Univerza v Ljubljani
Fakulteta za matematiko in fiziko
Seminar Ia
Activation of Water in Nuclear Reactors
Author: Andrej šohar
Mentor: doc. dr. Luka Sno j
Ljubljana, 2016
Abstract
In this seminar I will present the activation of cooling water in the Kr²ko nuclear power
plant and the International Thermonuclear Experimental Reactor (ITER). Activation of cooling
water occurs due to exposure to high neutron ux in the reactor vessel. Activated cooling water
circulates in the cooling system and is the main contributor of radiation outside of the reactor
vessel. The activation reaction rate was calculated using Monte Carlo Neutral Particle transport
program and dierent nuclear data libraries for the Kr²ko nuclear power plant and ITER. From
the results it is visible, that water is going to be in orders of magnitude more activated in ITER
than in Kr²ko. This will cause more damage to components and higher dose rates to workers in
ITER.
Contents
1
Introduction
2
2
Activation of cooling water
2
3
The Monte Carlo method
4
4
Kr²ko Nuclear Power Plant
6
5
International Thermonuclear Experimental Reactor (ITER)
9
6
Conclusion
1
3.1 Nuclear data libraries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1 Simulation of activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
7
5.1 16 N and 17 N production in ITER . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.2 Eects of 16 N and 17 N decay products . . . . . . . . . . . . . . . . . . . . . . . . 11
11
Introduction
Nuclear reactors use nuclear ssion or fusion to produce heat, which can be used to generate
electricity. This is the main purpose of nuclear power plants. Research reactors produce neutrons and gamma rays for irradiation of samples for studies of the eects of radiation, neutron
activation analysis, etc. All the reactors have a common feature. They need to be cooled to
prevent melting of the fuel in ssion reactors and to prevent melting of walls in fusion reactors.
There are many dierent uids and gasses that are cooling reactors, but the most common is
water (H2 O).
Due to intense neutron ux in nuclear reactors (> 1013 cm−2 s−1 ), the cooling uid becomes
radioactive. Because the cooling uid is circulating in the primary system, it is the main source of
radiation outside the reactor vessel. Due to this, the understanding of activation of cooling uid
in reactors is important for designing biological shielding and shielding of important equipment
inside and outside of the containment vessel.
In this seminar I will only focus on the activation of water, because this is the primary cooling
uid in the Slovenian nuclear power plant Kr²ko and ITER nuclear fusion research reactor and
it is the most common cooling liquid.
2
Activation of cooling water
During the cooling of the reactor core the water is exposed to intense neutron ux (> 1013
cm−2 s−1 ). This intense ux causes neutron reactions with nuclei in water molecules (Table 1).
Important reactions are 16 O(n,p)16 N, 17 O(n,p)17 N and 18 O(n,γ )19 O.
The most important radionuclide is 16 N due to a high natural abundance of 16 O, relatively
high cross section for (n,p) reaction and high energy gamma radiation. It also contributes the
most to the activity of cooling water and dose rate outside the reactor vessel. We also need
to take into account the activation of 18 O, due to 19 O gamma decay. 17 N emits high energy
neutrons which can activate components outside the reactor vessel and produce neutron induced
gamma rays. However the activity of 17 N is negligible in a ssion reactor due to lower neutron
ux and "softer" neutron spectrum, but needs to be included for fusion reactors. I decided to
include 17 N activation for the Kr²ko nuclear power plant in the seminar for completeness.
Due to the short half-life of radionuclides, they are important only during reactor operation.
2
Isotope
Natural
Dominant Activation
t1/2 [s]
abundance [%] reaction
product
16 O
99.76
(n,p)
16 N
7.13
17 O
0.04
(n,p)
17 N
4.14
18 O
0.20
(n,γ )
19 O
26.9
Dominant decay producs
(branching ratio)
6.129 MeV gamma (67%)
7.115 MeV gamma (5%)
0.383 MeV neutron (35%)
1.171 MeV neutron (53%)
0.197 MeV gamma (63%)
1.357 MeV gamma (33%)
Table 1: Activation products of water in reactor coolant. [1]
The threshold energy for activation of 16 O is at 10 MeV and for 17 O is 9 MeV. 18 O has no
threshold energy for activation and can be already activated with thermal neutrons.
Figure 1: Cross section energy dependence for activation of oxygen nuclide taken from JEFF-3.2
library. Dierent libraries and dierences between them are presented later in this seminar. [2]
To calculate the activation of a nuclide, we rst need to determine the reaction rates R(~r)
dened as[3]:
Z ∞
Ri (~r) =
N0 σi (E)φ(~r, E)dE.
(1)
0
N0 is the number density of target elements in cooling water [1/cm3 ], σi (E) is the macroscopic
cross section of the nuclide for reaction i [cm2 ] and φ(~r, E) is the energy dependent neutron ux
or neutron spectrum in the primary cooling water [1/MeVcm2 s−1 ].
To describe the change of radionuclide concentration in the primary cooling water N , we
need to solve the dierential equation[4]:
dN (t)
= R − λN,
dt
(2)
where λ is a decay constant [s−1 ] and R is an average reaction rate in region. To calculate the
specic activation of radionuclide we multiply the solution for N (t) with the decay constant (λ):
A(t) = N (t) λ = R(1 − e−λt ).
3
(3)
After long exposure the activation reaches saturation A = R = N0 σφ.
The cooling water circulates and is exposed to high neutron ux for a short time (∼ 3 s).
The new equilibrium of specic activation is:
A1 = A2 e−λte + R(1 − e−λte )
A2 = A1 e−λT ,
(4)
where A1 is the specic activation of cooling water on the outlet of the reactor vessel, A2 is the
specic activation of cooling water on the inlet of the reactor vessel, te is the exposure time and
T is time the cooling water needs from the outlet to the inlet of the reactor vessel.
If we combine equations in (4), we get the equilibrium value of specic activation on the
outlet of the reactor vessel:
1 − e−λte
A1 = R
.
(5)
−λ(te +T )
1−e
In the equations above we took average reaction rates over the whole reactor vessel. We
can also divide the reactor vessel into smaller subsections for better analysis of the contribution
of specic areas. In general we have n equations for n regions in the reactor vessel plus one
equation for the region outside the reactor vessel. The general form of equations is then:
A1 = An+1 e−λte1 + R1 (1 − e−λte1 )
..
.
An = An−1 e−λten + Rn (1 − e−λten )
(6)
An+1 = An e−λT .
From this equations we can calculate the equilibrium of the activity at the output of reactor
vessel (An ). In the equations we do not have the reaction rates. There are many ways to
calculate them, but in this seminar they were calculated using the Monte Carlo method.
3
The Monte Carlo method
The Monte Carlo method is very dierent from deterministic transport methods. Deterministic methods solve the transport equation for the average population behaviour. On the other
hand, the Monte Carlo method obtains solutions by simulating individual particles and recording some aspects (tallies) for their average behaviour. The average behaviour of particles in the
physical system is then inferred (using the central limit theorem) from the average behaviour
of the simulated particles. The Monte Carlo method is well suited for solving complicated
three-dimensional problems, which cannot be modeled by computer codes that use deterministic
methods. The individual probabilistic events that comprise a process are simulated sequentially.
The probability distributions of these events are statistically sampled to describe the total phenomenon. In general, the simulation is performed on a computer, because the number of particles
in simulation necessary to adequately describe the phenomenon is usually quite large, typically
on the order of 106 to 1012 . The statistical sampling process is based on the selection of random
numbers. The selection is analogous to throwing a dice in a gambling casino, hence the name
of the method. [5]
There are many programs for Monte Carlo simulation. In this seminar Monte Carlo Neutral
Particle (MCNP) program was used to calculate solutions. MCNP is a general-purpose Monte
Carlo transport code. It has been developing by Los Alamos National Laboratory since 1957. It
is primarily used for the simulation of nuclear processes, such as ssion, but has the capability
to simulate particle interactions involving neutrons, photons and electrons. It can also be used
to calculate kef f eigenvalues for the ssile system. The current version is MCNP 6.1.1. [5]
4
3.1 Nuclear data libraries
MCNP uses continuous-energy nuclear and atomic data libraries. The primary sources of
nuclear data are evaluated nuclear data libraries [1]. Nuclear data tables exist for neutron
interactions, neutron-induced photons, photon interactions, neutron dosimetry or activation and
thermal particle scattering. Over 836 neutron interaction tables are available for approximately
100 dierent isotopes and elements. More neutron interaction tables are constantly being added
as new and revised evaluations become available. [5]
Not all nuclear data is present in the standard MCNP ENDF/B-VII.0 library. Because of
that, there are several dierent atomic data libraries, which can be used in Monte Carlo for
calculations on missing atomic data. In the case of the activation of water, the standard MCNP
library does not have cross section energy dependence for 18 O(n,γ )19 O reaction. Also cross
section energy dependence for 17 O(n,p)17 N reaction is dierent in the standard MCNP library
than in other libraries. I used data from TENDL ([6]) and JEFF libraries ([7]), but there are
also others. In calculations for ITER, data from JENDL library was used.
The TENDL library uses the results from the TALYS nuclear model code system. TALYS
is a software for the simulation of nuclear reactions. TENDL is physically produced at the CEA
Bruyeres-le-Chatel and developed at PSI, the IAEA, CCFE and the CEA. It contains evaluations
for seven types of incident particles, for all isotopes living longer than 1 s (about 2800 isotopes)
up to 200 MeV. The current version is TENDL-2015. [6]
JEFF (Joint Evaluated Fission and Fusion File) is a collaboration between NEA Data Bank
member countries. The library combines the work of JEFF and EFF/EAF Working Groups to
produce a common sets of evaluated nuclear data, mainly for ssion and fusion applications.
The JEFF nuclear data library contains neutron and photon interaction data, radioactive decay
data, ssion yields, and thermal scattering law data. The current version is JEFF-3.2.[7]
As I mentioned above, there are dierences in cross section energy dependence for activation
17
of O and 18 O in the standard MCNP(ENDF/B-VII.0) library and JEFF and TENDL libraries.
The cross section energy dependence can be seen in Figure 2 and Figure 3. For the TENDL
library, the cross section energy dependence is taken from TENDL-2014, because the dependence
is the same in TENDL-2014 and TENDL-2015.
Figure 2: Cross section energy dependence for activation of
experimental data. [2]
5
17 O
from dierent libraries and
For the activation of 17 O the cross section energy dependence is almost the same for JEFF-3.2
and TENDL-2015 library, but dierent from standard MCNP library ENDF/B-VII.0. From the
graph, it is visible that the activation of 17 O is going to be higher with the use of the standard
MCNP library, but the dierence is not going to in orders of magnitude higher due to the shape
of the neutron spectrum in ssion reactors and the threshold in the neutron spectrum in the
fusion reactor. Some results of experimental measurements are shown on the graph, but they do
not tell us which dependence is the right one. Because of that, I used all of them for simulating
the activation in Kr²ko nuclear power plant.
For the activation of 18 O, there is a big dierence in cross section energy dependence in the
epithermal region (around 0.08 MeV) between JEFF-3.2 and TENDL-2015. TENDL-2015 has
a resonance in this region which contributes the majority to the activation of 18 O. From the
measurements, it is visible that there should be a resonance peak like in TENDL-2015 library,
but we cannot denitively claim that. Because of that, both libraries were used in the MCNP
simulation.
Figure 3: Cross section energy dependence for activation of
experimental data. [2]
4
18 O
from dierent libraries and
Kr²ko Nuclear Power Plant
Kr²ko nuclear power plant is a pressurised water ssion reactor with 2000 MW thermal
power and 696 MW power rating. It was built in 1981 in Slovenia near the city of Kr²ko. In
the reactor, the ssion of 235
92 U is the main source of energy in the beginning of fuel cycle, while
44% of energy is produced by ssion of 239
94 Pu at the end of fuel cycle.
235
92 U
239
94 Pu
+ 10 n →
+ 10 n →
A
ZX
A
ZX
+
+
236−A−ν
Y
92−Z
240−A−ν
Y
94−Z
+ ν 10 n + 200 MeV,
+ ν 10 n + 212 MeV,
(7)
where hνi = 2.43 for ssion of uranium and hνi = 2.88 for ssion of plutonium. In the equation
(7) the X and Y present the decay products. The products are not the same for every ssion.
For the X the atomic mass (AX ) number is most of the time between 90 and 100 and for Y
between 140 and 150, depending on the value for X . For better eciency of the reactor, the
239
fuel is enriched and contains between 2.1 and 4.3 % of 235
92 U. There is no 94 Pu in fresh fuel.
6
The reactor is cooled with water, which is circulated by two pumps having a ow rate of
22711 m3 /h ow rate. Due to this, the water is exposed to high ux (∼ 1.5 · 1013 cm−2 s−1 ) for
a short time ∼1 - 2 s. However, the whole cycle lasts only around 12 s, which means that some
activated water returns to the reactor and needs to be included in the nal calculated activation.
4.1 Simulation of activation
The neutron ux spectrum calculations was calculated by using Monte Carlo code on the
existing model of Kr²ko nuclear power plant. [8] The reactor vessel was divided in four areas:
downcomer (Region 1), lower plenum (Region 2), core (Region 3) and upper plenum (Region 4).
The scheme is shown in Figure 4 and lethargy spectrum in Figure 5.
Figure 4: Scheme of Kr²ko reactor vessel with marked areas for detailed analysis of activation.
Region 2
-2 -1
Lethargy spectrum [cm s ]
Lethargy spectrum [cm-2s-1]
Region 1
1.6x1012
1.4x1012
1.2x1012
1x1012
8x1011
6x1011
4x1011
2x1011
0
10-10 10-8 10-6 10-4 10-2 100 102
Energy [MeV]
Region 4
-2 -1
6x1013
5x1013
4x1013
3x1013
2x1013
1x1013
0
10-10 10-8 10-6 10-4 10-2 100
Energy [MeV]
Lethargy spectrum [cm s ]
Lethargy spectrum [cm-2s-1]
Region 3
2x109
1.8x109
1.6x109
9
1.4x10
1.2x109
1x109
8x108
6x108
4x108
2x108
0
10-10 10-8 10-6 10-4 10-2 100 102
Energy [MeV]
102
1x109
9x108
8x108
7x108
6x108
5x108
4x108
3x108
2x108
1x108
0
10-10 10-8 10-6 10-4 10-2
Energy [MeV]
100
102
Figure 5: Scheme of Kr²ko reactor vessel with lethargy spectrum in each region for detailed
analysis of activation.
7
MCNP 6.1 was used to calculated reaction rates. The results of reaction rates from MCNP
are normalized to one ssion neutron and are in units [cm−2 ]. To normalize the results by the
thermal power of the system, an appropriate scaling factor needs to be used. [9] The factor
depends on the thermal power of the system (2 GW in case of the Kr²ko nuclear power plant),
the average number of neutrons per ssion, the average energy released per ssion and eective
neutron multiplication factor. The exposure times were calculated from the known volume of
cooling water and ow. [10] With equation (3) and the system of equations (6) I calculated the
activation of cooling water. I also calculated the activity per litre instead of cm3 . The results
of the activation are presented in Table 2.
Library
Activation product
16 N
ENDF/B-VII.0
16 N
TENDL-2015
16 N
JEFF-3.2
17 N
ENDF/B-VII.0
17 N
TENDL-2015
17 N
JEFF-3.2
19 O
TENDL-2015
19 O
JEFF-3.2
Activity [Bq/l]
1.92·109 ± 3.62 · 106
1.92·109 ± 3.62 · 106
1.92·109 ± 3.62 · 106
4.21·105 ± 6.74 · 103
1.35·105 ± 2.03 · 103
1.48·105 ± 2.07 · 103
2.66·107 ± 4.10 · 105
8.42·106 ± 1.35 · 105
Table 2: Calculated specic activity.
The errors in results are due to the Monte Carlo method, the uncertainties in cross section energy dependence and uncertainties from calculating exposure time. The uncertainties in
exposure time calculation contributed the majority to the nal value error.
From Table 2 I can see the dierence in calculated activity using dierent libraries. For the
activation product 16 N there is no dierence in calculated activity due to the same cross section
energy dependence in all libraries. For 17 N the calculated activity for JEFF and TENDL is
almost three times lower than for ENDF/B-VII.0, due to a dierence in cross section energy
dependence presented in section 3.2. Also the calculated activity of 17 N from all libraries is
negligible compared to activity of 16 N. For 19 O the calculated activity from JEFF is more than
three times lower than from TENDL. The calculated activity is two orders of magnitude lower
than the activity of 16 N and contributes only a small portion to the whole activity of cooling
water.
19
O
7
3.0⋅10
Activity [Bq/l]
2.5⋅107
2.0⋅107
1.5⋅107
1.0⋅107
5.0⋅106
0.0⋅100
0
50
Figure 6: Time dependence of activity of
value.
100
150
Time [s]
19 O
8
200
250
from the start of a clean reactor to saturated
I also calculated the time and number of water recirculation cycles needed for the activity to
reach the saturated value, from inactivated state at full power (2 GW). The result for activity
of 19 O are presented in Figure 6. From the calculated data I found out that it takes 21 cycles
for 16 N to reach its saturated value (∼4.3 min), 14 cycles for 17 N to reach its saturated value
(∼2.9 min) and 70 cycles for 19 O to reach its saturated value (∼14.3 min).
Detailed eects of gamma rays dose eld and activation of components due to activated
cooling water in Kr²ko nuclear power plant have not been yet made.
5
International Thermonuclear Experimental Reactor (ITER)
ITER is a fusion reactor currently being built next to the Cadarache facility in the south of
France. In the reactor, deuterium and tritium will be fused together to form alpha particles and
high-energy neutrons.
2
3
4
1
(8)
1 D + 1 T → 2 He + 0 n + 17.6 MeV.
The reason for utilising this reaction is that the process requires the lowest activation energy,
while producing among the most energy per unit weight. The designed power output is 500
MW, while the reactor only needs 50 MW.
Construction of the ITER complex started in 2010 and is scheduled to be completed in 2019.
In 2020 the rst plasma experiments should begin and full deuterium-tritium fusion experiments
should begin in 2027.
As I mentioned in the introduction, water is going to be the coolant for blankets and vacuum
vessel in ITER. A typical cooling water loop is shown in Figure 7.
Figure 7: Schematic diagram of the water coolant ow path in ITER.[11]
Water circulates through the pipes at 4-8 m/s. This means that the exposure to high ux
at the rst wall in front of the blanket is short ∼ 0.4 - 1 s. The neutrons induce many reactions
in the cooling water, but the most important for ITER are the production of 16 N and 17 N. The
16 N decay gamma-rays will be an additional source of nuclear heat in the cryogenic components
inside the cryostat (superconducting magnets, cryostat walls, etc.) and could cause radiation
9
damage in unshielded diagnostic and electronic components. Fast neutrons from 17 N decay will
activate the cooling pipes and components inside the primary heat exchanger system. [11]
5.1
16
N and 17 N production in ITER
Neutron ux spectrum in fusion reactors is signicantly dierent from neutron ux in ssion
reactors. In ssion reactors, neutron ux is distributed according to the Watt spectrum. The
peak of the spectrum is at 0.7 MeV and the mean of the spectrum is at 2 MeV. In fusion reactors
the peak of the spectrum is at 14 MeV due to energy of neutron from D-T reaction. The
comparison can be seen in Figure 8. Also the neutron ux in fusion reactors is 8·1013 cm−2 s−1 ,
which is ve times higher than in the Kr²ko ssion reactor. This triggers more activation of 16 O
and 17 O.
Figure 8: Comparison between neutron ux in ssion (PWR) and fusion reactor (DEMO). [12]
The simulation was performed in MCNP and the detailed geometry and results are presented
in [11] and [13]. In this seminar I will only present the most important results of the simulation.
16 N activity was calculated for separate parts of cooling system. In the blanket modules the
water becomes most activated due to close proximity to plasma. The rate of activation is ten
times higher in the front section of the blanket module than in the remainder of the module.
The specic activity of 16 N at the exit of module was calculated to be[11]:
Ab = 5.6 · 1012 Bq/l.
In the divertor the water is activated in plasma facing components: dome, wings and targets.
Cooling water is in divertor about 7 s with high exposure about 1 s. The specic activity of 16 N
at the exit of divertor was calculated to be[11]:
Ad = 1.5 · 1012 Bq/l.
In both cases the time outside of the exposure area is greater than several half-life periods
of 16 N (50-70 s). This means that residual activity of 16 N can be neglected. This also means,
that cooling water reaches saturated value after just one cycle.
For 17 N the specic activity was calculated for blanket modules. The specic activation is
Ab = 6.7 · 109 Bq/l, three orders of magnitude lower then than of 16 N. Due to a short half-life,
around 50% of 17 N decays before leaving the blanket modules and specic activity at the outlet
of modules is[11]:
Ab = 1.7 · 109 Bq/l.
10
From the results I can see that the specic activity in fusion reactor is three orders of
magnitude higher for 16 N than in the Kr²ko ssion rector. In case of 17 N the specic activity is
four orders of magnitude higher. The reason for this is in the greater neutron ux and higher
energy spectrum of neutrons.
5.2 Eects of 16 N and 17 N decay products
The largest fraction of the 16 N gamma-ray energy is released in the pipe walls and the water
itself. About 5% is deposited in Toroidal Field coils and the remainder is deposited in the
cryostat and biological shield. The design specication limits the nuclear heating in the TF
coil case structure and superconductor to 2 and 1 mW/cm3 . The maximum gamma-ray energy
release was estimated to be 0.65 mW/cm3 in the TF coil casing and 0.004 mW/cm3 in the
superconductor. If additional shielding (8-cm SS + 5-cm H2 O) is introduced between the pipes
and the superconducting TF coil, the nuclear heating will be substantially lower.
The dose rates from gamma-rays in the outlet pipes during the reactor operation was also
calculated. The dose rate near the outlet pipe (behind cryostat) is estimated to be 650 Sv/h, in
the TF coil casing (near the front surface) 320 Sv/h and in the superconductor 2 Sv/h. All of
these are high doses and maintenance during operation is not going to be possible.
The expected 17 N fast neutron ux in the pipes inside the cryostat is about one magnitude
lower than behind the vacuum vessel. At 1 m from a single outlet pipe surface, the operational
dose rate in the cryostat from 17 N decay neutrons and secondary gamma-rays is 0.5 Sv/h, which
is signicantly lower than the dose rate from 16 N decay.
The residual activation of pipe walls was estimated after one year of operation. The contact
dose rates at a single outlet pipe surface as a function of time after shut down from activation
of steel are given in Table 3. [11]
Time after Shutdown Contact Dose Rate
[days]
[µSv/h]
0
1300
7
190
14
180
30
160
365
33
Table 3: The contact dose rate on a single outlet pipe. [11]
6
Conclusion
Water is the cooling liquid in ssion nuclear power plant Kr²ko and future fusion power plant
ITER. Due to high neutron ux, this cooling water becomes activated and is carried outside
reactor vessel, where it causes radiation problems. The activation was simulated with the Monte
Carlo method to determine activation of cooling water in reactor vessel for Kr²ko and ITER.
From the results we can see that cooling water is going to be in orders of magnitudes more
activated in ITER than in Kr²ko. This will cause more radiation damage to components and
higher dose rates for workers in ITER.
For ITER, the eects of activated cooling water decay was also carried out. From the results
it is visible that access to cryostat, while the reactor is going to operate, is not going to be
possible due to high radiation doses. Even outside of cryostat and biological shield, the doses
are going to be high and dangerous for workers. Nuclear heating from activated cooling water is
11
also going to be a problem for the reactor itself, especially for superconductors. Because of that,
sucient protection needs to be placed between coolant pipes and superconductor structure.
There are still some uncertainties in the Monte Carlo simulation for processes in nuclear
physics. The biggest uncertainty is for cross section energy dependence, especially for 18 O. This
eld needs the most attention and research in the future.
References
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, Nucl. Data Sheets 107(2006)2931.
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An Improved Version of the NEA Java-
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Centre for Fusion Energy, Abingdon, Oxfordshire OX14 3DB, UK, 20.11.2013.
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in the ITER Water Cooling System of the Shielding Blanket, Fusion Engineering, 17th
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12