Suitability of High Power Beam Dumps in Hall A and Hall C for use at 14.5 GeV Operation

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Suitability of High Power Beam Dumps in

Hall A and Hall C for use at 14.5

GeV
Operation


Erik Abkemeier, Radiation Control Department


The purpose of this technical note is to summarize and update information concerning
operability of the High Power Beam D
umps

(HPBDs)

located in Experimental Halls A
and C at an

energy of 14.5

GeV, and an operating beam power envelope of 900 kW (e.g.
a
n operating beam current of 62

microamps at an energy of 14.5

GeV) and a safety
envelope of 1 MW. An important fundamental co
ncept to grasp is that all aspects within
the realm or radiological protection (e.g., dose rate produced) essentially scale with
operating beam power, particularly for accelerators operating at energie
s above 3 Ge, as
shown in Figure 1. (Thomas, 2003;

Sull
ivan 1992). The proposed o
perating
and safety
envelope
s
for 14.5

GeV operation to either Experimental Hall A or C is identical to the
current operating envelope of 900 kW and a safety envelope of 1 MW for Experimental
Halls A and C with operation a approxi
mately 6 GeV, which is the current physical
accelerator operating limit at the time of this technical note
.


In order to explain how operating eithe
r Experimental Hall A or C at 14.5

GeV would
affect the surrounding groundwater, it is helpful to summarize
how the shielding for the
HPBD shielding was developed during the initial construction
. The principal factors
include
: (1) the radioisotopes potentially created, leached into groundwater, and the
groundwater standards to which Jefferson Lab was to be held;

(2) the assumed average
beam power operating envelope; (3) the groundwater flow rate; (4) the dose created from
impinging typical beam power on the beam coupled with a conversion to neutron flux at
energies necessary to create the radioisotopes. Each of t
hese factors will briefly be
discussed and compared as to the impact for the proposed

energy upgrade versus the
original shielding design criteria.


As to the driving concern (i.e., radionuclides produced in the groundwater outside of the
HPBD shielding en
closure)
,

the only long
-
lived radionuclides of concern capable of
being produced in soil and leached into water, or produced directly in groundwater
continue to be H
-
2 and Na
-
22. (Stapleton,

1989
). An increase in beam energy will not
introduce the potentia
l for more radionuclides of concern. The groundwater standards at
the time of the original design of the HPBD shielding enclosures were such that the total
dose to a person no
t exceed 1

mrem per year if ingestin
g the water at a rate of 2.2
lit
ers a
day.

Th
is corre
lated to a concentration of 2800 pCi/l for H
-
3, and 100

pCi/ for Na
-
22.
These standards have not changed since that time. The only peripheral point of note is
that Jefferson La
b currently is issued a permit
from the Commonwealth of Virginia
Departm
ent of Environmental Quality that references a number of groundwater wells in
three concentric rings varying from 3 to 5 meters from the accelerator ring and
experimental halls, and to the site boundary (VPDES, 2001). For the closest wells (in the
“A” ring
), an “action limit” of 5000 pCi/l for H
-
3 is enacted such that if during the
mandatory quarterly sampling which is required to be submitted to the DEQ.
Additionally, the “C” ring wells located at the site boundary have a limit of 1000 pCi/l
for H
-
3 and 61

pCi/l for Na
-
22.

As a note, during the conduct of mandatory sampling and
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analysis by a third party radiochemical analysis lab, no detectable concentrations of H
-
3
or Na
-
22 have been detected at any groundwater well site at any time since the inception
of
the sampling p
rotocol in 1989
( Virginia Commonwealth 1989
-
2006
). As the CEBAF
accelerator h
as been operating since 1994
, this would indicate that the HPBD shielding
enclosure (as well as other groundwater shielding
)

has performed more than adequately.


Con
cerning the assumed average beam power operating envelope, the constraints on
maximum normal beam power operations

will remain in effect
. That being the case, the
power assumptions from the initial design of the HPBD shielding enclosure remain valid.


The
groundwater flow is a cr
itical piece in determining concentrations of radionuclides in
the groundwater because

the build
-
up of radionuclides in the groundwater is directly
proportional to the residence time that

the water remains in the neutro
n flux field
that
creates the radionuclides. In other words, the slower the groundwater m
ovement rate, the
greater the radionuclide concentration

in the groundwater. At design time for the the
HPBD shielding enclosures, the estimated gr
oundwater flo
wrate was assumed to

be zero
,
i.e., the water would remain around the surfaces exposed to neutron flux so that the water
would reach a saturation concentration. (Stapleton 1989
). Post accelerator construction
and start up

hydrogeological survey

performed by

Malcolm Pirnie

ind
icate
s

that the
groundwater flo
w rate in the area o
f the HPBD at Hall A averages 68
me
ters per year

for
the period from 1996 to 2001 (Malcolm Pirnie 2001)
.
This same study shows an average
groundwater flow in the area of the HPBD for Ha
ll C of 142 meters p
er year

for the same
period.
To date, this flow has not been considered in the groundwater activation
calculations.
Be
cause the

experimental halls have been constructed below the water table,
in order to maintain structural integrity of the halls, it is ne
cessary to continuously pump
groundwater around the experimental halls to the surface

at a rate of approximate
ly
12
,000 t
o 24
,000 gallons per

day

(Malcolm Pirnie, 1995
)
.
These dewatering operations
cause the elevated groundwater flow rates in the areas sur
rounding the experimental
halls. (As a note, the de
-
watering consis
ts of water collecting in the End Station
Dewatering Sump

that is monitored for radionuclide concentration prior to discharge to
the surface).


The radionuclide production mechanism in gro
undwater at 14.5 GeV is the same as that
at 6 GeV, i.e., it is
based upon the neutron fluence

rate into the soil/groundwater
surrounding the concrete shielding surrounding the HPBDs for neutrons a
bove the energy
of approximately

20 MeV. This number is obta
ined conservatively by converting an
assumed neutron dose rate based upon empirical shielding calculations developed at
SLAC from an accelerator operating at 15 GeV, into a neutron flux rate using a selected
dose to neutron flux conversion factor

(Jenkins,

1979)
. As stated previously

and shown in
the SHIELD 11 coded

exposure rate formula, the dose rate is proportional
to power
envelope (Nelson, 2005
). As this power envelope remains the same at 14.5 GeV as it is at
current operating energy of 6 GeV, the calc
ulated dose rate will remain the same. The
dose rate to neutron flux conversion rate used for the original 6 GeV calculation was
based on a conservative approximation of 2.3 n/cm
2
sec per 1 mrem/hr
which continues to
be valid to energies up to 28 GeV
(Gil
ber
t

19
68
)
. Additionally, the
simplified
conservative model for the initial calculations was predicated on all of the neutron
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production emanating at a single point within the middle of the proposed HPBD.
A

more
complex model
ing was undertaken

that showed the

H
-
3

and Na
-
22 concentration levels
anticipated would be approximately a factor of 10 lower than calculated under the
original method

(Thomas 1989)
. This is further borne out in that no groundwater well in
any of the rings around the accelerator and experi
men
tal halls has ever had any

detectable

concentration of

H
-
3 or Na
-
22.
For H
-
3, this is approximately

700 pCi/l. For
Na
-
22, this
is approximately

30 pCi/l
. The past 4 years indicated that actual expe
rimental power for
the year was

on the order of 100 kW f
or the en
tire year (Degtiarenko 2002, 2003, 2004,
2005).


For an exercise in conservatively scaling radionuclide

concentration to power deposited
in the beam dumps based on radioanalytical results, we can assume
that the water levels
were actually at the
highest Minimum Detectable Concentrat
ions, and scaled power up to
750

kW in each
hall (which is highly unlikely

given that Scheduled Accelerator Downs
occur for at least 2 months out of any given year, and in order to average
750

kW, the
Halls would need t
o be operating at maximum capacity for the remaining 10 months of
the year.
)

Even given that entirely unrealistic set of conditions, this would scale to a
concentration approximately 5250 pCi/l for H
-
3, and 225

pCi/l for Na
-
22. Again this
assumes an unreal
istic extended run of maximum power accelerator beam delivered to a
hall, coupled with the assumption that water surro
unding the HPBDs is stationary
and at
saturation activity

(which is clearly not the case)
, and the radionuclide concentration in
the water

is just beneath the MDC. Even under this extremely conservative scenario the
concentrations projected are not entirely inconsistent with limits for the EPA. The
derivation of the concentration limits is described more fully in previous tech notes, but is
based upon one quarter of the EPA limit of 4 mrem/year assuming ingestion of the
groundwater in question. This correlates to a l
imit of 2800 pCi/l for H
-
3 and 1
00 pCi/l for
Na
-
22 in order to receive a dose of 1 mrem in a year.


For the purpose
s of obtainin
g more accurate (yet conservative)

numbers, now that
information of groundwater flow data is more readily available, the calculation will be
performed utilizing
the SHIELD 11 code formula, with only the high energy and mid
energy neutron co
mponents, as the
se are the only components active in activation of soil
and water. The formula for this is:




1
2
3
100
100
10
3
.
2
55
120













s
cm
n
x
MIDe
HENe
c
c
t
t




at one meter

Where

φ = activating neutron fluence rate

HEN = 22.5 rem h
-
1

kW
-
1

m
2

MID = 225 rem h
-
1

kW
-
1

m
2

t = thickness of concrete = 4.5 meters at 90 degrees

ρ
c

= density of concrete = 2.35 g/cm
3


This results in an activating neutron fluence rate for a saturation activit
y in the wate
r
normalized to power

of 7.7 n cm
-
2

s
-
1

per m
2
/kW
.

The assumed conservative power
envelope
(as described previously) is 750

kW averaged throughout the year.
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Additionally, the distance at 90 degrees from the point of beam interaction with the b
eam
dump which includes the thickness of the concrete and air space between the dump and
the concrete is approximately 5.5 meters. Using t
hese numbers yields a

neutron fluence
of 190

n cm
-
2

s
-
1
.


However, this number assumes that the water remains static i
n the regions adjacent to the
HPBDs (i.e., the water does not move.) We know from hydrogeologic studies that this is
not the case

(Malcolm Pirnie 2001)
. In fact
,

due to the continuous pumping action
necessary to maintain structural integrity of the Experim
ental halls, water moves rapidly
in that area. By

inspection of Fig. 2 which is
a simulated spectrum of neutrons traveling
through the concrete dimensions of the HPBD shielding enclosure

using GEANT
modeling
, one can see that only approximately half of the

surface area is exposed to the
neutron activating flux. In other words, groundwater cannot be activated unless it comes
in contact with this region.

For the sake of conservatism, we can assume a worst case
scenario in which groundwater flows along the lat
eral surface of one side of the HPBD
shielding enclosure

and down to a point under the center of the experimental hall where
the groundwater is collected in the end station dewatering sump
. We will also assume
that (contrary to modeling), the entire surfac
e of the HPBD shielding enclosure has an
activating neutron flux

of energy greater than 20 MeV
, such that groundwater can be

activated
during the entire time that it is in the region of the shielding wall

until the water
reaches the groundwater dewatering
sump
. We will then assume

that the

water flow in
the region of the HPBD shielding is the
average from the lower of the two halls (68

meters/year)
, and use that to determine the actual build
-
up of activity, and concentration
as the water passes through that

area. We can use the formula for determining activity at
time t
, which can be adjusted for neutron activating flux by canceling out like terms such
that the following correction factor is used.


(
t
e



1
)(
t
e


)


to accou
nt for building up activity

as well as decay

in the water, and hence concentration

(Bevelacqua, 1999)
.


In the case of H
-
3:

λ

= 0.69
3/(12.3 years)
]

t = (60

m) / (68

m/year
)

Which results in a correction factor of:

0.046
. This, in turn, results in
a

scaled down
neutron flux of 8.7

n cm
-
2

s
-
1


In the case of Na
-
22:

λ

= 0.6
93/[(2.6

years)

t

= (60

m)/(68

m/year
)

which results

in a
correction factor of 0.165
. This, in turn results in
a
scaled down
neutron flux of 31.3

n cm
-
2

s
-
1



From previous calculations determining radionuclide concentration in groundwater, the
limit for neutron fluence rate
to prevent groundwater activation that

would cause greater
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than 1 mrem/year dose
is 35 n cm
-
2

s
-
1

(Stapleton, 1997)
. As one can see, the neutron
fluence rates are again beneath those necessary to maintain the H
-
3 and Na
-
22
concentrations beneath one quarter of the EPA limits.


In conclusion, p
roduction of radionuclides of concern in groundwater (H
-
3 and Na
-
2
2) is

proportional to beam power. As the beam power operating envelope will not change in
the course of running 6GeV operations to 14.5 GeV operations
, radionuclide production
should not var
y significantly. Additionally, design of the original High Power Beam
Dump shielding enclosure did not take credit for the rapid groundwater flow rates in the
areas adjacent to the Experimental Halls, which is a parameter that significantly affects
radionu
clide production. Furthermore, quarterly sampling of installed groundwater wells
since the beginning of accelerator operations has not shown any detectable concentrations
of H
-
3 or Na
-
22.




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References


Bevelacqua, J. 1999. “Basic Health Physics”, John Wi
ley and Sons


Degtiarenko, P. 2002. “Radiation Dose Rates Resulting from the Experimental Program at
Jefferson Lab, July
-
December 2002” RCG Note 02
-
03, Newport News, Virginia


Degtiarenko, P. 2003. “Radiation Dose Rates Resulting from the Experimental Prog
ram at
Jefferson Lab, October
-
December 2003” RCG Note 03
-
04, Newport News, Virginia


Degtiarenko, P. 2004. “Radiation Dose Rates Resulting from the Experimental Program at
Jefferson Lab, October
-
December 2004” RCG Note 04
-
04, Newport News, Virginia


Degtia
renko, P. 2005
. “Radiation Dose Rates Resulting from the Experimental Program at
Jefferson Lab, July
-
December 2005” RCG Note 05
-
02, Newport News, Virginia


Gilbert
, W.S.

et al. 1968. CERN
-
LRL
-
RHEL Shielding Experiment at CERN, Lawrence
Radiation Laboratory

Report UCRL
-
17941, Sept. 1968


Jenkins, T. 1979. “Neutron and Photon Measurements Through Concrete from a 15 GeV electron
Beam on a Target


Comparison with Models and Calculations”, Nuclear Instruments and
Methods 159.


Malcolm Pirnie, Inc. 1995. CEBAF H
ydrogeologic Review, Newport News, Virginia, September.


Malcolm Pirnie, Inc. 2002. Groundwater Flow Directions and Velocities at Jefferson Lab,
Newport News, Virginia, February.


Nelson, W. et al. 2005. SLAC
-
Report
-
737 “The SHIELD11 Computer Code”, Februa
ry 2005.


Stapleton, G. 1987. “The Production of Radionuclides in the Groundwater”, Jefferson Lab Tech
Note, TN
-
0062, Newport News, Virginia


Stapleton, G. 1989. “Design of Shielding to Ensure Maximum Concentrations of H
-
3 and Na
-
22
in the Groundwater Rema
in Within Standards”, Jefferson Lab Tech Note TN
-
0155, Newport
News, Virginia


Stapleton, G. et al. 1997. “Occupational and Environment Aspects of the Radiation Control
Provisions at Jefferson Lab,” Jefferson Lab Tech Note, JLAB TN 97
-
017, Newport News,
Vi
rginia.


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Sullivan, A. H. 1992. “A Guide to Radiation and Radioactivity Levels Near High Energy Particle
Accelerators,” Nuclear Technical Publishing, Ashford, Kent, TN23 1JW, England, Chap 4.


Thomas R., et al. 1989. “Review of the Radiological Aspect of th
e Conceptual Design Report for
the CEBAF End Station and Switchyar
d, August 15
-
17, 1989”, Jefferson Lab Tech Note
TN
-
0174, Appendix 4 “Consideration of the Use of the EGS/FLUKA Monte Carlo Code to
Determine the Activity of H
-
3 and Na
-
22 in the Groundwater”.


Thomas R., et al., 2003. NCRP (National Council on Radiation Pr
otection and Measurement) No.
144, “Radiation Protection for Particle Accelerator Facilities”.


Virginia Commonwealth Department

of Environmental Quality (DEQ)

VPDES Monitoring
Reports 1989
-
2006.


VPDES (Virginia Pollutant Discharge Elimination System) Per
mit No. VA0089320. Thomas
Jefferson National Accelerator Facility/U.S. Department of Energy, Newport News,
Virginia. Effective July 16, 2001 to July 16, 2006

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Figure 1
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Figure 2