Radioactive Decay Of Fission Products Research Articles

DEVELOPMENT OF DECAY HEAT MODEL FOR RAST-K

Decay heat (DH) is the heat produced through a radioactive decay of fission products during or after a reactor operation. It is known as the second largest source of power in the core after fission. Being such a strong contributor to reactor power, it should be accurately determined at any time of reactor operation. Currently, there are two main approaches for DH estimation used in reactor simulation codes. One approach is based on careful inventorying of all produced target nuclides and their individual contributions to total power. Alternatively, the other popular approach is based on collapsing all target fission products into a small number of groups similar to delayed neutron estimation techniques. However, the last (multigroup) method currently has limitations when used in some transient scenarios such as transients occurred in fresh fuel. In this study, the multigroup method was further developed for reducing limitations while retaining the advantage in computation speed. Then, it was implemented into Reactor Analysis code for Steady state and Transient (RAST-K) and tested against other codes. As a result, the improved method was found capable of determining DH power at all tested stages of reactor operation under any tested operation scenario. In particular, the test simulations using the improved method showed better results in those scenarios that were under accuracy limitations of the original multigroup method. Overall, the quality of transient calculations in RAST-K was improved when using the newly implemented DH module.

Radiation and Thermal Ageing of Nuclear Waste Glass

The radioactive decay of fission products and actinides incorporated into nuclear waste glass leads to self-heating and self-radiation effects that may affect the stability, structure and performance of the glass in a closed system. Short-lived fission products cause significant self-heating for the first 600 years. Alpha decay of the actinides leads to self-radiation damage that can be significant after a few hundred years, and over the long time periods of geologic disposal, the accumulation of helium and radiation damage from alpha decay may lead to swelling, microstructural evolution and changes in mechanical properties. Four decades of research on the behavior of nuclear waste glass are reviewed.

Design and evaluation of Operation Grade Decay Heat Removal System of PFBR

The decay heat removal (DHR) system removes the decay heat generated (by radioactive decay of fission products) in the core after the reactor is shut down, thereby ensuring proper cooling of the core sub assemblies and limiting main vessel, internals and sodium temperature within safe limits. There are two diverse paths for removal of decay heat from the reactor, namely, Safety Grade Decay Heat Removal System (SGDHRS) and Operation Grade Decay Heat Removal System (OGDHRS). OGDHR circuit is used when at least one secondary sodium loop, DHR related steam water circuit and off site power supply is available and SGDHR circuit is used when OGDHR system is not available or when both the secondary loops are not available for DHR. This paper provides brief details of the design and evaluation of OGDHRS.

Occurrence of Fe–Mg-rich smectites and corrensite in the Morrón de Mateo bentonite deposit (Cabo de Gata region, Spain): A natural analogue of the bentonite barrier in a radwaste repository

► A hydrothermal process transformed Fe–Mg smectites into corrensite. ► This transformation was favoured by the intrusion of the Morrón de Mateo dome. ► The intrusion caused a temperature increased and a supply of Fe–Mg rich solutions. ► The system can be a good natural analogue of bentonite barrier in a radwaste disposal. ► Experimental studies of stability of bentonite are in agreement with the results. The Morrón de Mateo bentonite deposit is being studied as a natural analogue of the thermal and geochemical effects on a bentonite barrier in a deep geological repository of high level radioactive wastes. This bentonite deposit and its host rocks were intruded by a rhyodacitic volcanic dome that induced a hydrothermal metasomatic process affecting the biocalcarenite beds close to the dome. In this work, the mineralogical and chemical features of the clay minerals of the hydrothermally altered pyroclastic (white tuffs) and epiclastic rocks (mass flow), located in the NE sector of the Morrón de Mateo deposit are described. White tuffs have a high content of phyllosilicates, mainly composed of dioctahedral smectites, while mass flow have a higher proportion of inherited minerals, the neoformed phyllosilicates are dioctahedral smectites and an interlayer chlorite/smectite mineral of corrensite type. The chemical composition of smectites reflects the different nature of the parent rocks, in such a way that smectites from white tuffs have a quite homogeneous chemical composition and their structural formulae correspond to montmorillonite type, while smectites from mass flow show more chemical variability, higher Fe and Mg contents and a mean structural formulae corresponding to Fe–Mg-rich beidellite and/or to an intermediate smectite member between beidellite and saponite. In addition, chemical composition and textural features of corrensite-like clay minerals in relation to Fe–Mg-rich smectites in the samples have also been studied, suggesting that the former seems to be formed from Fe–Mg-rich smectites. The presence of corrensite in the epiclastic rocks suggests that in the Morrón de Mateo area a hydrothermal alteration process occurred after bentonite formation, which transformed Fe–Mg-rich smectites into corrensite. This transformation was probably favoured by the intrusion of the Morrón de Mateo volcanic dome, which produced a temperature increase in the geological media and a supply of Fe–Mg-rich solutions. These physicochemical conditions were also responsible for the metasomatic transformations observed in the biocalcarenite beds located on the top of the bentonite deposit. All these data suggest that the Morrón de Mateo natural system could be a good natural analogue of both thermal and chemical effects on a bentonite barrier related to the radioactive decay of fission products and the interaction between the corrosion products of steel over-packs and the bentonite. These circumstances would favour the transformation of the candidate Al-rich smectites into Fe–Mg-rich smectites and corrensite, as steps prior to formation of chlorite. In this case, all the physicochemical and mechanical properties of Al-rich smectites would disappear and the clayey barrier would fail.

Comparison of life of two types of decay heat exchangers used in prototype fast breeder reactor

The decay heat generated by radioactive decay of fission products in the core after reactor is shut down is to be removed for limiting of temperatures of various components. In prototype fast breeder reactor (PFBR), a dedicated safety grade decay heat removal (SGDHR) system consisting of four independent loops of each 8 MWt heat removal capacity is used for removing decay heat. Each loop consists of a sodium to sodium heat exchanger (DHX) and sodium to air heat exchanger (AHX). While DHX transfers heat from the radioactive primary sodium to intermediate sodium, AHX dissipates the heat to atmospheric air. Diversity in the design is adopted from reliability consideration. This paper compares the design life of two types of DHX (Type-A & B). Both DHX are shell and tube type with primary sodium on shell side and intermediate sodium on tube side. While Type-A has the tubesheet design similar to IHX, Type-B is a U ‘ tube two-pass heat exchanger. In this tubes are supported at the top by two separate tube sheets (split tubesheets). The tubesheets along with attached shells are analysed using CAST 3M, issued by CEA, France. The creep-fatigue damage is evaluated as per RCC-MR (2002). For the specified design life of 40 years, the fatigue damage is small and the creep damage evaluated is 0.26 in the type-A and 0.32 in the type-B, thus meeting design code limits. Two types of DHX are found to have 121y and 154y as allowable life after considering appropriate factor of safety given in design code RCC-MR.

Characteristics of conjugate free convection in a corium pool formed during a severe reactor accident

Conjugate natural convection flow in a thick walled semi-circular cavity containing molten core material has been analysed numerically. The lower plenum debris bed represents decay heating, heat conduction, and heat convection within the debris. The internal heat generation resulting from the radioactive decay of fission products ensures that most of the pool remains molten and also causes natural convection in the pool. Bottom circular wall of the cavity is taken to be thick with finite conductive properties, while top wall is considered to be isothermal. In carrying out the analysis, various constituents of the core material are assumed to be mixed homogenously. Thus a homogeneous fluid, for which Pr number is 0.45, has been considered while Ra number range from 3.2×10 7 to 3.2×10 12 has been investigated. Present results for non-conjugate semi-circular cavity compare very well with previously published experimental studies.

Numerical solution of the chain model of fission product nuclides. Effect on the reactivity of the reactor

Runge–Kutta algorithm of fourth order devised for the numerical solution of ordinary differential equations of fuel isotopic composition and fission products accumulation systems is described. Several hundreds fission product isotopes are included in partially depleted fuel as a result of direct yield from nuclear fission reaction, radioactive decay of fission products and/or neutron capture. The algorithm of the suggested method was coded and compared with both the analytical method and the international ORIGEN2 code system. The accuracy and speed of the calculations as obtained by implementing the method in a PCs with Linux system are found to be acceptable and faster than their resemblance. The effect of the calculated fission product concentrations on the reactivity of the reactor has been included.

Crystallization of Nuclear Waste Disposal Glass

The rate of bulk crystallization and shear viscosity η of a zinc borosilicate simulated nuclear waste disposal glass (SNWDG) were measured above the glass transition temperature Tg. Using recent theories and experimental data for the temperature dependence of η at <Tg, the crystallization rate below Tg was calculated; well below Tg the SNWDG (and, more generally, any high‐silica glass) will not crystallize appreciably over any important time period for radioactive decay of fission products.

Ionization of the atmosphere due to beta particles emitted by fission products

Persistent ionization of air at moderate heights, just below the E layer, after high-altitude nuclear detonations is predominantly due to radiations accompanying radioactive decay of fission products. The most important are BETA particles, which are restricted in their movements by the earth's magnetic field and thus create ionized clouds of high density in localized regions. A method of investigating this phenomenon is discussed, in which the geometry of the problem, the rate of loss of energy by a particle moving through gas of varying density, and the composite energy spectrum of radiation from fragments of an average fission of uranium-235 are considered jointly. Several simplifying assumptions are made, in particular, elastic scattering of the BETA particles by air is neglected. This simplification appears to be justified in the present circumstances. A technique is developed in which the ion-pair production rate is first computed as a function of the mass of gas traversed in the direction of a magnetic field line, and is then converted to any arbitrary situation described in terms of the actual height of the debris deposition and an arbitrary magnetic dip angle. Results of computations are normalized to one fission or one particle, and the method ofmore » conversion from one normalization to the other is given for any time after the detonation. (auth)« less