Naoteru Odano

Design of advanced integral-type marine reactor, MRX

Abstract The basic concept of an innovative advanced marine reactor with a passive safety system, MRX (Marine Reactor X) has been established for the primary application to ship propulsion. The design goals of the reactor system, to be lightweight and compact, and to be enhanced in safety and reliability, are achieved with adoption of new technologies. The MRX is of the integral-type PWR with 100 MW of thermal output. Adoption of a water-filled containment makes the MRX extremely lightweight and compact. The total weight and volume of MRX is about 1600 tons and 1210 m3, which is half that of the first Japanese nuclear ship, ‘Mutsu’, reactor, although the reactor power of MRX is three times greater than that of the ‘Mutsu’ reactor. Numbers of active components in the reactor system are greatly reduced, compared with loop type PWRs, by adopting an integral type reactor and the passive system. Safety was evaluated by both experiments and analyses. Core damage occurrence frequency estimated by probability safety analysis (PSA) is of two orders smaller than those of existing PWRs. Feasibility study on economics is conducted by comparing the total operation costs of a nuclear container ship installing the MRXs with a diesel engine ship. The nuclear ship has the advantage for greater speed and larger amounts of cargo carried.

JENDL-3.2 Covariance File

Covariances of neutron nuclear data have been estimated for 16 nuclides contained in JENDL-3.2. The physical quantities for which covariances are deduced are cross sections, resolved and unresolved resonance parameters, the first order Legendre-polynomial coefficients for the angular distribution of elastically scattered neutrons, and fission neutron spectra. As for 233,235,238U and 239,241Pu, covariances were obtained also for the average number of neutrons emitted in fission. Least-squares methods were applied to obtain the covariances of those cross sections which were based on experimental data. A simultaneous evaluation method yielded uncertainties in the fission cross sections of 235U, 238U, 239Pu, 240Pu and 241Pu. Covariances of nuclear model calculations were determined from uncertainties in model parameters. The covariance file thus obtained is processed by a system which has been developed, and used for the adjustment of group cross sections.

Distribution of local 137Cs anomalies on the seafloor near the Fukushima Dai-ichi Nuclear Power Plant

An estimated 3.5±0.7×10(15) Bq of (137)Cs is thought to have been discharged into the ocean following the melt down at Fukushima Dai-ichi Nuclear Power Plant (F1NPP). While efforts have been made to monitor seafloor radiation levels, the sampling techniques used cannot capture the continuous distribution of radionuclides. In this work, we apply in situ measurement techniques using a towed gamma ray spectrometer to map the continuous distribution of (137)Cs on the seafloor within 20 km of the F1NPP. The results reveal the existence of local (137)Cs anomalies, with levels of (137)Cs an order of magnitude higher than the surrounding seafloors. The sizes of the anomalies mapped in this work range from a few meters to a few hundreds of meters in length, and it is demonstrated that the distribution of these anomalies is strongly influenced by meter scale features of the terrain.

Development of a sonar system to estimate the seafloor subsurface burial depth of a towed gamma-ray spectrometer

A sonar system and image processing method have been developed to continuously estimate the seafloor subsurface burial depth of a gamma-ray spectrometer that is towed along the seafloor by a ship. The towed gamma-ray spectrometer, called the RESQ hose, is an instrument designed to measure radiation levels in seafloor sediments that were contaminated as a result of the accident at the Fukushima-Daiichi nuclear power plant. In situ measurements of subsurface burial depth can increase the accuracy of the radionuclide concentration levels estimated by the RESQ hose by allowing for more accurate modeling of the measurement conditions. The reliability of the system is verified through sea trials performed near the outlet of the Abukuma River. Continuous measurements of burial depth are achieved with an outlier ratio of less than 5%. The burial depths measured by the device show a strong correlation with the acoustic backscatter intensity of side-scan sonar images obtained along the surveyed transect, indicating that burial depth is dependent on sediment type. We examine the influence of burial depth on the conversion factors between detected gamma-ray count rates and the concentrations of 134Cs and 137Cs. For both nuclides, the calculated factors increase by nearly 80% as burial depth increases from 0 to 11 cm. The converted results from the measured 137Cs count rates are compared with a core sample obtained near the transect, with the nearest point within a factor of two of the sampling result.

Estimates of Collective Doses from a Hypothetical Accident of a Nuclear Submarine

The collective dose to the Japanese population has been estimated from a hypothetical accident of a nuclear submarine if it sinks in an offshore region around Japan. A computer code system DSOCEAN has been used for assessing the collective dose due to radionuclides released to the ocean from a sunken nuclear submarine. The estimated collective effective dose commitment from all of the radionuclides released after the break of the fuel pellets is estimated to be 2.5 x104 man-Sv. The contribution of 241Am to the total collective effective dose commitment is the highest, followed by 137Cs, 238Pu, 240Pu, 239Pu and 241 Pu. The maximum of the estimated collective effective dose by the annual intake of marine products after radionuclide releases for one year is approximately 0.3% of the annual average dose by the natural radiation that is reported by UNSCEAR.

Evaluation of Buildup of Activated Corrosion Products for Highly Compact Marine Reactor DRX without Primary Coolant Water Purification System

Japan Atomic Energy Research Institute has studied a highly compact reactor DRX for deep-sea research. The DRX has no purification system to achieve compact and light weight design by simplification of the system. The DRX is designed to operate for one month without purification of the primary coolant water. To quantitatively evaluate the validity of reactor operation without a purification system, a computer code CTAM-II has been developed to calculate accumulation of the activated corrosion products during and after reactor operation. The code is an improved and modified version of CTAM, which was developed for the shield modification project of the nuclear ship Mutsu. Validity of CTAM-II and parameters used in the code has been confirmed by comparison of the calculated data and experimental ones for measurement of the concentrations of radioactive materials in the primary coolant water. Estimation of buildup of the corrosion products for DRX using CTAM-II has been carried out and shielding calculations using source terms calculated from CTAM-II have been performed. A radiation safety assessment of the DRX without the purification system has been carried out by these shielding calculations.

Uniform Irradiation using Rotational-linear Scanning Method for Narrow Synchrotron Radiation Beam

At SPring-8, photon intensity monitors for synchrotron radiation have been developed. Using these monitors, the responses of radiation detectors and dosimeters to monoenergetic photons can be measured. In most cases, uniform irradiation to the sample is necessary. Here, two scanning methods are proposed. One is an XZ-linear scanning method, which moves the sample simultaneously in both the X and Z direction, that is, in zigzag fashion. The other is a rotational-linear scanning method, which rotates the sample moving in the X direction. To investigate the validity of the two methods, thermoluminescent dosimeters were irradiated with a broad synchrotron-radiation beam, and the readings from the two methods were compared with that of the dosimeters fixed in the beam The results for both scanning methods virtually agreed with that of the fixed method. The advantages of the rotational-linear scanning method are that low- and medium-dose irradiation is possible, uniformity is excellent and the load to the scanning equipment is light: hence, this method is superior to the XZ-linear scanning method for most applications.

Continuous mapping of radionuclides on the seafloor using a towed gamma ray spectrometer

An instrument to continuously map the distribution of radionuclides on the seafloor has been developed and applied to survey radioactive discharge from the Fukushima Dai-ichi Nuclear Power Plant following the M9.0 earthquake and tsunami that struck the east coast of Japan on March 11 2011. The towed gamma ray spectrometer has been deployed off Fukushima and has to date, been used to monitor radionuclide distribution over a total distance of more than 120 km. Here, we introduce the results of measurements made along three transects within an 80km radius of the plant, focusing on a 22km long transect off Iwaki. While the levels of the natural gamma ray emitter 40K remained constant, the levels of 137Cs and 134Cs were found to vary significantly with location. The in situ measurements show good agreement with laboratory analyzed samples obtained during the survey, indicating that the technique described provides an effective solution for rapid, low cost monitoring of radioactive material on the seafloor.

Passive Safety Small Reactor for Distributed Energy Supply: Heavy Water Mixing Core

The purpose of this paper is to study the core performance of passive safety small reactor for distributed energy supply by changing the heavy water (D{sub 2}O) concentration in the mixed coolant together with the fuel pitch. The long core life with conditions of the excessive reactivity of 2 %{delta}k/k, the reactivity shutdown margin of 1 %{delta}k/k and the negative coolant temperature reactivity coefficient is attained for the case of D{sub 2}O concentration of 60 % with 10 % enrichment gadolinia (Gd{sub 2}O{sub 3}) doped fuel rods. This D{sub 2}O core has a shorter core life 4.14 years than the original light water (H{sub 2}O) core 4.76 years, while it needs a larger core size. However, changing the D{sub 2}O concentration on the way during the burn-up shows a possibility of extending more the core life than that of the original H{sub 2}O core. (authors)

Shielding Design for Steam Generator of Advanced Marine Reactor MRX

Compactness and light weight are of importance for a reactor plant used for ship propulsion. An advanced marine reactor, MRX, which has been designed by Japan Atomic Energy Research Institute, has accomplished a passively safe, compact and light weight design by adoption of an integral PWR and a water filled containment vessel. Dose equivalent rates in all areas inside and outside a ship are required to be reduced below design criteria without any bulk shield outside the containment vessel. In this paper, a shielding design for installation of the steam generator in the vicinity of the reactor core to achieve a compact plant by reducing fast neutrons in the steam generator and to enable the engine room, where pipes of the secondary loop are installed, to be classified as surveillance area, not controlled area, is described. The main purpose to classify the engine room as the surveillance area is the improvement of working efficiency of crew. To accomplish the design goal, a relatively thick core barrel and a shield for the steam generator are introduced.