Calcium Sulfate Activated by Lead and Manganese for Thermoluminescence Dosimetry

CaSO₄: Pb, Mn has been found to be free from the serious disadvantage of rapid fading possessed by CaSO₄: Mn, the most sensitive thermoluminescence phosphor available so far. A study has been made on the effect of lead and manganese content on CaSO₄, and it is concluded that lead in CaSO₄ produces new traps for radiation energy, resulting in improvement of the properties of the phosphors for dosimetry. The optimum content of such activator additives was found to be 0.2 mol/₀ of lead and 3 mol/₀ of manganese.

The improved phosphor thus obtained produces glow peaks at 160° and 190°C, and the energy yield of the thermoluminescence is about which is twice that of CaSO₄: Mn. The more significant properties of this phosphor from the viewpoint of application to radiation dosimetry include:

(1) Linear responce from 50μR to 10⁴R

(2) Minimum detectable dose of 50μR±25% by experimental reader

(3) Fading rate of 5% in a week     

Properties of Autoregressive Model in Reactor Noise Analysis, (II)

Since the AR model widely used in the reactor noise analysis is mathematically a subset of the ARMA model, the characteristics of the AR model should be made clear for correct use of the resulting AR model. A simple tool called the PSD contour-map associated with the normalized PSD chart is proposed for interpretation and evaluation of the poles of the AR model from the viewpoint of the PSD. This tool not only visualizes the structure of the poles of the AR model but also provides us various information such as the importance of each pole of the AR model in the frequency region as well as the PSD difference of AR models with different model orders. The fundamental properties of the PSD of a pair of complex conjugate poles are extracted from the PSD contour-map in the form of rules. Model order reduction in AR model fitting is also discussed to show the effectiveness of the PSD contour-map.

It is also made clear in terms of the PSD that the information of the poles and zeros outside the convergence circle of the ARMA model is equivalently expressed in the AR model by relocation of the ring-poles representing the zero closest to the unit circle.     

Investigation of Resonant Power Oscillation in Halden Boiling Water Reactor Based on Noise Analysis

In the Halden Boiling Water Reactor (HBWR), a resonant power oscillation with a period around 0.04 Hz is observed at power levels higher than about 9.5 MWt. Although this resonant oscillation is not so strong as to affect the normal reactor operation, it is significant, from the viewpoint of reactor diagnosis, to reveal the cause of this oscillation as well as to understand its characteristics.

Noise analysis based on the autoregressive (AR) modeling technique together with spectral and correlation analyses is performed to investigate the driving source, which indicates that it is attributed to the dynamic interference with the reactor of heat exchange process in two parallel-connected steam transformers.

The present study demonstrates the effectiveness of the technique applied here for determining the so-called noise source inducing variations of quantities in a system together with its applicability to various problems in the field of reactor noise analysis and diagnosis.     

Effect of Moderator Density Distribution of Annular Flow on Fuel Assembly Neutronic Characteristics in Boiling Water Reactor Cores

The effect of the moderator density distribution of annular flow on the fuel assembly neutronic characteristics in a boiling water nuclear reactor was investigated using the SRAC95 code system. For the investigation, a model of annular flow for fuel assembly calculation was utilized. The results of the assembly calculation with the model (Method 1) and those of the fuel assembly calculation with the uniform void fraction distribution (Method 2) were compared. It was found that Method 2 underestimates the infinite multiplication factor in the fuel assembly including the gadolinia rod (type 1 assembly). This phenomenon is explained by the fact that the capture rate in the thermal energy region in gadolinia fuel is estimated to be smaller when the liquid film of annular flow at the fuel rod surface is considered. A burnup calculation was performed under the condition of a void fraction of 65% and a volumetric fraction of the liquid film in liquid phase of 1. It is found that Method 2 underestimates the infinite multiplication factor in comparison to Method 1 in the early stage of burnup, and that Method 2 becomes to overestimate the factor after a certain degree of burnup. This is because Method 2 overestimates the depletion rate of the gadolinia.