ML19341C523

Chapter 4 to Univ of Tx Triga Mark I RSAR, Rcs.

ML19341C523
Person / Time
Site:	05000192
Issue date:	01/31/1981
From:	TEXAS, UNIV. OF, AUSTIN, TX
To:
References
NUDOCS 8103030691
Download: ML19341C523 (8)

Text

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4. REACTOR COOLANT SYSTEM TRIGA is designed for operation with cooling provided by natural con-vective flow of demineralized water in the reactor pool. The suitability of this type of cooling at the power levels for this TRIGA has been demonstrated by numerous TRIGA installations throughout the world.

The prinary functions of the coolant system are:

1. To dissipate heat generated in the reactor

2. To provide vertical radiation shiciding over the core lleat dissipation is satisfied by natural convective flow of pool water through the reactor core and forced circulation of the pool water through

. an external heat exchanger. Vertical shielding is provided by 16 ft of water above the reactor core structure.

Other functions provided by the coolant system are:

1. Minimize corrosion of all reactor components, particularly the fuel elements

2. Maintain a minimal level of radioactivity in the reactor pool water

3. Maintain optical clarity of pool water The coolant system is supported in these three functions by a purification 3 circuit.

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4.1 DESIGN BASES The design basis for the reactor coolant system is predicated on its primary function, reactor cooling. Other coolant system functions establish .

the design bases for the purification circuit.

To assure adequate reactor cooling, the effectiveness of natural con-vective cooling has been evaluated with respect to the peak heat flux which may be achieved in the reactor. This evaluation then establishes the maxi-mum heat flux heyond which fuel element cladding integrity cannot be assured.

Based on these evaluations, which were discussed in Section 3, it is concluded that for steady-state operation the coolant inlet tempsrature and maximum heat flux at which fuel clad integrity is no longer assured is as shown in Fig. 3-22 on the curve relating heat flux and coolant temperature for the hottest channel. The design temperature of the coolant system, coolant inlet temperature, is 90*F. The maximum allowable peak heat flux at this temperature is 395,000 Btu /hr-ft2corresponding to a power level of 1700 kW (see Section 3.1.3). Since the maximum licensed power level is 110%

of design 'or 275 kW, the resulting maximun heat flux will be 61,900 Btu /hr-ft which is well below the value at which clad integrity may be questioned.

4.2. SYSTEM DESIGN Principal components of the coolant system are the aluminum reactor pool tank and the external cooling loop consisting of a pump and heat exchanger. A typical flow diagram for the system is shown in Fig. 4-1.

The total conlant volume in the reactor tank and cooling circuit is approx-imately 11,300 gallons.

4.2.1. -Syctem Operation The coolant pump suction pipe extends 3 f t below the top of the tank.

This limits the minimum level of the pool water with accidental pumping or 4-2

Centrifugal Pump 120 gpm Orifice Flowmeter ,

DY 66 F OT -13 psiAP p

12 psi 74op Thermocouple for Secondary 32 psi f Water Control V I

ap 250 KW Heat Exchanger -

Alarm E

48 psi 45 Psi

}7 Reactor Pool 40 F From University To University Chilling Chilling Fig. 4-1 PRIMARY COOLING SYSTEM

siphoning of water through the suction line. A summary of coolant system design conditions is given in Table 4-1. Valves provided in the external cooling circuit permit maintenance and removal of components as required.

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Coolant is returned to the reactor pool through a discharge pipe which ter-minates in a dif fuser nozzle approximately six feet above the reactor core.

Loss of pool water through the suction lines is prevented by 1/2-in.-diam siphon break holes located approximately one foot below normal water level.

TABLE 4-1 REACTOR COOLANT SYSTEM DESIGN SUl! MARY Reactor Tank Material Al plate, 1/4 in.(0.635 cm) min thick Water volume 11,300 gal (42,600 liters)

Coolant Pump Type Centrifugal (1-stage)

Materials Stainless steel Flow rate 12 gpm (7.6 1/sec)

Heat Exchanger Type Shell-and-tube Materials Shell Carbon steel Tubes 304 SS Heat duty 875,000 Btu /hr (250 kW)

Temps./ Pres.

Shell inlet 40 F (4.4)/48 psi (331 kPa)

Shell outlet 69 F (20.5)/45 psi (310 kPa)

Tube inlet 74 F (23.3)/32 psi (220 kPa)

Tube outlet 66 F (18.9)/12 psi (83 kPa)

The purification system is separate from the coolant system and is

. operated independently. A typical flow diagram for the system is shown in Fig. 4-2. Coolant purification is performed by circulating about 10 gpm of 4-4

32 psi l

~37 gpm -

GM Water Monitor Bypass l

Filter Centrifugal Pump Bypass Line n f,- 30 psi

(-

i v4 Z ~32 g,pm y i

e 1 8

" Freon j2 k)70F Water l Chiller X

y } - . - 50"Hg Flowmeter Purification

-5 gpm Filter Reactor 0"Hg Demineralizer Pool -

O O Thermister Conductivity O

Conductivity Cell Cell Fig. 4-2 WATER PURIFICATION SYSTEM

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coolant from the reactor pool through a pool surface skimmer, a small pump, a filter and mixed bed demineralizer, and back to the reactor pool. The filter removes particulate matter not retained in the surface skimmer while r the demineralizer removes dissolved material to maintain proper conductivity and radioactivity levels. The effectivenss of the demineralizer is measured by conductivity probes on the inlet and outlet side of the demfaeralizer.

Also included in the loop are a flowmeter radiation monitor, auxiliary i

filter, and supplementary cooling unit.

l 4.2.2 System Instrumentation j

tionitoring of the temperature of the coolant is provided by temperature i probes in the cooling and purification loops. Probes are mounted on inlets and outlets of the heat exchanger at the purification system inlet, and under the reactor bridge to measure bulk water temperature. The temperature

] probes are of various types and are indicated locally. The bulk water probe and a pool icvel monitor are connected to an alarm and annunicator light at the reactor console.

1 The pressure difference between the secondary cooling water and the primary reactor pool water in the heat exchanger is also monitored. To prevent pool water from entering the chilled water supply, the pressure at the chilled water outlet will be maintained in excess of the pressure at the pool water inlet to the heat exchanger. Pressure sensing probes connected to a differential prnrvure monitor activate an annunicator light and alarm when the dif ferential pressure drops below a preset level.

Water conductivity and demineralizer operation is monitored by two

conductivity probes and a Wheatstone bridge circuit. Each probe consists of a platinum-electrode conductivity cell shielded with glass. Each cell is connected by two conductors through a selector switch to the bridge circuit continalng a one-stage amplifier, an electric tuning-eye, and a

. recti fier power supply. The selector switch permits the operator to connect the desired conductivity cell to the bridge circuit. A thermister tempera-4

- ture probe provides for bridge temperature comper.sation.

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pool water radioactivity is monitored by a GM tube located in a tank of water in the inlet to the purification system. An alarm at the console 3

indicates an increase of radioactivity, in pc/cm , above calibrated values  !

normally observed during operation. Radioactivity in the reactor coolant  !

is also measured indirectly by the continuous air monitor and area monitors provided in the reactor room, e 4.3. WATER SYSTEM DESIGN EVALUATION

The water system including the reactor pool and the external cooling

and purification loops has the same design featurca as used in many other f operating TRIGA facilities. The demonstrated capability and integrity of this system provides assurance that the coolant system will perform its

, function properly and safely.

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Adequacy of reactor cooling is assured by the large amount of cooling capacity inherent in the reactor pool volume as well as the capacity of the external cooling circuit which can dissipate heat.at a rate equivalent to 250 kW steady-state operation. Without axternal cooling or other heat

, loss the bulk pool temperature will rise approximately 8 F af ter one hour of operation at a steady-state power level of 250 kW(t).

Availability of pool water for cooling and vertical shielding is assured by designing the system with siphon breaks on suction lines and dis-charge lines 6 ft or more above reactor core. Creater losses of pool water are extremely improbable, although they could conceivably be initiated by rupture of the reactor tank. As shown in the loss-of pool water accident analysis, even with complete loss of pool water fuel clad integrity is not

! threatened.

1 i Experience with this purification equipment in other TRIGA systems has

shown that coolant conductivity can be easily maintained at levels of less than six micromhos per centimeter using the materials contained in the

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, coolant system design. Furthermore, this experience has shown that no 4-7 l

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apparent corrosion of fuel clad or other components will occur if the con-

ductivity of the water does not exceed six micrombos per centimeter when averaged over a 30-day period.

I Control of radioacitivty in the coolant is provided by the purification

. system. Should radioactivity be released from a clad leak or rupture of an experiment, detection of the release would be signaled by the GM we.ter i monitor, reactor room area monitors, and by the continuous air monitor.

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Based on coolant transport time calculations in the safety analysis section, i

i these monitors should register an increase in coolant radioactivity within approximately 60 seconds of the time of radioactivity release. This re-sponse tine is more than adequate to permit room evacuation and ventilation system shutdown without excessive exposure. Subsequent reduction in coolant activity levels will occur through radioactive decay and removal of soluble radioactivity by the demineralizer in the purification loop.

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