Fuel fission product inventories

Under accident conditions of transport, irradiated fuel pins may fail, with subsequent radioactive release into the package containment system. Data on the fuel fission product inventory, possible failure rate of pin cladding and the mechanism of activity transfer from the failed pin into the containment system are therefore required to enable the package leaktightness to be assessed. 

A research reactor s power is usually in the range 0-100 MW thermal. The fission product inventory and the stored energy in research reactors are smaller than in power reactors. However, some of the research reactors have large power density (>5000 kW thermal per kg of fuel). The typical research reactor is of the swimming pool type, as shown in Fig. 7.6. 

Traditional large-scale nuclear power systems are quite proliferation-resistant. The fresh LEU fuel is of too low enrichment to be directly used in a weapon. The reactors are ill-suited for illicit irradiation and production of weapons material. Plutonium in spent fuel has poor isotopics for weapons applications, and is inherently protected by the significant radiation field arising from the fission product inventory. Even so, safeguards of LWR plants are needed because none of these barriers to proliferation risk is, by itself, completely effective. Diverted fresh fuel could be used to reduce the enrichment effort, given appropriate facilities. Fertile materials could, with difficulty, be irradiated in LWRs. The radiation barrier inherent to spent fuel decays with time, and plutonium from LWR spent fuel is considered a weapons-useable material, even if not ideal. 

For the U-Al alloy fuel assemblages, a faster rate of release was given to some of the more soluble and mobile radionuclides of the fission product inventory, typically 20% of the total activity. 

The radionuclide release rates predicted by the corrosion model took no account of any change in corrosion rates or fission product inventories due to possible criticality. However, since very little of the fuel had been used in the cores, it was decided to investigate the possibility of criticality being achieved as corrosion progressed. If a reactor core could achieve criticality, this could potentially have affected the predicted radionuclide release rates in two ways ... 

A containment gaseous radiation monitor is provided to measure the gamma radioactivity levels in the containment atmosphere by continuous sampling. Leakage is detected by this method, and to the extent practicable quantified, with a response time dependent on various factors such as the fraction of failed fuel, the fission product inventory in the core, and time of transit from the origin of the leak to the monitor. The activity is indicated in the Control Room by the DPS and averaged hourly. 

Use of a steam generator to separate the primary loop from the secondary loop largely confines the radioactive materials to a single building during normal power operation and eliminates the extensive turbine maintenance problems that would result from radio-actively contaminated steam. Radioactivity sources are the activation products from the small amount of corrosion that is present in the primary loop over the 12-18-month reactor cycle, as well as from the occasional (<1 in 10,000) fuel rod that develops a crack and releases a small portion of its volatile fission products. Uranium dioxide fuel is very resistant to erosion by the coolant, so the rod does not dump its entire fission product inventory into the RCS. 

Fluid Fueled Reactors. These concepts include the Molten Salt Reactor, the Molten Chloride Reactor, and aqueous solutions. Online refueling allows high capacity factors and low source terms. Online processing reduces fission product inventory. However, the possibility of selective removal of an essentially pure fissile stream during online reprocessing is a concern. These concepts can have strong negative temperature coefficients of reactivity... 

Safety. A large inventory of radioactive fission products is present in any reactor fuel where the reactor has been operated for times on the order of months. In steady state, radioactive decay heat amounts to about 5% of fission heat, and continues after a reactor is shut down. If cooling is not provided, decay heat can melt fuel rods, causing release of the contents. Protection against a loss-of-coolant accident (LOCA), eg, a primary coolant pipe break, is required. Power reactors have an emergency core cooling system (ECCS) that comes into play upon initiation of a LOCA. 

Uranium dioxide fuel is irradiated in a reactor for periods of one to two years to produce fission energy. Upon removal, the used or spent fuel contains a large inventory of fission products. These are largely contained in the oxide matrix and the sealed fuel tubing. 

The inventory of long-lived fission products is far less than an LWR because of the short exposure of the fuel to minimize Pu-240 production. But the health effects from an accident are comparable because it primarily results from short lived radionuclides. 

Overall, however, the release of refractory fission products from Windscale was less than the release of volatile elements by two or three orders of magnitude, relative to the inventories in the reactor fuel (Table 2.4). Alpha activity on the stack filters and environmental filters was mainly 210Po, derived from the bismuth irradiated in the isotope channels (Crouch Swainbank, 1958 Crooks et al., 1959). The 210Po/137Cs ratio on the environmental filters was about 0.2, with no significant change with distance, suggesting that both activities were carried on the same fume particles. 

Analysis of a sample of primary coolant taken on 30 March reported by English (1979) in The Kemeny Report, showed that about 10% of the inventory of rare gas, I and Cs fission products had been liberated from the fuel, but only about 0.1% of Te and less than 0.01 of alkaline and rare earths fission products (Table 2.6). Further samples of coolant taken 12 d later showed that leaching of refractory fission products had increased their concentrations by an order of magnitude. However,... 

The release of 131I and other fission products in reactor accidents has been considered in the previous chapter. In the Windscale accident, the temperature in the fire zone reached an estimated 1300°C and 8 tonne of uranium metal melted. Over 25% of the 1311 in the melted fuel escaped to atmosphere. In the Chernobyl accident, the fuel was U02, the temperature exceeded 2000°C, and about 25% of the total reactor inventory of 131I was released to atmosphere, as vapour or particulate aerosol. In the Three Mile Island accident, 131I remained almost completely in the reactor coolant. The activities of 131I released in reactor accidents, including that at Chernobyl, have totalled much less than the activities released from weapons tests (Table 2.3). 

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