Why has Japan's currency not devalued after the devastation?

In other words, why is Japanese currency the same as it was in March, namely it has not fallen in consideration of the damage done to the country. In other words, why has the IMF deemed Japan not good enough to devalue its currency? This would mean any foreign money going into Japan would be worth more money to the Japanese and Japanese currency going out would be worth less but at this time of failing crops and such this would be better, so why has this not happened?