Investigation Obtaining of Radionuclide Sulfur-35 without а Carrier from Chlorine Containing Compounds in a Reactor WWR-SM

Technical characteristics of the WWR-SM reactor

The WWR-SM (pressurized water reactor of serial modernization) reactor is one of the safest research reactors in the former Soviet Union. The WWR-SM reactor is a heterogeneous reactor in which ordinary distilled water serves as a neutron moderator, coolant, and biological shield [29]. The reconstruction of the WWR-SM reactor in 1978 led to an increase in its capacity from 2 to 10 MW, and in 2002, the WWR-SM reactor switched to using highly enriched uranium-235 (36%) fuel. IRT-3M allowed for a significant expansion of materials science research on it [30]. Table 1 shows the technical characteristics of the WWR-SM reactor.

Figure 1 shows a structural diagram of the WWR-SM reactor.

Figure 1: Structural diagram of WWR-SM reactor: 1 – active core; 2 - thermal graphite column; 3 – horizontal channel (total 9 channels); 4 – reactor bottom; 5 - subreactor space in the primary circuit pump room; 6 - first circuit water calming grid; 7 - platform for the reactor control and protection system equipment; 8 - supra-reactor space; 9 – embarrassment; 10 - primary circuit inlet pipe; 11 - primary circuit outlet pipe; 12 - one of the vertical channels with a guide pipe; 13 - electromagnetic rod for loading irradiated samples. Figure 2 shows a cartogram of the WWR-SM reactor core.

Figure 2 shows a cartogram of the active core of the WWR-SM reactor (example).

Figure 2: Cartogram of active core of the WWR-SM reactor: 1 - 6-tube IRT-4M; 2 - 8-tube IRT-4M; 3 - 6-tube IRT-4M with emergency protection rod; 4 - working element of the automatic control rod in a Be block; 5 - «dry channel» № 9 for samples irradiation; 6 - Be reflector block with plug (∅ = 44 mm); 7 - segmented Be reflector block with channel; 8 - Be reflector block with channel (∅ = 60 mm); 9 –irradiation horizontal «dry channel» №3; 10 - Be reflector block with irradiation channel (∅ = 44 mm); 11 - lateral Be displacer; 12 - coolant (first loop water).

Irradiation process of chlorine-containing targets

The radionuclide ³⁵S is obtained in a nuclear reactor during irradiation of organic and inorganic chlorine-containing compounds by thermal neutrons of a nuclear reactor. There is no reliable information in the literature on the activation cross-section and threshold energy of neutrons. The activation cross section of the above nuclear reaction ranges from 0.35 [31] barn to 0.575 [32] barn. In experiments, we used the activation cross-section average value 0,6375 barn.

The decay of the S-35 atom is accompanied by the formation of a Cl-35 atom, an electron, and an antineutrino in the nuclear reaction:

${}_{16}{}^{35}S\to {}_{17}{}^{35}CI+e^{-}+{\overline{v}}_e\text{ (1)}$

The process of irradiating chlorine-containing targets consists of the following stages:

Stage 1: Preparation of the target for irradiation. In the experiments, the neutron activation of the following chlorine-containing compounds was investigated: KCl - potassium chloride 99,9% (special purity); NaCl - sodium chloride 99,9% (chemically pure); MgCl₂·6H2O - magnesium chloride 6-hydrate (clean for analysis); СС14 - carbon tetrachloride (clean for analysis). All chlorine-containing compounds met the requirements of the state standard. Potassium chloride (KCl), sodium chloride (NaCl), magnesium chloride (MgCl₂), and carbon tetrachloride (CCl₄), each weighing 1.0 g, were placed in quartz ampoules with an inner diameter of Ø = 4 mm and a length of l = 50 mm, and then ware sealed at a temperature of 1600–1700 °C.

Stage 2: Preparation of targets of Co-60 monitors. Several batches of Co-59 tracking monitors were manufactured: Co-59 monitors (cobalt alloy with 0.1% Co-59) in the form of metal discs (Ø = 3 mm, h = 0.2 mm, m = 3.0 mg), each of which was individually wrapped in aluminum foil, and then each monitor together with targets of potassium chloride, sodium chloride, magnesium chloride, and carbon tetrachloride (each weighing 1.0 g) was placed in quartz ampoules and sealed.

Stage 3: Loading the aluminum block container with the target into the reactor’s active core and process irradiation them by neutrons. Sealed quartz ampoule with chlorine-containing target, standard. Potassium chloride (KCl), sodium chloride (NaCl), magnesium chloride (MgCl₂), and carbon tetrachloride (CCl₄), each weighing 1.0 g, were placed in quartz ampoules with an inner diameter of Ø = 4 mm and a length of l = 50 mm, and then ware sealed at a temperature of 1600–1700 °C.

Stage 2: Preparation of targets of Co-60 monitors. Several batches of Co-59 tracking monitors were manufactured: Co-59 monitors (cobalt alloy with 0.1% Co-59) in the form of metal discs (Ø = 3 mm, h = 0.2 mm, m = 3.0 mg), each of which was individually wrapped in aluminum foil, and then each monitor together with targets of potassium chloride, sodium chloride, magnesium chloride, and carbon tetrachloride (each weighing 1.0 g) was placed in quartz ampoules and sealed.

Stage 3: Loading the aluminum block container with the target into the reactor’s active core and process irradiation them by neutrons. Sealed quartz ampoule with chlorine-containing target, along with a cobalt monitor, was placed in a special aluminum block container (dimensions: length – L = 750 mm, diameter – Ø = 20 mm). Next, the block container was placed into the inner hollow space of the IRT-4M fuel assemblies in the reactor’s active core using an aluminum lifting boom 6 m long. Irradiation of the chlorine-containing target along with the Co-59 monitor was made under the following conditions: thermal neutron flux density ≥1·10¹⁴ n/cm²·s, irradiation time 2000 hours, nominal reactor power 10 MW.

Stage 4: Unloading the irradiated target from the reactor core after irradiation. After the target has been irradiated, it is unloaded from the reactor’s active core to ‘hot cells,’ and then the lid of the block container is opened. The irradiated target and monitors were removed from the quartz ampoules by cutting the quartz ampoules.

Stage 5: Measurement of sulfur-35 activity. After the radiochemical processing of the target (dissolution in water) and obtaining an aliquot, the aliquots were measured on a gamma-beta spectrometer of “Progress BG (II)” BDEB 3-2U with the software “Progress 5”. Co-59 monitors on the gamma-beta spectrometer were measured. The confidence limits of the total error of the measurement result of external beta radiation at a probability of 0.95 were within ± 20%.

The formula for the definition of the activity of radionuclide S-35 is:

$\text{Q} =\frac{0.6 ·F_t \sigma { }_t ·\theta · m · (1-e^{- \frac{0.693 · t_0}{T\frac{1}{2}}})}{A · 3,7·{10}^7}\text{ (2)}$

where: Q - activity of S-35, mCi; F_t - thermal neutrons flux density, neutrons/cm²sec;

t0 - irradiation time of sample, sec; T_1/2 - half-life S-35, sec; q - enrichment of S-35, %;

m - weight of target S-35, g; st - activation cross section of reaction 35Cl (n, p) ³⁵S, barn.

Stage 6: Measurement of Co-60 activity and determination of thermal neutron flux density. Thermal neutron flux density determination by the following formula was made:

${\text{F}}_{\text{n}} =\frac{\text{A }·{\text{ e}}^{\lambda ·{\text{ t}}_1}}{\text{N }·\sigma ·(1-{\text{e}}^{-\lambda ·{\text{t}}_0})}\text{ (3)}$

where: F_n − thermal neutrons flux density, neutrons/cm²·sec; A − measured activity of the monitor, imp/sec; N – number of kernels Со-59 (4.4⋅10¹⁶ kernels/cm²); t₁ − irradiation time, sec; σ − cross section of radionuclide ⁶⁰Co, barn; t₀ − irradiation time of monitor,sec; σ − activation cross section of reaction ⁵⁹Co (n, γ) ⁶⁰Co, barn; λ − decay constant (⁶⁰Cо); T_½ − half-life (⁶⁰Со), sec; e^λt = 0.693·t / T_1/2

The total effective activation cross-section (δ_eff) is formed by multiplying the thermal neutron activation cross-section by the coefficient of K_Cd. Therefore, the formula for calculating the induced activity of the radionuclide ³⁵S takes the following form:

$\text{Q}=\frac{0.6 ·F_t {\delta }_t·K_{Cd}·\theta ·m·{\theta }_1·(1-e^{- \frac{0.693·t_0}{T\frac{1}{2}}})}{A·3,7·{10}^7}\text{ (4)}$

where: Q - activity of S-35, mCi; F_t - thermal neutrons flux density, neutrons/cm²sec;

t₀ - irradiation time of sample, sec; T_1/2 - half-life S-35, sec; θ - enrichment of Cl-35, %;

θ₁- mass fraction of chlorine in a chlorine-containing compound; m - weight of target weight of chlorine-containing compound, g; δ_t - activation cross section of reaction ³⁵Cl(n, p)³⁵S, barn.

The final, most complete formula for determining the thermal neutron flux density is as follows:

${\text{F}}_{\text{n}}=\frac{\text{A}·{\text{e}}^{\lambda ·{\text{t}}_{\text{exp}}}·\text{S}}{0,6·\sigma ·{\text{R}}_{\gamma }{}_{}·\theta ·\epsilon ·\text{P}·\text{m}·(1-{\text{e}}^{-\lambda ·{\text{t}}_0})·{\tau }_{\text{m}}}\text{ - }\frac{\text{A}·{\text{e}}^{\lambda ·{\text{t}}_{\text{expСd}}}·{\text{S}}_{\text{Cd}}}{0,6·\sigma ·{\text{R}}_{\gamma }·\theta ·\epsilon ·\text{P}·{\text{m}}_{\text{Cd}}·(1-{\text{e}}^{-\lambda ·{\text{t}}_{0·\text{Cd}}})·{\tau }_{\text{mСd}}}\text{ (5)}$

where: F_n − thermal neutrons flux density, neutrons/cm²·sec; S - area of ​​the detector photopeak without Cd screen, imp/sec; Sc_d − area of ​​detector photo peak in Cd screen, imp/sec; A - atomic weight of Co-59; λ - decay constant of the isotope Co-60, sec^-1; σ - Co-60 activation cross section, barn (0,6375 x 1·10^-24 cm²); R_γ - gamma ray output; θ - isotope content in the detector; ε- analyzer sensor efficiency (3,53·10^-3); P- reactor power, MW; m – detector mass, g (m = 3·10^-5g); m_Cd - mass of the detector in the Cd screen, g; t0–irradiation time of detector without Cd screen, sec; t_{0 Cd} - irradiation time of detector with Cd screen, sec; t_exp – exposure time of detector without Cd screen, sec; t_expCd - exposure time of detector with Cd screen, sec; τ_m– measuring time of detector without Cd screen, sec; τ_{m Cd} - measurement time of detector with Cd-screen, sec.

Determination of the cadmium ratio value for isotope ³⁵S

When calculating the induced activity of sulfur-35 using the formula (4), there is no contribution of fast neutrons to the activation above the initial ingredient. In order to pay attention to the thermal neutrons on the activation of the irradiated target, we studied the cadmium ratio (R_Cd). The measurement of the cadmium ratio R_Cd is found from the results of two measurements. First of all, a target without a cadmium sheath is placed in the neutron field. It is activated by both thermal and resonant neutrons. After determining the induced activity A1 and holding for a period during which almost all active nuclei will decay, the target is wrapped in a cadmium sheath (screen) and placed back in the same place in the neutron field.

Cadmium has unusually high thermal neutron capture cross sections (7200 barns), and hence the radioactivity induced in cadmium-wrapped targets is approximately proportional to the intermediate neutron flux. The cadmium ratio R_Cd is equal to the ratio of the effect of activation by thermal neutrons to the effect of activation by resonant neutrons: To determine the cadmium ratio, 10 weighed portions of potassium chloride samples were irradiated with and without a cadmium screen under the same conditions: thermal neutron flux F_n = 1·10¹⁴ neutrons/cm²sec, and irradiation time is 1.0 hour. Knowing R_Cd, it is easy to take into account the contribution of neutrons when irradiating a sample above the cadmium region of the neutron spectrum using the formula:

${\text{K}}_{\text{Cd}}=\frac{{\text{R}}_{\text{Cd}}}{\text{Rcd}-1}\text{ (6)}$

where: R_Cd - cadmium ratio; K_Cd - target activation coefficient in excess of the cadmium region of the neutron spectrum (E_n1.0 eV). Table 2 shows a cadmium ratio value for radionuclide ³⁵S.

As can be seen from Table 2, the cadmium ratio for radionuclide ³⁵S is 9.37, which indicates that the nuclear reaction ³⁵Cl(n,p)³⁵S proceeds exclusively by thermal neutrons with energy E_n = 0.0253 eV [33].

Irradiation of carbon tetrachloride target in active core of reactor WWR-SM

The carbon tetrachloride target was placed in a sealed quartz ampoule, which was placed in a special aluminum block container with dimensions of length l = 750 mm and diameter ∅ = 20 mm. This block container has two openings in its body: one opening on the bottom and two openings on the lid. These openings are designed to cool the target with running water from the reactor’s primary circuit. The water of the first circuit enters through the upper inlet openings into the block-container, cools the irradiated target (carbon tetrachloride), and then exits through the lower outlet opening. The aluminum block container was loaded into the hollow central space of a 6-tube fuel assembly of the IRT-4M type in the active WWR-SM reactor using a 6-meter rod with a gripper. The targets were irradiated under various time conditions from 100 to 2000 hours at a nominal reactor power of 10 MW and a thermal neutron flux density of 1 10¹⁴ n/cm² s.

Figure 3 shows a 6-tube fuel assembly IRT-4M (a) and a special aluminum block container with irradiation target and design for fastening the irradiation channel to the reactor platform (b).

Figure 3: 6-tube FA IRT-4M and special aluminum block container with irradiation targets: (a) - 6-tube FA of the IRT-4M type (1- head; 2- fuel element; 3 – tail); (b) - special container for irradiation of carbon tetrachloride target (1- head; 2- fuel element; 3 – tail; 4 - quartz ampulla; 5 - target of liquid CCl₄; 6 – special block container; 7 - coolant inlet hole; 8 – coolant outlet hole; 9 - container head; 10 - vertical guide pipe; 11 - reactor platform; 12 - bracket for fixing the guide pipe.

Determination of thermal neutron flux density by using Co-59 monitors

Usually, in low-power reactors (including reactor WWR-SM) for measurement of thermal neutron flux densities, monitors Co-59, Na-24, Mo-98, Dy-164, In-113, In-115, Mn-55, Cu-63, Cu-65, Au-197, or monocrystalline silicon [34,35] can be used. But, for determining the thermal neutron flux density, the most convenient tracking monitors are the Co-59 and Au-197.

Table 3 shows the main nuclear-physical characteristics of neutron activation detectors of thermal and resonance neutrons.

Table 4 shows nuclear-physical characteristics of the tracking monitor (detector) Co-59. In our experiments, to determine the thermal neutron flux density, we used Co-59 monitors, which are stable at high temperatures and have the most convenient nuclear-physical characteristics.

Optimization of the target irradiation process

In the vertical channels of the WWR-SM reactor for local monitoring of the thermal neutron flux density, a small-sized thermal neutron sensor (TND-2.0) was used. TND-2.0 is operating depending on 5 × 1012 neutron/cm²sec to 5 × 10¹⁴ neutron/cm²sec in gaseous media and in water [36-39]. The TND-2.0 is made of two main functional parts: a sensitive element made of a material (alloy) containing a fissile material (uranium), and a non-volatile energy converter with an electrical output signal, which is a differential thermocouple. Under the influence of neutron irradiation, the energy of nuclear fission reactions is converted into thermal energy in the sensitive element, then part of the thermal energy is converted into electrical energy using a differential thermocouple in thermal contact with the sensitive element. The sensitive part of the TND-2.0 (uranium-nickel alloy) is connected to the current device by an insulated thermocouple cable.

Method for measuring the thermal neutron flux density is following: Thermoneutron sensor TND-2.0 (primary device) was rigidly fixed at the end of a 6 m long aluminum rod, the rod was loaded into a vertical irradiation channel and, at a reactor power of 300 kW was irradiated, than arising in TND-2.0 the potential difference was measured using a Z-4833 potentiometer (secondary device). Measurement of the potential difference at each point was carried out during 15÷20 second, and the distance between each fixed measurement point was 5 cm, starting from the top of the vertical channel and ending at the bottom of the irradiation channel. Next, at the measured point with the maximum value of the potential difference, Co-59 track monitors in the form of metal disks

(∅ = 3 mm, h = 0.2 mm, m = 3.0 mg) made of an alloy of aluminum and cobalt (0.1%) without cadmium screens and with cadmium screens of two types (KE-1 with a thickness of 0.5 mm and KE-2 with a thickness of 1.0 mm) were attached to the end of the aluminum rod. Table 5 shows the potential differences in the vertical channels of the WWR-SM reactor measured by the TND method.

Table 5 shows that the potential difference is greatest at a distance of 35-45 cm from the top of the vertical irradiation channel (the middle section of the fuel assembly). Accordingly, the thermal neutron flux density is also greatest in this section of the vertical channel.

Co-59 monitors, also at a reactor power of 300 kW for 15-20 seconds, were irradiated at the location where the maximum potential difference occurs (40 cm from the top of the channel). Activity of the formed radionuclide Co-60 (in the monitors Со-59) was measured by SU-01P multichannel pulse analyzer with a DGDK-100 Ge-Li detector with Aspekt, Angamma program and also by DSA1000 Canberra HP multichannel gamma spectrometer with Ge detector with the standard Genie 2000 software package between each fixed measurement point was been 5 cm starting from the top of the vertical channel and ending at the bottom of the irradiation channel. The efficiency of the detector of the gamma-spectrometer is defined by means of the standard Со-60 from a complete set of etalon spectrometric gamma sources (Reference-standard Closed Radionuclide Sources).

In the next step, in the vertical channels of the reactor, in the gap with the maximum potential differences (35-45 cm from the top of the fuel element), Co-59 monitors were irradiated during 48-50 hours at reactor power 10 MV and their activity was measured, and then the thermal neutron flux densities were determined using formula (5). Table 6 shows thermal neutron flux density (maximum values) in the reactor vertical channels determined by ⁵⁹Co monitors.

As shown in Table 6 highest thermal neutron flux density (6.9·10¹³ n/cm²·s) is observed in the vertical channel №5-5.

Figure 4 shows the distribution of the thermal neutron flux density in the vertical irradiation channel of the WWR-SM reactor, determined by the TND method.

Figure 4: Distribution of thermal neutron flux density in the vertical irradiation channel of the WWRS-SM reactor: 1- vertical channel 4-2; 2 – vertical channel 3-7.

From Figure 4, it can be seen that the thermal neutron flux density has its highest values at a distance of 35-45 cm from the top point of the vertical irradiation channel, which corresponds to the middle of the 6-tube fuel assembly IRT-4M.

Thus, to optimize the process of neutron irradiation of chlorine-containing targets, it is recommended to place them for irradiation with thermal neutrons in the central part of the irradiation channel, at a distance of 35-45 cm down from the top of the channel or 35-45 cm from the top of the head of the IRT-4M fuel assembly.

Induced activity of S-35 and the practical output activity of S-35

The induced activity and practical yield of the radionuclide C-35 depend on the target irradiation time and the thermal neutron flux density. Table 7 shows the value of the induced activity and the practical output activity of S-35 depending on the irradiation time (thermal neutron flux density 1· 10¹⁴ n/cm²·sec).

As can be seen from Table 8, the highest specific activity of the sulfur-35 radionuclide occurs when carbon tetrachloride targets are irradiated for 2000 hours. The obtained experimental results show that the maximum specific activity of the S-35 radionuclide (3.422 Ci/g) was obtained by irradiating carbon tetrachloride targets with thermal neutrons, which is 2 times greater than by irradiating with thermal neutrons of potassium chloride targets. This proves that the chlorine-containing compound carbon tetrachloride is the most suitable target for obtaining high activity carrier-free sulfur-35.

Figure 5 shows the accumulation curve of sulfur-35 at thermal neutrons flax density of 0.8·10¹⁴ n/cm²sec.

Figure 5: Accumulation curve of S-35 at thermal neutron flux density of 0.8⋅10¹⁴ n/cm²s.

Thus, by irradiating the target in the center of the active zone, or closer to the beryllium reflector, or inside the fuel assembly in a special container in the middle of the vertical channel, and with an irradiation time of up to 2000 hours at a reactor power of 10 MV, it is possible to achieve a high specific activity of the target sulfur-35 radionuclide.

Purification of S-35 from impurity radionuclide P-32

At the WWR-SM reactor, the value of integral fast neutrons with energy Fn >10 MeV in the overall neutron spectrum is 5–6 orders of magnitude lower than that of thermal neutrons. Therefore, during the irradiation of chlorine-containing targets at reactor, fast neutrons make the smallest contribution to the formation of radionuclides. In addition, when targets are irradiated with fast neutrons, several radionuclides with very short half-lives are formed, which decay within 20÷25 hours after irradiation (³⁴Cl, ³⁸Cl, ³⁴P, ⁴²K ) (Table 8).

As shown in Table 8, the most significant radionuclide that can be a radionuclide impurity for S-35 is the radionuclide ³²P with a half-life of 14.5 days, which is formed as a result of the nuclear reaction ³⁵Cl(n, α)³²P.

If you look at the energy spectrum of beta radiation of the radionuclide ³⁵S, you can see that it has a maximum energy of E_β = 0.167 MeV (Figure 6).

Figure 6: Beta radiation energy spectrum of the radionuclide ³⁵S [38].

From the energy spectrum of beta radiation of the radionuclide ³⁵S (Figure 6), it is clear that it has a maximum energy of E_β = 0.167 MeV, which is more than 10 times less than the energy spectrum of the radionuclide P-32 with a maximum energy of 1.71 MeV (Figure 7).

Figure 7: Beta radiation energy spectrum of the radionuclide ³²P [39]: relative number of electrons per unit energy range on the x-axis and the energy of the ³²P beta particle on the y-axis.

By comparing these two spectra (Figures 6,7), it can be concluded that the presence of phosphorus-32 in the target radionuclide greatly distorts the result of measuring the activity of the sulfur-35 radionuclide, so it is important to purify the target sulfur-35 radionuclide from the admixture of phosphorus-32 radionuclide.

To separate the target sulfur-35 radionuclide from radionuclide impurities, various radiochemical methods are used, such as precipitation, ion exchange, extraction, and sublimation.

A study of the literature showed that the review presents methods for the isolation of ³⁵S sulfur without a carrier from alkaline chloride matrices [40]. Study [41] presents a method for the separation of S-35 from a KCl target exposed to neutrons in a reactor, and the separation of S-35 from P-32 was carried out on a strongly basic anion exchanger Dowex-1. In, a carrier-free method was developed for the separation of radionuclides ³²P and ³⁵S from a neutron-activated potassium chloride target.

The irradiated potassium chloride target was dissolved in 0.1 N hydrochloric acid solution and then adsorbed on a chromatographic column with aluminum oxide (m = 5.0 g). Then the chromatographic column was washed with distilled water to remove K+ and Cl– ions. The ³⁵S radionuclide was eluted with 1.0 N ammonia solution. The eluate contained (NH₄)2³⁵SO4. It is known that for the extraction of radionuclide ³⁵S in the form of sulfate, a method is used based on the absorption of [³⁵SO₄]^-2 - ions on chromatographic aluminum oxide. A solution of irradiated NaCl in 0.5 N HCl is passed through a tube with Al2O3, on which both the target radionuclide ³⁵S and the impurity of radionuclide P-32 are sorbed. Then the column is washed with water, and then sulfur-35 is washed out with a 1.0 N ammonia solution. The radionuclide ³²P impurity remained sorbed on the sorbent. The sulfur yield is about 90%, and the radiochemical purity of radionuclide ³⁵S reaches 99.9% [31]. The irradiated targets of sodium chloride and magnesium chloride were processed similarly.

The method for purifying the target product from radionuclide impurities is as follows: carbon tetrachloride is chemically inert, does not react with air, and is stable to light, but its boiling at 76.7 °C, because irradiated CCl₄ target was opened after cooling in dry ice—a solid form of carbon dioxide kept at a temperature below –78.5 °C to prevent the CCl₄ from evaporating. Next, the ampoule containing the liquid irradiated CCl₄ was transferred to a separatory funnel, an aqueous solution of 1.0 N ammonia was added, and the separatory funnel and its contents were vigorously shaken. Since carbon tetrachloride is immiscible with the aqueous phase, the ³⁵S radionuclide, in the form of (NH₄)₂³⁵SO₄, quantitatively transfers into the aqueous phase upon shaking the contents of the separatory funnel. The water contains the target radionuclide ³⁵S, while the radionuclide impurity ³²P remains in the CCl₄. Thus, the extraction process was performed manually in a protective hood: irradiated liquid CCl₄ was placed in a glass separatory funnel, then an aqueous solution of 1.0 N ammonium hydroxide was added, and the funnel and its contents were shaken several times. This ensured quantitative transfer of the ³⁵S radionuclide into the aqueous phase.

During radiochemical processing of the target by the extraction method, the coefficient of extraction of the radionuclide ³⁵S from the organic phase into the aqueous phase has a high value:

${\text{K}}_{\text{ext}.}=\frac{1000 mCi}{80 mCi} =\text{12}.\text{5 (7)}$

The recovery factor of S-35 from the organic phase to the aqueous phase is 12.5, and the target carrier-free S-35 radionuclide has a yield of > 92%.

When comparing the activity of different irradiated targets, the specific activity of the S-35 radionuclide obtained from a carbon tetrachloride target is approximately 2 times higher than the specific activity of S-35 obtained from a sodium chloride target. This is due to the fact that the mass fraction of sulfur in CCl₄ has the highest value - 92.5%, while the proportion of sulfur in NaCl is 60%, in MgCl₂ - 74%, and in KCl - 45%.

When irradiating a target made of KCl, NaCl, or MgCl₂ with thermal neutrons, it is necessary to first dissolve the target and then adsorb it into a chromatographic column to isolate radionuclide S-35 and purify it from radionuclide impurities, the use of which reduces the yield of the target product S-35 to 60% - 70%. In the case of irradiation of a carbon tetrachloride target with thermal neutrons, the method of separating the activated target product (S-35) from the irradiated target (CCl₄) is the simplest (extraction) and does not require complex radioactive operations. However, carbon tetrachloride target samples must be irradiated in vertical reactor channels with mandatory cooling by water of the reactor’s primary circuit, since at high temperatures carbon tetrachloride passes from the liquid phase to the gaseous phase (boiling point = 76.6 °C), which we observed during the irradiation of targets in the “dry” channel (No. 9 and No. 3) of the reactor (the ampoule with carbon tetrachloride became depressurized).

The highest yield of radionuclide ³⁵S activity per 1.0 g of chlorine-containing compound, ~3.312 Ci/g, is achieved by irradiating CCl₄ targets under the following conditions: thermal neutron flux density ≥0.8·10¹⁴ n/cm²sec, irradiation time 2000 h, nominal reactor power 10 MW, and irradiation of a quartz ampoule with the target in a vertical reactor channel with mandatory cooling of the target. The separation of radionuclide ³⁵S without a carrier from irradiated CCl₄ and from the possible radionuclide ³²P is carried out by a simple method of extraction with water.

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have led to the results of the work presented in this article.