A technique for detection of WWER fuel failures by activity of Xe radionuclides during reactor operation

Fuel failures during operation of Nuclear Power Plants (NPPs) may lead to substantial economic losses. Negative effects of reactor operation with leaking fuel in the core may be reduced if fuel failures are detected in due time of the cycle. At present time, the ratio of the normalized release rates of 131I and 134I is used to detect fuel failures in WWERs during steady state operation. However, based on the activity of iodine radionuclides, it is not always possible to detect the fuel failure. This situation may occur in case of a small defect in cladding of a leaking fuel rod or for high burnup fuel if the defect is overlapped by the surface of the fuel pellet. If it is so, fuel deposits may be the dominant contributor to iodine activity, and the fuel failure may not be noticeable. In PWRs, fuel failures are detected by activity of radioactive noble gases. Noble gases are not adsorbed on cladding inner surface, as distinct from iodine radionuclides. Release of noble gases from the leaking fuel rod may be considerable even though defect in cladding is small. In this paper, a technique is proposed for detection of fuel failures at WWER reactors by activity of radioactive noble gases in the primary coolant. It is shown that radioactive noble gases may be a more sensitive indicator of fuel failures than iodine radionuclides. Detection of fuel failures is based on monitoring of the ratio between 133Xe and 135Xe activity. Some examples of practical applications are given.

Discussion of Tag Gas Method Using in China Experimental Fast Reactor for Failed Fuel Assembly Location

As the first fast reactor of China, the safety of China Experimental Fast Reactor (CEFR) is extremely important, and will decide the future of Chinese fast reactor project. The fuel failure detection system of CEFR provides surveillance and protection for the first barrier-fuel cladding of CEFR, so it is one of the most important systems for the safety of CEFR. As tag gas method is an important method for fuel-failure location in fast reactor, CEFR has a medium-term and long-term plan of using this method to locating failed fuel assemblies. This paper introduces the main principle of tag gas method, summarizes the application of this method, and compares the advantages and disadvantages of each fuel failure location method. Combining the design characteristics of CEFR, this work analyzes the selection principle of tag gas isotopes and the effects on heat transfer capability of fuel element while tag gas filled in. Meanwhile, according to the detection ability of mass spectrometer and the foreign advanced utilization experiences of tag gas method, some suggestions are provided.

Development of Security and Safety Fuel for Pu-Burner HTGR: Part 2 — Design Study of Fuel and Reactor Core

JAEA (Japan Atomic Energy Agency) has conducted feasibility studies of the fuel and of the reactor core for the plutonium-burner HTGR (High Temperature Gas-cooled Reactor). The increase of the internal pressure, which is caused by generations of CO gas and stable noble gases, is considered to be the one of the major causes of TRISO (TRI-structural ISO-tropic) fuel failure at high burn-up. The CO gas is generated by the chemical reaction of the graphite making up the buffer layer with the free-oxygen released from the fuel kernel by fission. The stable noble gases, which are fission products, are also released from the fuel kernel. Although it is considered very difficult to suppress the increase of the partial pressure of the stable noble gases because of its chemically inert nature, the increase of the CO gas partial pressure can be suppressed by reducing the free-oxygen mole concentration using a chemical reaction. ZrC acts an oxygen getter, which reduces the free-oxygen generated with fission reaction. An increase of the CO gas partial pressure with burn-up in a TRISO fuel is expected to be suppressed by coating ZrC on a fuel kernel. A PuO2-YSZ (Yttria Stabilized Zirconia) fuel kernel with a ZrC coating, which enhances safety, security and safeguard, namely: 3S-TRISO fuel, was proposed to introduce to the plutonium-burner HTGR. In this study, the efficiency of the ZrC coating as the free-oxygen getter under a HTGR temperature condition was examined based on a thermochemical calculation. A preliminary feasibility study on the 3S-TRISO fuel that enables to attain a high burn-up around 500 GWd/t was also conducted focusing on a fuel failure caused by an increase of the internal pressure. Additionally, a preliminary nuclear analysis was conducted for the plutonium-burner HTGR with a fuel shuffling in the radial direction. As a result, the thermochemical calculation result showed that all the amount of the free-oxygen is captured by a thin ZrC coating under 1600°C condition. The plutonium-burner HTGR will be designed to suppress fuel temperature to be lower than 1600°C under severe accident conditions, and hence it was confirmed that coating ZrC on the fuel kernel is very effective method to suppress the internal pressure. The internal pressure the 3S-TRISO fuel at 500 GWd/t is calculated to be lower than 60 MPa, which allows to prevent the fuel failure, and hence the feasibility of the 3S-TRISO fuel was also confirmed. Additionally, the results of the whole core burn-up calculations showed that the fuel shuffling in the radial direction allows to achieve the high burn-up around 500 GWd/t. It also showed that the temperature coefficient of reactivity is negative value during the rated power condition through the operation period.

Discussion of Tag Gas Method Using in China Experimental Fast Reactor for Failed Fuel Assembly Location

As the first fast reactor of China, the safety of CEFR is extremely important, and will decide the future of Chinese fast reactor project. The failed fuel detection system of CEFR provides surveillance and protection for the first barrier-fuel cladding of CEFR, so it is one of the most important systems for the safety of CEFR. As tag gas method is an important method for fuel-failure location in fast reactor, CEFR has a medium-term and long-term plan of using this method to locating failed fuel assemblies. This article introduces the main principle of tag gas method, summarizes the application of this method, and compares the advantages and disadvantages of each fuel failure location method. Combining the design characteristics of China Experimental Fast Reactor, this work analyzes the selection principle of tag gas isotopes and the effects on heat transfer capability of fuel element while tag gas filled in. Meanwhile, according to the detection ability of mass spectrometer and the foreign advanced utilization experiences of tag gas method, some suggestions are provided.

The Preliminary Performance Study of ATF With M3 Fuel and Zr-4 Cladding

Metal matrix microencapsulated (M3) fuel is one of the research directions on Accident Tolerant Fuel (ATF). In this article, it provides one of ATF design which consists of BISO (Bistructural ISOtropic) particles embedded in a zirconium alloy matrix, and the cladding coating with silicon carbon (SiC). The temperature distribution of the ATF element has been built, and then the center temperature has also been calculated based on the operation parameters of the large-scale pressurized-water reactor. Simultaneity, the several factors of fuel failure is preliminary analyzed and calculated, especially the pressure shell failure mechanism.