Calibration of CFUL01 fission chambers in the standard neutron fields of the BR1 reactor at SCK CEN

Recent subcritical VENUS-F experiments showed that fission chambers with a threshold deposit like U-238 can essentially improve the on-line sub-criticality measurments with the beam interruption method, which is currently supposed to be the main method for the ADS MYRRHA. To suppress the uncertainty caused by fissions in the U-235 impurities, the fraction of U-235 in the U deposit should be accurately known. Three PHOTONIS CFUL01 type fission chambers with U-238 deposit were purchased for sub-critical experiments in the VENUS-F reactor. To verify the purity of their deposits, the effective U-235 masses were measured in the empty cavity of the BR1 reactor with a well-known thermal neutron spectrum. It turned out that the measured effective U-235 mass in two fission chambers is lower than the declared mass (as it should be), but this is not the case for the third fission chamber. Then, the effective U-238 mass in these FCs was measured in the well-known fast spectrum of the MARK-III convertor in the BR1 reactor. Finally, the isotopic composition was obtained and it was found that the purity of two CFUL01 FCs is in agreement with the values declared in the certificates but it is not the case for the third fission chamber. As the length of the deposit is bigger than the length of the MARK-III convertor, necessary corrections were calculated with MCNP. The developed procedure using the BR1 standard irradiation fields can be applied for calibration and impurity determination of large fission chambers.

On Operational and Start-Up Transients in an Accelerator Driven Systems

Accelerator Driven Systems (ADS) are subcritical nuclear reactor cores driven by external spallation neutron source. These systems are currently under investigation to be used as minor actinide burners and also as transmuters of long lived fission products. The subcriticality level plays an important rule in the control of the system. Low subcriticality level more source dominant is the system. In this paper one analyzes two transients to analyze the feedback effect on power of the Accelerator Driven Systems, since they cover a wide range of temperature variations. These operational transients are a beam interruption in power and a startup one, when the ADS are powered up from zero power to power operation. Numerical results shown that ADS can not reach the nominal operation temperatures if the systems operate next to critical ones. In a beam interruption transient systems are not affected if operated as critical ones.

The Angular Distribution of UV-B Sky Radiance under Cloudy Conditions: A Comparison of Measurements and Radiative Transfer Calculations Using a Fractal Cloud Model

In recent years, global warming concerns have focused attention on cloud radiative forcing and its accurate encapsulation in radiative transfer measurement and modeling programs. At present, this process is constrained by the dynamic movement and inhomogeneity of cloud structure. This study attempts to quantify the UV sky radiance distribution induced by a partial and overcast stratiform cloud field while addressing some of the inherent spatial and temporal errors resulting from cloud. For this purpose, high-quality azimuthally averaged 2-min measurements of erythemal UV-B sky radiance distribution were undertaken by a variable sky-view platform at Hobart, Australia (42.90°S, 147.33°E). Measurements were subsequently compared with Monte Carlo radiative transfer simulations using both a multifractal and plane-parallel homogenous (PPH) cloud field. Data were also compared with several empirical parameterizations. Results at solar zenith angles of 30° and 50° show that for overcast conditions, the multifractal model is superior to the PPH model. For broken cloud conditions, the radiance measurements are biased toward higher instances of direct-beam interruption by cloud. This tends to smooth the near-sun sky radiance field whereas the multifractal model under the same conditions continues to exhibit the circumsolar effect, indicating that its performance may be still valid for radiation modeling. An empirical parameterization of the same multifractal model produced similar sky radiance profiles, warranting its use in radiative transfer models.

Factors Influencing the Microdistribution of Two Sympatric Species of Plecoptera: An Experimental Study

The responses of nymphs of two species of perlid stoneflies to manipulation of some important features of their microhabitats were studied in laboratory stream tanks fitted with an infrared beam interruption system. The activity of both Paragnetina media (Walker) and Phasganophora capitata (Pictet) was significantly affected by current speed and substrate size, and by an interaction between the two. Regression analysis showed that activity for P. capitata was highest at 15 and 75 cm/s and minimal between 25 and 50 cm/s, while that for P. media was highest at 15 cm/s and lowest at 25 cm/s and above. For P. media there was a significant negative relationship between substrate size and activity at a current speed of 15 cm/s. The relationship between the activity of P. capitata and substrate size was not linear. Addition of trout skin mucus to the stream tanks caused an increase in the levels of activity of P. media but a decrease in the activity of P. capitata. Addition of nymphs of the mayfly Ephemerella subvaria to the tank containing P. media produced no effect. These findings suggest mechanisms that establish different microdistributions for sympatric species.

Estrous Cycle and Temporal Pattern of Feeding in Female Rats

The effects of the estrous cycle and access to running wheels on the pattern of feeding were studied in 24 female albino rats using two methods, an indirect method (photocell beam interruption) and a direct observation of feeding. Although aspects of feeding differed with the method, the following results obtained with both methods. A minimum in the number of bursts of feeding and to a lesser extent the duration of bursts of feeding occurred in estrus. The estrous cycle and access-to-running wheel conditions interacted in affecting the organization of bursts into longer meal-taking periods. The results were considered consistent with the notion that estrus raises the threshold of feeding as a response to stimuli.