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Experimental study of swelling of irradiated solid methane during annealing
Nuclear Instruments and Methods in Physics Research B 266 (2008) 5126–5131
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Experimental study of swelling of irradiated solid methane during annealing E. Shabalin, A. Fedorov, E. Kulagin, S. Kulikov, V. Melikhov, D. Shabalin * Joint Institute for Nuclear Research, Joliot-Curie Street 6, 141980 Dubna, JINR, Moscow Region, Russia
a r t i c l e
i n f o
Article history: Received 22 April 2008 Received in revised form 2 September 2008 Available online 26 September 2008 PACS: 28.20.v 28.20.Gd 61.80.Hg Keywords: Solid methane Radiolytic hydrogen Pressure Swelling
a b s t r a c t Solid methane is still widely in use at pulsed neutron sources due to its excellent neutronic performance (IPNS, KENS, Second Target Station at ISIS), notwithstanding poor radiation properties. One of the specific problems is radiolytic hydrogen gas pressure on the walls of a methane chamber during annealing of methane. In this paper results of an experimental study of this phenomenon under fast neutron irradiation with the help of a specially made low temperature irradiation rig at the IBR-2 pulsed reactor are presented. The peak pressure on the wall of the experimental capsule during heating of a sample irradiated at 23–35 K appears to have a maximum of 2.7 MPa at an absorbed dose 20 MGy and then falls down with higher doses. The pressure always reached its peak value at the temperature range 72–79 K. Generally, three phases of methane swelling during heating can be distinguished, each characterized by a proper rate and intensity. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Solid methane, notwithstanding its poor radiation properties [1], is still widely in use at pulsed neutron sources as a cold neutron moderator [2–6]. One of the specific problems is radiolytic hydrogen gas pressure on the walls of a methane chamber during annealing of methane. Hydrogen, both in atomic and molecular form, is the main product of methane radiolysis. Four decomposed methane molecules give rise to about three molecules of hydrogen. The rate of hydrogen production (Rhyd, mol/g of CH4) is determined by the relation [4,5]: 4
RHyd ¼ 3 10
!! _ D_ Dt _ D tþ 1 expð Þ D0 D0
where D_ is the absorbed dose rate, MGy/h (1 W/g = 3.6 MGy/h), D0 is a constant 8 MGy and t is time of irradiation in hours. For low doses, RHyd is equal to 6 107 mol/J. At an irradiation temperature of 20–30 K, radiolytic hydrogen is in a condensed state, not building any measurable internal pressure and swelling of methane [4]. However, warming the condensed state after irradiation builds up an internal pressure of gaseous hydrogen inside the solid methane matrix, resulting in swelling of methane and a pressure load onto the methane
* Corresponding author. Tel./fax: +749 62165253. E-mail addresses: [email protected], [email protected] (D. Shabalin). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.09.022
chamber walls. Many chambers of solid methane moderators have been destroyed due to this effect [5–7]. For 20 years of solid methane moderator use, no attempt has been made to quantitatively study the characteristics of the phenomenon. This paper gives results of the only experimental study of pressure loads on the walls of a chamber with solid methane during warming after fast neutron irradiation.
2. Apparatus and methods This study was performed at the specially modified low temperature irradiation facility URAM-2 at the IBR-2 pulsed reactor [1,8]. The new irradiation rig and study programme were called the URAM-3M. The irradiation facility consists of four principal parts: – irradiation rig head with supporting parts (Figs. 1 and 2), – cooling system, based on using liquid helium as a coolant material, – vacuum-gas technology system, – control equipment. This facility enables the irradiation of solid methane condensed inside aluminum foam at 25–35 K for 35–40 h at dose rates of either 0.23 W/g, or 0.5 W/g. Solid methane samples were prepared by condensing methane gas into the chamber as liquid and freezing it afterwards.
E. Shabalin et al. / Nuclear Instruments and Methods in Physics Research B 266 (2008) 5126–5131
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Fig. 1. Front end of the irradiation rig, elevation through the central plane of the sample. 1 – Cylindrical shell of the chamber; 6 – aluminium foam filled with solid methane; 7 – coils of copper tube; 8 – steel tube for filling and evacuation gases; 9 – pockets for temperature sensors; 10 – steel string suspender of the sample chamber; 11 – helium pipes inside an evacuated 3 m-long tube.
Both the irradiation chamber and aluminum foam inside it, have a cylindrical shape. All walls of the chamber are made of stainless steel 12X18H10T (Russian standard), which has high resistance to radiation and good mechanical properties at low temperature. Two faces of the chamber represent plates of 1.5 mm thick steel and serve as membranes to follow methane expansion during hydrogen release (see the characteristics in Fig. 3). Displacement of the membranes was measured with two gauges of induction type (sensitivity – 0.63 V/mm). Inside an evacuated jacket of polished stainless steel, the chamber is kept strongly space oriented with a steel string under tension. Nevertheless, position of the irradiation chamber with respect to the displacement gauges depends on temperature. This temperature effect was compensated by estimation of displacement of the membranes as an average of indications of the two gauges. Because the P–X diagram (pressure on the membranes versus its displacement) at the first load after heating up to room temperature appeared to be nonlinear, ‘‘training” of membranes at 70–75 K was done each time before a new irradiation run, that is, triple loading up to 2.5 MPa. Such a procedure guaranteed a linear characteristic of the P–X diagram. To prevent overfilling of the aluminum foam with methane, a capillary tube made of copper was placed inside the methane feeding tube and was welded into the steel walls of the vacuum chamber. Due to high thermal conductivity of copper, the temperature of the capillary tube was near 300 K everywhere, preventing freezing of methane at the neck of the chamber. The temperature non-uniformity in the foam filled with solid methane was calculated with the 3D-heat transfer code COSMOS-M and appeared to be near 3 K. The temperature of the chamber walls was measured by thermocouples welded between
copper tube coils. Dose rates were measured in advance with a purpose-built test bench. Moreover, the dose rate in methane was checked every time by measuring the amount of released hydrogen gas as the hydrogen output per unit of absorbed dose is well known. Gas pressure monitoring was arranged with two gauges: 0–4 104 Pa and 0–2.5 MPa, both of 0.1% accuracy at full scale. 2.1. Principal procedure of the experiment 1. Before condensation of methane, the chamber is cooled down to 70–80 K and filled with helium under pressure varying from 0 to 2.5 MPa for calibration of the displacement gauges and training of the membranes. 2. The methane cavity is evacuated, heated up to 105–115 K and connected to a bottle of 5.73 L capacity charged with methane gas of 99.99% purity to a pressure between 0.19 MPa and 2.3 MPa (two irradiations were performed with methane doped with 0.65 mol% BF3 gas enriched with B10 up to 83%, which increased the dose rate up to 0.5 W/g). After condensation of 0.14–0.15 MPa, the gate between the bottle and the methane filling tube is shut. Until condensation is completed, the temperature of the methane is kept above 90.6 K to prevent solidification of methane before the sample chamber is filled with liquid methane. Then the methane is cooled down to 25–30 K. 3. Continuous irradiation of the solid methane sample for some predetermined time at the maximal temperature of the methane from 26 K up to 35 K proceeds. 4. The temperature of the methane sample is increased up to 90 K at a defined rate (from 0.3 to 5 K/min) by switching a helium heater on and depressing the helium flow rate. In the process
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E. Shabalin et al. / Nuclear Instruments and Methods in Physics Research B 266 (2008) 5126–5131
Fig. 2. Evacuated chamber with a sample, elevation along diameter. 1 – Cylindrical shell of the chamber; 2 – coaxial cables of displacement gauges (3); 4 – steel membrane; 5 – steel shell of a sample chamber; 6 – the sample (aluminium sponge filled with solid methane); 7 – coil of copper tube (heat exchanger); 8 – steel tube for filling and evacuation of methane and hydrogen gases.
stages 3 and 4, the temperature of the methane chamber and outgoing gas pressure and displacement of steel membranes (faces of the chamber) is recorded. 5. After saturation of the gas pressure in the bottle (that means that all hydrogen released) the temperature increases for evaporation of methane and cavity of the chamber is pumped out at 355 K to clean it of high boiling products of methane radiolysis. 6. The cavity of the chamber is filled with helium under pressure close to that observed in the experiment, for calibration of the displacement gauges. 7. Helium is pumped out, the temperature goes up, methane is evaporated, the chamber cavity is evacuated and the next experiment begins.
3. Experimental results The main characteristics of the experimental runs are summarized in Table 1. The lower limit of the value in column H in Table 1 reflects more or less exactly the beginning of hydrogen release;
the upper limit is the temperature corresponding to the peak of the pressure (column F). It should be noted that experiments #9 and #10 were made with methane doped with boron; experiment #9 was assigned to check the boron doping effect on the amount of hydrogen. The typical temporal behavior of parameters during heating of irradiated samples is given in Figs. 4 and 5. 3.1. Effect of heating rate In Fig. 6, the maximal pressure on the membranes versus the amount of accumulated hydrogen is given. Six out of nine experimental points lie strictly on the smooth curve which can be approximated as a parabola. All of these experiments are characterized by similar conditions (the chamber is not overfilled and rate of heating is in the range 0.5–15 K/min), except for different temperatures of irradiation. Three other points differ either in filling of the chamber (#3), or in the method of heating (in experiment #5 methane was heated by bleeding of a portion of warm methane gas), or by very low heating rate (#8). The last circumstance is not significant in practice – to gain about 20% in pressure
E. Shabalin et al. / Nuclear Instruments and Methods in Physics Research B 266 (2008) 5126–5131
Fig. 3. Response of the displacement gauge (Y-axis, mV) to the gas pressure load on the membrane (X-axis, 105 Pa) at 70–80 K. Curve 1 is for the first load after heating up to room temperature, curve 2 is for the second load and curve 3 – for the third load and the following.
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Fig. 5. Record of parameters during heating, experiment #7.
Table 1 Experimental data (titles of the columns see in the legend) No of run
A
B
C
D
E
F
G
H
I
1 2 3 4 5 6
430 406 450 400 400 406
6.0 5.7 9.16 15.5 6.7 24.2
25–27 25–30 30–35 25–26 27–30 30–35
18.8 17.1 28.3a 37.3 19.3 51.4
1.37 1.3 2.15 2.1 2.05 2.4
24 24 10 21 20 24
81–87 83–85 86–88 67–72 >85 66–74
3% 10% 10% 3% 10% 6%
7 8 9 10
400 391 383 367
29.5 9.8 7.5 18
30–39 30–31 85 23
60.0 23.4 39 80
2.5–3 1.5–2 4–5 0.5 –b 0.6– 1.4 1–2 0.3 – 10–15
20 21 – 16
64–72 69–79 – 68–74
11% 0.7% – 28%
2.45–2.5 1.3 – 1.97
A – charge of methane in mmol; B – irradiation time in hours; one hour of irradiation corresponds to absorbed dose of 0.81 MGy (for experiments ##1–8); C – temperature of irradiation, K; D – amount of H2 accumulated, mmol; E – heating rate, K/min; F – peak pressure on the membrane, Pmax, MPa; G – fraction of H2 (%) released before pressure reached Pmax ; H – temperature range of hydrogen release, K; I – maximal rate of hydrogen release, 1/min. a Calculated value. b Heating by filling the chamber with the warm methane gas.
Fig. 6. Peak value of pressure during heating of irradiated methane (Y-axis, 100 kPa) versus amount of hydrogen accumulated (mol/mol CH4). Black squares – experiments with the normal amount of methane in the chamber and with medium or high rate of heating; hollow squares – experiments with different parameters of heating.
on the membrane, one hour extra time for heating is needed. The rate of heating is not a critical factor for pressure on the membranes. 3.2. Effect of temperature of irradiation As is clear from Fig. 6 and Table 1, there is no visible evidence of irradiation temperature effect on maximal displacement of membrane. 3.3. Maximal value of pressure
Fig. 4. Record of parameters during heating, experiment #4.
For a dose >20 MGy the peak pressure on the membrane during heating reaches maximum of about 2.5 MPa with amount of accumulated hydrogen (the displacement of the membrane is 0.4 mm accordingly) and then falls, not depending on the rate of heating. Since the diameter of the membrane is greater than the diameter of the aluminum sponge, the maximal pressure was really 10% more (as shown by computer simulation), that is, about 2.7 MPa.
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3.4. Effect of burping In experiments #1, #4 and #10 induced burps were indicated. They did not affect markedly value of peak pressure on membranes. 4. Discussion and interpretation of the results Following regularities in the behavior of pressure on the membrane during heating of methane samples, it is noteworthy that: The pressure on the membrane always reaches peak value at a temperature in the range 72–79 K (after long irradiation) and, what is most intriguing, just up to this moment about 20% of the stored hydrogen is released in all the experiments. The rate of hydrogen release depends on temperature not monotonically: in the range of 64–72 K (for high doses of radiation) it sharply intensifies and returns to exponential (Arrhenius type) growth afterwards (see Figs. 4, 5, 7 and 8). Before hydrogen starts to escape at T > 64 K, the membranes exhibit displacement. The swelling of the methane, which is about half of the peak value, follows a jump in temperature from
40 K to 60 K. The balance of the heat in the chamber showed (Fig. 9) that at this moment the additional energy, released in the methane sample, is about 60 J/g. These regularities and other characteristics of the process of radiolytic hydrogen release can be explained if one assumes that the process of hydrogen release proceeds through three following phases: The first phase. Swelling of methane without releasing hydrogen. When solid methane is cooled to lower temperature after solidification, a lot of unpercolated pores and grains inside bulky methane are formed because of strong adhesion of methane to the metallic walls (the aluminum sponge enhances this effect). During heating, the pressure inside hydrogen gas bubbles goes up, until it becomes large enough to crack methane grains. Hydrogen is released partly (probably, 15–20%) out of the grains into the pores, building up an internal pressure that in turn causes expansion of the sample (the sponge, in our case). The process of methane grain disintegration and release of the compressed gas is favoured by fast heat release due to the recombination of radicals, which are partly (around 1%) still ‘‘alive” at T > 35 K (see Figs. 4 and 9). The second phase. When temperature of methane becomes large enough (about 64 K after long irradiation) to cause plastic deformation of methane under pressure of the gas inside the pores. The hydrogen gas escapes fast now because the cavities are percolated (the peak in Fig. 8). At this phase, swelling of the methane reaches a maximum because the remaining gas has free paths for escape and no longer can press on the methane. Methane at this stage is viscous-plastic, that is why its deformation decreases slowly after hydrogen gas bubbles escaped. After the second phase, most of the hydrogen is still kept dissolved in the methane matrix as isolated molecules. The temperature and pressure of the gas at the first and second phases of the process of hydrogen release are determined by the stress–stain diagram of solid methane, Fig. 10 [9]. The diagram for irradiated methane is unknown but analysis showed that if one uses that of fresh methane, the experimental facts more or less agree with this theory. At a low dose (6 h of irradiation, 4.9 MGy), the temperature of the start of hydrogen release (81 K) corresponds to the strength yield for uniaxial tension 1 MPa and experiment gives 1.3 MPa pressure on the membranes. At high dose (30 h of irradiation, 27 MGy), the temperature of the start of
Fig. 7. Rate of hydrogen release (relative to the amount of hydrogen unreleased).
Fig. 8. Same as Fig. 7, for broader temperature range.
Fig. 9. Internal heat release (curve 3, Watts) at the first stage of heating of irradiated methane. Curve 1 – hydrogen release, %; curve 2 – pressure on the membranes, 100 kPa.
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law (Fig. 7). Then, the molecules come out fast along percolated passes. A temperature increase of 2.1 K gives a twofold rate of hydrogen release.
Acknowledgements The authors are very thankful to Yu. Borzunov, I. Prokhorov, V. Ermilov, L. Edunov, N. Antsupov (JINR, LNPh) and to the staff of the Mechanics and Technology Dep. of the FLNP for their technical assistance. This work was performed with auspice of the CCLRC, Rutherford Appleton Laboratory under the Contract # 4144603. Results and conclusions of the work were used at the methane moderator development for the TS2 Target Station.
References Fig. 10. Uniaxial tensile stress–strain curves (Y-axis – r 107 Pa; X-axis – , %) at the deformation rate d/dt = 3.3 104 s1 on a polycrystal CH4: (1) T = 77 K, (2) T = 64 K, (3) T = 20 K, (4) T = 4.2 K.
hydrogen release (64–67 K) corresponds to the strength yield 1.6– 1.7 MPa and about 2.0 MPa was observed in experiment. The presence of a maximum of peak displacement of the membranes with absorbed dose is evident because decomposition of methane at doses 20–35 MGy reaches 20–30%. At such grade of decomposition, the remaining substance has mechanical properties totally different from that of methane and it seems to be more pliable. As a result, hydrogen escapes at a lower pressure. From experimental phenomena, it is possible to make an estimation of the volume of the pores at the temperature of irradiation (low temperature) – about 9–10%. During heating, this volume decreases down to 5.5–6.5% at 60–65 K and then increases up to 11– 12% when the methane undergoes maximal swelling. This value is close to elongation of methane at ultimate strength on the stress– stain diagram of fresh methane, which confirms the hypothesis of cracked methane. The third phase. When all hydrogen accumulated in gas bubbles is released, the remaining hydrogen molecules diffuse through fragments of the grains of the methane. Following the Arrhenius
[1] E.P. Shabalin, E.N. Kulagin, S.A. Kulikov, V.V. Melikhov, Nucl. Instr. and Meth. B 215 (2004) 181. [2] S. Ansell et al., Improved performance from the TS2 decoupled moderator, in: Proceedings of the 17th Meeting of International Collaboration on Advanced Neutron Sources, ICANS-XVII, V. II, p. 660. [3] A.A. Belyakov, V.V. Melikhov, Yu.N. Pepelyoshev, E.P. Shabalin, J. Neutron Res. 3 (1996) 209. [4] A.A.Belyakov, V.G.Ermilov, V.L.Lomidze, V.V.Melikhov, E.P.Shabalin, The first experience of a cold moderator and of solid methane irradiation at the IBR-2 pulsed reactor, in: The Proceedings of the 12th Meeting of International Collaboration on Advanced Neutron Sources, ICANS-XII, Abingdon, Oxon, England, May 25–28, 1993, RAL Report 94–025, V. II, p. 144. [5] E.P. Shabalin, A.A. Belyakov, V.G. Ermilov, V.V. Melikhov, Solid methane cold moderator at the IBR-2 reactor: test operation at 2 MW, in: Advanced Pulsed Neutron Sources, Proceedings of PANS-II, the Second International Meeting on Pulsed Advanced Neutron Sources, Dubna, 14–17 June 1994, JINR, Dubna, D395-169, 1995. [6] J. Carpenter, Cold moderator for pulsed neutron sources, in: Proceedings of International Workshop on Cold Neutron Sources. LANCE, Los-Alamos, US, 1990, LA-12146C. p. 131. [7] A.A. Belyakov, I.T. Tretyakov, E.P. Shabalin, First experience with the new solid methane moderator at the IBR-2 reactor, in: The Proceedings of the 15th Meeting of International Collaboration on Advanced Neutron Sources ICANS-XV, Tsukuba, Japan, November 6–9, 2000. [8] E. Shabalin, S. Kulikov, V. Melikhov, E. Kulagin, Some radiation effects in cold moderator materials, experimental study, in: Proceedings of 16th Meeting of the International Collaboration on Advanced Neutron Sources, May 12–15, 2003, Düsseldorf-Neuss, Germany. Vol. II, p. 911. [9] D.N. Bolshutkin, A.V. Leonteva, A.I. Prokhvatilov, Yu. E. Stetsenko, Fiz Kondens Sostoyan, Institute for Low Temperature Physics and Engineering, Kharkov, USSR, 1968, 2, 136.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Experimental study of swelling of irradiated solid methane during annealing E. Shabalin, A. Fedorov, E. Kulagin, S. Kulikov, V. Melikhov, D. Shabalin * Joint Institute for Nuclear Research, Joliot-Curie Street 6, 141980 Dubna, JINR, Moscow Region, Russia
a r t i c l e
i n f o
Article history: Received 22 April 2008 Received in revised form 2 September 2008 Available online 26 September 2008 PACS: 28.20.v 28.20.Gd 61.80.Hg Keywords: Solid methane Radiolytic hydrogen Pressure Swelling
a b s t r a c t Solid methane is still widely in use at pulsed neutron sources due to its excellent neutronic performance (IPNS, KENS, Second Target Station at ISIS), notwithstanding poor radiation properties. One of the specific problems is radiolytic hydrogen gas pressure on the walls of a methane chamber during annealing of methane. In this paper results of an experimental study of this phenomenon under fast neutron irradiation with the help of a specially made low temperature irradiation rig at the IBR-2 pulsed reactor are presented. The peak pressure on the wall of the experimental capsule during heating of a sample irradiated at 23–35 K appears to have a maximum of 2.7 MPa at an absorbed dose 20 MGy and then falls down with higher doses. The pressure always reached its peak value at the temperature range 72–79 K. Generally, three phases of methane swelling during heating can be distinguished, each characterized by a proper rate and intensity. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Solid methane, notwithstanding its poor radiation properties [1], is still widely in use at pulsed neutron sources as a cold neutron moderator [2–6]. One of the specific problems is radiolytic hydrogen gas pressure on the walls of a methane chamber during annealing of methane. Hydrogen, both in atomic and molecular form, is the main product of methane radiolysis. Four decomposed methane molecules give rise to about three molecules of hydrogen. The rate of hydrogen production (Rhyd, mol/g of CH4) is determined by the relation [4,5]: 4
RHyd ¼ 3 10
!! _ D_ Dt _ D tþ 1 expð Þ D0 D0
where D_ is the absorbed dose rate, MGy/h (1 W/g = 3.6 MGy/h), D0 is a constant 8 MGy and t is time of irradiation in hours. For low doses, RHyd is equal to 6 107 mol/J. At an irradiation temperature of 20–30 K, radiolytic hydrogen is in a condensed state, not building any measurable internal pressure and swelling of methane [4]. However, warming the condensed state after irradiation builds up an internal pressure of gaseous hydrogen inside the solid methane matrix, resulting in swelling of methane and a pressure load onto the methane
* Corresponding author. Tel./fax: +749 62165253. E-mail addresses: [email protected], [email protected] (D. Shabalin). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.09.022
chamber walls. Many chambers of solid methane moderators have been destroyed due to this effect [5–7]. For 20 years of solid methane moderator use, no attempt has been made to quantitatively study the characteristics of the phenomenon. This paper gives results of the only experimental study of pressure loads on the walls of a chamber with solid methane during warming after fast neutron irradiation.
2. Apparatus and methods This study was performed at the specially modified low temperature irradiation facility URAM-2 at the IBR-2 pulsed reactor [1,8]. The new irradiation rig and study programme were called the URAM-3M. The irradiation facility consists of four principal parts: – irradiation rig head with supporting parts (Figs. 1 and 2), – cooling system, based on using liquid helium as a coolant material, – vacuum-gas technology system, – control equipment. This facility enables the irradiation of solid methane condensed inside aluminum foam at 25–35 K for 35–40 h at dose rates of either 0.23 W/g, or 0.5 W/g. Solid methane samples were prepared by condensing methane gas into the chamber as liquid and freezing it afterwards.
E. Shabalin et al. / Nuclear Instruments and Methods in Physics Research B 266 (2008) 5126–5131
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Fig. 1. Front end of the irradiation rig, elevation through the central plane of the sample. 1 – Cylindrical shell of the chamber; 6 – aluminium foam filled with solid methane; 7 – coils of copper tube; 8 – steel tube for filling and evacuation gases; 9 – pockets for temperature sensors; 10 – steel string suspender of the sample chamber; 11 – helium pipes inside an evacuated 3 m-long tube.
Both the irradiation chamber and aluminum foam inside it, have a cylindrical shape. All walls of the chamber are made of stainless steel 12X18H10T (Russian standard), which has high resistance to radiation and good mechanical properties at low temperature. Two faces of the chamber represent plates of 1.5 mm thick steel and serve as membranes to follow methane expansion during hydrogen release (see the characteristics in Fig. 3). Displacement of the membranes was measured with two gauges of induction type (sensitivity – 0.63 V/mm). Inside an evacuated jacket of polished stainless steel, the chamber is kept strongly space oriented with a steel string under tension. Nevertheless, position of the irradiation chamber with respect to the displacement gauges depends on temperature. This temperature effect was compensated by estimation of displacement of the membranes as an average of indications of the two gauges. Because the P–X diagram (pressure on the membranes versus its displacement) at the first load after heating up to room temperature appeared to be nonlinear, ‘‘training” of membranes at 70–75 K was done each time before a new irradiation run, that is, triple loading up to 2.5 MPa. Such a procedure guaranteed a linear characteristic of the P–X diagram. To prevent overfilling of the aluminum foam with methane, a capillary tube made of copper was placed inside the methane feeding tube and was welded into the steel walls of the vacuum chamber. Due to high thermal conductivity of copper, the temperature of the capillary tube was near 300 K everywhere, preventing freezing of methane at the neck of the chamber. The temperature non-uniformity in the foam filled with solid methane was calculated with the 3D-heat transfer code COSMOS-M and appeared to be near 3 K. The temperature of the chamber walls was measured by thermocouples welded between
copper tube coils. Dose rates were measured in advance with a purpose-built test bench. Moreover, the dose rate in methane was checked every time by measuring the amount of released hydrogen gas as the hydrogen output per unit of absorbed dose is well known. Gas pressure monitoring was arranged with two gauges: 0–4 104 Pa and 0–2.5 MPa, both of 0.1% accuracy at full scale. 2.1. Principal procedure of the experiment 1. Before condensation of methane, the chamber is cooled down to 70–80 K and filled with helium under pressure varying from 0 to 2.5 MPa for calibration of the displacement gauges and training of the membranes. 2. The methane cavity is evacuated, heated up to 105–115 K and connected to a bottle of 5.73 L capacity charged with methane gas of 99.99% purity to a pressure between 0.19 MPa and 2.3 MPa (two irradiations were performed with methane doped with 0.65 mol% BF3 gas enriched with B10 up to 83%, which increased the dose rate up to 0.5 W/g). After condensation of 0.14–0.15 MPa, the gate between the bottle and the methane filling tube is shut. Until condensation is completed, the temperature of the methane is kept above 90.6 K to prevent solidification of methane before the sample chamber is filled with liquid methane. Then the methane is cooled down to 25–30 K. 3. Continuous irradiation of the solid methane sample for some predetermined time at the maximal temperature of the methane from 26 K up to 35 K proceeds. 4. The temperature of the methane sample is increased up to 90 K at a defined rate (from 0.3 to 5 K/min) by switching a helium heater on and depressing the helium flow rate. In the process
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Fig. 2. Evacuated chamber with a sample, elevation along diameter. 1 – Cylindrical shell of the chamber; 2 – coaxial cables of displacement gauges (3); 4 – steel membrane; 5 – steel shell of a sample chamber; 6 – the sample (aluminium sponge filled with solid methane); 7 – coil of copper tube (heat exchanger); 8 – steel tube for filling and evacuation of methane and hydrogen gases.
stages 3 and 4, the temperature of the methane chamber and outgoing gas pressure and displacement of steel membranes (faces of the chamber) is recorded. 5. After saturation of the gas pressure in the bottle (that means that all hydrogen released) the temperature increases for evaporation of methane and cavity of the chamber is pumped out at 355 K to clean it of high boiling products of methane radiolysis. 6. The cavity of the chamber is filled with helium under pressure close to that observed in the experiment, for calibration of the displacement gauges. 7. Helium is pumped out, the temperature goes up, methane is evaporated, the chamber cavity is evacuated and the next experiment begins.
3. Experimental results The main characteristics of the experimental runs are summarized in Table 1. The lower limit of the value in column H in Table 1 reflects more or less exactly the beginning of hydrogen release;
the upper limit is the temperature corresponding to the peak of the pressure (column F). It should be noted that experiments #9 and #10 were made with methane doped with boron; experiment #9 was assigned to check the boron doping effect on the amount of hydrogen. The typical temporal behavior of parameters during heating of irradiated samples is given in Figs. 4 and 5. 3.1. Effect of heating rate In Fig. 6, the maximal pressure on the membranes versus the amount of accumulated hydrogen is given. Six out of nine experimental points lie strictly on the smooth curve which can be approximated as a parabola. All of these experiments are characterized by similar conditions (the chamber is not overfilled and rate of heating is in the range 0.5–15 K/min), except for different temperatures of irradiation. Three other points differ either in filling of the chamber (#3), or in the method of heating (in experiment #5 methane was heated by bleeding of a portion of warm methane gas), or by very low heating rate (#8). The last circumstance is not significant in practice – to gain about 20% in pressure
E. Shabalin et al. / Nuclear Instruments and Methods in Physics Research B 266 (2008) 5126–5131
Fig. 3. Response of the displacement gauge (Y-axis, mV) to the gas pressure load on the membrane (X-axis, 105 Pa) at 70–80 K. Curve 1 is for the first load after heating up to room temperature, curve 2 is for the second load and curve 3 – for the third load and the following.
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Fig. 5. Record of parameters during heating, experiment #7.
Table 1 Experimental data (titles of the columns see in the legend) No of run
A
B
C
D
E
F
G
H
I
1 2 3 4 5 6
430 406 450 400 400 406
6.0 5.7 9.16 15.5 6.7 24.2
25–27 25–30 30–35 25–26 27–30 30–35
18.8 17.1 28.3a 37.3 19.3 51.4
1.37 1.3 2.15 2.1 2.05 2.4
24 24 10 21 20 24
81–87 83–85 86–88 67–72 >85 66–74
3% 10% 10% 3% 10% 6%
7 8 9 10
400 391 383 367
29.5 9.8 7.5 18
30–39 30–31 85 23
60.0 23.4 39 80
2.5–3 1.5–2 4–5 0.5 –b 0.6– 1.4 1–2 0.3 – 10–15
20 21 – 16
64–72 69–79 – 68–74
11% 0.7% – 28%
2.45–2.5 1.3 – 1.97
A – charge of methane in mmol; B – irradiation time in hours; one hour of irradiation corresponds to absorbed dose of 0.81 MGy (for experiments ##1–8); C – temperature of irradiation, K; D – amount of H2 accumulated, mmol; E – heating rate, K/min; F – peak pressure on the membrane, Pmax, MPa; G – fraction of H2 (%) released before pressure reached Pmax ; H – temperature range of hydrogen release, K; I – maximal rate of hydrogen release, 1/min. a Calculated value. b Heating by filling the chamber with the warm methane gas.
Fig. 6. Peak value of pressure during heating of irradiated methane (Y-axis, 100 kPa) versus amount of hydrogen accumulated (mol/mol CH4). Black squares – experiments with the normal amount of methane in the chamber and with medium or high rate of heating; hollow squares – experiments with different parameters of heating.
on the membrane, one hour extra time for heating is needed. The rate of heating is not a critical factor for pressure on the membranes. 3.2. Effect of temperature of irradiation As is clear from Fig. 6 and Table 1, there is no visible evidence of irradiation temperature effect on maximal displacement of membrane. 3.3. Maximal value of pressure
Fig. 4. Record of parameters during heating, experiment #4.
For a dose >20 MGy the peak pressure on the membrane during heating reaches maximum of about 2.5 MPa with amount of accumulated hydrogen (the displacement of the membrane is 0.4 mm accordingly) and then falls, not depending on the rate of heating. Since the diameter of the membrane is greater than the diameter of the aluminum sponge, the maximal pressure was really 10% more (as shown by computer simulation), that is, about 2.7 MPa.
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3.4. Effect of burping In experiments #1, #4 and #10 induced burps were indicated. They did not affect markedly value of peak pressure on membranes. 4. Discussion and interpretation of the results Following regularities in the behavior of pressure on the membrane during heating of methane samples, it is noteworthy that: The pressure on the membrane always reaches peak value at a temperature in the range 72–79 K (after long irradiation) and, what is most intriguing, just up to this moment about 20% of the stored hydrogen is released in all the experiments. The rate of hydrogen release depends on temperature not monotonically: in the range of 64–72 K (for high doses of radiation) it sharply intensifies and returns to exponential (Arrhenius type) growth afterwards (see Figs. 4, 5, 7 and 8). Before hydrogen starts to escape at T > 64 K, the membranes exhibit displacement. The swelling of the methane, which is about half of the peak value, follows a jump in temperature from
40 K to 60 K. The balance of the heat in the chamber showed (Fig. 9) that at this moment the additional energy, released in the methane sample, is about 60 J/g. These regularities and other characteristics of the process of radiolytic hydrogen release can be explained if one assumes that the process of hydrogen release proceeds through three following phases: The first phase. Swelling of methane without releasing hydrogen. When solid methane is cooled to lower temperature after solidification, a lot of unpercolated pores and grains inside bulky methane are formed because of strong adhesion of methane to the metallic walls (the aluminum sponge enhances this effect). During heating, the pressure inside hydrogen gas bubbles goes up, until it becomes large enough to crack methane grains. Hydrogen is released partly (probably, 15–20%) out of the grains into the pores, building up an internal pressure that in turn causes expansion of the sample (the sponge, in our case). The process of methane grain disintegration and release of the compressed gas is favoured by fast heat release due to the recombination of radicals, which are partly (around 1%) still ‘‘alive” at T > 35 K (see Figs. 4 and 9). The second phase. When temperature of methane becomes large enough (about 64 K after long irradiation) to cause plastic deformation of methane under pressure of the gas inside the pores. The hydrogen gas escapes fast now because the cavities are percolated (the peak in Fig. 8). At this phase, swelling of the methane reaches a maximum because the remaining gas has free paths for escape and no longer can press on the methane. Methane at this stage is viscous-plastic, that is why its deformation decreases slowly after hydrogen gas bubbles escaped. After the second phase, most of the hydrogen is still kept dissolved in the methane matrix as isolated molecules. The temperature and pressure of the gas at the first and second phases of the process of hydrogen release are determined by the stress–stain diagram of solid methane, Fig. 10 [9]. The diagram for irradiated methane is unknown but analysis showed that if one uses that of fresh methane, the experimental facts more or less agree with this theory. At a low dose (6 h of irradiation, 4.9 MGy), the temperature of the start of hydrogen release (81 K) corresponds to the strength yield for uniaxial tension 1 MPa and experiment gives 1.3 MPa pressure on the membranes. At high dose (30 h of irradiation, 27 MGy), the temperature of the start of
Fig. 7. Rate of hydrogen release (relative to the amount of hydrogen unreleased).
Fig. 8. Same as Fig. 7, for broader temperature range.
Fig. 9. Internal heat release (curve 3, Watts) at the first stage of heating of irradiated methane. Curve 1 – hydrogen release, %; curve 2 – pressure on the membranes, 100 kPa.
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law (Fig. 7). Then, the molecules come out fast along percolated passes. A temperature increase of 2.1 K gives a twofold rate of hydrogen release.
Acknowledgements The authors are very thankful to Yu. Borzunov, I. Prokhorov, V. Ermilov, L. Edunov, N. Antsupov (JINR, LNPh) and to the staff of the Mechanics and Technology Dep. of the FLNP for their technical assistance. This work was performed with auspice of the CCLRC, Rutherford Appleton Laboratory under the Contract # 4144603. Results and conclusions of the work were used at the methane moderator development for the TS2 Target Station.
References Fig. 10. Uniaxial tensile stress–strain curves (Y-axis – r 107 Pa; X-axis – , %) at the deformation rate d/dt = 3.3 104 s1 on a polycrystal CH4: (1) T = 77 K, (2) T = 64 K, (3) T = 20 K, (4) T = 4.2 K.
hydrogen release (64–67 K) corresponds to the strength yield 1.6– 1.7 MPa and about 2.0 MPa was observed in experiment. The presence of a maximum of peak displacement of the membranes with absorbed dose is evident because decomposition of methane at doses 20–35 MGy reaches 20–30%. At such grade of decomposition, the remaining substance has mechanical properties totally different from that of methane and it seems to be more pliable. As a result, hydrogen escapes at a lower pressure. From experimental phenomena, it is possible to make an estimation of the volume of the pores at the temperature of irradiation (low temperature) – about 9–10%. During heating, this volume decreases down to 5.5–6.5% at 60–65 K and then increases up to 11– 12% when the methane undergoes maximal swelling. This value is close to elongation of methane at ultimate strength on the stress– stain diagram of fresh methane, which confirms the hypothesis of cracked methane. The third phase. When all hydrogen accumulated in gas bubbles is released, the remaining hydrogen molecules diffuse through fragments of the grains of the methane. Following the Arrhenius
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