scholarly journals Fission Product Transport and Source Terms in HTRs: Experience from AVR Pebble Bed Reactor

2008 ◽  
Vol 2008 ◽  
pp. 1-14 ◽  
Author(s):  
Rainer Moormann
Keyword(s):  
Fission Product ◽  
Dominant Role ◽  
Fission Products ◽  
Normal Operation ◽  
Pebble Bed ◽  
Pebble Bed Reactor ◽  
Pebble Bed Reactors ◽  
Maintenance Work ◽  
Air Ingress ◽  
Gas Tight

Fission products deposited in the coolant circuit outside of the active core play a dominant role in source term estimations for advanced small pebble bed HTRs, particularly in design basis accidents (DBA). The deposited fission products may be released in depressurization accidents because present pebble bed HTR concepts abstain from a gas tight containment. Contamination of the circuit also hinders maintenance work. Experiments, performed from 1972 to 88 on the AVR, an experimental pebble bed HTR, allow for a deeper insight into fission product transport behavior. The activity deposition per coolant pass was lower than expected and was influenced by fission product chemistry and by presence of carbonaceous dust. The latter lead also to inconsistencies between Cs plate out experiments in laboratory and in AVR. The deposition behavior of Ag was in line with present models. Dust as activity carrier is of safety relevance because of its mobility and of its sorption capability for fission products. All metal surfaces in pebble bed reactors were covered by a carbonaceous dust layer. Dust in AVR was produced by abrasion in amounts of about 5 kg/y. Additional dust sources in AVR were ours oil ingress and peeling of fuel element surfaces due to an air ingress. Dust has a size of about 1  m, consists mainly of graphite, is partly remobilized by flow perturbations, and deposits with time constants of 1 to 2 hours. In future reactors, an efficient filtering via a gas tight containment is required because accidents with fast depressurizations induce dust mobilization. Enhanced core temperatures in normal operation as in AVR and broken fuel pebbles have to be considered, as inflammable dust concentrations in the gas phase.

scholarly journals A Safety Re-Evaluation of the AVR Pebble Bed Reactor Operation and Its Consequences for Future HTR Concepts

2008 ◽  
Author(s):  
Rainer Moormann
Keyword(s):  
Fission Product ◽  
End Of Life ◽  
Fission Products ◽  
Normal Operation ◽  
Primary Circuit ◽  
Pebble Bed ◽  
Pebble Bed Reactor ◽  
Hot Gas ◽  
Design Basis ◽  
Gas Tight

The AVR pebble bed reactor (46 MWth) was operated 1967–1988 at coolant outlet temperatures up to 990°C. Also because of a lack of other experience the AVR operation is a basis for future HTRs. This paper deals with insufficiently published unresolved safety problems of AVR and of pebble bed HTRs. The AVR primary circuit is heavily contaminated with dust bound and mobile metallic fission products (Sr-90, Cs-137) which create problems in current dismantling. The evaluation of fission product deposition experiments indicates that the end of life contamination reached several percent of a single core inventory. A re-evaluation of the AVR contamination is performed in order to quantify consequences for future HTRs: The AVR contamination was mainly caused by inadmissible high core temperatures, and not — as presumed in the past — by inadequate fuel quality only. The high AVR core temperatures were detected not earlier than one year before final AVR shut-down, because a pebble bed core cannot be equipped with instruments. The maximum core temperatures were more than 200 K higher than precalculated. Further, azimuthal temperature differences at the active core margin were observed, as unpredictable hot gas currents with temperatures > 1100°C. Despite of remarkable effort these problems are not yet understood. Having the black box character of the AVR core in mind it remains uncertain whether convincing explanations can be found without major experimental R&D. After detection of the inadmissible core temperatures, the AVR hot gas temperatures were strongly reduced for safety reasons. Metallic fission products diffuse in fuel kernel, coatings and graphite and their break through takes place in long term normal operation, if fission product specific temperature limits are exceeded. This is an unresolved weak point of HTRs in contrast to other reactors and is particularly problematic in pebble bed systems with their large dust content. Another disadvantage, responsible for the pronounced AVR contamination, lies in the fact that activity released from fuel elements is distributed in HTRs all over the coolant circuit surfaces and on graphitic dust and accumulates there. Consequences of AVR experience on future reactors are discussed. As long as pebble bed intrinsic reasons for the high AVR temperatures cannot be excluded they have to be conservatively considered in operation and design basis accidents. For an HTR of 400 MWth, 900°C hot gas temperature, modern fuel and 32 fpy the contaminations are expected to approach at least the same order as in AVR end of life. This creates major problems in design basis accidents, for maintenance and dismantling. Application of German dose criteria on advanced pebble bed reactors leads to the conclusion that a pebble bed HTR needs a gas tight containment even if inadmissible high temperatures as observed in AVR are not considered. However, a gas tight containment does not diminish the consequences of the primary circuit contamination on maintenance and dismantling. Thus complementary measures are discussed. A reduction of demands on future reactors (hot gas temperatures, fuel burn-up) is one option; another one is an elaborate R&D program for solution of unresolved problems related to operation and design basis accidents. These problems are listed in the paper.

scholarly journals Initial prediction of dust production in pebble bed reactors

2011 ◽  
Vol 2 (2) ◽  
pp. 189-195 ◽  
Author(s):  
M. Rostamian ◽  
S. Arifeen ◽  
G. P. Potirniche ◽  
A. Tokuhiro
Keyword(s):  
Computational Simulation ◽  
Fission Products ◽  
Initial Attempt ◽  
Pebble Bed ◽  
Pebble Bed Reactor ◽  
Pebble Bed Reactors ◽  
Safety Issues ◽  
Dust Generation ◽  
Very High Temperature Reactor ◽  
Very High

Abstract. This paper describes the computational simulation of contact zones between pebbles in a pebble bed reactor. In this type of reactor, the potential for graphite dust generation from frictional contact of graphite pebbles and the subsequent transport of dust and fission products can cause significant safety issues at very high temperatures around 900 °C in HTRs. The present simulation is an initial attempt to quantify the amount of nuclear grade graphite dust produced within a very high temperature reactor.

scholarly journals Physical Analysis of the Initial Core and Running-In Phase for Pebble-Bed Reactor HTR-PM

2017 ◽  
Vol 2017 ◽  
pp. 1-6
Author(s):  
Jingyu Zhang ◽  
Fu Li ◽  
Yuliang Sun
Keyword(s):  
Physical Parameters ◽  
Physical Analysis ◽  
Fuel Management ◽  
Fuel Temperature ◽  
Pebble Bed ◽  
Pebble Bed Reactor ◽  
Pebble Bed Reactors ◽  
Management Scheme ◽  
Typical Scheme ◽  
Scheme Evaluation

The pebble-bed reactor HTR-PM is being built in China and is planned to be critical in one or two years. At present, one emphasis of engineering design is to determine the fuel management scheme of the initial core and running-in phase. There are many possible schemes, and many factors need to be considered in the process of scheme evaluation and analysis. Based on the experience from the constructed or designed pebble-bed reactors, the fuel enrichment and the ratio of fuel spheres to graphite spheres are important. In this paper, some relevant physical considerations of the initial core and running-in phase of HTR-PM are given. Then a typical scheme of the initial core and running-in phase is proposed and simulated with VSOP code, and some key physical parameters, such as the maximum power per fuel sphere, the maximum fuel temperature, the refueling rate, and the discharge burnup, are calculated. Results of the physical parameters all satisfy the relevant design requirements, which means the proposed scheme is safe and reliable and can provide support for the fuel management of HTR-PM in the future.

The Fission Product Retention of Pebble-Bed Reactors in Ultimate Accidents

1979 ◽  
Vol 46 (2) ◽  
pp. 306-311 ◽  
Author(s):  
K. Petersen ◽  
H. Barthels ◽  
H. E. Drescher ◽  
C. B. Von Der Decken ◽  
N. Iniotakis ◽  
...  
Keyword(s):  
Fission Product ◽  
Pebble Bed ◽  
Pebble Bed Reactors

Spectral Zones Determination for Pebble Bed Reactor Cores Using Diffusion Theory Spectral Indices

2008 ◽  
Author(s):  
Ramatsemela Mphahlele ◽  
Abderrafi M. Ougouag ◽  
Kostadin N. Ivanov ◽  
Hans D. Gougar
Keyword(s):  
Diffusion Theory ◽  
Surface Fluxes ◽  
Spectral Indices ◽  
Pebble Bed ◽  
Pebble Bed Reactor ◽  
Absolute Value ◽  
Pebble Bed Reactors ◽  
Spectral Changes ◽  
Diffusion Constants

A practical methodology is developed for the determination of spectral zones in Pebble Bed Reactors (PBR). The methodology involves the use of spectral indices based on few-group diffusion theory whole core calculations. In this work a spectral zone is defined as made up of a number of nodes whose characteristics are collectively similar and that are assigned the same few-group diffusion constants. Therefore, spectral indices that reflect the physical behaviors of interest can be used to characterize said behaviors within each zone and thus to identify and distinguish the spectral zones. Several plausible spectral indices have been investigated in this work. Special emphasis and focus was placed on the trend or behavior of the spectral index at various positions along the radial and axial dimensions in the core. The ratio of group-wise surface currents to total surface fluxes, has been used to successfully identify spectral zone boundaries. A plot of the absolute value of this ratio versus position in the reactor exhibits a series of minima and maxima points. These extrema correlate with regions of significant spectral changes, and therefore are identified as plausible spectral zone boundaries.

Gas-Cooled Modular Reactors With Spherical Fuel Elements — Their Inherent Safety and Applications Not Just for Power Generation

2014 ◽  
Author(s):  
Walter Jaeger ◽  
H. J. Hamel ◽  
Heinz Termuehlen
Keyword(s):  
Power Generation ◽  
High Temperature ◽  
Reactor Design ◽  
Inherent Safety ◽  
Pebble Bed ◽  
Pebble Bed Reactor ◽  
Pebble Bed Reactors ◽  
Fuel Elements ◽  
High Temperature Gas

The gas-cooled reactor design with spherical fuel elements, referred to as high-temperature gas-cooled reactors (HTGR or HTR reactors) or pebble bed reactors has been already suggested by Farrington Daniels in the late 1940s; also referred to as Daniels’ pile reactor design. Under Rudolf Schulten the first pebble bed reactor, the 46MWth AVR Juelich reactor (Atom Versuchs-Reactor Jülich) was built in the late 1960s. It was in operation for 22 years and extensive testing confirmed its inherent safety.

Effect of Helium Injection on Natural Circulation Onset Time in a Simulated Pebble Bed Reactor

2008 ◽  
Author(s):  
Joseph P. Yurko ◽  
Katrina M. Sorensen ◽  
Andrew Kadak ◽  
Xing L. Yan
Keyword(s):  
High Temperature ◽  
Natural Circulation ◽  
Onset Time ◽  
Cfd Model ◽  
Pebble Bed ◽  
Analytical Estimate ◽  
Pebble Bed Reactor ◽  
Experimental Apparatus ◽  
Air Ingress ◽  
High Temperature Gas

This paper describes the experimental validation of a proposed method that uses a small amount of helium injection to prevent the onset of natural circulation in high temperature gas reactors (HTGR) following a depressurized loss of coolant accident. If this technique can be shown to work, air ingress accidents can be mitigated. A study by Dr. Xing L. Yan et al. (2008) developed an analytical estimate for the minimum injection rate (MIR) of helium required to prevent natural circulation. Yan’s study used a benchmarked CFD model of a prismatic core reactor to show that this method of helium injection would impede natural circulation. The current study involved the design and construction of an experimental apparatus in conjunction with a CFD model to validate Yan’s method. Based on the computational model, a physical experimental model was built and tested to simulate the main coolant pipe rupture of a Pebble Bed Reactor (PBR), a specific type of HTGR. The experimental apparatus consisted of a five foot tall, 2 inch diameter, copper U-tube placed atop a 55-gallon barrel to reduce sensor noise from outside air movement. Hot and cold legs were simulated to reflect the typical natural circulation conditions expected in reactor systems. FLUENT was used to predict the diffusion and circulation phases. Several experimental trials were run with and without helium injection. Results showed that with minimal helium injection, the onset of natural circulation was prevented which suggests that such a method may be useful in the design of high temperature gas reactors to mitigate air ingress accidents.

Development of Local Heat Transfer Models for Safety Assessment of High Temperature Gas-Cooled Reactor Cores—Part I: Pebble Bed Reactors

2009 ◽  
Vol 132 (1) ◽  
Author(s):  
Richard Stainsby ◽  
Matthew Worsley ◽  
Andrew Grief ◽  
Frances Dawson ◽  
Mike Davies ◽  
...  
Keyword(s):  
High Temperature ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Normal Operation ◽  
Fuel Particle ◽  
Subsequent Paper ◽  
Pebble Bed ◽  
Transient Conditions ◽  
Pebble Bed Reactors ◽  
Reactor Cores

This and the subsequent paper present models developed for determining fuel particle and fuel element temperatures in normal operation and transient conditions in high temperature reactor cores. Multiscale modeling concepts are used to develop the models for both pebble bed and prismatic core types. This paper, Part I, presents the development of the model for pebble bed reactors. Comparison is made with finite element simulations of an idealized “two-dimensional” pebble in transient conditions, and with a steady-state analytical solution in a spherical pebble geometry. A method is presented for determining the fuel temperatures in the individual batches of a multibatch recycle refuelling regime. Implementation of the multiscale and multibatch fuel models in a whole-core computational fluid dynamics model is discussed together with the future intentions of the research program.

scholarly journals The implication of Thorium fraction on neutronic parameters of pebble bed reactor

2021 ◽  
Vol 48 (3) ◽  
Author(s):  
Zuhair Zuhair ◽  
◽  
R. Andika Putra Dwijayanto ◽  
Suwoto Suwoto ◽  
Topan Setiadipura ◽  
...  
Keyword(s):  
Neutron Transport ◽  
Neutron Generation ◽  
Neutron Lifetime ◽  
Reactor Model ◽  
Pebble Bed ◽  
Pebble Bed Reactor ◽  
Pebble Bed Reactors ◽  
Fuel Burn ◽  
Neutronic Parameters ◽  
High Level

Thorium abundance in the Earth's crust is estimated to be three to four times higher than uranium. This is one potential advantage of Thorium as a provider of attractive fuel to produce nuclear energy. Fewer transuranics produced by Thorium during the fuel burn up in the reactor may also be another advantage for reducing the long-term burden of high-level long-lived waste. The scope of this paper is to study the implication of Thorium fraction on neutronic parameters of pebble bed reactor. The reactor model of HTR-10 was selected, and the (Th, 235U)O2 fuel was used in this study. The MCNP6 code was applied to solve a series of neutron transport calculations with various Thorium fractions in (Th,235U)O2 fuel based on the ENDF/BVII library. The calculation results show that the total temperature coefficient of reactivity of Thorium-added pebble bed reactors is generally more negative than those of LEU-fuelled one, except for 10% Thorium fraction. The kinetic parameters, especially prompt neutron lifetime and neutron generation time of pebble bed reactors, are higher, which means the addition of Thorium in the fuel makes the reactor more easily controlled. However, the burn-up calculations show that the introduction of Thorium in the same fuel kernel as LEU within the pebble bed reactor is unable to lengthen the fuel residence time, except for a minimum of 40% Thorium fraction.

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