Shutdown fission products

The xenon poisoning effect is well known in the field of nuclear engineering as the effect that prevented early reactors from rapid startup after shutdown. In addition to Xe being created in high relative yields as an independent yield fission product in the fission of U and Pu, it is also created in high amounts by a chain-yield fission product from decay of Te and I via ... 

Figure 2.9 Concentration changes of fission products during steady production and after shutdown,...

The parameter fp is the total number of fissions after irradiation time T per initial fissile atom, calculated by techniques described in Chap. 3. Equation (2.94) applies for operating times < 1.2614 X 10 s (4 years), shutdown times <10 s, and <3.0. A more detailed technique for calculating fission-product decay-heat power from an arbitrary time-dependent fission power, including contributions from the fission of U, U, and Pu, is given in the ANS Standard [A2]. 

The equations of Sec. 6.2 give the number of atoms of each fission product after a reactor has been run at stated conditions for a specified time. If the reactor is then shut down, the fission products build up and decay in accordance with the laws of simple radioactive decay, which were outlined in Sec. 3. If the nuclides in the decay chain are removed orJy by radioactive decay during reactor operations, the equations of Sec. 3 describe the changes with time of the number of atoms of any nuclide in the decay chain. If a member of a fission-product decay chain or its precursors in the decay chain are removed by neutron absorption, equations for the amount of each nuclide present at time t after shutdown may be obtained by applying the equations of radioactive decay to the amount present at shutdown. 

The standard reactor modules and "safety-related" buildings, structures, systems, portions of systems, and components dedicated to assuring reactor shutdown, decay heat removal, fission product retention, and security, including new (unirradiated) fuel. 

In the shutdown mode, the reactor vessel is fully pressurized or, at different times, in various stages of depressurization. Afterheat from fission product decay is generated at rates of up to about 7 percent of the core power level prior to shutdown, depending on the time interval since shutdown. The core decay heat is removed by the HTS. When the HTS is not available, the heat is removed by the Shutdown Cooling System (SCS). The outer control rods are normally fully Inserted during shutdown, and meet the required shutdown margin, with due allowances for uncertainties, even if the maximvim reactivity worth rod remains fully withdrawn. For cold shutdown, the control rods in the inner reflector are also Inserted and for this case, the maximum reactivity worth control rod is in the inner reflector. The neutron flux level is continuously monitored by the source range detectors. 

From 1992, the role of Phenix as an irradiation facility has been emphasized, particularly in support of the CEA R D programme in the context of line 1 of the December 30th 1991 law on long-lived radioactive waste management. The first experiment, called SUPERFACT, led to the incineration of minor actinides (neptunium and americium). This programme was further strengthened in 1997, to compensate for the shutdown of Superphenix. It involves transmutation of Minor Actinides and Long-Lived Fission Products. 

In order for the-reactor to satisfy these requirements, careful consideration had to be given to the minimum quantity of fissionable material which would be necessary at the flux level desired, the amount of foreign matter to be inserted, the need to reactivate the reactor within a few hours or less after shutdown and overcome fission product poisoning, temperature effect, and depletion of fissionable material, to name the most important. 

The hard y-ray activity (about 3 Mev) aftel shutdown, however, depends on the yield and decay of specific.fission product chains. These have been measured by the photoneutron threshold reacfioii in heaVy. water. The average lifetime and yield of the y rays of energies higher than this value are ... 

Although this intensity is one-half tolerance, the decay of fission products reduces the value to one-tenth tolerance at 1 hr after shutdown. 

Cooling After Shutdown. After - an operating, reactor is shut, down, the total heat liberated due to.the-absorption of gamma and beta radiation from the fission products is given-by the empirical equation ... 

Only the coohmt has to be circulate, and the same tope free from fission fragments. A schedule of replace-circulation cools the reaction and absorption zones. No 65 ment can readily be worked out to keep the reactor gas is evolved from the reaction zone and poisoning fac- operating with minimum shutdown times, in accordance tors, due to retention of fission product neutron absorb- with the power output and resultant use rate of the ers in the reaction zone, do not become critical because plutonium. 

This report examines the severe accident sequences and radionuclide source terms at the Sizewell pressurised water reactor with a piestressed concrete containment, the Konvoi pressurized water reactor with a steel primary contaimnent, the European Pressurised water Reactor (EPR) and a boiling water reactor with a Mark 2 containment. The report concludes that the key accident sequences for European plant designs are transient events and small loss-of-coolant accidents, loss of cooling during shutdown, and containment bypass sequences. The most important chemical and transport phenomena are found to be revaporisation of volatile radionuclides from the reactor coolant system, iodine chemistry, and release paths through the plant. Additional research is recommended on release of fission products from the fuel, release of fission products from the reactor coolant system, ehemistry of iodine, and transport of radionuclide through plants. 

The principal natural phenomena that influence transient operation are the temperature coefficients of the moderator and fuel and the buildup or depletion of certain fission products. Reactivity balancing may occur through the effects of natural phenomena or the operation of the reactor control system using the RCCs or chemical "shim." A change in the temperature of moderator or fuel (e.g., due to an increase or decrease in steam demand) will add or remove reactivity (respectively) and cause the power level to change (increase or decrease, respectively) xmtil the reactivity change is balanced out. RCC assemblies are used to follow fairly large load transients, such as load-follow operation, and for startup and shutdown. 

The chemical shim system uses the soluble neutron absorber boron (in the form of boric acid), which is inserted in the reactor coolant during cold shutdown, partially removed at startup, and adjusted in concentration during core lifetime to compensate for such effects as fuel consumption and accumulation of fission products which tend to slow the nuclear chain reaction. The control system allows the plant to accept step... 

Similar to other thermal reactor designs, there are three basic functions that are necessary to mitigate the consequences of fission product releases during a postulated accident. The functions, referred to as the 3 Cs, are control, cool, and contain. Control refers to safe reactor shutdown. Cool involves the removal of heat—from the fuel produced by the fission process (at power) or by the decay heat after reactor shutdown—and rejection of the heat to a heat sink. Contain is simply the physical means to prevent the release of radioactive material to the atmosphere by provision of containment systems. 

During operation of the MSRE however, an issue of modest concern did surface. That being that the fission product tellurium was depositing within the grain boundaries of Hastelloy N ". Upon observation of test samples and reactor components post-shutdown, shallow surface cracking, less than 15 mils deep, was evident on surfaces exposed to the fuel salt. 

The primary consequence of burnup is a drop in /c-effective as the fuel bums out and fission products are built up. This drop is compensated by the build-up of new fissile isotopes (notably Pu-239 from U-238 neutron absorption in uranium-fueled reactors). Generally, boiling water reactors and pressurized water reactors replace the fuel in stages, with fresh fuel assemblies replacing the most burned-out assemblies at scheduled shutdowns with nonreplaced assemblies often moved (shuffled) to new positions to optimize the reactor operating characteristics. 

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