner liner of reinforced concrete pipe. Poured concrete forms a bond between the outer steel liner and inner pipe. Structural design is in accordance with the local building codes and the National Building Code of Canada, Two 30" thick concrete shields over the top of the pool cover the reactor container and reduce the radiation fields imme- diately above the reactor to less than 0.35 mr/hr at 5 kW. In addition, a railing around the circumference of the pool limits access to authorized persons. The remaining equipment consists of a single control cabinet^ a loading station for samples, and separate water purification systems for the pool and container. Figure 2 shows a general view of the reactor room. The control cabinet is on the left, the reactor under concrete shielding in the centre and the sample station in the background.

3.2 Reactor

The main components of the reactor are 235U-Al fuel, beryllium reflectors, cadmium control absorbers, neutron flux detectors and thermocouples. These components are i 00 i

FIGURE 2. Reactor room at the University of Toronto. The reactor is in a pool under concrete shielding, centre, and the sample station in the background. - 9 -

TABLE 1

SUMMARY OF REACTOR SPECIFICATIONS

IRRADIATION SITES:

Capsule size O.D. (in.) 0.6 3 Length (in) 2.20 Sample volume (cm3) 7

Site in 3eryllium Side Reflector (Capsule is horizontal at reactor mid-plane) Average thermal flux over sample position at 5 kW (n/cm2.s);- 2.6 x 10ll Site in Pool (Capsule is vertical and located between 15" and 32" from axis of reactor)

Average thermal flux over sample position at 5 kW (n/cm2.s):- Site at 18" radius 1.1 x 109 Site at 24" radius 6.0 x 107 Site at 30" radius 5.3 x 106

REACTOR COMPONENTS:

Core Maximum licensed reactor power level (kW) 5 Maximum thermal neutron flux (n/cm2.s) 3 x 10 Right circular cylinder, diameter (in.) 9 height (in-) 10 volume U) 10

Fuel Pins 28 wt% U-Al alloy; U is 93% 235U diameter (in.) 0.15 Length (in.) 9.88 Total mass of 235U in core (g) 760 Cladding is aluminum, wall thickness (in.) 0.020

.. . continued - 10 -

Table 1 - SUMMARY OF REACTOR SPECIFICATIONS - continued

Moderator and Coolant

High purity H2O Volume of H20 in core region U) 8.6 Temperatures at normal 5 kW operation (pool at 68°F) Inlet (°F) 83 Outlet (°F) 100 Reflectors Material beryllium Radial, annular I.D. (in) 9. 03 O.D. (in) 16 .96 Height (in) 9. 89 Axial, bottom Thickness (in) 2. 0 Diameter (in) 13 .0

Axial, top Variable number of semi-circular shim plates each 6.5" radius and either 0.125" or 0.S0" thick, CONTAINING VESSEL AND POOL:

Vessel Aluminum alloy I.D. (in.) 23 .5 Length (in.) 16 8 Wall thickness (in.) 0. 25 Pool Concrete lined pool diameter (in.) 84 Depth below grade (in.) 240 Concrete thickness (in.) 14 BIOLOGICAL SHIELDING: Height of pool and reactor vessel water above core 10" 10" Thickness of concrete shielding blocks 21 6"

... continued - 11 -

Table 1 - SUMMARY OF REACTOR SPECIFICATIONS - continued REACTIVITY BALANCE: Assuming an excess reactivity for the cold reactor of 3.4 mk, then at the end of 18h operation at 5 kW:- Reactivity invested in temperature (ink) -0. 86 Xenon load (ink) -0. 86 Remaining excess reactivity to accommodate sample load, fuel burnup, samarium poison (mk) +1. 68 LONG TERM REACTIVITY CHANGES: Nominal core life (kW.year) 20 Net initial rate of reactivity loss due to burnup and samarium (mk/1000 kW.h) -0. 10 REACTIVITY CONTROL: Regulation and Shutdown Central control rod (auto); total worth (mk) 5.5 Outer control plate (manual) total worth (ink) 5.4 Core reactivity temperature coefficient at 120°F (mk/°F) -0.1 Inherent Safety Based upon negative coefficients Core temperature reactivity worth 6 8OF to 10OOF (mk) -2. 0 6 8C-F to 120OF (mk) -4. 0 6 8°F to 200°F (mk) -14 Void coefficient of reactivity (mk/cm3) -0. 04 Reactivity effect of 1% core voidage (mk) -4. 0 - 12 -

installed near the bottom of a cylindrical aluminum container, partially filled with water and suspended in the shielding pool. Water in the reactor container serves as coolant, moderator and shield. The fuel is cooled by natural convection. Heat in the container water is transferred to the outer pool through the aluminum walls of the container. The pool is cooled by city water flowing through a cooling coil. The purity of water in the reactor container and the shielding pool is controlled by periodic circulation through purification systems. The container is designed to be watertight so that water in contact with the fuel is not common with water in the pool. However, small leaks are not considered serious. The gas space above the water in the container is continuously purged with air from the room to remove hydrogen formed by radiolytic decomposition of the water. It is vented via an activated charcoal filter to the reactor room. Indicating instruments and controls are rack-mounted in a single cabinet. There are two independent control systems, one manually operated and one automatic. Neutron flux and coolant temperature are indicated on panel meters, and neutron flux is continuously recorded on a strip chart.

3.3 Irradiation Facilities

The main access to the reactor is provided by a sample irradiation system which has two transfer tubes extending from a sample station in the room to the reactor. One tube terminates in a horizontal hole in the beryllium reflector where a thermal neutron flux of 2.6 x 10 * * n/cm2.s is availabe at 5 kw. The other tube terminates in the pool. - 13 -

Its position may be adjusted laterally from the reactor container. At 18" from the centre of the core the flux is 1.1 x 109 n/cm2.s at 5 kW. Both positions are on the mid-plane of the core. The sample station shown in Figure 3 consists of a control panel, loading breeches and lead shielded receiving stations mounted below the shelf. The user's transfer flask is placed below the station to permit easy removal of irradiated samples. Bottles of nitrogen gas shown on the right propel samples to and from the reactor. The gas is vented through "absolute ' filters to the room. Each transfer tube has separate controls including a timer which allows pre-selected irradiation times from a few seconds to one hour. For longer irradiations, the timer may be placed on manual control. At the end of the pre-set time, the sample is automatically ejected from the reactor to the receiving station. A gamma health monitor measures the radiation fields from irradiated samples and alarms if they exceed 50 mR/h. Samples of material for irradiation are loaded into polyethylene capsules, 2.25" (58mm) long by 0.62" (16mm) O.D. The samples may be up to 1.9" (48mm) long and 0.56" (14mm) diameter with an approximate volume of 7 cm3. The capsule is fitted with a snap closing lid which is heat sealed with a special tool. Additional caps are squeezed on each end so the capsule is a snug fit in the transfer tube. Figure 4 shows a capsule being loaded prior to irradiation.

3.4 Fuel

Fuel is in the form of pencil-sized pins made of extruded uranium-aluminum alloy, 26 wt% 23 U. Each pin FIGURE 3. Sample station. Samples loaded in the breech, centre, are propelled to and from the reactor by nitrogen from bottles on the right, and return to shielded receivers, below. Timer-controls are located on the panel at the top. - 15 -

S": FIGURE 4. Sample loading. A capsule being loaded into the transfer tube breech prior to irradiation, - 16 -

is 0.15" diameter 10" long and extrusion clad with 0.020" of ASTM 1050 aluminum. End caps of ASTM 1100 aluminum are stud welded to the cladding which is coextruded with the meat. 298 fuel pins are arranged in a cylindrical array determined by matching holes in two aluminum grid plates; see Figure 5. The 9" diameter grid plates are welded to a 1" diameter central tube. Each pin is secured to the bottom plate but is free to expand upwards through the hole in the top plate. There are additional holes in the top plate to permit convection cooling.

3.5 Reflectors

A 4" thick beryllium annulus forms a side reflector for the fuel region and a 2" thick disc forms a bottom reflector. The top reflector consists of thin semi- circular plates which can be added or removed for reac- tivity adjustment using a long handling tool. The top and bottom reflectors are slightly separated from the side reflector to provide passages for coolant circulation by natural convection.

3.6 Control Systems

Two cadmium absorbers clad with aluminum are used for reactivity control. One is a curved plate located close to the outside surface of the beryllium side reflector. It is actuated by a motor-driven lead screw and is manually controlled. The second absorber is a rod which moves inside the central tube of the fuel cage. It is suspended by a steel cable which passes around a motor driven drum. - 17 -

FIGURE 5. Fuel pin loading. - 18 -

This rod is controlled automatically by a control unit which uses a flux detector signal. Both control absorbers have position indicators on the control panel. The manual and automatic control systems are com- pletely independent. The user is permitted to operate the manual control system only if a fault develops in the automatic control system. The main purpose of the manual system is to provide a convenient means of shutting the reactor down if the automatic control system fails in such a way that the central control rod remains fully up. During normal operation the outer control plate will be locked in the fully up position. When the reactor is shut down at the end of the day only the central control rod is lowered to the fully down position. Two self-powered flux detectors with cadmium emitters are used to measure neutron flux. One is used for auto- matic control and the other is connected to a strip chart recorder. The simplicity of reactor controls is emphasized by Figure 6. No ion chambers or neutron counters are required. Start-up to a predetermined power level is com- pletely automatic and takes approximately 2 minutes.

3.7 Automatic Shutdown

There are no mechanical safety devices triggered by flux measuring instruments as in other reactors. However, a thermocouple signal will automatically cause the central control rod to be motor-driven to its lower limit if the water temperature in the reactor core rises above its normal operating level. This single channel shutdown system which - 19 -

FIGURE 6. Reactor control panel Auto-control rod position, power and temperature are shown by the three lower indicators, manual control plate position is displayed digitally on the centre panel. The chart recorder logs reactor power. - 20 -

is considered temporary is provided as a backup to the automatic control system until the reliability of the latter has been adequately demonstrated. No automatic shutdowns took place during the first year of operation.

3.8 Radiation Monitoring

Three gamma radiation monitors with audible and visible alarms are installed in the reactor room. A high range detector, mounted below the concrete top shield, actuates an alarm when high radiation fields exist at the top of the reactor container. A second gamma detector, mounted near the water purification system, gives early warning of fission products escaping from defective fuel elements. The third detector, mounted near the sample station detects radiation from active samples. Remote indication and alarm for the high range detector is provided at the Radiation Control Centre, located in another building. A portable battery operated gamma monitor is used for general surveying and as a back-up for the other monitors on loss of electric power.

The reactor room is designed to handle occasional low level contamination that may occur during sample handling or water purification. A shielded end-window Geiger counter is provided in the reactor room to check the activity of filter paper "swipes". This Geiger counter is also used to detect fissile material in samples submitted for irradiation. - 21 -

3 _g Water Purification

Commerical demineralizers purify water in the reactor container and pool to reduce corrosion of the structural components and minimize the level of radioactive con- tamination . A two bed demineralizer operates continuously, purifying the pool water to 20 micro mho/cm at a pH of 7. The ion exchange resins are periodically regenerated as determined by water quality. A mixed bed demineralizer operating periodically during reactor shutdowns purifies the reactor container water to 0.3 micro mho/cm at pH of 6. The resin is not regenerated, and is discarded as radioactive waste when depleted.

3•10 Gas Purging

To ensure that hydrogen produced by radiolytic decom- position does not reach an explosive concentration, the gas space in the reactor container is continuously purged with a small flow of air (~1 ft3/h) . Some fission product noble gases are flushed by the air purge. Air is purged through the reactor and discharged directly to the reactor room via an activated charcoal filter (Refer to section 4.3) - 22 -

4. SAFETY ANALYSIS

The analysis presented here is not complete; it considers only the most severe hazardous conditions, and deals primarily with safety aspects which relate specifically to this reactor. Also note that the alarm features associated with loss of purge gas flow and loss of pool water will not be in operation until the reactor is licensed to operate unattended for ~18h; see Section 7.

4.1 Reactivity Transient

The maximum excess reactivity of _< 3.4 mk is estab- lished during construction of the reactor; the reactor container is then bolted and sealed to prevent persons from making changes to the reactor which could increase its excess reactivity. On this basis, the worst conceivable reactivity transient would be a rapid insertion of _< 3.4 mk. This situation could occur either by a failure of the auto- matic power controller during reactor start-up combined with a failure of the licensed user to recognise the situation and shut down the reactor, or a loss of A.C. power during reactor start-up. However, because of the inherent safety of the reactor system combined with efficient radiation shielding, a prolonged reactivity transient of 3.4 mk presents no hazard either to the reactor or via radiation fields in areas about the reactor. In fact, a series of reactivity transient tests was performed on the reactor whilst installed at CRNL. In these tests gradually increasing amounts of reactivity were inserted .nto the reactor; the results of these tests are summarized by Figures 7 to 10. Even with a reactivity - 23 -

FIG. 7 SLOWPOKE POWER TRANSIENT

zoo - EXCESS REACTIVITY + 6-8 mk

- 160 ' PROMPT' PEAK

E 140 Ul S ° 120 cr 'DELAYED' PEAK ° 100 o

UJ

60

40

20 I I 3 1

TIME (MINUTES)

4 Ul FIG. 8 SLOWPOKE TEMPERATURE TRANSIENT

EXCESS REACTIVITY + 6 8 mk 1- o

TIME (MINUTES) IOOC

FIG 9 PEAK FISSION POWER VERSUS REACTIVITY 500 FIG 10 PEAK COOLANT OUTLET TEMPERATURE VERSUS REACTIVITY

100

50

I o INITIAL TEMPERATURE PROMPT PEAK A , z 83°F A o DELAYED PEAK ° ' 87T O 70 °F D 10 NOTE: 6.8 mk TRANSIENT STARTS AT 70°F

ALL OTHER TRANSIENTS START AT~85°F

2 3 4 2 3 4 EXCESS REACTIVITY (mk) EXCESS REACTIVITY (mk) - 25 - insertion of +6.8 mk the transient is automatically limited to safe levels by the inherent negative void and moderator temperature coefficients. Note that the reactivity scale used is that defined by the "inhour equation" using an effective delayed neutron fraction g = 0.0082, ie. prompt critical on this scale corresponds to an excess reactivity of 8.2 mk. An analytical model has been developed to extend the range of studies to larger reactivity insertions. These calculations indicate that step reactivity insertions of up to 10.5 mk are safely limited by void formation in the reactor. Above 10.5 mk the reactor fuel may suffer perma- nent deformation. The insertion of fissile material into the reflector position sample irradiation site could also produce a reactivity excursion. Studies made on SLOWPOKE at CRNL indicate that the reactivity worth of 2 35 U in this sample site varies between ~0.4 mk/g of 235U in finely dispersed form, and 0.04 mk/g for 235U samples of mass > lOg. These results indicate that a quantity ~100g 235U is required to produce a reactivity transient comparable with the 3.4 mk transient considered above. Although there are no safety hazards associated with conceivable reactivity transients, should such a malfunction occur as to raise the reactor power significantly above the desired level whilst the reactor _s operating unat- tended, a remote alarm is given at the University 24h Emergency Service. This alarm will bring a licensed user to the reactor. - 26 -

4.2 Radiolytic Hydrogen Production

In any water cooled or moderated reactor, radiolytic decomposition of the water leads to the production of gaseous hydrogen. By maintaining high water purity the evolution rate of hydrogen into the gas space above the reactor is < 5 cm3/kw.h. To prevent the buildup of an explosive concentration of hydrogen in air (lower explosive limit ~4 vol% H2 in ~Ax) the air purge system is used. This system flows ~1 ft3/h of air through the gas space in the reactor vessel and vents via an activated charcoal filter to the reactor room. Loss of purge gas flow whilst the reactor is in operation but unattended could eventually result in an explosive concentration of hydrogen in the reactor container. Hov.7- ever, with normal hydrogen evolution rates and no purge gas flow it requires several days of continuous operation at power to produce an explosive concentration. Any significant loss of purge gas flow will cause a remote alarm in the 24h Emergency Centre, so bringing a licensed user to the facility.

4.3 Radionuclide Release

Due to the limited excess reactivity of the system, and the integrity of the reactor fuel and container, it is inconceivable that quantities of radioactive gases will ever escape to the atmosphere so suddenly or in such quantities that persons in areas about the reactor will be endangered. However, because of slight 2315U contamination of the outside surfaces of the fuel used in the University of - 27 -

Toronto reactor (associated with the stud welding of end caps to the fuel pins) there is a detectable level of fission product and daughter product nuclides in the reactor water. The gaseous fission products are also detectable in the gas purge space above the reactor. "*Ar is also detectable in the gas space; this is the direct result of the activation of "*0Ar present in the air dissolved in the reactor water. Because the gas stream purging the reactor vessel vents via an activated charcoal filter into the reactor room, h\r plus gaseous fission products are also vented to the reactor room. A detailed study of the radionuclides present in the gas purge stream at the University of Toronto reactor indicated the noble gas isotopes and daughter products 85inKr, 8flKr (+ 88Rb), 133Xe, 135Xe, 138Xe (+ 138Cs) and k xAr. No iodine isotopes were detected. When dispersed into the reactor room the total concentration of all these radionuclides is < 2% of the MPC based upon International a, Commission on Radiological Protection (ICRP) recommended values for a 40h week occupational exposure. 4.4 Loss of Electric Power to the Reactor

Loss of power to the reactor during a period of unat- tended operation does not cause the reactor to shut down. It will continue to operate although the power level will deviate slightly from the desired level; over a period of hours the reactor power wi]1 decrease due to pool tem- perature and xenon buildup effects. As explained in Section 4.2 a loss of purge gas flow due to an electrical power failure does not constitute an immediate hazard to the reactor. - 28 -

Loss of power causes a remote indication in the 24h Emergency Centre which will bring a licensed user to the reactor. Should the power failure be extended, the licensed user is able to shut down the reactor using an auxiliary power supply system.

4.5 Remote Monitoring During Unattended Operation

Although malfunction of the control system, loss of electric power and loss of purge gas flow during unat- tended operation of the reactor do not constitute hazardous situations, remote notification of such malfunctions in the reactor are considered desirable. For all these conditions a single alarm is given at the 24h Emergency Centre, which brings a licensed user to the reactor, usually within ~lh.

4.6 Loss of Pool Water

Although the Toronto area is not an abnormally active earthquake area, and the reactor pool is well constructed, the safety analysis of the system has considered the hazards associated with a complete loss of pool water during a period of unattended operation of the reactor. In fact, this incident has been investigated experimentally with the reactor operating at -250 W power. Shutting down the reactor under these conditions very quickly reduces dose rates in areas outside the reactor room to levels well below the ICRP recommended level of 0.25 mRem/h. The results of this experiment indicate that a complete loss of pool water with the reactor continuing to operate at 5 kW power, will produce total gamma and neutron dose - 29 -

rates in normally occupied rooms adjacent to the reactor of < 70 mRem/h. Comparing these dose rates with the ICRP* recommended total dose of 500 mRem to be received by an individual member of the general public over a 1 year period, we see that as long as the area about the reactor is vacated within say lh, or the reactor is shut down, radiation doses significantly less than 500 mRem will be absorbed. At the University of Toronto reactor an increase in radiation fields in rooms adjacent to the reactor room to -2.5 mR/h will give a specific remote alarm to the 24h Emergency Police service of the University; this alarm will bring police personnel to the area within 20 min. These officers will not enter the reactor room, but if necessary, will clear nearby areas of all persons.

Recommendations of the International Commission on Radiological Protection, ICRP Publication 6, Pergamon Press, 1962. - 30 -

5. THE FIRST YEAR'S OPERATION

5.1 Commissioning

Between March and June 1971 the prototype SLOWPOKE reactor which had undergone a year's experimental study at CRNL, was dismantled, shipped to Toronto and reassembled with a new fuel core in a pool that had been installed in a refurnished room within the Haultain Building of the Institute of Environmental Science and Engineering on the central St. George campus of the University. Operating licences were received from the Atomic Energy Control Board (AECB) on June 4, 1971; fuel loading was commenced and first criticality was achieved the same day. Both low and high power commissioning tests were com- pleted satisfactorily, and following approval of the facility by the AECB, operation of the reactor was formally transferred from AECL to the University of Toronto on June 25, 1971. Since that date the reactor has been in use as a almost daily and to date has generated over 7000 kW.h of energy. The greater part of the operation has been for sample irradiations and isotope production for university researchers and students. During this period extensive study has been made of reactor operating characteristics, radiolytically produced hydrogen, radionuclide activity in the gas purge stream and radiation fields about the facility, etc.

5 . 2 Operating Experience

No significant operating problems were encountered but a number of minor problems have occurred and these have been - 31 - corrected. There have been no unexpected reactivity excursions, the gas purge system has never failed, there has been no unusual loss of water from either the pool or the reactor container, and at no time during unattended operation was there any deviation from the preset power level. The safe operation and use of the facility has been controlled both by the reactor licenced operators and by the staff of the University of Toronto Radiation Control Centre. After ~ 3000 kW.h of energy generation the first adjustment of excess reactivity was successfully completed by AECL personnel by adding one beryllium plate to the top reflector. The reactor has operated mainly on an 8h-day, five day-week basis to a total of -1400 h, mostly at the maximum licenced power of 5 kW. Since November 1971, the reactor has been operated on average about 90% of the available time and has been used about 75% of this operating time for sample irradiations and isotope production. A total of 920 individual irradiations have been performed for a variety of experimenters within the chemical, physical, metallurgical, applied science, medical and environmental research depart- ments of the university as well as the teaching hospitals and some users outside the university community. The latter included the Ontario crime laboratory, New York court, scientists of two other universities and two Toronto industrial laboratories. - 32 -

Regular irradiations for the production of short- lived 21*Na, "*2K and 64Cu have been of 5 - 8 h duration whereas the average time for various other sample acti- vations was ~1 h. Many short irradiations were performed and rapid detection of short-lived radionuclides was completed within the reactor room.

5.3 Utilization of SLOWPOKE

Applications of SLOWPOKE during the first year have been varied and greater in number than expected; some were quite novel while others reflected recent developments reported elsewhere. Several users have explored research feasibility through a few irradiations while others have done more extensive studies involving a number of repeated irradiations. Regular production of 2 "*Na and kZK has been made for nuclear medicine, for administration to patients, for studies of ascites production in the Department of Medical Biophysics and for routine calibration of the Toronto General Hospital's whole body counter. Co-operative studies on the preparation of 18F-labelled glucose have also been done. Another isotope produced regularly was 61*Cu, for use by the Physics Depart- ment in positron annihilation studies. During the peak period of the university teaching session more than one hundred students used SLOWPOKE as part of assigned laboratory experiments in instrumental chemical analysis for copper and calcium, in neutron flux measurements by activation detectors and in preparation of trace nuclides for measuring surface exchange of solids and precipitates. In addition several undergraduates have used SLOWPOKE neutrons for project research extending over some weeks to a few months. - 33 -

The facility has also been visited by about 500 students and professors with varying interests in pure and applied nuclear science. In summary, SLOWPOKE has proved a very valuable asset in the central activity of the University, the teaching of undergraduate students. Other SLOWPOKE applications which emphasize its wide range of applicability are: - diffusion studies in alloys containing yttrium by means of beta counting of diffusion B9Y, assay of In in silver-indium alloys, analyses of ancient pottery fragments from the Royal Ontario Museum in order to distinguish elements such as Fe, Br, Mg, Na, Mn, Se, La, Co and rare earths, detection of trace heavy metals such as Hg, Cd, As, Se and Sb in environmental materials such as soil, river sediments, vegetation and tissues, possible routine clinical assay of trace metals in urine by neutron activation, comparison of seized narcotic specimens of cocain for a ISiew York court to establish a possible common source, survey of residual trace impurities in Connaught Laboratories final vaccine preparations, - analysis for approximately 0.01 - 0.1% chloride in industrial chromic acid pickling solution by activation to 38 min. 38C1, - development of a neutron activation technique to detect "weightless" aerosols (-0.1 g) of asbestos in the atmosphere by SLOWPOKE activation of 28Mg and 56Mn, - 34 -

evaluation of mixing efficiency in industrial equipment used for sands and crushed solids by sampling and assay for uniformity, studies on ballistic residues of barium and antimony by the crime laboratory, assessment of occupational exposure to toxic materials containing Hg, As and Se by hair activation, comparison of resonance neutron transmission analysis using Linac neutrons with thermal neutron activation in SLOWPOKE for tungsten alloys containing tantalum, hafnium and other components. Notable among the new applications that have developed during these first few months of operation of SLOWPOKE are: the detection in air pollution particulates. collected on filters, of submicrogram traces of halides from auto exhaust by means of 17 min. 80Br and 39 min. 38C1 nuclides, the controlled production of micro-holes in synthetic membranes for medical application by neutron induced fission tracks followed by chemical etching, the development of rapid instrumental neutron activation analysis by Ge(Li) counting for detecting cadmium occurring in polluted water bodies down to 1 ppb, by means of 49m lllItlCd, and traces of calcium impurity in COMINCO lead samples by detection of 9 min.lt9Ca. Further, studies with soils and sediments, with blood, urine and other fluids and chemical solutions, have been very much helped by the ability to irradiate in SLOWPOKE - 35 - damp solids, or samples containing appreciable moisture. Little pressure build-up in sample capsules has been evident, and problems due to sample heating have not been apparent. SLOWPOKE is probably the first nuclear reactor designed to operate without both conventional safety devices and a full-time operator. These features reduce significantly the capital and operating costs. Reliance on the reactor's inherent safety characteristics ensure the owner of the highest degree of safety without exacting maintenance and testing requirements. The success of the first years'opera- tion and the increasing use within the University is evidence that SLOWPOKE has a place as a low cost laboratory reactor. - 36 -

6 . UPRATING THE REACTOR TO EXTEND ITS USEFULNESS

As a result of satisfactory operating experience, high utilization and requests from present and potential users, a submission to uprate the reactor is in preparation and will be presented to the AECB for their consideration. This uprating is well within the capabilities of the reactor and will not affect the basic ideas of the SLOWPOKE concept as presented in Section 2. It is proposed to increase the operating power of the reactoi from 5 kW to 20 kW, without increasing the excess reactivity, and to extend the period of unattended opera- tion from its present limit of 4h, to 18h; this latter action permits convenient overnight irradiations to produce longer-lived isotopes such as 21*Na. It is also intended to add an extra, larger diameter sample irradiation site to the beryllium reflector where the thermal neutron flux will be ~5 x 10*1 n/cm2.s at 20 kW. Because of the limited excess reactivity of the system and its large inherent negative temperature coefficient, the reactor will be able to operate at 20 kW for only ~2h; operation at 10 kW can be maintained for ~10h. Future development of the concept will consider the feasibility of increasing the maximum excess reactivity of the system above its present maximum value of +3.4 mk; any increase in excess reactivity would permit longer operation at power levels of 10 kW and 20 kW. The elimination of the gas purge system venting to the reactor room is being con- sidered; it could be replaced by a closed system possibly incorporating a hydrogen recombiner. - 37 -

Subsequent SLOWPOKE reactors will be manufactured in Ottawa by the Commercial Products division of Atomic Energy of Canada Limited. The commercial model is designated SLOWPOKE-2. It can accommodate up to ten vertical irradiation tubes, five in the beryllium reflector and five in the water annulus outside the reflector. The Commercial Products division has been operating a SLOWPOKE-2 reactor since May 1971. It is being used for AECL's neutron activation analysis service, which is offered to the public on a commercial basis.

ACKNOWLEDGEMENTS

The authors wish to thank all those at Chalk River who assisted in the design and construction of the reactor. We also thank Mr. J.G. Thompson, SLOWPOKE project engineer, Mr. W.H. Ridge of the Radiation Control Centre, and the licenced users, Mr. H.P. Chung and Dr. R. Hancock, all at the University of Toronto. The assistance of the University's "SLOWPOKE Committee" is also acknowledged. We also acknowledge Mrs. G.D. Clark who typed the report. Additional copies of this document may be obtained from Scientific Document Distribution Office Atomic Energy of Canada Limited Chalk River, Ontario, Canada

Price - $1.00 per copy

1949-72