Fission products inventories

A reactor core s fission product inventory is the primary source of radioactivity from which the public is protected by the following independent barriers ... 

These results are the basis for optimism concerning the calculated distribution of fission products in fallout using the described method. A further test will be the application of the DELFIC particle distribution calculational system (3) using the calculated particle fission product inventories and comparing calculated and experimental radiation fields. 

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. 

R. H. Clarke and R. E. Utting, FISP 2 The Current Version of the FISP Program for Fission Product Inventories, CEGB Rep. RD/B/N1737 (1970). 

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. 

Category A Reactor Facilities means those production, test, and research reactors designated by DOE based on power level (e.g., design thermal power rating of 20 megawatts steady state and higher), potential fission product inventory, and exper i mentaI capab i I i ty,... 

The material in-process inventory is that of a maximally irradiated target. During the one hour initial processing period, the entire radiological inventory is potentially available for release. Once the iodine has been collected in the iodine trap, it would not be vulnerable to DBE effects. Thus, for the remainder of the processing period only the residual process liquid, containing about 75% of the total fission product inventory and totaling about 14,000 curies, is available as a potential source term. This source term is present until the waste is solidified (approximately 3 hours). 

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. 

Void reactivity effect calculations were performed for the cores near the end of the bumup cycle to determine the effect of the lower content and larger fission product inventory. The results showed that the void reactivity was about the same value as for the fresh core configurations. 

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. 

After this, the fission product release rate falls away quickly as the more mobile atoms are released, until the year 2500. After that, the rate is follows the less soluble atoms in the fission product inventory. The actinide release follows this slow degradation rate as time goes on, it is seen however, that the overall rate of activity release is dominated by the activation products. Between the years 2500 and 3360, the release rate is of the order of 0.2 GBq-a. ... 

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 prompt criticality with core disassembly in the far distant future would involve very little radioactivity compared to the present radionuclide inventory in these cores. By the year 2700, nearly all of the current fission product inventory in the cores would have decayed. Also, the amount of fission products produced in a prompt critical excursion is relatively small. For example, the amount of Cs generated in a 10" fission criticality excursion (about the same as the SL-1 accident in the USA) would be 0.044 GBq [31]. 

The power, and hence the fission product inventory, are generally low. 

Pressure vessel unit provided with isolation valves (retention of fission product inventory in primary helium at normal operation) and with safety valves (for overpressure protection, release into confinement)... 

If the leak is trough a containment penetration, the penetration should be isolated (e.g. by closing a valve in the leakage path). If this is not sufficient or not possible, the containment spray system should be started (if not yet done) in order to (1) reduce the airborne fission products inventory in the containment, and (2) reduce the driving force for the leak. 

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. 

Akers, D. W., McCardell, R. K., Russell, M. L., Worku, G. TMl-2 core materials and fission product inventory. Nucl. Engng. and Design 118, 451-461 (1990)... 

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