Karl Verfondern

Experimental and theoretical investigation of liquid hydrogen pool spreading and vaporization

The investigation of cryogenic pool spreading and vaporization as the initial step in an accident sequence with the spillage of liquefied hydrogen (LH2) is an essential part of a safety assessment to valuate the risks of hydrogen. Experiments have been conducted to examine the transient LH2 pool spreading behavior on a liquid (water) and solid (aluminum) ground. In a parallel effort, a computer model, LAuV, based on the shallow-layer differential equations has been developed to simulate the pool spreading and vaporization of a spilled cryogen under various conditions taking account of concomitant phenomena such as ice formation on the water surface. The objective of this paper is a description of the LH2 pool spreading test results as well as the performance of the LAuV calculation model. Furthermore a prediction of the consequences of the spill of a tanker truck load for different cryogens is given. The results represent pertinent information as source term for the subsequent analysis steps of dispersion and, in the case of ignition, combustion of the evolved gas cloud.

COATED PARTICLE FUEL FOR HIGH TEMPERATURE GAS COOLED REACTORS

Roy Huddle, having invented the coated particle in Harwell 1957, stated in the early 1970s that we know now everything about particles and coatings and should be going over to deal with other problems. This was on the occasion of the Dragon fuel performance information meeting London 1973: How wrong a genius be! It took until 1978 that really good particles were made in Germany, then during the Japanese HTTR production in the 1990s and finally the Chinese 2000-2001 campaign for HTR-10. Here, we present a review of history and present status. Today, good fuel is measured by different standards from the seventies: where initial free heavy metal fraction was typical for early AVR carbide fuel and initial free heavy metal fraction was acceptable for oxide fuel in THTR, we insist on values more than an order of magnitude below this value today. Half a percent of particle failure at the end-of-irradiation, another ancient standard, is not even acceptable today, even for the most severe accidents. While legislation and licensing has not changed, one of the reasons we insist on these improvements is the preference for passive systems rather than active controls of earlier times. After renewed HTGR interest, we are reporting about the start of new or reactivated coated particle work in several parts of the world, considering the aspects of designs/ traditional and new materials, manufacturing technologies/ quality control quality assurance, irradiation and accident performance, modeling and performance predictions, and fuel cycle aspects and spent fuel treatment. In very general terms, the coated particle should be strong, reliable, retentive, and affordable. These properties have to be quantified and will be eventually optimized for a specific application system. Results obtained so far indicate that the same particle can be used for steam cycle applications with helium coolant gas exit, for gas turbine applications at and for process heat/hydrogen generation applications with outlet temperatures. There is a clear set of standards for modem high quality fuel in terms of low levels of heavy metal contamination, manufacture-induced particle defects during fuel body and fuel element making, irradiation/accident induced particle failures and limits on fission product release from intact particles. While gas-cooled reactor design is still open-ended with blocks for the prismatic and spherical fuel elements for the pebble-bed design, there is near worldwide agreement on high quality fuel: a diameter kernel of 10% enrichment is surrounded by a thick sacrificial buffer layer to be followed by a dense inner pyrocarbon layer, a high quality silicon carbide layer of thickness and theoretical density and another outer pyrocarbon layer. Good performance has been demonstrated both under operational and under accident conditions, i.e. to 10% FIMA and maximum afterwards. And it is the wide-ranging demonstration experience that makes this particle superior. Recommendations are made for further work: 1. Generation of data for presently manufactured materials, e.g. SiC strength and strength distribution, PyC creep and shrinkage and many more material data sets. 2. Renewed start of irradiation and accident testing of modem coated particle fuel. 3. Analysis of existing and newly created data with a view to demonstrate satisfactory performance at burnups beyond 10% FIMA and complete fission product retention even in accidents that go beyond for a short period of time. This work should proceed at both national and international level.

Passive Safety Characteristics of Fuel for a Modular High-Temperature Reactor and Fuel Performance Modeling Under Accident Conditions

AbstractThe high fission product retention potential of coated particle fuel combined with inherently passive temperature controls guarantee almost complete fission product retention during an accident in a small modular high-temperature reactor. Extensive experimental results provide the basis for this claim to inherent safety.Models and codes have been developed to (a) predict realistic, or at least conservative, overall release rates from the primary circuit, (b) reduce the large number of experimental results to a small set of characteristic coefficients, and (c) predict release beyond experimental conditions. Conservative predictions of release from the core have been done using a traditional pressure vessel model for release from fuel particles and simplified diffusion models for fission product transport. This approach is based on experimental work that has been done on nearly all possible accident conditions and is limited by the finite number of experiments. Data reduction has been achieved with ...

Source Term Estimation for Small-Sized HTRs: Status and Further Needs, Extracted from German Safety Analyses

Abstract The stringent safety demands for advanced small pebble bed high-temperature reactors (HTRs) are outlined. Main results of German studies on source term estimation are discussed. Core heatup events are no longer dominant for modern fuel, but fission product transport during water ingress accidents (steam cycle plants) and He-circuit depressurizations are relevant, mainly due to remobilization of fission products that were plated out in the course of normal operation or that became dust borne. The following important lack of knowledge was identified: Data on plateout in normal operation are insufficient, as are data on behavior of dust-borne activity in total; better knowledge in these fields is also important for maintenance/repair and design/shielding. For core heatup events, the influence of burnup on temperature-induced fission product release has to be measured for future Pu-containing high burnup fuel; furthermore, transport mechanisms out of the He circuit into the environment require further examination. For water/steam ingress events, mobilization of plated-out fission products by steam or water has to be considered in detail along with steam interaction with kernels of particles with defective coatings. For source terms of depressurization, a more detailed knowledge of flow pattern and shear forces on surfaces is necessary. To improve the knowledge on plateout and dust in normal operation and to generate specimens for experimental remobilization studies, planning/design of plateout/dust examination facilities to be added to HTRs running in the next future reactors [HTR10 and the High-Temperature Engineering Test Reactor (HTTR)] is proposed. For severe air ingress and reactivity accidents, which belong to hypothetical events with frequencies <1 × 10-7 yr-1, behavior of future advanced fuel elements has to be experimentally tested.

Fission product behaviour and graphite corrosion under accident conditions in the HTR

Abstract This paper primarily gives an overview of methods and data in source term estimations for the HTR with pebble bed core. For medium size HTRs the risk dominating accidents are tied to core heat-up events, where a significant portion of the fission product inventory may be released from the coated fuel particles. Here the research mainly is focused on temperature-induced coated particle failure and the interaction of metallic fission products with the core graphite. For small HTRs, with their limitation of maximum temperatures below coated particle failure limits, core heat-up accidents virtually play no role with respect to source terms. Here the risk is dominated by accidents like water ingress or rapid depressurization which may lead to a partial release of fission products accumulated on primary circuit surfaces like the steam reformer. Deposition of fission products and remobilization under the conditions mentioned above are predominant research areas. It can be expected that the ongoing and planned improvements of models and data base, in particular for the medium size HTR, will result in a further reduction of the already low source terms. A principal possibility for core degradation and hence destruction of fission product barriers is graphite corrosion caused by massive air ingress. The research effort in this field as well as for graphite corrosion during water ingress accidents is described in Part B of this paper. From the viewpoint of risk for this type of accident no significant contribution to that of present reactor concepts was found.

Fission Product Release From HTGR Fuel Under Core Heatup Accident Conditions

Various countries engaged in the development and fabrication of modern fuel for the High Temperature Gas-Cooled Reactor (HTGR) have initiated activities of modeling the fuel and fission product release behavior with the aim of predicting the fuel performance under operating and accidental conditions of future HTGRs. Within the IAEA directed Coordinated Research Project CRP6 on “Advances in HTGR Fuel Technology Development” active since 2002, the 13 participating Member States have agreed upon benchmark studies on fuel performance during normal operation and under accident conditions. While the former has been completed in the meantime, the focus is now on the extension of the national code developments to become applicable to core heatup accident conditions. These activities are supported by the fact that core heatup simulation experiments have been resumed recently providing new, highly valuable data. Work on accident performance will be — similar to the normal operation benchmark — consisting of three essential parts comprising both code verification that establishes the correspondence of code work with the underlying physical, chemical and mathematical laws, and code validation that establishes reasonable agreement with the existing experimental data base, but including also predictive calculations for future heating tests and/or reactor concepts. The paper will describe the cases to be studied and the calculational results obtained with the German computer model FRESCO. Among the benchmark cases in consideration are tests which were most recently conducted in the new heating facility KUEFA. Therefore this study will also re-open the discussion and analysis of both the validity of diffusion models and the transport data of the principal fission product species in the HTGR fuel materials as essential input data for the codes.Copyright

High-Temperature Reactor Fuel Technology in the RAPHAEL European Project

Within the scope of the 5th EURATOM Framework Programme (FP) for the HTR-F and HTR-F1 projects, a new 4-year integrated project on very high temperature reactors (RAPHAEL: ReActor for Process Heat And Electricity) was started in April 2006 as part of the 6th Framework Programme. The Sub-Project on Fuel Technology (SP-FT) is one of eight sub-projects constituting the RAPHAEL project. R&D conducted in this sub-project focuses on understanding fuel behaviour, determining the limits of state-of-the-art fuel, and developing potential performance improvements. Fabrication processes were worked out for alternative fuel kernel composition (UCO instead of UO2 ) and coating (ZrC instead of SiC): i) UCO microstructure reduces fission product migration and is thus considered superior to UO2 under high burn-ups and high temperature gradients. For this reason, the manufacturing feasibility of UCO kernels using modified external sol-gel routes was addressed. The calcining and sintering steps were particularly studied. ii) For its better high temperature performance, ZrC is a candidate coating material for replacing SiC in TRISO (TRistructural ISOtropic) particles. One of the objectives was therefore to deposit a stoichiometric ZrC layer without impurities. An “analytical irradiation” experiment currently performed in the HFR — named PYCASSO for PYrocarbon irradiation for Creep And Swelling/Shrinkage of Objects — was set up to measure the changes in coating material properties as a function of neutron fluence, with samples coming from the new fabrication process. This experiment was started in April 2008 and will provide data on particle component behaviour under irradiation. This data is required to upgrade material models implemented in the ATLAS fuel simulation code. The PYCASSO irradiation experiment is a true Generation IV VHTR effort, with Korean and Japanese samples included in the irradiation. Further RAPHAEL results will be made available to the GIF VHTR Fuel and Fuel Cycle project partners in the future. Post-irradiation examinations and heat-up tests performed on fuel irradiated in an earlier project are being performed to investigate the behaviour of state-of-the-art fuel in VHTR normal and accident conditions. Very interesting results from destructive examinations performed on the HFR-EU1bis pebbles were obtained, showing a clear temperature (and high burn-up) influence on both kernel changes (including fission product behaviour) and the coating layers. Based on fuel particle models established earlier, the fuel modelling capabilities could be further improved: i) Modelling of fuel elements containing thousands of particles is expected to enable a statistical approach to mechanical particle behaviour and fission product release. ii) A database on historical and new fuel properties was built to enable validation of models. This paper reports on recent progress and main results of the RAPHAEL sub-project on fuel technology.Copyright

High temperature reactors for cogeneration applications

Abstract There is a large potential for nuclear energy also in the non-electric heat market. Many industrial sectors have a high demand for process heat and steam at various levels of temperature and pressure to be provided for desalination of seawater, district heating, or chemical processes. The future generation of nuclear plants will be capable to enter the wide field of cogeneration of heat and power (CHP), to reduce waste heat and to increase efficiency. This requires an adjustment to multiple needs of the customers in terms of size and application. All Generation-IV concepts proposed are designed for coolant outlet temperatures above 500 °C, which allow applications in the low and medium temperature range. A VHTR would even be able to cover the whole temperature range up to approx. 1 000 °C.

Qualification of pebble fuel for HTGRs

Abstract The German HTGR fuel development program for the HTR-Modul concept has resulted in a reference design based on LEU UO2 TRISO coated particle fuel in a spherical fuel element. The coated particles consist of minute uranium particle kernels coated with layers of carbon and silicon carbide. Analyses on quality of as-manufactured fuel, its behavior under HTR-Modul relevant operating and accident conditions have demonstrated excellent performance. Coated particles can withstand high internal gas pressure without releasing their fission products to the environment. International efforts are on-going for further improvement of coated particle fuel to meet the needs of future generation-IV HTR concepts.

The production of hydrogen by nuclear and solar heat

Both nuclear and solar energy represent significant carbon-free sources, which may contribute robust elements to a cleaner energy economy, to develop domestic energy sources for the purpose of energy security and stability, and to reduce national dependencies on imports of fossil fuels. Hydrogen, on the other hand, represents a fuel which is clean, powerful and an environmentally benign source of energy to the end-user. The current production of hydrogen is mainly based on hydrocarbons as feedstock, e.g. steam reforming of natural gas.