The experimental coolant exit temperature for each core ring is shown in Fig 14 as a function of the reactor power. The aluminum probe with thermocouple was inserted in each hole at top grid plate, and the coolant inlet temperature was about 38 oC in all measurements.

As mentioned earlier, to calculate the mean energies the feeding probability to the different levels populated in the decay is needed (Eqs. 6, 7). Now we can understand why TAS measurements can have an impact and should be applied to decay heat studies. If we have a beta decay that suffers from the pandemonium effect, the mean gamma energies will be underestimated and the beta energies will be overestimated. This is shown schematically in Fig. 4. Since the application of the TAS technique is the only way to avoid the pandemonium effect, this technique should be used for studying decays that are important for the decay heat problem.

Our interest in the decay heat topic was triggered by the work of Yoshida and coworkers (Yoshida, 1999). At the begining of the 1990s one of the most successful data bases for summation calculations was the JNDC-V2 (Japanese Nuclear Data Committee version 2) database. In this database the gross theory of beta decay (Takahashi & Yamada, 1969) was used to supplement experimental data that might suffer from the pandemonium effect. For example in (Tasaka, 1988) it is mentioned that the mean energies of many decays having a beta

decay Q value larger that 5 MeV were supplemented by theoretical values calculated using the gross theory. Even though the database in general worked well for the gamma component of the 239 Pu decay heat, there was a significant remaining discrepancy in the 300-3000 s cooling time. Yoshida studied the possible causes for the discrepancy and proposed several possible explanations. The most plausible was the possibility that the decay energies of some nuclides with half-lives in the range of 300-3000 s or with precursors with similar half-lives suffered from the pandemonium effect. After a careful evaluation of possible candidates he proposed that the decays of 102,104,105 Tc should be measured with the total absorption technique.

In the process of defining priorities for the TAS measurements related to decay heat, contact with specialists in the field was established and a series of meetings were held under the auspices of the International Atomic Energy Agency (IAEA). As a result of these meetings a list of nuclei that should be measured with the total absorption technique was defined (Nichols, 2007). This list included the Yoshida cases (102,104,105Tc) and some additional nuclei (in total 37). The nuclei were identified based on their contribution to the decay heat in different fuels, and in order to reduce the discrepancies between the major international databases (JENDL (Shibata, 2011), JEFF (Kellett, 2009), ENDF (Chadwick, 2006)). Additionally, this list included some cases that deserve to be measured for other reasons. For example there are TAS measurements that were performed in the past by Greenwood and coworkers (Greenwood, 1992). These TAS measurements were analyzed using different procedures from those used nowadays. It is important to verify the results and compare them with new measurements using different analysis techniques to look for systematic uncertainties. Similarly the measurements of (Rudstam, 1990), obtained from direct spectral measurements can be checked against TAS measurements (Tain & Algora, 2006).

2.3 Assessment of the developed correlation

The subcooling at OSV is evaluated by the present correlation and the previous correlations described in table 1 for (Whittle & Forgan, 1967) experiments. All the results and experimental data are plotted in Fig. 2. The solid line is a reference with the slope of one is drawn on the plot to give the relation between the predicted and measured data. The present correlation shows a good agreement with the experimental data, it gives only 6.6 % relative standard deviation from the experimental data while the others gives 20.2 %, 26.4 %, 27.4 % and 35.0 % for Khater et al., Lee & Bankoff, Sun et al. and Saha & Zuber correlations respectively as shown table 1.

The experimental data of (Whittle & Forgan, 1967) on light water cover the following operating conditions: [2]

The development of SCWRs requires an intensive study of convective heat transfer at supercritical pressures. Heat transfer at a supercritical pressure is different from that of a subcritical pressure because the thermophysical properties of a light-water coolant undergo significant variations as the temperature of the coolant passes through the pseudocritical point. Therefore, the traditional Nusselt number and other related non-dimensional parameters developed at subcritical pressures based on the bulk-fluid temperature cannot be used (Bae and Kim, 2009).

At a supercritical pressure, the thermophysical properties of a coolant at the sheath-wall temperature differ significantly from those at the bulk-fluid temperature. Although, a fluid does not undergo a phase change at a supercritical pressure, a low-density fluid separates the sheath-wall from a high-density fluid at high heat fluxes and low mass fluxes. This results in a reduction in the convective Heat Transfer Coefficient (HTC). Consequently, the sheath-wall temperature increases. This phenomenon is known as the Deteriorated Heat Transfer (DHT) regime. Therefore, the sheath-wall temperature must be reflected in a correlation, which is used to study the heat transfer at supercritical conditions.

Many correlations have been developed for the calculation of HTC at supercritical conditions. The most widely used correlations include those developed by Bishop et al. (1964); Swenson et al. (1965); Krasnoscheckov et al. (1967); Jackson (2002); and Mokry et al. (2009). Zahlan et al. (2011) compared the prediction capabilities of sixteen correlations including the aforementioned correlations. The conclusion of the Zahlan et al. (2011) comparison study showed that the Mokry et al. (2009) correlation resulted in the lowest Root-Mean-Square (RMS) error within the supercritical region compared to all other examined correlations.

The excess of neutrons due to their small absorption in lead-208 can be used for fuel breeding and transmutation of long-lived radiotoxic fission products. Here, as an example, we assume the radiation capture of neutrons by uranium-238 leading to creation of plutonium-239. The affectivity of this process will be as large as the value of one-group cross section of radiation neutron capture by uranium-238 nucleus is large. In Fig.5 microscopic cross sections of radiation neutron capture by U-238 taken from ENDF/B-VII.0 library are given.

From Fig.5 it can be seen that at neutron energies near to En=5-10 eV these cross sections have maximum equal to 170 barns. That is why if the neutron spectra contains an increased share of neutrons of small and intermediate energies the corresponding one-group will be large enough.

In table 2 the one-group cross sections of radiation neutron capture by U-238 averaged over neutron spectra of the 80 MW ADS and various FRs (BREST, BN-600 and RBEC-M) are given.

It can be noted that one-group cross section of neutron capture by uranium-238 in ADS spectrum is by 2.15 times bigger than for sodium reactor BN-600 spectrum and this fact indicates to the possibility of enhancing the breeding gain in the blanket of ADS 80 MW cooled with lead-208.

As an illustration, in Fig. 6 and 7 the results of burning 1 kg of uranium-238 placed in the one part of ADS 80 MW subcritical blanket (cell 3 near the blanket far margin) and corresponding accumulation of plutonium-239 are given. Calculations have been performed on the basis of code ACDAM [15] developed at the IPPE Centre of nuclear data.

Figure 15 shows the radial core coolant temperature profiles (inlet/outlet channel temperatures) at 265 kW. Theoretical results using the PANTERA code are also shown in the figure (Veloso, 2005).

We have started a research programme aimed at the study of nuclei included in the list of the WPEC-25 group of the IAEA (Nichols, 2007). To perform a succesful experiment, the first step is to define the best facility to carry it out. In recent years there have been extensive developments in the methods used to produce radioactive beams. Indeed, we are living a period of renaissence and renewal for these facilities, since new ones have been constructed and others are under development, construction and upgrade (FAIR, HIE-ISOLDE, HRIBF, etc.). In essence these facilities are based on two main methods of producing radioactive beams, the ISOL method and the fragmentation or In-Flight method. In the ISOL technique an intense particle beam, impinges on a thick target. After diffusion and effusion, the radioactive nuclei produced are mass selected, ionized and re-accelerated in a post-accelerator. This method in general produces cleaner nuclear species, but requires specific developments of the ion sources, which involve chemistry and physics aspects. Its major constrain is related to the production of very short-lived isotopes and the production of beams of refractory elements that are difficult to extract from ion sources. The fragmentation technique, as the name implies, is based on the fragmentation of high energy projectiles on target nuclei and the subsequent separation and selection in-flight of the radioactive nuclei produced using magnetic spectrometers. With this technique it is possible to study very short-lived isotopes, but typically the nuclei are produced in a less clean enviroment. In this case the experiments rely on the identification of the nuclear species produced on an ion-by-ion basis.

Some of the nuclei included in the WPEC-25 list are refractory elements, so it was not possible to produce them in "conventional" ISOL facilities such as ISOLDE(CERN). So for our experiments we decided to use the Ion-Guide Isotope Separator On-Line (IGISOL) facility of the University of Jyvaskyla (Aysto, 2001). In this facility the ion guide method was developed, which can be considered to be a "chemistry" independent ISOL method (Dendooven, 1997). The working principle of the ion guide method is that the radioctive nuclides are produced in a thin target after bombarment with the accelerator beam. The reaction products (recoils) fly out of the target and are transported by a differential pumping system to the first stage of the accelerator. The mean path of the recoils is optimized in such a way that they survive as singly charged ions. By this method we obtain a system, which is chemically insensitive and very fast (ms). A schematic picture of the ion-guide principle is presented in Fig. 5.

Fig. 5. The ion guide principle.

At the IGISOL facility several ion guides were developed. For our experiments we used a fission ion guide, which allows the extraction of fission products produced in proton-induced fission. This was the chosen method for the production of the radioactive nuclides of interest. Following the extraction, the ions can be separated using a dipol magnet. The magnetic field of the dipol bends the ions to different trajectories depending on their charge-to-mass ratio. Since most of the ions come out of the ion guide singly charged, mass separation is achieved. To characterize the separation quality, the mass resolving power of the system is used, which is a measure of how well species with different masses are separated (Дм/м). At IGISOL the mass resolving power typically varies from 200 to 500, depending on the experimental conditions. This is not enough to separate ions that have the same mass number (isobars), but it is adequate to separate isotopes of the same element. That is why the instrument is called an isotope separator. For the TAS measurements this separation is not enough, since you can have several isobars produced in fission that can not be separated with the isotope separator, and their decay will appear as contaminants in the TAS spectrum. An advantage of the IGISOL facility is that a Penning trap system (JYFLTRAP) (Kolhinen, 2004) can be used for further isobaric separation. Penning traps are devices for the storage of charged particles using a homogeneous static magnetic field and a spatially inhomogeneous static electric field. This kind of trap is particularly well suited for precision mass spectroscopy, but they can also be used as high resolution separators for "trap-assisted" spectroscopic studies as in our experiments.

With a Penning trap system a mass resolving power of the order of 105 and even 106 can be achieved. The good separation is shown in Fig 6, where a frequency scan in the Penning trap for mass A= 101 prior to one of our experiments is presented. Once a frequency in the trap is set for a particular isobar, a very pure radioactive beam can be obtained. The only disadvantage of this system is the relatively low intensity of the ion beam, since the transmission of the trap is only a few percent.

Figure 7 shows an schematic picture of the setup used in our experiments in Jyvaskyla. In this setup the radioactive beam coming from the trap is implanted in a tape system that allows us to transport the radioactive sources to the measuring position and to remove the undesired daughter activities. The cycles of the tape are optimized according to the half-life of the isotope of interest.