Makoto Teshigawara

Optimization of Coupled and Decoupled Hydrogen Moderators for a Short-pulse Spallation Source

Neutronic performance of coupled and decoupled hydrogen moderators for a short pulse spallation source was extensively studied by calculations to seek the optimal parameters, especially the para/ortho hydrogen ratio. A decoupled 100% para-hydrogen moderator gives excellent pulse characteristics at lower energies. Gadolinium (Gd) poisoning is useful to obtain narrower pulses up to about 70 meV at a reasonable intensity penalty. At higher energies, where the pulse tail decay characteristics become most important, the choice of the reflector material and the decoupling energy (E d) play an important role. One important technical issue is the radiation damage of a Gd-poison and a boron carbide decoupler. We discuss the possibility of using a composite decoupler material such as Cd+In (indium)+Eu (europium). In a coupled moderator a 100% para-hydrogen moderator of a larger volume gives an additional intensity gain over a normal one but the gain is not as large as in a decoupled one.

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.

A Challenge for a High-Resolution Ag-In-Cd Decoupler in Intensified Short-Pulsed Neutron Source

A sharp pulsed-neutron beam, which has narrow width and large attenuation, will enhance ability to separate peak position diffracted from crystal structure in time-of-flight measurement method. Traditional technique to satisfy the request above is to use Boron for decoupling higher cut-off energy. Disadvantage, however, will be an excess formation of He bubbles and large heat deposition in the materials as a result of (n,α) reaction, particularly in a MW class of neutron source. Alternative is to use a material combination of Ag-In-Cd, which has different energies for resonance absorption and often used in the control rod of pressurized water reactors with cladding stainless steel. For application of this material to pulsed-neutron source a challenge is to make sandwiches structure with Al alloy because a moderator is made of this material from points of views of heat removing. Technically hot isostatic pressing was a choice and devoted to realize a bonding strength through R&Ds. Final product was set up to MLF /J-PARC and a successful resolution was observed in a powder diffractometer.

Induced-radioactivity in J-PARC Spallation Neutron Source

Radioactivity estimation was performed on a MW-class spallation neutron source by using the DCHAIN-SP code system to design facility from both safety and maintenance points of view. Remote handling procedures and shielding parameters were considered based on the estimation. The results are reflected on the design of J-PARC neutron source facility.

Analysis of Heavy-Ion-Induced Deuteron-Deuteron Fusion in Solids

When energetic heavy ions irradiate a deuteride titanium target, a number of recoil deuterium atoms are produced in the solid. The recoil deuterium atoms cause deuteron-deuteron (d-d) fusion reactions in solids. The probability of the d-d fusion reaction has been calculated for the primary colliding deuterium atoms, as well as the collision cascade deuterium atoms. Based on calculated results, an experiment using a heavy-ion accelerator was proposed to study d-d fusion in solids. The enhancement effect on d-d fusion in solids is particularly interesting. The experimental parameters were as follows: The energy of the ion beam for the d-d fusion experiment was in the range from several to several tens of mega-electron-volts for an experiment with an iodine ion beam and a titanium target. The enhancement effect in the solid is evaluated by comparing the experimental results with the present calculated results. The existence of the enhancement at low energies can be confirmed by measuring the depth profile of the fusion probabilities. Reported experimental data have been analyzed by the calculated results. The enhancement has not been found in the data.

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.