# Fission Product Transport and Source Terms in HTRs: Experience from AVR Pebble Bed Reactor

Authors: Rainer Moormann
Journal: Science and Technology of Nuclear Installations
 (2008)

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## Abstract

Fission products deposited in the coolant circuit outside of the active core play a dominant role in source term estimations for advanced small pebble bed HTRs, particularly in design basis accidents (DBA). The deposited fission products may be released in depressurization accidents because present pebble bed HTR concepts abstain from a gas tight containment. Contamination of the circuit also hinders maintenance work. Experiments, performed  from 1972 to 88 on the AVR, an experimental pebble bed HTR, allow for a deeper insight into fission product transport  behavior. The activity deposition per coolant pass was lower than expected and was influenced by fission product chemistry and by presence of carbonaceous dust. The latter lead also to inconsistencies between Cs plate out experiments in laboratory and in AVR. The deposition  behavior of Ag was in line with present models. Dust as activity carrier is of safety relevance because of its mobility and of its sorption capability for fission products.  All metal surfaces in pebble bed reactors were covered by a carbonaceous dust layer. Dust in AVR was produced by abrasion in amounts of about 5 kg/y. Additional dust sources in AVR were ours oil ingress and peeling of fuel element surfaces due to an air ingress. Dust has a size of about 1μm, consists mainly of graphite, is partly remobilized by flow perturbations, and deposits with time constants of 1 to 2 hours. In future reactors, an efficient filtering via a gas tight containment is required because accidents with fast depressurizations induce dust mobilization. Enhanced core temperatures in normal operation as in AVR and broken fuel pebbles have to be considered, as inflammable dust concentrations in the gas phase.


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## Body

## 1. Introduction

Fission
product transport during HTR normal operation plays a major role in safety
examinations for normal operation and for accidents as also in maintenance and in
dismantling. Particularly the fission products released during reactor normal
operation and accumulated within the coolant circuit are most relevant
contributions to the source terms of design basis accidents (DBAs) for advanced
small HTRs. These activities are released into the coolant circuit during long-term
normal operation at high temperatures from intact- and defect coated fuel
particles and from uranium contamination in the graphite by diffusion through
kernel, coating layers, and graphite (metallic fission products) as well as by
diffusion out of the kernels of defective coated fuel particles without further
retention (nonmetals). The intact coated particle is a very efficient barrier
for nonmetals, but low melting metallic fission products diffuse through intact
coatings, particularly at temperatures>1000C°. Condensable fission products are mainly
deposited in the coolant circuit: plate out on metals competes with sorption on
graphitic dust. Except for noble gases, the gas-borne activities are low in steady-state
operation due to this deposition.Advanced
modular HTR abstain from a gas-tight containment. Accordingly, there remains
only one barrier between the accumulated activity and the environment, the
coolant circuit enclosure. DBA-like depressurizations, stand alone or followed
by a core heat-up phase, and steam/water ingress accidents may lead to a
partial release of these activities accumulated in the coolant circuit [1, 2].
Another still relevant source term contribution is the activity inventory of
coated particles with defective coatings (defect fraction: about 10−4).
Nonmetals as iodine are partly released from defective coated particles in
course of a water ingress accident due to an interaction between kernel and
steam. This holds also for oxide fuel [2, 3].In
contrast, the activity content of particles with intact coatings, which is calculated
for modern HTRs to about 99.9% of the total inventory, does not virtually dominate
DBA source terms of advanced small HTRs. Intact coated particles are not
affected by stand-alone depressurizations and by water ingress events of the DBA
range. However, there are some limited releases from fuel elements by accelerated
diffusion and by additional temperature-induced particle failure in case of
core heat-up events. Except for Ag-110 m, these releases remain low if core temperatures,
as claimed by design, do not exceed 1600C°, and if enhanced temperatures do not
last longer than about 200 hours. The total inventory of Ag-110 m is small for uranium
fuel but larger for plutonium fuel. Another release-limiting factor in core
heat-up events is caused by the fact that high temperatures and corresponding
releases occur subsequent to the depressurization of the coolant circuit: for that
there remains a very limited gas transport out of the coolant circuit, when
high temperatures are reached and releases start. Further, releases out of the
reactor building into the environment can be filtered because of the small
overpressure in the high-temperature phase. The only credible accidents, which
may induce a substantial fission product release from intact coated particles, are
severe air ingress with graphite burning [4] and rare reactivity transients.
However, these accident are far outside of the DBA range of well-designed HTRs.
Source terms related to the inventories outside of intact coated particles
dominate, therefore, doses in DBA and probably even the risk of small advanced
HTRs [1, 2].Fission
products accumulated in the coolant circuit have also to be taken into account
for maintenance purposes. This is particularly relevant for the gas turbine in
a direct cycle, which requires periodically hands on maintenance. Contamination
limits for gas turbines, allowing hand-on maintenance, are not easy to achieve
in HTRs [5] as will be discussed in Section 4.3.Dose limits for DBA
are comparatively low and have to be calculated in a pronounced conservative
manner. They allow only a very low activity release into the environment: following
German requirements, the maximum tolerable release for key nuclides is—for conditions as
assumed in the safety report of the HTR-Module200—in the range of
about10−7 (Cs-137) to about 10−8 (I-131 and Sr-90) of the total nuclide inventory in
that 200 MWth pebble-bed reactor design. Comparing these release
limits with inventories present outside of intact coated particles of up to 10−3,
which are subject of partial release in DBA due to nonexistence of a gas tight
containment, the relevance of fission product transport becomes obvious.
Respective experience from the AVR pebble-bed reactor is discussed in this paper.


## 2. Overview on AVR and its Fission Product Related Experiments

The AVR was the first pebble-bed HTR worldwide and was operated 1967–1988 in Juelich.
Main aim of this experimental reactor was the principal test of the pebble-bed
core, test of the fission product
transport in pebble-bed reactors, and test of many different types of pebble-shaped
fuel elements. Design data of the AVR are collected in Table1. Presently, AVR
undergoes dismantling and its vessel will be grouted with light concrete in 2009.
A rough scheme of the coolant circuit of this reactor is presented in Figure 1.Table 1
Main
data of the AVR reactor.
Power/Average power density46MWth(15MWel)/2.5 MW/m3CycleSteam cycle with the steam generator inside the reactor vesselCore height/diameter2.8 m/3 mCoolant/PressureHe/10.8 barGas outlet/inlet temperature700 to 950 (990)C°/275C°He-flow13 to 16 kg/s (depending on desired gas outlet temperature)Fuel95000 pebbles (6 cm diam.) diverse fuel types, at
end mainly modern TRISOFigure 1
Schematics of AVR and its fission product related experiments.AVR was operated until 1974 at coolant outlet temperatures of up to 850C°.
From February 1974, the maximum coolant temperature was increased to 950C°,
temporarily even up to 990C° (see top part of Figure 4). Whereas the release of
iodine and noble gases remained small after temperature increase, there was a
pronounced acceleration of release of metallic fission products (Sr-90, Cs-137,
Ag-110 m) by diffusion, which may have reached at end of life some percent of one
core inventory. There are two contributions for this release: on the one-hand
side until about 1975, mainly BISO fuel (two pyrocarbon coatings) with UC2-kernels
and large uranium contamination of the graphite was used. From 1985, modern UO2 TRISO fuel (one SiC layer between pyrocarbon layers) dominated the core
composition. Oxide kernels reveal a better Sr-retention than carbide kernels
and modern TRISO fuel elements improve the Cs-retention at least compared to some
early fuel elements used in AVR. On the other hand, core temperatures in AVR were
substantially higher than precalculated, as being discussed briefly in Section 6.2
and in more detail in [6], and have substantially accelerated the release of
metallic fission products [6]. The problem to which extent enhanced releases
of AVR have to be considered in future HTR concepts is studied in [6].Figure1 also contains the most relevant fission product-related
experimental facilities [3, 7–10]. All these experiments were operated in a
bypass to the main He circuit. These experiments are as follows.(1)
VAMPYR-I hot-gas filter: The condensable
nuclides activity in the hot gas was measured via a plate out section and of the
dust activity in a subsequent dust filter. Materials used in the plate out
section are not HTR ones. This is, because VAMPYR-I suffered from a strong
neutron activation of the inner plate out section. Mainly for that, plate out
sections consisting of Ti, which shows low activation, were used.(2)
Cold-gas filter: The total activity
(except noble gases, but including dust-borne activities also by a separate
dust filter) was measured in the cold-gas range.(3)
Noble gas measurement: These
experiments analyzed the total noble gas inventory in the coolant.(4)
VAMPYR-II plate out experiment in
the hot gas region: The plate out section consisted of metallic materials
representative for the THTR-300, and was equipped in certain experiments with
dust filters upflow and/or downflow. VAMPYR-II was operated only for short
periods at the end of AVR life.(5)
Dust experiment: Dust filter
equipment in the cold gas with the intention to examine the dust behavior  more
in detail, for example by several different filters parallel and in line. The
dust experiment was operated from 1984 to 1988 only. Unfortunately,
most of these experiments were not completely evaluated during and in the first
years after AVR operation. This paper gives an overview on the evaluation status
of these experiments, including those recently performed. At first, experiments
(1, 2, and 4) which cover fission product deposition and plate out, are
discussed. Sections 5 and 6 concentrate on details of dust behavior  (Experiment
5 and some dust-related results of other experiments) and some general AVR
experience on fission product behavior, including Experiment 3. Besides these
experiments, several other fission product-related experiments were performed
(Tritium reduction by gettering and permeation filters, fission product absorption
by metals [8]), which were however not completely successful and will not be
discussed here.


## 3. AVR Experiments on Fission Product Deposition in the Coolant Circuit

Condensable
nuclides released from fuel elements may be reabsorbed on fuel elements, absorbed
on graphite reflectors and plated out on metal surfaces, or—in particular in pebble-bed
reactors—absorbed on
graphitic dust. These deposition mechanisms will be discussed here in detail on
example of the AVR.


### 3.1. Deposition of Nuclides on Fuel
Elements and on the Graphite Reflector

Fission
product profiles in depth of such AVR fuel elements, which show a moderate
release from coated particles, reveal typically a minimum in concentration in
the outer fuel-free zone. This is an indication for fission product
reabsorption on fuel elements. Experimental profiles showing this concentration
reincrease towards the surface are presented in Figure2, which is for a GK (BISO-UC2)
fuel element. In the AVR core, fuel elements of different type and burn-up were
mixed together and accordingly release rates and sorption capabilities varied
between neighboring fuel elements, which led to this reabsorption effect. In
future reactors with more homogeneous fuel element composition, this effect is
expected to become less pronounced. A second contribution may be caused by gas-borne
dust-carrying fission products, which circulates in the primary circuit and
will adhere on fuel elements. Altogether, the shape of the profile is in line
with laboratory sorption experiments for Sr-90 and Cs-137 [3, 11].Figure 2
Concentration profiles of Cs, Eu and Sr in the outer fuel free shell of an AVR pebble fuel
element (17.6% fima, initial U-235/total heavy metal ratio = 0.17).As
an example, Figure3 compares the sorption capabilities of fuel element matrix
graphite A3-3 as used in AVR, of a standard nuclear graphite ASR-1RG, and of 2
coat mix materials containing different amounts of binder coke, for Cs partial
pressures as in accidents at 1100C°.Figure 3
Equilibrium concentrations of Cs at 1100C° in different carbonaceous materials (Cs-partial
pressure: 3⋅10−2Pa).Figure 4
Comparison of total Cs-137 activities in AVR hot gas and in cold gas.It becomes
obvious that the sorption capability of matrix graphite is about an order of
magnitude larger compared to the standard nuclear graphite ASR-1RG, which is—as shown by the
comparison with the coat mix materials—due to the
content of 10% of ungraphitized (coked) binder in the matrix graphite A3-3. The
binder coke absorbs more than 2 orders of magnitude more Cs than the graphite
filler, as results on the coat mix materials indicate. This result is
beneficial concerning fission product retention in the core, but problematic due
to the fission product sorption in the mobile graphitic dust. For Sr, the fuel
element matrix graphite reveals an even larger sorption capability than for Cs
[3, 11].Specific
activities of long-lived metallic fission products in the graphite/carbon brick
core top construction indicate that about 10% of the total inventories of
Sr-90 and Cs-137 released from active core are found in this component. The large He flow
velocity and the small geometrical surface of the reflector boreholes limit the
mass transfer-controlled sorption, despite of the large sorption capability of
graphite.


### 3.2. Deposition on the Steam
Generator

Most
condensable fission products are deposited along the steam generator, as
comparison of data of the hot-gas filter VAMPYR-I and of corresponding data of
the cold-gas filter indicates. This is due to the large geometrical surface of
the steam generator. Figure4 contains data (Cs-137) of both experiments for the
AVR operation time 1973–1988: The corresponding average hot gas temperatures
are also shown. Similar measurements are available for Cs-134 too. The
Cs-134/Cs-137-ratio in VAMPYR-I decreased from 1973–1978 almost continuously
from about 1 to 0.4, a value which remained constant in further operation. This
observation cannot yet be explained, the respective average ratio in the fuel
was about 0.8. The Cs-activity in the dust filter downflow of the plate out section
was substantially smaller than the activity in the plate out section. This
indicates that Cs occurs mainly in molecular form in the hot gas. This
assumption is supported by examination of the Co-60 behavior : Co-60 occurs only
dust borne (formed mainly by corrosion/abrasion from activated metallic
components). Its deposition behavior  in VAMPYR-II is very different from that
of Cs (see Section 4.2 and Figure 8). Further the deposition rate of AVR dust
is much smaller than that of Cs (see Figure 11 and Table 3). Also, the deposition
profile of Cs along the steam generator of Peach Bottom HTR with block-type-fuel
elements, which also contained some dust by oil cracking, indicates
predominantly molecular Cs-137 and even Sr-90 in the hot gas [12].VAMPYR-I has unfortunately not
sampled hot gas of an average activity composition. Its data were probably
influenced by sucking of cold-gas streams, as measurements of gas temperature
profiles in the surrounding of the entrance of VAMPYR-I revealed [13]. The
absolute activity values measured in VAMPYR-I were therefore too low.This underestimation becomes obvious
when the total deposition in the VAMPYR-II plate out experiment (see Section4.2)
is compared with respective VAMPYR-I results for about the same time period.
Table 2 contains specific activities measured in VAMPYR-II 1987-88 [3] at hot
gas temperatures of 700 to 930C°. The activity in the final charcoal filter of
VAMPYR-II was unfortunately not measured, so the data presented in Table 2 are
lower limits. This holds particularly for Cs-137: examinations of the 5 subsequent
segments of the fission product filters of VAMPYR-II-02 revealed no downflow declination
in Cs-137 concentration, which means that some unknown amount of Cs-137 passed
this facility [14].Table 2
Lower limits of specific hot gas
activities [Bq/m3STP] as measured in VAMPYR-II
(1987-88) for hot gas temperatures as given in brackets.
NuclideVII-01 (930C°)VII-02 (900C°)VII-03 (700C°)VII-04 (700C°)Cs-1374021125755Ag-110 m1264854Table 3
Deposition
degreeα in AVR for selected nuclides measured in 1982–1984.
NuclideαAg-110 m0.87I-1310.80Cs-134 and Cs-1370.80–0.98VAMPYR-II sucks gas under well-defined
conditions, that is, gas of about average hot gas temperature. Having in mind
that maximum coolant temperatures at the azimuthal position of VAMPYR-II were
measured to more than 100 K higher than average ones [13], an additional reason
for underestimations of coolant activities by VAMPYR-II may exist. This is
because fission product release follows an exponential temperature dependence. In
spite of this underestimation the specific hot gas activities of Cs in VAMPYR-II
are still higher than corresponding ones in VAMPYR-I by a factor of 30 at 700C°
and 3 at 930C°.VII-01
data correspond concerning Cs-137 to a release rate of greater than10−3 of the birth rate. The respective
value for Ag-110 m amounts to 2⋅10−2. Obviously, the main contribution stems from intact-coated
particles by diffusion through coatings, because in 1987 the uranium
contamination of fuel element graphite and the fraction of defective particles
were altogether less than 2×10−4 of the total inventory (see AVR Quarterly Progress Reports
1987 and 1988 in [15]). Even the Cs-releases of VII-03 and VII-04 at 700C° can
be explained by defect particles and uranium contamination only, if a complete
Cs release out of these fractions is assumed, which is unlikely. Accordingly,
diffusion through intact coatings has contributed here, too. Mainly unintentional
high core temperatures (see Section 6.2) have caused these high rates [6].Absolute
activity values of the cold-gas filter are also assumed to be too low due to
deposition along a 30 m pipe guiding to the filter. Comparison of cold-gas
filter data with those of the dust filter (10 m pipe length) indicates an
average underestimation by a factor of 2 with a large uncertainty scatter.Activities
deposited in the cold-gas filter were mainly found in dust except of iodine. The
dust filters in AVR cannot distinguish between molecular activities sorbed on
dust present in the filters, or already reach the filter bound on dust. However,
the dust concentration in these filters is so small (some mg) that a large
sorption effect of molecular activities on dust has not to be expected. Most
probably Cs occurs in molecular form in AVR hot gas but dust borne in the cold
gas.


### 3.3. Deposition of Nuclides per Pass
in Normal Operation

On basis of the data shown
in Figure4, the deposition degree α of Cs in the steam generator region was
calculated. α is shown in Figure 5 depending on hot gas
temperature. There is some tendency of α-increase with increasing hot gas
temperatures; one reason for that might be that at higher hot gas temperatures,
the flow velocities are smaller and therefore the residence times of the
coolant in the deposition range are larger. An average deposition degree of
about 0.85 is found. However, consideration of experimental errors as estimated
in Section 3.2 for VAMPYR-I and for the cold-gas filter will enlarge
average α-values to
about 0.95. Table 3 contains some estimates on α for relevant nuclides as found in AVR 1982 to
1984 [7].Figure 5
Deposition degreeα of gas borne Cs-137 activity (A) per pass of coolant.Computer
simulations based on laboratory scale plate out measurements on metals with
codes like SPATRA [14] lead to much higher Cs-deposition degrees (α greater
than 0.99), if no bypasses are considered. The deposition rate is controlled by mass transfer in the boundary layer, except for the hottest part of the steam generator, where an ad/desorption equilibrium is reached. Similar results are obtained, even if very low
deposition parameters—as derived in Section
4.2 from VAMPYR-II-data—are taken into
account. This discrepancy can only partly be explained by steam generator
bypass flows. A similar overestimation of α in deposition calculations was found for
several other nuclides in AVR and also in other HTR-reactors like DRAGON. Here,
experimental α-values for
I-131 as low as 0.25 are reported [16], whereas the Cs-deposition was similar
to AVR. A very low α for I-131 was observed also in the COMEDIE
loop [17]. This indicates that fission product chemistry (i.e., the chemical
form of the nuclides is not the elemental one) and/or particularly in case of pebble-bed
reactors the nuclide interaction with dust may play a relevant role in the
overall deposition behavior , as is discussed in Sections 4 and 5. Overestimation
of α underestimates the steady-state gas-borne
activities.Theα-value of
Sr-90 in AVR (1973-1974) was estimated to less than 0.5. This may be due to
the very strong affinity of Sr to dust. However, Sr-90 measurements in VAMPYR-I
are not available from mid of 1974 when Sr-90 measurements were replaced by
those of Eu-154, which creates some errors. In addition, as will be discussed in Section 5, the dust sampled by the cold-gas filter and by the dust filter contains not only the activities released in the sampling phase but to some extent also activities from prior release periods. This holds particularly for Sr-90.


## 3.1. Deposition of Nuclides on Fuel
Elements and on the Graphite Reflector

Fission
product profiles in depth of such AVR fuel elements, which show a moderate
release from coated particles, reveal typically a minimum in concentration in
the outer fuel-free zone. This is an indication for fission product
reabsorption on fuel elements. Experimental profiles showing this concentration
reincrease towards the surface are presented in Figure2, which is for a GK (BISO-UC2)
fuel element. In the AVR core, fuel elements of different type and burn-up were
mixed together and accordingly release rates and sorption capabilities varied
between neighboring fuel elements, which led to this reabsorption effect. In
future reactors with more homogeneous fuel element composition, this effect is
expected to become less pronounced. A second contribution may be caused by gas-borne
dust-carrying fission products, which circulates in the primary circuit and
will adhere on fuel elements. Altogether, the shape of the profile is in line
with laboratory sorption experiments for Sr-90 and Cs-137 [3, 11].Figure 2
Concentration profiles of Cs, Eu and Sr in the outer fuel free shell of an AVR pebble fuel
element (17.6% fima, initial U-235/total heavy metal ratio = 0.17).As
an example, Figure3 compares the sorption capabilities of fuel element matrix
graphite A3-3 as used in AVR, of a standard nuclear graphite ASR-1RG, and of 2
coat mix materials containing different amounts of binder coke, for Cs partial
pressures as in accidents at 1100C°.Figure 3
Equilibrium concentrations of Cs at 1100C° in different carbonaceous materials (Cs-partial
pressure: 3⋅10−2Pa).Figure 4
Comparison of total Cs-137 activities in AVR hot gas and in cold gas.It becomes
obvious that the sorption capability of matrix graphite is about an order of
magnitude larger compared to the standard nuclear graphite ASR-1RG, which is—as shown by the
comparison with the coat mix materials—due to the
content of 10% of ungraphitized (coked) binder in the matrix graphite A3-3. The
binder coke absorbs more than 2 orders of magnitude more Cs than the graphite
filler, as results on the coat mix materials indicate. This result is
beneficial concerning fission product retention in the core, but problematic due
to the fission product sorption in the mobile graphitic dust. For Sr, the fuel
element matrix graphite reveals an even larger sorption capability than for Cs
[3, 11].Specific
activities of long-lived metallic fission products in the graphite/carbon brick
core top construction indicate that about 10% of the total inventories of
Sr-90 and Cs-137 released from active core are found in this component. The large He flow
velocity and the small geometrical surface of the reflector boreholes limit the
mass transfer-controlled sorption, despite of the large sorption capability of
graphite.


## 3.2. Deposition on the Steam
Generator

Most
condensable fission products are deposited along the steam generator, as
comparison of data of the hot-gas filter VAMPYR-I and of corresponding data of
the cold-gas filter indicates. This is due to the large geometrical surface of
the steam generator. Figure4 contains data (Cs-137) of both experiments for the
AVR operation time 1973–1988: The corresponding average hot gas temperatures
are also shown. Similar measurements are available for Cs-134 too. The
Cs-134/Cs-137-ratio in VAMPYR-I decreased from 1973–1978 almost continuously
from about 1 to 0.4, a value which remained constant in further operation. This
observation cannot yet be explained, the respective average ratio in the fuel
was about 0.8. The Cs-activity in the dust filter downflow of the plate out section
was substantially smaller than the activity in the plate out section. This
indicates that Cs occurs mainly in molecular form in the hot gas. This
assumption is supported by examination of the Co-60 behavior : Co-60 occurs only
dust borne (formed mainly by corrosion/abrasion from activated metallic
components). Its deposition behavior  in VAMPYR-II is very different from that
of Cs (see Section 4.2 and Figure 8). Further the deposition rate of AVR dust
is much smaller than that of Cs (see Figure 11 and Table 3). Also, the deposition
profile of Cs along the steam generator of Peach Bottom HTR with block-type-fuel
elements, which also contained some dust by oil cracking, indicates
predominantly molecular Cs-137 and even Sr-90 in the hot gas [12].VAMPYR-I has unfortunately not
sampled hot gas of an average activity composition. Its data were probably
influenced by sucking of cold-gas streams, as measurements of gas temperature
profiles in the surrounding of the entrance of VAMPYR-I revealed [13]. The
absolute activity values measured in VAMPYR-I were therefore too low.This underestimation becomes obvious
when the total deposition in the VAMPYR-II plate out experiment (see Section4.2)
is compared with respective VAMPYR-I results for about the same time period.
Table 2 contains specific activities measured in VAMPYR-II 1987-88 [3] at hot
gas temperatures of 700 to 930C°. The activity in the final charcoal filter of
VAMPYR-II was unfortunately not measured, so the data presented in Table 2 are
lower limits. This holds particularly for Cs-137: examinations of the 5 subsequent
segments of the fission product filters of VAMPYR-II-02 revealed no downflow declination
in Cs-137 concentration, which means that some unknown amount of Cs-137 passed
this facility [14].Table 2
Lower limits of specific hot gas
activities [Bq/m3STP] as measured in VAMPYR-II
(1987-88) for hot gas temperatures as given in brackets.
NuclideVII-01 (930C°)VII-02 (900C°)VII-03 (700C°)VII-04 (700C°)Cs-1374021125755Ag-110 m1264854Table 3
Deposition
degreeα in AVR for selected nuclides measured in 1982–1984.
NuclideαAg-110 m0.87I-1310.80Cs-134 and Cs-1370.80–0.98VAMPYR-II sucks gas under well-defined
conditions, that is, gas of about average hot gas temperature. Having in mind
that maximum coolant temperatures at the azimuthal position of VAMPYR-II were
measured to more than 100 K higher than average ones [13], an additional reason
for underestimations of coolant activities by VAMPYR-II may exist. This is
because fission product release follows an exponential temperature dependence. In
spite of this underestimation the specific hot gas activities of Cs in VAMPYR-II
are still higher than corresponding ones in VAMPYR-I by a factor of 30 at 700C°
and 3 at 930C°.VII-01
data correspond concerning Cs-137 to a release rate of greater than10−3 of the birth rate. The respective
value for Ag-110 m amounts to 2⋅10−2. Obviously, the main contribution stems from intact-coated
particles by diffusion through coatings, because in 1987 the uranium
contamination of fuel element graphite and the fraction of defective particles
were altogether less than 2×10−4 of the total inventory (see AVR Quarterly Progress Reports
1987 and 1988 in [15]). Even the Cs-releases of VII-03 and VII-04 at 700C° can
be explained by defect particles and uranium contamination only, if a complete
Cs release out of these fractions is assumed, which is unlikely. Accordingly,
diffusion through intact coatings has contributed here, too. Mainly unintentional
high core temperatures (see Section 6.2) have caused these high rates [6].Absolute
activity values of the cold-gas filter are also assumed to be too low due to
deposition along a 30 m pipe guiding to the filter. Comparison of cold-gas
filter data with those of the dust filter (10 m pipe length) indicates an
average underestimation by a factor of 2 with a large uncertainty scatter.Activities
deposited in the cold-gas filter were mainly found in dust except of iodine. The
dust filters in AVR cannot distinguish between molecular activities sorbed on
dust present in the filters, or already reach the filter bound on dust. However,
the dust concentration in these filters is so small (some mg) that a large
sorption effect of molecular activities on dust has not to be expected. Most
probably Cs occurs in molecular form in AVR hot gas but dust borne in the cold
gas.


## 3.3. Deposition of Nuclides per Pass
in Normal Operation

On basis of the data shown
in Figure4, the deposition degree α of Cs in the steam generator region was
calculated. α is shown in Figure 5 depending on hot gas
temperature. There is some tendency of α-increase with increasing hot gas
temperatures; one reason for that might be that at higher hot gas temperatures,
the flow velocities are smaller and therefore the residence times of the
coolant in the deposition range are larger. An average deposition degree of
about 0.85 is found. However, consideration of experimental errors as estimated
in Section 3.2 for VAMPYR-I and for the cold-gas filter will enlarge
average α-values to
about 0.95. Table 3 contains some estimates on α for relevant nuclides as found in AVR 1982 to
1984 [7].Figure 5
Deposition degreeα of gas borne Cs-137 activity (A) per pass of coolant.Computer
simulations based on laboratory scale plate out measurements on metals with
codes like SPATRA [14] lead to much higher Cs-deposition degrees (α greater
than 0.99), if no bypasses are considered. The deposition rate is controlled by mass transfer in the boundary layer, except for the hottest part of the steam generator, where an ad/desorption equilibrium is reached. Similar results are obtained, even if very low
deposition parameters—as derived in Section
4.2 from VAMPYR-II-data—are taken into
account. This discrepancy can only partly be explained by steam generator
bypass flows. A similar overestimation of α in deposition calculations was found for
several other nuclides in AVR and also in other HTR-reactors like DRAGON. Here,
experimental α-values for
I-131 as low as 0.25 are reported [16], whereas the Cs-deposition was similar
to AVR. A very low α for I-131 was observed also in the COMEDIE
loop [17]. This indicates that fission product chemistry (i.e., the chemical
form of the nuclides is not the elemental one) and/or particularly in case of pebble-bed
reactors the nuclide interaction with dust may play a relevant role in the
overall deposition behavior , as is discussed in Sections 4 and 5. Overestimation
of α underestimates the steady-state gas-borne
activities.Theα-value of
Sr-90 in AVR (1973-1974) was estimated to less than 0.5. This may be due to
the very strong affinity of Sr to dust. However, Sr-90 measurements in VAMPYR-I
are not available from mid of 1974 when Sr-90 measurements were replaced by
those of Eu-154, which creates some errors. In addition, as will be discussed in Section 5, the dust sampled by the cold-gas filter and by the dust filter contains not only the activities released in the sampling phase but to some extent also activities from prior release periods. This holds particularly for Sr-90.


## 4. AVR Experiments on Plate out of Condensable Nuclides on Metals

### 4.1. Experimental Results

The main
facility for generation of fission product plate out data in AVR was VAMPYR-II
(Figure1), which contained an annular plate out section. The inner slab
consisted of Inconel617, whereas the tube was made from Incolloy800H. The total
length of the plate out section was about 3.5 m, the diameter of the slab was 9 mm and the inner diameter of the tube was 16 mm. Gas heating allowed operation
at similar temperature conditions. Nevertheless, the gas inlet temperature of
individual experiments (see Figure 9) varied slightly from 765 to 810C° depending
on the average hot gas temperature which is given in Table 2. Maximum wall
temperatures of the plate out section were between 660 and 725C°. A
temperature gradient along the length of the plate out section of about 100 K/m
was maintained by heaters. In contrast to VAMPYR-I, VAMPYR-II was installed in
a range without strong activation by neutrons. There were 4 plate out
experiments performed in VAMPYR-II; their duration was between 1 and 2 months.
Main nuclides examined in VAMPYR-II were
Cs-134, Cs-137, Ag-110 m, and Co-60. Some limited data on I-131 are available
for the first VAMPYR-II experiment. Two of the VAMPYR-II experiments (2 and 4)
were equipped with a dust filter upflow of the plate out section. Experiments 3
and 4 contained a dust filter downflow. Also, all experiments contained a so-called
fission product filter behind the plate out section or the second dust filter,
which however did not work properly [14].A general
result of all VAMPYR-II experiments was that Ag-110 m behaves as expected from
classic plate out theory, which means ad/desorption equilibrium at high
temperatures and mass transfer-controlled plate out at low temperatures which
means that the plate out section acts at low temperatures as a perfect sink
[14, 18]. Figure 6 contains as an example the deposition profile of Ag-110 m in Experiment
4 on Inconel617 together with postcalculations. A physical explanation of the
plate out model, used, for example, in the SPATRA plate out code, is discussed
in ANNEX I of [2].Figure 6
Measured and calculated plate-out profile for Ag-110 m on Inconel617 (VAMPYR-II).Model calculations with a desorption energy of 240 kJ/mol easily fit the
measured profile. An upper limit of a “penetration coefficient" of10−6 (see [14] and annex I of [2]) was extracted from these experiments too. The
penetration coefficient takes volume effects as absorption in the metal bulk into
account. Profiles of similar shape were obtained in the other three VAMPYR-II
experiments. Additional post examinations on the removal of plated out Ag-110 m
by water leaching were performed and are presented in Figure 6 too. Only a few
percent of the plated out activity can be removed by water leaching, that is,
decontaminations are not easy to perform. This is a relevant finding
particularly for maintenance and repair work in direct cycle facilities, where
plated out Ag-110 m belongs to the dose-determining nuclides (see Section 4.3).The more surprising output of VAMPYR-II is connected to cesium: in all
VAMPYR-II experiments, virtually flat (temperature independent) deposition
profiles were observed, connected to a very low deposition degree in the plate
out section as shown in Figure7. This deposition behavior  can be interpreted
by a low desorption energy in conjunction with a penetration coefficient of 10−5,
but one has to bear in mind that laboratory scale experiments on Cs-plate out
like LAMINAR-Loop [19] led to completely different results. Figure 7 also
contains a postcalculation of Cs-plate out using parameters measured in
laboratory scale experiments, which obviously do not fit to the VAMPYR-II
results. This also may be taken as an indication that Cs does not occur in
elemental gaseous form in the AVR. Taking that into account, there are two
explanations on this deviating Cs-behavior:Figure 7
Cs plate out in VAMPYR-II compared to calculations.Figure 8
Activity distribution in VAMPYR-II-04.Figure 9
Cs distribution in VAMPYR-II experiments.(i)
Cs is already dust borne in the hot gas;(ii)
Cs occurs as a chemical compound, which has a smaller desorption energy than
elemental Cs. Comparing
VAMPYR-II experiments with and without dust filter upflow of the plate out
section, however, does not lead to different results concerning Cs plate out,
because Cs passes this dust filter. This is taken as additional indication that
Cs is not mainly dust borne in the AVR hot gas. As also seen in Figure 8, Co-60
is retained with high efficiency already in the first dust filter in front of
the plate out section. Co-60 is known to be dust borne. However, as seen from Figure
8, most of the Cs in Experiment 4 is retained in the second dust filter, which
is situated downflow of the plate out section. The main difference between the
2 dust filters is the temperature, so condensation of a Cs-compound (CsOH) may
be the dominating deposition mechanism. Formation of CsOH was found to be
possible in AVR normal operation from kinetic and chemical thermodynamic point
of view. Similar holds for future reactors because a conservation of a protecting
oxide layer on metals requires a sufficient oxidation potential, which is
supplied by sufficiently high H2O/H2 ratio. The
deposition behavior  of those Cs-compounds under HTR conditions is not exactly
known but desorption energies are expected to be much smaller than for atomic
Cs. Figure 9 contains the distribution of Cs-137 onto the different components
of VAMPYR experiments.The fission
product deposition in VAMPYR-II occured mainly on metallic surfaces because a
complete dust layer could not be formed within the short period of operation,
and because some experiments contained a dust filter upflow of the plate out
section. As will be discussed more detailed in Section5, VAMPYR-II conditions
are therefore not completely representative for pebble-bed reactors.A similar plate out behavior  of Ag as above is observed in the COMEDIE
experiments [17, 20, 21]. Concerning Cs plate out, it is interesting to note that in
the first COMEDIE series, a flat profile with altogether low deposition was
observed too [20, 21], whereas in the second [17], a more classic (decreasing)
deposition profile combined with a higher deposition degree was found. A major source
for graphitic dust in COMEDIE is not seen, also because no movement of
graphitic components occurred. Thus, fission product chemistry seems to be the
reason for the “unusual" plate out behavior  of Cs in [20, 21]. It is worth to look
into differences in experimental conditions between both COMEDIE experimental series,
because here, one key for understanding the Cs-behavior  might be discovered.Iodine
plate out was measured on a stainless steel tube in VAMPYR-I [3, 14]. The plate
out profile can be crudely approximated by a desorption energy of 170 kJ/mol.


### 4.2. Influence of Dust on Fission
Product Plate out

Plate out
experiments for HTR metallic components are performed so far on metal specimen.
However, there is some doubt from HTR experience, whether that represents most
components of pebble-bed HTRs because of the presence of carbon-based dust. On
virtually all surfaces in the coolant circuit of the THTR-300 a graphitic dust
layer was present, which had a thickness of about 5-to-10μm (total full power
operation time of THTR-300: 16 months) [22]. A visible black dust layer is
present also on most AVR surfaces. Further, Cs and Sr in the coolant circuit of
the Peach Bottom HTR with block-type fuel were found to be greater than 80% in
carbonaceous layers, which covered almost all surfaces [12]. In the Peach
Bottom HTR, the carbonaceous layers were formed almost completely by continuous
small lubrication oil ingress from blowers. Examinations on the AVR blower nozzle
revealed that deposited fission products can be removed by wiping to the same
extent as by an electrolytic surface treatment of 1.8 μm in metal depth [23]. This
however does not hold for fast-moving parts of the blower, where shear forces
are too high for dust deposition. Accordingly, the activity may be present dust-bound
on the nozzle but not in other fast moving parts of the blower. Conclusions
drawn from blower examination suffer however from the fact that the operating
blower came in touch with liquid water for several days during a water ingress in
1978 and that the blower examinations occurred shortly after the water ingress.
There may have happened some decontamination of the blower already during the
water ingress.The
arguments presented before support the assumption that these fission products
are not plated out on the metal (and are thus less mobile) but mainly stuck in the
mobile graphitic dust layers. An additional argument lies in the very large
sorption capability of matrix graphite compared with metal surfaces for Cs and
Sr, as demonstrated in Figure3. The sorption capability of graphite is already
for a layer thickness of 1 μm substantial larger than that of the underlying
metal surface as parameter calculations using SPATRA revealed. These
calculations were performed on plate out of elemental Cs in an HTR-Module200-like
reactor, with and without a 1-μm matrix graphite layer on all metallic surfaces.
As expected, the results differ substantially in the high temperature range,
where deposition on graphitic dust dominates. Whereas for metal surfaces an
ad/desorption equilibrium is rapidly reached, the sorption capability of dust
is much larger and thus it acts as a perfect sink. In the low temperature range,
the deposition is mass transfer controlled for both, metals and graphite, that
is, perfect sink behavior . Accordingly, the deposition will be proportional to
the accessible geometrical surface of graphite and metal. The results are qualitatively
similar for desorption energies from LAMINAR-loop and from Experiment 4 of
VAMPYR-II. Because of the pronounced chemisorption enthalpy, associated with
sorption of Cs to graphite, and in line with chemical equilibrium calculations
[11], it has to be assumed that fission product chemistry does not change this
result in general although the deposition kinetics may be different.The
assumption of predominant fission product sorption on graphitic dust in pebble-bed
reactors changes, as outlined before, the deposition profile in a gas circuit,
but has also major safety implications. Because dust is easier to remobilize
than activities plated out on metals, the source terms in a fast depressurization
with its substantial shear forces will increase remarkably. A detailed
discussion of the dust behavior  in AVR and of its safety implications is
presented in Section5 of this paper.


### 4.3. Plate out Limits for a Gas Turbine

Plate out on metals/metal oxides
remains relevant for HTRs with block-type fuel and also for fast-rotating
components of pebble-bed reactors, particularly for a gas turbine. During the required
hand on maintenance of a gas turbine, the annual dose limit of a radiation
worker of 20 mSv has to be met, which limits the contamination of the turbine
during maintenance, for example, for Cs-137 to about10−6 of a core
inventory (400MWth reactor), as the dose rates of Table 4 indicate.Table 4
Dose rates on a gas turbine [5].
NuclideDose rate [mSv/(GBq⋅h)] in adistance of 1 m from a gas turbineCs-1370.10Ag-110 m0.45Consideration of AVR core release rates for the final stage
of operation with modern fuel at 950C° (see Figure 4), corrected with a factor
of 5 for the known underestimation of core release rates by VAMPYR-I and
assuming a plate out of 10% of the core release on the gas turbine results
leads to contamination values much higher than tolerable. Thus, satisfaction of
dose limits becomes a challenging item and requires further examinations.
Enhanced AVR core temperatures have to be discussed in this context too (see Section 6.2) [6].


## 4.1. Experimental Results

The main
facility for generation of fission product plate out data in AVR was VAMPYR-II
(Figure1), which contained an annular plate out section. The inner slab
consisted of Inconel617, whereas the tube was made from Incolloy800H. The total
length of the plate out section was about 3.5 m, the diameter of the slab was 9 mm and the inner diameter of the tube was 16 mm. Gas heating allowed operation
at similar temperature conditions. Nevertheless, the gas inlet temperature of
individual experiments (see Figure 9) varied slightly from 765 to 810C° depending
on the average hot gas temperature which is given in Table 2. Maximum wall
temperatures of the plate out section were between 660 and 725C°. A
temperature gradient along the length of the plate out section of about 100 K/m
was maintained by heaters. In contrast to VAMPYR-I, VAMPYR-II was installed in
a range without strong activation by neutrons. There were 4 plate out
experiments performed in VAMPYR-II; their duration was between 1 and 2 months.
Main nuclides examined in VAMPYR-II were
Cs-134, Cs-137, Ag-110 m, and Co-60. Some limited data on I-131 are available
for the first VAMPYR-II experiment. Two of the VAMPYR-II experiments (2 and 4)
were equipped with a dust filter upflow of the plate out section. Experiments 3
and 4 contained a dust filter downflow. Also, all experiments contained a so-called
fission product filter behind the plate out section or the second dust filter,
which however did not work properly [14].A general
result of all VAMPYR-II experiments was that Ag-110 m behaves as expected from
classic plate out theory, which means ad/desorption equilibrium at high
temperatures and mass transfer-controlled plate out at low temperatures which
means that the plate out section acts at low temperatures as a perfect sink
[14, 18]. Figure 6 contains as an example the deposition profile of Ag-110 m in Experiment
4 on Inconel617 together with postcalculations. A physical explanation of the
plate out model, used, for example, in the SPATRA plate out code, is discussed
in ANNEX I of [2].Figure 6
Measured and calculated plate-out profile for Ag-110 m on Inconel617 (VAMPYR-II).Model calculations with a desorption energy of 240 kJ/mol easily fit the
measured profile. An upper limit of a “penetration coefficient" of10−6 (see [14] and annex I of [2]) was extracted from these experiments too. The
penetration coefficient takes volume effects as absorption in the metal bulk into
account. Profiles of similar shape were obtained in the other three VAMPYR-II
experiments. Additional post examinations on the removal of plated out Ag-110 m
by water leaching were performed and are presented in Figure 6 too. Only a few
percent of the plated out activity can be removed by water leaching, that is,
decontaminations are not easy to perform. This is a relevant finding
particularly for maintenance and repair work in direct cycle facilities, where
plated out Ag-110 m belongs to the dose-determining nuclides (see Section 4.3).The more surprising output of VAMPYR-II is connected to cesium: in all
VAMPYR-II experiments, virtually flat (temperature independent) deposition
profiles were observed, connected to a very low deposition degree in the plate
out section as shown in Figure7. This deposition behavior  can be interpreted
by a low desorption energy in conjunction with a penetration coefficient of 10−5,
but one has to bear in mind that laboratory scale experiments on Cs-plate out
like LAMINAR-Loop [19] led to completely different results. Figure 7 also
contains a postcalculation of Cs-plate out using parameters measured in
laboratory scale experiments, which obviously do not fit to the VAMPYR-II
results. This also may be taken as an indication that Cs does not occur in
elemental gaseous form in the AVR. Taking that into account, there are two
explanations on this deviating Cs-behavior:Figure 7
Cs plate out in VAMPYR-II compared to calculations.Figure 8
Activity distribution in VAMPYR-II-04.Figure 9
Cs distribution in VAMPYR-II experiments.(i)
Cs is already dust borne in the hot gas;(ii)
Cs occurs as a chemical compound, which has a smaller desorption energy than
elemental Cs. Comparing
VAMPYR-II experiments with and without dust filter upflow of the plate out
section, however, does not lead to different results concerning Cs plate out,
because Cs passes this dust filter. This is taken as additional indication that
Cs is not mainly dust borne in the AVR hot gas. As also seen in Figure 8, Co-60
is retained with high efficiency already in the first dust filter in front of
the plate out section. Co-60 is known to be dust borne. However, as seen from Figure
8, most of the Cs in Experiment 4 is retained in the second dust filter, which
is situated downflow of the plate out section. The main difference between the
2 dust filters is the temperature, so condensation of a Cs-compound (CsOH) may
be the dominating deposition mechanism. Formation of CsOH was found to be
possible in AVR normal operation from kinetic and chemical thermodynamic point
of view. Similar holds for future reactors because a conservation of a protecting
oxide layer on metals requires a sufficient oxidation potential, which is
supplied by sufficiently high H2O/H2 ratio. The
deposition behavior  of those Cs-compounds under HTR conditions is not exactly
known but desorption energies are expected to be much smaller than for atomic
Cs. Figure 9 contains the distribution of Cs-137 onto the different components
of VAMPYR experiments.The fission
product deposition in VAMPYR-II occured mainly on metallic surfaces because a
complete dust layer could not be formed within the short period of operation,
and because some experiments contained a dust filter upflow of the plate out
section. As will be discussed more detailed in Section5, VAMPYR-II conditions
are therefore not completely representative for pebble-bed reactors.A similar plate out behavior  of Ag as above is observed in the COMEDIE
experiments [17, 20, 21]. Concerning Cs plate out, it is interesting to note that in
the first COMEDIE series, a flat profile with altogether low deposition was
observed too [20, 21], whereas in the second [17], a more classic (decreasing)
deposition profile combined with a higher deposition degree was found. A major source
for graphitic dust in COMEDIE is not seen, also because no movement of
graphitic components occurred. Thus, fission product chemistry seems to be the
reason for the “unusual" plate out behavior  of Cs in [20, 21]. It is worth to look
into differences in experimental conditions between both COMEDIE experimental series,
because here, one key for understanding the Cs-behavior  might be discovered.Iodine
plate out was measured on a stainless steel tube in VAMPYR-I [3, 14]. The plate
out profile can be crudely approximated by a desorption energy of 170 kJ/mol.


## 4.2. Influence of Dust on Fission
Product Plate out

Plate out
experiments for HTR metallic components are performed so far on metal specimen.
However, there is some doubt from HTR experience, whether that represents most
components of pebble-bed HTRs because of the presence of carbon-based dust. On
virtually all surfaces in the coolant circuit of the THTR-300 a graphitic dust
layer was present, which had a thickness of about 5-to-10μm (total full power
operation time of THTR-300: 16 months) [22]. A visible black dust layer is
present also on most AVR surfaces. Further, Cs and Sr in the coolant circuit of
the Peach Bottom HTR with block-type fuel were found to be greater than 80% in
carbonaceous layers, which covered almost all surfaces [12]. In the Peach
Bottom HTR, the carbonaceous layers were formed almost completely by continuous
small lubrication oil ingress from blowers. Examinations on the AVR blower nozzle
revealed that deposited fission products can be removed by wiping to the same
extent as by an electrolytic surface treatment of 1.8 μm in metal depth [23]. This
however does not hold for fast-moving parts of the blower, where shear forces
are too high for dust deposition. Accordingly, the activity may be present dust-bound
on the nozzle but not in other fast moving parts of the blower. Conclusions
drawn from blower examination suffer however from the fact that the operating
blower came in touch with liquid water for several days during a water ingress in
1978 and that the blower examinations occurred shortly after the water ingress.
There may have happened some decontamination of the blower already during the
water ingress.The
arguments presented before support the assumption that these fission products
are not plated out on the metal (and are thus less mobile) but mainly stuck in the
mobile graphitic dust layers. An additional argument lies in the very large
sorption capability of matrix graphite compared with metal surfaces for Cs and
Sr, as demonstrated in Figure3. The sorption capability of graphite is already
for a layer thickness of 1 μm substantial larger than that of the underlying
metal surface as parameter calculations using SPATRA revealed. These
calculations were performed on plate out of elemental Cs in an HTR-Module200-like
reactor, with and without a 1-μm matrix graphite layer on all metallic surfaces.
As expected, the results differ substantially in the high temperature range,
where deposition on graphitic dust dominates. Whereas for metal surfaces an
ad/desorption equilibrium is rapidly reached, the sorption capability of dust
is much larger and thus it acts as a perfect sink. In the low temperature range,
the deposition is mass transfer controlled for both, metals and graphite, that
is, perfect sink behavior . Accordingly, the deposition will be proportional to
the accessible geometrical surface of graphite and metal. The results are qualitatively
similar for desorption energies from LAMINAR-loop and from Experiment 4 of
VAMPYR-II. Because of the pronounced chemisorption enthalpy, associated with
sorption of Cs to graphite, and in line with chemical equilibrium calculations
[11], it has to be assumed that fission product chemistry does not change this
result in general although the deposition kinetics may be different.The
assumption of predominant fission product sorption on graphitic dust in pebble-bed
reactors changes, as outlined before, the deposition profile in a gas circuit,
but has also major safety implications. Because dust is easier to remobilize
than activities plated out on metals, the source terms in a fast depressurization
with its substantial shear forces will increase remarkably. A detailed
discussion of the dust behavior  in AVR and of its safety implications is
presented in Section5 of this paper.


## 4.3. Plate out Limits for a Gas Turbine

Plate out on metals/metal oxides
remains relevant for HTRs with block-type fuel and also for fast-rotating
components of pebble-bed reactors, particularly for a gas turbine. During the required
hand on maintenance of a gas turbine, the annual dose limit of a radiation
worker of 20 mSv has to be met, which limits the contamination of the turbine
during maintenance, for example, for Cs-137 to about10−6 of a core
inventory (400MWth reactor), as the dose rates of Table 4 indicate.Table 4
Dose rates on a gas turbine [5].
NuclideDose rate [mSv/(GBq⋅h)] in adistance of 1 m from a gas turbineCs-1370.10Ag-110 m0.45Consideration of AVR core release rates for the final stage
of operation with modern fuel at 950C° (see Figure 4), corrected with a factor
of 5 for the known underestimation of core release rates by VAMPYR-I and
assuming a plate out of 10% of the core release on the gas turbine results
leads to contamination values much higher than tolerable. Thus, satisfaction of
dose limits becomes a challenging item and requires further examinations.
Enhanced AVR core temperatures have to be discussed in this context too (see Section 6.2) [6].


## 5. AVR Experience on Dust and Dust-borne Activity

Dust played a major role in AVR fission product
transport and will be relevant for safety examinations of future pebble-bed
reactors too. This is due to the mobility of dust and due to its large specific
activities.


### 5.1. Dust:
Characterization and Formation Mechanisms

Maximum specific
activities in AVR dust were100GBqCs‐137/kg, 30GBqAg‐100m/kg
and 400GBqSr‐90/kg. These activities, which are not necessarily
typical for future reactors, were measured in 1986 on dust sampled from the
steam generator [24]. In 1976/1977 specific dust activities of even up to 2200GBqSr‐90/kg (700 ppm) were measured in the cold-gas filter. Part
of the activity is present already in dust during formation from fuel elements,
but the major part comes from sorption of molecular activities by the dust. The
latter is concluded from the observation that specific activities of dust sampled
from fuel element surfaces and from the fuel handling system are smaller than
maximum specific activities given above. In the past, it was assumed that only
about 10 to 15% of Cs and Sr are dust borne in AVR [24]. However this value is
based on the assumption that the deposition on the steam generator occurs as
plate out on metals and not on an overlying graphite layer. As already discussed before, most surfaces of pebble-bed
HTRs are covered with carbon layers, which predominantly absorb metallic
fission products.Analyses of dust sampled in
filters revealed a metal content of 5 to 15%, the main part is graphite. The
metal and metal oxide fraction was particularly high until 1975. Co-60 is formed by activation of
Co-containing alloys and of Co-impurities in carbon components, and distributed
dust borne in the whole circuit. Typical specific activities of Co-60 in AVR
dust were 1 to 5 GBq/kg, its stationary gas-borne activity was about10Bq/m3STP.
Because that hinders maintenance, Co-alloys should be avoided in HTRs as much
as possible.Dust formation in AVR by
abrasion occurred mainly in the active core and in the fuel element handling
system. Abrasion from reflectors plays some role too. Neutron irradiation may
enhance abrasive dust production. Besides by abrasion, a large amount of dust
was produced in AVR 1982 by a unintentional oil ingress (0.12m3) and subsequent
cracking of the oil, producing up to about 70 to 75 kg of loose carbon dust and
of carbonaceous layers on surfaces (see AVR Quarterly Progress Report III/1983
in [15]). Some soot-like dust was observed, but the consistence of the solid
cracking product depends on the cracking temperature and on the nature of the
solid substrate where the carbonaceous layer is formed on. In 1971 a unintentional feeding of 100 m3 of air into the
primary circuit induced a peeling of outer fuel element layers, which probably
increased the dust level by about 8 kg. The detailed mechanism of this peeling
effect is however not understood. The total dust amount is not exactly known,
but assumed to about 100 to 200 kg. A former published value of 60 kg [24] had
to be increased after inspections during dismantling and after consideration of
the oil ingress. Dust produced by abrasion is roughly estimated to 50 to 80 kg,
corresponding to a dust production rate of 5 kg/yoperation.
Neglecting reflector abrasion, which cannot yet be quantified, this is
equivalent to a fuel element weight loss of 0.2%. There were several out-of-pile
experiments on dust production rates, which were performed in air. These
experiments are not sufficiently representative for reactor conditions because
friction coefficients of
graphites are strongly reduced by a chemisorbed oxygen layer, which is not
present under reactor conditions.Size analyses of AVR dust performed
at the end of AVR life revealed an average number weighted diameter of 0.6μm
only, independent on the origin of dust (see Figure 10). This was originally not
expected for dust formed by abrasion, because typical grain sizes of fuel
element matrix and reflector graphites are much larger. However, experience on
interactions of dust particles with a turbine indicated that large dust
particles are crushed. The crushing probability increases strongly with the
particle diameter. Some crushing may have taken place in the AVR blowers too. Dust
found in filters consists partly of agglomerates. Agglomeration may have taken
place in filters and/or already gas circuit; a separation of agglomerates into
original particles was possible by ultrasonic treatment.Figure 10
Number weighted size distribution of AVR dust.Figure 11
Dust concentrations in AVR in blower transient T3 (measured by dust filter
experiment).It has to be noted that preliminary measurements
performed during the early AVR operation led to dust sizes in the range of 5μm. These measurements were however performed before of the oil ingress 1982. It
cannot be excluded that the dominating presence of dust less than or equal to 1 μm
is connected to the oil ingress. Visual inspections of dust just after the oil
ingress support this assumption. Only one large graphite particle was found in
an AVR dust filter: its weight was 130 mg. Large metal particles were more
frequently observed in these filters. A remarkable amount of coarse-grained
dust was found during AVR dismantling on the bottom reflector and in cracks of
it. The latter was however not examined in detail. Based on the early diameter
measurements, dust filters in AVR were mainly designed for average particle
sizes of greater than 2 μm. For that, an incomplete filtration of dust may have
taken place in some dust filters, but not in the cold-gas filter (Experiment 2
in Figure 1).


### 5.2. Results of Dust Filter Experiments

Besides dust sampling from VAMPYR-I and VAMPYR–II and from cold-gas
experiment for stationary operation, there are results from 15 stationary and 3
transient campaigns with the dust experiment 1984–1988. Condensable fission
products in the cold gas were found mainly dust borne, particularly for
transient operation.The most surprising results came from dust experiments
with blower transients with online detection of some relevant nuclides. The
blower was operated 2 days after reactor shut down at about 1500min⁡−1 until stationary conditions were achieved. Then, the blower frequency was
increased within less than 1 minute to 3000min⁡−1. Already for these
frequencies, which are smaller than in normal operation (3300 to 3600min⁡−1),
the dust concentration in the coolant increased by a factor of about 50 to 100
[7, 25]. Acceleration from 1500 to 4000min⁡−1 increased the dust
concentration even by a factor of 200 to 400. Obviously already small
perturbations in the flow pattern may lead to a significant dust remobilization.
Mobilization at low shear forces was observed also on dust layers out of pile [26].
Vibrations of components during transients may have played a role too. The
transient dust concentrations in the dust filter experiment are shown in Figure 11 indicating that dust redeposition occurs with time constants of 1 to 2 hours.In line with this slow dust
deposition is the postexamination result that no particle in these dust filter
experiments with a size greater than 1.5μm was found after ultrasonic treatment.
Provided that dust agglomeration happened mainly in the dust filter, this
result might support the hypothesis that no dust from dead water areas is remobilized
(where mainly large particles are expected to be sedimented), but dust
adhesively bound on surfaces. Dust is perhaps deposited on surfaces in
multilayers where adhesion forces decrease with increasing layer thickness; the
maximum layer thickness is in that case given by the shear forces of stationary
operation.Looking at the specific activities of dust in
transient experiments, it becomes obvious that I-131 activities are with up to
3.5 GBq/kg relatively high, having in mind that the total I-131 activity
outside the active core is in the range of 50 GBq [24]. Probably, the dust which
is remobilized contains some more short-lived nuclides than the remainder and
there is not much iodine plated out on metal surfaces (as already discussed
before for Cs), respectively, sorbed on graphite components. The specific
activities of long-lived nuclides as Cs-137 in the dust, mobilized during
transients, was by a factor of up to 4 larger than in comparable stationary
experiments and almost of the same order as in 1976, when specific activities
were highest. Here, we have to assume that old dust with larger specific
activities is mobilized in larger amounts too. The same conclusion has to be
drawn from dust sampling by the cold-gas filter under stationary conditions. Sr-90
shows for all cold-gas experiments the highest sampled activity, even for
experiments at low hot gas temperatures, when Sr-90 release is expected to be
low. For the time period 1986–1988, the sampled Sr-90 activity in the cold-gas
experiment is by about a factor of 4 to 8 higher than the Cs-137 one and by
about a factor of 20 to 200 higher than the Ag-110 m one. This may be explained
as follows: dust is settled and mobilized several times during operation until
a stable position is found. For the same reason, a correlation between sampled
Sr-90 activity in dust and hot gas temperatures is not always found. Precise
Sr-90 measurements are however not available for all runs of the cold-gas
experiment. Summing up, dust filters integrate particularly for Sr-90 to some
extent the prior release history of AVR. Not only fresh dust is sampled, but
also dust which is already present for a long time in the coolant circuit.The total stationary dust concentration in the
coolant, as measured by the dust filter of the cold-gas experiment, is shown in
Figure12, indicating its low values. The dust concentrations do not show a
dependence on coolant temperatures. Their large scatter reflects variations of
the flow pattern in the coolant circuit and of dust formation rates during
sampling. Dust concentrations measured in the cold-gas experiment are a factor
of 2 to3 smaller than in the dust filter experiment and by a factor of 3 to 4
smaller than in the dust filter of VAMPYR-I. This is due to differences in dust
deposition along the pipes to the filters.Figure 12
Gas borne dust concentrations in AVR operation (dust filter of the cold gas
experiment).A reliable model for dust behavior  in HTRs including
interaction with fission products does not yet exist. For that, there are no model-based
evaluations of AVR dust-related data.


### 5.3. Relevance
of Dust for Source Terms in Accidents of Pebble-Bed Reactors

In modern HTRs, friction forces in the active core are
about an order of magnitude larger than in AVR (inverse He flow, greater core
height), so dust production in the active core will increase. On the other
hand, future reactors will be supplied with a dust filter in the fuel handling system.
Conservative design values for dust production rates in modern small pebble-bed
HTRs(400MWth) are estimated to about 50 to 100 kg/y. Detailed
knowledge of the dust amount is not as relevant for analyses on dust-borne
activities as on dust explosions. The sorption capability of dust for fission
products is even for the lower uncertainty limit of dust production rates
sufficient for absorbing the activities in normal operation. On the other hand,
for dust explosions exists a concentration limit, which is 70 g/m3 for 4 μm
graphite particles in ambient air. The maximum overpressure of dust explosions is
6.5 bar, as experiments on related dust problems in fusion reactors revealed [27].
Dust explosions are mainly relevant for depressurizations into a building
containing air. Here, dust sedimentation and lift-off and their influence on
the probability of a local inflammable mixture is a problem, not present in
solely gaseous mixtures. The possibility of incineration or even explosion of
dust in filters, affecting their retention ability, should be examined. Matrix
graphite dust shows a higher reactivity in air at low temperatures [28], and thus
will have a lower ignition limit than the referenced 4 μm dusts.Altogether, despite of the small dust remobilization
in the transient AVR dust experiments the state of knowledge indicates that for
fast depressurizations with shear forces substantially higher than in normal
operation and/or very different flow pattern, a remarkable mobilization of the dust-bound
activity has to be considered excluded [17, 26], at least not within the
conservative treatment of design basis accidents (DBAs). Unfortunately, there
are no additional representative experiments on mobilization of dust as in pebble-bed
reactors available. A depressurization experiment of the AVR primary circuit
with measurement of the mobilized activity was planned for the last operation
year but was not commissioned. An experimental facility for lift-off
experiments on AVR specimen is at present in Juelich under construction.
However, it is not clear to which extent AVR specimen representative for future
pebble-bed reactors are still available. Representative specimen should consist
of dust formed by abrasion, but the incidental oil ingress of 1982 has probably
changed most AVR surfaces by oil cracking and has also changed the average dust
morphology. For that, specimens from THTR might be more suitable. The lift-off
behavior  of dust formed by abrasion is expected to be different from that of carbonaceous
layers formed by oil cracking. There is some experience on lift-off of
carbonaceous layers formed by unintentional oil ingress in the Peach Bottom HTR
[29]. Results of these lift-off experiments of 2-minute duration indicate that
lift-off of Cs and Sr remains less than or equal to 1% for shear forces up to
those present in normal operation of the reactor. Depending on temperature
history and on substrate composition of the specimen, a lift-off of 2 to 25%
was measured for shear forces of a factor 5 higher than in normal operation.
Even larger shear forces are expected for fast depressurizations.Plate out
activities on metals are much less mobilized in depressurizations than dust-borne
ones as lift-off experiments indicate [17]. Past safety analyses on pebble-bed reactors
assumed most activities in the coolant circuit to be plated out on metals and
only a minor fraction dust bound. Accordingly, source terms in
depressurizations were underestimated and the former conclusion that filtration
is superfluous due to low activity content of the released gas is no longer
valid. Thus, a carefully filtered release of
fast depressurizations is essential for pebble-bed HTRs. This is particularly
true for arrangements of several HTRs on one site, as is foreseen, because the
accident frequencies per site increase proportionally to the number of reactors
there. Such filtration is state of the art for nuclear systems equipped with a gas-tight
containment. However, it is not yet clear whether a sufficient fast and
efficient filtration is achievable for the vented, not pressure-retaining
confinements of present pebble-bed HTR concepts. Summing up the efficient
filtration of dust with its small particle diameter remains a challenging
problem.


## 5.1. Dust:
Characterization and Formation Mechanisms

Maximum specific
activities in AVR dust were100GBqCs‐137/kg, 30GBqAg‐100m/kg
and 400GBqSr‐90/kg. These activities, which are not necessarily
typical for future reactors, were measured in 1986 on dust sampled from the
steam generator [24]. In 1976/1977 specific dust activities of even up to 2200GBqSr‐90/kg (700 ppm) were measured in the cold-gas filter. Part
of the activity is present already in dust during formation from fuel elements,
but the major part comes from sorption of molecular activities by the dust. The
latter is concluded from the observation that specific activities of dust sampled
from fuel element surfaces and from the fuel handling system are smaller than
maximum specific activities given above. In the past, it was assumed that only
about 10 to 15% of Cs and Sr are dust borne in AVR [24]. However this value is
based on the assumption that the deposition on the steam generator occurs as
plate out on metals and not on an overlying graphite layer. As already discussed before, most surfaces of pebble-bed
HTRs are covered with carbon layers, which predominantly absorb metallic
fission products.Analyses of dust sampled in
filters revealed a metal content of 5 to 15%, the main part is graphite. The
metal and metal oxide fraction was particularly high until 1975. Co-60 is formed by activation of
Co-containing alloys and of Co-impurities in carbon components, and distributed
dust borne in the whole circuit. Typical specific activities of Co-60 in AVR
dust were 1 to 5 GBq/kg, its stationary gas-borne activity was about10Bq/m3STP.
Because that hinders maintenance, Co-alloys should be avoided in HTRs as much
as possible.Dust formation in AVR by
abrasion occurred mainly in the active core and in the fuel element handling
system. Abrasion from reflectors plays some role too. Neutron irradiation may
enhance abrasive dust production. Besides by abrasion, a large amount of dust
was produced in AVR 1982 by a unintentional oil ingress (0.12m3) and subsequent
cracking of the oil, producing up to about 70 to 75 kg of loose carbon dust and
of carbonaceous layers on surfaces (see AVR Quarterly Progress Report III/1983
in [15]). Some soot-like dust was observed, but the consistence of the solid
cracking product depends on the cracking temperature and on the nature of the
solid substrate where the carbonaceous layer is formed on. In 1971 a unintentional feeding of 100 m3 of air into the
primary circuit induced a peeling of outer fuel element layers, which probably
increased the dust level by about 8 kg. The detailed mechanism of this peeling
effect is however not understood. The total dust amount is not exactly known,
but assumed to about 100 to 200 kg. A former published value of 60 kg [24] had
to be increased after inspections during dismantling and after consideration of
the oil ingress. Dust produced by abrasion is roughly estimated to 50 to 80 kg,
corresponding to a dust production rate of 5 kg/yoperation.
Neglecting reflector abrasion, which cannot yet be quantified, this is
equivalent to a fuel element weight loss of 0.2%. There were several out-of-pile
experiments on dust production rates, which were performed in air. These
experiments are not sufficiently representative for reactor conditions because
friction coefficients of
graphites are strongly reduced by a chemisorbed oxygen layer, which is not
present under reactor conditions.Size analyses of AVR dust performed
at the end of AVR life revealed an average number weighted diameter of 0.6μm
only, independent on the origin of dust (see Figure 10). This was originally not
expected for dust formed by abrasion, because typical grain sizes of fuel
element matrix and reflector graphites are much larger. However, experience on
interactions of dust particles with a turbine indicated that large dust
particles are crushed. The crushing probability increases strongly with the
particle diameter. Some crushing may have taken place in the AVR blowers too. Dust
found in filters consists partly of agglomerates. Agglomeration may have taken
place in filters and/or already gas circuit; a separation of agglomerates into
original particles was possible by ultrasonic treatment.Figure 10
Number weighted size distribution of AVR dust.Figure 11
Dust concentrations in AVR in blower transient T3 (measured by dust filter
experiment).It has to be noted that preliminary measurements
performed during the early AVR operation led to dust sizes in the range of 5μm. These measurements were however performed before of the oil ingress 1982. It
cannot be excluded that the dominating presence of dust less than or equal to 1 μm
is connected to the oil ingress. Visual inspections of dust just after the oil
ingress support this assumption. Only one large graphite particle was found in
an AVR dust filter: its weight was 130 mg. Large metal particles were more
frequently observed in these filters. A remarkable amount of coarse-grained
dust was found during AVR dismantling on the bottom reflector and in cracks of
it. The latter was however not examined in detail. Based on the early diameter
measurements, dust filters in AVR were mainly designed for average particle
sizes of greater than 2 μm. For that, an incomplete filtration of dust may have
taken place in some dust filters, but not in the cold-gas filter (Experiment 2
in Figure 1).


## 5.2. Results of Dust Filter Experiments

Besides dust sampling from VAMPYR-I and VAMPYR–II and from cold-gas
experiment for stationary operation, there are results from 15 stationary and 3
transient campaigns with the dust experiment 1984–1988. Condensable fission
products in the cold gas were found mainly dust borne, particularly for
transient operation.The most surprising results came from dust experiments
with blower transients with online detection of some relevant nuclides. The
blower was operated 2 days after reactor shut down at about 1500min⁡−1 until stationary conditions were achieved. Then, the blower frequency was
increased within less than 1 minute to 3000min⁡−1. Already for these
frequencies, which are smaller than in normal operation (3300 to 3600min⁡−1),
the dust concentration in the coolant increased by a factor of about 50 to 100
[7, 25]. Acceleration from 1500 to 4000min⁡−1 increased the dust
concentration even by a factor of 200 to 400. Obviously already small
perturbations in the flow pattern may lead to a significant dust remobilization.
Mobilization at low shear forces was observed also on dust layers out of pile [26].
Vibrations of components during transients may have played a role too. The
transient dust concentrations in the dust filter experiment are shown in Figure 11 indicating that dust redeposition occurs with time constants of 1 to 2 hours.In line with this slow dust
deposition is the postexamination result that no particle in these dust filter
experiments with a size greater than 1.5μm was found after ultrasonic treatment.
Provided that dust agglomeration happened mainly in the dust filter, this
result might support the hypothesis that no dust from dead water areas is remobilized
(where mainly large particles are expected to be sedimented), but dust
adhesively bound on surfaces. Dust is perhaps deposited on surfaces in
multilayers where adhesion forces decrease with increasing layer thickness; the
maximum layer thickness is in that case given by the shear forces of stationary
operation.Looking at the specific activities of dust in
transient experiments, it becomes obvious that I-131 activities are with up to
3.5 GBq/kg relatively high, having in mind that the total I-131 activity
outside the active core is in the range of 50 GBq [24]. Probably, the dust which
is remobilized contains some more short-lived nuclides than the remainder and
there is not much iodine plated out on metal surfaces (as already discussed
before for Cs), respectively, sorbed on graphite components. The specific
activities of long-lived nuclides as Cs-137 in the dust, mobilized during
transients, was by a factor of up to 4 larger than in comparable stationary
experiments and almost of the same order as in 1976, when specific activities
were highest. Here, we have to assume that old dust with larger specific
activities is mobilized in larger amounts too. The same conclusion has to be
drawn from dust sampling by the cold-gas filter under stationary conditions. Sr-90
shows for all cold-gas experiments the highest sampled activity, even for
experiments at low hot gas temperatures, when Sr-90 release is expected to be
low. For the time period 1986–1988, the sampled Sr-90 activity in the cold-gas
experiment is by about a factor of 4 to 8 higher than the Cs-137 one and by
about a factor of 20 to 200 higher than the Ag-110 m one. This may be explained
as follows: dust is settled and mobilized several times during operation until
a stable position is found. For the same reason, a correlation between sampled
Sr-90 activity in dust and hot gas temperatures is not always found. Precise
Sr-90 measurements are however not available for all runs of the cold-gas
experiment. Summing up, dust filters integrate particularly for Sr-90 to some
extent the prior release history of AVR. Not only fresh dust is sampled, but
also dust which is already present for a long time in the coolant circuit.The total stationary dust concentration in the
coolant, as measured by the dust filter of the cold-gas experiment, is shown in
Figure12, indicating its low values. The dust concentrations do not show a
dependence on coolant temperatures. Their large scatter reflects variations of
the flow pattern in the coolant circuit and of dust formation rates during
sampling. Dust concentrations measured in the cold-gas experiment are a factor
of 2 to3 smaller than in the dust filter experiment and by a factor of 3 to 4
smaller than in the dust filter of VAMPYR-I. This is due to differences in dust
deposition along the pipes to the filters.Figure 12
Gas borne dust concentrations in AVR operation (dust filter of the cold gas
experiment).A reliable model for dust behavior  in HTRs including
interaction with fission products does not yet exist. For that, there are no model-based
evaluations of AVR dust-related data.


## 5.3. Relevance
of Dust for Source Terms in Accidents of Pebble-Bed Reactors

In modern HTRs, friction forces in the active core are
about an order of magnitude larger than in AVR (inverse He flow, greater core
height), so dust production in the active core will increase. On the other
hand, future reactors will be supplied with a dust filter in the fuel handling system.
Conservative design values for dust production rates in modern small pebble-bed
HTRs(400MWth) are estimated to about 50 to 100 kg/y. Detailed
knowledge of the dust amount is not as relevant for analyses on dust-borne
activities as on dust explosions. The sorption capability of dust for fission
products is even for the lower uncertainty limit of dust production rates
sufficient for absorbing the activities in normal operation. On the other hand,
for dust explosions exists a concentration limit, which is 70 g/m3 for 4 μm
graphite particles in ambient air. The maximum overpressure of dust explosions is
6.5 bar, as experiments on related dust problems in fusion reactors revealed [27].
Dust explosions are mainly relevant for depressurizations into a building
containing air. Here, dust sedimentation and lift-off and their influence on
the probability of a local inflammable mixture is a problem, not present in
solely gaseous mixtures. The possibility of incineration or even explosion of
dust in filters, affecting their retention ability, should be examined. Matrix
graphite dust shows a higher reactivity in air at low temperatures [28], and thus
will have a lower ignition limit than the referenced 4 μm dusts.Altogether, despite of the small dust remobilization
in the transient AVR dust experiments the state of knowledge indicates that for
fast depressurizations with shear forces substantially higher than in normal
operation and/or very different flow pattern, a remarkable mobilization of the dust-bound
activity has to be considered excluded [17, 26], at least not within the
conservative treatment of design basis accidents (DBAs). Unfortunately, there
are no additional representative experiments on mobilization of dust as in pebble-bed
reactors available. A depressurization experiment of the AVR primary circuit
with measurement of the mobilized activity was planned for the last operation
year but was not commissioned. An experimental facility for lift-off
experiments on AVR specimen is at present in Juelich under construction.
However, it is not clear to which extent AVR specimen representative for future
pebble-bed reactors are still available. Representative specimen should consist
of dust formed by abrasion, but the incidental oil ingress of 1982 has probably
changed most AVR surfaces by oil cracking and has also changed the average dust
morphology. For that, specimens from THTR might be more suitable. The lift-off
behavior  of dust formed by abrasion is expected to be different from that of carbonaceous
layers formed by oil cracking. There is some experience on lift-off of
carbonaceous layers formed by unintentional oil ingress in the Peach Bottom HTR
[29]. Results of these lift-off experiments of 2-minute duration indicate that
lift-off of Cs and Sr remains less than or equal to 1% for shear forces up to
those present in normal operation of the reactor. Depending on temperature
history and on substrate composition of the specimen, a lift-off of 2 to 25%
was measured for shear forces of a factor 5 higher than in normal operation.
Even larger shear forces are expected for fast depressurizations.Plate out
activities on metals are much less mobilized in depressurizations than dust-borne
ones as lift-off experiments indicate [17]. Past safety analyses on pebble-bed reactors
assumed most activities in the coolant circuit to be plated out on metals and
only a minor fraction dust bound. Accordingly, source terms in
depressurizations were underestimated and the former conclusion that filtration
is superfluous due to low activity content of the released gas is no longer
valid. Thus, a carefully filtered release of
fast depressurizations is essential for pebble-bed HTRs. This is particularly
true for arrangements of several HTRs on one site, as is foreseen, because the
accident frequencies per site increase proportionally to the number of reactors
there. Such filtration is state of the art for nuclear systems equipped with a gas-tight
containment. However, it is not yet clear whether a sufficient fast and
efficient filtration is achievable for the vented, not pressure-retaining
confinements of present pebble-bed HTR concepts. Summing up the efficient
filtration of dust with its small particle diameter remains a challenging
problem.


## 6. Other Source Term Relevant AVR Experience

### 6.1. Noble Gas Measurement and Related Quality Control
of Fuel

The noble
gas measurement is not directly related to the problems addressed here, because
noble gases are not deposited and do not create major safety problems. Their
tolerable release in DBA following German regulations is up to 8 orders of
magnitude larger than for key nuclides as Cs, Sr, and I. However, the noble gas
level was a direct indicator for the number of defective-coated particles in
the AVR core and its increase during operation. Accordingly, this experiment
allows a direct determination of the quality of the fuel elements present in
the reactor concerning coated particle failure; but in contrast to original expectations,
easily detectable noble gas release cannot be correlated with the release of
metallic fission products by diffusion: a late detected pronounced release of
metallic fission products leading to a heavy contamination of the coolant
circuit was a major problem of AVR operation.This item
has implications to the quality control of HTR fuel elements too: at present, the fuel manufacturing parameters
guaranteeing reasonable activity retention are known, but the related physical
fuel parameters are not sufficiently examined. Thus, monitoring of fuel
manufacture is the main quality-ensuring measure, but quality control after
manufacture is difficult. One reliable, but elaborate proof after manufacture
is irradiation of fuel specimen under conditions simulating normal operation,
and subsequent heatup to accident temperatures. This deficit in fuel quality
control abilities was thought to be compensable in AVR by a fast detection of
wrong fuel in operation by an enhanced noble gas release. However, as AVR
experience teaches, this does not hold for releases of metallic fission products
by diffusion. For that, additional effort in the fuel quality field is required.
Because the fuel element acts as the central element of the HTR barrier concept,
the presence of fuel giving no fast warning by a high noble gas level in normal
operation but failing in core heatup accidents has to be excluded at least for DBA.During AVR
operation, the total noble gas inventory in the coolant varied from 200–6000 GBq [9, 24],
depending on operational temperatures and was particularly high
during partial failure of a charge of BISO fuel elements with large fuel
content (GLE-1), which reached very high temperatures. A high noble gas level
was also observed even at lower temperatures after the water ingress of 1978
with its increased steam impurity level. This was due to the interaction
between steam and defective particles. Concerning safety, noble gas precursors
require some attention, particularly for Sr-90, but also for Cs-137.


### 6.2. Enhanced
AVR Core Temperatures

There is no way of online or local measurement of pebble-bed
(active core) temperatures in contrast to other reactors. This is, because the
moving pebble bed destroys all equipments. Average hot gas temperatures outside
of the active core were occasionally measured online in AVR at the same
positions as VAMPYR-I and VAMPYR-II as discussed in Section4.2. Talking about
temperatures in the active core, we have to distinguish between coated particle
temperatures, fuel element surface temperatures, and gas temperatures.
Depending on its power, the coated particle temperature of a fuel element
positioned near the hot gas exit was in AVR up to 120 K higher than the surface temperature. Gas
temperatures in this position were calculated to about 30±10 K lower
than fuel element surface temperatures. In this paper, core temperature means
fuel element surface temperature. Maximum (best estimate) core temperatures in
AVR were calculated to 1070C° for average hot gas temperatures of 950C° [30],
assuming a core composition as in 1974. In later operation with different core
compositions, higher maximum core temperatures of up to 1140C° were calculated
for small fuel-element fractions (less than 1% at temperatures greater than
1100C°) and limited operation time.Some effort was spent in measuring maximum
AVR core temperatures, which was however not completely successful before 1988:
In September 1986, 190 graphite pebbles equipped with melt wires (melting
points between 650 and 1280C°) were fed onto the core top. For most of the
time, when the monitor pebbles were calculated to pass the hot core top, the
AVR average hot gas temperature was at 950C°. These monitor pebbles record the
maximum gas temperature occurring around the monitor pebble, plus some minor
contribution of about 6 to 10 K by γ and neutron heating. Accordingly, the
corresponding core temperature is about 20 K higher than the recorded maximum
temperature. The results for 144 monitor pebbles, extracted until end of AVR
operation, are as follows [31–33]: except of about 1/3 of the pebbles fed
into the radial inner core zone, all pebbles revealed core higher temperatures
than the originally calculated maximum temperature of about 1100C°, 1/3 of the
pebbles fed into the radial outer zone even by more than 200 K, that is, all
melt wires were molten. Whereas almost 90% of the fuel elements fed onto the
inner core zone were evaluated, this holds for only about 55% of the monitor
pebbles fed onto the outer core zone. The unevaluated monitor pebbles belong
probably to the outmost part of the outer core zone, where pebble flow
velocities are expected to be lowest. Accordingly, a complete figure of maximum
temperatures in AVR is not yet available. Figure 4 reveals that Cs-release was
comparatively low in the last quarter of 1986, when monitor pebbles passed the
core top. The same holds for noble gas release [32]. Accordingly, the
impermissible high temperatures occurred probably during most of the AVR
operation.The unintentional high core
temperatures transgressed safety limits. Accordingly, the permit for an
operation at 950C° hot gas temperature was withdrawn and hot gas temperatures were
limited to 810C° for the rest of its operation time, that is, from February
1988, when first temperature results became available [6, 32]. Thus, a repetition
of temperature measurements at 950C° hot gas temperature in order to determine
the real maximum core temperature was not permitted.The reasons for the high AVR core
temperatures are not yet understood, several AVR specific but also other reasons
immanent to pebble beds are in discussion (gas bypass flow, power peaks near to
reflector noses, pebble bed stochastics and locally densified pebble bed, error
in axial power distribution, radial asymmetric power profile, etc.) [34]. At
present, some theoretical effort is spent in order to clarify this problem.The unintentional high temperatures are relevant also for
examinations of source terms: as discussed in Section2, a pronounced increase
in release rates of metallic fission products (Sr, Cs, Ag) was observed after
the hot gas temperature increase from 850 to 950C° in February, 1974. As
outlined in [6], this enhanced release has to be mainly attributed to the
unintentional high AVR temperatures and not, as assumed in the past, to
insufficient fuel quality only. Other safety-relevant implications of the unintentional
high core temperatures are discussed in [6] too.


### 6.3. Implications
of Water Ingress on Fission Product Behavior

A slow water ingress of 27500 kg in 1978 led to a
substantial transfer of activity (mainly Sr and H-3 to a minor extent Cs and I)
from primary circuit components into the water [35]. Some of the
water went unintentionally into the grounding of AVR, where it is still
present. Traces of Sr-90 were detected in the soil surrounding the reactor [36].
The exact size of this Sr-90 contamination is not yet known but will be
examined after dismantling of the AVR.There was
also some graphite oxidation during this accident and in the subsequent drying
phase. However, when large amounts of water had entered the primary circuit, core
temperatures were substantially lower than in normal operation. So the maximum
oxidation degree observed on fuel elements depending on the type was only 0.2–0.5%. After
water ingress, some standard oxidation tests (1 kPa steam in He, 1000C°) were
performed on fuel elements: A substantial acceleration of oxidation rates was
observed for some fuel elements by catalytic effects. On the one hand side metallic
corrosion products formed by the water ingress were absorbed by the fuel
elements and induced pitting corrosion. On the other hand, the large
Sr-concentration in the fuel element graphite increased oxidation rates in
steam homogeneously over the fuel element surface by up to a factor of 5 by
catalysis.It remains
to be examined, whether local high hot gas temperatures as observed [6, 13] may
have initiated the AVR steam generator leak.


### 6.4. Experiments on Coolant
Chemistry

There were
continuously experiments with impurity injections into the AVR coolant circuit (CO2, H2, H2O, N2, CH4, NH3) [32, 37].
Interesting for understanding of the H-3 behavior  are hydrogen injection
experiments, which led—by exchange
reactions with H-3 sorbed on graphite—to a large
emission of H-3 into the gas phase. Based also on such experiments, it was
concluded that nitrogen and hydrogen are present gas borne in HTRs only to
about 0.1%, but mainly sorbed on graphite.


### 6.5. Pebble Rupture in Operation

Rupture of fuel
element pebbles in operation leads to destruction of coated particles and to
the presence of actinides, particularly Pu-241, in dust. Similar was observed
in the THTR, where a far more pronounced pebble rupture than estimated (8000
pebbles in 16 months full power operation) [22] was observed. Whereas the majority
of the pebble rupture in THTR may be attributed to control rod insertion, some may
also be due to the pebble movement. Although the total actinide activities in
AVR are small (some ten GBq of Pu-241 in AVR dust), they cannot be neglected
because of their pronounced toxicity due to their α-activity
and because of the mobility of dust. Altogether, only 220 of 2.4 Mio cycled
pebbles ruptured in AVR [32]. There exists, however, no reliable model for
pebble rupture up to now, as also the THTR experience indicates [22]. Extrapolation
to modern facilities has to consider the same arguments as discussed before for
dust formation, that is, a higher rupture rate than in AVR has to be taken into
account.


## 6.1. Noble Gas Measurement and Related Quality Control
of Fuel

The noble
gas measurement is not directly related to the problems addressed here, because
noble gases are not deposited and do not create major safety problems. Their
tolerable release in DBA following German regulations is up to 8 orders of
magnitude larger than for key nuclides as Cs, Sr, and I. However, the noble gas
level was a direct indicator for the number of defective-coated particles in
the AVR core and its increase during operation. Accordingly, this experiment
allows a direct determination of the quality of the fuel elements present in
the reactor concerning coated particle failure; but in contrast to original expectations,
easily detectable noble gas release cannot be correlated with the release of
metallic fission products by diffusion: a late detected pronounced release of
metallic fission products leading to a heavy contamination of the coolant
circuit was a major problem of AVR operation.This item
has implications to the quality control of HTR fuel elements too: at present, the fuel manufacturing parameters
guaranteeing reasonable activity retention are known, but the related physical
fuel parameters are not sufficiently examined. Thus, monitoring of fuel
manufacture is the main quality-ensuring measure, but quality control after
manufacture is difficult. One reliable, but elaborate proof after manufacture
is irradiation of fuel specimen under conditions simulating normal operation,
and subsequent heatup to accident temperatures. This deficit in fuel quality
control abilities was thought to be compensable in AVR by a fast detection of
wrong fuel in operation by an enhanced noble gas release. However, as AVR
experience teaches, this does not hold for releases of metallic fission products
by diffusion. For that, additional effort in the fuel quality field is required.
Because the fuel element acts as the central element of the HTR barrier concept,
the presence of fuel giving no fast warning by a high noble gas level in normal
operation but failing in core heatup accidents has to be excluded at least for DBA.During AVR
operation, the total noble gas inventory in the coolant varied from 200–6000 GBq [9, 24],
depending on operational temperatures and was particularly high
during partial failure of a charge of BISO fuel elements with large fuel
content (GLE-1), which reached very high temperatures. A high noble gas level
was also observed even at lower temperatures after the water ingress of 1978
with its increased steam impurity level. This was due to the interaction
between steam and defective particles. Concerning safety, noble gas precursors
require some attention, particularly for Sr-90, but also for Cs-137.


## 6.2. Enhanced
AVR Core Temperatures

There is no way of online or local measurement of pebble-bed
(active core) temperatures in contrast to other reactors. This is, because the
moving pebble bed destroys all equipments. Average hot gas temperatures outside
of the active core were occasionally measured online in AVR at the same
positions as VAMPYR-I and VAMPYR-II as discussed in Section4.2. Talking about
temperatures in the active core, we have to distinguish between coated particle
temperatures, fuel element surface temperatures, and gas temperatures.
Depending on its power, the coated particle temperature of a fuel element
positioned near the hot gas exit was in AVR up to 120 K higher than the surface temperature. Gas
temperatures in this position were calculated to about 30±10 K lower
than fuel element surface temperatures. In this paper, core temperature means
fuel element surface temperature. Maximum (best estimate) core temperatures in
AVR were calculated to 1070C° for average hot gas temperatures of 950C° [30],
assuming a core composition as in 1974. In later operation with different core
compositions, higher maximum core temperatures of up to 1140C° were calculated
for small fuel-element fractions (less than 1% at temperatures greater than
1100C°) and limited operation time.Some effort was spent in measuring maximum
AVR core temperatures, which was however not completely successful before 1988:
In September 1986, 190 graphite pebbles equipped with melt wires (melting
points between 650 and 1280C°) were fed onto the core top. For most of the
time, when the monitor pebbles were calculated to pass the hot core top, the
AVR average hot gas temperature was at 950C°. These monitor pebbles record the
maximum gas temperature occurring around the monitor pebble, plus some minor
contribution of about 6 to 10 K by γ and neutron heating. Accordingly, the
corresponding core temperature is about 20 K higher than the recorded maximum
temperature. The results for 144 monitor pebbles, extracted until end of AVR
operation, are as follows [31–33]: except of about 1/3 of the pebbles fed
into the radial inner core zone, all pebbles revealed core higher temperatures
than the originally calculated maximum temperature of about 1100C°, 1/3 of the
pebbles fed into the radial outer zone even by more than 200 K, that is, all
melt wires were molten. Whereas almost 90% of the fuel elements fed onto the
inner core zone were evaluated, this holds for only about 55% of the monitor
pebbles fed onto the outer core zone. The unevaluated monitor pebbles belong
probably to the outmost part of the outer core zone, where pebble flow
velocities are expected to be lowest. Accordingly, a complete figure of maximum
temperatures in AVR is not yet available. Figure 4 reveals that Cs-release was
comparatively low in the last quarter of 1986, when monitor pebbles passed the
core top. The same holds for noble gas release [32]. Accordingly, the
impermissible high temperatures occurred probably during most of the AVR
operation.The unintentional high core
temperatures transgressed safety limits. Accordingly, the permit for an
operation at 950C° hot gas temperature was withdrawn and hot gas temperatures were
limited to 810C° for the rest of its operation time, that is, from February
1988, when first temperature results became available [6, 32]. Thus, a repetition
of temperature measurements at 950C° hot gas temperature in order to determine
the real maximum core temperature was not permitted.The reasons for the high AVR core
temperatures are not yet understood, several AVR specific but also other reasons
immanent to pebble beds are in discussion (gas bypass flow, power peaks near to
reflector noses, pebble bed stochastics and locally densified pebble bed, error
in axial power distribution, radial asymmetric power profile, etc.) [34]. At
present, some theoretical effort is spent in order to clarify this problem.The unintentional high temperatures are relevant also for
examinations of source terms: as discussed in Section2, a pronounced increase
in release rates of metallic fission products (Sr, Cs, Ag) was observed after
the hot gas temperature increase from 850 to 950C° in February, 1974. As
outlined in [6], this enhanced release has to be mainly attributed to the
unintentional high AVR temperatures and not, as assumed in the past, to
insufficient fuel quality only. Other safety-relevant implications of the unintentional
high core temperatures are discussed in [6] too.


## 6.3. Implications
of Water Ingress on Fission Product Behavior

A slow water ingress of 27500 kg in 1978 led to a
substantial transfer of activity (mainly Sr and H-3 to a minor extent Cs and I)
from primary circuit components into the water [35]. Some of the
water went unintentionally into the grounding of AVR, where it is still
present. Traces of Sr-90 were detected in the soil surrounding the reactor [36].
The exact size of this Sr-90 contamination is not yet known but will be
examined after dismantling of the AVR.There was
also some graphite oxidation during this accident and in the subsequent drying
phase. However, when large amounts of water had entered the primary circuit, core
temperatures were substantially lower than in normal operation. So the maximum
oxidation degree observed on fuel elements depending on the type was only 0.2–0.5%. After
water ingress, some standard oxidation tests (1 kPa steam in He, 1000C°) were
performed on fuel elements: A substantial acceleration of oxidation rates was
observed for some fuel elements by catalytic effects. On the one hand side metallic
corrosion products formed by the water ingress were absorbed by the fuel
elements and induced pitting corrosion. On the other hand, the large
Sr-concentration in the fuel element graphite increased oxidation rates in
steam homogeneously over the fuel element surface by up to a factor of 5 by
catalysis.It remains
to be examined, whether local high hot gas temperatures as observed [6, 13] may
have initiated the AVR steam generator leak.


## 6.4. Experiments on Coolant
Chemistry

There were
continuously experiments with impurity injections into the AVR coolant circuit (CO2, H2, H2O, N2, CH4, NH3) [32, 37].
Interesting for understanding of the H-3 behavior  are hydrogen injection
experiments, which led—by exchange
reactions with H-3 sorbed on graphite—to a large
emission of H-3 into the gas phase. Based also on such experiments, it was
concluded that nitrogen and hydrogen are present gas borne in HTRs only to
about 0.1%, but mainly sorbed on graphite.


## 6.5. Pebble Rupture in Operation

Rupture of fuel
element pebbles in operation leads to destruction of coated particles and to
the presence of actinides, particularly Pu-241, in dust. Similar was observed
in the THTR, where a far more pronounced pebble rupture than estimated (8000
pebbles in 16 months full power operation) [22] was observed. Whereas the majority
of the pebble rupture in THTR may be attributed to control rod insertion, some may
also be due to the pebble movement. Although the total actinide activities in
AVR are small (some ten GBq of Pu-241 in AVR dust), they cannot be neglected
because of their pronounced toxicity due to their α-activity
and because of the mobility of dust. Altogether, only 220 of 2.4 Mio cycled
pebbles ruptured in AVR [32]. There exists, however, no reliable model for
pebble rupture up to now, as also the THTR experience indicates [22]. Extrapolation
to modern facilities has to consider the same arguments as discussed before for
dust formation, that is, a higher rupture rate than in AVR has to be taken into
account.


## 7. Concluding Remarks

Major
discrepancies between in-pile and out-of-pile results of plate out experiments indicate
that common laboratory scale experiments on fission product deposition of HTRs should
be handled with care. They did not completely simulate the real deposition
behavior  in a reactor, because of a different coolant composition (impurities
like CO,CO2, steam, etc., and other fission or activation nuclides)
and because of the presence of fission product absorbing dust also on surfaces
preferently in a pebble-bed reactor. Also, for the same reason, a sufficient
understanding of the deposition degree α in physical or chemical terms is not yet
available. Plate out on a gas turbine is a challenging problem due to the
required hand-on maintenance. As consequence of their safety relevance, these
problems require detailed additional examinations.The data situation
concerning dust formation, interaction with fission products, and dust
behavior  in the coolant circuit of HTRs and in depressurization is still
insufficient. The same holds for respective modeling tools. Accordingly, major research and
development effort on dust is required. Oil ingress events as a
dust source need additional attention. As long as uncertainties are unresolved,
the need for mitigation of dust-related problems advises a pebble-bed reactor
design equipped with a gas tight containment and measures against dust
explosions. A gas tight containment may diminish many of the problems
associated with enhanced normal operation temperatures as observed in AVR and with
pebble rupture, as problems due to the insufficient fuel element quality control,
but not those associated with maintenance and dismantling.



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*Source: 597491-2008-08-17.xml*