# Megawatt-day (unit MWd) – Energy Unit

Energy is generally defined as the potential to do work or produce heat. This definition causes the SI unit for energy to be the same as the unit of work – the joule (J). Joule is a derived unit of energy, and it is named in honor of James Prescott Joule and his experiments on the mechanical equivalent of heat. In more fundamental terms, 1 joule is equal to:

1 J = 1 kg.m2/s2

Since energy is a fundamental physical quantity and is used in various physical and engineering branches, there are many energy units in physics and engineering.

## Megawatt-day (unit: MWd)

Megawatt-day (unit: MWd).  Megawatt-day is a derived unit of energy. It is used to measure the energy produced, especially in power engineering. One megawatt day equals one megawatt of power produced by a power plant for one day (megawatts multiplied by the time in days). 1 MWd = 24,000 kWh. At nuclear power plants, there are also gigawatt-days because it approximately corresponds to energy produced by power plants for one day. This unit (MWd) was also used to derive the unit of fuel burnup. The most commonly used measure of fuel burnup is the fission energy release per unit mass of fuel. Therefore fuel burnup of nuclear fuel normally has units of megawatt-days per metric tonne (MWd/MTU), where tonne refers to a metric ton of uranium metal (sometimes MWd/tU HM as Heavy Metal). In this field, the megawatt-day refers to the reactor’s thermal power, not the fraction converted to electricity. For example, for a typical nuclear reactor with thermal power of 3000 MWth, about ~1000MWe of electrical power is generated in the generator. For example, a reactor with 100 000 kg of fuel operating at 3000MWth power level for 1000 days would have a burnup increase of 30,000 MWd/MTU. In words of fissions, the fissioning of about 1 g of U-235 produces about 1 MWd of thermal energy (see: Energy Release per Fission).

• 1 MWd = 8.64 x 1010 J
• 1 MWd = 2.06 x 1010 cal
• 1 MWd = 8.19 x 107 BTU

References:
Reactor Physics and Thermal Hydraulics:
1. J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
3. W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
4. Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
5. Todreas Neil E., Kazimi Mujid S. Nuclear Systems Volume I: Thermal Hydraulic Fundamentals, Second Edition. CRC Press; 2 edition, 2012, ISBN: 978-0415802871
6. Zohuri B., McDaniel P. Thermodynamics in Nuclear Power Plant Systems. Springer; 2015, ISBN: 978-3-319-13419-2
7. Moran Michal J., Shapiro Howard N. Fundamentals of Engineering Thermodynamics, Fifth Edition, John Wiley & Sons, 2006, ISBN: 978-0-470-03037-0
8. Kleinstreuer C. Modern Fluid Dynamics. Springer, 2010, ISBN 978-1-4020-8670-0.
9. U.S. Department of Energy, THERMODYNAMICS, HEAT TRANSFER, AND FLUID FLOW. DOE Fundamentals Handbook, Volume 1, 2, and 3. June 1992.

Energy