Decay heat power

When performing calculations, the following parameters were taken as the initial condition of the container with SRU core subcriticality 16.7 Pefh intensity of the own neutron source 4-10 N/s decay heat power 1.1 kW siurounding-air temperature 20°Q calculated mean temperature in the core 65.3°C. 

Table 2.11 Decay-heat power from fission products from thermal fission of and for near-infinite reactor operating time ...

Time after reactor shutdown, s Decay-heat power E ( . t), (MeV/s)/ (fission/s) Percent uncertainty... 

Source American Nuclear Society Standards Committee Working Group ANS-5.1, American National Standard for Decay Heat Power in Light Water Reactors, Standard ANSI/ANS-5.1, American Nuclear Society, La Grange Park, III., 1979. With permission of the publisher, the American Nuclear Society. 

The total decay-heat power Pa(T, t) for fission products from a reactor operating at constant total thermal power Pf, and neglecting neutron absorption in fission products, is given by the following simplified method, from the ANS Standard ... 

Neutron absorption in fission products has a small effect on decay-heat power for r < 10 s and is treated by a correction factor G. The corrected total decay-heat power is given by the ANS Standard, in terms of thermal-neutron flux (in neutrons/cm s), reactor operating time T (in s), and cooling time t (in s) as... 

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]. 

ISO (1992) Nuclear energy - Light water reactors Calculation of the decay heat power in nuclear fuels, ISO 10645. 

ANSI/ANS 5.1, "Decay Heat Power In Light Water Reactors , American National Standards Institute, 1979. 

The decay heat power comes mainly from five sources (1) unstable fission products, which decay via a, p-, p+, and y ray emission to stable isotopes (2) unstable actinides that are formed by successive neutron capture reactions in the uranium and plutonium isotopes present in the fuel (3) fissions induced by delayed neutrons (4) reactions induced by spontaneous fission neutrons (5) structural and cladding materials in the reactor that may have become radioactive. Heat production due to delayed neutron-induced fission or spontaneous fission is usually neglected. Activation of light elements in structural materials plays a role only in special cases. 

Based on several experimental data American Nuclear Society (ANS) has assembled decay heat standard ANS-5.1-1979 that contained a single curve to represent all uranium-fueled reactors (Schrock, 1979). The latest version of the standard is ANS-5.1-1994 (Current Standard, Revision of ANSl/ANS-5.1-1979 R1985). The standard was developed to fulfill a need for evaluations of fission reactor performance dependent upon knowledge of decay heat power in the fuel elements. The ANS-5.1 standard provides bases for determining the shutdown decay heat power and its uncertainty following shutdown of LWRs. 

Schrock, V.E. 1979. A revised ANS standard for decay heat from fission products. Nucl. Technol. 46 323, and ANSI/ANS-5.1-1979. 1979. Decay Heat Power in Light Water Reactors. Hindsdale, IL American Nuclear Society. 

A further decrease in primary coolant temperature would cause a rise of reactivity. At the 25 minute of the transient, the reactivity would become slightly positive and the neutron power would start to increase at a high rate. Due to the low absolute value of reactivity, the first stage of this power rise is not characterized by a feedback the latter manifests itself when the neutron power (power immediately released in U nuclei fission) becomes comparable to decay heat power. In this situation the neutron power would be subject to secondary over-control, and then all reactor parameters are characterized by damped oscillations. 

The strong reactivity feedback from the fast neutron spectrum core with transuranic nitride fuel and lead coolant results in passive core power reduction to decay heat power levels while system temperatures remain within structural limits, in the event of loss-of-normal heat removal to the secondary side through the in-reactor lead-to-C02 heat exchangers. 

External natural convection driven passive air-cooling over the guard/containment vessel (surrounding the reactor vessel) that is always in effect and removes decay heat power levels. 

Table 4,5 lists decay heat power for various times following reactor shutdown. Immediately at shutdown, decay heat is 7.5 percent of operating power. For a power reactor operating at 3300 Mw, that amounts to 48 Mw, Decay heat power decays fairly rapidly for approximately one hour, down to i. 8 percent of operating power, but then decays much more slowly so that, after one year, fission product decay heat still generates 3 Mw in a reactor that had been operating at 3300 Mw. 

Figure 4.6 Reactor Fission Power and Fission Product Decay Heat Power Time Behavior Following A Reactor Scram...

The polonium production in an LBE-cooled reactor is so high that in the 80 MW, LBE-cooled ADS developed in the 5th Framework Program of Euratom, the polonium inventory within the primary coolant circuit was evaluated to be 2 kg at equilibrium. This amount of polonium generates a decay heat in the primary system that, 5 days after a reactor shutdown, would equal the decay heat power of the fuel itself (Cinotti et al., 2011). 

Pure lead is not completely exempt firom polonium formation because Pb (the most abundant natural isotope of lead) transmutes into Bi, and Po is eventually produced from neutron capture by ° Bi. The rate of polonium production in pure lead is, however, much lower than in the case of LBE, and it is negligible in terms of decay heat power. In fact, the polonium inventory at equilibrium in the primary system of a 1500 MWth, pure lead-cooled reactor (ie, ELSY) has been calculated to be less than 1 g after 40 years of irradiation (Cinotti et al., 2011). 

Figures 3.3-4 and 3.3-5 illustrate the potential contribution of the zirconium oxidation energy to the overall energy release rate in the core region, as a function of oxidation temperature. Decay heat transfer to residual saturated water below the uncovered portion of the core results in a steam production rate that is proportional to the below-water portion of the decay heat power, As indicated in Figure...

The input data to APRIL.MOD3 included the plant geometry and pre-accident operating conditions as well as some parameters concerning the acdd t sequmce. The latter included, in particular, flie decay heat power curve used before in BWRSAR calculations (see Fig. 1) and the prescribed timing of selected events, as shown in Table 1. 

The depressurization of the reactor pressure vessel when tiie core is already parti y uncovered leads to a sudden and complete core dryout As a resiilt of this, the water level drops below the lower core plate, whidi in turn minimizes the evaporation rate from the pool and practically stops oxidation of the metallic core constituents. Consequently, the decay heat power becomes the only source of energy, and the core heatup process is much slower compared to the scenario witiiout RPV depressurization. 

Figure 1. Decay heat power used in the APRIL.MOD3 simulation of a short-term station blackout accident at Peach Bottom. 

Anon., 2005. Decay Heat Power in Light Water Reactors, vANSI/ANS-5.1-2005, LaGrange, IL American Nuclear Society. 

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