Kashering a Microwave:
Does it need to be Kashered? Some Rabbanim say that a microwave does not need to be Kashered at all prior to use for meat/milk/Pesach, and simply needs to be wiped clean. Accordingly, it may be used intermittently for meat and then dairy and Pesach, after cleaning it. The majority consensus of Poskim however is that a microwave requires Kashering, and thus cannot be used with meat/dairy/Pesach in its un-Kashered state. Practically, one must abide by the latter opinion, as the former opinion does not take all factors into account and is hence inaccurate. According to all, a combination microwave oven, which is both an oven and a microwave, requires Kashering. We will now discuss if and how a microwave can be Kashered:
How to Kasher it: Some Poskim rule that it is not possible to Kasher a microwave. Other Poskim rule it can be Kashered through steaming water inside it. This is accomplished through the following steps: The microwave is to be cleaned, not used for 24 hours, and have water with bleach/soap placed in it and heated for about 10-20 minutes, until it steams out. Practically, one is to purchase a new microwave for Pesach. If this is not possible, and one is in need to use the microwave, then one is to do as stated below, and use a hermetically sealed container to cook the food, as stated next.
Cooking in a hermetically sealed container: In all cases, it is permitted to cook in a microwave food [meat/dairy/Pesach] that is contained within a hermetically sealed container which does not allow any vapor to escape or enter. This applies even if the microwave has not been Kashered, and certainly if it has been Kashered in the method mentioned above. Some Poskim however discourage using the microwave in this method as it occurs that the hermetic sealing tears or opens during the cooking, which would then pose a Kashrus issue. It is therefore best to only use the microwave in this method after first Kashering it.
Some Poskim rule it is not possible to Kasher a microwave. Other Poskim rule a microwave does not need to be Kashered at all. Other Poskim rule it can be Kashered through steeming water inside. This is accomplished trhough the following steps: The microwave is to be cleaned, not used for 24 hours and have water with bleach placed in it and heated for about 10-20 minutes, until it steams out. Practcially, one is to purchase a new microwave for Pesach. If this is not possible, then one is to do as stated above.
 See Piskeiy Tehsuvos 451/22; Sefer Hakashrus [Fuchs] 1/47-50 [pp. 48-49]; Nitei Gavriel 80/16; See also Hadarom Choveret 6 Nissan 5722; Kovetz Beis Ahron Yisrael 4/3
How does a microwave cook? A microwave is a rapid cooking element, which can warm and cook food much quicker than traditional cooking methods. Now, how does the microwave achieve its rapid cooking? The microwave does not use the heat of a fire or electricity to cook but rather cooks the food using radiation, or radio electromagnetic waves, which is projected from a vacuum tube and bounced off the metal lined walls of the microwave which penetrate the food from all sides. These waves hasten the movement of the water molecules in the food to atomic levels hence generating heat. [Heat is generated from movement and friction.] This form of cooking cooks the food much quicker than fire or electricity, as the radioactive waves hits the food equally in all areas and furthermore, penetrates the inside of the food molecules hence making the entire mass of the food an equal recipient of the heat. This is unlike fire or electric cooking which heats the external part of the food, and that heat then must travel to the inner part of the food in order to cook it. Likewise, this form of cooking only heats the actual food, as it does not actually send heat to the food but causes the food to heat itself up. Accordingly, all other areas and items of the microwave might remain cold, including the walls and certain plastic or glass containers which cover the food. The only way these items will become hot is if they are in contact with the food itself. The radio waves harmlessly pass through these containers into the food and do not cause any heating within them being they do not contain water molecules or other polar charge component. [See Hakashrus ibid footnote 100; See here for an educational video on how a microwave works. https://www.youtube.com/watch?v=kp33ZprO0Ck]
 Rav Yitzchak Yosef
 The reason: As the actual walls of the microwave do not heat up at all, and only the food itself heats up.
 See all Gedolei HaPoskim mentioned in Hakashrus ibid and Piskeiy Teshuvos ibid, which include Rav Wozner, Rav Elyashiv, Rav Sheinberg, Rav Halbershtam; Nitei Gavriel ibid footnote 26 in name of Rav Neiman of Montreal;
 The reason: a) Although the walls of the microwave do not heat, nevertheless the steam and spills of the food inside make the walls absorb the food and hence it must be Kashered. [Piskeiy Teshuvos ibid; See Admur 451/41, Michaber 451/14, and M”B 451/81 that a vessel which absorbed the steam of an Issur requires Hagala] b) As there is a vent duct in the microwave that contains actual steam of food, and that area is not Kasherable. [Rav Neiman ibid
 As although the walls don’t heat up the microwave receives steam from the foods and hence must be Kashered.
 See Piskeiy Teshuvos ibid; Siddur Pesach Kehilchaso that 8/3 that it can only be Kashered through Iruiy Keli Rishon and Even Meluban
 The reason: a) Although the walls of the microwave do not heat, nevertheless the steam and spills of the food inside make the walls absorb the food and hence it must be Kashered. Now, it is not possible to Kasher the microwave as one cannot blow torch it, place it in boiling water, and many Poskim rule an item cannot be Kashered through steaming water inside it. [See Shoel Umeishiv Telisa 3/125; Sdei Chemed Mareches Hei 24; Chmaetz Umatzah 17/12 that it is not possible to Kasher through vapor] It therefore has no viable path for Kashering. A second reason is b) As there is a vent duct in the microwave that contains actual steam of food, and that area is not cleanable or Kasherable. [Rav Neiman ibid]
 Teshuvos Vehanhagos 2/212; Yalkut Yosef Pesach p. 360; Piskeiy Teshuvos ibid regarding Shaas Hadechak
 The reason: As we rule that it is possible to Kasher an item through steam, and just like the microwave absorbed the food through steam, so too it can be Kashered through steam. [See Peri Chadash 121; P”M Y.D. 94 M”Z 1; Tevuos Hasadeh 3/3]
 See Hakashrus ibis footnote 105 that so ruled Rav Wozner, Rav Shternbuch and Rav Mordechai Eliyahu
 Piskeiy Teshuvos ibid writes to leave it on for one hour; Sefer Hakashrus ibid writes [based on Mitbach Kehalacha p. 58 and Techumin 8/21] to enter a half a liter of water and leave the microwave on until it steams out; Rav Yitzchak Yosef says to leave it on for six minutes
 Teshuvos Vehanhagos 2/212; Koveitz Mibeis Levi 3/22; 7/25; Yalkut Yosef Pesach p. 360; Piskeiy Teshuvos ibid; Hakashrus ibid
 Piskeiy Teshuvos ibid; Hakashrus ibid footnote 105; Nitei Gavriel ibid concludes that due to the many opinions, one is not to use it for Pesach without directive from a Rav
 Hakashrus ibid; Nitei Gavriel ibid in name of Rav Neiman
 Pischeiy Halacha Kashrus p. 28; Piskeiy Teshuvos ibid footnote 106
 See Kovetz Mibeis Levi 3/22-9; Hadarom ibid; Beis Ahron Veyisrael ibid