Right now the main concern in Japan is cleaning up this mess. They wont have access to the melted nuclear material for quite a few years now, from I have read. So basically Japan has weighed the costs of this one accident and have determined that the risks are just too great. That is their decision to make.
When one weighs the risk of an action one also must weigh the risk of alternative actions.
If they actually take the action to shut down their nuclear power plants they will need to replace that generation with another power source. Renewables simply can not fill that void, nor can improved efficiency.
Their choices will be increased reliance on fossil fuel (that must be imported) or to become a preindustrial society.
Reliance on fossil fuel for electrical generation will make them more vulnerable to political and military constrictions of their energy supply lines. Their increased use of fossil fuels will also put upward pressure of the price of these commodities increasing the cost of fuel for other less affluent countries.
Although this accident is severe and cleanup will be costly it does not prove the inviability of nuclear power in Japan. As I said earlier they can learn from the vulnerabilities revealed by this accident and make improvements to their existing plants. They can also make sure that new power plants are designed to more robust requirements.
Japan needs nuclear power to remain a world class industrial country. With out nuclear Japan will be come a second or third tier economy.