Kiichi Ohtsu

Operational characteristics of the J-PARC cryogenic hydrogen system for a spallation neutron source

The J-PARC cryogenic hydrogen system provides supercritical hydrogen with the para-hydrogen concentration of more than 99 % and the temperature of less than 20 K to three moderators so as to provide cold pulsed neutron beams of a higher neutronic performance. Furthermore, the temperature fluctuation of the feed hydrogen stream is required to be within ± 0.25 K. A stable 300-kW proton beam operation has been carried out since November 2012. The para-hydrogen concentrations were measured during the cool-down process. It is confirmed that para-hydrogen always exists in the equilibrium concentration because of the installation of an ortho-para hydrogen convertor. Propagation characteristics of temperature fluctuation were measured by temporarily changing the heater power under off-beam condition to clarify the effects of a heater control for thermal compensation on the feed temperature fluctuation. The experimental data gave an allowable temperature fluctuation of ± 1.05 K. It is clarified through a 286-kW and a 524-kW proton beam operations that the heater control would be applicable for the 1-MW proton beam operation by extrapolating from the experimental data.

Design Result of the Cryogenic Hydrogen Circulation System for 1 MW Pulse Spallation Neutron Source (JSNS) in J‐PARC

A cryogenic hydrogen circulation system to cool cryogenic hydrogen moderators for the spallation neutron source in J‐PARC has been designed. This system consists of a helium refrigerator system and a hydrogen circulation system. The refrigeration capacity required for the cryogenic system is specified to be around 6 kW at 17 K. The hydrogen circulation system is composed of a hydrogen‐helium heat exchanger, two circulation pumps, multiple transfer lines, three moderator vessels, an Ortho‐Para hydrogen converter, an accumulator, a heater and others. The system adopts a centrifugal‐type hydrogen pump that can circulate the cryogenic hydrogen (20 K, 0.5 to 1.5 MPa) with the mass flow up to 162 g/s through the three moderators. This forced‐flow circulation can remove the nuclear heating from the moderators and can keep the temperature difference through the moderators within 3 K. The Ortho‐Para hydrogen converter will be installed to maintain the Para‐hydrogen concentration of more than 99% at the inlet of th...

PRESSURE FLUCTUATION BEHAVIOR IN THE CRYOGENIC HYDROGEN SYSTEM CAUSED BY A 100 kW PROTON BEAM INJECTION

Supercritical hydrogen (1.5 MPa and around 20 K) has been selected as a moderator material for the intense spallation neutron source (JSNS) in J‐PARC. The cryogenic hydrogen system provides the supercritical hydrogen for the moderators and removes the nuclear heating at the moderators, which is estimated to be 3.8 kW for a proton beam power of 1 MW. The pressure control system was designed to mitigate pressure fluctuation caused by suddenly turning a proton beam on and off, which is composed of a heater as an active controller for thermal compensation and an accumulator as a passive volume controller. A 109 kW proton beam was injected to the JSNS in December 2007. The pressure fluctuation behaviors were studied for the 109 kW proton beam operation. As soon as the proton beam was injected, the accumulator spontaneously started to constrict. The heater control succeeded in maintaining a constant heat load applied to the cryogenic hydrogen system. The pressure control system successfully reduced the pressure...

Dynamic behavior of the cryogenic hydrogen system using only a heater control

The J-PARC cryogenic hydrogen system provides supercritical hydrogen to three moderators and absorbs a nuclear heating of 3.75 kW for a 1-MW proton beam operation. A pressure control system, which consists of a heater for the thermal compensation and a cryogenic accumulator with a bellows acting as a volume controller, is prepared to mitigate a pressure fluctuation caused by the sudden heat load of kW-order. Stable operation with a 120-kW proton beam power, where the heat load is 450 W, has been conducted since November 2009. However, a major problem with cryogenic accumulator occurred during a short maintenance period in February 2010. In order to resume the 120-kW proton beam operation as soon as possible, an operational approach without using the cryogenic accumulator was studied and the hydrogen loop was partially altered. It was confirmed through an on-beam commissioning that only the approach could successfully mitigate the pressure fluctuation below the allowable value of 0.1 MPa, although it was e...

Development of a simulation code for a cool-down process of the cryogenic hydrogen system

Supercritical hydrogen with a pressure of 1.5 MPa and a temperature of 20 K has been selected as a moderator material in an intense spallation neutron source (JSNS), which is one of main experimental facilities in J‐PARC. The cryogenic hydrogen system, in which a hydrogen circulation system is cooled by a helium refrigerator with the refrigeration power of 6.45 kW at 15.5 K, has been designed to provide the supercritical hydrogen to the moderator and to remove the nuclear heating generated there. In this study, we have developed a simulation code that predicts temperature behaviors in the hydrogen circulation system during its cool‐down process. Cool‐down process analyses have been performed, and an operational method for the cool‐down process has been studied. The analytical results indicate that the hydrogen circulation system would be able to be cooled down to 18 K within 19 hours.

Operational Experiences of J-PARC cryogenic hydrogen system for a spallation neutron source

The Japan Proton Accelerator Research Complex (J-PARC) cryogenic hydrogen system was completed in April 2008. The proton beam power was gradually increased to 500 kW. A trial 600-kW proton beam operation was successfully completed in April 2015. We achieved long-lasting operation for more than three months. However, thus far, we encountered several problems such as unstable operation of the helium refrigerator because of some impurities, failure of a welded bellows of an accumulator, and hydrogen pump issues. Furthermore, the Great East Japan Earthquake was experienced during the cryogenic hydrogen system operation in March 2011. In this study, we describe the operation characteristics and our experiences with the J-PARC cryogenic hydrogen system.

Pressure and temperature fluctuation simulation of J-PARC cryogenic hydrogen system

The J-PARC cryogenic hydrogen system provides supercritical cryogenic hydrogen to the moderators at a pressure of 1.5 MPa and temperature of 18 K and removes 3.8 kW of nuclear heat from the 1 MW proton beam operation. We prepared a heater for thermal compensation and an accumulator, with a bellows structure for volume control, to mitigate the pressure fluctuation caused by switching the proton beam on and off. In this study, a 1-D simulation code named DiSC-SH2 was developed to understand the propagation of pressure and temperature propagations through the hydrogen loop due to on and off switching of the proton beam. We confirmed that the simulated dynamic behaviors in the hydrogen loop for 300-kW and 500-kW proton beam operations agree well with the experimental data under the same conditions.

Pressure drop evaluation of the hydrogen circulation system for JSNS

In J‐PARC, an intense spallation neutron source (JSNS) driven by a proton beam of 1 MW has selected supercritical hydrogen with a temperature of around 20 K and the pressure of 1.5 MPa as a moderator material. A hydrogen‐circulation system, which consists of two pumps, an ortho‐para hydrogen converter, a heater, an accumulator and a helium‐hydrogen heat exchanger, has been designed to provide supercritical hydrogen to the moderators and remove the nuclear heating there. A hydrogen‐circulation system is cooled through the heat exchanger by a helium refrigerator with the refrigeration power of 6.45 kW at 15.5 K. It is important for the cooling design of the hydrogen‐circulation system to understand the pressure drops through the equipments. In this work, the pressure drop through each component was analyzed by using a CFD code, STAR‐CD. The correlation of the pressure drops through the components that can describe the analytical results within 14% differences has been derived. It is confirmed that the press...

DEVELOPMENT OF THE CRYOGENIC HYDROGEN SYSTEM FOR A SPALLATION NEUTRTON SOURCE IN J‐PARC

An intense spallation neutron source (JSNS) driven by a proton beam of 1‐MW has been constructed as one of the main experimental facilities in J‐PARC. Supercritical hydrogen at around 20 K and 1.5 MPa was selected as a moderator material in JSNS. Three kinds of hydrogen moderators (coupled, decoupled, and poisoned) were installed to provide pulsed neutron beam of higher neutronic performance. The total nuclear heating in the moderators was estimated to be 3.75 kW for a proton beam power of 1 MW. The cryogenic hydrogen system, where the hydrogen circulation system is cooled by a helium refrigerator system with the refrigerator capacity of 6.45 kW at 15.6 K, provides the supercritical hydrogen for the moderators and absorbs nuclear heating in the moderators. The off‐beam commissioning has confirmed that the cryogenic hydrogen system can be cooled down to 18 K within 19 hours. The supercritical hydrogen with a mass flow rate of 190 g/s can be circulated in the rated condition. It was verified that the cryoge...

THERMAL STRESS ANALYSIS FOR A TRANSFER LINE OF HYDROGEN MODERATOR IN J-PARC

An intense spallation neutron source (JSNS) driven by a 1-MW proton beam was constructed, as one of the main experimental facilities in J-PARC. In JSNS, supercritical hydrogen (1.5 MPa, 20 K) was selected as a moderator material. Three kinds of hydrogen moderator are installed (coupled, decoupled, and poisoned) to provide pulsed neutron beam with higher neutronic performance. The moderators contain cryogenic hydrogen transfer lines located in a radioactive area. Therefore, the transfer lines should be designed to have minimum pipe size and elbow-type bend sections to reduce the potential for radiation dose by radiation streaming. The design should also consider mechanical stress concentrations, deformation, and touching between the pipes due to the thermal shrinkage at the cryogenic hydrogen temperature. A FEM code analysis determined the appropriate locations of piping supporting spacers to keep the thermal stress below the allowable stress and to also avoid touching between the pipes.