Helium Management of the ESS Target and Monolith Systems

Helium Management of the ESS Target and Monolith Systems P. Nilsson, R. Linander, A. Lundgren, C. Kharoua, P. Sabbagh, F. Plewinski, F. Mezei and E. Pitcher European Spallation Source ESS AB, SE-22100 Lund, Sweden Abstract The European Spallation Source (ESS), being designed and built in Lund, Sweden, will be a 5 MW long-pulse neutron spallation research facility [1]. At the heart of the facility is the solid rotating tungsten target [2], which converts high-energy protons into neutrons for delivery to a suite of research instruments. The preferred option for the primary cooling system for the spallation target is helium. The target cooling helium system is connected to another helium system, for the flow in the monolith containment, which is surrounding the target wheel [3]. The connection goes via the rotary target feed-through and seals, where helium is injected to prevent any helium from leaking out from the primary cooling system. This is a standard buffer gas sealing solution. A small fraction of the injected helium goes into the primary cooling system, which is pressurized at 4 bar(a). A larger fraction goes into the surrounding monolith containment, which operates slightly below atmospheric pressure. In order to regulate the pressure of these two systems, helium thus has to be continuously removed from them at the same rate as it is injected. Also, in order to save helium and minimize radioactive emissions, the removed helium should be continuously reused in the helium seal injection. The target cooling helium circuit with its two-tank pressure control system, the monolith atmosphere circuit with its two-tank pressure control system, the connection of the systems via the rotary seals and a simplified purification system have all been modeled using the Modelica language. The results from these first simulations are presented in this paper as a function of time, in seconds. HELIUM FLOW BALANCE IN TARGET AND MONOLITH The target cooling helium circuit and the monolith atmosphere circuit are connected by the injection in the rotating seals, see Fig. 1. In the figure, there are three seals with flow indicated. The one at the top is the by-pass leakage within the target circuit. The one at the bottom, is the seal flow down through the target shaft bearings into the monolith. The one in the middle is the seal flow into the target circuit. The injected seal flow is thus the sum of the latter two. The seal tank is used to control the seal pressure and thus ensure the seal injection flow rate. By pass flow Seal flows Monolith Target Bearings W Target cooling circuit Seal Tank Monolith atmosphere circuit Figure 1: Sketch of target rotary seal injection principle. A model was built using the Modelica language [4] with the software Dymola [6] using libraries from Modelon [5]. It is a lumped parameter model for solving the fluid and heat transport equations in time. This means that there is no spatial resolution, except as modeled in certain components, and most components are basically modeled as volumes, discrete losses or combinations of these. The modeled balance of the target and monolith systems is described below in normal operation and for clarity of description excluding all ancillaries. A chart of the modeled systems, which includes purification, is shown in Fig. 2. Several connections, e.g. to helium supply and exhaust are simplified in this model, because these circuits shall work as a closed system in normal operation. The main components will be outlined in the following paragraphs, leaving out the associated control systems. It should be noted that there will be further optimization of these systems and the associated control, but the model here demonstrates one possible principle. per.nilsson@esss.se

Figure 2: Flow chart of helium in target, monolith and seal injection. The Target Circuit in the upper left corner contains the cyclone filters, the cooling heat exchanger before and after the compressor and the compressor itself. In the heat exchangers, the helium is cooled by an intermediate system, which is modeled as ideal. There are also a single loss and a volume, added to represent system volume and pressure losses in that circuit. Just before the target, there are some components to model species source terms such as spallation products. The Target Pressure Control in the upper right corner contains the low and high pressure tanks and valves, the small pressure control / purification compressor and the target helium Purification system. The target purification mass flow rate has to be at least the sum of the seal injection mass flow rate into the target circuit, in order to limit the target pressure. The seal injection rate from the target pressure control is controlled in the Target Seal Flow System in the centre. Purified flow in excess of what goes into the seal flow system can be injected back into the target circuit via the High Pressure Tank. The Seal Tank pressure is controlled by connection to the Monolith Pressure Control. The Monolith Circuit in the lower left corner contains a cooler and a circulator, plus a single loss to represent pressure losses in this circuit. The Monolith Pressure Control in the lower right corner contains the high and low pressure tanks and the small pressure control compressor. In this model, no special purification is included for the monolith. Such purification is planned, but will be primarily to remove moisture and air and is therefore different from the included target purification, which is mainly for spallation products.

The target circuit flow rate is set to 3 kg/s and the monolith circuit circulation rate to 40 g/s, but these values have no large influence in these simulations. The target seal flow Table 1: Modelled system part volumes and pressures Part System m 3 bar(a) Target System 20 4 Target PC Low Pressure Tank 2 1 Target PC High Pressure Tank 1 30 Monolith Atmosphere System 40 1 Monolith PC High Pressure Tank 1 30 Monolith PC Low Pressure Tank 4 0.1 Seal tank 4 4.5 Figure 4: Target heat source as a function of simulation time from 3000 to 6000 s. rate design value is 10 g/s, where 7 g/s goes into the monolith and 3 g/s to the target circuit [1]. The volumes and normal operating pressures of the system parts are summarized in Table 1. In order to investigate if the pressure control system tanks and compressor capacities are appropriate, some arbitrary sequential transients were tested after initializing the system numerically for 3000 s. The input that is prescribed to change with time is essentially described in Fig. 3 and 4. First the compressor speed is increased to Figure 5: Pressures as a function of simulation time from 3000 to 6000 s. Figure 3: Compressor speed and mass flow rate as a function of simulation time from 3000 to 6000 s. the nominal value at 3500 s, see Fig. 3. Then the target heating, i.e. 3 MW due to protons from the accelerator, is started at 4100 s, see Fig. 4. It is turned off again between 4600 s and 6000 s. The compressor speed is decreased at 5200 s and then re-increased at 5300 s, see Fig. 3. Some basic results are presented in Fig. 5 and 6. The diagrams in Fig. 5 show that the pressures in the target circuit, the monolith circuit and the target seal volume remain relatively constant. In Fig. 6, it can be seen that the target seal flow rates are held stable at the design values, 3 g/s and 7 g/s. This implies that the modeled tank sizes, compressor capacities and control systems are able to keep the pressures through the transients.

Figure 6: Flow rates as a function of simulation time from 3000 to 6000 s. PURIFICATION ESTIMATES The helium purification systems will not be described or modeled in detail here. For more details, see ESS Technical Design Report (TDR) [1]. In the models here, the purification systems are ideal, i.e. removing contamination down to prescribed concentrations. The purpose is to investigate the principles of connecting the target and monolith circuit and thus purifying a small fraction in a by-pass, a fraction which is then used for injection in the target rotary seal. In the model, the contaminants are transported as inert mass fractions. It implies that chemical interaction and accumulation within the systems is neglected. Also, the contaminants do not affect on the bulk fluid properties which are solely based on helium. As a test basis for the inert transport and balance of some contaminant species, hydrogen isotope source terms were taken from neutronic calculations in the TDR [1]. First the source terms are converted to mole rates in unit nano gram moles per second, i.e. 10 9 moles, where one mole of protium has a mass of 1 gram. Then those mole rates of hydrogen isotope atoms are combined into di-hydrogen mass rates, see table 2. In addition to these hydrogen iso- Table 2: Source terms in nano grams per second PP PD PT H2 Target 143 140 88.9 372 Monolith 95.1 40.6 7.94 144 topes produced in the target and the monolith, a continuous injection of hydrogen is modeled after the purification to keep a reducing atmosphere of about 10 ppmv in the helium circuits. The transport and balance model of H2, PD and PT has been implemented, as follows, in the same Modelica model that was used for the helium flow balance in the section above. The transport and balance of PP follows from these other three species. The source terms as a function of time in the model are show in Fig. 7. The three val- Figure 7: Hydrogen molecule source terms as a function of time in seconds. From the top: total hydrogen (H2), protium-deuterium (PD) and protium-tritium (PT). ues correspond to estimated production rates of total hydrogen (H2), protium-deuterium (PD) and protium-tritium (PT). They are added into the target and monolith volumes respectively. The model purification system is located in the target pressure control system, see the upper right of Fig. 2. In the model the purification removes total hydrogen (H2) down to 2 ppmv, i.e. all hydrogen isotopes independently while maintaining their relative ratio. Then after the removal, the total hydrogen concentration (H2) is continuously held at 10 ppmv by injection of pure protiumprotium (PP) after the purification system. Some transients have been tested also for the purification. The diagrams in Fig. 7 shows how contaminant production is started at 6000 s. The purification rate is not shown in diagrams, but at 25000 s, the purification is turned off, to be turned on 3600 s later at 28600 s.

The resulting contaminant mass fraction development as a function of time is given in Fig. 8. It can be seen that the will be added to the model to increase the performance. CONCLUSIONS AND OUTLOOK The simulations show the feasibility of connecting the target and monolith systems via a common injection in the target rotary seal with the current sizing. The pressures and flow rates are well within bounds during the simulations, which implies that the modeled systems are sufficient for these transients. Further optimization in terms of e.g. tank volumes, compressor capacities and control system settings can give even smoother results. The models will also be further elaborated, e.g. in terms of more complex contamination transport, accumulation and purification. ACKNOWLEDGEMENT The Dymola simulations have been performed with support by Carl Wilhelmsson, Modelon. Figure 8: Hydrogen molecule mass fractions as a function of time in seconds. From the top: total hydrogen (H2), protium-deuterium (PD) and protium-tritium (PT). REFERENCES [1] S. Peggs et. al., ESS Technical Design Report, ESS-doc- 274, April 23, 2013. [2] C. Kharoua et al., The ROtating Tungsten HElium Cooled TArget (ROTHETA) Concept, WEOTA08, AccApp13, Bruges, 2013. [3] R. Linander et al., ESS Target Station, An Overview of the Monolith Layout and Design, WEOTA03, AccApp13, Bruges, 2013. [4] https://www.modelica.org/ [5] https://www.modelon.com/ [6] https://www.3ds.com/ hydrogen fraction is very low, similar to the 10ppmV continuously added after purification. When the purification is turned off at 25000 s, the contamination concentrations increase smoothly in the target volume. That is due to that the major amount is produced here. In the seal and the monolith, the contamination levels instead increase sharply by such turn off. That is due to that helium with contamination is injected from the target system into the seal when the purification system is turned off. When the purification system is turned back on at 28600 s, the concentrations of all contaminants decrease. After some time, the concentrations approach the steady levels they were at before the purification was turned off. These levels are given by the production rate of contamination and injection rate of pure hydrogen (PP). The time it takes to reach the steady levels is given by the size of the system and the purification rate. It should also be noted that the model is rough, excluding e.g. surface adsorption, i.e. the results should not be taken as quantitative, but as indications of trends. More details