CHAPTER 9
9-1
DESIGN AGAINST RADIATION HAZARDS
Radiation Shielding
Out of all these radiations, the ones of importance in shielding analysis are neutral
particles: photons and neutrons since the stopping powers of charged particles are very
big while those of neutral particles are not. In particular, the photons are major concern
in practice since neutrons mostly exist in and around nuclear reactors.
In radiation shielding, we should have the information as follows:
1. source specification: S (r, E, , t )
-
strength (particles emitted per second)
-
energy spectrum
-
angular distribution
-
spatial distribution
-
time dependence
2. source-shield-detector geometry
-
geometrical configuration
-
material composition, and densities of material
-
any simplifications (ex. heterogeneous  homogeneous)
3. data collections
-
cross sections
-
attenuation coefficients
-
buildup factors
-
secondary particle generation
-
response functions (dose factors, damage factors, etc.)
4. choice of computational techniques
-
available computer programs
-
complexity of the problem
-
accuracy requirements
5. economic factors
-
choice of shield materials
-
safe vs. penalty
-
fixed vs. mobile shields
-
advantages of using existing structural materials
6. measures
-
verification by actual measurements
-
compare with other data
7. others
-
requirement of thermal insulation
-
erection methods
-
inspection and test procedures
-
streaming through penetrations (labyrinths, etc.)
-
structural requirements
8. shield materials
material
lead
iron
concrete
density (g/cm3)
yield strength
(psi)
tensile strength
(psi)
maximum
temp. (oC)
radiation
damage
remarks
11.35
860
7.87
25,000
2.2~2.4
-
1,900
40,000
327
(melting)
nil
shrinks
about 4% in
volume on
solidification
heavy
concrete
3.7~4.8
-
water
1
-
polyethylene
0.95
-
<500
-
2,800
1,535
(melting)
small
93 (dehydration)
negligible
100
(boiling)
small
40
(softening)
significant
strong
inelastic &
prompt
capture ’s
on neutron
careful curing required
(7-28 days)
container
required
9-2
Ventilation
The appropriate ventilation system reduces the airborne radioactive material in the
enclosure. For example, the systems that we could consider for ventilation in a nuclear
power plant are as follows:
-
containment purge or ventilation for operator access
-
PAB ventilation for operator access
-
Control room ventilation for habitability
1. containment atmosphere
In general, the containment ventilation or purge systems is provided to control the
airborne radioactive material before access of personnel. The system is briefly described
as follows:
containment, Vc
direct vent exhaust
filtered
reactor
discharge
recirculation
Let,
lpc = primary-to-containment leak rate during normal power operation,
Vc = containment free volume, ft3
Fx = containment vent or purge rate, ft3/hr
Fr = containment recirculation flow rate, ft3/hr, and
 = recirculation and exhaust filter (HEPA-Charcoal-HEPA) efficiency.
The rate of change in airborne containment activity of nuclide i is
dK i (t )
  pc Ci  Rri  i  Rx K i (t )
dt
(9-2.1)
where,
Ki = activity of nuclide i in the containment, Ci
Rri = containment recirculation cleanup rate, vol/hr;
Rri   i
Fr
Vc
Rx = containment vent-out rate, vol/hr
Rx 
Fx
Vc
In an equilibrium state,
the activity concentration (Ci/c.c.) in the containment air
becomes:
Ki 
 pc Ci
(9-2.2)
(i  Rri  Rx )Vc
Example 9-2.1: Containment purge. Consider a single closed containment of a typical
PWR plant. Prior to the access to containment inside following plant shutdown, it is
required to purge the containment air to the environment. No recirculation cleanup of
containment air operates during power operation. Assume the containment free volume
is 5.7x1010 c.c. Find the equilibrium activity of I-133 in the containment just prior to
purge.
Data:
Primary-to-containment leak rate = 8x10-6 of reactor coolant per day
Reactor coolant activity of I-133 = 1.4x10-1 Ci/g
Solution:
The activity accumulated inside the containment right before purge is give by:
Ki 
 pc Ci
(i  Rr  Rx )Vc

 pc Ci
iVc
(8 x10 6 / day )(1.4 x10 1 Ci / g )(5.5 x10 5 lb )( 453.6 g / lb )
Ki 
 4.278 x10 9 Ci / c.c.
1
10
3
(0.0476 hr )( 24 hr / day )(5.7 x10 cm )
2. PAB (primary auxiliary building) atmosphere
CVCS
Fv: vent rate
BRS,etc.
primary auxiliary bldg., VPAB
The primary auxiliary building encloses the several systems which handle the primary
coolant. Typical systems are CVCS (chemical and volume control system) and BRS
(boron recycle system).
The rate of change in airborne PAB activity of nuclide i is
dPi
  pp Ci ( PF ) PAB  i Pi  Rv Pi
dt
(9-2.4)
where,
Pi = activity of nuclide i in the PAB, Ci
 pp = primary-to-PAB leak rate, lb/day (normally, 160 lb/day)
Rv = PAB vent exhaust rate, vol/hr
Rv 
Fv
.
VPAB
(PF)PAB = partition factor in PAB which is dependent on the RC temperature in the PAB.
In an equilibrium state,
the airborne concentration of radioactivity in the PAB
atmosphere is:
Pi 
 pp Ci ( PF ) PAB
(i  Rv )VPAB
(9-2.5)
Except very short-lived isotopes; Rv>>I, and the airborne concentration of radioactivity
in the PAB atmosphere becomes:
Pi 
 pp Ci ( PF ) PAB
RvVPAB

 pp Ci ( PF ) PAB
Fv
Example 9-2.2: PAB atmosphere. Consider a continuously vented PAB. No
recirculation cleanup during power operation.
Data:
PF for I-131 in PAB = 0.005
Ventilation exhaust rate = 5x109 c.c./hour
Solution:
Pi 
 pp Ci ( PF ) PAB
Fv
(160lb / day )(1.4 x10 1 Ci / g )( 453.6 g / lb )(0.005)

(5 x10 9 c.c. / hr )( 24hr / day )
= 4.23x10-10 Ci/c.c.
9-3
Radioactive Waste Management Systems
1. waste gas processing system
The process systems used for radioactive gaseous waste treatment in a nuclear power
plant are as follows:
-
decay tanks for holdup and decay,
-
charcoal delay systems for delayed discharge,
-
cover gas recycle to VCT (volume control tank) for recycle back to the primary
coolant, and
-
cryogenic distillation for volume reduction using low temperature separation.
The input to the gaseous waste system (GWS) are from:
-
degassing of reactor coolant following cold shutdowns; and volume degassing in the
VCT or via degassifier,
-
evaporators of BRS (boron recycle system) and, LWS (liquid waste system)
condenser venting, and
-
venting of RCDT (reactor coolant drain tank) or pressurizer relief tank.
1) decay-tank system
Decay tanks could be used for the purpose of holdup and decay of radioactive gases
before their discharges to the atmosphere. Short-lived nuclides will be rapidly decayed
during holdup.
The filling time of a decay tank is expressed by:
Tf 
VP
F
days
(9-3.1)
where,
V = volume of a decay tank, ft3
P = storage pressure (usually 70% of design pressure), atm
F = average gas flow rate per reactor to the storage tank, ft3/day (STP)
Once a decay tank is completely filled with gases, the tank will be sealed off and
isolated for letting the radioactive gases decay in the tank. The holdup time following
the isolation is given by:
Th 
VP(n  2)
F
where,
days
(9-3.2)
n = total number of tanks including standby tanks in the system.
2) charcoal delay systems:
Sometimes, charcoal delay systems are used for the purpose of holdup and decay of
radioactive gases. Short-lived nuclides will be also decayed during holdup. Charcoal
adsorbers are used for delay of their releases.
The holdup time of the charcoal adsorber is given by:
T = 0.011 M K / F
(9-3.3)
where,
0.011 = (103 lb)(454 g/lb)(3.53x10-5 ft3/cm3)(1 d/24x60 min)
F = system flow rate, ft3/min
M = mass of charcoal adsorber, 103 lb
K = dynamic adsorption coefficient, cm3/g
77oF
77oF
77oF
0oF
Dew point
45oF
0oF
-40oF
-20oF
Kr
18.5
25
70
105
Xe
330
440
1160
2410
Operating
temperature
3) Cryogenic distillation:
Inlet gases are liquefied under liquid-N2 temperature. And the liquid is boiled off using
distillation process. According to the different boiling point of each gas, it will be
separated via distillation process. The decontamination factor (DF) of each nuclide
using cryogenic distillation is given as follows:
DF
I, Xe
1x104
4x103
Kr
Example 9-3.1:
Decay tanks
Assume the decay tank system as follows:
-
Number of decay tanks = 3
-
Design pressure of the tank = 200 psia
-
Volume of the tank = 900 ft3
-
System flow rate = 140 ft3/day
Solution:
- filling time;
Tf 
VP (900)( 200 x0.7  14.7)

 61
F
140
- holdup time;
Th 
VP(n  2) VP

 61 days
F
F
days
2. Liquid Waste Treatment
The liquid treatment methods and their respective DF (decontamination factor)s are
summarized as follows:
1. demineralization (ion exchange)*
anion
Cs-Rb others
mixed bed
RC letdown (Li+BO3-)
100
2
50
radwaste (H+OH-)
100(10) 2(10)** 100(10)
evap.condensate polish
5
1
10
BRS
10
2
10
S/G blowdown
100(10) 10(10) 100(10)
Cation bed (any systems)
1(1)
10(10) 10(10)
Anion bed (any systems)
100(10) 1(1)
1(1)
Powdex (any systems)
10(10)
2(10)
10(10)
2. evaporation***
iodines
all others
misc. wastes
100
1000
boric acid recovery
100
1000
3. reverse osmosis
all nuclides
laundry wastes
30
other liquid wastes
10****
4. filtration
all nuclides
1
*
DF is a function of relative ion concentration and resin volume.
**
Higher due to lower relative concentration of other nuclides.
***
Evaporators are assumed to be unavailable for 3 consecutive
days per week.
****
Lower due to I and Cs contents.
1) Reverse osmosis:

brine flow
dense membrane (cellulose acetate)
< 1 m thick dense layer
 water flow
porous membrane
feed
brine (liquor)
- batch process permeate
osmotic pressure
reverse pressure
effluent
Let,
DFm  membrane DF 
nuclide concentration in liquor stream
nuclide concentration in effluent stream
= 100 for others
30 for iodines
F  fractional re cov ery 
effluent volume processed through membrane
inlet batch volume
In this case, the system DFs which is defined by
DFs 
nuclide concentration in feed stream
F

1 / DF
nuclide concentration in effluent stream
1  1  F  m
for DFm =100;
DFs 
0.95
 30
1 / 100
1  1  0.95
for DFm =30;
DFs 
0.95
 10
1 / 30
1  1  0.95
~ 95%
Example 9-3.2:
Liquid waste system
A sample process system of dirty wastes is given as follows:
Input
other input (ex. detergent waste)
Dirty Waste
Collection
Filter
Evaporator
Demineralizer
Waste Sample
(mixed bed) 1
Tanks (2)
Tanks (2)
discharge
Demineralizer
(mixed bed) 2
The system DF :
filter
evaporator
1
1
1
100
1000
1000
iodine
Cs-Rb
others
demi.1
5
1
10
The collection time for liquid wastes in a collection tank is:
Tc 
0.8Vc
F
days
where,
Tc = dirty waste collection time in the collection tank, days
Vc = tank volume of a collection tank, gal
F = dirty waste input flow rate, gal/day
And the process time of the waste is:
Tp 
0.8Vc
Fp
days
where,
Tp = waste process time of a redundant collection tank, days
Fp = dirty waste process flow rate, gal/day
demi.2
(optional)
1
1
1
system
DF
500
1000
10000
Finally, the discharge time, if 50% credit for decay on average taken, is:
Td 
0.8Vs
Fd
days
where,
Td = waste discharge time of a sample tank, days
Vs = tank volume of a sample tank, gal
Fd = dirty waste discharge flow rate, gal/day
If the capacity of a sample tank is bigger than tank input during process time, the decay
time of wastes is the sum of the process time and one half of the sample tank discharge
time, i.e.,
If 0.8 Vs > Tp (Fp + Fo), then decay time = Tp + 0.5 Td
where,
Fo = flow rate of additional waste input to the sample tank, g/d
Let,
Item
symbol
quantity
Dirty waste input flow rate
F
1000 gal/day
Waste collection tank volume
Vc
20000 gal per tank
Evaporator process rate
Fevap
15 gal/min.
Demineralizer process rate
Fdemi
100 gal/min.
Waste sample tank volume
Vs
40000 gal per tank
Other input flow rate
Fo
540 gal/day
Discharge flow rate
Fd
10 gal/min
Collection time:
Tc 
(0.8)( 20000)
 16
1000
Process time:
Tp 
(0.8)( 20000)
 0.7
(15 gal / min)( 60 x 24 min/ day )
days
days
Discharge time:
Check for decay credit,
Td 
(0.8)( 40000)
 2.2
(10 gal / min)( 60 x24 min/ day )
0.8Vs
0.8 x 40000

 1.45
Fp  Fo 15 x60 x 24  540
days
days > Tp
Hence, the credit for decay time is (Tp + 0.5 Td) = 0.7 + 0.5x2.2 = 1.8 days.