Research in Support of European Radio-isotope Power System Development at the European Commission's Joint Research Centre in Karlsruhe
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atw Vol. 65 (2020) | Issue 4 ı April
Serial | Major Trends in Energy Policy and Nuclear Power
Research in Support of European Radio
198
isotope Power System Development
at the European Commission’s Joint
SERIAL | MA JOR TRENDS IN ENERGY POLIC Y AND NUCLEAR POWER
Research Centre in Karlsruhe
Daniel Freis, Jean-François Vigier, Karin Popa and Rudy J.M. Konings
1 Introduction The urge to discover the unknown, to explore the unexplored and to broaden our knowledge
beyond the limits of the present is inherent to human nature. One of the most interesting and fascinating fields of
science is the exploration of the cosmos, either from Earth using telescopes or by sending automated probes to other
planets and into the vastness of space.
s cientific instruments on the Moon enables long-lasting missions [20].
(Figure 1), and they were used for Unfortunately, there is a global
many of the most famous and exiting shortage of this isotope, the efforts
exploratory endeavours, such as associated with its production are
the Pioneer missions to Saturn and high [21] and there are currently no
Jupiter [9,10], the Viking missions to facilities for its synthesis in Europe.
Mars [11], and the Voyager 1 & 2 An alternative is the americium
spacecraft, which travelled beyond isotope Am-241, which is more easily
the boundaries of our solar system available, since it is produced through
and are still delivering scientific decay from Pu-241 and can be
results from the interstellar medium, extracted isotopically pure from
more than 40 years after their launch existing stocks of civil plutonium in
[12,13]. More recently, the radio France or the United Kingdom via
isotope- powered missions Galileo, chemical extraction [22]. Therefore,
Ulysses and Cassini-Huygens were
ESA has decided to study the use of
exploring Jupiter, the Sun and
Am-241 for its RPS development
Saturn/Titan, respectively. These [5,16,17]. However, Am-241 has some
more recent missions were performed disadvantages compared to Pu-238,
in collaboration between the National such as a lower power density of
| Fig. 1. Aeronautics and Space Administra- 0.114 W/g and slightly higher radia-
Apollo astronaut photo of a SNAP-27 RTG on the Moon. Photo: NASA. tion (NASA) and the European Space tion levels. In addition, the oxide
Agency (ESA) [14]. Historically, this shows chemical instability at high
A basic requirement to operate auto- was the only way for Europe to gain temperatures, the experience with
mated spacecraft successfully over the access to space nuclear power s ystems. Am241 is limited and additional
course of an exploratory mission, is In 2005 a European Working Group research and safety assessment is
the reliable supply of long-lasting on Nuclear Power Sources for Space needed before it can be used for space
power. If independence of solar radia- identified RPS as a “key enabling applications.
tion is required, e.g. when travelling technology for future European activi- Within the ESA research pro-
into deep space or to the dark side of ties in space” [15], and suggested the gramme, the UK’s National Nuclear
planetary bodies, nuclear energy establishment of an European safety Laboratory (NNL) is exploring the
becomes advantageous compared to framework for space nuclear power cost effective production of Am-241
other potential sources of energy. The sources and the development of and the University of Leicester (UoL)
utilisation of nuclear power for appli- the technical capabilities to perform is developing a European Radio
cations in space had been considered nuclear powered missions indendently isotope Heater Unit (RHU) and Radio-
since the early beginnings of space [16,17]. As a consequence, a research isotope Thermoelectric Generator
flight in the late 1940s, and the first and development programme was (RTG) [16,23,24]. To support these
Radioisotope Power System (RPS) in launched by ESA for the production of efforts, the European Commission’s
space was already launched by the European RPS to satisfy thermal Joint Research Centre (JRC) in Karls-
U.S. Navy in 1961, onboard the Transit management and electrical power
ruhe is investigating methods to
4A navigational satellite [1,2]. Since needs for spacecraft [16,17,18,19]. stabilize americium in the oxide
then RPS have enabled some of the In the past, most RPS for space form and to establish a safe and
most spectacular missions in the missions were based on the plutonium reliable pelletizing process [20,25].
history of space exploration [1-7],
isotope Pu-238 [1-5], a radionuclide In collaboration with UoL, safety
mostly performed by the USA but also which is superior to other isotopes, relevant properties and behaviour of
by the former Soviet Union, China and because of its high specific power of Americium oxide are assessed under
Europe (via cooperation with the 0.567 W/g, low radiation, compa representative conditions for storage
USA). RPS were used on satellites for tibility with cladding materials and on Earth, operations in space as
navigation, meteorology and commu- chemical stability as oxide. Pu-238 well as hypothetical accident and
nication [8], they have powered has a half-life of 87.7 years which post-accident environments [16,26],
Serial | Major Trends in Energy Policy and Nuclear Power
Research in Support of European Radioisotope Power System Development at the European Commission’s Joint R
esearch Centre in Karlsruhe ı Daniel Freis, Jean-François Vigier, Karin Popa and Rudy J.M. Konings
atw Vol. 65 (2020) | Issue 4 ı April
and the compatibility with the clad- used when onboard power is needed Energy source Energy density, MJ/kg
ding material is tested. Complementa- for no more than a few weeks, or as
199
2 H2 + O2 13.33
ry work is performed to develop a rechargeable energy buffer to supply
qualified welding methodology of the peak loads or to bridge periods with- N2H4* + O2 9.75
safety encapsulation. out sunlight. For missions, which 2 Li + O2 12.2
require continuous power supply for
Li-ion battery 0.9
SERIAL | MA JOR TRENDS IN ENERGY POLIC Y AND NUCLEAR POWER
2 Energy Supply in Space an extended time, only solar or
In order to operate spacecraft, a nuclear energy sources are feasible, Fission of U-235** 8.2 · 106
reliable source of power in the form of since the payload associated with Decay of Pu-238*** 3.3 · 105
electricity and heat is required. chemical fuels or batteries would
Decay of Am-241*** 7.1 · 104
Electricity is needed to power onboard simply become too high.
electronic systems such as navigation Solar energy is principally un | Tab. 1.
and manoeuvring systems, onboard limited, as long as the solar cells, used Energy densities of typical energy sources.
*Hydrazine, **total fission, ***over 20 years mission time
computers, lighting, robotics, scien to convert the radiation energy into
tific instruments and communication electricity, are not degrading, and as
systems. In some cases, spacecraft are long as they can be adjusted in the controlled fission of fissile isotopes,
equipped with electric propulsion direction of the sunlight and the
such as U235 or Pu-239, and Radio
systems, and electricity for life s upport spacecraft is not in the shadow of isotope Power Sources or Systems
is needed if the mission is manned. planetary bodies or too far away from (RPS), which obtain their energy from
Heat is needed in cold environments the sun. However, their effective area, the spontaneous decay of radioactive
to keep sensitive spacecraft com the conversion efficiency and the isotopes. Both types can generate
ponents at minimum operational or intensity of the solar radiation deter- heat for temperature control and/or
survival temperature, e.g. during mine the power density of solar cells. electricity via additional energy
lunar nights. This intensity decreases inversely with conversion systems.
Primary energy sources can be the square of the distance from the While nuclear reactors are
chemical, solar or nuclear. Some of sun, as shown in Figure 2, and if the generally suited for applications,
these are limited with respect to their distance becomes too large solar which need significant power levels
power or energy density, and the energy becomes unpractical. For
above 10 kW, RPS are employed
profile of each individual mission example, while the solar constant is whenever a limited amount of solar-
determines which energy sources can 1.367 kW/m2 at the semi-major axis of independent power, up to 5 kW, is
be utilized. Table 1 shows typical Earth, at one Astronomical Unit (AU) needed for a longer time period. RPS
energy densities for different energy distance to the sun, it decreases are compact, long-lived, reliable,
sources. to only 51 W/m2 or 3.7 % at the semi- robust, radiation resistant, solar-
Chemical energy sources in the major axis of Jupiter (5.2 AU). There- independent, maintenance-free, and
form of solid or liquid fuels can release fore, solar arrays are not an option for they have energy densities, which are
a large quantity of energy in a very all research missions to the outer solar several orders of magnitude above
short time, but they are limited with system, but also not if a system is to be chemical power sources (Table 1)
respect to total energy density. For in- operated during long periods of dark- [1,2,3]. Figure 3 shows qualitatively
stance, chemical fuels for propulsion ness, for example during the lunar the different regimes of power levels
can be used whenever high thrust is nights, which last 14 days. and durations, where different energy
needed for a short time, e.g. as rocket Nuclear energy has the highest sources are applicable.
fuel to overcome the gravity field of power densities of all possible on- Space power systems, which are
earth, but they have shortcomings if board energy sources, and can deliver based on the decay heat of radio
long-lasting power or long accelera- reliable power over very long time isotopes, can be distinguished into
tion times are needed, e.g. to reach periods. Most importantly, it is inde- systems which make direct use of
the high velocities necessary for inter- pendent of sunlight. There are two the thermal energy, and systems
planetary or even interstellar travel. types of space nuclear power systems; which convert heat into electricity
Chemical energy in the form of Reactor power systems (small nuclear (Figure 4). In both cases the em-
batteries or fuel for fuel cells can be reactors), which generate power by ployed radioisotope is the power
| Fig. 2. | Fig. 3.
Intensity of solar radiation. Application of different energy sources (reproduced from references 1 and 3).
Serial | Major Trends in Energy Policy and Nuclear Power
Research in Support of European Radioisotope Power System Development at the European Commission’s Joint R
esearch Centre in Karlsruhe ı Daniel Freis, Jean-François Vigier, Karin Popa and Rudy J.M. Konings
200
SERIAL | MA JOR TRENDS IN ENERGY POLIC Y AND NUCLEAR POWER atw Vol. 65 (2020) | Issue 4 ı April
| Fig. 4. | Fig. 5.
Elements of radioisotope power systems. Thermoelectric circuit.
source while the heat sink is provided thermo electric couple or thermo
by space. Systems which make direct couple. In addition, static heat con
use of the thermal energy are called version is also possible by thermionic
Radioisotope Heater Units (RHU). conversion, where a flow of electrons
They provide heat to the space craft is induced from a hot to a cool surface
to keep sensitive electronics warm via thermionic emission.
without using heavy and complicated Thermoelectric conversion is not
heat distribution systems, and with- very efficient and practical RTG
out creating electromagnetic inter systems using SiGe or PbTe/TAGS
ference. thermocouples show typical power
Heat-to-electricity conversion sys- conversion efficiencies of 6 % to 7 % | Fig. 6.
tems can be classified into dynamic [5]. To improve efficiency, develop- Radioisotope power source.
and static systems. The dynamic ment of high temperature thermo
systems employ moving parts and use electric materials including skutter highly refractory noble metal like
a thermodynamic cycle to convert udites and Zintl-based systems is iridium or platinum-rhodium alloys
heat into electricity, e.g. a Stirling ongoing [28,29]. However, if operated (Figure 6), which can withstand the
engine [27], and show principally the at low temperatures or under a protec- most extreme conditions (e.g. launch
highest conversion efficiencies. The tive gas cover, existing RTG are very pad explosion, Earth re-entry acci-
earliest efforts were focussed on the reliable, show low degradation and dents). The cladding is surrounded by
development of dynamic conversion can provide power over many decades. thermal insulation, usually made of
systems, and the first RPS SNAP-1 Because of the importance of reliable pyrolytic graphite, which shall protect
(SNAP stands for Systems for Nuclear and maintenance-free systems for the cladding from reaching peak
Auxiliary Power) in 1959 was based automated space probes, relatively
temperatures during aerodynamic
on a Ce-144 powered mercury low conversion efficiencies are usually heating. Finally, the thermal insula-
Rankine cycle [1]. But despite their accepted. tion is surrounded by an aeroshell
high conversion efficiency, dynamic The most important design made of carbon-carbon composite
conversion systems have not yet criterion for any space nuclear power (fine-weaved pierced fabrice), which
reached the reliability required for the system, including all forms of RPS, is provides additional protection from
operation of a space probe, which safety. If an RPS shall be employed on postulated launch vehicle explosions
might be on a mission for several a space application, it must comply or against impacts on hard surfaces at
decades without the possibility for
with resolution 47/68 of the United terminal velocity [1,5,14].
maintenance or repair, and no Nations General Assembly on the
dynamic conversion system was ever Principles Relevant to the Use of 3 Radioisotopes for RPS
used in space. Nuclear Power Sources in Outer
The selection of suitable radioisotopes
The static heat-to-electricity con- Space. The resolution defines that the for application in space RPS is based
version systems are called Radio use of RPS shall be restricted to those on a number of criteria; among them
isotope Thermal Generators (RTG). space missions, which cannot be are a long half-life, high isotopic p
ower
RTG have no moving parts and use the performed by non-nuclear energy
and a low level of penetrating radia-
thermoelectric principle, also known sources in a reasonable way. It also tion. In addition, a chemically stable
as the Seebeck effect, to generate states that the design and use of the compound with high density should
electricity. This phenomenon was
RPS shall ensure that the hazards exist, which can serve as stable host for
discovered in 1794 by the Italian
during operation and foreseeable
the decay products and is compatible
scientist A
lessandro Volta and, inde- accidents are kept below acceptance to the encapsulation materials and the
pendently, in 1821 by the German levels and that radioactive material potential operating or post-accident
physicist Thomas Johann Seebeck. does not cause a significant contami- environments. Furthermore, the com-
If two dissimilar materials are con- nation of the biosphere and outer pound should resist high temperatures
nected in a closed circuit and a space [30]. and should not disperse into inhalable
temperature difference is applied over In order to comply with these small particles, in case of an accident.
the two junctions, a voltage can be requirements, RPS are designed to the A low solubility in the environment
measured and electricity is generated meet highest safety standards. The (water) and the h uman body are also
(Figure 5). Such a device is called a fuel is encapsulated in a cladding of a advantageous [4,5].
Serial | Major Trends in Energy Policy and Nuclear Power
Research in Support of European Radioisotope Power System Development at the European Commission’s Joint R
esearch Centre in Karlsruhe ı Daniel Freis, Jean-François Vigier, Karin Popa and Rudy J.M. Konings
atw Vol. 65 (2020) | Issue 4 ı April
Isotope Half-life (y) Isotopic power (W/g) Principal decay mechanism Comment Shielding (typically)
201
Am-241 432.8 0.114 alpha Soft gamma radiation 2 mm lead equivalent
Cs-137 30.04 0.417 beta Gamma emitter Heavy
Ce-144 285 2.08 beta Gamma emitter Heavy
Cm-242 0.45 122 alpha Strong neutron emitter Heavy
SERIAL | MA JOR TRENDS IN ENERGY POLIC Y AND NUCLEAR POWER
Cm-244 18 2.84 alpha Strong neutron emitter Heavy
Po-208 2.93 18.1 alpha Short half-life None
Po-210 0.38 144 alpha Short half-life None
Pu-238 87.7 0.567 alpha Very soft gamma radiation None
Sr-90 28.79 0.907 beta Bremsstrahlung Significant
| Tab. 2.
Radioisotopes for space applications [31,32].
Since the specific power correlates which is artificially created in nuclear yield of circa 13 400 n/(s g) in pluto
inversely to the half-life, a compro- reactors. It was the first isotope of nium oxide [34]. The (α-n) reactions
mise has to be found between specific plutonium, which was discovered by can only occur in the low abundancy
weight and volume of the heat source Glenn T. Seaborg in 1940 [33], and it isotopes O-17 and O-18, while the
on one side, and a long-lasting stable is the most important and most widely threshold energy of O-16 is too high
power output during the required mis- adopted radioisotope for space power (15.2 MeV) [34]. The neutron yield in
sion time on the other side. Therefore, applications, due to its high power pure Pu-238 oxide can be reduced to
the selection of a suitable radioisotope density, long half-life, chemical sta circa 2700 n/s-g [5] if the oxygen is
always depends on the concrete bility as oxide, and its low neutron and depleted in O-17 and O-18 by 98 %
application. Alpha emitters tend to be soft gamma emissions. So far, Pu-238 [21].
better suited than beta emitters are, is the only radioisotope used for RPS Metallic plutonium has a density
because the alpha decay energy is in space by the USA and China, while of 19.77 g/cm3 (Pu-238, α-phase at
typically in the range between 5 MeV the former Soviet Union also utilized room temperature and atmospheric
and 6 MeV per decay event, for Po-210. pressure), which is the highest density
example compared to 0.546 MeV for Pu-238 has a half-life of 87.7 years form and would allow a volumetric
the beta decay of Sr-90, and the alpha and an isotopic power of 0.567 W/g. power density of up to 11.21 W/cm3.
particles do not generate Brems The isotope decays primarily via alpha However, the metal shows six different
strahlung when stopped in the sur- decay to U-234, with a decay energy structural modifications at ambient
rounding matter. of 5.59 MeV. The main radiation emis- pressure in the temperature range
Other important criteria are the sions of Pu-238 are alpha particles from 0 K (-273.15 °C) to 640 °C (melt-
availability of the selected isotope, as with an average energy of 5.49 MeV. ing point), and the phase transitions
well as the production costs and In addition, the isotope emits soft cause significant dimensional changes
necessary infrastructure and effort
gamma radiation (main energies: as well as alterations of the mechani-
to process the radioactive material. 43.5 keV & 99.85 keV) with low emis- cal and thermal properties (Figure 7).
Table 2 gives an overview over the sion probability, and Auger electrons, In addition, plutonium metal is burn-
most common radioisotopes for space which in turn generate X-rays at able, and the powder is extremely
RPS, of which Pu-238 is by far the 17.11 keV and 13.61 keV (main lines), pyrophoric. The first RTGs in the
most significant. If not mentioned also with relatively low emission 1960s were still fuelled with metallic
otherwise, all nuclear data were taken probabilities. For a sintered and en- Pu-238, and designed to burn-up into
from the JEFF-3.1 nuclear data library capsulated pellet, the self-shielding fine particles below 30 nm and dis-
[31] via Nucleonica.com [32]. effect and encapsulation are more perse into the atmosphere in case of
than sufficient to shield these radia- an re-entry accident1. This safety
4 Properties of Plutonium- tions. Spontaneous fission occurs with philosophy had to prove itself, when
238 & Americium-241 a negligible probability of 1.86E-09, the Transit satellite 5BN-3 failed
The development of European RPS is and the spontaneous fission reaction to achieve orbit in 1964, and the
based on the strategic decision of ESA creates a neutron yield of circa SNAP-9A RTG re-entered the atmo
to utilize Am-241 and to take advan- 2300 n/(s g) (oxide). However, alpha- sphere, carrying about 1 kg Pu-238
tage of the existing nuclear infrastruc- neutron (α-n) reactions in natural metal. As predicted, the metallic
ture related to the civil reprocessing of oxygen cause an additional neutron fuel completely burned-up and was
spent nuclear fuel in Europe, rather
than to establish an expensive pro Pu-Metal PuO2 Am-Metal AmO2
duction capability for Pu-238 [16,17].
Half-life: 87.7 y 432.8 y
In order to understand the impact of
this choice on the RPS design charac- Density: 19.85 g/cm3 11.46 g/cm3 13.67 g/cm3 11.68 g/cm3
teristics and the production process, a 0.500 W/g 0.101 W/g
Power density: 0.567 W/g 0.114 W/g
comparison of both isotopes has to be (0.418 W/g)* (0.088 W/g)**
made. Table 3 summarizes the most Thermal conductivity: 6 – 16 W/m K35 3 – 10 W/m K37 10 W/m K ~ 2.0 W/m K38***
important properties of Am-241 com- 36
Melting point: 640 °C 2744 °C 1176 °C35 2113 °C42
pared to Pu-238.
Pu-238 is an isotope of the chemi- | Tab. 3.
cal element plutonium, an actinide Properties of Pu-238 and Am-241 (Metal and Oxide). *Typical isotopic composition, **Stabilized with 12 % U, ***Sub-stoichiometric
Serial | Major Trends in Energy Policy and Nuclear Power
Research in Support of European Radioisotope Power System Development at the European Commission’s Joint R
esearch Centre in Karlsruhe ı Daniel Freis, Jean-François Vigier, Karin Popa and Rudy J.M. Konings
atw Vol. 65 (2020) | Issue 4 ı April
good host for U-234, the decay pro emits additional neutrons from the
duct of Pu-238, which is also crystal alpha-neutron (α-n) reaction in natu-
202
lizing in the fcc structure. ral oxygen. This reaction causes an
Am-241 is an isotope of the radio- additional neutron yield of circa
active element americium. It belongs 2700 n/(s g)34. While this is signifi-
also to the actinides and has the cantly lower compared to Pu-238 by a
SERIAL | MA JOR TRENDS IN ENERGY POLIC Y AND NUCLEAR POWER
atomic number 95. Like plutonium, factor of six, it has to be considered
americium is an artificial element and that for the same thermal power about
was discovered by the group of Glenn five times more Am-241 is needed.
T. Seaborg in 1944. Americium is Therefore, also in the case of Am-241
usually created in nuclear reactors
oxide O-17 and O-18 depletion is
by neutron capture and radioactive needed to reduce the neutron yield to
decay (Figure 8). However, during acceptable levels [5].
irradiation not only Am-241 but also Metallic americium has similar
other americium isotopes are created, disadvantages as metallic plutonium
| Fig. 7. which are unwanted in RPS. Isotopi- for the use in RPS. In addition, its
Thermal conductivity of metallic plutonium structural modifications [35]. cally pure Am-241 is produced con density is relatively low compared to
tinuously in the stocks of civil plutoni- the oxide (13.67 g/cm3 α-phase at
um through beta decay of Pu-241 room temperature). Elemental ameri-
(t1/2=14.33 y), from where it can be cium is a soft metal, which oxidises
separated using chemical methods. quickly in the atmosphere forming
A cost efficient separation and purifi- a protective oxide layer. At room
cation process (AMPEX) was devel- temperature americium forms a
oped and demonstrated by the UK’s stable hexagonal α-phase (space
National Nuclear Laboratory (NNL) group P63/mmc) [35], at 769 °C it
to separate ingrown Am-241 from changes into the cubic β-phase (space
plutonium [22,39]. group Fm3̄m), and at 1077 °C it
Am-241 has a half-life of 432.8 converts to the γ-phase showing a
years and an isotopic power of body- centered cubic structure [35].
0.114 W/g. The isotope decays The phase transitions cause dimen-
primarily via alpha decay to Np-237. sional changes, however not as signifi-
In a final repository Am-241 is one of cant as in the case of plutonium. The
the most important drivers for the melting point of metallic americium is
medium-term heat load, and there- at 1176 °C. Americium metal powder
fore the required gallery space, and is also very pyrophoric and metallic
| Fig. 8. Np-237 (t1/2=2.14∙106 y) is one of the americium would definitively burn-up
Production of Am-241 by neutron capture and significant isotopes driving long-term in case of an uncontained re-entry
β-decay.
radiotoxicity [40,41]. The main radia- accident.
tion of Am-241 are alpha particles The preferred form of Am-241 for
dispersed into the atmosphere where with an average energy of 5.47 MeV. space applications is americium oxide,
it was diluted to low concentrations Unlike Pu-238, Am-241 emits signifi- a ceramic material with a density of
and did not cause any unacceptable cant gamma radiation at 59.54 keV 11.68 g/cm3 which can be sintered
health hazard [1,8,14]. (main line) with a probability of 36 % into pellets [6,16,20,25]. However,
After the Transit 5BN-3 accident and also Auger electrons, which cause there are two major compounds which
and with the upcoming of larger RPS X-rays at 14.44 keV (probability are relevant for applications in space,
with higher radioactive inventory the 33 %). Even though the radiation is the cubic dioxide (AmO2) (α-phase,
dispersion approach was no longer still relatively soft, shielding and space group Fm3̄m) and the
accepted and a new fuel form, which remote handling tools are required if sesquioxide (Am2O3), which exists in
would stay intact at re-entry was larger amounts are to be processed, the hexagonal form (Aphase, space
needed. Since then, the preferred and especially when the material is in group P3̄m1) and the cubic form
chemical form of Pu-238 for RPS is solution and self-shielding is not effec- (C-phase, space group Ia3̄). Unfor
plutonium dioxide (PuO2), safely tive. Once the pellets have been tunately, the americium-oxygen sys-
encapsulated in a cladding of high
sintered and encapsulated the radia- tem shows a complex behaviour. The
refractory material which can safely tion decreases. However, it remains melting point of AmO2 is at 2113 °C
contain the radioactivity under all still significant and adequate radia- [42], but at high temperatures and
credible circumstances [8]. PuO2 is a tion protection measures for workers in the vacuum of space americium
ceramic material with a high melting need to be in place. A dose rate of dioxide loses oxygen. Due to this
point of 2744 °C [36], which can be circa 150 µSv/h in 10 cm distance is process, it transforms into sub-
sintered into stable pellets. The com- estimated for a typical full scale RHU stoichiometric AmO2-x, thereby slowly
pound crystallizes in the face centred containing 26 g AmO2 encapsulated in increasing its volume, until it finally
cubic (fcc) structure (space group 1.8 mm Pt30Rh. converts into the hexagonal sesqui
Fm3̄m) [37], has a high chemical Am-241 has a very low probability oxide [37,43,44]. This process causes
stability, a low solubility in water
for spontaneous fission of 4.3E-12, significant dimensional changes and
and does not react with the typical and the spontaneous fission reaction can lead to decomposition of the pel-
cladding materials, such as iridium or creates a negligible neutron yield of lets [25,45]. In addition, the release of
platinum alloys (e.g. Pt20Rh or circa 1.2 n/s-g (oxide). However, as in oxygen can cause pressure build-up
Pt30Rh). In addition, it serves as a the case of Pu-238, Am-241 oxide and potentially corrode surrounding
Serial | Major Trends in Energy Policy and Nuclear Power
Research in Support of European Radioisotope Power System Development at the European Commission’s Joint R
esearch Centre in Karlsruhe ı Daniel Freis, Jean-François Vigier, Karin Popa and Rudy J.M. Konings
atw Vol. 65 (2020) | Issue 4 ı April
structures. On the other higher, methods to stabilize americium in the actinide containing samples from the
americium sesquioxide is prone to
oxide form and to establish a safe and base material to the fully encapsu
203
oxidation at lower temperatures and reliable p
elletizing process [20,25]. lated sample, and the MA-Lab repre-
will slowly convert into the dioxide in In colla boration with UoL, safety sents an ideal infrastructure for prepa-
air, again undergoing significant relevant properties and behaviour
ration of highly radiating americium
structural modifications. After several of americium oxide are assessed pellets and fully qualified fuel pins.
SERIAL | MA JOR TRENDS IN ENERGY POLIC Y AND NUCLEAR POWER
weeks of storage, even under low [16,26]. The synthesis of the base material
oxygen partial pressures, this e ffect, Am-241 emits a significant amount (powder) is performed in the glove
in combination with self- irradiation of gamma radiation, and working box named “infiltration”. The process
from α-decay, will cause Am2O3 with Am-241 can result in high dose is dust-free, based on the so-called gel
pellets to disintegrate into black
rates for the operating personnel. supported precipitation [47] and the
dioxide powder. Both phenomena, the Remote operated and shielded
porous bead infiltration technique
reduction of AmO2 at e levated tem- equipment is advantageous or even [48]. This process is highly flexible
peratures and the oxidation of Am2O3 necessary in order to prepare and easily adapted to the require-
at low temperatures, are problematic americium-based pellets for RPS. The ments and specifications of new
with respect to the integrity of the fuel JRC in Karlsruhe has a unique infra- sample compositions. The next glove
pellets and can cause increased dis- structure for handling of highly box contains a calcination furnace and
persion of radioactive material in case radiative actinide materials, the so- other equipment for powder prepara-
of certain accident scenarios [25]. called M inor Actinide Laboratory tion. The prepared powders are dust
(MA-Lab) [46]. It is of high relevance free, the individual beads typically
5 The Minor Actinide Labo- for safety research on fuels for trans- show heterogeneous size distributions
ratory at the Joint Re- mutation in Europe, as it is one of the between 30 µm to 120 µm and are
search Centre Karlsruhe only dedicated facilities for the ideal for pressing pellets. The ready to
The Joint Research Centre is the synthesis of minor a ctinide containing press powder can be transferred via
European Commission’s science and materials, either for property an automated channel to the next
knowledge service. Its mission is to measurements or for the preparation glove box, where it can be pressed to
support EU policies with independent of irradiation experiments. pellets. After sintering in reducing
evidence throughout the whole policy The MA-lab consists of seven or oxidizing atmosphere (glove box
cycle. Its work has a direct impact on glove-boxes with protection walls “Sintering”) the pellets are fully
the lives of citizens by contributing forming two separate chains. A characterized and inserted into a
with its research outcomes to a schematic lay-out of the Ma-Lab is
cladding.
Finally, pin welding and
healthy and safe environment, secure shown in Figure 9. The glove boxes non-destructive weld examination are
energy supplies, sustainable mobility are shielded by 50 cm neutron performed in the two last, alpha free
and consumer health and safety. The shielding and 5 cm of lead. Based on glove boxes.
JRC hosts specialist laboratories and the thickness of the water and lead
unique research facilities and is home wall, the mass limits have been 6 Stabilisation of
to thousands of scientists working to calculated to 150 g of Am-241 or 5 g of Americium Oxide &
support EU policy (https://ec.europa. Cm-244. The glove boxes can be RHU-Size Prototype Pellet
eu/jrc/en). accessed manually from the back if Production
The JRC in Karlsruhe belongs to radiation levels are low enough to
Unlike plutonium oxide, which is
the Directorate for Nuclear Safety and perform experiment pre
paration or stable in a broad range of tem
Security (Directorate G), where JRC’s maintenance. In addition, tele-mani peratures and oxygen potentials,
nuclear work programme, funded by pulators and remote operated auto- americium oxide is prone to phase
the EURATOM Research and Training mated equipment can be used for changes and disintegration in chang-
Programme, is carried out. The operation at high dose rates. ing environments [25]. If americium
Directorate contributes to the scien The glove boxes of the minor oxide is sintered under oxidizing
tific foundation for the protection of actinide laboratory are configured as conditions into AmO2, it releases
the European citizen against risks complete preparation chain for minor oxygen at elevated temperatures in
associated with the handling and
storage of highly radioactive material,
and scientific and technical support
for the conception, development,
implementation and monitoring of
community policies related to nuclear
energy. Research and policy support
activities of Directorate G contribute
towards achieving effective safety and
safeguards systems for the nuclear
fuel cycle, to enhance nuclear security
then contributing to achieving the
goal of low carbon energy production.
The JRC supports the ESA
research programme on developing a
European Radioisotope Heater Unit
(RHU) and Radioisotope Thermo
electric Generator (RTG). These acti | Fig. 9.
vities are focussed on investigating Minor Actinide Laboratory at JRC-Karlsruhe [46].
Serial | Major Trends in Energy Policy and Nuclear Power
Research in Support of European Radioisotope Power System Development at the European Commission’s Joint R
esearch Centre in Karlsruhe ı Daniel Freis, Jean-François Vigier, Karin Popa and Rudy J.M. Konings
atw Vol. 65 (2020) | Issue 4 ı April
was observed, only low crystallo-
graphic swelling occurred due to
204
self-irradiation, which saturated after
circa 60 days. Overall the pellet
showed good long-term structural
and dimensional stability under self-
SERIAL | MA JOR TRENDS IN ENERGY POLIC Y AND NUCLEAR POWER
irradiation conditions (Figure 11)
[25].
7 Development of Welding
Methodology
In order to be able to meet the launch
safety requirements and safely ship
| Fig. 10. RPS sealed sources to sites, where they
Small (left side) and large (right side) scale prototype pellet. can be assembled into RHUs or RTGs
and subsequently be installed onto a
spacecraft, it is necessary to develop
containment technologies that meet
these requirements. The first layer of
containment immediately surrounding
the fuel pellets is the cladding, which
must ensure the enclosure of radio
activity during storage, normal opera-
tion and accident scenarios. The
encapsulation has to be performed in a
nuclear installation, ideally the manu-
facturing site, and it has to be ensured
that the fueled clads are free of e xternal
contamination.
In order to test the feasibility of
our welding equipment and to gain
experience in the welding of Pt30Rh
capsules, two types of Pt30Rh-en
capsulation were constructed and
| Fig. 11. welding tests were performed. The
Crystallographic swelling of (Am0.80U0.12Np0.06Pu0.02)O1.8 mixed oxide under alpha self-irradiation. first capsule design had similar
Values for PuO2 and AmO2 swelling are reported for comparison [25].
dimensions as the US LWRHU (Figure
12) and the second capsule design
the vacuum of space and changes into reducing atmospheres. A solution was was made according to input by UoL
sesquioxide (Am2O3), thereby under- found by inserting 12 % of uranium to host (Am,U)O2 pellets of 15 mm
going strong structural reorgani into the americium oxide (in addition diameter and 20 mm height.
zation, density changes and dis to 6 % Np and 2 % Pu already present). The capsules were welded using
integration. If americium oxide is Thereby, it was possible to stabilize established Tungsten Inert Gas
sintered under reductive conditions
the cubic phase and to sinter a number welding equipment, which is also
into Am2O3, it will transform into the of discs and pellets, including a used in the frame of qualified welding
dioxide under accident conditions, prototype pellet in the dimensions of of fuel rodlets for irradiation experi-
but also under the influence of self- the US LWRHU [49] under mois ments [41]. Non-destructive as well as
irradiation even at low oxygen poten- turized Ar/H2 atmosphere and a destructive weld examinations were
tials, which leads to its total dis larger disc representative for a future performed and showed that good
integration. European RHU [16] (Figure 10). welding results were achieved;
JRC has investigated possibilities Due to the absence of phase indicating that future welding quality
to stabilize americium dioxide in its changes, the stabilized material
criteria can be met (Figure 13).
cubic form under a broad range of showed a good sintering behaviour
temperatures, in oxidizing as well as and it was possible to sinter a number 8 Conclusions
of discs and pellets with good quality Radioisotope power sources are a key
and without cracking. After sintering enabling technology for exploratory
oxidation testing was performed and missions into deep space or to the dark
the material proved stable up to side of planetary bodies, and the
1000 °C. This result represents a European Space Agency is sponsoring
significant improvement with respect the development of Am-241 based
to safety of the material against radio- radioisotope power systems. The
active material dispersion in case of development of a new RPS based on
accidental conditions. In addition, the americium is a challenging task. The
macroscopic and crystallographic optimization of the fuel is a key issue,
swelling was assessed on the small as the oxide of americium has signifi-
| Fig. 12. scale protype pellet (Figure 10) over cantly different properties compared
Small scale prototype Pt30Rh encapsulation. time. While no macroscopic swelling to that of Pu-238, both in terms of
Serial | Major Trends in Energy Policy and Nuclear Power
Research in Support of European Radioisotope Power System Development at the European Commission’s Joint R
esearch Centre in Karlsruhe ı Daniel Freis, Jean-François Vigier, Karin Popa and Rudy J.M. Konings
atw Vol. 65 (2020) | Issue 4 ı April
205
SERIAL | MA JOR TRENDS IN ENERGY POLIC Y AND NUCLEAR POWER
| Fig. 13.
Large scale prototype Pt30Rh encapsulation (left side) and dye penetrant test of weld (right side).
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