Abstract: A neutron poison test device for verifying that high density spent nuclear fuel racks contain neutron poison (a substance that absorbs neutrons without inducing a nuclear fission which produces more neutrons) comprises a can having a square cross-section, which contains a fast neutron source and neutron moderator, and four square cans each containing a thermal neutron detector, one detector can positioned opposite each of the four walls of the source can. The five cans are carried at the lower ends of five elongated tubes, which are connected together at the top by a structure having means for receiving the lifting hook of a hoist or crane. A neutron shield having a hollow square cross-section is supported for sliding movement on the center tube and is adapted to envelop the source can when the test device is outside the high density storage rack. Each of the four walls of the sliding shield contains a Boral plate or other thermal neutron absorber plate.