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Solid methane in neutron radiation: Cryogenic moderators and cometary cryo volcanism
Cryogenics 88 (2017) 101–105
Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Research paper
Solid methane in neutron radiation: Cryogenic moderators and cometary cryo volcanism
MARK
⁎
O. Kirichek , C.R. Lawson, D.M. Jenkins, C.J.T. Ridley, D.J. Haynes ISIS Facility, STFC, Rutherford Appleton Laboratory, Harwell, Didcot, UK
A R T I C L E I N F O
A B S T R A C T
Keywords: Solid methane Neutron radiation Moderator
The effect of ionizing radiation on solid methane has previously been an area of interest in the astrophysics community. In the late 1980s this interest was further boosted by the possibility of using solid methane as a moderating medium in spallation neutron sources. Here we present test results of solid methane moderators commissioned at the ISIS neutron source, and compare them with a model based on the theory of thermal explosion. Good agreement between the moderator test data and our model suggests that the process of radiolysis defect recombination happens at two different temperature ranges: the “lower temperature” recombination process occurs at around 20 K, with the “higher temperature” process taking place between 50 and 60 K. We discuss consequences of this mechanism for the designing and operation of solid methane moderators used in advanced neutron sources. We also discuss the possible role of radiolysis defect recombination processes in cryovolcanism on comets, and suggest an application based on this phenomenon.
1. Introduction The properties of frozen methane, when exposed to ionising radiation, have attracted significant interest in the planetary and interstellar astrophysics community since the early 1980s [1,2]. In the late 1980s this interest was further boosted by the possibility of using solid methane as a moderating medium in spallation neutron sources [3]. Solid methane possesses unique neutronic properties that enable the conversion of hot, energetic neutrons into cold neutrons with an efficiency approximately 3.5 times that of liquid hydrogen based moderators [4]. However, practical applications of solid methane as a neutron moderator material turned out to be much more challenging than initially expected. Early developments in methane moderators were carried out by the IPNS neutron source based at Argonne National Laboratory. Here solid methane at around 10 K was exposed to neutron radiation for few days, leading to a build-up of radiolysis products in the solid methane matrix. It was then shown that at some critical number of defects a spontaneous self-accelerated recombination process would take place, described by J. Carpenter as the ‘burp’ phenomenon [3,4]. This effect, in combination with the expansion of hydrogen built up in bulk solid methane during irradiation, was believed to be responsible for the moderator’s breakdowns. In the late 1990s the effects of neutron radiation on solid methane were systematically studied at the IBR neutron source in Dubna [4,5].
⁎
Corresponding author. E-mail address: [email protected] (O. Kirichek).
http://dx.doi.org/10.1016/j.cryogenics.2017.10.017 Received 14 September 2017; Accepted 12 October 2017 Available online 14 October 2017 0011-2275/ © 2017 Published by Elsevier Ltd.
The results of this research confirmed the assumptions of Carpenter’s model and gave some estimations of the scale of the process. After these experiments the consensus in the neutron scattering community was that all of the radiolysis products accumulated in solid methane (irradiated at temperatures around 10 K) start to recombine either spontaneously, if the density of the defects exceeds critical value, or by thermal activation mechanism during warming up if temperature exceeds 20 K. In either case, all defect recombination processes should be completed at temperatures below 40 K [3–6] and the design of the Target Station 2 Solid Methane Moderator (TS2 SMM), commissioned by the ISIS neutron source in 2009, was based on this assumption. However, the very first test of this moderator operated at 38 K ended in a similar burp-like event, which damaged the moderator. This observation reignited interest in the results of the IPNS neutron source and suggested that the defect recombination process might be more complex that initially thought, with multiple defect types recombining at different temperature ranges. Further support for this concept came from results obtained by researchers from the Verkin Institute in Kharkiv, Ukraine [7], who studied the radiation defect relaxation mechanisms in solid methane films, pre-irradiated by an electron beam. This experiment was able to measure both mass ejection and electromagnetic luminescence events. An expected peak of particle ejection (post-desorption) was found in solid methane at temperatures as low as 15 K, but another maximum was also observed at higher temperatures around 50 K.
Cryogenics 88 (2017) 101–105
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operation cycle. During testing, the moderator was irradiated by a neutron flux of ∼1016 neutron cm−2s−1, at a temperature of 38 K, for intervals of between 8 to 24 h. In three instances the self-accelerated heating process was triggered at ∼50 K during an intentional warmup of the moderator (this is a standard procedure, carried out with the aim of annealing the solid methane) leading to moderator damage. In all three cases the damage occurred at temperatures around 65 K. In the current moderator design (mark two), the thickness of the aluminium walls of the vessel has been increased from 3 mm to 10 mm. However this reduces the methane capacity of the moderator by almost half of the mark one prototype – 225 g of CH4. Such radical design change has completely eliminated the possibility of moderator damage, but at the same time significantly reduced the cold neutron flux.
Here we present and discuss the results of commissioning tests of solid methane moderators used in Target Station 2 at the ISIS neutron source, and compare them with a model based on the theory of thermal explosion [8]. We also compare the results of our model simulation with the IPNS solid methane moderator test data published in [3]. The temperature range of the TS2 SMM commissioning tests was 37–100 K. The strong agreement between measured moderator temperature data and our simulation, combined with methane irradiation data from [7], suggests that the process of radiolysis defect recombination happens in two stages, at two different temperature ranges. The “lower temperature” recombination process occurs at around 20 K, with the “higher temperature” process taking place between 50 and 60 K. We then use our model to simulate the pressure experienced by the walls of the TS2 SMM vessel produced by the rapid expansion of solid methane during self-accelerating heating, driven by the recombination process. We argue that this pressure spike can be intense enough to damage the vessel, which applies certain limitations on solid methane moderator design and operational procedure. We also discuss the possible role of radiolysis defect recombination processes in cryo-volcanism on comets, and suggest an application based on this phenomenon.
3. Model of solid CH4 in neutron radiation Our model utilises an approach similar to that used by Carpenter in [3] based on a simplified version of the theory of thermal explosion [8]. In order to reflect the presence of radiation defect recombination occurring at higher temperatures (50–70 K) we have introduced secondary binary collisions, with a rate coefficient K2 that also has Arrhenius-form temperature dependence. Then the process of accumulation and recombination of radiolysis defects can be described by the equations:
2. Design and commissioning of the ISIS Target Station 2 solid methane moderator
dN1,2 (t ) 2 (t ) = R1,2 (t )−K1,2 (T ) N1,2 dt
An illustrative sketch of the decoupled TS2 SMM is presented in Fig. 1. The moderator cryostat consists of an outer vacuum can (3), infrared radiation shield (4) and the solid methane moderator vessel (5). The “mark one” version of the moderator vessel was made of aluminium alloy 5083–0 with internal dimensions 124 × 140 × 70 mm and wall thickness 3 mm. The aluminium foam block (6) from aluminium alloy 1199-F and 8% density has been pressure fitted into the moderator vessel (5). The moderator comprises a heat exchanger coil (7) made of Ø 10 mm aluminium tube welded into the vessel’s body. The fully loaded TS2 SMM (mark one) contained 405 g of CH4. The high purity methane gas is condensed into the moderator through the inlet pipe (1) and exhausted through the outlet pipe (2) at the end of the
MC (T )
(1)
dT = P (t ) + ε1 K1 (T ) N12 (t ) + ε2 K2 (T ) N22 (t ) dt −AH (T )(T (t )−Tcool (t ))
K1,2 (T ) = K o1,2 e−E1,2/ kB T
(2) (3)
Here, T is temperature, N1 is the number of defects recombining around 20 K and N2 is the number of defects recombining at around 50 K. Equation (1) is the same for both species, so for simplicity we use N1,2 where index 1 and index 2 are for lower and higher temperatures of recombination respectively. R1,2 is the defect production rate for each species. In our case of solid methane irradiated by fast neutrons, we assume that production rate is the same R1 ≂ R2 for both species. M is the mass of the system, C is heat capacity of solid methane, H is the thermal conductivity, A is the area available for heat-exchange between the system and coolant, Tcool is the coolant temperature and P is the heating power provided by the external source. Eq. (3) gives radiation defect recombination rate coefficients K1,2 for each species, where K0 1,2 are the recombination rates at infinite temperature for each species. ΔE1,2 are the activation energies for defect diffusion. We have used the activation energy of dislocation motion in solid methane at different temperatures [9] as an estimation for choosing ΔE1/kB = 108.6 K and ΔE2/kB = 235 K. In Eq. (2) ε1 and ε2 represent the heat released per defect annihilation. In our calculations we have used 218 kJ mole−1 for H + H → H2 reaction (which, we assume, is the main reaction at lower temperatures) and 368 kJ mole−1 for CH3 + CH3 → C2H6 [3] (which is expected to happen around 50 K) ignoring all other reactions. Our model also makes use of linear interpolations from the solid methane thermal conductivity data in [10] and heat capacity in [11]. The differential Eqs. (1)–(3) are inherently non-linear and so we solve them numerically in order to produce a simulation of the system’s behaviour under different parameters. Starting at some known temperature, with a fixed defect production rate R1,2, we calculate the new number of defects N1,2 from Eq. (1) over our suitable chosen time step dt. Eq. (2) shows the balance of power into, and out from, the system, from which calculating the step increase in T over dt is simple. The process then loops over the chosen length of time.
Fig. 1. Illustrative sketch of the decoupled TS2 Solid Methane Moderator. The description of the components is given in the text.
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Fig. 2. Time dependence of IPNS solid methane moderator temperature: solid triangles represent averaged temperature data from the IPNS moderator [3]; results of calculations based on our model are presented by the solid curve.
4. Moderators test results and outcome of the model
determined using the cyclic relation that:
dP dP dV =− dT V dV T dT P
In Fig. 2 we present averaged temperature data from the IPNS solid methane moderator after three days irradiation with fast neutrons at 5·1012 neutron cm−2 s−1 [3]. The moderator’s temperature was maintained at 12 K. A rapid rise in temperature was purposely induced by throttling down the coolant flow. The temperature rose to 40 K in ∼ 20 s. Results of calculations based on our model (presented by the solid curve) demonstrate a good agreement with the real data. The test data (solid triangles) obtained during commissioning of the TS2 SMM mark two (with methane capacity 225 g of CH4) are presented in Fig. 3 together with the results of our model calculations. The moderator was irradiated by fast neutrons with a flux of 1016 neutron cm−2 s−1 for 8 h. The temperature of the solid methane during irradiation was maintained around 37 K. A moderator warm-up was then initiated by reduction of the coolant circulation in the heat-exchanger. The blue curve represents the solid methane temperature over time, whilst the red curve gives the number of radiation defects as a function of time. At 90.7 K the solid methane melts, adsorbing latent melting energy. The deviation of the calculated curve from the test data can be explained by not including the melting energy into our model. As in the previous case, our simulation results demonstrate a good agreement with the test data.
(4)
The (1/V)∂V/∂TP term is related to the isobaric thermal expansion coefficient of solid methane [12], and -V∂P/∂VP to the isothermal bulk modulus [9]. The relation (4) allows us to convert the temperature time dependence into the pressure in the ideal case of absolutely rigid walls of the container. In reality the internal volume of a thick walled pressure vessel is pressure dependent [13]. Finite element analysis (FEA) of the TS2 SMM (mark one) was performed using the ANSYS static structural module. The model assumed a purely elastic model for the aluminium body to determine the percentage volume change as a function of internal pressure. The determined stresses converged within 1% after mesh refinement, and indicated that absolute failure of the vessel would be expected at approximately 30 bar. By making a first order approximation that the volume of the moderator will always be equal to the volume of solid methane within, we are able to estimate the pressure experienced by the TS2 SMM with 3 mm wall thickness throughout the simulation (Fig.4). This estimation indicates that the ‘burp’ phenomenon is more than capable of generating the pressures necessary to permanently damage the moderator vessel. In this example the 30 bar failure point is reached at a temperature of approximately 67 K.
5. Pressure generated by thermal expansion of solid methane 6. Consequences for solid methane moderator design and operation
The outcome of our model is the time dependences of few parameters of interest, most important of which is the temperature. The obtained temperature time dependence can be translated into the time dependence of the internal pressure applied on the moderator vessel’s wall and generated by thermal expansion of solid methane. Assuming that the equation of state of solid methane can be expressed as function of state variables f(P,V,T) isochoric pressure derivative can be
As shown in the previous section, the quick expansion of solid methane has the potential to generate significant pressure on the walls of the moderator vessel, which in some cases could easily lead to structural damage. We would suggest that rapid warming up of the coolant circulated through the cooling loop, prior to beginning the Fig. 3. The TS2 SMM (mark two) test data are presented by solid triangles. The results of calculations based on our model are presented by the solid blue curve. The red curve gives the number of radiation defects as a function of time.
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Fig. 4. Simulation of the TS2 SMM (mark one) showing both temperature and pressure. The dashed red line shows the expected failure point at 30 bar.
the accumulation of heavy hydrocarbons (produced by methane radiolysis) on the surfaces of the heat-exchanger and aluminium foam. The usual life-span of a solid methane moderator is ∼60 days.
annealing process, might prevent this build-up of pressure. This quick heating would warm up the solid methane layer close to the heat exchanger’s surface. The rest of the bulk solid methane is expected to remain at low temperatures due to the poor thermal conductivity of solid methane [10]. At temperatures above 50 K solid methane significantly changes its mechanical properties [14], becomes soft [15] and can be squeezed out of the warm layer by the rising pressure, releasing the pressure on the moderator walls. Another recommendation would be to never fill the moderator 100% with methane, always leave some empty space where soft methane from the warm layer can be squeezed out. The last approach has been already incorporated into the ISIS TS2 solid methane moderator operational procedure. After a number of TS2 SMM (mark one) failures it was decided to increase the thickness of the aluminium walls of the vessel, from 3 mm to 10 mm. As mentioned previously, such radical design change has completely eliminated the possibility of moderator damage, but at the same time significantly reduced the cold neutron flux. Typical operation data for TS2 SMM (mark two) is presented in Fig. 5. The full operational cycle of TS2 SMM lasts 19 h, in which time the moderator will experience an integrated proton beam exposure of 650 µA. The cycle begins with a full warm up of the moderator from 47 K (base temperature) to 150 K, during which the solid methane initially melts and then evaporates. After the vessel is completely empty the temperature is reduced below ∼60 K and new, pure methane gas is condensed into the moderator vessel. Once the condensation process is finished, the solidified methane undergoes an annealing procedure. Annealing is required in order to improve the solid methane’s thermal conductivity, which accelerates all processes and significantly reduces the base temperature. This entire procedure (exhausting, condensing and annealing) takes ∼2.5 h. After half an hour of thermal equilibration the moderator is routinely used as a source of cold neutrons for another 16 h. Then the cycle is repeated. All the operational procedures are automated and do not require human supervision. After a number of cycles the base temperature of the moderator starts to increase, due to
7. Role of “burp” phenomenon in cryo-volcanism on comets In 1980 M. B. Neidner suggested that reactive species could be synthesised in cometary snows by long duration exposure to cosmic rays during the comet’s time as part of the Oort-Opik cloud complex. The author further suggested that these snows may then explode when exposed to high energy solar wind protons, observable as comet outbursts [16]. Seven years later Carpenter suggested that the ‘burp’ phenomenon described above [3] might play a significant role in the generation of cryo-jets, such as those observed from comets. Comets spend a considerable amount of time in the low temperature environment of the Kuiper Belt and the Oort-Opik cloud, exposed to the continuous radiation of outer space. In these conditions the bulk solid methane inclusions (whose existence is suggested in [17]) might accumulate a substantial number of radiation defects, which in turn could lead to spontaneous self-accelerated heating. However, the role of reactive species in cryo-volcanism on comets was later downplayed [18,19]. In the case of Carpenter’s scenario, the main concern would be a difference in temperature ranges: the ‘burp’ effect takes place around 20 K, but the temperature of a comet is never thought to drop below 30 K, with cryo-eruptions usually happening above 50 K. The latest and most plausible explanation for the driving mechanism behind cometary cryo-volcano eruption was offered by R. Miles [17,18]. According to this scenario cryo-volcanism is mostly driven by the release of the enthalpy of solution, also known as ‘heat of solution’ [18]. However, to start the process the system would still require an initial heat release in combination with some pressure (above 1 kPa) at temperatures as low as 40 K. We suggest that the higher temperature self-accelerated heating effect reported here at 50 K provides the necessary heating and pressure to trigger the eruption of a cryo-volcano. The eruption can be started by spontaneous self-accelerating heating in bulk solid methane inclusions [17] exposed to space radiation (galactic cosmic radiation and solar particle events) long enough to accumulate the number of radiation defects required for activating the process. Alternatively it could be triggered thermally, if during the comet’s orbit around the Sun the temperature exceeds the activation energy of radiation defect recombination (in our case somewhere between 40 and 50 K). Our observations allow us to assume that solid methane exposed to radiation can accumulate enough energy to warm up and even melt. Rapid warming should be accompanied by thermal expansion of solid methane which, in the case of rigid low temperature water ice walls, could generate high pressures of the order of tens of MPa. Therefore the initial stage of the cryo-eruption based on radiation defects recombination provides enough energy, along with a suitable temperature and high pressure environment, for initiating the ‘heat of
1140 min (19 h) or 650 Amps 150 K
93 K
60 K 47 K
47 K ~2.5 h
Fig. 5. TS2 Solid Methane Moderator (mark two) operation data: time dependences of temperatures of the vessel (black line) and solid methane (red line).
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Kulikov for valuable and stimulating discussions. We also would like to thank members of laboratory of spectroscopy of molecular systems in B. Verkin Institute, Kharkiv, Ukraine and particularly E.V. Savchenko for sharing their data and ideas with us and S.R. Wakefield for interest and support to the methane moderator project. We appreciate Oksana Kirichek’s original idea of possibility of triggering cryogenic eruption by thermal activation. OK was supported by the project TUMOCS. This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant No. 645660.
solution’ scenario [18] leading to a full scale cryo-volcano eruption. Here we would also like to mention that radiation defect recombination in water ice [6] might also play noticeable role in cryo-volcanism on outer celestial bodies. The thermally activated initiation of radiation defect recombination could be utilised for deliberate triggering of a cryo-volcano eruption, by the deposition of relatively small amounts of energy into the bulk solid methane inclusions of a comet. This could be achieved by shining an infrared light laser beam, generated by a moderate energy laser installed on a spacecraft similar to Rosetta [20], which is orbiting the comet of interest. An estimation based on our model suggests that 30 kJ of energy deposed into ∼ 1 L volume within few minutes might trigger a ‘burp’ effect in cometary methane previously exposed to a space radiation for few months. Each eruption triggered by the laser beam produces an intense discharge of fast-moving gas/dust jets. We can speculate that if cryo eruptions are triggered multiple times at defined positions on a comet, and in known direction of jet ejection, it could be possible to change the comet’s trajectory.
References [1] [2] [3] [4]
[5] [6] [7]
8. Conclusions
[8] [9]
In our paper we have presented and discussed results arising from commissioning tests of the solid methane moderator design used in Target Station 2 at the ISIS neutron source, and compared them to a model based on the theory of thermal explosion. The good agreement between moderator test data and our simulation suggests that the process of radiolysis defect recombination happens in two stages, at two different temperature ranges. The “lower temperature” recombination process occurs at around 20 K, with the “higher temperature” process taking place between 50 and 60 K. We also used our model to simulate the pressure experienced by the walls of the TS2 SMM vessel. The pressure spike produced by the rapid expansion of solid methane during self-accelerating heating can be intense enough to damage the vessel, which applies certain limitations on solid methane moderator design and operational procedure. We also discuss the possible role of radiolysis defect recombination processes in cryo-volcanism on comets, and suggest an application based on this phenomenon.
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
[20]
Acknowledgements The authors are grateful to J.M. Carpenter, E.P. Shabalin and S.A.
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Foti G, Calcagno L, Sheng K, Strazzulla G. Nature 1984;310:126. Foti G, Calcagno L, Zhou FZ, Strazzulla G. Nucl Instr Meth 1987;B24(25):522–5. Carpenter JM. Nature 1987;330:358. Scott TL, Carpenter JM, Miller ME. The development of solid methane neutron moderators at the intense pulsed neutron source facility of Argonne National Laboratory Preprint ANL/IPNS/CP-98533, 1999. Shabalin EP. JINR Commun. 1995;E17–95-142. Kulagin E, Kulikov S, Melikhov V, Shabalin E. Nucl Instr Meth 2004;B215:181–6. Kirichek O, Savchenko EV, Lawson CR, Khyzhniy IV, Jenkins DM, Uyutnov SA, et al. 2017. Available from: arXiv: 1709.01994 [cond-mat.mtrl-sci]. Bowden FP, Yoffe AD. Fast reactions in solids. London: Butterworth; 1958. Prokhvatilov AI. Plasticity and elasticity of cryocrystals (New York: Begell House Inc.) 2001;118–32. Manzhelii VG, Krupskii IN. Fiz Tverd Tela 1968;10:281. Clusius KZ. Phys Chem 1929;B3:41. Bol’shutkin DN, Gasan VM, Prokhvatilov AI. Zhurnal Strukturnoi Khimii 1971;12:734–6. Lewis WF, Benson D, Crawford RK, Daniels WB. J Phys Chem Solids 1974;35:383–91. Zakharov AYu, Leont’eva AV, Prokhorov AY, Erenburg AI. J Low Temp. Phys. 2017;187:140–7. Kirichek O, Church AJ, Thomas MG, Cowdery D, Higgins SD, Dudman MP, et al. Cryogenics 2012;52:325–30. Niedner MB. Interplanetary gas. XXV. A solar wind and interplanetary magnetic field interpretation of commentary light outbursts. Astrophys J 1980;241:820–9. Miles R. Icarus 2016;272:387–413. Miles R. Icarus 2016;272:356–86. Hughes DW. Possible mechanisms for cometary outbursts. In: Newburn RL, Neugebauer M, Rahe J editor. Comets in the Post-Halley Era, vol. 2. Dordrecht: The Netherlands; Kluwer Academic Publishers; 1991, 825–51. Gibney E. Mission accomplished: Rosetta crashes into comet. Nature 2016;538:13–4.
Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Research paper
Solid methane in neutron radiation: Cryogenic moderators and cometary cryo volcanism
MARK
⁎
O. Kirichek , C.R. Lawson, D.M. Jenkins, C.J.T. Ridley, D.J. Haynes ISIS Facility, STFC, Rutherford Appleton Laboratory, Harwell, Didcot, UK
A R T I C L E I N F O
A B S T R A C T
Keywords: Solid methane Neutron radiation Moderator
The effect of ionizing radiation on solid methane has previously been an area of interest in the astrophysics community. In the late 1980s this interest was further boosted by the possibility of using solid methane as a moderating medium in spallation neutron sources. Here we present test results of solid methane moderators commissioned at the ISIS neutron source, and compare them with a model based on the theory of thermal explosion. Good agreement between the moderator test data and our model suggests that the process of radiolysis defect recombination happens at two different temperature ranges: the “lower temperature” recombination process occurs at around 20 K, with the “higher temperature” process taking place between 50 and 60 K. We discuss consequences of this mechanism for the designing and operation of solid methane moderators used in advanced neutron sources. We also discuss the possible role of radiolysis defect recombination processes in cryovolcanism on comets, and suggest an application based on this phenomenon.
1. Introduction The properties of frozen methane, when exposed to ionising radiation, have attracted significant interest in the planetary and interstellar astrophysics community since the early 1980s [1,2]. In the late 1980s this interest was further boosted by the possibility of using solid methane as a moderating medium in spallation neutron sources [3]. Solid methane possesses unique neutronic properties that enable the conversion of hot, energetic neutrons into cold neutrons with an efficiency approximately 3.5 times that of liquid hydrogen based moderators [4]. However, practical applications of solid methane as a neutron moderator material turned out to be much more challenging than initially expected. Early developments in methane moderators were carried out by the IPNS neutron source based at Argonne National Laboratory. Here solid methane at around 10 K was exposed to neutron radiation for few days, leading to a build-up of radiolysis products in the solid methane matrix. It was then shown that at some critical number of defects a spontaneous self-accelerated recombination process would take place, described by J. Carpenter as the ‘burp’ phenomenon [3,4]. This effect, in combination with the expansion of hydrogen built up in bulk solid methane during irradiation, was believed to be responsible for the moderator’s breakdowns. In the late 1990s the effects of neutron radiation on solid methane were systematically studied at the IBR neutron source in Dubna [4,5].
⁎
Corresponding author. E-mail address: [email protected] (O. Kirichek).
http://dx.doi.org/10.1016/j.cryogenics.2017.10.017 Received 14 September 2017; Accepted 12 October 2017 Available online 14 October 2017 0011-2275/ © 2017 Published by Elsevier Ltd.
The results of this research confirmed the assumptions of Carpenter’s model and gave some estimations of the scale of the process. After these experiments the consensus in the neutron scattering community was that all of the radiolysis products accumulated in solid methane (irradiated at temperatures around 10 K) start to recombine either spontaneously, if the density of the defects exceeds critical value, or by thermal activation mechanism during warming up if temperature exceeds 20 K. In either case, all defect recombination processes should be completed at temperatures below 40 K [3–6] and the design of the Target Station 2 Solid Methane Moderator (TS2 SMM), commissioned by the ISIS neutron source in 2009, was based on this assumption. However, the very first test of this moderator operated at 38 K ended in a similar burp-like event, which damaged the moderator. This observation reignited interest in the results of the IPNS neutron source and suggested that the defect recombination process might be more complex that initially thought, with multiple defect types recombining at different temperature ranges. Further support for this concept came from results obtained by researchers from the Verkin Institute in Kharkiv, Ukraine [7], who studied the radiation defect relaxation mechanisms in solid methane films, pre-irradiated by an electron beam. This experiment was able to measure both mass ejection and electromagnetic luminescence events. An expected peak of particle ejection (post-desorption) was found in solid methane at temperatures as low as 15 K, but another maximum was also observed at higher temperatures around 50 K.
Cryogenics 88 (2017) 101–105
O. Kirichek et al.
operation cycle. During testing, the moderator was irradiated by a neutron flux of ∼1016 neutron cm−2s−1, at a temperature of 38 K, for intervals of between 8 to 24 h. In three instances the self-accelerated heating process was triggered at ∼50 K during an intentional warmup of the moderator (this is a standard procedure, carried out with the aim of annealing the solid methane) leading to moderator damage. In all three cases the damage occurred at temperatures around 65 K. In the current moderator design (mark two), the thickness of the aluminium walls of the vessel has been increased from 3 mm to 10 mm. However this reduces the methane capacity of the moderator by almost half of the mark one prototype – 225 g of CH4. Such radical design change has completely eliminated the possibility of moderator damage, but at the same time significantly reduced the cold neutron flux.
Here we present and discuss the results of commissioning tests of solid methane moderators used in Target Station 2 at the ISIS neutron source, and compare them with a model based on the theory of thermal explosion [8]. We also compare the results of our model simulation with the IPNS solid methane moderator test data published in [3]. The temperature range of the TS2 SMM commissioning tests was 37–100 K. The strong agreement between measured moderator temperature data and our simulation, combined with methane irradiation data from [7], suggests that the process of radiolysis defect recombination happens in two stages, at two different temperature ranges. The “lower temperature” recombination process occurs at around 20 K, with the “higher temperature” process taking place between 50 and 60 K. We then use our model to simulate the pressure experienced by the walls of the TS2 SMM vessel produced by the rapid expansion of solid methane during self-accelerating heating, driven by the recombination process. We argue that this pressure spike can be intense enough to damage the vessel, which applies certain limitations on solid methane moderator design and operational procedure. We also discuss the possible role of radiolysis defect recombination processes in cryo-volcanism on comets, and suggest an application based on this phenomenon.
3. Model of solid CH4 in neutron radiation Our model utilises an approach similar to that used by Carpenter in [3] based on a simplified version of the theory of thermal explosion [8]. In order to reflect the presence of radiation defect recombination occurring at higher temperatures (50–70 K) we have introduced secondary binary collisions, with a rate coefficient K2 that also has Arrhenius-form temperature dependence. Then the process of accumulation and recombination of radiolysis defects can be described by the equations:
2. Design and commissioning of the ISIS Target Station 2 solid methane moderator
dN1,2 (t ) 2 (t ) = R1,2 (t )−K1,2 (T ) N1,2 dt
An illustrative sketch of the decoupled TS2 SMM is presented in Fig. 1. The moderator cryostat consists of an outer vacuum can (3), infrared radiation shield (4) and the solid methane moderator vessel (5). The “mark one” version of the moderator vessel was made of aluminium alloy 5083–0 with internal dimensions 124 × 140 × 70 mm and wall thickness 3 mm. The aluminium foam block (6) from aluminium alloy 1199-F and 8% density has been pressure fitted into the moderator vessel (5). The moderator comprises a heat exchanger coil (7) made of Ø 10 mm aluminium tube welded into the vessel’s body. The fully loaded TS2 SMM (mark one) contained 405 g of CH4. The high purity methane gas is condensed into the moderator through the inlet pipe (1) and exhausted through the outlet pipe (2) at the end of the
MC (T )
(1)
dT = P (t ) + ε1 K1 (T ) N12 (t ) + ε2 K2 (T ) N22 (t ) dt −AH (T )(T (t )−Tcool (t ))
K1,2 (T ) = K o1,2 e−E1,2/ kB T
(2) (3)
Here, T is temperature, N1 is the number of defects recombining around 20 K and N2 is the number of defects recombining at around 50 K. Equation (1) is the same for both species, so for simplicity we use N1,2 where index 1 and index 2 are for lower and higher temperatures of recombination respectively. R1,2 is the defect production rate for each species. In our case of solid methane irradiated by fast neutrons, we assume that production rate is the same R1 ≂ R2 for both species. M is the mass of the system, C is heat capacity of solid methane, H is the thermal conductivity, A is the area available for heat-exchange between the system and coolant, Tcool is the coolant temperature and P is the heating power provided by the external source. Eq. (3) gives radiation defect recombination rate coefficients K1,2 for each species, where K0 1,2 are the recombination rates at infinite temperature for each species. ΔE1,2 are the activation energies for defect diffusion. We have used the activation energy of dislocation motion in solid methane at different temperatures [9] as an estimation for choosing ΔE1/kB = 108.6 K and ΔE2/kB = 235 K. In Eq. (2) ε1 and ε2 represent the heat released per defect annihilation. In our calculations we have used 218 kJ mole−1 for H + H → H2 reaction (which, we assume, is the main reaction at lower temperatures) and 368 kJ mole−1 for CH3 + CH3 → C2H6 [3] (which is expected to happen around 50 K) ignoring all other reactions. Our model also makes use of linear interpolations from the solid methane thermal conductivity data in [10] and heat capacity in [11]. The differential Eqs. (1)–(3) are inherently non-linear and so we solve them numerically in order to produce a simulation of the system’s behaviour under different parameters. Starting at some known temperature, with a fixed defect production rate R1,2, we calculate the new number of defects N1,2 from Eq. (1) over our suitable chosen time step dt. Eq. (2) shows the balance of power into, and out from, the system, from which calculating the step increase in T over dt is simple. The process then loops over the chosen length of time.
Fig. 1. Illustrative sketch of the decoupled TS2 Solid Methane Moderator. The description of the components is given in the text.
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Fig. 2. Time dependence of IPNS solid methane moderator temperature: solid triangles represent averaged temperature data from the IPNS moderator [3]; results of calculations based on our model are presented by the solid curve.
4. Moderators test results and outcome of the model
determined using the cyclic relation that:
dP dP dV =− dT V dV T dT P
In Fig. 2 we present averaged temperature data from the IPNS solid methane moderator after three days irradiation with fast neutrons at 5·1012 neutron cm−2 s−1 [3]. The moderator’s temperature was maintained at 12 K. A rapid rise in temperature was purposely induced by throttling down the coolant flow. The temperature rose to 40 K in ∼ 20 s. Results of calculations based on our model (presented by the solid curve) demonstrate a good agreement with the real data. The test data (solid triangles) obtained during commissioning of the TS2 SMM mark two (with methane capacity 225 g of CH4) are presented in Fig. 3 together with the results of our model calculations. The moderator was irradiated by fast neutrons with a flux of 1016 neutron cm−2 s−1 for 8 h. The temperature of the solid methane during irradiation was maintained around 37 K. A moderator warm-up was then initiated by reduction of the coolant circulation in the heat-exchanger. The blue curve represents the solid methane temperature over time, whilst the red curve gives the number of radiation defects as a function of time. At 90.7 K the solid methane melts, adsorbing latent melting energy. The deviation of the calculated curve from the test data can be explained by not including the melting energy into our model. As in the previous case, our simulation results demonstrate a good agreement with the test data.
(4)
The (1/V)∂V/∂TP term is related to the isobaric thermal expansion coefficient of solid methane [12], and -V∂P/∂VP to the isothermal bulk modulus [9]. The relation (4) allows us to convert the temperature time dependence into the pressure in the ideal case of absolutely rigid walls of the container. In reality the internal volume of a thick walled pressure vessel is pressure dependent [13]. Finite element analysis (FEA) of the TS2 SMM (mark one) was performed using the ANSYS static structural module. The model assumed a purely elastic model for the aluminium body to determine the percentage volume change as a function of internal pressure. The determined stresses converged within 1% after mesh refinement, and indicated that absolute failure of the vessel would be expected at approximately 30 bar. By making a first order approximation that the volume of the moderator will always be equal to the volume of solid methane within, we are able to estimate the pressure experienced by the TS2 SMM with 3 mm wall thickness throughout the simulation (Fig.4). This estimation indicates that the ‘burp’ phenomenon is more than capable of generating the pressures necessary to permanently damage the moderator vessel. In this example the 30 bar failure point is reached at a temperature of approximately 67 K.
5. Pressure generated by thermal expansion of solid methane 6. Consequences for solid methane moderator design and operation
The outcome of our model is the time dependences of few parameters of interest, most important of which is the temperature. The obtained temperature time dependence can be translated into the time dependence of the internal pressure applied on the moderator vessel’s wall and generated by thermal expansion of solid methane. Assuming that the equation of state of solid methane can be expressed as function of state variables f(P,V,T) isochoric pressure derivative can be
As shown in the previous section, the quick expansion of solid methane has the potential to generate significant pressure on the walls of the moderator vessel, which in some cases could easily lead to structural damage. We would suggest that rapid warming up of the coolant circulated through the cooling loop, prior to beginning the Fig. 3. The TS2 SMM (mark two) test data are presented by solid triangles. The results of calculations based on our model are presented by the solid blue curve. The red curve gives the number of radiation defects as a function of time.
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Fig. 4. Simulation of the TS2 SMM (mark one) showing both temperature and pressure. The dashed red line shows the expected failure point at 30 bar.
the accumulation of heavy hydrocarbons (produced by methane radiolysis) on the surfaces of the heat-exchanger and aluminium foam. The usual life-span of a solid methane moderator is ∼60 days.
annealing process, might prevent this build-up of pressure. This quick heating would warm up the solid methane layer close to the heat exchanger’s surface. The rest of the bulk solid methane is expected to remain at low temperatures due to the poor thermal conductivity of solid methane [10]. At temperatures above 50 K solid methane significantly changes its mechanical properties [14], becomes soft [15] and can be squeezed out of the warm layer by the rising pressure, releasing the pressure on the moderator walls. Another recommendation would be to never fill the moderator 100% with methane, always leave some empty space where soft methane from the warm layer can be squeezed out. The last approach has been already incorporated into the ISIS TS2 solid methane moderator operational procedure. After a number of TS2 SMM (mark one) failures it was decided to increase the thickness of the aluminium walls of the vessel, from 3 mm to 10 mm. As mentioned previously, such radical design change has completely eliminated the possibility of moderator damage, but at the same time significantly reduced the cold neutron flux. Typical operation data for TS2 SMM (mark two) is presented in Fig. 5. The full operational cycle of TS2 SMM lasts 19 h, in which time the moderator will experience an integrated proton beam exposure of 650 µA. The cycle begins with a full warm up of the moderator from 47 K (base temperature) to 150 K, during which the solid methane initially melts and then evaporates. After the vessel is completely empty the temperature is reduced below ∼60 K and new, pure methane gas is condensed into the moderator vessel. Once the condensation process is finished, the solidified methane undergoes an annealing procedure. Annealing is required in order to improve the solid methane’s thermal conductivity, which accelerates all processes and significantly reduces the base temperature. This entire procedure (exhausting, condensing and annealing) takes ∼2.5 h. After half an hour of thermal equilibration the moderator is routinely used as a source of cold neutrons for another 16 h. Then the cycle is repeated. All the operational procedures are automated and do not require human supervision. After a number of cycles the base temperature of the moderator starts to increase, due to
7. Role of “burp” phenomenon in cryo-volcanism on comets In 1980 M. B. Neidner suggested that reactive species could be synthesised in cometary snows by long duration exposure to cosmic rays during the comet’s time as part of the Oort-Opik cloud complex. The author further suggested that these snows may then explode when exposed to high energy solar wind protons, observable as comet outbursts [16]. Seven years later Carpenter suggested that the ‘burp’ phenomenon described above [3] might play a significant role in the generation of cryo-jets, such as those observed from comets. Comets spend a considerable amount of time in the low temperature environment of the Kuiper Belt and the Oort-Opik cloud, exposed to the continuous radiation of outer space. In these conditions the bulk solid methane inclusions (whose existence is suggested in [17]) might accumulate a substantial number of radiation defects, which in turn could lead to spontaneous self-accelerated heating. However, the role of reactive species in cryo-volcanism on comets was later downplayed [18,19]. In the case of Carpenter’s scenario, the main concern would be a difference in temperature ranges: the ‘burp’ effect takes place around 20 K, but the temperature of a comet is never thought to drop below 30 K, with cryo-eruptions usually happening above 50 K. The latest and most plausible explanation for the driving mechanism behind cometary cryo-volcano eruption was offered by R. Miles [17,18]. According to this scenario cryo-volcanism is mostly driven by the release of the enthalpy of solution, also known as ‘heat of solution’ [18]. However, to start the process the system would still require an initial heat release in combination with some pressure (above 1 kPa) at temperatures as low as 40 K. We suggest that the higher temperature self-accelerated heating effect reported here at 50 K provides the necessary heating and pressure to trigger the eruption of a cryo-volcano. The eruption can be started by spontaneous self-accelerating heating in bulk solid methane inclusions [17] exposed to space radiation (galactic cosmic radiation and solar particle events) long enough to accumulate the number of radiation defects required for activating the process. Alternatively it could be triggered thermally, if during the comet’s orbit around the Sun the temperature exceeds the activation energy of radiation defect recombination (in our case somewhere between 40 and 50 K). Our observations allow us to assume that solid methane exposed to radiation can accumulate enough energy to warm up and even melt. Rapid warming should be accompanied by thermal expansion of solid methane which, in the case of rigid low temperature water ice walls, could generate high pressures of the order of tens of MPa. Therefore the initial stage of the cryo-eruption based on radiation defects recombination provides enough energy, along with a suitable temperature and high pressure environment, for initiating the ‘heat of
1140 min (19 h) or 650 Amps 150 K
93 K
60 K 47 K
47 K ~2.5 h
Fig. 5. TS2 Solid Methane Moderator (mark two) operation data: time dependences of temperatures of the vessel (black line) and solid methane (red line).
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Kulikov for valuable and stimulating discussions. We also would like to thank members of laboratory of spectroscopy of molecular systems in B. Verkin Institute, Kharkiv, Ukraine and particularly E.V. Savchenko for sharing their data and ideas with us and S.R. Wakefield for interest and support to the methane moderator project. We appreciate Oksana Kirichek’s original idea of possibility of triggering cryogenic eruption by thermal activation. OK was supported by the project TUMOCS. This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant No. 645660.
solution’ scenario [18] leading to a full scale cryo-volcano eruption. Here we would also like to mention that radiation defect recombination in water ice [6] might also play noticeable role in cryo-volcanism on outer celestial bodies. The thermally activated initiation of radiation defect recombination could be utilised for deliberate triggering of a cryo-volcano eruption, by the deposition of relatively small amounts of energy into the bulk solid methane inclusions of a comet. This could be achieved by shining an infrared light laser beam, generated by a moderate energy laser installed on a spacecraft similar to Rosetta [20], which is orbiting the comet of interest. An estimation based on our model suggests that 30 kJ of energy deposed into ∼ 1 L volume within few minutes might trigger a ‘burp’ effect in cometary methane previously exposed to a space radiation for few months. Each eruption triggered by the laser beam produces an intense discharge of fast-moving gas/dust jets. We can speculate that if cryo eruptions are triggered multiple times at defined positions on a comet, and in known direction of jet ejection, it could be possible to change the comet’s trajectory.
References [1] [2] [3] [4]
[5] [6] [7]
8. Conclusions
[8] [9]
In our paper we have presented and discussed results arising from commissioning tests of the solid methane moderator design used in Target Station 2 at the ISIS neutron source, and compared them to a model based on the theory of thermal explosion. The good agreement between moderator test data and our simulation suggests that the process of radiolysis defect recombination happens in two stages, at two different temperature ranges. The “lower temperature” recombination process occurs at around 20 K, with the “higher temperature” process taking place between 50 and 60 K. We also used our model to simulate the pressure experienced by the walls of the TS2 SMM vessel. The pressure spike produced by the rapid expansion of solid methane during self-accelerating heating can be intense enough to damage the vessel, which applies certain limitations on solid methane moderator design and operational procedure. We also discuss the possible role of radiolysis defect recombination processes in cryo-volcanism on comets, and suggest an application based on this phenomenon.
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
[20]
Acknowledgements The authors are grateful to J.M. Carpenter, E.P. Shabalin and S.A.
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