Calculation of Decay Heat

The quantity of decay heat being produced in a fuel assembly at any time after shutdown can be computed in two ways. The primary way is to compute the quantity of fission products present at the time of shutdown. This is a pretty explained process and is based upon power history. For a given kind of fuel, the focuses, decay energies, and half lives of fission products are known. By beginning from a recognized value, depend on power history at shutdown, the decay heat generation rate can be computed for any time after shutdown.

The precise solution should take into account the fact that there are hundreds of various radio nuclides present in the core, each with its own focus and decay half-life. It is possible to make a rough estimation by using a single half-life which represents the whole decay of the core over a firm period of time. The equation which uses this estimation is the equation shown below:

Here:

Q = decay heat generation rate at some time after shutdown

Q_o = initial decay heat immediately after shutdown

time = quantity of time since shutdown

half-life = overall decay half-life of the core

The second technique is much simpler to employ, though is not helpful for forecasting heat loads in the future. To compute the decay heat load at a given point after shutdown, secure any heat elimination components from the prime system or spent fuel pool and plot the heat up rate. When the mass of the coolant and the specific heat of the coolant are recognized, the heat generation rate can be precisely computed.

Here:

Q = decay heat (Btu/hr)

m = mass of coolant (lbm)

c_p = specific heat capacity of coolant (Btu/lbm-^oF)

ΔT = temperature change of coolant (^oF)

Δt = time over which heat up occurs (hr)