ML19341C526
| ML19341C526 | |
| Person / Time | |
|---|---|
| Site: | 05000192 |
| Issue date: | 01/31/1981 |
| From: | TEXAS, UNIV. OF, AUSTIN, TX |
| To: | |
| References | |
| NUDOCS 8103030693 | |
| Download: ML19341C526 (24) | |
Text
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.
. 5. REACTOR ROOM
- 5.1 DESIGN BASES The design of a structure to contain a TRIGA reactor depends on the protection requirements for the fuel elements and the control of radioactive materials. Fuel elements and other special nuclear materials are protected by physical containment and surveillance. The physical containment will also control the release of radioactive materials during routine operation or potential accident conditions. Release of airborne radioactivity from routine operation consists mostly of air activation products, while non-routine releases from a fuel element failure will consist of fission product materiais. Other potential releases may be associated with specific types of experiments. The reactor room contain-ment is designed to control the exposure of operation personnel and the
~
public from radioactive material releases caused by reactor cperation.
Release criterion are based on Title 10 Chapter 20 of the U.S. Code of
. Federal Regulations.
5.2 ROOM DESIGN The TRIGA Mark I reactor is located in a modified room of an engi-neering building that includes laboratory space, of fice and classrooms.
Room 131 of Taylor Hall contains the reactor with adjacent rooms to the north (rooms 125, 127, and 129) providing additional laboratory space and rooms to the south (133 and 135) providing office space. Normal laboratory access is from a building corridor east of the laboratory.
The laboratory is accessibic from the office areas to the south and laboratory areas to the north. An outside doubic door exit provides for equipment movement and emergencies. Only with the reactor shutdown
~
and under direct supervision of authorized personnel, will the outside doors be used. The room is of brick and reinforced concrete construction
-
5-1
!8103030 %
__ - ..
. _ _ _ _ _
I including the floor. The roof is of steci girder and gypsum construc-
, tion. Partitions are metal lath and cement plaster, fireproof construction.
,
Figure 5-1 shows the arrangement of the major pieces of equipment in the laboratory, and Figure 5-2 shows the arrangement of the pit rreative to the existing floor, ground, and outside wall of the building. The
. floor in most of the room is of 3" and 3.5" suspended concrete slab con-struction with strengthening concrete beams underneath. A small portion of the flocr along the cast side of the room is a 6" concrete slab on grade. Around the periphery of this portion of Taylor llall, including the south and west sides of the laboratory, is a service trench for various utility lines. This trench provides more space under the floor than would otherwise be available, as the floor slab is only about 3 or 4 feet above grade here and even less at other locations.
The reactor is located near the bottom of a pit below grotud level
- (see Figure 5-2). The pit is lined with an aluminum tank 0.25 inches thick. Outside the aluminum is a poured concrete shield varying in thick-
,
ness from 48 inches in the vicinity of the core to 14 inches around the upper portion of the tank. The tank assembly rests upon a 24-inch poured concrete slab as shown. The reactor is located near one end of the clongated pit with the remainder of the pit is available for experimental purposes. Three 10 foot deep storage wells provide isolated storage for fuel elements or radiation sources.
The exterior walls to the south and west are solid brick 17" thick with window sections replaced by concrete block and brick construction.
The north wall and the outer cast wall are solid brick 13" thick. The walls of the storage room and the adjacent office are of structural clay tile 6" thick. The interior surfaces of all of these walls are com-pletely covered with approximately 3/4" of gypsum plaster. Secondary
-
walls on the east side of the laboratory are of solid cement plaster 2.25" thick on steel channel and expanded metal lath. These secondary
, walls enclose the ventilation equipment that supplies other areas adjacent 5-2 i
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to the reactor room and create a restricted access entry passage to the
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laboratory (see Figure 5-3).
The ceiling height in the main part of the laboratory is about 16 feet
.
to the bottom of the steel girders and about 17 feet between the girders.
Over the east side of the room the ceiling is of saw-tooth shape and varies from about 12 to 20 feet in height. The ceiling (roof) is supported by steel girders and consists of 4.5-inch gypsum board and fill, one inch of rigid insulation, wilerproofing, and built-up roofing. The floor area of the room 131 area is approximately 1725 ft and the room volume is about 29,000 ft . Plaster and wood partition structures in the laboratory area subdivide the reactor room volume into a 24,000 f t reactor area, a 2,400 ft console area, 1,750 ft sample handling area, and 440 ft for a dark room. An additional 750 ft with a volume of approximately 12,000 f 3 is contained in the shop, entry wsy, and ventilation room.
,
. All doors to the reactor laboratory are of solid core construction 4
with intrusion alarms. Two entrances may be electronically controlled.
.
Each entrance, the utility access trench and roof area are provided with visual warning signals of reactor operation.
5.3 ROOM ISOLATION Although the original analysis by General Atomic based on experimcatal data had shown that no special requirements for a building were required to install a TRIGA reactor, several modifications have beer. made to assure reasonable control of potential radioactive martrial releases. The immediate area of the reactor room contains a clcsed ventilation system with both heated and cooled air generated in the reactor room by supplies of steam and chilled water respectively. Other utility supplies to the reactor room include heat exchanger chilled water, high pressure air, and
. elactrical power.
.
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NORTH Fig. 5-3 TAYLOR HALL FLOOR PLAN '
ADJACENT ROOMS TO 131 5- 6
- _
-
- . _ _ _ _ = _ - _ ___ . _ _ _ _ _ - . - _-_
i The ventilation air in the reactor room area (-29,000 ft3) is re-
,
stricted to the room by recirculation. Leakage of air from the reactor room directly to the environment through the emergency exit to the south is limited by weatherstripped doors that will remain closed during reactor
,
operation. Other potential direct leakage paths to the building through doors or penetrations are either weatherstripped or scaled. Besides the
, emergency exit, two laboratory staf f of fices with a volume of 4,000 f t are located to the south. Leakage from the north side of the reactor room ~
3 is into a controlled access laboratory area of approximately 12,400 f t i (plus an adjacent room with a volume of ~25,000 ft3),
Isolation of the laboratory to the east is provided by a buffer zone consisting of the entry way and adjoining ventilation equipment room.
The entry way provides two doors to restrict air leakage to building corridors. The ventilation system, whose intake is located in the 3
corridor outside the entry way, has a capacity of 15,600 ft / min.
. and ventilates adjacent offices, laboratories, classrooms, and corridors consisting of about 78,400 cubic feet.
.
An air exhaust system for the reactor room will consist of high ef ficiency particulate air filter, pneumatically operated damper, thin wall CM tube detector, 1500 cubic feet per minute fan and exhaust stack.
The exhaust outlet will be above the adjacent second story roof line.
The exhaust radiation detector will provide an isolation signal to shut off exhaust fan, close pneumatic damper, and close any room air intakes.
Air intake will be through room leakage effectively creating a negative pressure in the controlled reactor room area during exhaust fan operation.
.
A secondary low volume exhaust system vents the air from a fume hood and a glove box. The exhaust fans for each are Lanually operated and vented outside the building af ter passing through absolute filters.
,
1
.
5-7
_
__ _ _ _ _ _ _.
- - - .
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'
5.4 ROOM CONTAINMENT EVALUATION
.
Containment evaluation depends on the quantity of airborne radio-activity release possible from the air and water that are in the region 1 .
of the reactor during operation. Calculation, measurement, and experience of similar research reactors support the evaluation.
} 5.4.1. Release of Argon-41 and Nitrogen-16 from Pool Water 1
Argon-41 is produced in the reactor pool as a result of the (n,y) re-
} action with argon-40 dissolved in the pool water. Most of this argon-41 4
remains in solution but some of it is transferred to the reactor room air
- at the pool surface. Calculations based on experimental measurements pre-dict the equilibrium concentration of argon-41 generated by 250 kw operation
-0
,
will be about 4 x 10 uCi/cm3 in a room volume of 6.8 x 10 cm . This ,
measurement includes the effects of incidental releases from the rotary I
, specimen rack and pneumatic transfer tube experimental facilities.
.
The nitrogen-16 produced through the (n,p) reaction with the oxygen in the water molecule has a very short half-life (7 see) so only a very small fraction of that produced in the core will find its way.to the pool surface.
The principal radio' logical effect of the nitrogen-16 is as a contributor to i
1 the radiation level at the pool stirface (measured as approximately 50mr/hr
( at 250 kw).
l l
- 5.4.1.1. Argon-41 Activity in Reactor Room. The argon-41 activity in the reactor pool water results from irradiation of the air dissolved in the
, water. The following calculations were performed to evaluate the rate at which argon-41 escapes from the reactor pool water into the reactor room.
The calculations show that the argon-41 decays while in the water, and
,
,
most of the radiation is safely absorbed in the water. The changes in
, argon-41 concentration in the core region, in the pool water external to
-
the reactor, and in the air of the reactor room, are calculated using the variables as defined below:
- i
.I
+
5-8 s
_ . . . _ _ . __
V volume of region (cm )
N atomic density (at./cm )
- A decay constant (sec_1) o absorption cross section (cm ) - 0.61 b 33
- "
-
q volume flow rate from reactor room exhaust l w mass flow rate (gm/sec) see) p density (gm/cm3) v volume flow rate through the core
( 3) f average thermal neutron flux in the core (n/cm -sec)
The volume flow rate through the core is
.E. 3.5 x 10 g/s 3 y
P
= 3.5 x 10 cm /sec 1 g/cm 3 From the flow channel volume, fA 1, the exposure time in the core is t = V/V = A ge E /v = 46.0 x 38.1
= 3.8 sec 3.5 x 10 3
.
It remains to find the atom density h for dissolved argon-40 in the reactor pool water.
According to Henry's law for gases in contact with liquids the equilibrium concentration in the liquid is proportional to the partial pressure of the gas. The saturated concentration of argon in water at one atomsphere of standard air is given in table 5-1.
The argen-40 concentration (N) in the water at the core inlet tempera-ture (32*C) is 15 3
,
N = 7.7 x 10 ,g At 48.9 C, the core exit-water temperature, the concentration of
~
argon in 110 is 6.3 x 10 2
at./cm .
5-9
TABLE 5-1 SATURATED ARGON CONCENTRATION IN WATER (Ref. 1)
.
_
Temperature 3
. (*C) S(atoms A-40/cm H 2O) 16
,
10 1.14 x 10 16 20 0.94 x 10 6
30 0.79 x 10 16 40 0.69 x 10 16 50 0.62 x 10 16 60 0.56 x 10 16 70 0.52 x 10 10 80 0.48 x 10 The argon-41 density (at equilibrium) at the exit from the core is given by
~
A, = Ag e" + N o$ (1 - e~ E) ,
'
and at the entrance A,= A,e~ ,
where t is the expositre time in the core (3.8 sec), and T is the cycle time in the pool.
The average out-of-core cycle time T is given by V
T=l= 4.06 x 10 cm 4
= 1. 6 x 10 sec, 9 3 3 3.5 x 10 cm /sec whe re V is the pool volume and v is, again, the volume flow rate through the core . The solution to tlis set of equations is
~ '
-At
^= = No)
, A(t+T) ,
-
5-10
_ _
_ . . . - . . . _ . _ . _ - . , . - ~ . - - . - _ . . . . .. .-. . . - - . - - -
Substituting the valuec from above one obtains
. A ,=-(7.7 x'1015) (0.61 x 10-24) (0.38 x 10 3) j exp -(1.06 x 10~ ) (3.8)]
~
1 X -
4-1 - exp
_
-(1.06 x 10-4) 1.16 x 10 ) _
'
A = 10.2 dps/cm ,
4 3 j and N 43
= 10.2 = 9.58 x 10 atoms /cm .
~
- 11.06 x 10 '
f 1
>
One source of argon-41 in the room results from the reduced solubility of argon in water.as the temperature increases. Considering the expected
'
temperature rise of the water passing through the core, an immediate release of about 18% of the argon-41 made could be expected during passage. Some
,
,
of this argon-41 might be redissolved as it is transported into cooler
! water, but since the coole. aater is in equilibrium with the air above, it is nearly saturated with argon and will not absorb all of the argon
- released. Measurements of argon-41 in the water as a function of height
- above the core-indicate that approximately 60% of the released argon-41 is reabsorbed.
1 A combination of the two sources of argon-41 mentioned above will give the upper limit of the fraction of total argon atoms that can leave the
,
water per second.
1
. Assuming that the 7% (i.e., 40% of 18%) of the argcn-41 comes out of 1
j solution, remains undissolved after leaving the core, and escapes to the
- air, this source would be
,
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5-11 4
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. _ _ _ - _ _ _ - , _ .. _ - _ - . . - - . - - , _ - - . . - . , , _ . , . . , - _ . . - __ _ ._. . - - . - ,
S = 0.07 N 41 *
, , , ,
- #E#** **
= 0.07 x 9.58 x 10 x 3.5 x 10 sec,
, em ,
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= 2.35 x 10 g sec ,
The tendency of the balance of the argon activity in the paol to escape to the air owing to its proximity to the water-air boundary will constitute the additional source of argon-41 at the water surface.
Estimates of the surface exchange coefficient S(i.e., the gas in a unit volume that is exchanged at the surface per unit time per unit surface area) for argon vary considerably. One method of arriving at a value for this parameter is through the diffusion coefficient of the gas in water. The mean square distance traversed by a molecule is
. __.n AX~ = 2Dt ,
'
where D = diffusion coefficient (cm /sec),
t = time (sec).
The exchange coefficient is assumed to be S = (5X ) /t= (2D/t) evaluated for t = 1 sec. The diffusion coefficient at 40 C is about 1.1 x
-5 2 10 cm /sec, and, if one assumes that only one-half of the argon atoms within one diffusion length of the surface escape, S
=f(2x1.1x10 )12 = 2.35 x 10~ cm/sec
.
~
5-12
Measurements have been made of the argon-41 activity in a TRIGA Mark III reactor pool and from the data acquired f roe these measurements it was
.
possible to construct a value for the surface exchange coefficient. This value at 40 C is about 2.9 x 10~ cm/sec.
.
Values for the surface exchange coef ficient have been reported by Dorsey (Ref. 2) for air, 0 , and N . The values for these three gases are all about equal. Assuming argon behaves as do these gases, a value for 6 of 5.7 x 10~ cm/ set is obtained.
In this analysis the largest of the three values was used for the surf ace exchange coef ficient, i.e., 5.7 x 10~ cm/sec.
The rate at which argon atoms are transferred across the water-air interface is determined by
, S2 = 0.93 SN31Ag
~3
. = 0.93 x 5.7 x 10 x 9.58 x 10 x 7.01 x 10'
= 3.56 x 10 atoms /sec ,
4 2 where Ag is the surface area of the pool (7.01 x 10 cm ).
The total transfer rate of argon-41 nucici to the reactor room is 7 7 nuclei S = 2.35 x 10 + 3.56 x 10 = 5.93 x 10
=S1+S2 sec .
In air-filled volumes ' rom which air is removed at the rate of q cm /sec, the accumulation of argon-41 as a function of operating tice is given by
=
S /V - (A + q/V)N ,
-
where S is the source of radioactive atoms released to the air.
5-13
_
Integrating from N = 0 at t = 0, one obtains 41 -
N 41(t) =
S /V 1- exp -(A 41 + q/V)t
.
.
A^1 + q/V - -
"
For prolonged reactor operation at a steady power level in which t > >1/(A + q/V) ,
the source term and removal rates of argon-41 are in equilibrium, in which case 01 41 S /V 3 N =
at./cm .
A47 + q/V The argon-41 concentration in the reactor room and buildin, exhaust air
-
S/V R S
~
"R A+ q/V 9 R R
.
3
= 5.9 3 x 10 = 652 atoms /cm
~
1.06 x 10 ' x 6. 8 x 10 + 1.89 x 10 8 3 where the ef fective room volume is 6.8 x 10 cm and a room air exhaust 4 3 rate
- is 1.89 x 10 cm /sec. This corresponds to an activity concentration in the room of A= = 1.06 x 10- x 652 -6
= 1.87 x 10 C1/cm .
3.7 x 10
, *The room exhaust rate assumes that the air exchange rate in an unventilated room is at least 10% of the volume per hour.
.
5-14
i 5
For operation of a laboratory ventilation system at 7.08 x 10 cm 3/sec (3.75 volume exchanges per hour) the previous analysis predicts a room activity of 2.18 x 10~ pCi/cm and the activity discharge rate would be
-
Aq = 2.2 x 10 ~ x 7.1 x 10 = 0.16 pCi/sec = 1.6 x 10~ Ci/sec .
The concentration downwind from the point at which the activity is discharged from the building is A = Aq $(x) ,
D where $ = the dilution factor at the distance x, (sec/m ),
A = the activity concentration in the discharge (Ci/m ),
q = the building exhaust rates (m /sec).
The dilutio- factor in the lee of the building (x=0), if it is assumed that the disch. is at the roof line, is given (Ref. 3) by:
.
t(0) = csu
.
where c = a constant (0.5),
s = building cross-sectional area normal to the wind direction (m ),
u = wind velocity (m/sec).
The minimum cross-sectional area is 47.6 m (32 x 16 ft at laboratory roof Icvel) and, for a wind velocity of 1m/sec,
"*
$(0) = 1 = 4.2 x 10~
3
.
0.5 x 4 7.6 x 1.0 m The maximum argon-41 concentration outside the building would be AD = 1.6 x 10~ x 4.2 x 10~ = 6.7 x 10 ~9 Ci/m .
.
i 5-15
_
The whole body gamma ray dose rate to a person immersed in a semi-infinite cloud of radioactive gases can be approximated by
.
D = 900 EA D
.
where E = the photon energy.
Thus, the maximum downwind dose rate resulting from discharge of argon-41 produced in the reaction is D = 900 EA D
~
= 9^9 x 1. 3 (6. 7 x 10 ')
-6
= 7.8 x 10 rads /hr = 7.8 prad/hr .
I.cakage of laboratory air to adjacent areas of the building without a laboratory ventilation system operating would be diluted by the high flow rate of the adjacent area ventilation unit. Assuming 100% leakage
'
of laboratory air to adjacent areas at a rate of 2 air volume exchanges per hour the previous analysis yields a leakage source term of 132 argon-
-
41 atoms /cm or 3.78 x 10~ pCi/cm . The activity discharge rate to adjacent areas is Aq = 3.78 x 10~ x 3.78 x 10 = 1.43 x 10~ Ci/sec The ventilation flow rate of adjacent building areas provides a dilution factor, t, of (7.08 m /sec)~ . Argon-41 concentration in leakage air is Aq $ = 1.43 x 10~ /7.08 = 2.02 x 10~ pCi/cm .
The recirculacion of adjacent area air results in a possible maximum concentration of (24000/78000) x 3.78 x 10~ = 1.17 x 10~ pc/cm .
o Averaged over one year of 250 kw full power operation 5 days / week, 8 hrs / day the minimal averaged concentration is
- -7 1.17 x 10 x (5/7) x (8/24) = 2.79 x 10
-8 cjc,3 ,
5-16
.
The average dose rate represented by the leakage would be
. D = 900 x 1.3 (2.79 x 10-8) = 3.26 x 10~ rads /hr,
, or an average of 32.6 prad/hr, and a peak of 137 prad/hr.
De actual effect of argon-41 releases from the reactor pool would be substantially less than those estimated as a result of the various conservative estimates in the calculation. Among the major conservative assumptions are the transfer amounts of argon from the pool surface, period of full power operation, release rates and volumes.
S.4.1.2. Nitrogen-16 Activity in Reactor Room. The cross-section threshold for the oxygen-16 (n,p) nitrogen-16 reactions is 9.4 McV; however, the minimum energy of the incident neutrons must be about 10.2 MeV because of center of mass corrections. This high threshold limits the production of nitrogen-16 since only about 0.1% of all fission neutrons
, have an energy in excess of 10 MeV. Moreover, a single
- hydrogen scattering event will reduce the energy of these high-energy neutrons to below the
- threshold. The effective cross-section of oxygen-16 (n,p) nitrogen-16 reaction averaged over the TRIGA spectrum is 2.1 x 10-29 cm2 . This value agrees well with the value obtained from integrating the effective cross section over the fission spectrum.
The concentration of nitrogen-16 atoms per em of water as it leaves the reactor core is given by N O -A
$".
N, = 1-e ,
where N = nitrogen-16 atoms per cm of water, 2
12
& = neutron flux (0.6 - 15 MeV) = 3.0 x 10 n/cm -sec, 3
N 1
= oxygen atoms per cm of water = 3.3 x 10 atoms /cm ,
ay =
(n,p) cross section of oxygen = 2.1 x 10 -29 cm 2 (averaged over 0.6 - 15 MeV),
-
A = nitrogen-16 decay constant = 9.35 x 10~ -1 2
sec ,
t = ave rage time of exposure in reactor.
5-17
- - - _ _ _ _ _ _ _ -
The average exposure time in the core (3.8 sec), was derived in the discussion on argon activity. Solving for N in the equation above, 2
one obtains
.
-2
-9.35 x 10 x 3.8 N = 2.22 x 10 1-e = 0.66 x 10 at cm ,
2 as the density of nitrogen-16 in the water leaving the core.
If it is assumed that the water continues to flow at the same velocity to the surface, a distance of ~457 cm, the transit time from core to surface is
"
457 t = 50 sec rise 9.2 ,
where the flow velocity, 9.2 cm/sec (Tabic 3-7), was given in the discussion on heat t ransfe r.
This assumption is quite conservative as energy losses from the fluid
-
stream resulting from turbulent mixing will reduce the velocity sigulficantly.
Furthermore, delays in transit time resulting from operation of the diffuser
, pump are sizeable. Measurements made of the dose rates at the pool surface of several TRICA reactors show that the operation of the diffuser pump reduces the nitrogen-16 contribution to the surface dose rate by an order of magnitude of more depending on the size of the pool.
-3 In 50 seconds the nitrogen-16 decays to 9.25 x 10 times the value of the activity leaving the core. Thus the concentration of nitrogen-16 atoms that reach the region near the surface of the pool is no greater than about 6.10 x 10 at/cm .
Only a .wmall propottion of the nitrogen-16 atoms present near the pool surface are transferred into the air of the reactor room. When a nitrogen-16 atom is formed, it appears as a recoil atom with various degrees
- of ionization. For high-purity water (approximately 2 pmho) practically
.
5-18
all of the nitrogen-16 combines with oxygen and hydrogen atoms of the water.
Most of it combines in an anion form, which has a tendency to remain in
, the water (see Ref. 4) . It is assumed that a least one-half of all ions formed are anions. Because of its 7.1-sec half-life, the nitrogen-16
. . decays before reaching a uniform concentration in the tank water. The activity will be dispersed over the surface area of the pool and much of it will decay during the lateral movement.
For the purpose of the analysis it is postulated that the water-bearing nitrogen-16 rises from the core to the surface and then spreads across a disk source with an equivalent radius of 150 cm. For a constant velocity of 9.2 cm/sec the cycle time for distributing the nitrogen-16 over the pool surface would be t s= 150 cm/9.2 cm/sec = 16.3 sec .
The average concentration during this time is
.
N =
ft" Ne ~
\
dt /t o s ko Y N { 6.10 x 30 4
-9.35 x 10
-2 x 16. 3
= 'l - e ~AE s i = y_ ,
s ( ) 9.35 x 10~ x 16.3
( )
=
3.1 x 10 atom /cm .
The thickness of the layer of nitrogen-16-bearing water is
" * * *
- h= = = 0.81 cm ,
s 7.1 x 10 3 3 where the volume flow rate 3.5 x 10 cm /sec was given in the discussion on heat transfer.
.
.
5-19
.
The dose rate at the pool surface arising from the nitrogen-16 near the surface is
. -
A5 D = 2 pK ~ 2 (ph) ,
- -
, .
where p is the attenuation coefficient for 6 MeV photons in water (0.0275
~
cm ).
K is the flux to dose rate conversion l 1.6 x 10 5 photons rad hr
( cm /see and E 2 is the second exponential integral. 'diisyields,approximately, D = 32 mr/hr .
This is a value comparable to those which can be extrapolated from measurements made on other TRIGA reactors.
.
The i n t e re s t from the point of safety is then the number of nitrogen-16 atoms escaping into the air from the diffusing surface source above
- the core. The number escaping to the air would be about (3.1 x 10 atoms /cc) (0.9 x 10~ cm/sec) = 279/cm -sec ,
-2 where the escape velocity 0.9 x 10 cm/sec is from Dorsey (Ref. 5).
In the room, the activity is af fected by dilution, ventilatiot.,
and decay. Thus the rate of accumulation of nitrogen-16 in the room as a whole is given by
= s - (A + q/V) N V ,
-
where S = number of nitrogen-16 atoms entering the room from the pool per 2 4 2 7 -1 second (279/cm ) (7.1 x 10 cm ) = 2.0 x 10 sec ,
8 3 V = volume of the reactor room (effective) = 6.8 x 10 cm ,
4 3
-
q = volume f low ra t e f rom the reactor room exhaust = 1.9 x 10 cm /sec 5-20
For saturation conditions
.
7 yn 16 , S ,
2 x 10 0
- 2.1 x 10 nuclei .
-5
,
(A + q/V) 9.35 x 10 + 2.8 x 10 This corresponds to an activity concentration of 7.8 x 10
~
pc/cm .
The gamma dose rate from this concentration of nitrogen-16 in the air I8 3.7 x 10 photons x 7.8 x 10~ (pc/cm ) x 546 cm D= sec pc 2 x 1.6 x 105 (photons /sec cm / rad-hr)
= 4.9 x 10 -5 rad /hr = 49.2 prad/hr ,
'
when the effective radius of the room, taken to be a hemisphere with 8 3 a volume of 6.8 x 10 cm , is 546 cm.
.
5.4.2. Activation of Air in the Experimental Facilities In the TRIGA reactor installation, the following experimental facili-ties contain air: pneumatic transfer tube, rotary specimen rack, and vertical beam tubes. Of the radioisotopes produced in these air cavities, argon-41 (1.83 hr half-life) is the most significant with respect to airborne radioactivity hazards. Nitrogen 16 (7.11 sec half-life) and oxygen-19 (26.9 see half-life) are considerably less significant. In saturated activity of argon-41 in an experimental cavity is A =N 4 lA41 , ___ A S CI/cm ,
- C(A + q/V) i
.
where c = 3.7 x 10 disintegrations /sec-pci, and the source term S is
$
3
~[a n/cm -sec.
5-21
- - - . _ . - . -- . .
_- .- - . . _ . - . ~ _ - - - . - - . . .
1
.
e a
d'
, The effective air volumes of the various experimental facilities are
- listed in Table 5-2. Also given are conservatively high estimates of the
'
"
average thermal neutron fluxes for 250 kW operation (E 41 = 1.59 x 10-7 cm
-l)*
,
'
[ TABLE 5-2 VOLUMES AND TilERMAL FLUXES OF FACILITIES' Effective Average Thermal 4
Air Volume Flux at 250 kW j Region (cm3) (n/cm2 -sec) 4 Rotary Specimen rack 3.3 x 10 1.8 x 10 3
Pneuentic tube 1.6 x 10 1.8 x 10 12 1
Vertical beam tube 3.6 x 10 0.5 x 10 12 Central thimble 5. 3 x 10 3 1. 3 x 10 At prolonged 250 kW operation with no ventilation, the argon activity in the pneumatic transfer tube is 1.2 x 10 pC1, 4 and in the 5
. rotary specimen rack it is 2.5 x 10 pC1. Measurements at the Torrey l Pines TRICA Mark I reactor indicate no measurable argon-41 release from i the rotary specimen rack.
,
,
Discharging all of the equilibrium argon-41 produced in the pneumatic 8
f
transfer tube into the reactor room (with a volume of 6.8 x 10 cm )
-Sr
[ would produce an average concentration of 1.8 x 10 pCi/cm3 . Venting 1
the rotary specimen rack to the room would produce a concentration of 3.7 x 10 pCi/cm .
I i
In an experiment performed with a TRICA Mark I reactor, air from
'
the pneumatic transfer tube and the rotary specimen rack was discharged
- . into a reactor room and the argon-41 concentration in the room was found
,
to be well below the maximum permissible concentration (MPC) for restricted ,
-6 areas (2 x 10 pC1/cm3 ). The experiment consisted of operating the 1
'
}
l . reactor at 100 kW for several hours, discharging the air from the pneumatic
- .
I transfer tube into .the reactor room (2.6 x 10 9cm 3) and lowering and j
.
raising a sample container in the access tube to pump out some of the air
,
5-22 l .. ,- -
from various positions of the rotary specimen rack. Based on these 8
results the concentration expected for a room volume of 6.8 x 10 cm3
. is estimated to be less than 2.5% of the maximum permissible concentration for restricted areas. The argon-41 produced in the pneumatic transfer
. tube and the rotary specimen rack that has been observed indicates that the amounts discharged from them is very small when averaged over time.
If these facilities are heavily used or used in such a way as to release large amounts of argon-41, this should be considered.
A modification of the pneumatic transfer system to a closed loop containing carbon dioxide allows for operation of the inlet / outlet terminal in a restricted room volume. Calculations indicate saturated activities of 7.7 x 10" pC1/cm of nitrogen-16 and 0.2 x 10~ pCi/cm of oxygen-19 may be produced in the outer ring of the core matrix.
The total activity that could be produced in the pneumatic tube during steady-state operation is therefore,12.6 pC1. Additional calculations show that the total activity induced in the CO during a $2.00 pulse is 2
'
60 pC1. An instantaneous release of this activity to a restricted room volume of 4.96 x 10 cm would yield a concentration of 1.21 x 10 -6 pCi/cm ,
. which is less than 2.5% of the maximum permissible concentration for restricted areas.
.
i l
- 5-23
)
Chapter 5 References
-
- 1. Dorsey, N.E. , Prope rties of Ordinary Water-Substance , Reinhold Publishing Corn., New York p. 537.
. 2. Ibid., p. 554.
- 3. Slade, D .ll . , (ed.), " Meteorology and Atomic Energy," USAEC Reactor Develop. and Tech. Div. Report TID-24190, DFSTI, Springfield, Virginia, 1968.
- 4. Mitti, R.L. , and M.ll. Theys, "N-16 Concent ration in EBWR,"
Nucleonics p. 81 (1961).
- 5. Dorsey, N.E., op cit., p. 554.
.
O
.
- 5-24
,
l l