A first year undergraduate engineering student would be able to build a reliable temperature monitor.
Right. Because there are so many combinations of materials that can withstand temperatures in the thousands of degrees F and the intense neutron flux in a commercial reactor core for any prolonged period. Core status is measured by the temperature of the water entering and leaving the core - the core power can be calculated by how much the water heats up. Safety limits are usually given in terms of power, because the behavior has to be calculated.
To invent, you need a good imagination and a pile of junk. -- Thomas Edison